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Sample records for afar geoscientific lithospheric

  1. Seismically Imaging the Destruction of Continental Lithosphere beneath Afar

    NASA Astrophysics Data System (ADS)

    Rychert, C. A.; Hammond, J. O.; Kendall, J. M.; Harmon, N.; Keir, D.; Ebinger, C. J.; Stuart, G. W.; Belachew, M.

    2011-12-01

    The onset of continental rifting is often accompanied by production of large volumes of molten rock. However, the influence of magmatism on the deforming lithosphere during the breakup process is not well understood. In particular, whether lithosphere is predominantly thinned by mechanical stretching or thermal destruction from melt infiltration, and how this impacts melt production during the breakup process remains unconstrained. Here we use S-to-P (Sp) receiver functions to image the onset of decompression melting beneath Afar, Ethiopia; a region where continental breakup gives way to oceanic spreading. We analyze three broadband datasets using S-to-p (Sp) imaging, which provide high resolution imaging beneath the rift and surrounding regions: the Ethiopia/Kenya Broadband Seismic Experiment (EKBSE), the Ethiopia Afar Geophysical Lithospheric Experiment (EAGLE), and a new UK/US led deployment of 46 stations in the Afar depression and surrounding area. We use two methodologies to investigate structure and locate robust features: 1) binning by conversion point and then simultaneous deconvolution in the frequency domain, and 2) extended multitaper followed by migration and stacking. At ~75 km depth we image a strong, sharp, velocity reduction on the flank of the rift that likely represents the lithosphere-asthenosphere boundary, versus a strong velocity increase with depth beneath the rift. The sharpness of the negative gradient can only be explained by melt ponded at the base of the lithosphere. The depth and magnitude of the positive gradient resemble those expected from numerical estimates for the onset of decompression melting in a mid-ocean ridge environment where ~1% melt is retained in the mantle. This implies that the mantle lithosphere beneath Afar has been destroyed; melt intrusion likely played a key role in the initial destruction of continental lithosphere, but the degree of influence from a thermal plume today in Afar is minimal.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  3. Plume- Ridge Lithospheric Interactions: Cases of Afar (Africa)

    NASA Astrophysics Data System (ADS)

    Montagner, J. P.; Stutzmann, E.; Sicilia, D.; Sebai, A.; Beucler, E.; Silveira, G.; Cara, M.; Debayle, E.; Leveque, J. J.

    2003-04-01

    Detection of mantle plumes in geophysical and geochemical data is controversial and trigger vigorous debates. It remains unclear how plumes are formed, their origin at depth, and whether they act independently from plate tectonics. We may learn about the role of plumes in mantle dynamics by studying their interactions with lithosphere and crust below ridges and the way in which they perturb the flow pattern in the uppermost mantle. Several regional tomographic studies of seismic velocity and anisotropy around several hotspots were obtained during the last 2 years. Their lateral resolution is smaller than 1000km and they enable to make qualitative intercomparison between Afar (Horn of Africa Program), Azores (COSEA project) in the Atlantic, La Reunion in the Indian Ocean and Pacific provinces hotspots. These models demonstrate that there is not only one family of plumes but several ones. Some plumes are confined in the uppermost 200km but a few can originate in the transition zone and even at the Core-mantle Boundary for superplumes. Seismic anisotropy which is a good marker of deformation processes and mantle flow pattern, shows that the interaction between a plume and a ridge below the lithosphere can occur over distances larger than 1000km, via sublithospheric channels. The existence of LACs (Low Anisotropy Channels) below the Pacific plate seems to be intimately related to the active hotspots in Central Pacific and indicate a future reorganization of plate boundaries. Another important consequence of the interaction between plume and ridge is the triggering of secondary convection in the asthenosphere, which will be discussed during the presentation.

  4. Receiver function imaging of the lithosphere-asthenosphere boundary and melt beneath the Afar Rift in comparison to other systems

    NASA Astrophysics Data System (ADS)

    Rychert, Catherine A.; Harmon, Nicholas

    2015-04-01

    Heating, melting, and stretching destroy continents at volcanic rifts. Mantle plumes are often invoked to thermally weaken the continental lithosphere and accommodate rifting through the influx of magma. However the relative effects of mechanical stretching vs. melt infiltration and weakening are not well quantified during the evolution of rifting. S-to-p (Sp) imaging beneath the Afar Rift provides additional constraints. We use two methodologies to investigate structure and locate robust features: 1) binning by conversion point and then simultaneous deconvolution in the frequency domain, and 2) extended multitaper followed by migration and stacking. We image a lithosphere-asthenosphere boundary at ~75 km beneath the flank of the Afar Rift vs. its complete absence beneath the rift. Instead, a strong velocity increase with depth at ~75 km depth is imaged. Beneath the rift axis waveform modeling suggests the lack of a mantle lithosphere with a velocity increase at ~75 km depth. Geodynamic models that include high melt retention and suppress thermal convection easily match the required velocity-depth profile, the velocity increase arising from a drop in melt percentage at the onset of decompression melting. Whereas, models with conservative melt retention that include thermal buoyancy effects cannot reproduce the strong velocity increase. The shallow depth of the onset of melting is consistent with a mantle potential temperature = 1350 - 1400°C, i.e., typical for adiabatic decompression melting. Trace element signatures and geochemical modeling have been used to argue for a thick lithosphere beneath the rift and slightly higher mantle potential temperatures ~1450°C, although overall, given modeling assumptions, the results are not in disagreement. Therefore, although a plume initially destroyed the mantle lithosphere, its influence directly beneath Afar today is not strong. Volcanism continues via adiabatic decompression melting assisted by strong melt buoyancy

  5. A kinematic model for Afar Depression lithospheric thinning and its implications for hominid evolution: an exercise in plate-tectonic paleoanthropology

    NASA Astrophysics Data System (ADS)

    Redfield, T.; Often, M.; Wheeler, W. H.

    2002-12-01

    We present a detailed Nubia-Arabia-Somalia (NU-AR-SOM) kinematic reconstruction based on magnetic sea floor isochrons in the Gulf of Aden and Red Sea and piercing points along the Red Sea margins. The reconstruction is combined with digital topographic and depth-to-Moho data to constrain in 4D the Late Oligocene to present-day evolution of the Afar supra-Moho crust. Opposite end-member models for crustal evolution are described. We conclude that less than 20% of the present-day Afar supra-Moho crust was constructed by magmatic processes such as diking and underplating. The reconstructions indicate that the greater percentage of crustal thinning (extension) occurred before 6.2 Ma. We model the thinning of the effective elastic lithosphere that accompanied extension, and show that the regional-scale topographic development of the Afar depression was virtually complete by Mid Pliocene time. The plate-tectonic model has paleoanthropological implications. Prior to 6.2 Ma the proximal positions of NU-SOM, AR, and the Danakil block suggest subaerial conditions prevailed between Yemen and Ethiopia. Uninhibited Africa-Eurasia faunal exchange through Afar and Arabia (corroborated by isotopic and paleontologic data) was tectonically permissible until the time of the earliest hominids. Continued stretching caused the Afar land bridge(s) to disappear during Early to Mid Pliocene time. Primitive hominid populations living within the Afar Depression became isolated from AR sometime before ~3.2 Ma. With the plateau becoming less habitable due to long-term Late Neogene cooling, hominids that remained in the Afar Depression were required to adapt to a smaller range that was effectively bounded by the already well-developed NU-SOM escarpments and the newly opened Straits of Bab el Mandeb. The combination of high quality habitat,topographic confinement, and a gradual (tectonic) reduction in range, exacerbated by potentially severe fluctuations in local climate (well documented by land

  6. Mapping the evolving strain field during continental breakup from crustal anisotropy in the Afar Depression.

    PubMed

    Keir, Derek; Belachew, M; Ebinger, C J; Kendall, J-M; Hammond, J O S; Stuart, G W; Ayele, A; Rowland, J V

    2011-01-01

    Rifting of the continents leading to plate rupture occurs by a combination of mechanical deformation and magma intrusion, yet the spatial and temporal scales over which these alternate mechanisms localize extensional strain remain controversial. Here we quantify anisotropy of the upper crust across the volcanically active Afar Triple Junction using shear-wave splitting from local earthquakes to evaluate the distribution and orientation of strain in a region of continental breakup. The pattern of S-wave splitting in Afar is best explained by anisotropy from deformation-related structures, with the dramatic change in splitting parameters into the rift axis from the increased density of dyke-induced faulting combined with a contribution from oriented melt pockets near volcanic centres. The lack of rift-perpendicular anisotropy in the lithosphere, and corroborating geoscientific evidence of extension dominated by dyking, provide strong evidence that magma intrusion achieves the majority of plate opening in this zone of incipient plate rupture. PMID:21505441

  7. Mapping the evolving strain field during continental breakup from crustal anisotropy in the Afar Depression

    PubMed Central

    Keir, Derek; Belachew, M.; Ebinger, C.J.; Kendall, J.-M.; Hammond, J.O.S.; Stuart, G.W.; Ayele, A.; Rowland, J.V.

    2011-01-01

    Rifting of the continents leading to plate rupture occurs by a combination of mechanical deformation and magma intrusion, yet the spatial and temporal scales over which these alternate mechanisms localize extensional strain remain controversial. Here we quantify anisotropy of the upper crust across the volcanically active Afar Triple Junction using shear-wave splitting from local earthquakes to evaluate the distribution and orientation of strain in a region of continental breakup. The pattern of S-wave splitting in Afar is best explained by anisotropy from deformation-related structures, with the dramatic change in splitting parameters into the rift axis from the increased density of dyke-induced faulting combined with a contribution from oriented melt pockets near volcanic centres. The lack of rift-perpendicular anisotropy in the lithosphere, and corroborating geoscientific evidence of extension dominated by dyking, provide strong evidence that magma intrusion achieves the majority of plate opening in this zone of incipient plate rupture. PMID:21505441

  8. Seismically imaging the Afar plume

    NASA Astrophysics Data System (ADS)

    Hammond, J. O.; Kendall, J. M.; Bastow, I. D.; Stuart, G. W.; Keir, D.; Ayele, A.; Ogubazghi, G.; Ebinger, C. J.; Belachew, M.

    2011-12-01

    Plume related flood basalt volcanism in Ethiopia has long been cited to have instigated continental breakup in northeast Africa. However, to date seismic images of the mantle beneath the region have not produced conclusive evidence of a plume-like structure. As a result the nature and even existence of a plume in the region and its role in rift initiation and continental rupture are debated. Previous seismic studies using regional deployments of sensors in East-Africa show that low seismic velocities underlie northeast Africa, but their resolution is limited to the top 200-300km of the Earth. Thus, the connection between the low velocities in the uppermost mantle and those imaged in global studies in the lower mantle is unclear. We have combined new data from Afar, Ethiopia with 6 other regional experiments and global network stations across Ethiopia, Eritrea, Djibouti and Yemen, to produce high-resolution models of upper mantle P- and S- wave velocities to the base of the transition zone. Relative travel time tomographic inversions show that the top 100km is dominated by focussed low velocity zones, likely associated with melt in the lithosphere/uppermost asthenosphere. Below these depths a broad SW-NE oriented sheet like upwelling extends down to the top of the transition zone. Within the transition zone two focussed sharp-sided low velocity regions exist: one beneath the Western Ethiopian plateau outside the rift valley, and the other beneath the Afar depression. The nature of the transition zone anomalies suggests that small upwellings may rise from a broader low velocity plume-like feature in the lower mantle. This interpretation is supported by numerical and analogue experiments that suggest the 660km phase change and viscosity jump may impede flow from the lower to upper mantle creating a thermal boundary layer at the base of the transition zone. This allows smaller, secondary upwellings to initiate and rise to the surface. Our images of secondary upwellings

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

    NASA Astrophysics Data System (ADS)

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

    2013-02-01

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

  10. Which mantle below the active rift segments in Afar?

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  11. Receiver function imaging of the onset of melting, implications for volcanism beneath the Afar Rift in contrast to hotspot environments

    NASA Astrophysics Data System (ADS)

    Rychert, C. A.; Harmon, N.; Hammond, J. O.; Laske, G.; Kendall, J.; Ebinger, C. J.; Shearer, P. M.; Bastow, I. D.; Keir, D.; Ayele, A.; Belachew, M.; Stuart, G. W.

    2012-12-01

    Heating, melting, and stretching destroy continents at volcanic rifts. Mantle plumes are often invoked to thermally weaken the continental lithosphere and accommodate rifting through the influx of magma. However the relative effects of mechanical stretching vs. melt infiltration and weakening are not well quantified during the evolution of rifting. S-to-p (Sp) imaging beneath the Afar Rift and hotspot regions such as Hawaii provides additional constraints. We use data from the Ethiopia/Kenya Broadband Seismic Experiment (EKBSE), the Ethiopia Afar Geophysical Lithospheric Experiment (EAGLE), a new UK/US led deployment of 46 stations in the Afar depression and surrounding area, and the PLUME experiment. We use two methodologies to investigate structure and locate robust features: 1) binning by conversion point and then simultaneous deconvolution in the frequency domain, and 2) extended multitaper followed by migration and stacking. We image a lithosphere-asthenosphere boundary at ~75 km beneath the flank of the Afar Rift vs. its complete absence beneath the rift, where the mantle lithosphere has been totally destroyed. Instead a strong velocity increase with depth at ~75 km depth matches geodynamic model predictions for a drop in melt percentage at the onset of decompression melting. The shallow depth of the onset of melting is consistent with a mantle potential temperature = 1350 - 1400°C, i.e., typical for adiabatic decompression melting. Therefore although a plume initially destroyed the mantle lithosphere, its influence directly beneath Afar today is minimal. Volcanism continues via adiabatic decompression melting assisted by strong melt buoyancy effects. This contrasts with a similar feature at much deeper depth, ~150 km, just west of Hawaii, where a deep thermal plume is hypothesized to impinge on the lithosphere. Improved high resolution imaging of rifting, ridges, and hotspots in a variety of stages and tectonic settings will increase constraints on the

  12. Upper mantle temperature and the onset of extension and break-up in Afar, Africa

    NASA Astrophysics Data System (ADS)

    Armitage, John J.; Ferguson, David J.; Goes, Saskia; Hammond, James O. S.; Calais, Eric; Rychert, Catherine A.; Harmon, Nicholas

    2015-05-01

    It is debated to what extent mantle plumes play a role in continental rifting and eventual break-up. Afar lies at the northern end of the largest and most active present-day continental rift, where the East African Rift forms a triple junction with the Red Sea and Gulf of Aden rifts. It has a history of plume activity yet recent studies have reached conflicting conclusions on whether a plume still contributes to current Afar tectonics. A geochemical study concluded that Afar is a mature hot rift with 80 km thick lithosphere, while seismic data have been interpreted to reflect the structure of a young, oceanic rift basin above mantle of normal temperature. We develop a self-consistent forward model of mantle flow that incorporates melt generation and retention to test whether predictions of melt chemistry, melt volume and lithosphere-asthenosphere seismic structure can be reconciled with observations. The rare-earth element composition of mafic samples at the Erta Ale, Dabbahu and Asal magmatic segments can be used as both a thermometer and chronometer of the rifting process. Low seismic velocities require a lithosphere thinned to 50 km or less. A strong positive impedance contrast at 50 to 70 km below the rift seems linked to the melt zone, but is not reproduced by isotropic seismic velocity alone. Combined, the simplest interpretation is that mantle temperature below Afar is still elevated at 1450 °C, rifting started around 22-23 Ma, and the lithosphere has thinned from 100 to 50 km to allow significant decompressional melting.

  13. A new model for the development of the active Afar volcanic margin

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    Volcanic passive margins, that represent more than the three quarters of continental margins worldwide, are privileged witnesses of the lithospheric extension processes thatform new oceanic basins. They are characterized by voluminous amounts of underplated, intruded and extruded magmas, under the form of massive lavas prisms (seaward-dipping reflectors, or SDR) during the course of thinning and stretching of the lithosphere, that eventually form the ocean-continent transition. The origin and mechanisms of formation of these objects are still largely debated today. We have focussed our attention in the last few years on the Afar volcanic province which represents an active analogue of such volcanic margins. We explored the structural and temporal relationships that exist between the development of the major thinning and stretching structures and the magmatic production in Central Afar. Conjugate precise fieldwork analysis along with lavas geochronology allowed us to revisit the timing and style of the rift formation, since the early syn-rift period of time in the W-Afar marginal area to present days. Extension is primarily accommodated over a wide area at the surface since the very initial periods of extension (~ 25 Ma) following the emplacement of Oligocene CFBs. We propose in our reconstruction of central Afar margin history that extension has been associated with important volumes of underplated mafic material that compensate crustal thinning. This has been facilitated by major crustal-scale detachments that help localize the thinning and underplating at depth. In line with this 'magmatic wide-rift' mode of extension, we demonstrate that episodic extension steps alternate with more protracted magmatic phases. The production of syn-rift massive flood basalts (~ 4 Ma) occurs after early thinning of both the crust and the lithosphere, which suggests that SDR formation, is controlled by previous tectonic event. We determined how the melting regime evolved in

  14. The mantle sources beneath the Afar volcanic province and their interplay with extension

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    The evolution of mantle sources beneath the Ethiopian volcanic province has long been discussed and debated with a long-lived controversy in identifying mantle reservoirs and locating them in the mantle. One interpretation of the isotopic composition of erupted lavas considers that the Afar mantle plume composition is best expressed by recent lavas from Afar and Gulf of Aden (e.g. Erta Ale, Manda Inakir and the 45°E torus anomaly on the Gulf of Aden) implying that all other volcanics (including other active segments and the initial flood basalt province) result from mixing of this plume component with additional lithospheric and asthenospheric components. A completely opposite view considers that the initial Oligocene continental flood basalts best represent the isotopic composition of the Afar mantle plume, which is subsequently mixed in various proportions with continental lithospheric mantle for generating some of the specific signature of Miocene and Quaternary volcanics. The precise and correct identification of mantle components involved in the generation of magmas is of particular importance because this is the only way to document the participation of mantle during extension and its potential role in break-up processes. In this contribution we provide new isotopic data for central Afar and we revisit the whole data set of the Ethiopian volcanic province and African/Arabian intraplate volcanics in order to: (i) precisely identify the distinct mantle components implicated, (ii) discuss their location and evolution in space and time, and (3) link the evolution of mantle with extensional processes beneath the Afar province. This new interpretation of geochemical data allows reconsidering the evolution of mantle in the course of rift evolution. In terms of mantle sources, two populations of active segments are frontally opposed in the volcanic province: those that share exactly the same composition with plume related CFBs (e.g. the Manda Hararo and the Main

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

    PubMed

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

    2013-07-01

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

  16. Public understanding of geoscientific topics

    NASA Astrophysics Data System (ADS)

    Münch, Ute; Lauterjung, Jörn

    2014-05-01

    Geoscientific topics and their consequences for the society are becoming more and more important for our daily life. Natural hazards such as flood and storm or the consequences of the climate change are urgent tasks and great challenges we have to tackle. Cascading natural hazards or environmental risks, can't be evaluated as single events by one scientific discipline, they rather need the expertise of different experts. The same applies for slowly progressive processes such as the climate change and its different aftereffects. More than ever politicians, decision makers, but also the public are asking for comprehensive background information and data to discuss activity options and to develop sustainable solutions. The improvement of public knowledge about science, their assets and drawbacks, chances and risks is getting crucial. To paint a comprehensive picture of different factors, correlations and dependencies the pooling of expertise is required. Thus eight research centres of the research field "Earth and Environment" of the Helmholtz-Association, Germany's largest scientific research organisation are currently building up a knowledge platform. Scientists of different disciplines will provide background information and explain their latest findings in an understandable way. Infographics, maps and animations will be applied to simplify and interpret complicated facts and findings. In addition to the web presence target group-specific products and activities will be organized. To meet the expectations of the different stakeholders an intensive dialog is aspired: round table discussions, exhibitions in museums and public places, tweeds are envisaged. In the beginning the partners will concentrate on the topics "consequences of the climate change", "pollutant dispersion" and "natural hazards/meteorological extreme events". The project is called Earth System Knowledge Platform (ESKP) coordinated by the German Research Centre for Geosciences (GFZ) and the Helmholtz

  17. Mantle structure beneath the Afar triple junction derived from surface wave tomography

    NASA Astrophysics Data System (ADS)

    Gallacher, R. J.; Keir, D.; Harmon, N.; Stuart, G. W.; Leroy, S. D.; Hammond, J. O.; Kendall, J. M.; Wondem, A. A.; Gezahegn, B. G.; Ogubazghi, G.

    2013-12-01

    Continental breakup in Afar is generally magma-rich and occurs near the triple junction of the Gulf of Aden (GOA), Red Sea rift and the Main Ethiopian rift (MER). Hypotheses for the source of magmatism associated with this rifting include elevated mantle temperatures resulting from northward migration of hot African Superplume material, and also due to phases of increased decompression melting from rapid plate thinning. To evaluate these hypotheses we conducted a surface wave tomographic experiment using 571 events and 290 stations from 15 seismic networks deployed over the past 12 years. From these data we produced a 3D shear velocity model which constrains the upper 350 km of the Earth, including the lithosphere and uppermost asthenosphere where melt is produced. At 30-100 second periods, our images show a significant (~0.1 km/s) decrease in velocity from the rift flanks into the Afar depression, showing the signature of breakup between Africa and Arabia is still present throughout the mantle. Within Afar, seismic velocities are low, with particularly localised slow anomalies at the 40-second period, beneath the Asal rift (3.57 km/s), Ayelu segment of MER (3.63 km/s) and Dabbahu rift (3.63 km/s) and fast velocities on the rift flanks (3.70-3.80 km/s). These slow anomalies show localised decompression melting and intrusion beneath the rift axis of Afar. Low velocities are also present throughout the mantle beneath the northern section of the MER and in the GOA. Our results show that the mantle beneath Afar still preserves structure from rifting 30 Ma. In addition our results show that localised plate thinning beneath zones of strain focus magmatism to a narrow rift axis.

  18. Introduction: geoscientific knowledgebase of Chernobyl and Fukushima

    NASA Astrophysics Data System (ADS)

    Yamauchi, Masatoshi; Voitsekhovych, Oleg; Korobova, Elena; Stohl, Andreas; Wotawa, Gerhard; Kita, Kazuyuki; Aoyama, Michio; Yoshida, Naohiro

    2013-04-01

    Radioactive contamination after the Chernobyl (1986) and Fukushima (2011) accidents is a multi-disciplinary geoscience problem. Just this session (GI1.4) contains presentations of (i) atmospheric transport for both short and long distances, (ii) aerosol physics and chemistry, (ii) geophysical measurement method and logistics, (iv) inversion method to estimate the geophysical source term and decay, (v) transport, migration, and sedimentation in the surface water system, (vi) transport and sedimentation in the ocean, (vii) soil chemistry and physics, (viii) forest ecosystem, (ix) risk assessments, which are inter-related to each other. Because of rareness of a severe accident like Chernobyl and Fukushima, the Chernobyl's 27 years experience is the only knowledgebase that provides a good guidance for the Fukushima case in understanding the physical/chemical processes related to the environmental radioactive contamination and in providing future prospectives, e.g., what we should do next for the observation/remediation. Unfortunately, the multi-disciplinary nature of the radioactive contamination problem makes it very difficult for a single scientist to obtain the overview of all geoscientific aspects of the Chernobyl experience. The aim of this introductory talk is to give a comprehensive knowledge of the wide geoscientific aspects of the Chernobyl contamination to Fukushima-related geoscience community.

  19. Block rotation and continental extension in Afar: A comparison to oceanic microplate systems

    NASA Astrophysics Data System (ADS)

    Acton, Gary D.; Stein, Seth; Engeln, Joseph F.

    1991-06-01

    The reorganization of oceanic spreading centers separating major plates often appears to occur by a process in which discrete microplates form and evolve by rift propagation. To see whether such microplate behavior has implications for continental rifting, we investigate the application of a microplate model to the Afar region at the Nubia-Somalia-Arabia triple junction. Studies of marine magnetic anomalies, volcanic ages, bathymetry, and seismicity suggest that the westward propagating Gulf of Aden spreading center has propagated into eastern Afar within the past 2 m.y., causing rifting and extension within the continent. We derive constraints on the extension history from the geometry and timing of rift formation and from paleomagnetic data indicating that Pliocene to Pleistocene age rocks have undergone a clockwise rotation of ˜11°. We suggest that the history of rifting, the rotation, and several other features of the regional geology can be described by combining features of an oceanic microplate model and the concept of rift localization previously proposed for Afar. In this scenario, motion occurring on several rifts within an extensional zone preceding the propagating spreading center is gradually transferred to a single rift. While motion is transferred, the overlap region between the growing and dying rifts acts as one or more microplates or blocks that rotate relative to the surrounding major plates. The rifting history and rotations in eastern Afar are thus related to the rift propagation and localization that occurs as the plate boundary evolves. Provided the constraints we use are appropriate, our model better describes the regional kinematics than alternative block models including one based on "bookshelf" faulting. If the tectonics of Afar are typical for continental breakup, they have interesting implications for the geometry of passive margins. In particular, asymmetric rifted margins can be produced if the final location of the rift axis is not

  20. Exposed guyot from the afar rift, ethiopia.

    PubMed

    Bonatti, E; Tazieff, H

    1970-05-29

    A series of originally submarine volcanoes has been found in the Afar Depression. Some of the volcanic structures are morphologically similar to oceanic guyots. One of them consists of strata of finely fragmented and pulverized basaltic glass. The fragmentation of the lava is probably the result of stream explosions taking place during the submarine eruption. The flat top of this guyot is considered to be a constructional feature; by analogy, it is suggested that not all oceanic guyots are necessarily the result of wave truncation of former volcanic islands. PMID:17833451

  1. Lithospheric processes

    SciTech Connect

    Baldridge, W.

    2000-12-01

    The authors used geophysical, geochemical, and numerical modeling to study selected problems related to Earth's lithosphere. We interpreted seismic waves to better characterize the thickness and properties of the crust and lithosphere. In the southwestern US and Tien Shari, crust of high elevation is dynamically supported above buoyant mantle. In California, mineral fabric in the mantle correlate with regional strain history. Although plumes of buoyant mantle may explain surface deformation and magmatism, our geochemical work does not support this mechanism for Iberia. Generation and ascent of magmas remains puzzling. Our work in Hawaii constrains the residence of magma beneath Hualalai to be a few hundred to about 1000 years. In the crust, heat drives fluid and mass transport. Numerical modeling yielded robust and accurate predictions of these processes. This work is important fundamental science, and applies to mitigation of volcanic and earthquake hazards, Test Ban Treaties, nuclear waste storage, environmental remediation, and hydrothermal energy.

  2. Upper mantle structure of shear-waves velocities and stratification of anisotropy in the Afar Hotspot region

    NASA Astrophysics Data System (ADS)

    Sicilia, D.; Montagner, J.-P.; Cara, M.; Stutzmann, E.; Debayle, E.; Lépine, J.-C.; Lévêque, J.-J.; Beucler, E.; Sebai, A.; Roult, G.; Ayele, A.; Sholan, J. M.

    2008-12-01

    The Afar area is one of the biggest continental hotspots active since about 30 Ma. It may be the surface expression of a mantle "plume" related to the African Superswell. Central Africa is also characterized by extensive intraplate volcanism. Around the same time (30 Ma), volcanic activity re-started in several regions of the African plate and hotspots such as Darfur, Tibesti, Hoggar and Mount Cameroon, characterized by a significant though modest volcanic production. The interactions of mantle upwelling with asthenosphere, lithosphere and crust remain unclear and seismic anisotropy might help in investigating these complex interactions. We used data from the global seismological permanent FDSN networks (GEOSCOPE, IRIS, MedNet, GEO- FON, etc.), from the temporary PASSCAL experiments in Tanzania and Saudi Arabia and a French deployment of 5 portable broadband stations surrounding the Afar Hotspot. A classical two-step tomographic inversion from surface waves performed in the Horn of Africa with selected Rayleigh wave and Love wave seismograms leads to a 3D-model of both S V velocities and azimuthal anisotropy, as well as radial SH/ SV anisotropy, with a lateral resolution of 500 km. The region is characterized by low shear-wave velocities beneath the Afar Hotspot, the Red Sea, the Gulf of Aden and East of the Tanzania Craton to 400 km depth. High velocities are present in the Eastern Arabia and the Tanzania Craton. The results of this study enable us to rule out a possible feeding of the Central Africa hotspots from the "Afar plume" above 150-200 km. The azimuthal anisotropy displays a complex pattern near the Afar Hotspot. Radial anisotropy, although poorly resolved laterally, exhibits S H slower than S V waves down to about 150 km depth, and a reverse pattern below. Both azimuthal and radial anisotropies show a stratification of anisotropy at depth, corresponding to different physical processes. These results suggest that the Afar hotspot has a different and

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

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

  4. Regional kinematic models for the development of the Afar depression

    NASA Astrophysics Data System (ADS)

    Redfield, T. F.; Wheeler, W. H.; Often, M.

    2003-04-01

    Few reconstructions of the Afar rift combine plate kinematics with analyses of the rift basin evolution. The Afar rift is a highly-extended region of continental to transitional oceanic crust lying at the junction of the Red Sea, Gulf of Aden and Ethiopian rifts. Here, we present a new Afar reconstruction taking into account plate kinematics, crustal thinning and magmatic construction. We use a regional plate reconstruction incorporating Nubia, Arabia, Somalia and Danakil to constrain the regional-scale extension and subsidence of the rift and relative movement of Danakil. The plate model is temporally and spatially well constrained at the onset of rifting (ca. 20 Ma) and from sea-floor spreading anomalies in the Red Sea (ca. 6 Ma-present) and Gulf of Aden (ca. 10 Ma-present). The Red Sea pre-rift fit is constrained by piercing points along the Red Sea margins (Sultan et al. 1993). We model the Late Oligocene to present-day evolution of the Afar crust by volume balance using a crustal model based on published topographic and depth-to-Moho interpretations as well as volume estimates of extrusive and sedimentary rocks. Errors stemming from plate boundary uncertainties are small in relation to the reconstructed volume. We partition Afar magmatism into pre-extensional and syn-extensional volumes. From thermal modeling and flexural considerations we infer that the regional-scale subsidence of the Afar depression was virtually complete by Mid Pliocene time. Our model supports the interpretation that the escarpments bounding the Afar Depression achieved nearly their present height (ca. 3 km) by the Late Miocene. Erosional considerations suggest the Late Miocene escarpments were steeper than they are today. Our model does not support the interpretation found in the paleo-anthropological literature that Late Miocene and Pliocene vertical movements were sufficiently large (ca. 2 km) to cause small fault blocks such as Hadar to migrate through climatic temperature zones

  5. Recent rift-related volcanism in Afar, Ethiopia

    NASA Astrophysics Data System (ADS)

    Ferguson, David J.; Barnie, Talfan D.; Pyle, David M.; Oppenheimer, Clive; Yirgu, Gezahegn; Lewi, Elias; Kidane, Tesfaye; Carn, Simon; Hamling, Ian

    2010-04-01

    Rift zones are the most common magmatic environment on Earth. However opportunities to observe active rifting are rare, and consequently the volcanological characteristics of rift systems are not well understood. An ongoing phase of magmatic rifting along a section of the Red Sea system in Afar, Ethiopia, presents an exceptional opportunity to constrain relationships between volcanism and crustal growth. Here, by integrating analyses of satellite images (i.e. MODIS, OMI, ASTER, and ALI) with field observations, we characterise two recent (August 2007 and June 2009) basaltic fissure eruptions in Afar and evaluate the role and significance of volcanism in the rifting process. Both events were brief (36-72 h) and erupted 4.4-18 × 10 6 m 3 of lava from a fissure system 4-6.5 km in length. Data from the spaceborne Ozone Monitoring Instrument (OMI) suggests total SO 2 emissions for each eruption of 26 ± 5 kt (2007) and 34 ± 7 kt (2009), consistent with complete degassing of the erupted magma volumes. Using geodetic models for the intrusive activity in Afar we estimate the partitioning of magma between intrusive and extrusive components, up to July 2009, to be ˜ 180:1. Comparing the first-order volcanic characteristics and the intrusive-extrusive volume balance for the Afar volcanism with data from the 1975-1984 Krafla rifting cycle (Iceland) suggests that the volcanic flux in Afar will rise significantly over the next few years as the stresses are increasingly relieved by dyking, and subsequent dykes are able to propagate more easily to the surface. As a consequence, basaltic fissure eruptions in this section of the Afar rift will become of increasing large magnitude as the rifting event matures over the next 5-10 yr. Using available models of magmatic rifting we forecast the likely size and location of future eruptions in Afar.

  6. Lithospheric processes

    SciTech Connect

    Baldridge, W.S.; Wohletz, K.; Fehler, M.C.

    1997-11-01

    This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The main objective was to improve understanding of the origin and evolution of the Earth`s lithosphere by studying selected processes, such as deformation and magmatic intrusion during crustal extension, formation and extraction of mantle melts, fluid transport of heat and mass, and surface processes that respond to deep-seated events. Additional objectives were to promote and develop innovative techniques and to support relevant educational endeavors. Seismic studies suggest that underplating of crust by mantle melts is an important crustal-growth mechanism, that low-angle faults can be seismogenic, and that shear deformation creates mantle anisotropy near plate boundaries. Results of geochemical work determined that magmas from oceanic intraplate islands are derived from a uniform depth in the upper mantle, whereas melts erupted at mid-ocean ridges are mixed from a range of depths. The authors have determined the extent and style of fluid infiltration and trace-element distribution in natural magmatic systems, and, finally, investigated {sup 21}Ne as a tool for dating of surficial materials.

  7. Lithospheric structure of Africa: insights from its effective elastic thickness variations.

    NASA Astrophysics Data System (ADS)

    Pérez-Gussinyé, M.; Metois, M.; Fernández, M.; Vergés, J.; Fullea, J.

    2009-04-01

    Detailed images of lithospheric structure can help understand how surface deformation is related to Earth's deep structure. A proxy for lithospheric structure is its effective elastic thickness, Te, which mainly depends on its thermal state and composition. We present a new effective elastic thickness, Te, map of the African lithosphere estimated using the coherence function between topography and Bouguer anomaly. The Bouguer anomaly used in this study derives from the EGM 2008 model, which constitutes the highest resolution gravity database over Africa, allowing a significant improvement on lateral resolution in Te. Our map shows that Te is high > 100 km, in the West African, Congo, Kalahari and Tanzania cratons. Of these, the Kalahari presents the thinnest elastic thicknesses and, based on additional seismic and mineral physics studies, we suggest this may reflect modification of the lithosphere by anomalously hot mantle beneath the lithosphere. The effective elastic thickness is lowest beneath the Afar and Main Ethiopian rifts, where the maximum extension and thinnest lithosphere of Africa occur. The Tanzania craton appears as two rigid blocks separated by a relatively low Te area located southwest of lake Victoria. This coincides with the centre of seismic radial anisotropy beneath the craton, suggested to be the Victoria plume head by Weertrane et al. [2003]. Along the eastern branch of the East African rift Te is low and increases abruptly at 2 to 3 degrees South, coinciding with a deepening of earthquake depocenter and a change from narrow to wide rifting. These and other considerations suggest that the southern part of the eastern branch is underlain by thick, rigid cratonic lithosphere. Finally, the northern part of Africa is characterised by low Te on the Darfur, Tibesti, Hoggar and Cameroon line volcanic provinces, suggesting that the underlying lithospheric mantle has been thermally thinned. Corridors of low Te connect these volcanic provinces

  8. Database of Geoscientific References Through 2007 for Afghanistan, Version 2

    USGS Publications Warehouse

    Eppinger, Robert G.; Sipeki, Julianna; Scofield, M.L. Sco

    2007-01-01

    This report describes an accompanying database of geoscientific references for the country of Afghanistan. Included is an accompanying Microsoft? Access 2003 database of geoscientific references for the country of Afghanistan. The reference compilation is part of a larger joint study of Afghanistan's energy, mineral, and water resources, and geologic hazards, currently underway by the U.S. Geological Survey, the British Geological Survey, and the Afghanistan Geological Survey. The database includes both published (n = 2,462) and unpublished (n = 174) references compiled through September, 2007. The references comprise two separate tables in the Access database. The reference database includes a user-friendly, keyword-searchable, interface and only minimum knowledge of the use of Microsoft? Access is required.

  9. Asthenosphere–lithosphere interactions in Western Saudi Arabia: Inferences from 3He/4He in xenoliths and lava flows from Harrat Hutaymah

    USGS Publications Warehouse

    Konrad, Kevin;; Graham, David W; Thornber, Carl; Duncan, Robert A; Kent, Adam J.R.; Al-Amri, Abdulla

    2016-01-01

    Elevated 3He/4He in the western harrats has been observed only at Rahat (up to 11.8 RA; Murcia et al., 2013), a volcanic field situated above thinned lithosphere beneath the Makkah-Medinah-Nafud volcanic lineament. Previous work established that spinel lherzolites at Hutaymah are sourced near the lithosphere-asthenosphere boundary (LAB), while other xenolith types there are derived from shallower depths within the lithosphere itself (Thornber, 1992). Helium isotopes are consistent with melts originating near the LAB beneath many of the Arabian harrats, and any magma derived from the Afar mantle plume currently appears to be of minor importance.

  10. The R software environment in reproducible geoscientific research

    NASA Astrophysics Data System (ADS)

    Pebesma, Edzer; Nüst, Daniel; Bivand, Roger

    2012-04-01

    Reproducibility is an important aspect of scientific research, because the credibility of science is at stake when research is not reproducible. Like science, the development of good, reliable scientific software is a social process. A mature and growing community relies on the R software environment for carrying out geoscientific research. Here we describe why people use R and how it helps in communicating and reproducing research.

  11. Geodetic measurements and numerical models of the Afar rifting sequence 2005-2010

    NASA Astrophysics Data System (ADS)

    Ali, T.; Feigl, K.; Calais, E.; Hamling, I. J.; Wright, T. J.

    2012-12-01

    Rifting episodes are characterized by magma migration and dike intrusions that perturb the stress field within the surrounding lithosphere, inducing viscous flow in the lower crust and upper mantle that leads to observable, transient surface deformation. The Manda Hararo-Dabbahu rifting episode that occurred in the Afar depression between 2005 and 2010 is the first such episode to unfold fully in the era of satellite geodesy, thus providing a unique opportunity to probe the rheology of lithosphere at a divergent plate boundary. GPS and SAR measurements over the region since 2005 show accelerated surface deformation rates during post-diking intervals [Wright et al., Nature Geosci., 2012]. Using these observations in combination with a numerical model, we estimate model parameters that best explain the deformation signal. Our model accounts for three distinct processes: (i) secular plate spreading between Nubian and Arabian plates, (ii) time dependent post-rifting viscoelastic relaxation following the 14 dike intrusions that occurred between 2005 and 2010, including the 60 km long mega dike intrusion of September 2005, and (iii) magma accumulation within crustal reservoirs that feed the dikes. To model the time dependent deformation field, we use the open-source unstructured finite element code, Defmod [Ali, 2011, http://defmod.googlecode.com/]. Using a gradient-based iterative scheme [Ali and Feigl, Geochem. Geophys. Geosyst., 2012], we optimize the fit between observed and modeled deformation to estimate parameters in the model, including the locking depth of the rift zone, geometry and depth of magma reservoirs and rheological properties of lower crust and upper mantle, along with their formal uncertainties.

  12. Magmatism on rift flanks: insights from Ambient-Noise Phase-velocity in Afar region

    NASA Astrophysics Data System (ADS)

    Korostelev, Félicie; Weemstra, Cornelis; Leroy, Sylvie; Boschi, Lapo; Ren, Yong; Ahmed, Abdulhakim; Keir, Derek; Stuart, Graham W.; Rolandone, Frédérique; Khanbari, Khaled; Hammond, James O. S.; Kendall, J. Michael; Doubre, Cécile; Ganad, Ismail Al

    2015-04-01

    During the breakup of continent in the presence of magma, strain is commonly assumed to initially occur by border faulting, and progressively migrate in space and time towards axial magma intrusion. Magmatic processes near the rift flanks are commonly ignored. We present phase-velocity maps of the crust and uppermost mantle of the conjugate margins of the southern Red Sea (Afar and Yemen) using ambient noise tomography to constrain crustal modification during breakup. Our images show that the low seismic velocities characterize not only upper crust beneath the axial volcanic systems, but also both upper and lower crust beneath rift flanks where ongoing volcanism and hydrothermal activity occurs at the surface. The results show that magmatic modification of the crust beneath rift flanks likely occurs for a protracted period of time during the breakup process, and may persist through to early seafloor spreading. Since ongoing flank magmatism during breakup impacts the thermal evolution of the lithosphere, it has implications for the subsidence history of the rift.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  14. Spaceborne radar for geoscientific applications in North China

    NASA Astrophysics Data System (ADS)

    Guo, Hua-Dong; Wu, Guo-Xiang; Wang, Zhen-Song

    1993-05-01

    The Shuttle Imaging Radar-A and -B (SIR-A and SIR-B) carried on the Space Shuttle Columbia in Nov. 1981 and the Challenger in Oct. 1984 acquired images of test sites of North China. The Russian ALMAZ SAR also acquired imagery of part of this test site in Sep. 1992. In Nov. 1990, the airborne SAR developed by the Chinese Academy of Sciences (CAS/SAR) covered this area for the purpose of Chinese spaceborne radar development. By studying and analyzing these SAR data, positive results in geoscientific applications were achieved.

  15. Extensible query processing and data communication in geoscientific information systems

    NASA Astrophysics Data System (ADS)

    Shek, Eddie Chi-Ho

    Research interest in geoscientific data management, processing, and dissemination has significantly increased over the last few years. Geoscientific applications are characterized by the massive size and spatio-temporal nature of their datasets, as well as the complexity and variety of their operations. In this dissertation, we study issues concerning data modeling, query processing, and data communication in an extensible distributed information system designed to satisfy the requirements imposed by the characteristics of geoscientific applications. Data model and computation architecture form the basis of an information system. It is important that they fit the requirements imposed by the application to be supported. We define a conceptually simple yet expressive scientific data modeling framework, called the Field data model, that captures the structure of a wide variety of scientific and traditional data structures. In addition, it models a number of common higher-order scientific computation patterns hence allowing application-specific operations fitting these patterns to be naturally and seamless introduced. Parallel evaluation is one of the most effective approaches to deliver high query performance. At the same time, geoscientific information systems have to be extensible to accommodate user-defined application-specific operations. To support extensibility in parallel query optimization, we introduce a model that allows the implicit parallelism available in scientific operations to be declaratively captured. Query parallelism algorithms are developed to demonstrate how these information can be exploited to systematically generate efficient parallel query execution plans. The cost of data communication in a scientific information system can be expensive despite the advancement of networking technology, especially during the delivery of data to user over a wide-area network such as the Internet. We present a flexible object-oriented framework called FALCON to

  16. Spaceborne radar for geoscientific applications in North China

    NASA Technical Reports Server (NTRS)

    Guo, Hua-Dong; Wu, Guo-Xiang; Wang, Zhen-Song

    1993-01-01

    The Shuttle Imaging Radar-A and -B (SIR-A and SIR-B) carried on the Space Shuttle Columbia in Nov. 1981 and the Challenger in Oct. 1984 acquired images of test sites of North China. The Russian ALMAZ SAR also acquired imagery of part of this test site in Sep. 1992. In Nov. 1990, the airborne SAR developed by the Chinese Academy of Sciences (CAS/SAR) covered this area for the purpose of Chinese spaceborne radar development. By studying and analyzing these SAR data, positive results in geoscientific applications were achieved.

  17. Historical geoscientific collections - requirements on digital cataloging and problems

    NASA Astrophysics Data System (ADS)

    Ehling, A.

    2011-12-01

    The Federal Institute for Geosciences and Natural Resources maintains comprehensive geoscientific collections: the historical collections of Prussian Geological Survey in Berlin (19th and 20th century; about 2 mio specimen) and the geoscientific collections of the 20th century in Hannover (about 800.000 specimen). Nowadays, where financial support is strictly bound to efficiency and rentability on one side and the soaring (among young people - nearly exclusive) use of the web for the research, it is mandatory to provide the information about the available stock of specimen on the web. The digital cataloging has being carried out since 20 years: up to now about 40 % of the stock has been documented in 20 access-databases. The experiences of 20 years digital cataloging as well as the contact with professional users allow to formulate the requirements on a modern digital database with all accordingly problems. The main problems are different kinds of specimen: minerals, rocks, fossils, drill cores with diverging descriptions; obsolescent names of minerals, rocks and geographical sites; generations of various inventory numbers; inhomogeneous data (quantity and quality). Out of it result requirements to much, well educated manpower on the one side and an intelligent digital solution on the other side: it should have an internationally useable standard considering all the described local problems.

  18. Analyzing Geoscientific Data with R: Past, Present, Future

    NASA Astrophysics Data System (ADS)

    Pebesma, E. J.

    2014-12-01

    The talk will look back at the largely successful growth of R-spatial, the community of scientists who use R [1,2] for analysing spatial [3] and spatio-temporal [4] data, over the last decade. It will reflect on strengths and weaknesses of the current system and development model, and reveal a number of ambitions several active developers agreed upon earlier this year. I will also relate to the broader contexts of reproducibility of geoscientific research [2], community software development, and the need for scientists who develop software to get recognized for their developments. [1] Bivand, R.S., E.J. Pebesma, V. Goméz-Rubio, 2013. Applied Spatial Data Analysis with R, Second edition. Springer, NY.[2] Pebesma, E., D. Nüst, R. Bivand, 2012. The R software environment in reproducible geoscientific research. Eos, Transactions American Geophysical Union 93, vol 16, p. 163-164[3] http://cran.r-project.org/web/views/Spatial.html[4] http://cran.r-project.org/web/views/SpatioTemporal.html

  19. Rhyolites associated to Ethiopian CFB: Clues for initial rifting at the Afar plume axis

    NASA Astrophysics Data System (ADS)

    Natali, Claudio; Beccaluva, Luigi; Bianchini, Gianluca; Siena, Franca

    2011-12-01

    A comprehensive tectono-magmatic model based on new geochemical and field data is discussed in order to highlight the significance of the high-TiO 2 bimodal picrite basalt/rhyolite association in the north-eastern sector of the Ethiopian Plateau, which is considered to be the axial zone of the 30 Ma Continental Flood Basalt activity related to the Afar plume (Beccaluva et al., 2009). In this area the volcanic sequence consists of approximately 1700 m of high TiO 2 (4-6.5%) picrite basalts, covered by rhyolitic ignimbrites and lavas, with an average thickness of 300 m, which discontinuously extend over an area of nearly 13,500 km 2 (ca. 3600 km 3). Petrogenetic modelling, using rock and mineral chemical data and phase equilibria calculations by PELE and MELTS, indicates that: 1) picrite basalts could generate rhyolitic, sometimes peralkaline, residual melts with persistently high titanium contents (TiO 2 0.4-1.1%; Fluorine 0.2-0.3%; H 2O 2-3%; density ca. 2.4) corresponding to liquid fractions 9-16%; 2) closed system fractional crystallisation processes developed at 0.1-0.3 GPa pressure and 1390-750 °C temperature ranges, under QFM fO 2 conditions; 3) the highest crystallisation rate - involving 10-13% of Fe-Ti oxide removal - in the temperature range 1070-950 °C, represents a transitory (short-lived) fractionation stage, which results in the absence of erupted silica intermediate products (Daly gap). The eruption of low aspect ratio fluorine-rich rhyolitic ignimbrites and lavas capping the basic volcanics implies a rapid change from open- to closed-system tectono-magmatic conditions, which favoured the trapping of parental picrite basalts and their fractionation in upwardly zoned magma chambers. This evolution resulted from the onset of continental rifting, which was accompanied by normal faulting and block tilting, and the formation of shallow - N-S elongated - fissural chambers parallel to the future Afar Escarpment. The eruption of large volumes of rhyolitic

  20. The geochemical and Sr-Nd-Pb-He isotopic characterization of the mantle source of Rungwe Volcanic Province: comparison with the Afar mantle domain

    NASA Astrophysics Data System (ADS)

    Castillo, P. R.; Hilton, D. R.; Halldorsson, S. A.; Wang, R.

    2012-12-01

    The ultimate source of heat and magmatism associated with continental rifting in the East African Rift System (EARS) is generally viewed to be the African Superplume, but there is continuing debate on the surface expression of this large anomalous feature, which originates in the lower mantle. Previous studies have demonstrated an insignificant role for crustal contamination thereby identifying a single mantle plume signature in Quaternary basalts from the Main Ethiopian Rift in the northern EARS. This is designated to be the Afar plume and is characterized by, e.g., 3He/4He >15 RA, 206Pb/204Pb = 19.5 and 87Sr/86Sr = 0.7035 [Rooney et al., J. Pet. 53, 2012]. In contrast, the signature of plume(s) in the southern EARS is less constrained. Rogers et al. [EPSL 176, 2000] proposed a plume in the sub-lithospheric Kenyan mantle with characteristically lower 43Nd/144Nd than the Afar plume whereas Furman [JAES 48, 2007] advocated a high μ [HIMU] plume based primarily on the high 206Pb/204Pb ratios of lavas in all areas within and south of the Turkana Depression: both models assume a 3He/4He lower than the Afar plume. Here we report the trace element and Sr-Nd-Pb isotopic composition of basaltic lavas from the Rungwe Volcanic Province (RVP) in the southern extreme of the Western Rift previously identified as a high 3He/4He locality (~15 RA; [Hilton et al., GRL 38, 2011]). Trace element analyses are within the previously reported range of lava compositions that include a relatively large lithospheric component. More importantly, we identify correlations among incompatible trace element and isotopic ratios (e.g., 3He/4He vs 206Pb/204Pb, Rb/Sr, Nb/Ta; 87Sr/86Sr vs 208Pb/204Pb). Our new results suggest the presence of a distinct, high 3He/4He mantle source beneath RVP that is more radiogenic (e.g., 206Pb/204Pb up to ~19.8; 87Sr/86Sr up to 0.7055) than the Afar mantle plume. There is also very little or no HIMU signature in RPV basalts based on their high Sr and low Nd isotopic

  1. Generating performance portable geoscientific simulation code with Firedrake (Invited)

    NASA Astrophysics Data System (ADS)

    Ham, D. A.; Bercea, G.; Cotter, C. J.; Kelly, P. H.; Loriant, N.; Luporini, F.; McRae, A. T.; Mitchell, L.; Rathgeber, F.

    2013-12-01

    This presentation will demonstrate how a change in simulation programming paradigm can be exploited to deliver sophisticated simulation capability which is far easier to programme than are conventional models, is capable of exploiting different emerging parallel hardware, and is tailored to the specific needs of geoscientific simulation. Geoscientific simulation represents a grand challenge computational task: many of the largest computers in the world are tasked with this field, and the requirements of resolution and complexity of scientists in this field are far from being sated. However, single thread performance has stalled, even sometimes decreased, over the last decade, and has been replaced by ever more parallel systems: both as conventional multicore CPUs and in the emerging world of accelerators. At the same time, the needs of scientists to couple ever-more complex dynamics and parametrisations into their models makes the model development task vastly more complex. The conventional approach of writing code in low level languages such as Fortran or C/C++ and then hand-coding parallelism for different platforms by adding library calls and directives forces the intermingling of the numerical code with its implementation. This results in an almost impossible set of skill requirements for developers, who must simultaneously be domain science experts, numericists, software engineers and parallelisation specialists. Even more critically, it requires code to be essentially rewritten for each emerging hardware platform. Since new platforms are emerging constantly, and since code owners do not usually control the procurement of the supercomputers on which they must run, this represents an unsustainable development load. The Firedrake system, conversely, offers the developer the opportunity to write PDE discretisations in the high-level mathematical language UFL from the FEniCS project (http://fenicsproject.org). Non-PDE model components, such as parametrisations

  2. Geoscientific Characterization of the Bruce Site, Tiverton, Ontario

    NASA Astrophysics Data System (ADS)

    Raven, K.; Jackson, R.; Avis, J.; Clark, I.; Jensen, M.

    2009-05-01

    Ontario Power Generation is proposing a Deep Geologic Repository (DGR) for the long-term management of its Low and Intermediate Level Radioactive Waste (L&ILW) within a Paleozoic-age sedimentary sequence beneath the Bruce site near Tiverton, Ontario, Canada. The concept envisions that the DGR would be excavated at a depth of approximately 680 m within the Ordovician Cobourg Formation, a massive, dense, low- permeability, argillaceous limestone. Characterization of the Bruce site for waste disposal is being conducted in accordance with a four year multi-phase Geoscientific Site Characterization Plan (GSCP). The GSCP, initially developed in 2006 and later revised in 2008 to account for acquired site knowledge based on successful completion of Phase I investigations, describes the tools and methods selected for geological, hydrogeological and geomechanical site characterization. The GSCP was developed, in part, on an assessment of geoscience data needs and collection methods, review of the results of detailed geoscientific studies completed in the same bedrock formations found off the Bruce site, and recent international experience in geoscientific characterization of similar sedimentary rocks for long-term radioactive waste management purposes. Field and laboratory work related to Phase 1 and Phase 2A are nearing completion and have focused on the drilling, testing and monitoring of four continuously cored vertical boreholes through Devonian, Silurian, Ordovician and Cambrian bedrock to depths of about 860 mBGS. Work in 2009 will focus on drilling and testing of inclined boreholes to assess presence of vertical structure. The available geological, hydrogeological and hydrogeochemical data indicate the presence of remarkably uniform and predictable geology, physical hydrogeologic and geochemical properties over well separation distances exceeding 1 km. The current data set including 2-D seismic reflection surveys, field and lab hydraulic testing, lab petrophysical and

  3. Arctic lithosphere - A review

    NASA Astrophysics Data System (ADS)

    Pease, V.; Drachev, S.; Stephenson, R.; Zhang, X.

    2014-07-01

    This article reviews the characteristics of Arctic lithosphere and the principal tectonic events which have shaped it. The current state-of-knowledge associated with the crust, crustal-scale discontinuities, and their ages, as well as knowledge of the lithosphere as a whole from geophysical data, permits the division of Arctic lithosphere into discrete domains. Arctic continental lithosphere is diverse in age, composition, and structure. It has been affected by at least two periods of thermal overprinting associated with large volumes of magmatism, once in the Permo-Triassic and again in the Aptian. In addition, it was attenuated as the result of at least five phases of rifting (in the late Devonian-early Carboniferous, Permo-Triassic, Jurassic, Early Cretaceous, and Late Cretaceous-Cenozoic). Older phases of consolidation are associated with continental lithosphere and occurred through a series of continent-continent collisions in the Paleozoic. Jurassic and Cretaceous extensional phases are related to the dismembering of Pangea and Eurasia, and were concentrated in the Norway-Greenland and Canadian-Alaskan Arctic regions. Large areas of submarine, hyperextended continental (?) lithosphere developed in parts of the Amerasia Basin. After continental breakup and the accretion of new oceanic lithosphere, the Eurasia and Canada basins were formed.

  4. Late Pliocene - Early Pleistocene geologic history of Eastern Ledi-Geraru, Ethiopia: implications for the evolution of the southern Afar Depression and hominin paleoenvironments

    NASA Astrophysics Data System (ADS)

    DiMaggio, E.; Arrowsmith, R.; Campisano, C. J.; Reed, K.; Deino, A.

    2012-12-01

    During the late Pliocene and early Pleistocene (~ 3-2.5 Ma), the Afar region of Ethiopia was undergoing major structural reorganization (e.g., change in extension direction, increased spreading rate) leading to significant landscape modification. Concurrent with these changes in paleogeography, regional trends towards a cooler and drier climate coincide with a clustering of first appearance and extinction events in the faunal record, including the diversification of the early hominin genus Australopithecus and the emergence of our own genus, Homo. However, sediments that span the 3 to 2.5 Ma interval are sparse in eastern Africa, and are especially rare at paleoanthropological sites in the Afar. Here we present new geologic mapping results that indicate extensive deposits of late Pliocene sediments in a previously unmapped region of the lower Awash Valley referred to as the Eastern Ledi-Geraru (ELG). Numerous interbedded airfall tephras enable geochemical comparisons to the existing regional tephrostratigraphic framework as well as high precision 40Ar/39Ar dating of tephras with suitable feldspars. Feldspars from 8 such tephra deposits span the time period of 3.0 to 2.8 Ma, providing the first glimpse of depositional environments and associated landscapes that existed at that time. Geologic mapping and stratigraphic analysis shows that over a 100 meter thick section of lacustrine to fluvial sediments are exposed along faulted basalt flows following both the Red Sea Rift and Main Ethiopian Rift structural trends. We interpret the geology at ELG to reflect a northeastern migration of paleo Lake Hadar, possibly into a series of smaller basins responding to the migration of the triple junction, a thinning lithosphere, and an increased period of volcanism. Combined with recently collected paleontological assemblages this work provides an opportunity to test proposed links between biotic events, global/regional climate change, and local tectonic events during a critical

  5. Determinants of delivery practices among Afar pastoralists of Ethiopia

    PubMed Central

    Mekonnen, Medhanit Getachew; Yalew, Kassahun Negash; Umer, Jemal Yesouf; Melese, Muluken

    2012-01-01

    Introduction In a previous qualitative study in Afar, it was reported that most mothers deliver at home, assisted by Traditional Birth Attendants (TBAs). However, determinant factors of delivery practices in this region remain scarcely explored. The objective of this study was to elucidate these factors among the Afar community. Methods This cross sectional study was conducted in April 2011 in zone 3 of Afar regional state, Ethiopia. Quantitative data were collected from 478 women who had given birth during the preceding one year. Results Out of 478 interviewed mothers in urban/rural areas, 398 (83.3%) gave birth to the youngest child while at home; 370 (92.5%) were assisted by TBAs. Only 3.2% of them were assisted by Health Extension Workers/nurses in health posts or at home during delivery. We found an association between health facility delivery and ante-natal care (ANC) attendance (p<0.001), educational status (p<0.001) and occupation of the husband/wife (mother) and gravidity (p=0.003); but there was no association with the number of wives the husband had (p=0.566). In the adjusted model, ANC attendance and education status of mother were significantly associated with health facility delivery (p=0.036; p<0.001). Conclusion Most deliveries in the study community took place at home. Educated mothers and ANC attending mothers have high tendency for health facility delivery. Programmes need to strengthen the capacities of mothers to attend ANC services, as well as build the capacity of Health Extension Workers (HEWs) and nurses working in health posts, in order to win the confidence of the community. PMID:23467618

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

    PubMed

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

    2006-07-20

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

  7. Influence of the Afar plume on the deep structure of Aden and Red Sea margins - Insight from teleseismic tomography in western Yemen

    NASA Astrophysics Data System (ADS)

    Korostelev, Félicie; Basuyau, Clémence; Leroy, Sylvie; Ahmed, Abdulhakim; Keir, Derek; Stuart, Graham; Rolandone, Frédérique; Ganad, Ismail Al; Khanbari, Khaled

    2013-04-01

    Continental rupture processes under mantle plume influence are still poorly known although extensively studied. The Afar plume has been largely investigated in Ethiopia to study early stages of continental break-up. Here we imaged the lithospheric structure of western continental Yemen to evaluate the role of the Afar plume on the evolution of the continental margin and its extent towards the East. A part of the YOCMAL project (YOung Conjugate MArgins Laboratory) permitted the deployment of twenty-three broadband stations in Yemen (from 2009 to 2010). Using a classical teleseismic tomography (Aki et al., 1974) on these stations together with a permanent GFZ station, we image the relative velocity variations of P-waves in the crust and lithosphere down to 300 km depth, with a maximum lateral resolution of about ~20 km. The model thus obtained shows (1) a dramatic and localized thinning of the crust in the vicinity of the Red Sea and the Gulf of Aden (2) the presence of magmatic underplating related to seaward dipping reflectors under those two volcanic margins (3) two granitic syn-rift intrusions on the border of the great escarpment (4) a low velocity anomaly in which with evidence of partial melting, just below thick Oligocene trapps series and other volcanic events (from 15 Ma to present). This low velocity anomaly could correspond to an abnormally hot mantle and could be responsible for dynamic topography and recent magmatism in western Yemen. (5) Finally, we infer the presence of hot material under the Southwestern corner of Yemen that could be related to Miocene volcanism in Jabal an Nar.

  8. Tectonics of the Afar triple junction from InSAR and GPS derived strain maps and seismicity

    NASA Astrophysics Data System (ADS)

    Pagli, Carolina; Ebinger, Cynthia; Yun, Sang-Ho; Keir, Derek; Wang, Hua

    2016-04-01

    Strain and seismicity show us the mode by which deformation is accommodated in rifting continents. Here we present a combined analysis of InSAR and GPS derived strain maps and seismicity to understand the tectonics of the current Afar triple junction plate boundary zone. Our results show that that the plate spreading motion is accommodated in different ways in the Red Sea Rift after jumping southeastward along the Gulf of Aden Rift. At the Red Sea Rift, extension and shear are coupled with seismicity, occurring both along-rift but also in areas off-rift. In the Gulf of Aden Rift extension and normal faulting occur in the central parts of the rifts while at the rifts tips strike-slip earthquakes are observed. The extensional strains occur over a broad zone encompassing several overlapping rifts. Conversely the strike-slip earthquakes are focused along a narrow EW trending lineament. The pattern suggests that the recent history of magmatic intrusions in the Red Sea Rift still dominates the plate boundary deformation inducing earthquakes even in areas off-rift and with no previous faults mapped. On the other hand, in the Gulf of Aden Rift our strain and seismicity maps are consistent mainly with extensional tectonics occurring over an exceptionally broad zone (over 200 km). We interpret the strike-slip earthquakes observed at the rift tips as the result of shearing at the rifts tips where the extension terminates against continental lithosphere.

  9. The international lithosphere program

    NASA Astrophysics Data System (ADS)

    Flinn, Edward A.

    The International Lithosphere Program is a new international interdisciplinary research program in the solid earth sciences that has been established by the International Council of Scientific Unions (ICSU) at the joint request of the International Union of Geodesy and Geophysics (IUGG) and the International Union of Geological Sciences (IUGS). Its goal is a better understanding of the development of the earth, particularly those aspects upon which human society depends for its well-being.The International Lithosphere Program (ILP) is a natural sequel to a series of international cooperative projects in the geosciences that began with the International Geophysical Year in 1957-58 and continued with the Upper Mantle Project in the 1960's and the International Geodynamics Project (IGP) in the 1970's. In 1977, IUGG and IUGS established an inter-union task group to consider the possibility of a successor to the IGP for the 1980's. The task group, under cochairmen Carl Kisslinger (Cooperative Institute for Research in Environmental Sciences, University of Colorado), foreign secretary of the American Geophysical Union, and J. Henning Illies (Geophysical Institute, University of Karlsruhe, Federal Republic of Germany), invited suggestions and comments from the two unions and the national committees in the member countries. Their report, which was completed late in 1978, proposed a new project on the dynamics, origin, and evolution of the lithosphere. This proposal was approved by the IUGS Executive Committee in December 1979 and by the IUGS Council in June 1980. An inter-union steering committee, established in 1979 under the joint chairmanship of Kisslinger and Illies, developed the organizational framework and constitution of the new program. These were approved by resolution of the ICSU Governing Board in September 1980, and the Inter-Union Commission on the Lithosphere (ICL) was established to implement the program. National members of ICSU were urged to establish

  10. Lithospheric and crustal thinning

    NASA Technical Reports Server (NTRS)

    Moretti, I.

    1985-01-01

    In rift zones, both the crust and the lithosphere get thinner. The amplitude and the mechanism of these two thinning situations are different. The lithospheric thinning is a thermal phenomenon produced by an asthenospherical uprising under the rift zone. In some regions its amplitude can exceed 200%. This is observed under the Baikal rift where the crust is directly underlaid by the mantellic asthenosphere. The presence of hot material under rift zones induces a large negative gravity anomaly. A low seismic velocity zone linked to this thermal anomaly is also observed. During the rifting, the magmatic chambers get progressively closer from the ground surface. Simultaneously, the Moho reflector is found at shallow depth under rift zones. This crustal thinning does not exceed 50%. Tectonic stresses and vertical movements result from the two competing effects of the lithospheric and crustal thinning. On the one hand, the deep thermal anomaly induces a large doming and is associated with extensive deviatoric stresses. On the other hand, the crustal thinning involves the formation of a central valley. This subsidence is increased by the sediment loading. The purpose here is to quantify these two phenomena in order to explain the morphological and thermal evolution of rift zones.

  11. Intraplate Harrat Volcanism and Neogene Evolution of the Lithosphere-Asthenosphere Boundary beneath Western Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Kent, A. J.; Duncan, R. A.; Graham, D. W.; Al-Amri, A. M.; Alshalntoni, S. A.

    2015-12-01

    Continental extension is a fundamental plate tectonic process, and extensional environments are associated with significant production of basaltic magmas. Although tholeiitic magmatism produced by mantle decompression is common, dispersed, less voluminous and compositionally variable basaltic and related magmas also occur in association with continental extension. One of the most voluminous, best-preserved and least studied examples of the latter is the volcanic harrats of western Saudi Arabia. Uplift, crustal erosion and harrat volcanism occurred from ~15 Ma to recent over a considerable region of western Arabia. Volcanism trends north from the Red Sea along the Makkah-Madinah-Nafud (MMN) line as a number of discrete harrats, and geophysical evidence suggests this region is underlain by hot upwelling asthenosphere. Larger harrats occur along the central axis of the MMN line, with smaller fields on the periphery. We present initial results of an ongoing study of harrat volcanism, including more than 50 new 40Ar-39Ar ages together with geochemical and isotopic data. Magma storage and fractionation appears to have occurred at a range of crustal levels, including deep storage and transport sufficiently rapid to preserve a range of lithospheric and crustal xenoliths. Melting commenced within the garnet peridotite field, however modeling of REE element abundances suggest that considerable differences exist in the thickness of the overlying lithospheric lid. Thinner lithosphere (<40 km) and higher degrees of melting occurs along the MMN line, whereas harrats peripheral to the MMN line have thicker lithosphere (>60-80 km) and magmas result from lower degree melts. The observed variability can be explained by a process of progressively thinning lithosphere along the main axis of the MMN line, as a result of regional lithospheric extension and mantle decompression melting, coupled with northward asthenospheric flow from the Red Sea and/or Afar hot spot.

  12. The Axum-Adwa basalt-trachyte complex: a late magmatic activity at the periphery of the Afar plume

    NASA Astrophysics Data System (ADS)

    Natali, C.; Beccaluva, L.; Bianchini, G.; Siena, F.

    2013-08-01

    CFB event, characterized by comparatively lower volume of more alkaline products, conforms to the progressive vanishing of the Afar plume thermal effects and the parallel decrease of the partial melting degrees of the related mantle sources. This evolution is also concomitant with the variation of the tectono-magmatic regime from regional lithospheric extension (CFB eruption) to localized rifting processes that favoured magmatic differentiation.

  13. The volcano seismic crisis in Afar, Ethiopia, starting September 2005

    NASA Astrophysics Data System (ADS)

    Ayele, Atalay; Jacques, Eric; Kassim, Mohammed; Kidane, Tesfaye; Omar, Ahmed; Tait, Stephen; Nercessian, Alexandre; de Chabalier, Jean-Bernard; King, Geoffrey

    2007-03-01

    We describe the seismo-volcanic crisis that occurred in northern Afar in late 2005, which involved 15 earthquakes greater than M5 and a small explosive silicic eruption from a vent called Da'Ure (at 12.651°N., 40.519°N) close to Dabbahu volcano. The purpose is to pull together the different sources of information into a coherent preliminary interpretation of what happened. The main geophysical data are the locations of the largest earthquakes, and a radar interferogram of unusually high quality that reveals injection of a 60 km long dyke with surface deformation expressed as normal faulting. Subsidence occurred around the Dabbahu volcanic edifice. Most of the dyke is likely to have been basaltic rather than silicic although the eruption was silicic. The volume of the subsidence represents at most 25% of the magma injected into the dyke. The silicic eruption was possibly triggered by interaction of basaltic magma with a shallow silicic reservoir. At about the same time as the eruption and dyking episode, some activity appears also to have taken place at the lava lake at Erta Ale volcano, some 150 km to the north of the eruption site. We evaluate the possibility that there may be some link by calculating stresses associated with opening of the fissure and looking at the activity of the lava lake as revealed by the thermal anomaly seen by weather satellites.

  14. Regional and local tectonics at Erta Ale caldera, Afar (Ethiopia)

    NASA Astrophysics Data System (ADS)

    Acocella, Valerio

    2006-10-01

    Erta Ale volcano lies along the on-shore Red Sea Rift (northern Afar, Ethiopia), separating the Nubia and Danakil plates. Erta Ale has a NNW-SSE elongated caldera, with a subvertical rim scarp, hosting a lava lake. Structural field work was aimed at defining the deformation pattern around the caldera. The caldera consists of along-rim and across-rim structures, resulting from local and regional (maximum extension ˜NE-SW) stress fields, respectively. These structures cross-cut each other at high angles, suggesting that the two stress fields remain distinct, each prevailing during rifting or caldera collapse. The local along-rim extensional fractures are gravity-driven structures that formed due to the retreat of the caldera wall after collapse, and are confined to the region of caldera subsidence. The across-rim structures are mainly located to the N and S of the caldera, where they form rift zones each accommodating a similar amount of extension (˜6.3 m), but displaying different trends and extension directions. Analogue models of interacting fractures are consistent with the Southern Rift being representative of the regional fault kinematics, while the Northern Rift is a local perturbation, resulting from the interaction between two right-stepping rift segments along the Erta Ale Range.

  15. Spectral analysis of dike-induced earthquakes in Afar, Ethiopia

    NASA Astrophysics Data System (ADS)

    Tepp, Gabrielle; Ebinger, Cynthia J.; Yun, Sang-Ho

    2016-04-01

    Shallow dike intrusions may be accompanied by fault slip above the dikes, a superposition which complicates seismic and geodetic data analyses. The diverse volcano-tectonic and low-frequency local earthquakes accompanying the 2005-2010 large-volume dike intrusions in the Dabbahu-Manda Hararo rift (Afar), some with fault displacements of up to 3 m at the surface, provide an opportunity to examine the relations among the earthquakes, dike intrusions, and surface ruptures. We apply the frequency index (FI) method to characterize the spectra of swarm earthquakes from six of the dikes. These earthquakes often have broad spectra with multiple peaks, making the usual peak frequency classification method unreliable. Our results show a general bimodal character with high FI earthquakes associated with deeper dikes (top > 3 km subsurface) and low FI earthquakes associated with shallow dikes, indicating that shallow dikes result in earthquakes with more low-frequency content and larger-amplitude surface waves. Low FI earthquakes are more common during dike emplacement, suggesting that interactions between the dike and faults may lead to lower FI. Taken together, likely source processes for low FI earthquakes are shallow hypocenters (<3 km) possibly with surface rupture, slow rupture velocities, and interactions with dike fluids. Strong site effects also heavily influence the earthquake spectral content. Additionally, our results suggest a continuum of spectral responses, implying either that impulsive volcano-tectonic earthquakes and the unusual, emergent earthquakes have similar source processes or that simple spectral analyses, such as FI, cannot distinguish different source processes.

  16. GIS-based Conceptual Database Model for Planetary Geoscientific Mapping

    NASA Astrophysics Data System (ADS)

    van Gasselt, Stephan; Nass, Andrea; Neukum, Gerhard

    2010-05-01

    concerning, e.g., map products (product and cartograpic representation), sensor-data products, stratigraphy definitions for each planet (facies, formation, ...), and mapping units. Domains and subtypes as well as a set of two dozens relationships define their interaction and allow a high level of constraints that aid to limit errors by domain- and topologic boundary conditions without limiting the abilitiy of the mapper to perform his/her task. The geodatabase model is part of a data model currently under development and design in the context of providing tools and definitions for mapping, cartographic representations and data exploitation. The database model as an integral part is designed for portability with respect to geoscientific mapping tasks in general and can be applied to every GIS project dealing with terrestrial planetary objects. It will be accompanied by definitions and representations on the cartographic level as well as tools and utilities for providing easy accessible workflows focussing on query, organization, maintainance, integration of planetary data and meta information. The data model's layout is modularized with individual components dealing with symbol representations (geology and geomorphology), metadata accessibility and modification, definition of stratigraphic entitites and their relationships as well as attribute domains, extensions for planetary mapping and analysis tasks as well as integration of data information on the level of vector representations for easy accessible querying, data processing in connection with ISIS/GDAL and data integration.

  17. Triple junctions and multi-directional extension of the lithosphere

    NASA Astrophysics Data System (ADS)

    Gerya, Taras; Burov, Evgenii

    2016-04-01

    new oceanic crust and lithosphere accretion. We document initial formation and destabilization of quadruple R-R-R-R junctions as initial plate rifting structures under bi-directional extension. In most cases, quadruple plate rifting junctions rapidly (typically within 1-2 Myr) evolve towards formation of two diverging triple oceanic spreading junctions connected by a linear spreading center lengthening with time. Asymmetric stretching results in various configurations, for example formation of "T-junctions" with trans-extensional components and combination of fast and slow spreading ridges. Numerical experiments also suggest that several existing oceanic spreading junctions form as the result of plate motions rearrangements after which only one of two plates spreading along the ridge becomes subjected to bi-directional spreading. Combined with plume impingement, this scenario evolves in realistic patterns closely resembling observed plate dynamics. In particular, opening of the Red Sea and of the Afar rift system find a logical explanation within a single model. We also demonstrate that the development of triple junctions should be an intrinsic feature of subduction and plate tectonics initiation triggered by plume-lithosphere interactions.

  18. Stress field during early magmatism in the Ali Sabieh Dome, Djibouti, SE Afar rift

    NASA Astrophysics Data System (ADS)

    Sue, Christian; Le Gall, Bernard; Daoud, Ahmed Mohamed

    2014-09-01

    The so-called Ali Sabieh range, SE Afar rift, exhibits an atypical antiform structure occurring in the overall extensional tectonic context of the Afar triple junction. We dynamically analyzed the brittle deformation of this specific structural high using four different methods in order to better constrain the tectonic evolution of this key-area in the Afar depression. Paleostress inversions appear highly consistent using the four methods, which a posteriori validates this approach. Computed paleostress fields document two major signals: an early E-W extensional field, and a later transcurrent field, kinematically consistent with the previous one. The Ali Sabieh range may have evolved continuously during Oligo-Miocene times from large-scale extensional to transcurrent tectonism, as the result of probable local stress permutation between σ1 and σ2 stress axes.

  19. Afar-wide Crustal Strain Field from Multiple InSAR Tracks

    NASA Astrophysics Data System (ADS)

    Pagli, C.; Wright, T. J.; Wang, H.; Calais, E.; Bennati Rassion, L. S.; Ebinger, C. J.; Lewi, E.

    2010-12-01

    Onset of a rifting episode in the Dabbahu volcanic segment, Afar (Ethiopia), in 2005 renewed interest in crustal deformation studies in the area. As a consequence, an extensive geodetic data set, including InSAR and GPS measurements have been acquired over Afar and hold great potential towards improving our understanding of the extensional processes that operate during the final stages of continental rupture. The current geodetic observational and modelling strategy has focused on detailed, localised studies of dyke intrusions and eruptions mainly in the Dabbahu segment. However, an eruption in the Erta ‘Ale volcanic segment in 2008, and cluster of earthquakes observed in the Tat Ale segment, are testament to activity elsewhere in Afar. Here we make use of the vast geodetic dataset available to obtain strain information over the whole Afar depression. A systematic analysis of all the volcanic segments, including Dabbahu, Manda-Hararo, Alayta, Tat ‘Ale Erta Ale and the Djibouti deformation zone, is undertaken. We use InSAR data from multiple tracks together with available GPS measurements to obtain a velocity field model for Afar. We use over 300 radar images acquired by the Envisat satellite in both descending and ascending orbits, from 12 distinct tracks in image and wide swath modes, spanning the time period from October 2005 to present time. We obtain the line-of-sight deformation rates from each InSAR track using a network approach and then combine the InSAR velocities with the GPS observations, as suggested by Wright and Wang (2010) following the method of England and Molnar (1997). A mesh is constructed over the Afar area and then we solve for the horizontal and vertical velocities on each node. The resultant full 3D Afar-wide velocity field shows where current strains are being accumulated within the various volcanic segments of Afar, the width of the plate boundary deformation zone and possible connections between distinct volcanic segments on a

  20. SEASAT observations of lithospheric flexure

    NASA Technical Reports Server (NTRS)

    Mcadoo, D. C.

    1984-01-01

    Models of lithospheric flexure were tested on SEASAT altimetric observations of the geoid over Outer Rises. These altimeter data were found to provide significant new information about the strength of the oceanic lithosphere. Among the significant results derived from altimeter data is confirmation of the proposition that the effective elastic thickness, T sub e, of the lithosphere increases with age in approximate accord with the relation T sub E approximately equals C times one half the age. SEASAT altimeter data over Outer Rises provide an important constraint on mechanical models of the oceanic lithosphere. These data are quite consistent with an experimentally predicted mechanical model of the lithosphere which indicates that this model may be useful in other geodynamic investigations.

  1. Are Archean lithospheric keels inverted?

    NASA Astrophysics Data System (ADS)

    Percival, J. A.; Pysklywec, R. N.

    2007-02-01

    Post-tectonic granite "blooms" in many Archean cratons manifest widespread crustal melting that cannot be explained by lithosphere removal, considering the long-term buoyancy of Archean cratonic keels and low mantle geotherms indicated by Archean diamondiferous lithosphere. Rather, heat may have been transferred from the mantle through a process of lithosphere inversion, driven by basal tractive forces acting on gravitationally unstable lithosphere. The top-heavy cell comprises eclogitic lower crust ( ρ = 3500 kg/m 3) lying above depleted mantle lithosphere ( ρ = 3300 kg/m 3), whose aggregate density remained less than that of surrounding asthenosphere ( ρ = 3340 kg/m 3). Parameterized numerical models of the inversion process show a > 40 m.y. pulse of maximum 1060 °C temperatures in the lower crust, sufficient to drive anhydrous melting, when overturned basal lithosphere reaches the Moho. In equilibrating to lower lithosphere conditions, the eclogitic cap may have yielded siliceous melts that infiltrated overlying, previously depleted mantle, producing the high Si/Mg characteristic of some cratonic peridotites. P-T paths calculated for the central part of the inverting cell criss-cross the graphite-diamond boundary, explaining development of large diamond crystals through numerous growth increments. The craton stabilization process follows terminal tectonism by about 50 m.y. as a result of initial cooling, eclogite formation and lithosphere stiffening. Related consequences in the crust include low-pressure regional metamorphism and widespread hydrothermal effects including some gold mineralization. Craton stability is attributed to the high strength of the depleted mantle lithosphere, its rectified density profile, and the presence of refractory compositions at the lithosphere-asthenosphere boundary.

  2. the role of magmatism and segmentation in the structural evolution of the Afar Rift

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    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 Central Afar magmatic rift (Ethiopia), the structures related to rifting 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 Central 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 Central Afar appears to be largely distributed through space and time ("magmatic wide rift"). It has been accommodated in a 200-300 km wide strip being a diffuse incipient plate boundary during the whole rifting 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 Central Afar has been re-thickened during extension by the syn

  3. Geotectonic modeling of lithosphere dynamics and deformation: advances and challenges (inspired by geological observations)

    NASA Astrophysics Data System (ADS)

    Burov, Evgueni

    2014-05-01

    Thermo-mechanical numerical modeling becomes a universal tool for studying short- and long-term lithosphere processes, validating and verifying geodynamic and geological concepts and putting stronger constraints on the observational data. State-of-the-art models are focused on most complete integration of geoscientific methods and geological observations and account for complex thermo-rheological and mineralogical structure of the lithosphere, implement high resolution calculations allowing for direct match of their outputs with the geological and geophysical observations. Challenges of these models are vast including understanding of the behavior of complex geological systems and processes, parameterization of rheological parameters and other rock properties for geological conditions, not forgetting a large number of future methodological breakthroughs such as the development of ultra-high resolution 3D models coupled with thermodynamic processes, fluid circulation and surface processes. We here discuss both geological and geodynamic applications of the models, their principals, and major results of regional modeling studies focused on rifting, convergent and transform plate boundaries and mantle-lithosphere interactions.

  4. Degassing of carbon dioxide from basaltic magma at spreading centers: I. Afar transitional basalts

    NASA Astrophysics Data System (ADS)

    Gerlach, Terrence M.

    1989-11-01

    This study investigates the hypothesis that a significant fraction of the CO 2 in basalt supplied to axial volcanic ranges of spreading centers in the Afar depression, escapes by degassing during residence in crustal magma reservoirs. The investigation employs volcanic gas data to test the degassing hypothesis. Volcanic gases emitted from source vents at Erta'Ale lava lake are used to represent volatiles present in the basalt supplied to magma reservoirs underlying Afar spreading centers. For comparison, volcanic gases from a large fissure eruption at Ardoukoba are used to represent volatiles in basalt after a period of storage in the Afar magma reservoirs. The results confirm the hypothesis. Gases from the lava lake and fissure eruption are the same except for CO 2. They lie along a common CO 2 control line. The fissure eruption gases are six-fold depleted in CO 2 compared to gases from the continuously supplied lava lake. This difference corresponds to a loss of approximately 85% of the initial CO 2. Moreover, gases from the fissure eruption are nearly identical to those emitted by Kilauea basalts that have lost CO 2 by magma reservoir degassing. Mass balance modeling indicates an initial CO 2 content for Afar basalt of 0.12 wt.% compared to CO 2 concentrations as low as 0.02 wt.% after degassing at depth.

  5. Canada's Deep Geological Repository For Used Nuclear Fuel -The Geoscientific Site Evaluation Process

    NASA Astrophysics Data System (ADS)

    Hirschorn, S.; Ben Belfadhel, M.; Blyth, A.; DesRoches, A. J.; McKelvie, J. R. M.; Parmenter, A.; Sanchez-Rico Castejon, M.; Urrutia-Bustos, A.; Vorauer, A.

    2014-12-01

    The Nuclear Waste Management Organization (NWMO) is responsible for implementing Adaptive Phased Management, the approach selected by the Government of Canada for long-term management of used nuclear fuel generated by Canadian nuclear reactors. In May 2010, the NWMO published and initiated a nine-step site selection process to find an informed and willing community to host a deep geological repository for Canada's used nuclear fuel. The site selection process is designed to address a broad range of technical and social, economic and cultural factors. The suitability of candidate areas will be assessed in a stepwise manner over a period of many years and include three main steps: Initial Screenings; Preliminary Assessments; and Detailed Site Characterizations. The Preliminary Assessment is conducted in two phases. NWMO has completed Phase 1 preliminary assessments for the first eight communities that entered into this step. While the Phase 1 desktop geoscientific assessments showed that each of the eight communities contains general areas that have the potential to satisfy the geoscientific safety requirements for hosting a deep geological repository, the assessment identified varying degrees of geoscientific complexity and uncertainty between communities, reflecting their different geological settings and structural histories. Phase 2 activities will include a sequence of high-resolution airborne geophysical surveys and focused geological field mapping to ground-truth lithology and structural features, followed by limited deep borehole drilling and testing. These activities will further evaluate the site's ability to meet the safety functions that a site would need to ultimately satisfy in order to be considered suitable. This paper provides an update on the site evaluation process and describes the approach, methods and criteria that are being used to conduct the geoscientific Preliminary Assessments.

  6. Ethnobotanical study of plants used in management of livestock health problems by Afar people of Ada’ar District, Afar Regional State, Ethiopia

    PubMed Central

    2013-01-01

    Background The great majority of the Afar people of Ethiopia are pastoralists, highly dependent on livestock and livestock products. Livestock productivity is, however, frequently affected by different diseases. Although many districts in the Region have veterinary clinics, they lack basic facilities. As a result, the Afar people are still dependent on local materials, mainly plants, and traditional knowledge to manage livestock health problems. However, there is a serious threat to such local resources mainly due to recurrent drought and influence of modernization. Hence there is a need for proper documentation and evaluation of the existing ethnoveterinary knowledge in the Region. This study was aimed at documenting and analysing ethnoveterinary knowledge of people in Ada’ar District of the Afar Region associated with the use of plants. Methods The study involved interviewing selected knowledgeable Afar people in Ada’ar District on the use of plants to manage livestock ailments. Fidelity Level (FL) values were calculated for the reported medicinal plant to estimate their healing potentials. Specimens of reported medicinal plant were collected, identified and deposited at the National Herbarium, Addis Ababa University. Results The study revealed 49 medicinal plants as being used by the Afar people of Ada’ar District for the treatment of various livestock ailments, the majority of which (67.3%) were shrubs. Highest number of medicinal plants was used to treat blackleg, contagious caprine pleuropneumonia (CCPP), sudden sickness and pneumonia. Leaf was the most frequently sought plant part, accounting for 47% of the reported plants. All the medicnal plants used in the District were uncultivated ones growing in semi-disturbed and disturbed habitats as remnant plants and weeds. Cissus quadrangularis and Solanum incanum were the plants scoring the highest fidelity level values for their use to treat blackleg and respiratory tract problems, respectively. Conclusion

  7. Canada's Deep Geological Repository for Used Nuclear Fuel - Geo-scientific Site Evaluation Process - 13117

    SciTech Connect

    Blyth, Alec; Ben Belfadhel, Mahrez; Hirschorn, Sarah; Hamilton, Duncan; McKelvie, Jennifer

    2013-07-01

    The Nuclear Waste Management Organization (NWMO) is responsible for implementing Adaptive Phased Management (APM), the approach selected by the Government of Canada for long-term management of used nuclear fuel generated by Canadian nuclear reactors. The ultimate objective of APM is the centralized containment and isolation of Canada's used nuclear fuel in a Deep Geological Repository in a suitable rock formation at a depth of approximately 500 meters (m) (1,640 feet [ft]). In May 2010, the NWMO published a nine-step site selection process that serves as the road map to decision-making on the location for the deep geological repository. The safety and appropriateness of any potential site will be assessed against a number of factors, both technical and social in nature. The selected site will be one that can be demonstrated to be able to safely contain and isolate used nuclear fuel, protecting humans and the environment over the very long term. The geo-scientific suitability of potential candidate sites will be assessed in a stepwise manner following a progressive and thorough site evaluation process that addresses a series of geo-scientific factors revolving around five safety functions. The geo-scientific site evaluation process includes: Initial Screenings; Preliminary Assessments; and Detailed Site Evaluations. As of November 2012, 22 communities have entered the site selection process (three in northern Saskatchewan and 18 in northwestern and southwestern Ontario). (authors)

  8. System for Automated Geoscientific Analyses (SAGA) v. 2.1.4

    NASA Astrophysics Data System (ADS)

    Conrad, O.; Bechtel, B.; Bock, M.; Dietrich, H.; Fischer, E.; Gerlitz, L.; Wehberg, J.; Wichmann, V.; Böhner, J.

    2015-02-01

    The System for Automated Geoscientific Analyses (SAGA) is an open-source Geographic Information System (GIS), mainly licensed under the GNU General Public License. Since its first release in 2004, SAGA has rapidly developed from a specialized tool for digital terrain analysis to a comprehensive and globally established GIS platform for scientific analysis and modeling. SAGA is coded in C++ in an object oriented design and runs under several operating systems including Windows and Linux. Key functional features of the modular organized software architecture comprise an application programming interface for the development and implementation of new geoscientific methods, an easily approachable graphical user interface with many visualization options, a command line interpreter, and interfaces to scripting and low level programming languages like R and Python. The current version 2.1.4 offers more than 700 tools, which are implemented in dynamically loadable libraries or shared objects and represent the broad scopes of SAGA in numerous fields of geoscientific endeavor and beyond. In this paper, we inform about the system's architecture, functionality, and its current state of development and implementation. Further, we highlight the wide spectrum of scientific applications of SAGA in a review of published studies with special emphasis on the core application areas digital terrain analysis, geomorphology, soil science, climatology and meteorology, as well as remote sensing.

  9. System for Automated Geoscientific Analyses (SAGA) v. 2.1.4

    NASA Astrophysics Data System (ADS)

    Conrad, O.; Bechtel, B.; Bock, M.; Dietrich, H.; Fischer, E.; Gerlitz, L.; Wehberg, J.; Wichmann, V.; Böhner, J.

    2015-07-01

    The System for Automated Geoscientific Analyses (SAGA) is an open source geographic information system (GIS), mainly licensed under the GNU General Public License. Since its first release in 2004, SAGA has rapidly developed from a specialized tool for digital terrain analysis to a comprehensive and globally established GIS platform for scientific analysis and modeling. SAGA is coded in C++ in an object oriented design and runs under several operating systems including Windows and Linux. Key functional features of the modular software architecture comprise an application programming interface for the development and implementation of new geoscientific methods, a user friendly graphical user interface with many visualization options, a command line interpreter, and interfaces to interpreted languages like R and Python. The current version 2.1.4 offers more than 600 tools, which are implemented in dynamically loadable libraries or shared objects and represent the broad scopes of SAGA in numerous fields of geoscientific endeavor and beyond. In this paper, we inform about the system's architecture, functionality, and its current state of development and implementation. Furthermore, we highlight the wide spectrum of scientific applications of SAGA in a review of published studies, with special emphasis on the core application areas digital terrain analysis, geomorphology, soil science, climatology and meteorology, as well as remote sensing.

  10. A kinematic model for the development of the Afar Depression and its paleogeographic implications

    NASA Astrophysics Data System (ADS)

    Redfield, T. F.; Wheeler, W. H.; Often, M.

    2003-11-01

    The Afar Depression is a highly extended region of continental to transitional oceanic crust lying at the junction of the Red Sea, the Gulf of Aden and the Ethiopian rifts. We analyze the evolution of the Afar crust using plate kinematics and published crustal models to constrain the temporal and volumetric evolution of the rift basin. Our reconstruction constrains the regional-scale initial 3D geometry and subsequent extension and is well calibrated at the onset of rifting (˜20 Ma) and from the time of earliest documented sea-floor spreading anomalies (˜6 Ma Red Sea; ˜10 Ma Gulf of Aden). It also suggests the Danakil block is a highly extended body, having undergone between ˜200% and ˜400% stretch. Syn-rift sedimentary and magmatic additions to the crust are taken from the literature. Our analysis reveals a discrepancy: either the base of the crust has not been properly imaged, or a (plume-related?) process has somehow caused bulk removal of crustal material since extension began. Inferring subsidence history from thermal modeling and flexural considerations, we conclude subsidence in Afar was virtually complete by Mid Pliocene time. Our analysis contradicts interpretations of late (post 3 Ma) large (˜2 km) subsidence of the Hadar area near the Ethiopian Plateau, suggesting paleoclimatic data record regional, not local, climate change. Tectonic reconstruction (supported by paleontologic and isotopic data) suggests that a land bridge connected Africa and Arabia, via Danakil, up to the Early to Middle Pliocene. The temporal constraints on land bridge and escarpment morphology constrain Afar paleogeography, climate, and faunal migration routes. These constraints (particularly the development of geographic isolation) are fundamentally important for models evaluating and interpreting biologic evolution in the Afar, including speciation and human origins.

  11. 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.

  12. Geo-Seas - building a unified e-infrastructure for marine geoscientific data management in Europe

    NASA Astrophysics Data System (ADS)

    Glaves, H.; Schaap, D.

    2012-04-01

    A significant barrier to marine geoscientific research in Europe is the lack of standardised marine geological and geophysical data and data products which could potentially facilitate multidisciplinary marine research extending across national and international boundaries. Although there are large volumes of geological and geophysical data available for the marine environment it is currently very difficult to use these datasets in an integrated way due to different nomenclatures, formats, scales and coordinate systems being used within different organisations as well as between countries. This makes the direct use of primary data very difficult and also hampers use of the data to produce integrated multidisciplinary data products and services. The Geo-Seas project, an EU Framework 7 funded initiative, is developing a unified e-infrastructure to facilitate the sharing of marine geoscientific data within Europe. This e-infrastructure is providing on-line access to both discovery metadata and the associated federated data sets from 26 European data centres via a dedicated portal. The implementation of the Geo-Seas portal is allowing a range of end users to locate, assess and access standardised geoscientific data from multiple sources which is interoperable with other marine data types. Geo-Seas is building on the work already done by the existing SeaDataNet project which currently provides a data management e-infrastructure for oceanographic data which allows users to locate and access federated oceanographic data sets. By adopting and adapting the SeaDataNet methodologies and technologies the Geo-Seas project has not only avoid unnecessary duplication of effort by reusing existing and proven technologies but also contributed to the development of a multidisciplinary approach to ocean science across Europe through the creation of a joint infrastructure for both marine geoscientific and oceanographic data. This approach is also leading to the development of

  13. Mechanical heterogeneities and lithospheric extension

    NASA Astrophysics Data System (ADS)

    Duretz, Thibault; Petri, Benoit; Mohn, Geoffroy; Schenker, Filippo L.; Schmalholz, Stefan

    2016-04-01

    Detailed geological and geophysical studies of passive margins have highlighted the multi-stage and depth-dependent aspect of lithospheric thinning. Lithospheric thinning involves a variety of structures (normal faults, low angle detachments, extensional shear zones, extraction faults) and leads to a complex architecture of passive margins (with e.g. necking zone, mantle exhumation, continental allochthons). The processes controlling the generation and evolution of these structures as well as the impact of pre-rift inheritance are so far incompletely understood. In this study, we investigate the impact of pre-rift inheritance on the development of rifted margins using two-dimensional thermo-mechanical models of lithospheric thinning. To first order, we represent the pre-rift mechanical heterogeneities with lithological layering. The rheologies are kept simple (visco-plastic) and do not involve any strain softening mechanism. Our models show that mechanical layering causes multi-stage and depth-dependent extension. In the initial rifting phase, lithospheric extension is decoupled: as the crust undergoes thinning by brittle (frictional-plastic) faults, the lithospheric mantle accommodates extension by symmetric ductile necking. In a second rifting phase, deformation in the crust and lithospheric mantle is coupled and marks the beginning of an asymmetric extension stage. Low angle extensional shear zones develop across the lithosphere and exhume subcontinental mantle. Furthemore, crustal allochthons and adjacent basins develop coevally. We describe as well the thermal evolution predicted by the numerical models and discuss the first-order implications of our results in the context of the Alpine geological history.

  14. Optical-radar-DEM remote sensing data integration for geological mapping in the Afar Depression, Ethiopia

    NASA Astrophysics Data System (ADS)

    Thurmond, Allison K.; Abdelsalam, Mohamed G.; Thurmond, John B.

    2006-02-01

    The advantages of integrating optical (Landsat Enhanced Thematic Mapper Plus (ETM+) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)) and radar (Shuttle Imaging Radar (SIR) - C, X-band Synthetic Aperture Radar (SAR) and RADARSAT-1) remote sensing data, and digital elevation models (DEMs) (Shuttle Radar Topography Mission (SRTM)) for geological mapping in arid regions such as the Afar Depression in Ethiopia are demonstrated. The Afar Depression in NE Africa is a natural laboratory for studying processes of sea-floor spreading and the transition from rifting to true sea-floor spreading. It is ideal for geological remote sensing because of its vastness, remoteness and inaccessibility together with almost continuous exposure, and lack of vegetation and soil cover. Optical-radar-DEM remote sensing data integration is used for: (1) Distinguishing spatial and temporal distribution of individual lava flows in the Quaternary Erta 'Ale Volcanic Range in the northern part of the Afar Depression, by integrating band-ratios of ASTER thermal infrared (TIR) data with Landsat ETM+ visible and near infrared (VNIR) and SIR-C/X-SAR L-band ( λ = 24 cm) data with horizontally transmitted and horizontally received (HH) polarization. (2) Visualizing and interpreting extensional imbrication fans that constitute part of the Dobe Graben in the central part of the Afar Depression by integrating Landsat ETM+ VNIR data with RADARSAT C-band ( λ = 6 cm) data with HH polarization and SRTM DEMs. These imbrication fans were developed as layer-parallel gravitational slip of the border fault hanging-wall towards the graben center. (3) Mapping morphologically defined structures in rhyolite flows exposed on the flanks of the Tendaho Rift by merging ASTER VNIR and short wave infrared (SWIR) with RADARSAT C-band data with HH polarization. The Tendaho Rift constitutes part of the Tendaho-Gobaad Discontinuity that separates the southern and the central eastern parts of the Afar

  15. Lithospheric architecture beneath Hudson Bay

    NASA Astrophysics Data System (ADS)

    Porritt, Robert W.; Miller, Meghan S.; Darbyshire, Fiona A.

    2015-07-01

    Hudson Bay overlies some of the thickest Precambrian lithosphere on Earth, whose internal structures contain important clues to the earliest workings of plate formation. The terminal collision, the Trans-Hudson Orogen, brought together the Western Churchill craton to the northwest and the Superior craton to the southeast. These two Archean cratons along with the Paleo-Proterozoic Trans-Hudson internides, form the core of the North American craton. We use S to P converted wave imaging and absolute shear velocity information from a joint inversion of P to S receiver functions, new ambient noise derived phase velocities, and teleseismic phase velocities to investigate this region and determine both the thickness of the lithosphere and the presence of internal discontinuities. The lithosphere under central Hudson Bay approaches ˜350 km thick but is thinner (˜200-250 km) around the periphery of the Bay. Furthermore, the amplitude of the LAB conversion from the S receiver functions is unusually large for a craton, suggesting a large thermal contrast across the LAB, which we interpret as direct evidence of the thermal insulation effect of continents on the asthenosphere. Within the lithosphere, midlithospheric discontinuities, significantly shallower than the base of the lithosphere, are often imaged, suggesting the mechanisms that form these layers are common. Lacking time-history information, we infer that these discontinuities reflect reactivation of formation structures during deformation of the craton.

  16. Uppermost mantle (Pn) velocity model for the Afar region, Ethiopia: an insight into rifting processes

    NASA Astrophysics Data System (ADS)

    Stork, A. L.; Stuart, G. W.; Henderson, C. M.; Keir, D.; Hammond, J. O. S.

    2013-04-01

    The Afar Depression, Ethiopia, offers unique opportunities to study the transition from continental rifting to oceanic spreading because the process is occurring onland. Using traveltime tomography and data from a temporary seismic deployment, we describe the first regional study of uppermost mantle P-wave velocities (VPn). We find two separate low VPn zones (as low as 7.2 km s-1) beneath regions of localized thinned crust in northern Afar, indicating the existence of high temperatures and, potentially, partial melt. The zones are beneath and off-axis from, contemporary crustal magma intrusions in active magmatic segments, the Dabbahu-Manda-Hararo and Erta'Ale segments. This suggests that these intrusions can be fed by off-axis delivery of melt in the uppermost mantle and that discrete areas of mantle upwelling and partial melting, thought to characterize segmentation of the uppermost mantle at seafloor spreading centres, are initiated during the final stages of break-up.

  17. A User-Friendly, Keyword-Searchable Database of Geoscientific References Through 2007 for Afghanistan

    USGS Publications Warehouse

    Eppinger, Robert G.; Sipeki, Julianna; Scofield, M.L. Sco

    2008-01-01

    This report includes a document and accompanying Microsoft Access 2003 database of geoscientific references for the country of Afghanistan. The reference compilation is part of a larger joint study of Afghanistan?s energy, mineral, and water resources, and geologic hazards currently underway by the U.S. Geological Survey, the British Geological Survey, and the Afghanistan Geological Survey. The database includes both published (n = 2,489) and unpublished (n = 176) references compiled through calendar year 2007. The references comprise two separate tables in the Access database. The reference database includes a user-friendly, keyword-searchable interface and only minimum knowledge of the use of Microsoft Access is required.

  18. The 2004 dyke-fault interaction at Dallol, northern Afar (Ethiopia)

    NASA Astrophysics Data System (ADS)

    Nobile, A.; Pagli, C.; Keir, D.; Wright, T.; Ruch, J.; Acocella, V.

    2012-04-01

    The 2005 Dabbahu intrusive event in Afar highlighted the importance of magma in controlling rifting along divergent plate boundaries. Here we show that a further intrusive event occurred in late 2004 at Dallol, along the axis of the Red Sea Rift in northern Afar. Dallol consists of an hydrothermal area lacking significant volcanic activity at the northern end of the Erta Ale volcanic segment. We use InSAR and seismicity data to constrain the geometric and kinematic features of the intrusion, as well as its possible effects. The dyke intruded a 6 km portion of the rift, from Dallol volcano, which subsided, to the south. The intrusion was accompanied by a seismic sequence, including a Mw 5.5 earthquake on 22 October. The larger events were registered by several seismic stations worldwide and located by the International Seismological Center (ISC). InSAR data from Envisat allow us to measure the surface deformation associated with these events. An inversion for a simple elastic model suggests that a ~3m wide and ~46km3 volume dyke was accompanied by fault slip. Seismicity observations suggest that the Mw 5.5 earthquake was triggered by the dyke intrusion. These results at Dallol provide the northernmost evidence of dyke-induced extension in Afar, occurring shortly before the 2005 event at Dabbahu.

  19. Large magnitude silicic volcanism in north Afar: the Nabro Volcanic Range and Ma'alalta volcano

    NASA Astrophysics Data System (ADS)

    Wiart, Pierre; Oppenheimer, Clive

    2005-02-01

    Much of the volcanological work carried out in north Afar (Ethiopia and Eritrea) has focused on the nature of Quaternary basaltic volcanic ranges, which have been interpreted by some as incipient oceanic ridges. However, we show here that comparable volumes of silicic magmas have been erupted in the region. In particular, the virtually undocumented Nabro Volcanic Range, which runs NNE for more than 100 km from the margin of the Danakil Depression to the Red Sea coast, has a subaerial volume of the order of 550 km3, comparable to the volume of the much better known Erta’Ale axial volcanic range. Nabro volcano itself forms part of an enigmatic double caldera structure with a neighbouring volcano, Mallahle. The twin caldera may have formed simultaneously with the eruption of between 20 and 100 km3 of ignimbrite, which is readily identified in Landsat Thematic Mapper imagery. This may have been the largest explosive eruption in north Afar, and is certain to have deposited a regionally distributed tephra layer which could in the future be located in distal sections as a stratigraphic marker. An integrated analysis of optical and synthetic aperture radar imagery, digital topographic data, field observations and limited geochemical measurements, permits here descriptions and first order inferences about the structure, stratigraphy and compositions of several major volcanoes of the Afar Triangle, and a reappraisal of their regional significance.

  20. Magmatic expressions of continental lithosphere removal

    NASA Astrophysics Data System (ADS)

    Wang, Huilin; Currie, Claire A.

    2015-10-01

    Gravitational lithosphere removal in continental interior has been inferred from various observations, including anomalous surface deflections and magmatism. We use numerical models and a simplified theoretical analysis to investigate how lithosphere removal can be recognized in the magmatic record. One style of removal is a Rayleigh-Taylor-type instability, where removal occurs through dripping. The associated magmatism depends on the lithosphere thermal structure. Four types of magmatism are predicted: (1) For relatively hot lithosphere (e.g., back arcs), the lithosphere can be conductively heated and melted during removal, while the asthenosphere upwells and undergoes decompression melting. If removal causes significant lithospheric thinning, the deep crust may be heated and melted. (2) For moderately warm lithosphere (e.g., average Phanerozoic lithosphere) in which the lithosphere root has a low density, only the lithosphere may melt. (3) If the lithosphere root has a high density in moderately warm lithosphere, only asthenosphere melt is predicted. (4) For cold lithosphere (e.g., cratons), no magmatism is induced. An alternate style of removal is delamination, where dense lithosphere peels along Moho. In most cases, the lithosphere sinks too rapidly to melt. However, asthenosphere can upwell to the base of the crust, resulting in asthenospheric and crustal melts. In delamination, magmatism migrates laterally with the detachment point; in contrast, magmatism in Rayleigh-Taylor-type instability has a symmetric shape and converges toward the drip center. The models may explain the diversity of magmatism observed in areas with inferred lithosphere removal, including the Puna Plateau and the southern Sierra Nevada.

  1. New geodetic measurements in central Afar constraining the Arabia-Somalia-Nubia triple junction kinematics

    NASA Astrophysics Data System (ADS)

    Doubre, C.; Deprez, A.; Masson, F.; Socquet, A.; Lewi, E.; Grandin, R.; Calais, E.; Wright, T. J.; Bendick, R. O.; Pagli, C.; Peltzer, G.; de Chabalier, J. B.; Ibrahim Ahmed, S.

    2014-12-01

    The Afar Depression is an extraordinary submerged laboratory where the crustal mechanisms involved in the active rifting process can be studied. But the crustal movements at the regional scale are complicated by being the locus of the meeting of three divergent plate boundaries: the oceanic spreading ridges of the Red Sea and the Aden Ridge and the intra-continental East-African Rift (EAR). We present here the first GPS measurements conducted in a new network in Central Afar, complementing existing networks in Eritrea, around the Manda-Harraro 2005-2010 active segment, in the Northern part of the EAR and in Djibouti. Even if InSAR data were appropriate for mapping the deformation field, the results are difficult to interpret for analyzing the regional kinematics because of the atmospheric conditions, the lack of complete data catalogue, the acquisition configuration and the small velocity variations. Therefore, our measurements in the new sites are crucial to obtain an accurate velocity field over the whole depression, and focus specifically on the spatial organization of the deformation to characterize the tripe junction. These first results show that a small part of the motion of the Somalia plate with respect to the Nubia plate or the Arabia plate (2-3 mm/yr) occurs south of the Tadjura Gulf and East of the Adda-do segment in Southern Afar. The complex kinematic pattern involves a clockwise rotation of this Southeastern part of the Afar rift and can be related to the significant seismic activity regularly recorded in the region of Jigjiga (northern Somalia-Ethiopia border). The western continuation of the Aden Ridge into Afar extends West of the Asal rift segment and does not reach the young active segment of Manda-Inakir (MI). A slow gradient of velocity is observed across the Dobi Graben and across the large systems of faults between Lake Abhe and the MI rift segment. A striking change of the velocity direction occurs in the region of Assaïta, west of Lake

  2. The lithospheric shear-wave velocity structure of Saudi Arabia: Young volcanism in an old shield

    NASA Astrophysics Data System (ADS)

    Tang, Zheng; Julià, Jordi; Mai, P. Martin

    2016-04-01

    We are utilizing receiver function and surface wave dispersion data to investigate the lithospheric shear-wave velocity structure of Saudi Arabia. The Arabian plate consists of the western Arabian shield and the eastern Arabian platform. The Arabian shield is a complicated mélange of several Proterozoic terrains, separated by ophiolite-bearing suture zones and dotted by outcropping Cenozoic volcanic rocks (so-called harrats). The Arabian platform is covered by thick Paleozoic, Mesozoic and Cenozoic sedimentary rocks. To understand the geo-dynamics and present-day geology in western Saudi Arabia, the origin and activity of the harrats needs to be investigated: are they controlled primarily by a local mantle plume underneath western Saudi Arabia or by lateral mantle flow from the Afar and (perhaps) Jordan hotspots? In our study, we first estimate Vp/Vs ratios by applying the H-κ stacking technique and construct local shear-wave velocity-depth profiles by jointly inverting teleseismic P-receiver functions and Rayleigh wave group velocities at 56 broadband stations deployed by the Saudi Geological Survey (SGS). Our results reveal significant lateral variations in crustal thickness, S-velocity, and bulk Vp/Vs ratio. The Arabian shield has, on average a ~34 km thick crust with Vs ~3.72 km/s and Vp/Vs ~1.73. Thinner crust (~25 - 32 km thick) with strong lateral variations is present along the Red Sea coast. In contrast, the Arabian platform reveals a ~41 km thick crust with Vs ~3.52 km/s and Vp/Vs ~1.77. We find anomalously high Vp/Vs ratios at Harrat Lunayyir, interpreted as solidified magma intrusions. Slow shear-velocities in the upper-mantle lid throughout the southernmost and northernmost Arabian shield suggest lateral heating from hot mantle upwellings centered beneath Afar and (perhaps) Jordan. Our findings on crustal S-velocity structures, Vp/Vs ratios, and upper-mantle lid velocities support the hypothesis of lateral mantle flow from the Afar and (perhaps

  3. Effective elastic thickness of Africa and its relationship to other proxies for lithospheric structure and surface tectonics

    NASA Astrophysics Data System (ADS)

    Pérez-Gussinyé, M.; Metois, M.; Fernández, M.; Vergés, J.; Fullea, J.; Lowry, A. R.

    2009-09-01

    Detailed information on lateral variations in lithospheric properties can aid in understanding how surface deformation relates to deep Earth processes. The effective elastic thickness, Te, of the lithosphere is a proxy for lithospheric strength. Here, we present a new Te map of the African lithosphere estimated from coherence analysis of topography and Bouguer anomaly data. The latter data set derives from the EGM 2008 model, the highest resolution gravity database over Africa, enabling a significant improvement in lateral resolution of Te. The methodology used for Te estimation improves upon earlier approaches by optimally combining estimates from several different window sizes and correcting for an estimation bias term. Our analysis finds that Te is high, ~ 100 km, in the West African, Congo, Kalahari and Tanzania cratons. Of these, the Kalahari exhibits the lowest Te. Based in part on published seismic and mineral physics constraints, we suggest this may reflect modification of Kalahari lithosphere by anomalously hot asthenospheric mantle. Similarly, the Tanzania craton exhibits relatively lower Te east of Lake Victoria, where a centre of seismic radial anisotropy beneath the craton has been located and identified with a plume head, thus suggesting that here too, low Te reflects modification of cratonic lithosphere by an underlying hot mantle. The lowest Te in Africa occurs in the Afar and Main Ethiopian rifts, where lithospheric extension is maximum. In the western Ethiopian plateau a local Te minimum coincides with published images of a low P and S seismic velocity anomaly extending to ~ 400 km depth. Finally, the Darfur, Tibesti, Hoggar and Cameroon line volcanic provinces are characterised by low Te and no deep-seated seismic anomalies in the mantle. Corridors of relatively low Te connect these volcanic provinces to the local Te minima within the western Ethiopian plateau. We interpret the low Te to indicate thinner lithosphere within the corridors than in

  4. Effective elastic thickness of Africa and its relationship to other proxies for lithospheric structure and surface tectonics

    NASA Astrophysics Data System (ADS)

    Perez-Gussinye, M.; Metois, M.; Fernandez, M.; Verges, J.; Fullea, J.; Lowry, A. R.

    2009-12-01

    Detailed information on lateral variations in lithospheric properties can aid in understanding how surface deformation relates to deep Earth processes. The effective elastic thickness, Te, of the lithosphere is a proxy for lithospheric strength. Here, we present a new Te map of the African lithosphere estimated from coherence analysis of topography and Bouguer anomaly data. The latter data set derives from the EGM 2008 model, the highest resolution gravity database over Africa, enabling a significant improvement in lateral resolution of Te. The methodology used for Te estimation improves upon earlier approaches by optimally combining estimates from several different window sizes and correcting for an estimation bias term. Our analysis finds that Te is high, ~ 100 km, in the West African, Congo, Kalahari and Tanzania cratons. Of these, the Kalahari exhibits the lowest Te. Based in part on published seismic and mineral physics constraints, we suggest this may reflect modification of Kalahari lithosphere by anomalously hot asthenospheric mantle. Similarly, the Tanzania craton exhibits relatively lower Te east of Lake Victoria, where a centre of seismic radial anisotropy beneath the craton has been located and identified with a plume head, thus suggesting that here too, low Te reflects modification of cratonic lithosphere by an underlying hot mantle. The lowest Te in Africa occurs in the Afar and Main Ethiopian rifts, where lithospheric extension is maximum. In the western Ethiopian plateau a local Te minimum coincides with published images of a low P and S seismic velocity anomaly extending to ~400 km depth. Finally, the Darfur, Tibesti, Hoggar and Cameroon line vo provinces lcanic are characterised by low Te and no deep-seated seismic anomalies in the mantle. Corridors of relatively low Te connect these volcanic provinces to the local Te minima within the western Ethiopian plateau. We interpret the low Te to indicate thinner lithosphere within the corridors than in

  5. Lithospheric dynamics near plate boundaries

    NASA Astrophysics Data System (ADS)

    Solomon, Sean C.

    1992-09-01

    The progress report on research conducted between 15 Mar. - 14 Sep. 1992 is presented. The focus of the research during the first grant year has been on several problems broadly related to the nature and dynamics of time-dependent deformation and stress along major seismic zones, with an emphasis on western North America but with additional work on seismic zones in oceanic lithosphere as well. The principal findings of our research to date are described in the accompanying papers and abstract. Topics covered include: (1) Global Positioning System measurements of deformations associated with the 1987 Superstition Hills earthquake: evidence for conjugate faulting; (2) Global Positioning System measurements of strain accumulation across the Imperial Valley, California: 1986-1989; (3) present-day crustal deformation in the Salton Trough, southern California; (4) oceanic transform earthquakes with unusual mechanisms or locations: relation to fault geometry and state of stress in the lithosphere; and (5) crustal strain and the 1992 Mojave Desert earthquakes.

  6. Lithospheric dynamics near plate boundaries

    NASA Technical Reports Server (NTRS)

    Solomon, Sean C.

    1992-01-01

    The progress report on research conducted between 15 Mar. - 14 Sep. 1992 is presented. The focus of the research during the first grant year has been on several problems broadly related to the nature and dynamics of time-dependent deformation and stress along major seismic zones, with an emphasis on western North America but with additional work on seismic zones in oceanic lithosphere as well. The principal findings of our research to date are described in the accompanying papers and abstract. Topics covered include: (1) Global Positioning System measurements of deformations associated with the 1987 Superstition Hills earthquake: evidence for conjugate faulting; (2) Global Positioning System measurements of strain accumulation across the Imperial Valley, California: 1986-1989; (3) present-day crustal deformation in the Salton Trough, southern California; (4) oceanic transform earthquakes with unusual mechanisms or locations: relation to fault geometry and state of stress in the lithosphere; and (5) crustal strain and the 1992 Mojave Desert earthquakes.

  7. Crustal Structure of the Gulf of Aden Continental Margins, from Afar to Oman, by Ambient Noise Seismic Tomography

    NASA Astrophysics Data System (ADS)

    Korostelev, F.; Weemstra, C.; Boschi, L.; Leroy, S. D.; Ren, Y.; Stuart, G. W.; Keir, D.; Rolandone, F.; Ahmed, A.; Al Ganad, I.; Khanbari, K. M.; Doubre, C.; Hammond, J. O. S.; Kendall, J. M.

    2014-12-01

    Continental rupture processes under mantle plume influence are still poorly known although extensively studied. The Gulf of Aden presents volcanic margins to the west, where they are influenced by the Afar hotspot, and non volcanic margins east of longitude 46° E. We imaged the crustal structure of the Gulf of Aden continental margins from Afar to Oman to evaluate the role of the Afar plume on the evolution of the passive margin and its extent towards the East. We use Ambient Noise Seismic Tomography to better understand the architecture and processes along the Gulf of Aden. This recent method, developed in the last decade, allows us to study the seismic signal propagating between two seismic stations. Ambient Noise Seismic Tomography is thus free from artifacts related to the distribution of earthquakes. We collected continuous records from about 200 permanent or temporary stations since 1999 to compute Rayleigh phase velocity maps over the Gulf of Aden.

  8. The strength of Miranda's lithosphere

    NASA Technical Reports Server (NTRS)

    Pappalardo, Robert; Greeley, Ronald

    1991-01-01

    In attempting to understand the endogenic processes which have shaped the surface of an icy satellite, it is desirable to quantify the failure strength of the satellite's lithosphere. In a crust that is fractured on a large scale, frictional sliding along pre-existing fractures occurs in response to lower differential stresses than required to initiate fracture of pristine rock, thus governing failure of a brittle lithosphere. Failure is predicted along favorably oriented fracture planes; if fractures of all orientations are assumed to be present in the crust (as is expected of a heavily cratered lithosphere), frictional failure relations are directly applicable. The Coulomb criterion predicts that the shear stress (sigma sub t) and normal stress (sigma sub n) components on a fracture plane at failure are related as sigma sub t = mu-sigma sub n + S sub o, where S sub o is the cohesion and mu is the coefficient of friction. At moderate to high pressures, the frictional sliding strength of most materials is found to be sigma sub t = 0.85 sigma sub n.

  9. Characteristics of the Bab al Mandab-Northern Afar area of the southern Red Sea

    SciTech Connect

    Allen, R.B. ); Sikander, A.H. ); Abouzakhm, A.G.

    1991-08-01

    The southern Red Sea and adjacent Afar area represent an enigmatic portion of the Red Sea/Gulf of Aden basin system. Although the topographic rift shoulders of the Red Sea and Gulf of Aden can be traced through this elbow, and appear to suggest that a similar width for the zone of extension is maintained across the region, the character of the floor of the rift zone changes. The distinctive character of the southern Red Sea-Afar area results in part from a topographically elevated region, possibly associated with the Red Sea-Gulf of Aden-East African rift triple junction. In addition, however, seismic data from offshore Ethiopia suggests that the distinctive character is due a complex pattern of rifting. The central axial trough of the Red Sea decreases in depth to the south toward the strait of Bab al Mandab, suggesting that rifting is dying out southward. Farther to the west at the same latitude, a major but narrow half-graben can be seen on seismic in the Gulf of Zula, bounded to the east by a large west-dipping normal fault. This structure continues south into the Danakil Depression of the Afar area. Between these two en echelon rift trends, the Danakil Alps form a long-lived high. Seismic data from the southern Red Sea of Ethiopia show southward thinning and pinch-out of the Miocene syn-rift evaporite sequence onto the northern Danakil block. Thus, it appears that the Danakil block has largely escaped Red Sea extension and subsidence. Instead, it forms a large unextended terrain located between overlapping en echelon rift trends, and may represent an accommodation zone structure associated with offset in the rift axis of the southern Red Sea.

  10. The August 2002 earthquake sequence in north Afar: Insights into the neotectonics of the Danakil microplate

    NASA Astrophysics Data System (ADS)

    Ayele, Atalay; Stuart, Graham; Bastow, Ian; Keir, Derek

    2007-06-01

    In August 2002, there was high seismic activity in Afar concentrated at the plateau margin of the northern Ethiopian rift east of Mekele, near the western part of the Danakil microplate. The spatial and temporal distributions of this seismic activity over four weeks indicate the NNW propagation of the Gulf of Aden rift across the Afar Depression towards the western Ethiopian plateau. Fault plane solutions for six larger earthquakes from the August 2002 sequence are estimated from moment tensor inversion of local broadband waveform data. The results show only normal faulting on NNW trending and NE dipping faults, which agree with tectonics of the area and distribution of aftershocks. No strike-slip component is observed in any of our fault plane solutions or those of other workers including Harvard CMT solutions in the region. Such motion would be indicative of oblique-slip deformation between the Nubian plate and the Danakil microplate consistent with counter-clockwise rotation of the microplate. Hypocentral depths of well-constrained events are 5-7 km, which is the approximate elastic plate thickness in the Main Ethiopian rift, possibly indicating the depth to the brittle-ductile transition zone in this part of the Afar Depression. The shallowness of the depth estimates agree with the macroseismic reports available from a wide area in northern Ethiopia. Potential future shallow crustal deformation may cause significant loss of human life and damage to property in the densely populated highland region around Mekele unless measures are taken in improving building standards. The b-value for this sequence is estimated to be 0.66 using a least squares fit, while it is 0.67 ± 0.16 from a maximum-likelihood approach. This estimated b-value is low or the frequency of occurrence of relatively larger magnitude events is high indicating that it is a highly stressed region as evidenced by the recent increase of the seismicity in the area.

  11. First Evidence of Epithermal Gold Occurrences in the SE Afar Rift, Republic of Djibouti

    NASA Astrophysics Data System (ADS)

    Moussa, Nima; Fouquet, Yves; Caminiti, Antoine Marie; Le Gall, Bernard; Rolet, Joel; Bohn, Marcel; Etoubleau, Joel; Delacourt, Christophe; Jalludin, Mohamed

    2010-05-01

    The Republic of Djibouti, located at the SE part of the Afar volcanic Triangle, is characterized by intense tectonic and bimodal volcanic activity, and is emplaced over an earlier magmatic rift system, as old as 25-30 Ma. Each magmatic event is accompanied by hydrothermal activity. Few works have been so far published on hydrothermal mineralization in the Afar area. Mineralization generally occur as veins and are mainly associated with acidic volcanic intrusions along the fractures at the edges of grabens established during the last 4 Ma. Eighty samples from hydrothermal quartz ± carbonate veins and breccias were studied on 9 different sites representative of 4 main volcanic events ranging in age from early Miocene up to Present. Gold was found in excess of 200 ppb in 30% of the samples. Mineralogical analyses based on optical reflected light microscopy, X-Ray diffractometry, X-Ray fluorescence, inductively coupled plasma mass spectroscopy and electron microprobe, led us to identify two types of gold mineralization (i) native gold, electrum, hessite and sulfides (chalcopyrite, pyrite, bornite, ± sphalerite, and galena) in massive quartz breccias and banded chalcedony, (ii) gold, electrum, hematite, magnetite, trace minerals (argentite) and adularia in banded chalcedony. Another group without gold is characterized by quartz, pyrite ± goethite. Secondary minerals are characterized by goethite, native silver and native copper. Arsenic is enriched in pyrite in samples with a high gold content. The bimodal volcanism, the occurrence of adularia, the native gold and electrum in banded silica veins, are classically observed in neutral epithermal systems. The discovery of this type of mineralization in a recent-active continental rift system supplies new insights about hydrothermal processes associated with volcanic activity in a spreading context. Keywords: Republic of Djibouti, Afar Triangle, Hydrothermal, Epithermal system, Gold

  12. Mantle convection in the Middle East: Reconciling Afar upwelling, Arabia indentation and Aegean trench rollback

    NASA Astrophysics Data System (ADS)

    Faccenna, Claudio; Becker, Thorsten W.; Jolivet, Laurent; Keskin, Mehmet

    2013-08-01

    The Middle East region represents a key site within the Tethyan domain where continental break-up, collision, backarc extension and escape tectonics are kinematically linked together. We perform global mantle circulation computations to test the role of slab pull and mantle upwellings as driving forces for the kinematics of the Arabia-Anatolia-Aegean (AAA) system, evaluating different boundary conditions and mantle density distributions as inferred from seismic tomography or slab models. Model results are compared with geodetically inferred crustal motions, residual topography, and shear wave splitting measurements. The AAA velocity field with respect to Eurasia shows an anti-clockwise toroidal pattern, with increasing velocities toward the Aegean trench. The best match to these crustal motions can be obtained by combining the effect of slab pull exerted in the Aegean with a mantle upwelling underneath Afar and, more generally, with the large-scale flow associated with a whole mantle, Tethyan convection cell. Neogene volcanism for AAA is widespread, not only in the extensional or subduction settings, but also within plates, such as in Syria-Jordan-Israel and in Turkey, with geochemical fingerprints similar of those of the Afar lava. In addition, morphological features show large uplifting domains far from plate boundaries. We speculate that the tectonic evolution of AAA is related to the progressive northward entrainment of upwelling mantle material, which is itself associated with the establishment of the downwelling part of a convection cell through the segmented Tethyan slab below the northern Zagros and Bitlis collision zone. The recently established westward flow dragged Anatolia and pushed the Aegean slab south-westward, thus accelerating backarc extension. Our model reconciles Afar plume volcanism, the collision in the Bitlis mountains and northern Zagros, and the rapid increase of Aegean trench rollback in a single coherent frame of large scale mantle

  13. Destruction of the North China Craton: Lithosphere folding-induced removal of lithospheric mantle?

    NASA Astrophysics Data System (ADS)

    Zhang, Kai-Jun

    2012-01-01

    High heat flow, high surface topography, and widespread volcanism indicate that the lithospheric mantle of typical cratonic character of the North China Craton has been seriously destroyed in its eastern half. However, the mechanism of this process remains open to intense debate. Here lithosphere folding-induced lithospheric mantle removal is proposed as a new mechanism for the destruction of the craton. Four main NNE-SSW-striking lithospheric-scale anticlines and synclines are recognized within North China east of the Helan fold-and-thrust belt. The lithosphere folding occurred possibly during the Late Triassic through Jurassic when the Yangzi Craton collided with the North China Craton. It was accompanied or followed by lithospheric dripping, and could have possibly induced the lithosphere foundering of the North China Craton. The lithosphere folding would have modified the lithosphere morphology, creating significant undulation in the lithospheric base and thus causing variations of the patterns of the small-scale convection. It also could have provoked the formation of new shear zones liable to impregnation of magma, producing linear incisions at the cratonic base and resulting in foundering of lithospheric mantle blocks. Furthermore, it generated thickening of the lithosphere or the lower crust and initiated the destabilization and subsequent removal of the lithospheric mantle.

  14. Death Valley regional groundwater flow model calibration using optimal parameter estimation methods and geoscientific information systems

    USGS Publications Warehouse

    D'Agnese, F. A.; Faunt, C.C.; Hill, M.C.; Turner, A.K.

    1996-01-01

    A three-layer Death Valley regional groundwater flow model was constructed to evaluate potential regional groundwater flow paths in the vicinity of Yucca Mountain, Nevada. Geoscientific information systems were used to characterize the complex surface and subsurface hydrogeological conditions of the area, and this characterization was used to construct likely conceptual models of the flow system. The high contrasts and abrupt contacts of the different hydrogeological units in the subsurface make zonation the logical choice for representing the hydraulic conductivity distribution. Hydraulic head and spring flow data were used to test different conceptual models by using nonlinear regression to determine parameter values that currently provide the best match between the measured and simulated heads and flows.

  15. Evolution of Northeast Atlantic Magmatic Continental Margins from an Ethiopian-Afar Perspective

    NASA Astrophysics Data System (ADS)

    England, R. W.; Cornwell, D. G.; Ramsden, A. M.

    2014-12-01

    One of the major problems interpreting the evolution of magmatic continental margins is that the structure which should record the pre-magmatic evolution of the rift and which potentially influences the character of the rifting process is partially or completely obscured by thick basalt lava flows and sills. A limited number of deep reflection seismic profiles acquired with tuned seismic sources have penetrated the basalts and provide an image of the pre-magmatic structure, otherwise the principle data are lower resolution wide-angle/refraction profiles and potential field models which have greater uncertainties associated with them. In order to sidestep the imaging constraints we have examined the Ethiopian - Afar rift system to try to understand the rifting process. The Main Ethiopian rift contains an embryonic magmatic passive margin dominated by faulting at the margins of the rift and en-echelon magmatic zones at the centre. Further north toward Afar the rift becomes in-filled with extensive lava flows fed from fissure systems in the widening rift zone. This rift system provides, along its length, a series of 'snapshots' into the possible tectonic evolution of a magmatic continental margin. Deep seismic profiles crossing the NE Atlantic margins reveal ocean dipping reflector sequences (ODRS) overlying extended crust and lower crustal sill complexes of intruded igneous rock, which extend back beneath the continental margin. The ODRS frequently occur in fault bounded rift structures along the margins. We suggest, by analogy to the observations that can be made in the Ethiopia-Afar rift that these fault bounded basins largely form at the embryonic rift stage and are then partially or completely filled with lavas fed from fissures which are now observed as the ODRS. Also in the seismic profiles we identify volcanic constructs on the ODRS which we interpret as the equivalent of the present day fissure eruptions seen in Afar. The ocean ward dip on the ODRS is

  16. Multiple mantle upwellings through the transition zone beneath the Afar Depression?

    NASA Astrophysics Data System (ADS)

    Hammond, J. O.; Kendall, J. M.; Stuart, G. W.; Thompson, D. A.; Ebinger, C. J.; Keir, D.; Ayele, A.; Goitom, B.; Ogubazghi, G.

    2012-12-01

    Previous seismic studies using regional deployments of sensors in East-Africa show that low seismic velocities underlie Africa, but their resolution is limited to the top 200-300km of the Earth. Thus, the connection between the low velocities in the uppermost mantle and those imaged in global studies in the lower mantle is unclear. We have combined new data from Afar, Ethiopia with 7 other regional experiments and global network stations across Kenya, Ethiopia, Eritrea, Djibouti and Yemen, to produce high-resolution models of upper mantle P- and S-wave velocities to the base of the transition zone. Relative travel time tomographic inversions show that within the transition zone two focussed sharp-sided low velocity regions exist: one beneath the Western Ethiopian plateau outside the rift valley, and the other beneath the Afar depression. Estimates of transition zone thickness suggest that this is unlikely to be an artefact of mantle discontinuity topography as a transition zone of normal thickness underlies the majority of Afar and surrounding regions. However, a low velocity layer is evident directly above the 410 discontinuity, co-incident with some of the lowest seismic velocities suggesting that smearing of a strong low velocity layer of limited depth extent may contribute to the tomographic models in north-east Afar. The combination of seismic constraints suggests that small low temperature (<50K) upwellings may rise from a broader low velocity plume-like feature in the lower mantle. This interpretation is supported by numerical and analogue experiments that suggest the 660km phase change and viscosity jump may impede flow from the lower to upper mantle creating a thermal boundary layer at the base of the transition zone. This allows smaller, secondary upwellings to initiate and rise to the surface. These, combined with possible evidence of melt above the 410 discontinuity can explain the seismic velocity models. Our images of secondary upwellings suggest that

  17. Multi-purpose presentation techniques for geoscientific data in various media

    NASA Astrophysics Data System (ADS)

    Rink, Karsten; Bilke, Lars

    2014-05-01

    The intuitive presentation of the progression of complex geoscientific phenomena is often an underrated part of the modelling- and simulation workflow. Compiling such a presentation allows to easily communicate progress in joint research projects between participants with different backgrounds. Also, adequate 3D visualisations are usually easier to understand when presenting research results to stakeholders as well as the general public and critical information is conveyed in a more comprehensible manner. We established a workflow that is based on integration and preprocessing of multiple geoscientific data sets in a suitable framework such as the OpenGeoSys Data Explorer or ParaView. After choosing an adequate visual representation of the data in these frameworks, custom-made interfaces are employed to export the data to presentation frameworks. For instance, using the Unity 3D Engine allows to implement interaction techniques such as adding camera paths, concentrating on specific subsets of the data or scene, blending multiple data sets, etc. While a general sequence of the presentation can be predefined, interactive navigation is still possible and allows to focus on particular interests of the audience. Established interfaces and frameworks allow to display existing presentations in multiple ways, including virtual reality environments, novel hardware such as head-mounted displays like the Occulus Rift, or even websites presenting 3D content. Furthermore, the content can be redistributed as an executable for use on arbitrary machines. This versatility enables the use of prepared presentations for a multitude of occasions including exchange of intermediary result to partners in cooperate projects, reports at conferences, the defense of research projects, or use in training courses or for tutorials.

  18. Lithospheric instabilities. [associated with mantle geoid

    NASA Technical Reports Server (NTRS)

    Turcotte, D. L.; Haxby, W. F.; Ockendon, J. R.

    1977-01-01

    In this paper we define a mantle geoid. This is the height that hot solid mantle rock from the asthenosphere would attain if it were not confined by the lithosphere. The mantle geoid lies 3.25 km below the hydrogeoid (sea level). Hot mantle rock cannot entirely penetrate the continental lithosphere. One consequence of this partial penetration is rifting; as a result of rifting an accreting plate margin may be created. Hot mantle rock from the asthenosphere can penetrate through the oceanic lithosphere if the sea floor lies below the mantle geoid. Penetration of the oceanic lithosphere by this solid mantle rock is a necessary condition for the initiation of subduction. We argue that the same processes that are associated with rifting in continental lithosphere will be associated with behind arc spreading and the initiation of subduction in the oceanic lithosphere.

  19. Magnetotelluric imaging of upper crustal partial melt at Tendaho graben in Afar, Ethiopia

    NASA Astrophysics Data System (ADS)

    Didana, Yohannes Lemma; Thiel, Stephan; Heinson, Graham

    2014-05-01

    We report on a recent magnetotelluric (MT) survey across the Manda Hararo magmatic segment (MHMS) within the Tendaho graben in the Afar Depression in northeastern Ethiopia. Twenty-two broadband MT sites with ˜1 km station spacing were deployed along a profile with the recorded data covering a period range from 0.003 s to 1000 s. A two-dimensional (2-D) resistivity model reveals an upper crustal fracture zone (fault) and partial melt with resistivity of 1-10Ωm at a depth of >1 km. The partial melt has a maximum horizontal width of 15 km and extends to a depth of 15 km within the Afar Stratoid Series basalts. We estimate a melt fraction of about 13% based on geochemical and borehole data, and bulk resistivity from the 2-D MT inversion model. The interpreted upper crustal partial melt may have been formed by either a magma intrusion from mantle sources or a large volume of continental crust that has been fluxed by a small amount of mantle melt and heat. Within the MHMS and Tendaho graben, a magma intrusion is a plausible explanation for the upper crustal conductor. The inferred presence of a conductive fracture zone or fault with hydrothermal fluid and shallow heat sourcing magma reservoir also makes the Tendaho graben a promising prospect for the development of conventional hydrothermal geothermal energy.

  20. 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.

  1. Anatomy of lithosphere necking during orthogonal rifting

    NASA Astrophysics Data System (ADS)

    Nestola, Yago; Cavozzi, Cristian; Storti, Fabrizio

    2013-04-01

    The evolution of lithosphere necking is a fundamental parameter controlling the structural architecture and thermal-state of rifted margin. The necking shape depends on several parameters, including the extensional strain-rate and thermal layering of the lithosphere. Despite a large number of analogue and numerical modelling studies on lithosphere extension, a quantitative description of the evolution of necking through time is still lacking. We used analogue modelling to simulate in three-dimension the progression of lithosphere thinning and necking during orthogonal rifting. In our models we simulated a typical "cold and young" 4-layer lithosphere stratigraphy: brittle upper crust (loose quartz sand), ductile lower crust (silicon-barite mixture), brittle upper mantle (loose quartz sand), and ductile lower mantle (silicon-barite mixture). The experimental lithosphere rested on a glucose syrup asthenosphere. We monitored model evolution by periodic and coeval laser scanning of both the surface topography and the lithosphere base. After model completion, each of the four layers was removed and the top of the underlying layer was scanned. This technical approach allowed us to quantify the evolution in space and time of the thinning factors for both the whole lithosphere (βz) and the crust (γ). The area of incremental effective stretching (βy) parallel to the extensional direction was obtained from the βz maps.

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

    NASA Astrophysics Data System (ADS)

    Bourgeois, O.; Dauteuil, O.

    2010-12-01

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

  3. Editorial: The publication of geoscientific model developments v1.0

    NASA Astrophysics Data System (ADS)

    Executive Editors, GMD

    2013-08-01

    In 2008, the first volume of the European Geosciences Union (EGU) journal Geoscientific Model Development (GMD) was published. GMD was founded because we perceived there to be a need for a space to publish comprehensive descriptions of numerical models in the geosciences. The journal is now well established, with the submission rate increasing over time. However, there are several aspects of model publication that we believe could be further improved. In this editorial we assess the lessons learned over the first few years of the journal's life, and describe some changes to GMD's editorial policy, which will ensure that the models and model developments are published in such a way that they are of maximum value to the community. These changes to editorial policy mostly focus on improving the rigour of the review process through a stricter requirement for access to the materials necessary to test the behaviour of the models. Throughout this editorial, "must" means that the stated actions are required, and the paper cannot be published without them; "strongly encouraged" means that we encourage the action, but papers can still be published if the criteria are not met; "may" means that the action may be carried out by the authors or referees, if they so wish. We have reviewed and rationalised the manuscript types into five new categories. For all papers which are primarily based on a specific numerical model, the changes are as follows: - The paper must be accompanied by the code, or means of accessing the code, for the purpose of peer-review. If the code is normally distributed in a way which could compromise the anonymity of the referees, then the code must be made available to the editor. The referee/editor is not required to review the code in any way, but they may do so if they so wish. - All papers must include a section at the end of the paper entitled "Code availability". In this section, instructions for obtaining the code (e.g. from a supplement, or from a

  4. The lithospheric shear-wave velocity structure of Saudi Arabia: Young volcanism in an old shield

    NASA Astrophysics Data System (ADS)

    Tang, Zheng; Julià, Jordi; Zahran, Hani; Mai, P. Martin

    2016-06-01

    We investigate the lithospheric shear-wave velocity structure of Saudi Arabia by conducting H-κ stacking analysis and jointly inverting teleseismic P-receiver functions and fundamental-mode Rayleigh wave group velocities at 56 broadband stations deployed by the Saudi Geological Survey (SGS). The study region, the Arabian plate, is traditionally divided into the western Arabian shield and the eastern Arabian platform: The Arabian shield itself is a complicated mélange of crustal material, composed of several Proterozoic terrains separated by ophiolite-bearing suture zones and dotted by outcropping Cenozoic volcanic rocks (locally known as harrats). The Arabian platform is primarily covered by 8 to 10 km of Paleozoic, Mesozoic and Cenozoic sedimentary rocks. Our results reveal high Vp/Vs ratios in the region of Harrat Lunayyir, which are interpreted as solidified magma intrusions from old magmatic episodes in the shield. Our results also indicate slow velocities and large upper mantle lid temperatures below the southern and northern tips of the Arabian shield, when compared with the values obtained for the central shield. We argue that our inferred patterns of lid velocity and temperature are due to heating by thermal conduction from the Afar plume (and, possibly, the Jordan plume), and that volcanism in western Arabia may result from small-scale adiabatic ascent of magma diapirs.

  5. Comparing the composition of the earliest basalts erupted by the Iceland and Afar mantle plumes.

    NASA Astrophysics Data System (ADS)

    Stuart, Finlay M.

    2013-04-01

    The first basalts erupted by mantle plumes are typically generated by mantle melting at temperatures 200-300°C higher than average ambient mantle. This is consistent with the derivation of from a thermal boundary layer at the core-mantle boundary. Mantle plume temperatures decrease with time, likely as large plume heads give way to thin plume conduits. Consequently the early, hot plume basalts are a window into the deep mantle. At it's simplest they provide a test of whether the discrete plume source regions are primordial mantle that have been isolated since soon after Earth accretion, or have substantial contributions from subducted slabs. Here I present new isotopic and trace element determinations of the earliest picritic basalts from the ~30 Ma Afar plume in Ethiopia. They will be compared with similar material from the ~60 Ma proto-Iceland plume (PIP) in an effort to test prevailing models regarding the source of mantle plumes. The extremely primordial nature of the helium in the PIP picrites (3He/4He ~ 50 Ra) contrasts with much lower values of the Ethiopian flood basalt province (~21 Ra). The Iceland plume 3He/4He has decreased (linearly) with time, mirroring the secular cooling of the Iceland mantle plume identified by decreasing MgO and FeO in primary melts. In 60 million years the Iceland plume 3He/4He is still higher than the maximum Afar plume value. The Sr-Nd-Pb isotopic composition of the high 3He/4He Ethiopian flood basalt province picrites are remarkably homogenous (e.g. 87Sr/86Sr = 0.70396-0.70412; 206Pb/204Pb = 18.82-19.01). In comparison the PIP picrites have ranges that span nearly the global range of E-MORB and N-MORB. The Afar and proto-Iceland mantle plumes are clearly not initiated in a single deep mantle domain with the same depletion/enrichment and degassing histories, and the same scale of heterogeneity. This implies that there is more than one plume source region/mechanism that is capable of generating comparable volumes of basalt melt

  6. 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

  7. Planet Press: an EGU initiative to bring geoscientific research to children

    NASA Astrophysics Data System (ADS)

    Ferreira, Bárbara

    2016-04-01

    Planet Press (http://www.egu.eu/education/planet-press/) is an EGU educational project that aims to get children (mainly 7-13 year olds), as well as their parents and educators, interested in and engaged with up-to-date scientific research and news. Planet Press articles are short versions of EGU press releases written in child-friendly language. Because EGU press releases cover research published in the various EGU scientific journals, Planet Press focuses on topics as varied as air pollution, glaciers, climate change, earthquakes, ocean sciences, droughts and floods, or space sciences. The texts are reviewed by both scientists and educators to make sure they are accurate and clear to their target audience. By sharing new and exciting geoscientific research with young kids, we hope to inspire them to develop an interest in the Earth, planetary and space sciences. In this presentation, we describe how the Planet Press idea came about, how the project is run, and the challenges and lessons learnt since the launch of this educational initiative in 2014. Planet Press, which has the support of the EGU Committee on Education, is made possible by the work of volunteer scientists and educators who review and translate the texts. We are grateful for the help of Jane Robb, former EGU Educational Fellow, with launching the project. Planet Press is inspired by Space Scoop (http://www.spacescoop.org/), an initiative by UNAWE, the EU-Universe Awareness organisation, that brings astronomy news to children every week.

  8. OIL—Output input language for data connectivity between geoscientific software applications

    NASA Astrophysics Data System (ADS)

    Amin Khan, Khalid; Akhter, Gulraiz; Ahmad, Zulfiqar

    2010-05-01

    Geoscientific computing has become so complex that no single software application can perform all the processing steps required to get the desired results. Thus for a given set of analyses, several specialized software applications are required, which must be interconnected for electronic flow of data. In this network of applications the outputs of one application become inputs of other applications. Each of these applications usually involve more than one data type and may have their own data formats, making them incompatible with other applications in terms of data connectivity. Consequently several data format conversion utilities are developed in-house to provide data connectivity between applications. Practically there is no end to this problem as each time a new application is added to the system, a set of new data conversion utilities need to be developed. This paper presents a flexible data format engine, programmable through a platform independent, interpreted language named; Output Input Language (OIL). Its unique architecture allows input and output formats to be defined independent of each other by two separate programs. Thus read and write for each format is coded only once and data connectivity link between two formats is established by a combination of their read and write programs. This results in fewer programs with no redundancy and maximum reuse, enabling rapid application development and easy maintenance of data connectivity links.

  9. Magmatism on rift flanks: Insights from ambient noise phase velocity in Afar region

    NASA Astrophysics Data System (ADS)

    Korostelev, Félicie; Weemstra, Cornelis; Leroy, Sylvie; Boschi, Lapo; Keir, Derek; Ren, Yong; Molinari, Irene; Ahmed, Abdulhakim; Stuart, Graham W.; Rolandone, Frédérique; Khanbari, Khaled; Hammond, James O. S.; Kendall, J. M.; Doubre, Cécile; Ganad, Ismail Al; Goitom, Berhe; Ayele, Atalay

    2015-04-01

    During the breakup of continents in magmatic settings, the extension of the rift valley is commonly assumed to initially occur by border faulting and progressively migrate in space and time toward the spreading axis. Magmatic processes near the rift flanks are commonly ignored. We present phase velocity maps of the crust and uppermost mantle of the conjugate margins of the southern Red Sea (Afar and Yemen) using ambient noise tomography to constrain crustal modification during breakup. Our images show that the low seismic velocities characterize not only the upper crust beneath the axial volcanic systems but also both upper and lower crust beneath the rift flanks where ongoing volcanism and hydrothermal activity occur at the surface. Magmatic modification of the crust beneath rift flanks likely occurs for a protracted period of time during the breakup process and may persist through to early seafloor spreading.

  10. Late Miocene hominin teeth from the Gona Paleoanthropological Research Project area, Afar, Ethiopia.

    PubMed

    Simpson, Scott W; Kleinsasser, Lynnette; Quade, Jay; Levin, Naomi E; McIntosh, William C; Dunbar, Nelia; Semaw, Sileshi; Rogers, Michael J

    2015-04-01

    Since 2000, significant collections of Latest Miocene hominin fossils have been recovered from Chad, Kenya, and Ethiopia. These fossils have provided a better understanding of earliest hominin biology and context. Here, we describe five hominin teeth from two periods (ca. 5.4 Million-years-ago and ca. 6.3 Ma) that were recovered from the Adu-Asa Formation in the Gona Paleoanthropological Research Project area in the Afar, Ethiopia that we assign to either Hominina, gen. et sp. indet. or Ardipithecus kadabba. These specimens are compared with extant African ape and other Latest Miocene and Early Pliocene hominin teeth. The derived morphology of the large, non-sectorial maxillary canine and mandibular third premolar links them with later hominins and they are phenetically distinguishable and thus phyletically distinct from extant apes. PMID:25795338

  11. Caring from Afar: Asian H1B Migrant Workers and Aging Parents.

    PubMed

    Lee, Yeon-Shim; Chaudhuri, Anoshua; Yoo, Grace J

    2015-09-01

    With the growth in engineering/technology industries, the United States has seen an increase in the arrival of highly skilled temporary migrant workers on H1B visas from various Asian countries. Limited research exists on how these groups maintain family ties from afar including caring for aging parents. This study explores the experiences and challenges that Asian H1B workers face when providing care from a distance. A total of 21 Chinese/Taiwanese, Korean, and Indian H1B workers participated in in-depth qualitative interviews. Key findings indicate that despite distance, caring relationships still continue through regular communications, financial remittances, and return visits, at the same time creating emotional, psychological, and financial challenges for the workers. Findings highlight the need for further research in understanding how the decline of aging parent's health impacts the migrants' adjustment and health in the United States. PMID:26267591

  12. Ground deformation near Gada ‘Ale Volcano, Afar, observed by radar interferometry

    NASA Astrophysics Data System (ADS)

    Amelung, Falk; Oppenheimer, Clive; Segall, P.; Zebker, H.

    2000-10-01

    Radar interferometric measurements of ground-surface displacement using ERS data show a change in radar range, corresponding to up to 12 cm of subsidence near Gada ‘Ale volcano in northern Afar, Ethiopia, that occurred between June 1993 and May 1996. This is the area of lowest topography within the Danakil Depression (-126 m). Geodetic inverse modeling and geological evidence suggest a volcanic origin of the observed deformation; it was probably caused by a combined process of magma withdrawal from a larger reservoir and normal faulting. There is no evidence of subaerial eruption. This is the only identifiable deformation event during June 1993-October 1997 in the 80 km long Erta ‘Ale volcanic range, indicating surprising inactivity elsewhere in the range.

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

    NASA Astrophysics Data System (ADS)

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

    2008-12-01

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

  14. Reconstructing recent basaltic fissure eruptions in the Afar Depression, Ethiopia, using satellite imagery

    NASA Astrophysics Data System (ADS)

    Barnie, T. D.; Ferguson, D. J.; Oppenheimer, C.

    2009-12-01

    Since 2005, there have been numerous dike intrusions in the Manda-Hararo rift segment in central Afar, Ethiopia, as part of an on-going rifting event similar to that which occurred at Krafla, Iceland between 1975 and 1984. Two of the dikes have been associated with basaltic fissure eruptions, in August 2007 and in June 2009. A large fissure eruption also occurred in the Erta ‘Ale volcanic range in northern Afar in November 2008, which may be related to the activity further south. These eruptions took place in remote areas and lasted only a few days, making field observation almost impossible. We must therefore rely on the geomorphology and geochemistry of the erupted deposits, and syn-eruption satellite observations of thermally emitted radiance and gas emissions to reconstruct the evolution of these eruptions. Reconstruction of the eruptions is important for i) understanding the relationship between dike intrusion and fissure eruption dynamics ii) understanding the relationship between eruption dynamics and the resulting deposits, iii) interpreting the deposits of pre-historic eruptions from previous rifting events, and iv) estimating the hazard posed to the local population by further activity. In this study we use pre- and post-eruption high resolution ASTER and ALI images to map the morphology of the erupted material, and SEVIRI and MODIS time series of the eruptions to track evolution of the vent dynamics and lava flows. These datasets are augmented by oblique aerial and ground based photographic and FLIR surveys of the cooling lava flows and tephra. ASTER and ALI images are also used to map older erupted materials, which allow us to compare this rifting event with previous events to anticipate future activity.

  15. 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

  16. Rheology of the lithosphere: selected topics.

    USGS Publications Warehouse

    Kirby, S.H.; Kronenberg, A.K.

    1987-01-01

    Reviews recent results concerning the rheology of the lithosphere with special attention to the following topics: 1) the flexure of the oceanic lithosphere, 2) deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins, 3) the rheological stratification of the continents, 4) strain localization and shear zone development, and 5) strain-induced crystallographic preferred orientations and anisotropies in body-wave velocities. We conclude with a section citing the 1983-1986 rock mechanics literature by category.-Authors

  17. Lithospheric Discontinuities Beneath North America (Invited)

    NASA Astrophysics Data System (ADS)

    Fischer, K. M.; Abt, D. L.; Yuan, H.; Romanowicz, B. A.

    2009-12-01

    The goal of this study is to compare lithospheric discontinuities between the stable cratonic core of North America and surrounding regions that have experienced more recent tectonic activity. Are the properties of the cratonic lithosphere-asthenosphere boundary (LAB) fundamentally different from the LAB in regions with thinner lithosphere? Do significant discontinuities exist within the cratonic lithosphere? Sp and Ps converted seismic waves from 93 permanent seismic stations spanning North America, including stations of the EarthScope Reference Array, were used to image the discontinuity structure of the upper mantle. Receiver functions were calculated with frequency-domain deconvolution and migrated to depth with 1D models that account for variations in crustal structure and mantle velocities between stations. Prominent Sp phases from a negative velocity contrast were found at depths of 50-120 km. To interpret these Sp phases as either the LAB or a mid-lithospheric discontinuity (MLD), we compared their depth to the transition from the low-velocity asthenosphere to the high velocity lithospheric lid in the absolute shear velocity model from surface wave tomography that was used to migrate the receiver functions. In the tectonically active western U.S., the negative Sp phases were interpreted as the LAB at depths of 50-105 km. On average, the amplitudes of these Sp phases are the largest in North America. They are consistent with a large and rapid LAB velocity gradient and an anomalously hot and shallow asthenosphere that is very rich in water or contains partial melt. In the regions of the southern and eastern U.S where the Sp phases were interpreted as the LAB, the discontinuity lies at depths of 75-110 km and also implies the presence of water or melt in the asthenosphere. In contrast, no Sp phases were observed at depths comparable to the base of the thick high velocity lithosphere that lies beneath cratonic North America and portions of the Phanerozoic

  18. Asthenosphere-lithosphere interactions in Western Saudi Arabia: Inferences from 3He/4He in xenoliths and lava flows from Harrat Hutaymah

    NASA Astrophysics Data System (ADS)

    Konrad, Kevin; Graham, David W.; Thornber, Carl R.; Duncan, Robert A.; Kent, Adam J. R.; Al-Amri, Abdullah M.

    2016-04-01

    Extensive volcanic fields on the western Arabian Plate have erupted intermittently over the last 30 Ma following emplacement of the Afar flood basalts in Ethiopia. In an effort to better understand the origin of this volcanism in western Saudi Arabia, we analyzed 3He/4He, and He, CO2 and trace element concentrations in minerals separated from xenoliths and lava flows from Harrat Hutaymah, supplemented with reconnaissance He isotope data from several other volcanic fields (Harrat Al Birk, Harrat Al Kishb and Harrat Ithnayn). Harrat Hutaymah is young (< 850 ka) and the northeasternmost of the volcanic fields. There is a remarkable homogeneity of 3He/4He trapped within most xenoliths, with a weighted mean of 7.54 ± 0.03 RA (2σ, n = 20). This homogeneity occurs over at least eight different xenolith types (including spinel lherzolite, amphibole clinopyroxenite, olivine websterite, clinopyroxenite and garnet websterite), and encompasses ten different volcanic centers within an area of ~ 2500 km2. The homogeneity is caused by volatile equilibration between the xenoliths and fluids derived from their host magma, as fluid inclusions are annealed during the infiltration of vapor-saturated magmas along crystalline grain boundaries. The notable exceptions are the anhydrous spinel lherzolites, which have a lower weighted mean 3He/4He of 6.8 ± 0.3 RA (2σ, n = 2), contain lower concentrations of trapped He, and have a distinctly depleted light rare earth element signature. 3He/4He values of ~ 6.8 RA are also commonly found in spinel lherzolites from harrats Ithnayn, Al Birk, and from Zabargad Island in the Red Sea. Olivine from non-xenolith-bearing lava flows at Hutaymah spans the He isotope range of the xenoliths. The lower 3He/4He in the anhydrous spinel lherzolites appears to be tied to remnant Proterozoic lithosphere prior to metasomatic fluid overprinting. Elevated 3He/4He in the western harrats has been observed only at Rahat (up to 11.8 RA; Murcia et al., 2013), a

  19. Yellowstone hotspot-continental lithosphere interaction

    NASA Astrophysics Data System (ADS)

    Jean, Marlon M.; Hanan, Barry B.; Shervais, John W.

    2014-03-01

    The Snake River Plain represents 17 m.y. of volcanic activity that took place as the North American continent migrated over a relatively fixed magma source, or hotspot. We present new Pb, Sr, and Nd data for a suite of 25 basalts collected from Western and Central Snake River Plain (SRP). The new isotope data, combined with previously published data from the SRP, provide a traverse of the Wyoming craton margin, from the 87Sr/86Sr = 0.706 line boundary of western SRP with Phanerozoic accreted terranes, east through the central and eastern SRP, to the Yellowstone Plateau. Low-K basalts from the western SRP, overlain by high-K basalts, provide a temporal record of regional source variation from ∼16.8 to 0.2 Ma. Principal Component Analysis (PCA) of the new and previously published SRP basalt Pb isotopes reveals that >97% of the total variability is accounted for by mixing between three end-members and is consistent with a sublithospheric Yellowstone hotspot mantle source with a radiogenic isotope composition similar to the mantle source of the early Columbia River Basalt Group (CRBG) and two continental lithosphere end-members, heterogeneous in age and composition. We use the SRP Pb, Sr, and Nd isotope data to model the Yellowstone Hotspot-continental lithosphere interaction by three component mixing between two continental lithospheric components, Archean lithosphere (CL1) that represents older lithosphere underlying the Yellowstone Plateau in the east, and Paleoproterozoic lithosphere (CL2) representing the younger lithosphere underlying the SRP in the west near the craton margin, and a sublithospheric end-member, representing the Yellowstone hotspot (PL). The results suggest a continuous flow of PL material westward as the NA continental lithosphere migrated over the upwelling hotspot along a shoaling gradient in the sub-continental mantle lithosphere. The model shows a decrease in Total Lithosphere end-members (CL1 + CL2) and the Lithosphere Ratio (CL1/CL2

  20. Deformation and instability of underthrusting lithospheric plates

    NASA Technical Reports Server (NTRS)

    Liu, H.

    1972-01-01

    Models of the underthrusting lithosphere are constructed for the calculation of displacement and deflection. First, a mathematical theory is developed that rigorously demonstrates the elastic instability in the decending lithosphere. The theory states that lithospheric thrust beneath island arcs becomes unstable and suffers deflection as the compression increases. Thus, in the neighborhood of the edges where the lithospheric plate plunges into the asthenosphere and mesosphere its shape will be contorted. Next, the lateral displacement is calculated, and it is shown that, before contortion, the plate will thicken and contract at different positions with the variation in thickness following a parabolic profile. Finally, the depth distribution of the intermediate and deep focus earthquakes is explained in terms of plate buckling and contortion.

  1. Lithosphere-asthenosphere boundary: Where and why?

    NASA Astrophysics Data System (ADS)

    Aryasova, Olga; Khazan, Yakov

    2015-04-01

    A necessary condition of the lithosphere steady state is that the convective boundary layer (CBL) accommodating a transition from the lithosphere to the convecting mantle is on the verge of instability. The common practice of solving the stationary heat equation with boundary conditions (temperature and heat flow) imposed on the surface provides a solution which does not necessarily satisfy the marginal stability condition (MSC) of the CBL and therefore does not necessarily describe a valid steady state. We suggest the approach to the thermal modeling that uses the MSC instead of the heat flow boundary condition, which guarantees that the solution describes the steady-state lithosphere. In addition, in contrast to the commonly used approach, the MSC-based solution only weakly depends on the uncertainty of the crustal heat production in the sense that any two steady-state geotherms corresponding to different crustal heat production, but the same potential temperature and lithosphere structure, converge at depth. We demonstrate that if there is no obstacle to the mantle convection like chemical boundary layer (ChBL) comprising the crust and the layer of depleted rock then the lithosphere base occurs at the rheological depth, Hrh, which is of 70 to 50 km under the potential temperature of 1300 to 1350oC. This situation is characteristic of the mantle beneath the old oceanic crust areas far from disturbed regions, with the heat flow and the seafloor depth depending only on the potential temperature,Tp. An absence of noticeable distinctions between the heat flows in different oceanic basins suggests a global constancy of the potential temperature Tp at least in suboceanic mantle. Beneath continents, the ChBL thickness, Hdepl, exceeds Hrh even in Phanerozoic regions and, all the more so, in Precambrian ones. Therefore, in the subcontinental mantle the lithosphere is the same as the chemical boundary layer and the CBL is immediately adjacent to the lithosphere base. We

  2. The structure of the Ionian lithosphere

    NASA Technical Reports Server (NTRS)

    Carr, M. H.; Clow, G. D.

    1984-01-01

    Exploratory work on the structure of the Ionian lithosphere is reported. The approach is to examine temperature profiles within the lithosphere that result from different distributions of sulfur and silicates and different conductive heat fluxes, then compare such profiles with observations in the expectation that only a limited set of the profiles are possible. In this preliminary work some rather simplistic assumptions were taken and the report should be viewed more as a demonstration of a method rather than a presentation of results.

  3. Lithospheric layering in the North American craton.

    PubMed

    Yuan, Huaiyu; Romanowicz, Barbara

    2010-08-26

    How cratons-extremely stable continental areas of the Earth's crust-formed and remained largely unchanged for more than 2,500 million years is much debated. Recent studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at depths too shallow to be consistent with the lithosphere-asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. Here we show that changes in the direction of azimuthal anisotropy with depth reveal the presence of two distinct lithospheric layers throughout the stable part of the North American continent. The top layer is thick ( approximately 150 km) under the Archaean core and tapers out on the surrounding Palaeozoic borders. Its thickness variations follow those of a highly depleted layer inferred from thermo-barometric analysis of xenoliths. The lithosphere-asthenosphere boundary is relatively flat (ranging from 180 to 240 km in depth), in agreement with the presence of a thermal conductive root that subsequently formed around the depleted chemical layer. Our findings tie together seismological, geochemical and geodynamical studies of the cratonic lithosphere in North America. They also suggest that the horizon detected in receiver function studies probably corresponds to the sharp mid-lithospheric boundary rather than to the more gradual lithosphere-asthenosphere boundary. PMID:20740006

  4. In-field Access to Geoscientific Metadata through GPS-enabled Mobile Phones

    NASA Astrophysics Data System (ADS)

    Hobona, Gobe; Jackson, Mike; Jordan, Colm; Butchart, Ben

    2010-05-01

    Fieldwork is an integral part of much geosciences research. But whilst geoscientists have physical or online access to data collections whilst in the laboratory or at base stations, equivalent in-field access is not standard or straightforward. The increasing availability of mobile internet and GPS-supported mobile phones, however, now provides the basis for addressing this issue. The SPACER project was commissioned by the Rapid Innovation initiative of the UK Joint Information Systems Committee (JISC) to explore the potential for GPS-enabled mobile phones to access geoscientific metadata collections. Metadata collections within the geosciences and the wider geospatial domain can be disseminated through web services based on the Catalogue Service for Web(CSW) standard of the Open Geospatial Consortium (OGC) - a global grouping of over 380 private, public and academic organisations aiming to improve interoperability between geospatial technologies. CSW offers an XML-over-HTTP interface for querying and retrieval of geospatial metadata. By default, the metadata returned by CSW is based on the ISO19115 standard and encoded in XML conformant to ISO19139. The SPACER project has created a prototype application that enables mobile phones to send queries to CSW containing user-defined keywords and coordinates acquired from GPS devices built-into the phones. The prototype has been developed using the free and open source Google Android platform. The mobile application offers views for listing titles, presenting multiple metadata elements and a Google Map with an overlay of bounding coordinates of datasets. The presentation will describe the architecture and approach applied in the development of the prototype.

  5. Geoscientific Site Evaluation Approach for Canada's Deep Geological Repository for Used Nuclear Fuel

    NASA Astrophysics Data System (ADS)

    Sanchez-Rico Castejon, M.; Hirschorn, S.; Ben Belfadhel, M.

    2015-12-01

    The Nuclear Waste Management Organization (NWMO) is responsible for implementing Adaptive Phased Management, the approach selected by the Government of Canada for long-term management of used nuclear fuel generated by Canadian nuclear reactors. The ultimate objective of APM is the centralized containment and isolation of Canada's used nuclear fuel in a Deep Geological Repository in a suitable crystalline or sedimentary rock formation. In May 2010, the NWMO published and initiated a nine-step site selection process to find an informed and willing community to host a deep geological repository for Canada's used nuclear fuel. The site selection process is designed to address a broad range of technical and social, economic and cultural factors. The site evaluation process includes three main technical evaluation steps: Initial Screenings; Preliminary Assessments; and Detailed Site Characterizations, to assess the suitability of candidate areas in a stepwise manner over a period of many years. By the end of 2012, twenty two communities had expressed interest in learning more about the project. As of July 2015, nine communities remain in the site selection process. To date (July 2015), NWMO has completed Initial Screenings for the 22 communities that expressed interest, and has completed the first phase of Preliminary Assessments (desktop) for 20 of the communities. Phase 2 of the Preliminary Assessments has been initiated in a number of communities, with field activities such as high-resolution airborne geophysical surveys and geological mapping. This paper describes the approach, methods and criteria being used to assess the geoscientific suitability of communities currently involved in the site selection process.

  6. Receiver function constraints on crustal seismic velocities and partial melting in the Red Sea Rift, Central Afar

    NASA Astrophysics Data System (ADS)

    Reed, C. A.; Almadani, S.; Gao, S. S.; Elsheikh, A. A.; Cherie, S.; Thurmond, A. K.; Liu, K. H.

    2013-12-01

    The Afar Depression is currently a unique locale for the investigation of crustal and mantle processes involved in the transition from continental to oceanic rifting. To provide high-quality images of the crust and upper mantle beneath the Red Sea Rift in Central Afar, we deployed an array of 18 broadband seismic stations in 2010 and 2011. Here we report receiver function stacking results to investigate crustal properties of this presumably incipient oceanic rift. Stacking of approximately 2200 radial receiver functions along the ~200 km long array reveals an average crustal thickness of 22 km, ranging from nearly 18 km within the Red Sea Rift axis to approximately 30 km within the overlap zone between the Red Sea and Gulf of Aden rift axes. The resulting anomalously high Vp/Vs ratios systematically decrease toward the northeast, ranging from 2.40 southwest of the Tendaho Graben to 1.85 within the overlap zone. We utilize theoretical Vp and melt fraction relationships to constrain a highly reduced average crustal P-velocity of 5.0 km/s within the rift axis, which is characterized by a melt percentage of ~11% confined primarily to the lower crust while the overlap zone contains relatively minor quantities of partial melt. An observed asymmetric distribution of high Vp/Vs values within the Tendaho Graben, as well as regionally maximum values on the southwestern rift flank, suggest crustal magmas either delivered from off-axis subcrustal magma chambers or as material present as residuum from the Red Sea Rift axis migration. Comparisons of these crustal properties beneath the Red Sea Rift and those found beneath mature mid-ocean ridges suggest the locus of extensional strain within the Central Afar is currently diffuse and in the process of localizing toward the Tendaho Graben accompanying the northeastward migration of the Afar Triple Junction.

  7. 2003-2010 InSAR time series at Dallol, a proto-volcanic magmatic system in Afar (Ethiopia)

    NASA Astrophysics Data System (ADS)

    Nobile, Adriano; Pagli, Carolina; Keir, Derek; Wright, Tim; Acocella, Valerio

    2013-04-01

    Quaternary extension and volcanism in Afar focus along magmatic segments, like the Erta Ale Ridge. Here we study the ground deformation at Dallol, in northern Afar (Ethiopia) along the Afar rift axis. Dallol rift lies in a depressed area ~120 m below sea level, it includes a ~40 m high mound and an active hydrothermal field. In 1993 a seismic swarm occurred NW of Dallol and was probably associated to a dyke intrusion. In 2004 another seismic swarm occurred SE of Dallol. In a previous study we used InSAR data to show that this seismicity was induced by a dyke intrusion in October 2004, probably fed by a shallow (1.5-3.3 km) magma chamber. Here we analyze the ground deformation at Dallol between 2003 and 2010, using InSAR time series computed with pi-rate software. These data suggest that after the dyking event, during ~3 years, the mound slowly uplifted, until 2008, then subsided very quickly up to 2010. Preliminary inversion of the InSAR data between 2008 and 2010 for the subsidence period shows a deformation source shallower than the dike source. We tested different sources (Mogi, ellipsoid, Sill). Depths vary between 0.8-1.8 km depending on the chosen source. Preliminary analysis highlights a mean-term correlation (2007-2010) between long term deformations in Dallol and other volcanoes belonging to the Erta Ale Ridge.

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

    NASA Technical Reports Server (NTRS)

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

    2014-01-01

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

  9. Current plate boundary deformation of the Afar rift from a 3-D velocity field inversion of InSAR and GPS

    NASA Astrophysics Data System (ADS)

    Pagli, Carolina; Wang, Hua; Wright, Tim J.; Calais, Eric; Lewi, Elias

    2014-11-01

    Extension, faulting, and magmatism are the main controls on the magnitude and localization of strain at mid-ocean ridges. However, the temporal and spatial patterns of such processes are not clear since the strain distribution has not been resolved in the past at sufficient spatial resolution and over extended areas. Interferometric synthetic aperture radar (InSAR) and GPS data with unprecedented resolution are now available to us from the Afar rift of Ethiopia. Here we use a velocity field method to combine InSAR and GPS to form the first high-resolution continuous three-dimensional velocity field of Afar. We study an area that is 500 km wide and 700 km long, covering three branches of the Afar continental rift and their triple junctions. Our velocity field shows that plate spreading is currently achieved in Afar in contrasting modes. A transient postdiking deformation is focused at the Dabbahu rift segment, while in central Afar, spreading is distributed over several overlapping segments and southern Afar exhibits an interdiking deformation pattern focused at the Asal-Ghoubbet segment. We find that current spreading rates at Dabbahu, following the 2005-2010 intrusions, are up to 110 mm/yr, 6 times larger than the long-term plate divergence. A segment-centered uplift of up to 80 mm/yr also occurs, indicating that magma flow is still a primary mechanism of deformation during postdiking. On the other hand, no vertical displacements are currently observed in central and southern Afar, suggesting lack of significant magmatic activity at shallow levels.

  10. Complexity of the Fennoscandian lithosphere

    NASA Astrophysics Data System (ADS)

    Vinnik, Lev; Kozlovskaya, Elena; Oreshin, Sergey; Kosarev, Grigoriy; Silvennoinen, Hanna; Vaganova, Natalia; Kiselev, Sergey

    2014-05-01

    -explosion profile crossing this part of Fennoscandia. At the stations of the POLENET/LAPNET array the P-wave velocities are generally close to the IASPEI91 velocities down to a depth of about 200 km. The S-wave velocities are close to the IASPEI91 velocities in the depth interval from the Moho to 100 km and higher than the IASPEI91 velocities in a depth range from 100 to 200 km. The most complex lithosphere structure is obtained for stations where the anomalously thick (more than 60-km) crust is known from earlier controlled-source seismology experiments. For some stations the S-wave and P-wave crust-mantle boundaries are located at different depths (around 45 km and 75-80 km, respectively). In summary, the structure of the Fennoscandian lithosphere appears to be very complex and deserving further multi-disciplinary studies.

  11. Permeability Barrier Generation in the Martian Lithosphere

    NASA Astrophysics Data System (ADS)

    Schools, Joe; Montési, Laurent

    2015-11-01

    Permeability barriers develop when a magma produced in the interior of a planet rises into the cooler lithosphere and crystallizes more rapidly than the lithosphere can deform (Sparks and Parmentier, 1991). Crystallization products may then clog the porous network in which melt is propagating, reducing the permeability to almost zero, i.e., forming a permeability barrier. Subsequent melts cannot cross the barrier. Permeability barriers have been useful to explain variations in crustal thickness at mid-ocean ridges on Earth (Magde et al., 1997; Hebert and Montési, 2011; Montési et al., 2011). We explore here under what conditions permeability barriers may form on Mars.We use the MELTS thermodynamic calculator (Ghiorso and Sack, 1995; Ghiorso et al., 2002; Asimow et al., 2004) in conjunction with estimated Martian mantle compositions (Morgan and Anders, 1979; Wänke and Dreibus, 1994; Lodders and Fegley, 1997; Sanloup et al., 1999; Taylor 2013) to model the formation of permeability barriers in the lithosphere of Mars. In order to represent potential past and present conditions of Mars, we vary the lithospheric thickness, mantle potential temperature (heat flux), oxygen fugacity, and water content.Our results show that permeability layers can develop in the thermal boundary layer of the simulated Martian lithosphere if the mantle potential temperature is higher than ~1500°C. The various Martian mantle compositions yield barriers in the same locations, under matching variable conditions. There is no significant difference in barrier location over the range of accepted Martian oxygen fugacity values. Water content is the most significant influence on barrier development as it reduces the temperature of crystallization, allowing melt to rise further into the lithosphere. Our lower temperature and thicker lithosphere model runs, which are likely the most similar to modern Mars, show no permeability barrier generation. Losing the possibility of having a permeability

  12. The thermal structure and thermal evolution of the continental lithosphere

    NASA Technical Reports Server (NTRS)

    Morgan, P.

    1984-01-01

    The thermal structure and evolution of the continental lithosphere are examined. Surface heat flow data and the factors which modify them are addressed, and the diversity of thermal phenomena in the lithosphere is discussed in the framework of plate interactions. The lithosphere is divided into three sections for the purposes of discussion. In the upper, near-surface zone, temperatures can be strongly affected by near-surface processes, which must be taken into account in the measurement and evaluation of surface heat flow. The thermal structure of the middle, internal zone of the lithosphere responds to the heat balance and thermal properties of the lithosphere, which define its steady state thermal structure. Internal deformation and magmatic intrusion within this zone, and interaction between the lithosphere and the asthenosphere in the lower boundary zone of the lithosphere cause transient thermal disturbances in the lithosphere. The criteria for defining the base of the thermal lithosphere are briefly discussed.

  13. Tephrochronology of rare Plio-Pleistocene fossiferous strata in south-central Afar, Ethiopia

    NASA Astrophysics Data System (ADS)

    DiMaggio, E.; Arrowsmith, R.; Campisano, C. J.; Deino, A. L.

    2013-12-01

    Sedimentary basins in the south and central Afar Depression archive the complex structural, climatic, volcanic, and biologic development of the region during the Pliocene and Pleistocene. The lower Awash Valley in central Afar has long served as a focus for these investigations, including the extensive work conducted to place fossil assemblages (including hominins) into stratigraphic and temporal context. Here we present a detailed analysis of tephra chemistry, correlations, and ages of the newly mapped and fossiliferous area of eastern Ledi-Geraru (ELG) in the lower Awash Valley (~3-2.5 Ma). Our results allow us to construct a tephrostratigraphic framework that provides important constraints for regional studies previously lacking a calibrated sedimentary record spanning 3 to 2.7 Ma. Based on glass chemistry and morphology, 40Ar/39Ar dating of feldspars, and stratigraphic mapping, we identified 23 distinct tephras (8 of which were dated) in >100 m of newly mapped fluvial and lacustrine sediments at ELG. The oldest tuff at ELG (Kuhulta Tuff; 2.994 Ma) is exposed in lake sediments (diatomite) that lie 3-5 m above basalt flows dated to ca. 3 Ma. The youngest ELG tephra (ca. 2.44 Ma) outcrops as a lenticular channel tuff in sediments faulted against older strata (~2.7 Ma). Between these two tephras lies the Gurumaha Tuff (ca. 2.82 Ma) and the Daáma and Bulinan Tuffs (both ca. 2.85 Ma), which provide excellent stratigraphic ties across a distance of 7.5 km, allowing us to document a lateral facies change from lacustrine in northern ELG to more nearshore in the south. These tuffs also confirm the presence of a fossiferous sedimentary record spanning the late Pliocene sedimentary gap in lower Awash Valley stratigraphy (ca. 2.94 - 2.7 Ma). While the youngest and oldest tephras at ELG temporally overlap with dated tephras from the well-described Hadar (3.8 - 2.94 Ma) and Busidima (2.7 - 0.016 Ma) Formations, we have yet to confirm geochemical correlates to any tephra

  14. Observations and modeling of the current deformation in Afar using Synthetic Aperture Radar Interferometry

    NASA Astrophysics Data System (ADS)

    Tomic, Jelena

    The Afar system is a unique place on Earth where a triple rift junction may be emerging. As the three rifts separating Arabia, Nubia and Somalia plates have not achieved a complete connection at present, I observe a 200 km wide area of complex surface deformation. A variety of extensional structures including a network of faults, fissures, dikes, and volcanic centers are collectively accommodating far field movement of the surrounding plates. Understanding the nature and distribution of the deformation over this vast region is critical since here I observe the transition between established oceanic ridges (the Red Sea and the Aden-Goubbet ridges) and continental deformation. In this study I use the technique of Synthetic Aperture Radar Interferometry (InSAR) to analyze radar data of the Afar region, and to construct a 10 yr timeline of surface displacement over a 200 km by 400 km area. By combining data acquired from ascending and descending passes I construct a two-dimensional velocity maps of the region. The maps show localized extensional deformation across the Asal-Ghoubbet rift segment accommodating the diverging motion of the Arabia-Somalia plates, as well as regional uplift asymmetrically distributed north and south of the Asal Rift area. The vertical velocity map in the rift indicates subsidence of the rift floor with respect to the rift shoulders, accommodated by fault creep. To interpret the observed velocity across the Asal rift I develop a 2-dimensional and a 3-dimensional dislocation model using a combination of dikes, sill and faults embedded in an elastic half space. The forward modeling allows me to place the overall geometry of sub-surface structures and estimate rates of dike and sill inflation, and fault movement. Then I construct a 3-dimensional model to perform a least-squares inversion of the radar-derived velocity maps. The results show an inflating body centered under the Fieale volcano expanding at a rate of 2 106 m3/yr. Faults bordering

  15. Lithospheric buoyancy and continental intraplate stresses

    USGS Publications Warehouse

    Zoback, M.L.; Mooney, W.D.

    2003-01-01

    Lithospheric buoyancy, the product of lithospheric density and thickness, is an important physical property that influences both the long-term stability of continents and their state of stress. We have determined lithospheric buoyancy by applying the simple isostatic model of Lachenbruch and Morgan (1990). We determine the crustal portion of lithospheric buoyancy using the USGS global database of more than 1700 crustal structure determinations (Mooney et al., 2002), which demonstrates that a simple relationship between crustal thickness and surface elevation does not exist. In fact, major regions of the crust at or near sea level (0-200 m elevation) have crustal thicknesses that vary between 25 and 55 km. Predicted elevations due to the crustal component of buoyancy in the model exceed observed elevations in nearly all cases (97% of the data), consistent with the existence of a cool lithospheric mantle lid that is denser than the asthenosphere on which it floats. The difference between the observed and predicted crustal elevation is assumed to be equal to the decrease in elevation produced by the negative buoyancy of the mantle lid. Mantle lid thickness was first estimated from the mantle buoyancy and a mean lid density computed using a basal crust temperature determined from extrapolation of surface heat flow, assuming a linear thermal gradient in the mantle lid. The resulting values of total lithosphere thickness are in good agreement with thicknesses estimated from seismic data, except beneath cratonic regions where they are only 40-60% of the typical estimates (200-350 km) derived from seismic data. This inconsistency is compatible with petrologic data and tomography and geoid analyses that have suggested that cratonic mantle lids are ??? 1% less dense than mantle lids elsewhere. By lowering the thermally determined mean mantle lid density in cratons by 1%, our model reproduces the observed 200-350+ km cratonic lithospheric thickness. We then computed

  16. Insolation driven variations of Mercury's lithospheric strength

    NASA Astrophysics Data System (ADS)

    Williams, Jean-Pierre; Ruiz, Javier; Rosenburg, Margaret A.; Aharonson, Oded; Phillips, Roger J.

    2011-01-01

    Mercury's coupled 3:2 spin-orbit resonance in conjunction with its relatively high eccentricity of ˜0.2 and near-zero obliquity results in both a latitudinal and longitudinal variation in annual average solar insolation and thus equatorial hot and cold regions. This results in an asymmetric temperature distribution in the lithosphere and a long wavelength lateral variation in lithosphere structure and strength that mirrors the insolation pattern. We employ a thermal evolution model for Mercury generating strength envelopes of the lithosphere to demonstrate and quantify the possible effects the insolation pattern has on Mercury's lithosphere. We find the heterogeneity in lithosphere strength is substantial and increases with time. We also find that a crust thicker than that of the Moon or Mars and dry rheologies for the crust and mantle are favorable when compared with estimates of brittle-ductile transition depths derived from lobate scarps. Regions of stronger and weaker compressive strength imply that the accommodation of radial contraction of Mercury as its interior cooled, manifest as lobate scarps, may not be isotropic, imparting a preferential orientation and distribution to the lobate scarps.

  17. The hydrothermal power of oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Grose, C. J.; Afonso, J. C.

    2015-10-01

    We have estimated the power of ventilated hydrothermal heat transport, and its spatial distribution, using a set of recently developed plate models which highlight the effects of axial hydrothermal circulation and thermal insulation by oceanic crust. Testing lithospheric cooling models with these two effects, we estimate that global advective heat transport is about 6.6 TW, significantly lower than most previous estimates, and that the fraction of that extracted by vigorous circulation on the ridge axes (< 1 My old) is about 50 % of the total, significantly higher than previous estimates. These new estimates originate from the thermally insulating properties of oceanic crust in relation to the mantle. Since the crust is relatively insulating, the effective properties of the lithosphere are "crust dominated" near ridge axes (a thermal blanketing effect yielding lower heat flow) and gradually approach mantle values over time. Thus, cooling models with crustal insulation predict low heat flow over young seafloor, implying that the difference of modeled and measured heat flow is due to the heat transport properties of the lithosphere, in addition to ventilated hydrothermal circulation as generally accepted. These estimates may bear on important problems in the physics and chemistry of the Earth because the magnitude of ventilated hydrothermal power affects chemical exchanges between the oceans and the lithosphere, thereby affecting both thermal and chemical budgets in the oceanic crust and lithosphere, the subduction factory, and the convective mantle.

  18. Editorial: The publication of geoscientific model developments v1.1

    NASA Astrophysics Data System (ADS)

    Executive Editors, GMD

    2015-10-01

    Version 1.0 of the editorial of the EGU (European Geosciences Union) journal, Geoscientific Model Development (GMD), was published in 2013. In that editorial an assessment was made of the progress the journal had made since it started, and some revisions to the editorial policy were introduced. After 2 years of experience with this revised editorial policy there are a few required updates, refinements and clarifications, so here we present version 1.1 of the editorial. The most significant amendments relate to the peer-review criteria as presented in the Framework for GMD manuscript types, which is published as an appendix to this paper and also available on the GMD manuscript types webpage. We also slightly refine and update the Publication guide and introduce a self-contained code and data policy. The changes are summarised as follows: - All manuscript types are now required to include code or data availability paragraphs, and model code must always be made available (in the case of copyright or other legal issues, to the editor at a minimum). - The role of evaluation in GMD papers is clarified, and a separate evaluation paper type is introduced. Model descriptions must already be published or in peer review when separate evaluation papers are submitted. - Observationally derived data should normally be published in a data journal rather than in GMD. Syntheses of data which were specifically designed for tasks such as model boundary conditions or direct evaluation of model output may, however, be published in GMD. - GMD publishes a broad range of different kinds of models, and this fact is now more explicitly acknowledged. - The main changes to the Publication guide are the addition of guidelines for editors when assessing papers at the initial review stage. Before sending papers for peer review, editors are required to make sure that papers comply with the Framework for GMD paper types and to carefully consider the topic of plagiarism. - A new appendix, the GMD

  19. Mentoring from Afar: Nurse Mentor Challenges in the Canadian Armed Forces.

    PubMed

    Neal, Laura D M

    2015-06-01

    There is an integral connection between leadership, mentoring and professional career progression within the nursing profession. The purpose of this article is to examine recommendations and best practices from the literature and provide a basis to construct a formalized successful mentoring dyad program with guidelines on establishing and maintaining a productive mentoring relationship over long distance. Canadian Armed Forces (CAF) nurses practice within a unique domain both domestically and abroad. The military environment incorporates many aspects of mentoring that could benefit significantly by distance interchange. Supported through examining literature within nursing, CAF publications and other professions along with contrasting successful distance mentoring programs, the findings suggest that a top-down, leadership-driven formal mentoring program could be beneficial to CAF nurses. The literature review outlines definitions of terms for mentorship and distance mentoring or e-mentoring. A cross section of technology is now embedded in all work environments with personal communication devices commonplace. Establishing mentoring relationships from afar is practical and feasible. This article provides a guided discussion for nursing leaders, managers and grassroots nurses to implement mentoring programs over distances. The recommendations and findings of this article could have universal applications to isolated nursing environments outside of Canadian military operational frameworks. PMID:26560257

  20. The Afar triple junction accommodation zone from InSAR derived strain and seismicity

    NASA Astrophysics Data System (ADS)

    Pagli, C.; Ebinger, C. J.; Keir, D.; Wang, H.

    2015-12-01

    Strain and seismicity show us the mode by which deformation is accommodated in rifting continents. Here we present a combined analysis of InSAR derived strain maps and seismicity of the Afar triple junction from 2006 to 2010. Our analysis shows that that the plate spreading motion is accommodated in different modes. A dogbone-shaped seismicity and strain distribution dominates the northern Red Sea branch of the triple junction, likely as a result of repeated dike intrusions 2005-2010. East of the triple junction, in the Gulf of Aden branch the strain and seismicity distribution appears decoupled. The strain focuses across the central part of several overlapping rifts, while the seismicity mainly occurs at the rift tips. Conversely, the Main Ethiopian Rift branch shows a narrow and elongated zone of both high strain and seismicity. The pattern suggests that the recent history of magmatic intrusions in the northern branch and mainly tectonic extension in the other branches creates a diverse triple junction accommodation zone.

  1. Chronostratigraphy of the Miocene-Pliocene Sagantole Formation, Middle Awash Valley, Afar rift, Ethiopia

    SciTech Connect

    Renne, P.R. |; WoldeGabriel, G.; Heiken, G.; Hart, W.K.; White, T.D.

    1999-06-01

    The Sagantole Formation comprises more than 200 m of lacustrine, alluvial, and volcaniclastic sediments, plus compositionally bimodal tephras and basaltic lavas, exposed in a domelike horst named the Central Awash Complex in the southwestern Afar rift of Ethiopia. The Sagantole Formation is widely known for abundant vertebrate faunas, including the 4.4 Ma primitive hominid Ardipithecus ramidus. New lithostratigraphic data are used to subdivide the Sagantole Formation into the Kuseralee, Gawto, Haradaso, Aramis, Beidareem, Adgantole, and Belohdelie Members, in ascending order. The members are defined on the basis of lithologic differences and laterally continuous bounding tephras. {sup 40}Ar/{sup 39}Ar dating of 12 intercalated volcanic units firmly establishes the age of the Sagantole Formation to be 5.6 to 3.9 Ma, significantly older than previous proposals based on erroneous correlations. Magnetostratigraphic data reveal eight paleomagnetic polarity zones, which can be correlated unambiguously with the Thvera, Sidufjall, Nunivak, and Cochiti Subchrons of the Gilbert Chron. Thus, by reference to the geomagnetic polarity time scale, seven additional chronological datums can be placed in the Sagantole Formation. With a total of 19 such datums, the age resolution anywhere in the Sagantole Formation is better than {+-}100 k.y., making this the best-dated Miocene-Pliocene succession in Africa.

  2. Mapping Distribution and Forecasting Invasion of Prosopis juliflora in Ethiopia's Afar Region

    NASA Astrophysics Data System (ADS)

    West, A. M.; Wakie, T.; Luizza, M.; Evangelista, P.

    2014-12-01

    Invasion of non-native species is among the most critical threats to natural ecosystems and economies world-wide. Mesquite (which includes some 45 species) is an invasive deciduous tree which is known to have an array of negative impacts on ecosystems and rural livelihoods in arid and semi-arid regions around the world, dominating millions of hectares of land in Asia, Africa, Australia and the Americas. In Ethiopia, Prosopis juliflora (the only reported mesquite) is the most pervasive plant invader, threatening local livelihoods and the country's unique biodiversity. Due to its rapid spread and persistence, P. juliflora has been ranked as one of the leading threats to traditional land use, exceeded only by drought and conflict. This project utilized NASA's Earth Observing System (EOS) data and species distribution modeling to map current infestations of P. juliflora in the Afar region of northeastern Ethiopia, and forecast its suitable habitat across the entire country. This project provided a time and cost-effective strategy for conducting risk assessments of invasive mesquite and subsequent monitoring and mitigation efforts by land managers and local communities.

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

    NASA Astrophysics Data System (ADS)

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

    2004-07-01

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

  4. Implications of longeval lava lakes for geomorphological and plutonic processes at Erta 'Ale volcano, Afar

    NASA Astrophysics Data System (ADS)

    Oppenheimer, Clive; Francis, Peter

    1998-01-01

    Erta 'Ale volcano, sited within the Afar Triangle of Ethiopia, is one of the least visited of perennially active, subaerial volcanoes. Satellite images recorded over the past thirty years illustrate the sustained activity of the volcano and complement the few brief periods of field observation. We consider that Erta 'Ale's lava lakes have probably persisted for ≥ 90 years, placing them among the longest-lived yet recorded. Despite sustained surface heat fluxes of 100-400 MW, the net surface growth rate integrated over the interval 1968-1995 was only ≈ 10 kg s -1. We speculate that this minimal lava discharge cannot be used to infer a low magma supply rate to the volcano, since heat losses from die lava lakes inhibit eruption by increasing magma density. This implies growth of the underlying crust by formation of dikes and sills containing cumulates, accommodated, and possibly promoted, by regional extension and intrusion loading. Open vent degassing and rift kinematics are clearly reflected in Erta 'Ale's morphology which is characterised by very gentle (< 3 °) slopes and a summit caldera.

  5. Segment linkage in Afar via magma intrusion: the birth of a transform fault?

    NASA Astrophysics Data System (ADS)

    Aronovitz, A. C.; Ebinger, C. J.; Campbell, E.; Keir, D. B.; Ayele, A.; Mitra, G.

    2007-12-01

    Both continental and oceanic rifts are segmented along their length, but the relation between transfer faults and transform faults linking segments remains unclear. How and when do transform faults initiate to link rift segments? Does magma intrusion achieve some of the strain transfer between segments? A temporary seismic array in the volcanically and seismically active Afar rift of Ethiopia provides insights into these two fundamental questions. We analyze the spatial and temporal patterns of earthquakes, and compare these to patterns in high-resolution satellite imagery and space geodetic data from the ongoing seismo-volcanic episode that began in 2005. We integrate these results to understand how stresses are transferred between ridge segments and how this possibly relates to the initiation of transform faults. Earthquake swarms from October 2005 to March 2006 form narrow bands coinciding with NW-SE striking fault zones linking the active Erta' Ale and Tat `Ale magmatic rift segments, and the Dabbahu and Alayta magmatic segments. Step over distances are ~15 km and ~20 km respectively. The time and spatial distribution of these seismic events as well as the correlation of events with magmatic centers suggests that earthquakes are triggered by magma intrusion. These patterns offer insight to magma accommodation along faults and between rift segments, suggesting magma intrusion facilitates transform fault initiation. We compare and contrast active structures during the 2005- 2006 episode with segment linkage patterns preserved in the rock record to understand transform evolution.

  6. Children with learning disabilities in the paediatric clinic, Hospital Tuanku Ja'afar Seremban: an overview.

    PubMed

    Mariana, A M Aina; Wong, S L

    2011-12-01

    The aim of the study was to document the prevalence of learning disability among the children attending the Paediatric Clinic in Hospital Tuanku Ja'afar Seremban. The demographic distribution of these patients; the age of detection of the problem; the associated medical conditions and types of intervention received by these patients were documented. Patients who were between the ages of five to twelve years were included in the study. Learning disability was divided into three categories: speech and articulation problems, academic skills disorder and other categories which included developmental delay. Children with cerebral palsy were excluded from the study. Out of 1320 patients screened, 355 were found to have learning disorders. Majority were Malays, with the male to female ratio of 1.9:1. Most of the patients stayed in Seremban. The learning problem was most commonly detected at the age of 4 years and below. The commonest type of learning disorder was developmental delay, followed by academic skills disorder, speech and academic skills problems and speech disorders. Problems that were detected early were speech problems and developmental delay. Majority of the children had associated medical conditions. Most of the patients received some form of intervention but 11.3% did not attend any intervention program at all. A strategy should be formulated and implemented to help this group of children. PMID:22390107

  7. Lithospheric stress patterns: A global view

    NASA Astrophysics Data System (ADS)

    Zoback, Mary Lou; Burke, Kevin

    The present-day lithospheric stress state is the result of a variety of forces that act on and within the tectonic plates forming the Iithosphere. Knowledge of this stress state provides important constraints on forces acting at a variety of scales and, hence, helps to solve scientific problems of interest to a wide spectrum of scientists and engineers.Six years of effort by scientists from all over the world (listed at end of article) brought together under the International Lithosphere Program (ILP) of the joint International Union of Geodesy and Geophysics/International Union of Geological Sciences (IUGG/IUGS) Interunion Commission on the Lithosphere culminated in the July 1992 publication of the World Stress Map and nineteen accompanying research papers in a special issue of the Journal of Geophysical Research-Solid Earth (volume 87, number B8). Figure 1 shows a reduced version of the published 1:40,000,000 color map.

  8. Viscosity of the lithosphere of Enceladus

    NASA Technical Reports Server (NTRS)

    Passey, Q. R.

    1983-01-01

    Regions of the Enceladus surface are shown by high resolution Voyager II images to be highly cratered, as if by heavy bombardment, with crater forms similar to those of fresh lunar surfaces but often shallower in depth. The flattening of these craters and the bowing up of their floors indicate viscous relaxation of the topography. Viscosity at the top of the lithosphere is suggested by crater form analysis to lie between 10 to the 24th and 10 to the 25th P. The zones where flattened craters occur may be regions of past or present heat flow that is higher than in adjacent terrains. Encedalus probably has a mixture of ammonia ice and water ice in the lithosphere, while the lithospheres of Ganymede and Callisto are primarily composed of water ice

  9. Lithospheric structure in the Pacific geoid

    NASA Technical Reports Server (NTRS)

    Marsh, B. D.; Hinojosa, J. H.

    1985-01-01

    The high degree and order SEASAT geoid in the central Pacific correlates closely with the structure of the cooling lithosphere. Relative changes in plate age across major fracture zones in relatively young seafloor frame the east-west trending pattern formed by the geoid anomalies. The field removal in bathymetry corresponds to removal of some of the low degree and order geoidal components, the step like structure across fracture zones is also removed. The regional thermal subsidence was removed from the bathymetry by subtracting a mean subsidence surface from the observed bathymetry. This produces a residual bathymetry map analogous to the usual residual depth anomaly maps. The residual bathymetry obtained in this way contains shallow depths for young seafloor, and larger depths for older seafloor, thus retaining the structure of the lithosphere while removing the subsidence of the lithosphere.

  10. Water in the Cratonic Mantle Lithosphere

    NASA Technical Reports Server (NTRS)

    Peslier, A. H.

    2016-01-01

    The fact that Archean and Proterozoic cratons are underlain by the thickest (>200 km) lithosphere on Earth has always puzzled scientists because the dynamic convection of the surrounding asthenosphere would be expected to delaminate and erode these mantle lithospheric "keels" over time. Although density and temperature of the cratonic lithosphere certainly play a role in its strength and longevity, the role of water has only been recently addressed with data on actual mantle samples. Water in mantle lithologies (primarily peridotites and pyroxenites) is mainly stored in nominally anhydrous minerals (olivine, pyroxene, garnet) where it is incorporated as hydrogen bonded to structural oxygen in lattice defects. The property of hydrolytic weakening of olivine [4] has generated the hypothesis that olivine, the main mineral of the upper mantle, may be dehydrated in cratonic mantle lithospheres, contributing to its strength. This presentation will review the distribution of water concentrations in four cratonic lithospheres. The distribution of water contents in olivine from peridotite xenoliths found in kimberlites is different in each craton (Figure 1). The range of water contents of olivine, pyroxene and garnet at each xenolith location appears linked to local metasomatic events, some of which occurred later then the Archean and Proterozoic when these peridotites initially formed via melting. Although the low olivine water contents (<10 ppm wt H2O) at > 6 GPa at the base of the Kaapvaal cratonic lithosphere may contribute to its strength, and prevent its delamination, the wide range of those from Siberian xenoliths is not compatible with providing a high enough viscosity contrast with the asthenophere. The water content in olivine inclusions from Siberian diamonds, on the other hand, have systematically low water contents (<20 ppm wt H2O). The xenoliths may represent a biased sample of the cratonic lithosphere with an over-­abundance of metasomatized peridotites with

  11. Uppermantle anisotropy and the oceanic lithosphere

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.; Regan, J.

    1983-01-01

    Published Rayleigh and Love wave phase and group velocity data have been inverted taking into account sphericity, anelastic dispersion, and transverse isotropy. For a PREM-type modular parameterization, the thickness of the high velocity mantle seismic lithosphere (LID) varies in thickness from about 30 km for young ocean to about 50 km for old ocean, much less than previous estimates based on isotropic inversion of similar data. This LID thickness is comparable to the elastic or flexural thickness found from studies of seamount loading and flexure at trenches, suggesting that the thickness of the lithosphere may be controlled by mineralogy, composition, or crystal orientation rather than by temperature alone.

  12. Lithospheric thinning beneath rifted regions of Southern California.

    PubMed

    Lekic, Vedran; French, Scott W; Fischer, Karen M

    2011-11-11

    The stretching and break-up of tectonic plates by rifting control the evolution of continents and oceans, but the processes by which lithosphere deforms and accommodates strain during rifting remain enigmatic. Using scattering of teleseismic shear waves beneath rifted zones and adjacent areas in Southern California, we resolve the lithosphere-asthenosphere boundary and lithospheric thickness variations to directly constrain this deformation. Substantial and laterally abrupt lithospheric thinning beneath rifted regions suggests efficient strain localization. In the Salton Trough, either the mantle lithosphere has experienced more thinning than the crust, or large volumes of new lithosphere have been created. Lack of a systematic offset between surface and deep lithospheric deformation rules out simple shear along throughgoing unidirectional shallow-dipping shear zones, but is consistent with symmetric extension of the lithosphere. PMID:21979933

  13. Lithospheric Thickness Modeled From Long Period Surface Wave Dispersion

    NASA Astrophysics Data System (ADS)

    Pasyanos, M. E.

    2007-12-01

    The behavior of surface waves at long periods is indicative of subcrustal velocity structure. Using recently published dispersion models, we invert surface wave group velocities for lithospheric structure, including lid velocity and lithospheric thickness, over much of the Eastern Hemisphere, encompassing Eurasia, Africa, and the Indian Ocean. Thicker lithosphere keels and faster upper mantle velocities under Precambrian shields and platforms are clearly observed, not only under the large cratons (West African Craton, Congo Craton, Baltic Shield, Russian Platform, Siberian Platform, Indian Shield, Kalahari Craton), but also under smaller blocks like the Tarim Basin and Yangtze Craton. There are also interesting variations within cratons like the Congo Craton. As expected, the thinnest lithospheric thickness is found under oceanic and continental rifts, and also along convergence zones. We compare our results to thermal lithospheric models of the continents, lithospheric cooling models of oceanic lithosphere, lithosphere-asthenosphere boundary (LAB) estimates from S-wave receiver functions, and velocity variations of global tomography models.

  14. On the importance of debate in (geo-)scientific research (Arthur Holmes Medal Lecture)

    NASA Astrophysics Data System (ADS)

    Courtillot, V.

    2012-04-01

    It is of course a great honor to receive the Holmes medal from EGU. As past (founding) treasurer and later president of EUG, the medal carries special significance for me. It may be a good time to look back on the scientific path I have followed, pursuing research in the geosciences, with outstanding support from a number of family members (foremost my wife Michèle), mentors, colleagues and students. Chance, not planning, led me to attend a French school that trained mining engineers, then a US University that made me fall in love with geophysics and plate tectonics at a time when this scientific revolution was still going on, and finally the marvelous Institut de Physique du Globe de Paris (IPGP), where I have spent the rest of my career to this day. To pursue on this path, I selected the rather separate fields of paleomagnetism (then linked to geology) and geomagnetism (then linked to physics). I have devoted much of my time to make sure that the two specialties would closely interact, including in the structure of our groups at IPGP. Geo- and paleo-magnetism have turned out (in a way reminiscent of geochemistry) to be powerful tools to explore a broad range of exciting scientific questions. Equipped with them, I have had the pleasure and good fortune to navigate from the discovery of geomagnetic secular variation impulses (with Jean-Louis Le Mouël), now inelegantly called "geomagnetic jerks", to that of propagating rifting of continents in the Afar depression, to fascinating work on the India-Asia collision in the Tibetan plateau and the Cenozoic paleogeography of the Indian ocean bordering continents, to the reconstruction of synthetic apparent polar wander paths for major continental masses (with Jean Besse) that have been widely used, to the understanding of the significance of the volume, age and short duration of massive flood basalt volcanism in the Deccan traps of India and their potential link to the biological mass extinction at the Cretaceous

  15. The Quaternary volcanic rocks of the northern Afar Depression (northern Ethiopia): Perspectives on petrology, geochemistry, and tectonics

    NASA Astrophysics Data System (ADS)

    Hagos, Miruts; Koeberl, Christian; van Wyk de Vries, Benjamin

    2016-05-01

    The northern Afar Depression is one of the most volcano-tectonically active parts of the East African Rift system, a place where oceanic rifting may be beginning to form an incipient oceanic crust. In its center, over an area that is ∼80 km long and ∼50 km wide, there are seven major NNW-SSE-aligned shield volcanoes/volcanic edifices surrounded by compositionally distinct fissure-fed basalts. The Quaternary lavas in this area range from transitional to tholeiitic basalts, with significant across-axis variation both in mineralogy and chemistry. The variation in the contents of the major elements (TiO2, Al2O3, and Fe2O3), incompatible trace elements (Nd, Hf, Th, Ta), and the contents and ratios of the rare earth elements (REE) (e.g., (La/Yb)n = 5.3-8.9) indicate some variation in the petrogenetic processes responsible for the formation of these basalts. However, the variation in isotopic compositions of the mafic lavas is minimal (87Sr/86Sr = 0.7036-0.7041, 143Nd/144Nd = 0.51286-0.51289), which suggests only one source for all the Danakil Depression basalts. These basalts have isotope and incompatible trace element ratios that overlap with those of the Oligocene High-Ti2 flood basalts from the Ethiopian Plateau, interpreted as being derived from the last phase/tail of the Afar mantle plume source. Moreover, the Ce/Pb, Ba/U ratios indicate that the involvement of continental crust in the petrogenesis of the basaltic rocks is minimal; instead, both depth and degree of melting of the source reservoir underneath the northern Afar Depression played a major role for the production of incompatible element-enriched basalts (e.g., AleBagu Shield basalts) and the incompatible element-depleted tholeiitic basalts (e.g., Erta'Ale and Alu Shield basalts).

  16. Imaging Lithospheric Structure beneath the Indian continent

    NASA Astrophysics Data System (ADS)

    Maurya, S.; Montagner, J. P.; Mangalampally, R. K.; Stutzmann, E.; Burgos, G.; Kumar, P.; Davuluri, S.

    2015-12-01

    The lithospheric structure and thickness to the LAB are the most debated issues, especially beneath continents. In this context, the structure and thickness of the Indian lithosphere has been controversial. Paleomagnetic data reveals that the Indian continent moved northwards at exceptionally high speeds (18-20 cm/year) and subsequently slowed down to 4-5 cm/year after its collision with Asia ≈40 Myr ago. This super mobility has been explained by an unusually thin Indian lithosphere (≈100 km; Kumar et al., 2007) in contradiction with the thick lithosphere that commonly underlies old cratonic nuclei. It is pertinent to note that the thermobarometric estimates on the ultramafic xenoliths from 65 Myr kimberlites of the Central India (Babu et al. 2009) suggest an approximately 175 km thick lithosphere. Also, recent results of P and S wave travel time tomography of India suggest that the lithospheric roots are not uniformly thick on a regional scale. Although high velocity roots typical of Precambrian shields are preserved beneath a few cratons of the Indian shield, they seem to have suffered attrition, in the plume ravaged regions like the NDVP and the Southern SGT (Singh et al., 2014). We assembled a new massive surface wave database towards obtaining 3D isotropic and anisotropic models for the Indian sub-continent, using surface waves. This necessitated processing of data from more than 500 seismic broadband stations across India and surrounding regions. Surface waves group and phase dispersion measurements are performed in a broad frequency range (16-250s). Our phase velocity anomaly maps recover most of the known geological structures. The cratons are associated with high velocity (4-6%) anomalies till 200 sec, with the WDC being faster than the EDC. Slow velocities in NW India and very high velocity anomalies (6-8%) beneath the central part of the Indo-Gangetic plains are possibly associated with the subducting Indian lithosphere. The LAB depths inferred from

  17. Filling the temporal gap in Plio-Pleistocene sedimentological records from the southern Afar Depression, Ethiopia

    NASA Astrophysics Data System (ADS)

    Dimaggio, E.; Arrowsmith, R.; Reed, K.; Campisano, C. J.

    2009-12-01

    The structural development of the Afar Depression controls sedimentary basin architecture and thus directly influences the spatial and temporal pattern of depositional environments. Over the past 5 Myr these processes, in addition to climate variations and volcanic eruptions, influenced the composition and variation of early human habitats and their record is preserved in the sediment record of synchronous rift basins. Here we present results from field investigations at Geraru located within the Ledi-Geraru field site, part of the greater Hadar sedimentary basin in the Afar region of Ethiopia. We aim to characterize local basin structure and expand and refine interpretations of the complex Plio-Pleistocene history of local and regional-scale landscape change during a time of critical importance for understanding hominin evolution. We mapped Geraru geology (1:7000), measured stratigraphic sections, and sampled 20 tephra deposits for absolute age dating (40Ar/39Ar) and for compositional studies and correlation using glass shard chemistry. We mapped over 16 km2 and identified lacustrine to fluvial sediments that are well-exposed along NW-SE to NE-SW trending faulted basalt hills that offset sediment blocks up to ~30 m. Although stratigraphic sections are not continuous, faulting relationships and marker beds suggest that at least 65m of strata are accessible. The lower ~40m of sediments are lacustrine deposits indicated by finely laminated silts and clays, two <2m thick diatomite layers, and gastropod shells. Conformably overlying them, we found ~25m of coarse fluvial sands and gravels that are locally cross-bedded and contain fossils of terrestrial fauna. Compositional results of glass shards from 4 tephra samples indicate that they not chemically similar to tephras from the Hadar (ca. 3.8-2.9 Ma) or Busidima (ca. 2.7-0.16 Ma) Formations. One tephra sample collected from the central portion of the stratigraphic section yielded a plateau age of 2.814 ± 0.017 Ma

  18. Magnetic mineralogy of the Mercurian lithosphere

    NASA Astrophysics Data System (ADS)

    Strauss, Becky; Feinberg, Joshua; Johnson, Catherine

    2016-04-01

    Mercury and Earth are the only inner solar system planets with present-day core-dynamo magnetic fields, in contrast to the past fields of Mars and the Moon and the absence of evidence for a past or present field at Venus. Recently, the MESSENGER mission also measured magnetic fields from lithospheric magnetization on Mercury for the first time. These fields are consistent with remanent magnetization held by rocks exposed to an ancient, internally generated planetary magnetic field. However, the conditions for magnetization in the lithosphere of Mercury are unique among terrestrial planets, and the mechanisms for the acquisition (induced versus remanent) and alteration of magnetization are still unknown. We investigate the physical and chemical environment of Mercury's crust, past and present, to establish the conditions in which magnetization may have been acquired and subsequently modified. Three factors are particularly crucial to the determination of crustal composition and iron mineralogy: the temperature profile of the lithosphere and its evolution over time, redox conditions in the planet's crust and mantle, and the iron content of the lithosphere. We explore potential mechanisms for remanence acquisition and alteration on Mercury, whose surface environment is distinct from that of other inner solar system planets in that it is both very hot and highly reducing. The long-term thermal history of Mercury's crust plays an important role in the longevity of any crustal magnetization, which may be subject to remagnetization through thermal, viscous, and shock mechanisms. This thermal and compositional framework isused to constrain plausible candidate magnetic mineralogies, which can then be analyzed in terms of their capacity to acquire and retain magnetic remanence that is detectable from satellite orbit. We propose a suite of minerals and materials that could be carriers of remanence in the lithosphere of Mercury, including iron alloys, silicides, and sulfides.

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

    NASA Astrophysics Data System (ADS)

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

    2014-04-01

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

  20. Magmatic cycles pace tectonic and morphological expression of rifting (Afar depression, Ethiopia)

    NASA Astrophysics Data System (ADS)

    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.

    2016-07-01

    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 rift 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 rift 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 central parts of DMH rift, 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 rift 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.

  1. Prevalence of Schistosoma haematobium Infection among School-Age Children in Afar Area, Northeastern Ethiopia.

    PubMed

    Degarege, Abraham; Mekonnen, Zeleke; Levecke, Bruno; Legesse, Mengistu; Negash, Yohannes; Vercruysse, Jozef; Erko, Berhanu

    2015-01-01

    In this study, the prevalence and intensity of Schistosoma haematobium infection was determined among school-age children living in the Middle and Lower Awash Valley, Afar Regional State of Ethiopia. Between February and May 2014, urine samples were collected from 885 school-age children (5-16 years of age) from the Middle (n = 632; 4 villages) and Lower (n = 253; 3 villages) Awash Valley. All samples were processed using urine filtration to detect and quantify S. haematobium eggs. In addition, a subset of the urine samples was tested for hematuria using a urine dipstick (n = 556). The overall prevalence was 20.8% (95% Confidence Interval (CI) = 18.1%, 23.5%), based on urine filtration but the prevalence considerably varied across villages both in the Middle (from 12.5% to 37.0%) and Lower Awash Valley (from 0 to 5.3%). The overall mean urine egg count (UEC) among the infected children was 4.0 eggs/10 ml of urine (95% CI = 2.43, 5.52). The infection intensity varied from 0.4 eggs/10 ml of urine to 7.7 eggs/10 ml of urine in the Middle Awash Valley, and from 0 to 1.1 eggs/10 ml of urine in Lower Awash Valley. Age and sex were not associated with S. haematobium infection based on the multivariable logistic regression model. The prevalence of hematuria was 56.3% (95% CI = 52.2%, 60.4%) among a subset of the study participants (556) examined using the urine dipstick. The prevalence of hematuria also varies with villages from 8.3% to 93.2%. In conclusion, the prevalence of S. haematobium infection in the Middle Awash Valley was high and it varies across villages. Hence, children living in the present study villages of the Middle Awash Valley need to be treated with praziquantel to reduce morbidity and disrupt transmission. PMID:26252615

  2. Phylogeny of early Australopithecus: new fossil evidence from the Woranso-Mille (central Afar, Ethiopia)

    PubMed Central

    Haile-Selassie, Yohannes

    2010-01-01

    The earliest evidence of Australopithecus goes back to ca 4.2 Ma with the first recorded appearance of Australopithecus ‘anamensis’ at Kanapoi, Kenya. Australopithecus afarensis is well documented between 3.6 and 3.0 Ma mainly from deposits at Laetoli (Tanzania) and Hadar (Ethiopia). The phylogenetic relationship of these two ‘species’ is hypothesized as ancestor–descendant. However, the lack of fossil evidence from the time between 3.6 and 3.9 Ma has been one of its weakest points. Recent fieldwork in the Woranso-Mille study area in the Afar region of Ethiopia has yielded fossil hominids dated between 3.6 and 3.8 Ma. These new fossils play a significant role in testing the proposed relationship between Au. anamensis and Au. afarensis. The Woranso-Mille hominids (3.6–3.8 Ma) show a mosaic of primitive, predominantly Au. anamensis-like, and some derived (Au. afarensis-like) dentognathic features. Furthermore, they show that, as currently known, there are no discrete and functionally significant anatomical differences between Au. anamensis and Au. afarensis. Based on the currently available evidence, it appears that there is no compelling evidence to falsify the hypothesis of ‘chronospecies pair’ or ancestor–descendant relationship between Au. anamensis and Au. afarensis. Most importantly, however, the temporally and morphologically intermediate Woranso-Mille hominids indicate that the species names Au. afarensis and Au. anamensis do not refer to two real species, but rather to earlier and later representatives of a single phyletically evolving lineage. However, if retaining these two names is necessary for communication purposes, the Woranso-Mille hominids are best referred to as Au. anamensis based on new dentognathic evidence. PMID:20855306

  3. Phylogeny of early Australopithecus: new fossil evidence from the Woranso-Mille (central Afar, Ethiopia).

    PubMed

    Haile-Selassie, Yohannes

    2010-10-27

    The earliest evidence of Australopithecus goes back to ca 4.2 Ma with the first recorded appearance of Australopithecus 'anamensis' at Kanapoi, Kenya. Australopithecus afarensis is well documented between 3.6 and 3.0 Ma mainly from deposits at Laetoli (Tanzania) and Hadar (Ethiopia). The phylogenetic relationship of these two 'species' is hypothesized as ancestor-descendant. However, the lack of fossil evidence from the time between 3.6 and 3.9 Ma has been one of its weakest points. Recent fieldwork in the Woranso-Mille study area in the Afar region of Ethiopia has yielded fossil hominids dated between 3.6 and 3.8 Ma. These new fossils play a significant role in testing the proposed relationship between Au. anamensis and Au. afarensis. The Woranso-Mille hominids (3.6-3.8 Ma) show a mosaic of primitive, predominantly Au. anamensis-like, and some derived (Au. afarensis-like) dentognathic features. Furthermore, they show that, as currently known, there are no discrete and functionally significant anatomical differences between Au. anamensis and Au. afarensis. Based on the currently available evidence, it appears that there is no compelling evidence to falsify the hypothesis of 'chronospecies pair' or ancestor-descendant relationship between Au. anamensis and Au. afarensis. Most importantly, however, the temporally and morphologically intermediate Woranso-Mille hominids indicate that the species names Au. afarensis and Au. anamensis do not refer to two real species, but rather to earlier and later representatives of a single phyletically evolving lineage. However, if retaining these two names is necessary for communication purposes, the Woranso-Mille hominids are best referred to as Au. anamensis based on new dentognathic evidence. PMID:20855306

  4. Formation and stability of ridge-ridge-ridge triple junctions in rheologically realistic lithosphere model

    NASA Astrophysics Data System (ADS)

    Gerya, Taras; Burov, Evgueni

    2015-04-01

    -branch junction formation and evolution by using high-resolution 3D numerical mechanical experiments that take into account realistic thermo-rheological structure and rheology of the lithosphere. We find that two major types of quadruple and triple junctions are formed under bi-directional or multidirectional far-field stress field: (i) plate rifting junctions are formed by the initial plate fragmentation and can be subsequently re-arranged into (ii) oceanic spreading junctions controlled by the new oceanic crust accretion. In particular, we document initial formation and destabilization of quadruple R-R-R-R junctions as initial plate rifting structures under bi-directional extension. In most cases, quadruple plate rifting junctions rapidly (typically within 1-2 Myr) evolve towards formation of two diverging triple oceanic spreading junctions connected by a linear spreading center lengthening with time. This configuration remains stable over long time scales. However, under certain conditions, quadruple junctions may also remain relatively stable. Asymmetric stretching results in various configurations, for example formation of "T-junctions" with trans-extensional components and combination of fast and slow spreading ridges. Combined with plume impingement, this scenario evolves in realistic patterns closely resembling observed plate dynamics. In particular, opening of the Red Sea and of the Afar rift system find a logical explanation within a single model. Numerical experiments also suggest that several existing oceanic spreading junctions form as the result of plate motions rearrangements after which only one of two plates spreading along the ridge become subjected to bi-directional spreading.

  5. Determination of thrust compression systems with a multisource geoscientific data processing in the southern margin of Ordos basin, China

    SciTech Connect

    Yongjie, T.; Genderen, J.L. van

    1996-08-01

    In this paper, the multisource geoscientific data, such as TM data, NOAA data, the data of Bouguer gravity anomaly and aeromagnetic anomaly, the multisource geophysical processing methods, such as upward continuations, vertical derivative and gradient image, and synthetic image processing method - remote sensing image processing and geophysical image processing under I{sup 2}S600, are used to study the reverse faults or thrusts in the southern margin of Ordos basin, China. On the basis of these results and with geological investigation in the field, the authors have determined three thrust compression systems, named EW thrust compression system, NE thrust compression system and NW thrust compression system. These three systems reveal the regularity of geological structural evolution in the southern margin of Ordos basis and in the north side of Qinling Orogenic Belt.

  6. Mantle lithosphere fabrics around the TESZ

    NASA Astrophysics Data System (ADS)

    Vecsey, L.; Plomerova, J.; Babuska, V.; Passeq Working Group

    2012-04-01

    Though the lithosphere-asthenosphere boundary (LAB) represents the first order structural interface in the upper mantle, its nature remains puzzling. By modelling structure of the mantle lithosphere we aim at contributing to endeavours to better understand what the LAB represents. We examine lateral variations of shear-wave splitting evaluated from data recorded during the PASSEQ (2006-2008) passive seismic experiment spanning across the Trans-European suture Zone (TESZ). SKS waves split in the Bohemian Massif (BM) with an average delay time of the slow shear wave ~1.2 s., while null splits were evaluated for waves from the NE at stations located in the Polish Platform between the BM and TESZ. Further to the NE, eastward of the TESZ, a weak splitting with the fast shear-wave polarized in the SW azimuth was detected. The TESZ represents a distinct ~3500 km long tectonic feature, which can be traced through north-western to south-eastern Europe in various seismic velocity (e.g., Bijwaard et al., JGR 1998, Goes et al., JGR 2000) as well as in seismic anisotropy (e.g., Babuska et al., PAGEOPH 1998). Models of seismic anisotropy around the western part of TESZ (Plomerova et al., 2002; Babuska and Plomerova, 2004) delimited three lithospheric domains with different structures and thickness: (1) north of the TESZ, the high velocities of the anisotropic structures dip to the NE in the thick lithosphere of Fennoscandia; (2) the sharply bounded fragment of a thinner lithosphere between the northern (Sorgenfrei-Tornquist Zone) and southern branch (Thor Suture) of the TESZ, where anisotropic structures dip to the WNW; (3) south of the TESZ, a domain belonging to a very thin lithosphere of Avalonia exhibits the high velocities dipping to the SW-W. In this contribution we present 3D self-consistent anisotropic models of the upper mantle around the central part of TESZ. The models meet both the spatial variations of the teleseismic shear-wave splitting and P-wave travel time

  7. Mapping current and potential distribution of non-native Prosopis juliflora in the Afar region of Ethiopia

    USGS Publications Warehouse

    Wakie, Tewodros; Evangelista, Paul H.; Jarnevich, Catherine S.; Laituri, Melinda

    2014-01-01

    We used correlative models with species occurrence points, Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation indices, and topo-climatic predictors to map the current distribution and potential habitat of invasive Prosopis juliflora in Afar, Ethiopia. Time-series of MODIS Enhanced Vegetation Indices (EVI) and Normalized Difference Vegetation Indices (NDVI) with 250 m2 spatial resolution were selected as remote sensing predictors for mapping distributions, while WorldClim bioclimatic products and generated topographic variables from the Shuttle Radar Topography Mission product (SRTM) were used to predict potential infestations. We ran Maxent models using non-correlated variables and the 143 species-occurrence points. Maxent generated probability surfaces were converted into binary maps using the 10-percentile logistic threshold values. Performances of models were evaluated using area under the receiver-operating characteristic (ROC) curve (AUC). Our results indicate that the extent of P. juliflora invasion is approximately 3,605 km2 in the Afar region (AUC = 0.94), while the potential habitat for future infestations is 5,024 km2 (AUC = 0.95). Our analyses demonstrate that time-series of MODIS vegetation indices and species occurrence points can be used with Maxent modeling software to map the current distribution of P. juliflora, while topo-climatic variables are good predictors of potential habitat in Ethiopia. Our results can quantify current and future infestations, and inform management and policy decisions for containing P. juliflora. Our methods can also be replicated for managing invasive species in other East African countries.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  9. Mapping of lithologic and structural units using multispectral imagery. [Afar-Triangle/Ethiopia and adjacent areas (Ethiopian Plateau, Somali Plateau, and parts of Yemen and Saudi Arabia)

    NASA Technical Reports Server (NTRS)

    Kronberg, P. (Principal Investigator)

    1974-01-01

    The author has identified the following significant results. ERTS-1 MSS imagery covering the Afar-Triangle/Ethiopia and adjacent regions (Ethiopian Plateau, Somali Plateau, and parts of Yemen and Saudi Arabi) was applied to the mapping of lithologic and structural units of the test area at a scale 1:1,000,000. Results of the geological evaluation of the ERTS-1 imagery of the Afar have proven the usefullness of this type of satellite data for regional geological mapping. Evaluation of the ERTS images also resulted in new aspects of the structural setting and tectonic development of the Afar-Triangle, where three large rift systems, the oceanic rifts of the Red Sea and Gulf of Aden and the continental East African rift system, seem to meet each other. Surface structures mapped by ERTS do not indicate that the oceanic rift of the Gulf of Aden (Sheba Ridge) continues into the area of continental crust west of the Gulf of Tadjura. ERTS data show that the Wonji fault belt of the African rift system does not enter or cut through the central Afar. The Aysha-Horst is not a Horst but an autochthonous spur of the Somali Plateau.

  10. Flexural deformation of the continental lithosphere

    NASA Technical Reports Server (NTRS)

    1990-01-01

    Prior work focused primarily on the Adriatic and northern Ionian regions. The results of these studies have been summarized previously, and so are only briefly discussed. More recent work focuses on two different topics: (1) analysis of foredeep basin geometry, sedimentary style, and thrust belt structure in light of the kinematics at the associated plate boundary and subduction zone dynamics; and (2) the evolution and plate strength of early Proterozoic lithosphere.

  11. The electrical Lithosphere of the Alboran Domain

    NASA Astrophysics Data System (ADS)

    Garcia, X. A.; Evans, R. L.; Elsenbeck, J.; Jegen, M. D.; Matsuno, T.

    2011-12-01

    On the Western edge of the Mediterranean, the slow convergence of the Iberian and African plates is marked by very intricate tectonic activity, marked by a combination of small-scale subduction and sub-lithospheric downwelling. Delamination or convective instability has also been proposed to have occurred beneath this domain during the past 25 My. And different geodynamic models have been proposed to explain the lithospheric structure of the arc-shaped belt (Betic and Rif orogenies) and the opening of the Alboran Basin. As part of several international projects carried out in this area, magnetotelluric (MT) methods have been used to explore the crust and upper mantle. The measurements of mantle electrical conductivity are a well known complement to measurements of seismic velocity. Conductivity is sensitive to temperature, composition and hydration of the mantle, and therefore MT is widely used to provide constraints on mantle processes. We present results of electromagnetic studies in the Western Mediterranean, focusing specially in the recently work on the Alboran sea as part of a marine MT survey. Land MT studies have already imaged an area of low resistivity coincident with an area of low velocities without earthquake hypocenters, interpreted as asthenospheric material intruded by the lateral lithospheric tearing and breaking-off of the east-directed subducting Ligurian slab under the Alboran Domain. The model suggests that the most likely scenario for the opening of the Alboran Basin is related to the westward rollback of the Ligurian subducting slab. The marine data show complex MT response functions with strong distortion due to seafloor topography and coast effect, suggesting a fairly resistive lithosphere beneath the seafloor.

  12. Flexure and rheology of Pacific oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Hunter, Johnny; Watts, Tony

    2016-04-01

    The idea of a rigid lithosphere that supports loads through flexural isostasy was first postulated in the late 19th century. Since then, there has been much effort to investigate the spatial and temporal variation of the lithosphere's flexural rigidity, and to understand how these variations are linked to its rheology. We have used flexural modelling to first re-assess the variation in the rigidity of oceanic lithosphere with its age at the time of loading, and then to constrain mantle rheology by testing the predictions of laboratory-derived flow laws. A broken elastic plate model was used to model trench-normal, ensemble-averaged profiles of satellite-derived gravity at the trench-outer rise system of circum-Pacific subduction zones, where an inverse procedure was used to find the best-fit Te and loading conditions. The results show a first-order increase in Te with plate age, which is best fit by the depth to the 400 ± 35°C plate-cooling isotherm. Fits to the observed gravity are significantly improved by an elastic plate that weakens landward of the outer rise, which suggests that bending-induced plate weakening is a ubiquitous feature of circum-Pacific subduction zones. Two methods were used to constrain mantle rheology. In the first, the Te derived by modelling flexural observations was compared to the Te predicted by laboratory-derived yield strength envelopes. In the second, flexural observations were modelled using elastic-plastic plates with laboratory-derived, depth-dependent yield strength. The results show that flow laws for low-temperature plasticity of dry olivine provide a good fit to the observations at circum-Pacific subduction zones, but are much too strong to fit observations of flexure in the Hawaiian Islands region. We suggest that this discrepancy can be explained by differences in the timescale of loading combined with moderate thermal rejuvenation of the Hawaiian lithosphere.

  13. Adakites from collision-modified lithosphere

    NASA Astrophysics Data System (ADS)

    Haschke, M.; Ben-Avraham, Z.

    2005-08-01

    Adakitic melts from Papua New Guinea (PNG) show adakitic geochemical characteristics, yet their geodynamic context is unclear. Modern adakites are associated with hot-slab melting and/or remelting of orogenic mafic underplate at convergent margins. Rift-propagation over collision-modified lithosphere may explain the PNG adakite enigma, as PNG was influenced by rapid creation and subduction of oceanic microplates since Mesozoic times. In a new (rift) tectonic regime, decompressional rift melts encountered and melted remnant mafic eclogite and/or garnet-amphibolite slab fragments in arc collisional-modified mantle, and partially equilibrated with metasomatized mantle. Alternatively, hot-slab melting in a proposed newborn subduction zone along the Trobriand Trough could generate adakitic melts, but recent seismic P-wave tomographic models lack evidence for subducting oceanic lithosphere in the adakite melt region; however they do show deep subduction zone remnants as a number of high P-wave anomalies at lithospheric depths, which supports our proposed scenario.

  14. Fennoscandian lithosphere - electromagnetic and seismic constraints

    NASA Astrophysics Data System (ADS)

    Korja, T.; Kozlovskaya, E.; Smirnov, M.

    2009-04-01

    Knowledge of the present-day structure of the Earth's mantle is essential to our understanding of plate tectonics as well as Earth's thermomechanical evolution over long periods of geological time. Several factors including temperature, chemical composition, presence of partial melt or water influence seismic velocities and electrical conductivity in the upper mantle. Similarly, anisotropy may have a profound effect on seismic and magnetotelluric observations. During last ten years, several large scale multinational and national seismic and magnetotelluric experiments have been carried out in Fennnoscandia including e.g. the SVEKALAPKO seismic tomography experiment, Swedish National Seismic Network (SNSN) array monitoring, BEAR and EMMA magnetotelluric array studies and magnetotelluric profiling such as TOR and Jamtland. Altogether these studies cover most of Fennoscandia and make it possible to correlate two different data sets and to study lithospheric structures in Fennoscandia. In particular, we will compare the thickness of the lithosphere obtained from seismic anisotropy studies and from magnetotelluric studies. We will also correlate spatially sparse indications on seismic reflectors and electrically conducting layers in the mantle lithosphere. Finally, we aim to compare directly absolute values of seismic velocity and electrical conductivity.

  15. Thermal classification of lithospheric discontinuities beneath USArray

    NASA Astrophysics Data System (ADS)

    Hansen, Steven M.; Dueker, Ken; Schmandt, Brandon

    2015-12-01

    Broadband seismic data from the United States were processed into Ps and Sp receiver function image volumes for the purpose of constraining negative velocity gradients (NVG) at depths between the Moho and 200 km. Moho depth picks from the two independent datasets are in good agreement, however, large discrepancies in NVG picks occur and are attributed to free-surface multiples which obscure deep NVG arrivals in the Ps data. From the Sp data, shallow NVG are found west of the Rockies and in the central US while deep and sporadic NVG are observed beneath the Great Plains and northern Rockies. To aid the interpretation of the observed NVG arrivals, the mantle thermal field is estimated by mapping surface wave tomography velocities to temperature assuming an anelastic olivine model. The distribution of temperature versus NVG depth is bi-modal and displays two distinct thermal populations that are interpreted to represent both the lithosphere-asthenosphere boundary (LAB) and mid-lithosphere discontinuities (MLD). LAB arrivals occur in the western US at 60-85 km and 1200-1400 °C depth suggesting that they manifest partial melt near the base of the thermal plate. MLD arrivals primarily occur at 70-110 km depth and 700-900 °C and we hypothesize that these arrivals are caused by a low-velocity metasomatic layer containing phlogopite resulting from magma crystallization products that accumulate within long-lived thick lithosphere.

  16. Petrological and geochemical data of volcanic rocks from the southern Afar Depression, Ethiopia.

    NASA Astrophysics Data System (ADS)

    Urbanek, Ch.; Faupl, P.; Richter, W.; Seidler, H.

    2003-04-01

    The geological and petrological investigations (FWF Project P15196) in the southern Afar Depression of Ethiopia support an international palaeoanthropological research-team (PAR) under the leadership of Horst Seidler. Mount Galila is the conspicuous centre of the research area [N 9° 44.101', E 40° 27.368'], situated about 20 km E of the NNE-SSW striking, recently active Hertale Graben, which represents a northernmost segment of the Main Ethiopian Rift (MER). Stratigraphically, the fossiliferous lacustrine and fluvial deposits, as well as the intercalated volcanic layers of the Galila area, belong to the "Upper Stratoid Series" (5-1.4 Ma) and will be named the Mount Galila Formation. They are similar to the Awash Group, from which very famous early hominid fossils have been described. In the Mount Galila Fm., 7 main volcanic horizons serve as marker beds comprising basalts, ignimbrites, tuffs and tuffaceous sands. The basalt horizons in the research area represent basaltic lava flows each consisting of one single flow unit c. 5 meters thick with maximum 5 cooling units. A first set of geochemical data from XRF spectrometry comprising main and trace element analysis shows characteristics for the volcanic marker beds as following: The basalts are clearly tholeiitic in the main elements (FeO/MgO/Alk) and show typical trace element distributions (e.g. Zr/Y-Zr; Ti/100-Yx3-Zr) as Within Plate Tholeiit Basalts. All basalt samples contain access 40Ar which can be explained by specific erruption mechanisms that leads to analytical problems for 40Ar/39Ar dating. In the TAS diagram after LeMaitre 1984 the ignimbrites vary at high alkali levels (7-9%) from trachytic to dacitic and rhyolitic composition, whereas at low alkali contents (<7%) they plot into the andesitic field. Compared to the basalts, the geochemistry of the ignimbrites is much more inhomogenous. Tuffs and tuffaceous sands are relevant as marker beds especially for the palaeoanthropological excavations in the

  17. Magmatic cycles pace tectonic and morphological expression of rifting (Afar depression, Ethiopia)

    NASA Astrophysics Data System (ADS)

    Medynski, Sarah; Pik, Raphael; Burnard, Peter; Blard, Pierre-Henri

    2016-04-01

    Dyking and faulting at mid-oceanic ridges are concentrated in narrow axial volcanic zones due to focussing of both melt distribution and tectonic strain along the plate boundary. Due to the predominantly submarine location of oceanic ridges, the interplay between these processes remain poorly constrained in time and space. In this study, we use the Dabbahu-Manda Hararo (DMH) magmatic rift segment (MRS) (Afar, Ethiopia) to answers the long debated chicken-egg question about magmatic and tectonic processes in extensive context: which on comes first, and how those two processes interplay to finally form oceanic ridges? The DMH MRS is an oceanic ridge analogue and here we present quantitative slip rates on major and minor normal fault scarps for the past 40 kyr in the vicinity of a recent (September 2005) dike intrusion. Our data show that the long-term-vertical slip rates of faults that ruptured in 2005 are too low to explain the present rift topography and that the 2005 strain distribution is not the main stress accommodating mechanism in the DMH segment. Instead, we show that the axial valley topography is created by enhanced slip rates which occur only when the amount of magma available in magma reservoirs is limited, thus preventing dykes from reaching the surface. Our results suggest that development of the axial valley topography is regulated by the magma reservoir lifetime and, thus, to the magmatic cycles of replenishment/differentiation (< 100 ky). This implies that in the DMH rift system (with a magma supply typical of an intermediate spreading centre), significant topography of the axial rift valley is transient, and is expressed only when magma available in the reservoirs decreases. The absence of tilting on the rift margins over the last 200 kyr also suggests that amagmatic accommodation of extension is not required over this time period. Extension instead is accommodated by dykes injected laterally from multiple ephemeral reservoirs located along the DMH

  18. Afar unrest: the 2008 Alu eruption in the Erta `Ale volcanic system (Ethiopia)

    NASA Astrophysics Data System (ADS)

    Pagli, C.; Wright, T. J.; Ayele, A.; Barnie, T.

    2009-04-01

    The Alu volcano is located within the Erta ‘Ale volcanic system in northern Afar (Ethiopia), about 30 km to the north of the Erta ‘Ale volcano whose summit caldera hosts a lava lake. Aster, Hotspot and S02 emissions images formed on 3rd November, 2008 showed that a fissural eruption was occurring east of the Alu volcano. Here we present an InSAR study of the area, using data from four tracks of the Envisat satellite both in descending ad ascending orbits, also including data in Wide Swath acquisition mode. The interferograms span different time periods allowing us to separate pre-, co- and post-eruptive deformation. In particular, an acquisition made on 3rd November at 19:20 (the day of the eruption) is used to observe deformation over two different co-eruptive periods. The co-eruptive interferograms show two closely-spaced but distinct concentric deformation patterns, both consistent with deflation, on the Alu volcano and on another unnamed volcano 3 km south of Alu. Most deformation in both volcanoes occurred during the first part of 3rd November (until 19:20) but significant deflation, up to ~ 45 cm, is also observed in the following interferogram from 3rd November at 19:20 to 8th December. Interestingly, no significant pre-eruptive deformation was observed in the area, nor any deformation was observed at the Erta ‘Ale lava lake prior to, during or after the Alu eruption. Preliminary modelling results, using two deflating Mogi sources, suggest that the two shallow magma chambers at ~1-1.5 km depth deflated during the eruption, with a volume change of the sources of ~-0.01 km3. Lack of pre-eruptive deformation suggests that the erupted magma was sitting at shallow depth under the Alu volcanoes. The lava lake in the nearby Erta 'Ale volcano also indicates that shallow magma reservoirs are a common feature in the area. We plan to model the observed deformations using different models, i.e. sill, penny shaped crack and ellipsoidal source and to compare our

  19. Lithospheric Thickness Modeled from Long Period Surface Wave Dispersion

    SciTech Connect

    Pasyanos, M E

    2008-05-15

    The behavior of surface waves at long periods is indicative of subcrustal velocity structure. Using recently published dispersion models, we invert surface wave group velocities for lithospheric structure, including lithospheric thickness, over much of the Eastern Hemisphere, encompassing Eurasia, Africa, and the Indian Ocean. Thicker lithosphere under Precambrian shields and platforms are clearly observed, not only under the large cratons (West Africa, Congo, Baltic, Russia, Siberia, India), but also under smaller blocks like the Tarim Basin and Yangtze craton. In contrast, it is found that remobilized Precambrian structures like the Saharan Shield and Sino-Korean Paraplatform do not have well-established lithospheric keels. The thinnest lithospheric thickness is found under oceanic and continental rifts, as well as along convergence zones. We compare our results to thermal models of continental lithosphere, lithospheric cooling models of oceanic lithosphere, lithosphere-asthenosphere boundary (LAB) estimates from S-wave receiver functions, and velocity variations of global tomography models. In addition to comparing results for the broad region, we examine in detail the regions of Central Africa, Siberia, and Tibet. While there are clear differences in the various estimates, overall the results are generally consistent. Inconsistencies between the estimates may be due to a variety of reasons including lateral and depth resolution differences and the comparison of what may be different lithospheric features.

  20. Lithospheric thickness modeled from long-period surface wave dispersion

    NASA Astrophysics Data System (ADS)

    Pasyanos, Michael E.

    2010-01-01

    The behavior of surface waves at long periods is indicative of subcrustal velocity structure. Using recently published dispersion models, we invert surface wave group velocities for lithospheric structure, including lithospheric thickness, over much of the Eastern Hemisphere, encompassing Eurasia, Africa, and the Indian Ocean. Thicker lithospheres under Precambrian shields and platforms are clearly observed, not only under the large cratons (West Africa, Congo, Baltic, Russia, Siberia, India), but also under smaller blocks like the Tarim Basin and Yangtze craton. In contrast, it is found that remobilized Precambrian structures like the Saharan Shield and Sino-Korean Paraplatform do not have well-established lithospheric keels. The thinnest lithospheric thickness is found under oceanic and continental rifts, as well as along convergence zones. We compare our results to thermal models of continental lithosphere, lithospheric cooling models of oceanic lithosphere, lithosphere-asthenosphere boundary (LAB) estimates from S-wave receiver functions, and velocity variations of global tomography models. In addition to comparing results for the broad region, we examine in detail the regions of Central Africa, Siberia, and Tibet. While there are clear differences in the various estimates, overall the results are generally consistent. Inconsistencies between the estimates may be due to a variety of reasons including lateral and depth resolution differences and the comparison of what may be different lithospheric features.

  1. Integrative Analysis of Mantle Lithosphere Rheology

    NASA Astrophysics Data System (ADS)

    Hirth, G.; Collins, J. A.; Molnar, P. H.; Kelemen, P. B.

    2014-12-01

    We will present an analysis of the rheology of mantle lithosphere based on extrapolation of lab-based flow laws, microstructural characterization of mantle shear zones and xenoliths, and the spatial distribution of mantle earthquakes and seismic anisotropy. As a starting point, we illustrate the similarity in the evolution of olivine lattice preferred orientation (LPO) for cm-scale lab samples (e.g., Zhang et al., 2000) and 100 meter-scale shear zones (e.g., Warren et al., 2008; Skemer et al., 2010). This correlation provides strong support for the extrapolation of lab data in both time and scale. The extrapolation of these results to plate-scale processes is supported by the analysis of shear wave splitting across the Alpine Fault on the South Island of New Zealand and its surrounding ocean basins (Zietlow et al., 2014). For the same region, the similarity in the fast Pn azimuth with the fast shear wave polarization directions indicates high strain deformation of relatively cold (~500-700oC) mantle lithosphere across a region 100-200 km wide (Collins and Molnar, 2014). This latter observation suggests that the lithosphere is significantly weaker than predicted by the extrapolation of dislocation creep or Peierls creep flow laws. Weakening via promotion of grain size sensitive creep mechanisms (diffusion creep and DisGBS) is likely at these conditions; however, studies of exhumed mantle shear zones generally indicate that the activation of these processes leads to strain localization at scales <<200 km. These observations motivate us to consider rheological constraints derived from geodetic studies and earthquake depths in regions where deformation of the lithosphere occurs at similar conditions. At face value, these data provide additional support for the extrapolation of lab data; the depth extent of earthquakes is consistent with estimates for the conditions where a transition from stable to unstable frictional sliding occurs (e.g., Boettcher et al., 2007) - and

  2. LAB and other lithospheric discontinuities below Cratons

    NASA Astrophysics Data System (ADS)

    Sodoudi, Forough

    2013-04-01

    Cratons are extremely stable continental areas of the Earth's crust, which have been formed and remained largely unchanged since Precambrian. However, their formation and how they survived destruction over billions of years remains a subject of debate. Seismic properties of the cratonic lithosphere reflect its composition and physical state and obtain basic constraints on processes of the formation and evolution of continents. Insight on these issues may be gained by determining the depth and the nature of the Lithosphere-Asthenosphere Boundary (LAB), which is a necessary element of the plate tectonic theory. However, It has proved quite "elusive" beneath the oldest continental areas. What is missing to date is a consensus on the feature that would correspond to the LAB and whether such a feature exists everywhere beneath cratons. The relatively recently developed S receiver function technique employing S-to-P conversions appears promising for detecting the LAB with a sufficiently high resolution and density. A growing number of regional observations obtained from S receiver function studies has detected discontinuities characterized by a significant negative velocity contrast in the upper mantle. However, challenges still remain in detecting the S-to-P conversions from the LAB beneath the Precambrian cratons. Some recent SRF studies observed a deep (> 160 km) negative velocity contrast beneath cratons and interpreted it as the LAB. For example, a deep LAB at about 250 km was reported beneath the Kalahari craton by different authors. Similar results were also obtained beneath some parts of the Canadian shield, East European Craton, Australia, the Arabian Shield and Tanzania craton. In contrast, other SRF studies found no evidence for negative discontinuities at these depths in the North American craton, in Kalahari craton or in Australia. Instead they revealed a very sharp negative velocity gradient at much shallower depth (60-150 km), leading some authors to infer

  3. The Quaternary volcanic rocks of the northern Afar Depression (northern Ethiopia): Perspectives on petrology, geochemistry, and tectonics

    NASA Astrophysics Data System (ADS)

    Hagos, Miruts; Koeberl, Christian; van Wyk de Vries, Benjamin

    2016-05-01

    The northern Afar Depression is one of the most volcano-tectonically active parts of the East African Rift system, a place where oceanic rifting may be beginning to form an incipient oceanic crust. In its center, over an area that is ∼80 km long and ∼50 km wide, there are seven major NNW-SSE-aligned shield volcanoes/volcanic edifices surrounded by compositionally distinct fissure-fed basalts. The Quaternary lavas in this area range from transitional to tholeiitic basalts, with significant across-axis variation both in mineralogy and chemistry. The variation in the contents of the major elements (TiO2, Al2O3, and Fe2O3), incompatible trace elements (Nd, Hf, Th, Ta), and the contents and ratios of the rare earth elements (REE) (e.g., (La/Yb)n = 5.3-8.9) indicate some variation in the petrogenetic processes responsible for the formation of these basalts. However, the variation in isotopic compositions of the mafic lavas is minimal (87Sr/86Sr = 0.7036-0.7041, 143Nd/144Nd = 0.51286-0.51289), which suggests only one source for all the Danakil Depression basalts. These basalts have isotope and incompatible trace element ratios that overlap with those of the Oligocene High-Ti2 flood basalts from the Ethiopian Plateau, interpreted as being derived from the last phase/tail of the Afar mantle plume source. Moreover, the Ce/Pb, Ba/U ratios indicate that the involvement of continental crust in the petrogenesis of the basaltic rocks is minimal; instead, both depth and degree of melting of the source reservoir underneath the northern Afar Depression played a major role for the production of incompatible element-enriched basalts (e.g., AleBagu Shield basalts) and the incompatible element-depleted tholeiitic basalts (e.g., Erta'Ale and Alu Shield basalts).

  4. Preface to "Insights into the Earth's Deep Lithosphere"

    SciTech Connect

    Pasyanos, M E

    2009-11-19

    Dear Readers: I am pleased to present a special issue of Tectonophysics entitled 'Insights into the Earth's Deep Lithosphere.' This compilation sought to capture the flavor of the increasing number of studies that are emerging to investigate the complex lithospheric structure of the earth. This issue evolved out of a Fall 2007 AGU special session entitled 'Understanding the Earth's Deep Lithosphere' that I organized with Irina Artemieva from the University of Copenhagen. For that session, we solicited talks that discussed the increasing number of methods that have surfaced to study various aspects of the earth's deep lithosphere. These methods include seismic, gravity, thermal, geochemical, and various combinations of these methods. The quality of the presentations (2 oral sessions with 16 talks and 23 associated poster presentations) was such that we felt that the emerging topic deserved a dedicated forum to address these questions in greater detail. The availability of new data sets has also improved the number and quality of lithospheric studies. With many new studies and methodologies, a better understanding of both continental and oceanic lithospheres is starting to emerge. Questions remain about the thickness and evolution of the lithosphere, the presence of lithospheric keels, the density and anisotropy of lithospheric roots, mechanisms of lithospheric thinning, and differences between mechanical, thermal and chemical boundary layers. While we did not get contributions on the full gamut of methods and regions, a lot of ground was covered in this issue's manuscripts. Like any collection of papers on the deep lithosphere, the topics are quite varied in methodology, geographic location, and what aspect of the lithosphere being studied. Still, the results highlight the rewarding aspects of earth structure, history, and evolution that can be gleaned. A brief synopsis of the papers contained in this issue is given.

  5. Foundering lithosphere imaged beneath the southern Sierra Nevada, California, USA.

    PubMed

    Boyd, Oliver S; Jones, Craig H; Sheehan, Anne F

    2004-07-30

    Seismic tomography reveals garnet-rich crust and mantle lithosphere descending into the upper mantle beneath the southeastern Sierra Nevada. The descending lithosphere consists of two layers: an iron-rich eclogite above a magnesium-rich garnet peridotite. These results place descending eclogite above and east of high P wave speed material previously imaged beneath the southern Great Valley, suggesting a previously unsuspected coherence in the lithospheric removal process. PMID:15286370

  6. Death valley regional ground-water flow model calibration using optimal parameter estimation methods and geoscientific information systems

    USGS Publications Warehouse

    D'Agnese, F. A.; Faunt, C.C.; Hill, M.C.; Turner, A.K.

    1999-01-01

    A regional-scale, steady-state, saturated-zone ground-water flow model was constructed to evaluate potential regional ground-water flow in the vicinity of Yucca Mountain, Nevada. The model was limited to three layers in an effort to evaluate the characteristics governing large-scale subsurface flow. Geoscientific information systems (GSIS) were used to characterize the complex surface and subsurface hydrogeologic conditions of the area, and this characterization was used to construct likely conceptual models of the flow system. Subsurface properties in this system vary dramatically, producing high contrasts and abrupt contacts. This characteristic, combined with the large scale of the model, make zonation the logical choice for representing the hydraulic-conductivity distribution. Different conceptual models were evaluated using sensitivity analysis and were tested by using nonlinear regression to determine parameter values that are optimal, in that they provide the best match between the measured and simulated heads and flows. The different conceptual models were judged based both on the fit achieved to measured heads and spring flows, and the plausibility of the optimal parameter values. One of the conceptual models considered appears to represent the system most realistically. Any apparent model error is probably caused by the coarse vertical and horizontal discretization.A regional-scale, steady-state, saturated-zone ground-water flow model was constructed to evaluate potential regional ground-water flow in the vicinity of Yucca Mountain, Nevada. The model was limited to three layers in an effort to evaluate the characteristics governing large-scale subsurface flow. Geoscientific information systems (GSIS) were used to characterize the complex surface and subsurface hydrogeologic conditions of the area, and this characterization was used to construct likely conceptual models of the flow system. Subsurface properties in this system vary dramatically, producing

  7. Study of the time evolution of the lithosphere

    NASA Technical Reports Server (NTRS)

    Roufosse, M. C.

    1983-01-01

    The behavior and mechanical properties of the lithosphere were studied. This is a prerequisite to an understanding of the mechanisms and processes that occur in the Earth's mantle, which are masked by the lithospere. Geoid heights derived from the GEOS-3 and SEASAT radar altimeters were used. The correlation between bathymetry and geoid heights gives information on the mechanical properties of the lithosphere, such as its thickness, which is related to the age of the lithospheric plate. By probing in several locations spanning various temporal situations, the time evolution of the lithospheric plates were retraced.

  8. Transient creep and convective instability of the lithosphere

    NASA Astrophysics Data System (ADS)

    Birger, Boris I.

    2012-12-01

    Laboratory experiments with rock samples show that transient creep, at which strain grows with time and strain rate decrease at constant stress, occurs while creep strains are sufficiently small. The transient creep at high temperatures is described by the Andrade rheological model. Since plate tectonics allows only small deformations in lithospheric plates, creep of the lithosphere plates is transient whereas steady-state creep, described by non-Newtonian power-law rheological model, takes place in the underlying mantle. At the transient creep, the effective viscosity, found in the study of postglacial flows, differs significantly from the effective viscosity, which characterizes convective flow, since timescales of these flows are very different. Besides, the transient creep changes the elastic crust thickness estimated within the power-law rheology of the lithosphere. Two problems of convective stability for the lithosphere with the Andrade rheology are solved. The solution of the first problem shows that the state, in which large-scale convective flow in the mantle occurs under lithospheric plates, is unstable and must bifurcate into another more stable state at which the lithospheric plates become mobile and plunge into the mantle at subduction zones. If the lithosphere had the power-law fluid rheology, the effective viscosity of the stagnant lithospheric plates would be extremely high and the state, in which large-scale convection occurs under the stagnant plates, would be stable that contradicts plate tectonics. The mantle convection forms mobile lithospheric plates if the effective viscosity of the plate is not too much higher than the effective viscosity of the underlying mantle. The Andrade rheology lowers the plate effective viscosity corresponding to the power-law fluid rheology and, thus, leads to instability of the state in which the plates are stagnant. The solution of the second stability problem shows that the state, in which the lithospheric plate

  9. Lithospheric models of the North American continent

    NASA Astrophysics Data System (ADS)

    Tesauro, Magdala; Kaban, Mikhail; Mooney, Walter; Cloetingh, Sierd

    2015-04-01

    We constructed NACr14, a 3D model of the North American (NA) crust, based on the most recent seismic data from the USGS database. In comparison with the global crustal model CRUST 1.0, NACr14 is more heterogeneous, showing a larger spatial variability of the thickness and average velocities of the crustal layers. Velocities of the lower crust vary in a larger range than those of the other layers, while the thickness of all the three layers is on average between 11 and 13 km. The largest velocities of the crystalline crust (>6.6 km/s) reflect the presence of a 7.x layer (>7.0 km/s) in the lowermost part of the crust. Using NACr2014, a regional (NA07) and a global (SL201sv) tomography model, and gravity data, we apply an iterative technique, which jointly interprets seismic tomography and gravity data, to estimate temperature and compositional variations in the NA upper mantle. The results obtained demonstrate that temperature of the cratonic mantle is up to 150°C higher than when using a uniform compositional model. The differences between the two tomography models influence the results more strongly than possible changes of the depth distribution of compositional variations. Strong negative compositional density anomalies, corresponding to Mg # >92, characterize the upper mantle of the northwestern part of the Superior craton and the central part of the Slave and Churchill craton. The Proterozoic upper mantle of the western and more deformed part of the NA cratons, appears weakly depleted (Mg# ~91) when NA07 is used, in agreement with the results based on the interpretation of xenolith data. When we use SL2013sv, the same areas are locally characterized by high density bodies, which might be interpreted as the effect due to fragments of subducted slabs, as those close to the suture of the Appalachians and Grenville province. We used the two thermal models to estimate the integrated strength and the effective elastic thickness (Te) of the lithosphere. In the

  10. Global equivalent magnetization of the oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Dyment, J.; Choi, Y.; Hamoudi, M.; Lesur, V.; Thebault, E.

    2015-11-01

    As a by-product of the construction of a new World Digital Magnetic Anomaly Map over oceanic areas, we use an original approach based on the global forward modeling of seafloor spreading magnetic anomalies and their comparison to the available marine magnetic data to derive the first map of the equivalent magnetization over the World's ocean. This map reveals consistent patterns related to the age of the oceanic lithosphere, the spreading rate at which it was formed, and the presence of mantle thermal anomalies which affects seafloor spreading and the resulting lithosphere. As for the age, the equivalent magnetization decreases significantly during the first 10-15 Myr after its formation, probably due to the alteration of crustal magnetic minerals under pervasive hydrothermal alteration, then increases regularly between 20 and 70 Ma, reflecting variations in the field strength or source effects such as the acquisition of a secondary magnetization. As for the spreading rate, the equivalent magnetization is twice as strong in areas formed at fast rate than in those formed at slow rate, with a threshold at ∼40 km/Myr, in agreement with an independent global analysis of the amplitude of Anomaly 25. This result, combined with those from the study of the anomalous skewness of marine magnetic anomalies, allows building a unified model for the magnetic structure of normal oceanic lithosphere as a function of spreading rate. Finally, specific areas affected by thermal mantle anomalies at the time of their formation exhibit peculiar equivalent magnetization signatures, such as the cold Australian-Antarctic Discordance, marked by a lower magnetization, and several hotspots, marked by a high magnetization.

  11. Distributed and localized faulting in extensional settings: Insight from the North Ethiopian Rift-Afar transition area

    NASA Astrophysics Data System (ADS)

    Soliva, Roger; Schultz, Richard A.

    2008-04-01

    Extensional fault systems in the Earth's crust can exhibit two end-member geometries that we identify as distributed and localized faulting regimes. A satellite image analysis of fault populations from the Main Ethiopian Rift-Afar area reveals that the rift architecture contains these two faulting regimes. The occurrence of these regimes reveals a jump in the scale of fault segmentation and linkage. Strain localization at rift border zones exhibits particularly large-scale fault linkage and a power law size distribution. This regime replaces prior distributed fault systems, showing small-scale fault linkage and an exponential size distribution. The distributed faulting is interpreted as confined to the thick trap basalt carapace. We show that continental fault systems can develop by a combination of these two geometries, and we demonstrate how to quantitatively decipher the jump between them.

  12. Satellite observations of the eruption of Dala Filla volcano in the Afar Depression, Ethiopia, during November 2008

    NASA Astrophysics Data System (ADS)

    Barnie, T.; Oppenheimer, C.

    2009-04-01

    The Dala Filla volcano lies in the north of the Erta Ale volcanic range in the Afar Depression, Ethiopia, and was the site of an effusive fissure eruption in November 2008. The volcano lies in a remote, sparsely populated region where access is difficult, so satellite images are the main source of information for reconstructing the eruption and estimating duration, volumes and rates. We present medium resolution thermal images acquired during the eruption to characterise its evolution and high resolution visual images acquired after the eruption to characterise the geomorphology of the resulting deposits. The significance of the eruption is assessed in the context of previous activity in the region using a revised geological interpretation of the area from satellite images.

  13. Lithospheric delamination underneath Far East Russia

    NASA Astrophysics Data System (ADS)

    Ntaflos, Theodoros; Koutsovitis, Petros; Aschchepkov, Igor; Hauzenberger, Christoph; Prikhodko, Vladimir; Asseva, Anna

    2013-04-01

    In the back-arc environment of Far East Russia, mantle xenoliths from Sikhoti-Alin( Komku area, KO) and Primorie (Sviyaginsky area, SV), Far East Russia are fertile spinel lherzolites with traces of amphibole, phlogopite and hyalophane in some of the studied samples. Though samples from both localities are fertile there is a systematic difference in their fertility. The KO samples have mg# varying from 0.891 to 0.899 and are slightly more fertile than the SV samples that have mg# ranging from 0.898 to 0.904. LA-ICP-MS analyses on clinopyroxenes confirm this trend as the (La/Yb)N in KO samples range from 0.1to 1.0 and in SV samples from 0.15 to 1.73. The estimated equilibration temperatures for the KO suite range from 940 °C to 1035 °C and for the SV suite from 770 to 945. The differences in the estimated equilibrium temperatures between the KO and SV suites suggest that the less fertile SV suite originated in shallower depths than the more fertile KO suite. Pargasitic amphibole, kaersutite, and extremely Ti-rich phlogopite, up to 14 wt% TiO2, are associated with intergranular glass indicating clearly metasomatism of undersaturated hydrous alkaline melts. Incompatible element abundances, besides Ba, Sr and Ti that are slightly enriched in the amphibole, are similar in both phases suggesting minor metasomatism due to percolation of small amounts of water-rich fluids. The Sr and Nd cpx isotopic ratios range from 0.702599 to 0.703567 and 0.512915 to 513153, repectively and the model Nd isotope age range from 1.5 to 2.2 Ga indicating an old (Proterozoic?) partial melt event. The lithospheric mantle beneath the studied area represents the residue after partial melting of up to 2 % of a primitive mantle and is comparable to that of Mongolia. Despite the fact that the studied area experienced several subducting episodes, the lithospheric mantle appears to be unaffected from the upwelling fluids/melts of the subducted slab(s). Since there is no indication for plume

  14. The mantle transition zone beneath the Afar Depression and adjacent regions: Implications for mantle plumes and hydration

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

    The Afar Depression and its adjacent areas are underlain by an upper mantle marked by some of the world's largest negative velocity anomalies, which are frequently attributed to the thermal influences of a lower-mantle plume. In spite of numerous studies, however, the existence of a plume beneath the area remains enigmatic, partially due to inadequate quantities of broadband seismic data and the limited vertical resolution at the mantle transition zone (MTZ) depth of the techniques employed by previous investigations. In this study, we use an unprecedented quantity (over 14,500) of P-to-S receiver functions (RFs) recorded by 139 stations from 12 networks to image the 410 and 660 km discontinuities and map the spatial variation of the thickness of the MTZ. Non-linear stacking of the RFs under a 1-D velocity model shows robust P-to-S conversions from both discontinuities, and their apparent depths indicate the presence of an upper mantle low-velocity zone (LVZ) beneath the entire study area. The Afar Depression and the northern Main Ethiopian Rift are characterized by an apparent 40-60 km depression of both MTZ discontinuities and a normal MTZ thickness. The simplest and most probable interpretation of these observations is that the apparent depressions are solely caused by velocity perturbations in the upper mantle and not by temperature or hydration anomalies within the MTZ. Thickening of the MTZ on the order of 15 km beneath the southern Arabian Plate, southern Red Sea and western Gulf of Aden, which comprise the southward extension of the Afro-Arabian Dome, could reflect long-term hydration of the MTZ. A 20 km thinning of the MTZ beneath the western Ethiopian Plateau is observed and is interpreted as evidence for a possible mantle plume stem originating from the lower mantle.

  15. Assessment of Late Quaternary strain partitioning in the Afar Triple Junction: Dobe and Hanle grabens, Ethiopia and Djibouti

    NASA Astrophysics Data System (ADS)

    Polun, S. G.; Stockman, M. B.; Hickcox, K.; Horrell, D.; Tesfaye, S.; Gomez, F. G.

    2015-12-01

    As the only subaerial exposure of a ridge - ridge - ridge triple junction, the Afar region of Ethiopia and Djibouti offers a rare opportunity to assess strain partitioning within this type of triple junction. Here, the plate boundaries do not link discretely, but rather the East African rift meets the Red Sea and Gulf of Aden rifts in a zone of diffuse normal faulting characterized by a lack of magmatic activity, referred to as the central Afar. An initial assessment of Late Quaternary strain partitioning is based on faulted landforms in the Dobe - Hanle graben system in Ethiopia and Djibouti. These two extensional basins are connected by an imbricated accommodation zone. Several fault scarps occur within terraces formed during the last highstand of Lake Dobe, around 5 ka - they provide a means of calibrating a numerical model of fault scarp degradation. Additional timing constraints will be provided by pending exposure ages. The spreading rates of both grabens are equivalent, however in Dobe graben, extension is partitioned 2:1 between northern, south dipping faults and the southern, north dipping fault. Extension in Hanle graben is primarily focused on the north dipping Hanle fault. On the north margin of Dobe graben, the boundary fault bifurcates, where the basin-bordering fault displays a significantly higher modeled uplift rate than the more distal fault, suggesting a basinward propagation of faulting. On the southern Dobe fault, surveyed fault scarps have ages ranging from 30 - 5 ka with uplift rates of 0.71, 0.47, and 0.68 mm/yr, suggesting no secular variation in slip rates from the late Plestocene through the Holocene. These rates are converted into horizontal stretching estimates, which are compared with regional strain estimated from velocities of relatively sparse GPS data.

  16. Receiver function constraints on crustal seismic velocities and partial melting beneath the Red Sea rift and adjacent regions, Afar Depression

    NASA Astrophysics Data System (ADS)

    Reed, Cory A.; Almadani, Sattam; Gao, Stephen S.; Elsheikh, Ahmed A.; Cherie, Solomon; Abdelsalam, Mohamed G.; Thurmond, Allison K.; Liu, Kelly H.

    2014-03-01

    The Afar Depression is an ideal locale for the investigation of crustal processes involved in the transition from continental rifting to oceanic spreading. To provide relatively high resolution images of the crust beneath the Red Sea rift (RSR) represented by the Tendaho graben in the Afar Depression, we deployed an array of 18 broadband seismic stations in 2010 and 2011. Stacking of about 2300 receiver functions from the 18 and several nearby stations along the ~200 km long array reveals an average crustal thickness of 22±4 km, ranging from ~17 km near the RSR axis to 30 km within the overlap zone between the Red Sea and Gulf of Aden rifts. The resulting anomalously high Vp/Vs ratios decrease from 2.40 in the southwest to 1.85 within the overlap zone. We utilize theoretical Vp and melt fraction relationships to obtain an overall highly reduced average crustal Vp of ~5.1 km/s. The melt percentage is about 10% beneath the RSR while the overlap zone contains minor quantities of partial melt. The observed high Vp/Vs values beneath most of the study area indicate widespread partial melting beneath the southwest half of the profile, probably as a result of gradual eastward migration of the RSR axis. Our results also suggest that the current extensional strain in the lower crust beneath the region is diffuse, while the strain field in the upper crust is localized along narrow volcanic segments. These disparate styles of deformation imply a high degree of decoupling between the upper and lower crust.

  17. Mapping current and potential distribution of non-native Prosopis juliflora in the Afar region of Ethiopia.

    PubMed

    Wakie, Tewodros T; Evangelista, Paul H; Jarnevich, Catherine S; Laituri, Melinda

    2014-01-01

    We used correlative models with species occurrence points, Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation indices, and topo-climatic predictors to map the current distribution and potential habitat of invasive Prosopis juliflora in Afar, Ethiopia. Time-series of MODIS Enhanced Vegetation Indices (EVI) and Normalized Difference Vegetation Indices (NDVI) with 250 m2 spatial resolution were selected as remote sensing predictors for mapping distributions, while WorldClim bioclimatic products and generated topographic variables from the Shuttle Radar Topography Mission product (SRTM) were used to predict potential infestations. We ran Maxent models using non-correlated variables and the 143 species- occurrence points. Maxent generated probability surfaces were converted into binary maps using the 10-percentile logistic threshold values. Performances of models were evaluated using area under the receiver-operating characteristic (ROC) curve (AUC). Our results indicate that the extent of P. juliflora invasion is approximately 3,605 km2 in the Afar region (AUC  = 0.94), while the potential habitat for future infestations is 5,024 km2 (AUC  = 0.95). Our analyses demonstrate that time-series of MODIS vegetation indices and species occurrence points can be used with Maxent modeling software to map the current distribution of P. juliflora, while topo-climatic variables are good predictors of potential habitat in Ethiopia. Our results can quantify current and future infestations, and inform management and policy decisions for containing P. juliflora. Our methods can also be replicated for managing invasive species in other East African countries. PMID:25393396

  18. Mapping Current and Potential Distribution of Non-Native Prosopis juliflora in the Afar Region of Ethiopia

    PubMed Central

    Wakie, Tewodros T.; Evangelista, Paul H.; Jarnevich, Catherine S.; Laituri, Melinda

    2014-01-01

    We used correlative models with species occurrence points, Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation indices, and topo-climatic predictors to map the current distribution and potential habitat of invasive Prosopis juliflora in Afar, Ethiopia. Time-series of MODIS Enhanced Vegetation Indices (EVI) and Normalized Difference Vegetation Indices (NDVI) with 250 m2 spatial resolution were selected as remote sensing predictors for mapping distributions, while WorldClim bioclimatic products and generated topographic variables from the Shuttle Radar Topography Mission product (SRTM) were used to predict potential infestations. We ran Maxent models using non-correlated variables and the 143 species- occurrence points. Maxent generated probability surfaces were converted into binary maps using the 10-percentile logistic threshold values. Performances of models were evaluated using area under the receiver-operating characteristic (ROC) curve (AUC). Our results indicate that the extent of P. juliflora invasion is approximately 3,605 km2 in the Afar region (AUC  = 0.94), while the potential habitat for future infestations is 5,024 km2 (AUC  = 0.95). Our analyses demonstrate that time-series of MODIS vegetation indices and species occurrence points can be used with Maxent modeling software to map the current distribution of P. juliflora, while topo-climatic variables are good predictors of potential habitat in Ethiopia. Our results can quantify current and future infestations, and inform management and policy decisions for containing P. juliflora. Our methods can also be replicated for managing invasive species in other East African countries. PMID:25393396

  19. Merging Disparate Data Sources Into a Paleoanthropological Geodatabase for Research, Education, and Conservation in the Greater Hadar Region (Afar, Ethiopia)

    NASA Astrophysics Data System (ADS)

    Campisano, C. J.; Dimaggio, E. N.; Arrowsmith, J. R.; Kimbel, W. H.; Reed, K. E.; Robinson, S. E.; Schoville, B. J.

    2008-12-01

    Understanding the geographic, temporal, and environmental contexts of human evolution requires the ability to compare wide-ranging datasets collected from multiple research disciplines. Paleoanthropological field- research projects are notoriously independent administratively even in regions of high transdisciplinary importance. As a result, valuable opportunities for the integration of new and archival datasets spanning diverse archaeological assemblages, paleontological localities, and stratigraphic sequences are often neglected, which limits the range of research questions that can be addressed. Using geoinformatic tools we integrate spatial, temporal, and semantically disparate paleoanthropological and geological datasets from the Hadar sedimentary basin of the Afar Rift, Ethiopia. Applying newly integrated data to investigations of fossil- rich sediments will provide the geospatial framework critical for addressing fundamental questions concerning hominins and their paleoenvironmental context. We present a preliminary cyberinfrastructure for data management that will allow scientists, students, and interested citizens to interact with, integrate, and visualize data from the Afar region. Examples of our initial integration efforts include generating a regional high-resolution satellite imagery base layer for georeferencing, standardizing and compiling multiple project datasets and digitizing paper maps. We also demonstrate how the robust datasets generated from our work are being incorporated into a new, digital module for Arizona State University's Hadar Paleoanthropology Field School - modernizing field data collection methods, on-the-fly data visualization and query, and subsequent analysis and interpretation. Armed with a fully fused database tethered to high-resolution satellite imagery, we can more accurately reconstruct spatial and temporal paleoenvironmental conditions and efficiently address key scientific questions, such as those regarding the

  20. The mantle transition zone beneath the Afar Depression and adjacent regions: implications for mantle plumes and hydration

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

    The Afar Depression and its adjacent areas are underlain by an upper mantle marked by some of the world's largest negative velocity anomalies, which are frequently attributed to the thermal influences of a lower-mantle plume. In spite of numerous studies, however, the existence of a plume beneath the area remains enigmatic, partially due to inadequate quantities of broad-band seismic data and the limited vertical resolution at the mantle transition zone (MTZ) depth of the techniques employed by previous investigations. In this study, we use an unprecedented quantity (over 14 500) of P-to-S receiver functions (RFs) recorded by 139 stations from 12 networks to image the 410 and 660 km discontinuities and map the spatial variation of the thickness of the MTZ. Non-linear stacking of the RFs under a 1-D velocity model shows robust P-to-S conversions from both discontinuities, and their apparent depths indicate the presence of an upper-mantle low-velocity zone beneath the entire study area. The Afar Depression and the northern Main Ethiopian Rift are characterized by an apparent 40-60 km depression of both MTZ discontinuities and a normal MTZ thickness. The simplest and most probable interpretation of these observations is that the apparent depressions are solely caused by velocity perturbations in the upper mantle and not by deeper processes causing temperature or hydration anomalies within the MTZ. Thickening of the MTZ on the order of 15 km beneath the southern Arabian Plate, southern Red Sea and western Gulf of Aden, which comprise the southward extension of the Afro-Arabian Dome, could reflect long-term hydration of the MTZ. A 20 km thinning of the MTZ beneath the western Ethiopian Plateau is observed and interpreted as evidence for a possible mantle plume stem originating from the lower mantle.

  1. Examining the Causes of Low-frequency Hybrid Earthquakes During Dike Intrusions in the Afar Rift, Ethiopia

    NASA Astrophysics Data System (ADS)

    Tepp, G.; Ebinger, C. J.; Belachew, M.

    2014-12-01

    Between 2005 and 2012, there were 14 large dike intrusions into the Dabbahu rift segment in the Afar rift, Ethiopia. Swarms of earthquakes with local magnitudes between 1.45Afar hybrids are largely a result of path effects based on high attenuation (Q ~ 200) and azimuthal dependence of spectral content. However, large (~3m) surface displacements on short faults indicate that unusual source processes, such as slow rupture times, may also be a factor in these hybrid events. The aims of this study are to distinguish between path and source effects, to characterize the source processes of these events, and to explore the relation between hybrid and normal tectonic events in the region - are the differences in the source or only in the path? For closely located earthquakes, an Empirical Green's Function approach is a great method to isolate the source-time function. Spectral analysis of the source-time function can be used to provide insights into the rupture time, stress drop, and scaling relations of the earthquakes. These results will be used to further refine earthquake classifications and determine if there are any defining characteristics of the classes that associate them with specific faulting processes, such as surface

  2. Investigation of volcanic gas analyses and magma outgassing from Erta' Ale lava lake, Afar, Ethiopia

    NASA Astrophysics Data System (ADS)

    Gerlach, T. M.

    1980-05-01

    compositions. This change is qualitatively compatible with magma outgassing controlled by solubility differences and could reflect a longer term trend. The chemistry of the Erta' Ale gases is consistent with models invoking the generation of basalts resembling mid-ocean ridge tholeiites along axial zones of crustal spreading in the Afar Triangle. The CO 2-rich vesicle gases of submarine basalts and the relatively H 2O-rich volcanic gases of Erta'Ale appear to be end-member compositions of an outgassing trend controlled largely by the effect of pressure on volatile solubilities in tholeiitic basalts from spreading zones. By analogy with submarine basalts H 2O and sulfur in the Erta' Ale gases are mainly contributed by magma outgassing down to shallow depths (50-250 m); CO 2 contributions probably come from larger volumes of magma down to depths of at least 1000 m.

  3. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

    Levander, A.; Bezada, M. J.; Niu, F.; Palomeras, I.; Thurner, S.; Humphreys, E.; Miller, M. S.; Carbonell, R.; Gallart, J.; Schmitz, M.

    2014-12-01

    While subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, recycling continental lithosphere appears far more complicated and is less well understood. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we describe another process that can lead to the loss of continental lithosphere adjacent to a subduction zone: Subducting oceanic plates can entrain and recycle lithospheric mantle from an adjacent continent and disrupt the continental lithosphere far inland from the subduction zone. Seismic images from recent dense broadband arrays on opposite sides of the Atlantic show higher than expected volumes of positive anomalies identified as the subducted Atlantic (ATL) slab under northeastern South America (SA), and the Alboran slab beneath the Gibraltar arc region (GA). The positive anomalies lie under and are aligned with the continental margins at depths greater than 200 km. Closer to the surface we find that the continental margin lithospheric mantle is significantly thinner than expected beneath the orogens adjacent to the subduction zones. Thinner than expected lithosphere extends inland as far as the edges of nearby cratonic cores. These observations suggest that subducting oceanic plates viscously entrain and remove continental mantle lithosphere from beneath adjacent continental margins, modulating the surface tectonics and pre-conditioning the margins for further deformation. The latter can include delamination of the entire lithospheric mantle, as around GA, inferred by results from active and passive seismic experiments. Secondary downwellings develop under the continental interior inland from the subduction zone: We image one under SA and one or more in the past were likely under GA. The process of subduction-driven continental margin lithosphere removal reconciles numerous, sometimes mutually

  4. Lithospheric Mantle Deformation beneath the Indian Cratons.

    PubMed

    Pandey; Agrawal

    1999-11-01

    The nature of deformation of the deep continental roots beneath the Archean-Early Proterozoic terrains opens the question whether these ancient terrains have had stable roots since the Precambrian or whether recent plate motions have deformed them. In view of this, we make an attempt to study the thermal structure beneath the cratonic regions of the Indian shield, which vary in lithospheric thickness from 65 km in the Singhbhum craton to 148 km in the Archean Dharwars. The average depth of 104 km to the top of the underlying asthenosphere is consistent with other termination methods and is in fact less than half the 200-400-km depth found in other stable areas of the earth. Similarly, the average reduced heat flow of about 35 mW/m2 and Moho temperature of about 550 degrees C (range: 400 degrees -730 degrees C) for the Indian cratons are also much higher than their counterparts elsewhere. Our study indicates a large-scale deformation of the cratonic mantle lithosphere beneath the Indian shield since the Mesoproterozoic caused by various geodynamic causes, challenging the idea of stability of deep continental roots. PMID:10517883

  5. Sedimentary loading, lithospheric flexure and subduction initiation at passive margins

    SciTech Connect

    Erickson, S.G. . Dept. of Earth Sciences)

    1992-01-01

    Recent theoretical models have demonstrated the difficulty of subduction initiation at passive margins, whether subduction is assumed to initiate by overcoming the shear resistance on a thrust fault through the lithosphere or by failure of the entire lithosphere in bending due to sedimentary loading. A mechanism for subduction initiation at passive margins that overcomes these difficulties incorporates the increased subsidence of a marginal basin during decoupling of a previously locked margin. A passive margin may decouple by reactivation of rift-related faults in a local extensional or strike-slip setting. Flexure of marginal basins by sedimentary loading is modeled here by the bending of infinite and semi-infinite elastic plates under a triangular load. The geometry of a mature marginal basin fits the deflection produced by loading of an infinite plate in which the flexural rigidity of continental lithosphere is larger than that of oceanic lithosphere. Decoupling of such a locked passive margin by fault reactivation may cause the lithospheric bending behavior of the margin to change from that of an infinite plate to that of a semi-infinite plate, with a resultant increase in deflection of the marginal basin. The increase in deflection depends on the flexural rigidities of continental and oceanic lithosphere. For flexural rigidities of 10[sup 30]-10[sup 31] dyn-cm (elastic lithosphere thicknesses 24--51 km), the difference in deflections between infinite and semi-infinite plates is 15--17 km, so that decoupling sinks the top of the oceanic lithosphere to depths of ca 35 km. Additional sedimentation within the basin and phase changes within the oceanic crust may further increase this deflection. Subduction may initiate if the top of the oceanic lithosphere sinks to the base of the adjacent elastic lithosphere.

  6. The model of lithospheric thickness beneath China from gravity data

    NASA Astrophysics Data System (ADS)

    Zhang, H.; Ravat, D.

    2015-12-01

    We compare estimates of lithospheric thickness from several studies in China and examine whether the available gravity field anomalies can constrain these estimates. Ma (1987) suggested based on integrated geophysics that the lithospheric thickness varies from ~130 km in Qinling Dabie orogenic belt to ~60 km in Beijing, and ~50 km in Bohai bay. Lebedev and Nolet (2003) determined the lithospheric thickness in Bohai bay to be ~140 km from S wave tomography. Sodoudi et al.'s (2006) estimate of the lithospheric thickness is 72 km in Qinling Dabie orogenic belt and ~60 km in north China block. Since physical character differences exist between lithosphere and asthenosphere, it is possible to determine the thickness of lithospheric though gravity data. In this study, we use the crustal thickness obtained from teleseismic receiver functions (Li et al., 2014) to model the Moho gravity field variation and then remove this variation from the observed gravity field. Based on the residual field, the lithospheric thickness is obtained by the Parker inversion. Results show that the lithospheric thickness beneath China varies from ~80 km in the north of XinJiang to ~140 km in Tibet, and it changes to ~100 km in Eastern China. The residual field used for inversion is smooth which results in a smooth lithosphere-asthenosphere boundary (LAB). The LAB is generally in agreement with the previous seismic inversion result along profiles in eastern China (e.g. Li et al., 2011) and suggests that our method could be used to estimate the regional lithospheric variation in other areas in China, and somewhere else.

  7. Subduction-driven recycling of continental margin lithosphere.

    PubMed

    Levander, A; Bezada, M J; Niu, F; Humphreys, E D; Palomeras, I; Thurner, S M; Masy, J; Schmitz, M; Gallart, J; Carbonell, R; Miller, M S

    2014-11-13

    Whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, the recycling of continental lithosphere appears to be far more complicated and less well understood. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings. We have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted Atlantic slab under northeastern South America and the Alboran slab beneath the Gibraltar arc region; the anomalies are under, and are aligned with, the continental margins at depths greater than 200 kilometres. Rayleigh wave analysis finds that the lithospheric mantle under the continental margins is significantly thinner than expected, and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores. Taking these data together, here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone. Subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins. This drives surface tectonics and pre-conditions the margins for further deformation by creating topography along the lithosphere-asthenosphere boundary. This can lead to development of secondary downwellings under the continental interior, probably under both South America and the Gibraltar arc, and to delamination of the entire lithospheric mantle, as around the Gibraltar arc. This process reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these subduction zones

  8. Perennial plate tectonics with lasting mantle lithosphere scars

    NASA Astrophysics Data System (ADS)

    Heron, P.; Pysklywec, R. N.; Stephenson, R.

    2015-12-01

    Although the conventional theory of plate tectonics can explain non-rigid behaviour at plate boundaries, it cannot adequately explain the processes involved in deformation and seismicity within plate interiors. Here, we consider that the pre-existing deformation or "scarring" within the mantle lithosphere may have a very long lived presence that could incorporate deformation of the plate interior and plate boundary. Mantle lithosphere scars from continent-continent collisions could generate virtual plate boundaries that remain over long timescales, producing "perennial" plate tectonics. Local geophysical studies can map the crustal environment well, and global whole mantle tomography models are rapidly improving, yet high-resolution images of the mantle lithosphere are often not available in regions where scarring may be present. Where mantle lithosphere heterogeneities have been observed (usually interpreted simply as subduction scars), the same attention has not been afforded to them as, for example, re-activation of faults within the Earth's crust. In idealized numerical simulations, we compare how relic scarring at varying depths in the lithosphere affects patterns of deformation. High-resolution thermal-mechanical numerical experiments explore continental lithospheric deformation featuring a weakened crust and mantle lithosphere scars. Our models show that deep lithospheric scars can control the tectonic evolution of a region over shallow geological features, indicating the importance of mantle lithosphere heterogeneities. The Altyn Tagh Fault (ATF) in central China is an example of an ancient continental collision zone that undergoes periodic deformation during times of regional compression. We suggest that the ATF may be a locale where a long-lasting mantle lithosphere scar can control the subsequent crustal evolution and deformation, with ancient plate boundaries having a "perennial" plate tectonic presence.

  9. Temperature-dependent transient creep and dynamics of cratonic lithosphere

    NASA Astrophysics Data System (ADS)

    Birger, Boris I.

    2013-11-01

    Large-scale mantle convection forms the upper boundary layer (lithosphere) where the vertical temperature drop is about 1300 K. Theoretical rheology and laboratory experiments with rock samples show that transient creep occurs while creep strains are sufficiently small. The transient creep is described by the temperature-dependent Andrade rheological model. Since plate tectonics allows only small deformations in lithospheric plates, creep of the lithosphere plates is transient whereas steady-state creep, described by non-Newtonian power-law rheological model, takes place in the underlying mantle. The solution of stability problem shows that the lithosphere is stable but small-scale convective oscillations are attenuated very weakly in regions of thickened lithosphere beneath continental cratons (subcratonic roots) where the thickness of the lithosphere is about 200 km. These oscillations create small-scale convective cells (the horizontal dimensions of the cells are of the order of the subcratonic lithosphere thickness). Direction of motion within the cells periodically changes (the period of convective oscillations is of the order of 3 × 108 yr). In this study, the oscillations of cratonic lithosphere caused by initial relief perturbation are considered. This relief perturbation is assumed to be created by overthrusting in orogenic belts surrounding cratons. The perturbation of the Earth's surface relief leads to a fast isothermal process of isostatic recovery. In the presence of vertical temperature gradient, vertical displacements, associated with the recovery process in the lithosphere interior, instantly produce the initial temperature perturbations exciting thermoconvective oscillations in the cratonic lithosphere. These small-amplitude convective oscillations cause oscillatory crustal movements which form sedimentary basins on cratons.

  10. Lithosphere mapping beneath the North American plate

    NASA Astrophysics Data System (ADS)

    Griffin, W. L.; O'Reilly, Suzanne Y.; Doyle, B. J.; Pearson, N. J.; Coopersmith, H.; Kivi, K.; Malkovets, V.; Pokhilenko, N.

    2004-09-01

    Major- and trace-element analyses of garnets from heavy-mineral concentrates have been used to derive the compositional and thermal structure of the subcontinental lithospheric mantle (SCLM) beneath 16 areas within the core of the ancient Laurentian continent and 11 areas in the craton margin and fringing mobile belts. Results are presented as stratigraphic sections showing variations in the relative proportions of different rock types and metasomatic styles, and the mean Fo content of olivine, with depth. Detailed comparisons with data from mantle xenoliths demonstrate the reliability of the sections. In the Slave Province, the SCLM in most areas shows a two-layer structure with a boundary at 140-160 km depth. The upper layer shows pronounced lateral variations, whereas the lower layer, after accounting for different degrees of melt-related metasomatism, shows marked uniformity. The lower layer is interpreted as a subcreted plume head, added at ca. 3.2 Ga; this boundary between the layers rises to <100 km depth toward the northern and southern edges of the craton. Strongly layered SCLM suggests that plume subcretion may also have played a role in the construction of the lithosphere beneath Michigan and Saskatchewan. Outside the Slave Province, most North American Archon SCLM sections are less depleted than similar sections in southern Africa and Siberia; this may reflect extensive metasomatic modification. In E. Canada, the degree of modification increases toward the craton margin, and the SCLM beneath the Kapuskasing Structural Zone is typical of that beneath Proterozoic to Phanerozoic mobile belts. SCLM sections from several Proterozoic areas around the margin of the Laurentian continental core (W. Greenland, Colorado-Wyoming district, Arkansas) show discontinuities and gaps that are interpreted as the effects of lithosphere stacking during collisional orogeny. Some areas affected by Proterozoic orogenesis (Wyoming Craton, Alberta, W. Greenland) appear to retain

  11. Cratonic lithosphere: an electrifying view (Invited)

    NASA Astrophysics Data System (ADS)

    Jones, A. G.

    2013-12-01

    Deep-probing electromagnetic studies reveal the vertical and lateral electrical conductivity structure of cratonic lithosphere. At lithospheric temperatures and for silicate mantle minerals, semi-conduction is from small polaron hopping and, in the presence of water, proton conduction, both of which is thermally-driven and can be described by an Arrhenius equation. There is little compositional sensitivity, save for the far higher water contents prevalent in pyroxenes (typically 200-400 wt ppm) compared to olivine (typically 10-100 wt ppm), with the latter exhibiting a depth dependence and the former none. Seismological methods are sensitive to temperature and composition, and virtually insensitive to low amounts of water. Taken together, the two are highly complementary. Seismological and magnetotelluric studies across Southern Africa can be employed together to constraint temperature and water content, where there is a significant difference at 100 km depth between the Kaapvaal Craton compared to the Angola (Congo) Craton compared to the Zimbabwe Craton. The Congo Craton is driest but hottest, the Kaapvaal Craton exhibits laterally-varying water content and is coldest, and the Zimbabwe Craton is the wettest and intermediate in temperature. Such thermal and water content variation results in lateral rheological variation, with the wettest and warmest being more deformable. For the Canadian Shield, there is again significant lateral variation at 100 km, with the Superior Province being mostly dry, and the Slave Craton and Rae/Hearne Provinces being far wetter. By 200 km however, all of the Canadian Shield is dry, consistent with the xenolith-based observations from the Kaapvaal Craton of far higher water contents in olivine at 100 km (50-100 wt ppm) decreasing with depth to around 10 wt ppm at 200 km. The lithosphere beneath the Kimberley area of the Kaapvaal Craton has been forward modeled in a manner that is self-consistent not only with the seismological and

  12. Electromagnetic imaging of lithosphere permeable zones

    NASA Astrophysics Data System (ADS)

    Litvinova, Tamara; Petrova Petrova, Alevtina; Petrishchev Petrishchev, Maxim

    2014-05-01

    By way of strong minima of magnetic anomalies studies we are investigated the features of the lithosphere structure by magnetic and gravity data. Exploration methods included the application of existing and open source near-surface aeromagnetic (WDMAM) with satellite data both at 100 km and 400 km in altitude (CHAMP) and gravity satellite data (GRACE). Aeromagnetic data have been used for the 2D geomagnetic model for a depth range from 3 to 50 km plotting. Gravity data has allowed to study the 2D density model for a depth range from 5 to 200 km plotting. At the heart of the geomagnetic and density model plotting lies the technique of the spectral-spatial representation of a geomagnetic field converted in a deep geomagnetic model. The technique of the spectral-spatial analysis (SPAN) is used to differentiate the weakly magnetic heterogeneities within the basement. In this paper we have studied the structure of the lithosphere in the area of deep magnetic minima in the vicinity of the eastern part of the Fennoscandian Shield, Central Europe and the northern part of South America. We have found powerful (more than 10 km) permeable feeble magnetic zones in the middle crust (20-30 km in depth) that are detected as feebly magnetic layer using the geomagnetic data. The magnetic minimum at 100 and 400 km in altitude corresponds to this feeble magnetic layer. It stands out as the low density layer at depth 20-35 km and, after the break, at depth 60-100 km. Ground-based magnetotelluric survey has allowed to allocate the high-conductivity layer at depth 15-30 and 60-110 km. It suggests that the detected layers can be rheological weak. The same is for the regions of Central Europe and South America. The powerful feebly magnetic layers have been detected in the middle and bottom crust (30-50 km for the Central Europe and 30-40 km for South America). The low density layers have been found for 20-35 km and 50-80 km in depth. The ground based measurement has confirmed the presence

  13. Lithospheric Instabilities within the Northern Basin and Range Province

    NASA Astrophysics Data System (ADS)

    Porter, R. C.; Fouch, M. J.

    2012-12-01

    Flat slab subduction and the subsequent removal of the subducting Farallon slab beneath the western United States has had a profound impact on the state of the North American lithosphere. In order to provide new constraints on the structure and evolution of the region's crust and upper mantle, we use surface wave tomography and receiver functions to image the earth beneath the Northern Basin and Range (NBR) and surrounding regions. We combine these results with published geophysical and geochemical data to further characterize lithospheric and asthenospheric processes and relate these to geological observations at the surface. Our initial results show high-velocity upper mantle, interpreted as lithosphere, beneath the center of the NBR and thinner lithosphere along its western, southern, and eastern margins. The zone of thickest lithosphere corresponds to areas with relatively thick crust, high elevations, and an absence of historic seismicity greater than magnitude 5.0. This region of thicker lithosphere also underlies a zone of reduced volcanic rock exposure relative to surrounding regions. Further, within the region there are no volcanic rocks in the NAVDAT database younger than 10 Ma, with the exception of Lunar Craters located in the south-central NBR. The shallow lithosphere-asthenosphere boundary observed along the margins of the NBR suggests that significant lithospheric thinning has occurred in these areas. This thinning is likely related to either extensional shear stresses or to the removal of lithospheric material, perhaps leading to accentuated strain along the margins of the Basin and Range. We interpret these data in the context of gravitational instabilities resulting in the removal of lithospheric material. A lithospheric downwelling has previously been identified within the center of the NBR based on high-velocity upper-mantle material observed in body wave tomography, a zone of weak or absent horizontal anisotropic fabric observed in shear

  14. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

    Levander, Alan; Bezada, Maximiliano; Niu, Fenglin; Palomeras, Imma; Thurner, Sally; Humphreys, Eugene; Carbonell, Ramon; Gallart, Josep; Schmitz, Michael; Miller, Meghan

    2015-04-01

    Subduction recycling of oceanic lithosphere, a central theme of plate tectonics, is relatively well understood, whereas recycling continental lithosphere is more difficult to recognize, and appears far more complicated. Delamination and localized convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we describe another process that can lead to the loss of continental lithosphere adjacent to a subduction zone: Subducting oceanic plates can entrain and recycle lithospheric mantle from an adjacent continent and disrupt the continental lithosphere far inland from the subduction zone. Seismic images from recent dense broadband seismograph arrays in northeastern South America (SA) and in the western Mediterranean show higher than expected volumes of positive anomalies identified as the subducted Atlantic slab under northeastern SA, and the Alboran slab beneath the Gibraltar arc region (GA). The positive anomalies lie under and are aligned with the continental margins at depths greater than 200 km. Closer to the surface we find that the continental margin lithospheric mantle is significantly thinner than expected beneath the orogens adjacent to the subduction zones. The thinner than expected lithosphere extends inland as far as the edges of nearby cratonic cores. These observations suggest that subducting oceanic plates viscously entrain and remove continental mantle lithosphere from beneath adjacent continental margins, modulating the surface tectonics and pre-conditioning the margins for further deformation. The latter can include delamination of the entire lithospheric mantle, as around GA, inferred by results from active and passive seismic experiments. Viscous removal of continental margin lithosphere creates lithosphere-asthenosphere boundary (LAB) topography which can give rise to secondary downwellings under the continental interior far inland from the subduction

  15. Limiting depth of magnetization in cratonic lithosphere

    NASA Technical Reports Server (NTRS)

    Toft, Paul B.; Haggerty, Stephen E.

    1988-01-01

    Values of magnetic susceptibility and natural remanent magnetization (NRM) of clino-pyroxene-garnet-plagioclase granulite facies lower crustal xenoliths from a kimberlite in west Africa are correlated to bulk geochemistry and specific gravity. Thermomagnetic and alternating-field demagnetization analyses identify magnetite (Mt) and native iron as the dominant magnetic phases (totaling not more than 0.1 vol pct of the rocks) along with subsidiary sulfides. Oxidation states of the granulites are not greater than MW, observed Mt occurs as rims on coarse (about 1 micron) Fe particles, and inferred single domain-pseudosingle domain Mt may be a result of oxidation of fine-grained Fe. The deepest limit of lithospheric ferromagnetism is 95 km, but a limit of 70 km is most reasonable for the West African Craton and for modeling Magsat anomalies over exposed Precambrian shields.

  16. Arctic Ocean Databases With a Dual Use: Geoscientific Investigations and the Delimitation of the Outer Continental Shelf

    NASA Astrophysics Data System (ADS)

    Macnab, R.; Jakobsson, M.; Glebovsky, V.; Poselov, V.; Kovacs, L.

    2005-12-01

    For decades, a lack of information from the Arctic Ocean stymied investigators seeking to understand the geological framework and the tectonic history of the region. The reasons were environmental and political. Climate and permanent ice cover presented significant obstacles to mapping and sampling missions, while competing strategic interests militated against open research and the free exchange of information. The political dimension of this situation changed in the 1990s, and conditions in the Arctic Ocean are now more favourable to open research. Ice and climate still present obstacles, but in the form of technical challenges to be overcome through international collaboration. A clearer picture is emerging of major geological and tectonic characteristics, such as depth and shape of the seabed, thickness and distribution of underlying sediments, and variations in the gravity and magnetic fields. This information is complemented by a growing accumulation of bottom samples that shed light on the nature of sedimentary deposits on and beneath the seafloor. Where political considerations once constrained the acquisition and dissemination of data, they now promote cooperative activities to unlock the geological and tectonic secrets that remain. Article 76 of UNCLOS outlines the criteria and a methodology for qualified coastal states to project their sovereign rights beyond the usual EEZ limit. Based on the analysis and interpretation of bathymetric and geological information, these developments have prompted fresh efforts to enhance existing databases through the accumulation and assimilation of new observations. This presentation will review existing public geoscientific databases that describe the characteristics of the Arctic Ocean basin. It will also transcend the focused issues of Article 76 by discussing how the databases need to be amplified with complementary information in order to help answer a fundamental question that still baffles Arctic geoscientists, i

  17. Geoscientific Model Development: A new EGU Journal for Descriptions of Numerical Models of the Earth System and its components

    NASA Astrophysics Data System (ADS)

    Rutt, I.; Lunt, D.; Hargreaves, J.; Annan, J.; Sander, R.

    2007-12-01

    Geoscientific Model Development (GMD), launching in January 2008, will be an international scientific journal dedicated to the publication and public discussion of the description, development and benchmarking of numerical models of the Earth System and its components. Manuscript types considered for peer-reviewed publication will be: model descriptions, model inter-comparisons, benchmarking papers, and technical papers. In encouraging full publication of Earth System Models we have two main goals. The primary goal is to promote the efficient and effective development of the models, through the clear presentation of the techniques from which all other developers can improve their own models. A secondary goal is to provide increased credibility to the Earth System Science field by creating a space within which models can be openly presented and critically discussed, and their results reproduced and validated. A welcome side-effect will be the formal, peer-reviewed, recognition of the work of Earth System Model developers. It is anticipated that model description papers will form the backbone of GMD. These will comprehensively describe the underlying science behind the models, and will also include details often omitted from more traditional papers, such as the numerical schemes employed. The papers should be somewhat more advanced than internal technical reports. For example, the inclusion of discussion of the scope of applicability and limitations of the approach adopted is expected. In order to enable full peer review of the models, evidence of model output should also be provided, with comparison to standard benchmarks, observations and/or other model output included as appropriate. The publication will potentially consist of three parts: the main paper, a user manual, and the source code (ideally supported by some summary outputs from test case simulations).

  18. Moho, seismogenesis, and rheology of the lithosphere

    NASA Astrophysics Data System (ADS)

    Chen, Wang-Ping; Yu, Chun-Quan; Tseng, Tai-Lin; Yang, Zhaohui; Wang, Chi-yuen; Ning, Jieyuan; Leonard, Tiffany

    2013-12-01

    The Moho is not always a sharp interface; but seismic phase SsPmp yields robust, physically averaged estimates of crustal thickness (virtual deep seismic sounding, VDSS). In S. Tibet where the Moho is as deep as 75 km, bimodal distribution of earthquake depths, with one peak in the upper crust and the other below the Moho, generated much interest in how lithological contrast affects seismicity and rheology. Generally seismicity is limited by distinct temperatures (Tc): 350 ± 50 °C in the crust and 700 ± 100 °C in the mantle (Earthquake Thermometry). Laboratory experiments show that distinct Tc reflect the onset of substantial crystal plasticity in major crustal and mantle minerals, respectively. Above these Tc, frictional instability ends due to velocity weakening of slip. So the seismic to aseismic transition is closely linked with brittle-ductile transitions in the crust and in the uppermost mantle, where the strength of the continental lithosphere is expected to peak (“Jelly Sandwich”). Plasticity depends exponentially on temperature (which evolves over time), so interplay between the geotherm and crustal thickness could result in concentrated seismicity in the upper crust - the only portion of a very warm lithosphere where temperature is below ~ 350 °C (“Crème Brûlée”). Conversely, where the entire crust is below ~ 350 °C (and the uppermost mantle is also below ~ 700 °C), then earthquakes could occur over a wide range of depths, including the entire crust and the uppermost mantle (“Caramel Slab”).

  19. Moho, Seismogenesis, and Rheoloy of the Lithosphere

    NASA Astrophysics Data System (ADS)

    Chen, W.; Tseng, T.; Yang, Z.; Wang, C.; Yu, C.; Ning, J.; Leonard, T.

    2012-12-01

    The Moho is not always a sharp interface; but seismic phase SsPmp yields robust, physically averaged estimates of crustal thickness (deep, virtual seismic sounding, DVSS). In S. Tibet where the Moho is as deep as 75 km, bimodal distribution of earthquake depths, with one peak in the upper crust and the other below the Moho, generated much interest in how lithological contrast affects seismicity and rheology. Generally seismicity is limited by distinct temperatures (Tc): 350±50oC in the crust and 700±100oC in the mantle (Earthquake Thermometry). Lab experiments show that distinct Tc reflect the onset of substantial crystal plasticity in major crustal and mantle minerals, respectively. At these Tc, frictional instability ends due to velocity weakening of slip. So the seismic to aseismic transition is closely linked with brittle-ductile transitions in the crust and in the uppermost mantle, where the strength of the continental lithosphere is expected to peak ("Jelly Sandwich"). Plasticity depends exponentially on temperature (which evolves over time), so interplay between the geotherm and crustal thickness could result in concentrated seismicity in the upper crust - the only portion of a very warm lithosphere where Tc is below ~350oC ("Crème Brûlée"). Conversely, where the entire crust is below ~350oC and the uppermost mantle is also below ~700oC, then earthquakes could occur over a wide range of depths, including the entire crust and the uppermost mantle ("Caramel Slab").

  20. Lithospheric Architecture of the Hudson Bay Region

    NASA Astrophysics Data System (ADS)

    Eaton, D.; Darbyshire, F.

    2009-05-01

    Hudson Bay is a vast inland sea that penetrates deeply into north-central Canada, forming a conspicuous element of the coastline and concealing several fundamental tectonic elements of North America, including most of the Paleoproterozoic Trans Hudson orogen (THO) and the Paleozoic Hudson Bay basin. The THO formed due to a collision between two Archean domains, the Superior and Churchill Provinces of the Canadian Shield, and is similar in scale and tectonic style to the modern Himalayan-Karakorum orogen. Tectonic reconstructions suggest that the lobate shape of the indentor (Superior Province) formed an orogenic template that exerted a persistent influence on the tectonic evolution of the region, resulting in anomalous preservation of juvenile crustal material. Based on analysis of gravity and magnetic data, we propose a model in which juvenile crust in the southeastern part of Hudson Bay formed within an island-arc setting proximal to the Superior Province, in contrast to the Reindeer Zone of Saskatchewan and Manitoba which accreted first to the Churchill Province. Thick, cold and refractory lithosphere that underlies the Bay is well imaged by surface-wave studies and comprises a large component of the cratonic mantle keel that forms the nucleus of the North American continent. The existence of an unusually thick mantle root beneath Hudson Bay indicates that subduction and collision are root-forming (or at least root-preserving) processes. Although the Hudson Bay basin is the largest by surface area of four major intracratonic basins in North America, it is also the shallowest. Available evidence suggests that basin subsidence may have been triggered by eclogitization of crust that was previously thickened during the Trans-Hudson orogeny. Relatively stiff Early Paleozoic lithosphere may have inhibited subsidence of the Hudson Bay basin relative to other basins of similar age in North America.

  1. Tectonic determinations of lithospheric thickness on Ganymede and Callisto

    NASA Technical Reports Server (NTRS)

    Croft, S. K.

    1985-01-01

    The concept of the Maxwell time of a viscoelastic material (4.5) is used in conjunction with calculated thermal profiles to evaluate the significance of tectonic estimates of lithospheric thickness. Thermal lithospheric thicknesses provide fundamental constraints on planetary thermal histories that complement the constraints provided by dateable surface deposits of endogenic origin. Lithospheric constraints are of particular value on the icy satellites where our understanding of both rheology and surface ages is considerably poorer than it is for the terrestrial planets. Certain extensional tectonic features can and have been used to estimate lithospheric thicknesses on Ganymede and Callisto. These estimates, however, refer to the depth of the elastic lithosphere defined by the zone of brittle failure. The relation between the elastic lithosphere and the thermal lithosphere (generally defined by the zone of conductive heat transport) is not straightforward, because the depth of brittle failure depends not only on the thermal profile, but also on rheology and strain rate (or the characteristic time over which stresses build towards failure). Characteristic time considerations are not trivial in this context because stresses generating brittle failure on the icy satellites may be produced by impacts, with characteristic times of seconds to days, or by geologic processes with time scales of hundreds of millions of years.

  2. The Lithospheric Structure of Southern Africa from Magnetotelluric Sounding

    NASA Astrophysics Data System (ADS)

    Evans, R. L.; Jones, A. G.; Atekwana, E. A.

    2014-12-01

    Measurements of mantle electrical conductivity, made through the magnetotelluric method, offer considerable insight into the structure of cratonic lithosphere. A particularly expansive data set has been collected in Southern Africa, started through the Southern Africa Magnetotelluric Experiment (SAMTEX) experiment, now continuing north through Zambia as part of the Project for Rift Initiation Development and Evolution (PRIDE) experiment. The combined data set highlights large variability in lithospheric structure that broadly correlates with surface geology: cratonic lithosphere is generally thick and electrically resistive, while much thinner lithosphere is seen beneath mobile belts. In areas of relatively uniform resistivity structure, we have constructed resistivity-depth profiles and use new laboratory data to place constraints on the water content of lithospheric mantle. Uncertainty in our estimates arises from differences between different laboratory results, but our data are generally consistent with a slightly damp upper lithospheric mantle above a dry and strong cratonic root. Other areas show complexity of structure that is difficult to understand using current knowledge of conductivity -the Bushveld complex, where the mantle is highly conductive, is one such example. In southwestern Zambia, the lithosphere is seen to be very thin (around 50km) beneath mobile belt terrain, as was inferred nearly 40 years ago on the basis of high heatflow. The mantle is highly conductive, most likely due to a combination of elevated temperatures, water content and perhaps a trace amount of melting. This anomalous structure may be linked to the southwest propagation of the East African Rift system.

  3. SEASAT observations of lithospheric flexure seaward of trenches

    NASA Technical Reports Server (NTRS)

    Mcadoo, D. C.; Martin, C. F.

    1983-01-01

    Lithospheric flexure seaward of deep ocean trenches in SEASAT altimeter observations of the marine geoid. In fact, mechanical models of lithospheric flexure can be tested directly on the SEASAT altimeter data. A simple elastic model was used for the oceanic lithosphere and, after least squares adjustments, estimates of model parameters were recovered including Outer Rise (OR) amplitude, OR wavelength, and effective lithospheric thickness. Effective lithospheric thickness was recovered for five regions: the Mariana, the Kuril, the Philippine, the Aleutian and the Middle America OR. These results support the suggestion of Bodine et al. (1981) that effective thickness, T, increased with age of lithosphere in approximate accord with the relation T approximately equals x age to the 1/2 power where C approximately equals 4 km x my to the -1/2 power. Altimetric results agree more closely with this relation than do published results based on bathymetric data. The close agreement with the thickness-age relation suggests that there is no longer any need to assume that significant horizontal compression acts across the Kuril, Marianas and Izo-Bonin trenches. This thickness-age relation implies that flexural strength of the oceanic lithosphere is temperature controlled.

  4. Lithospheric structure of Venus from gravity and topography

    NASA Astrophysics Data System (ADS)

    Jiménez-Díaz, Alberto; Ruiz, Javier; Kirby, Jon F.; Romeo, Ignacio; Tejero, Rosa; Capote, Ramón

    2015-11-01

    There are many fundamental and unanswered questions on the structure and evolution of the venusian lithosphere, which are key issues for understanding Venus in the context of the origin and evolution of the terrestrial planets. Here we investigate the lithospheric structure of Venus by calculating its crustal and effective elastic thicknesses (Tc and Te, respectively) from an analysis of gravity and topography, in order to improve our knowledge of the large scale and long-term mechanical behaviour of its lithosphere. We find that the venusian crust is usually 20-25 km thick with thicker crust under the highlands. Our effective elastic thickness values range between 14 km (corresponding to the minimum resolvable Te value) and 94 km, but are dominated by low to moderate values. Te variations deduced from our model could represent regional variations in the cooling history of the lithosphere and/or mantle processes with limited surface manifestation. The crustal plateaus are near-isostatically compensated, consistent with a thin elastic lithosphere, showing a thickened crust beneath them, whereas the lowlands exhibit higher Te values, maybe indicating a cooler lithosphere than that when the venusian highlands were emplaced. The large volcanic rises show a complex signature, with a broad range of Te and internal load fraction (F) values. Finally, our results also reveal a significant contribution of the upper mantle to the strength of the lithosphere in many regions.

  5. Rejuvenation of the lithosphere by the Hawaiian plume.

    PubMed

    Li, Xueqing; Kind, Rainer; Yuan, Xiaohui; Wölbern, Ingo; Hanka, Winfried

    2004-02-26

    The volcanism responsible for creating the chain of the Hawaiian islands and seamounts is believed to mark the passage of the oceanic lithosphere over a mantle plume. In this picture hot material rises from great depth within a fixed narrow conduit to the surface, penetrating the moving lithosphere. Although a number of models describe possible plume-lithosphere interactions, seismic imaging techniques have not had sufficient resolution to distinguish between them. Here we apply the S-wave 'receiver function' technique to data of three permanent seismic broadband stations on the Hawaiian islands, to map the thickness of the underlying lithosphere. We find that under Big Island the lithosphere is 100-110 km thick, as expected for an oceanic plate 90-100 million years old that is not modified by a plume. But the lithosphere thins gradually along the island chain to about 50-60 km below Kauai. The width of the thinning is about 300 km. In this zone, well within the larger-scale topographic swell, we infer that the rejuvenation model (where the plume thins the lithosphere) is operative; however, the larger-scale topographic swell is probably supported dynamically. PMID:14985758

  6. Hudson Bay Lithospheric Experiment: Constraints on Lithospheric Thickness From Surface Waves

    NASA Astrophysics Data System (ADS)

    Darbyshire, F. A.

    2008-12-01

    HuBLE (Hudson Bay Lithospheric Experiment) is an international initiative to study the structure, dynamics and evolution of the Hudson Bay region. In particular, we seek to understand the interaction between the Archean cratons surrounding the region and the underlying Paleoproterozoic Trans-Hudson Orogen, which formed during the collision of the Superior and Churchill Provinces at 1.9-1.8 Ga. Global and continental- scale tomographic models indicate a thick, cold and refractory lithosphere beneath Hudson Bay. Most tomographic models suggest that this region is associated with the highest velocities and thickest seismological lithosphere of the Canadian Shield. The HuBLE project commenced in 2006, with the deployment of a number of telemetered broadband seismograph stations on the east and west coasts of Hudson Bay. Along with existing stations from the POLARIS/FedNor initiative in northern Ontario, and permanent Canadian stations, the deployment ringed Hudson Bay on three sides. A second phase of deployment in 2007, using non-telemetered broadband stations, completed the coverage of the region. A considerable number of large teleseismic earthquakes have been recorded by the array since its installation, and the data are generally of high quality. We measure Rayleigh wave phase velocities for paths crossing Hudson Bay, using the two-station cross-correlation method of Meier et al. (2004). Average phase velocity dispersion curves are constructed using data from multiple earthquakes along each path, resulting in a set of reliable dispersion measurements in the period range ~15--250~seconds. The data set therefore permits constraint of lithospheric shear wave velocity structure from mid-crustal to asthenospheric depths beneath the continent. Preliminary 1D shear wave velocity models of path-averaged structure are estimated using a smooth linearised inversion technique (Maupin & Cara, 1992). The models show a typically 'shield- type' signature, with a high

  7. Effective elastic thickness of the continental lithosphere in China from heat flow: Implications for the lithospheric rheology and active tectonics

    NASA Astrophysics Data System (ADS)

    Liu, S.; Wang, L.

    2009-04-01

    The effective elastic thickness (Te) of continental lithosphere is one important parameter that describes the response of the lithosphere to long-term loads. However, the estimation of Te is still controversial and various forward and inverse methods have been proposed since the last 20 years. Besides the general application of gravity-topography based inverse method, thermal aspect and related technique is more emphasized, since the mechanical behavior of lithosphere is obviously influenced by temperature. Here we present the effective elastic thickness of the continental lithosphere in China from heat flow data by the method proposed by Burov et al, J. Geophys. Res., 1995, 100(B3):3905-3927. Our results show that Te varies much in different areas of China due to diverse and complicated geological evolution and associated change in thermal regime. Te is much larger than the crustal thickness in the regions where the heat flow is really low (usually less than 50mW/m2) and the lithosphere is relatively thick, indicating much more contribution from the upper mantle to the whole strength of lithosphere. Under this condition, the rheology of the mantle with olivine dominates the deformation manner and processes of the lithosphere and the typical cases in China are those blocks (Tarim, Junggar, Ordos and Sichuan) in central-western China. For instance, the Te of the Tarim basin is 66

  8. Olympus Mons shaped by lithospheric flexure

    NASA Astrophysics Data System (ADS)

    Byrne, P. K.; van Wyk de Vries, B.; Holohan, E. P.; Murray, J. B.; Troll, V.

    2009-12-01

    Olympus Mons is the biggest volcano on Mars, towering 22 km above the NW flank of the Tharsis-Syria upland. Olympus has a characteristic morphology featuring a flat summit, convex upper flanks, concave lower flanks, and a basal escarpment. The summit hosts a six-caldera collapse complex. The upper flanks are terraced in a circumferential, imbricate pattern, whilst the lower flanks are covered in leveed lava channels and lava fans. The basal scarp surrounds much of the shield and reaches heights of 6 km in places. Beyond the escarpment lie aureole deposits, particularly prominent to the NW, downslope from the Tharsis Rise. Although the caldera complex probably developed due to multi-cyclic recharge and evacuation of magma chambers within the edifice, the origin of the remaining features is contentious. Flank terraces have been ascribed to lithospheric flexure, magma chamber tumescence, or volcano spreading. The basal scarp has been linked to tectonic, eruptive, and mass movement processes. The aureole deposits are generally regarded as causally related to the escarpment, but they too have been attributed to a range of formation mechanisms. Previous attempts to explain the genesis of this suite of structures have primarily invoked lithospheric flexure or volcano spreading, but no holistic model yet exists. Here we show that lithospheric flexure, with coeval slip along a basal décollement, is the leading candidate mechanism for the formation of the terraces, scarp, and aureole deposits of Olympus Mons. In a set of scaled analogue experiments, we loaded a ductile silicone putty “lithosphere” with a brittle sand cone “edifice”; a thin layer of putty below the cone served as a detachment surface. Flexure of the underlying silicone produced imbricate, outward-verging convexities on the cone’s upper- and mid flanks, a concave-upward annular trough on its lower flanks, and a prominent scarp at its base. The convexities closely resembled the geometry of terraces

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

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

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

  10. Lithospheric loading and tectonics of the lunar irregular maria

    NASA Technical Reports Server (NTRS)

    Hall, J. Lynn; Solomon, Sean C.

    1987-01-01

    Many of the tectonic features associated with the circular maria are the result of stresses generated by loading of the lunar lithosphere by mare basalt units. The hypothesis is tested that the irregular lunar maria share with the circular maria the processes leading to the formation of associated graben and mare ridge systems. A formulation of the lithospheric flexure problem that accounts for the variable distribution of basalt loads for the irregular maria is applied. On the basis of the tectonic structures and geologic history of each of the major irregular maria, as well as models for the distribution of mare basalt for each region, the predicted stress fields are compared with the distribution of tectonic features for a range of assumed values of the thickness of the elastic lithosphere. The best fitting values for lithospheric thickness beneath the regular maria are then compared with those previously inferred for the circular maria.

  11. Application of MAGSAT to lithospheric modeling in South America

    NASA Technical Reports Server (NTRS)

    Keller, G. R.; Lidiak, E. G. (Principal Investigator)

    1983-01-01

    Progress in the determination of relations of MAGSAT anomalies to lithospheric structures is reported. The prime emphasis was on a Rayleigh wave study and the determination of both group and phase velocity dispersion.

  12. Determination of the Earth's lithospheric magnetic field with satellite data

    NASA Astrophysics Data System (ADS)

    Kotsiaros, Stavros; Olsen, Nils; Finlay, Christopher

    2016-07-01

    Satellites such as Magsat, Ørsted, CHAMP and Swarm provide the most effective means of determining on a global scale the Earth's lithospheric magnetic field. In particular, the Swarm three-satellite constellation mission aims at capturing the smallest-scale features of the lithospheric field that have ever been captured from space. To achieve that, explicit advantage of the constellation aspect of Swarm has to be taken by using gradient estimates. We derive lithospheric field models using more than one year of magnetic gradient data, which are approximated by first differences of field vector data between the two lower Swarm satellites and along each satellite orbit, respectively. We find that gradient data are less sensitive to large-scale external field fluctuations. Moreover, gradient data appear to be a very efficient way of increasing the resolution of lithospheric field models and thus providing an initial validation of the gradient concept underlying the Swarm mission.

  13. Shield volcanism and lithospheric structure beneath the Tharsis plateau, Mars

    NASA Technical Reports Server (NTRS)

    Blasius, K. R.; Cutts, J. A.

    1976-01-01

    The heights of four great shield volcanoes, when interpreted as reflecting the local hydrostatic head on a common source of upwelling magma, provide significant constraints on models of lithospheric structure beneath the Tharsis plateau. If Bouguer gravity anomalies are modeled in terms of a variable thickness crust, and a two-component (crust/mantle) earth-like structure is assumed for the Martian lithosphere, the derived model lithosphere beneath the Tharsis plateau has the following properties: (1) the upper low-density 'crustal' component is thickened beneath the Tharsis plateau; (2) the lower high-density 'mantle' component is thinned beneath the Tharsis plateau; and (3) there is a net gradient on the base of the Martian lithosphere directed downward away from beneath the summit of the Tharsis plateau. A long history of magmatic intrusion is hypothesized to have been the cause of the updoming of the Tharsis plateau and the maintenance of the plateau in a state of only partial compensation.

  14. Lithospheric Loading by the Northern Polar CAP on Mars

    NASA Astrophysics Data System (ADS)

    Johnson, C. L.; Solomon, S. C.; Head, J. W.; Smith, D. E.; Zuber, M. T.

    1999-03-01

    Loading of the martian lithosphere by the northern polar cap is investigated using elastic and viscoelastic models and constraints from MOLA data and geology. Implications for basement topography, polar cap volume and the gravity field are discussed.

  15. An Outbreak of Acute Febrile Illness Caused by Sandfly Fever Sicilian Virus in the Afar Region of Ethiopia, 2011

    PubMed Central

    Woyessa, Abyot Bekele; Omballa, Victor; Wang, David; Lambert, Amy; Waiboci, Lilian; Ayele, Workenesh; Ahmed, Abdi; Abera, Negga Asamene; Cao, Song; Ochieng, Melvin; Montgomery, Joel M.; Jima, Daddi; Fields, Barry

    2014-01-01

    In malaria-endemic regions, many medical facilities have limited capacity to diagnose non-malarial etiologies of acute febrile illness (AFI). As a result, the etiology of AFI is seldom determined, although AFI remains a major cause of morbidity in developing countries. An outbreak of AFI was reported in the Afar region of Ethiopia in August of 2011. Retrospectively, 12,816 suspected AFI cases were identified by review of medical records. Symptoms were mild and self-limiting within 3 days after the date of onset; no fatalities were identified. All initial test results of AFI patient specimens were negative for selected pathogens using standard microbiological and molecular techniques. High-throughput sequencing of nucleic acid extracts of serum specimens from 29 AFI cases identified 17 (59%) of 29 samples as positive for Sandfly Fever Sicilian Virus (SFSV). These results were further confirmed by specific reverse transcription polymerase chain reaction. This is the first study implicating SFSV as an etiological agent for AFI in Ethiopia. PMID:25266349

  16. Lithospheric thermal structure and rheology of the eastern China

    NASA Astrophysics Data System (ADS)

    Wang, Yang; Cheng, Su-Hua

    2012-03-01

    Temperature distributions of the lithosphere beneath the eastern China were estimated by local isostasy equilibrium constrained geothermal calculation in this study. Maps of the lateral temperature variation at depths of 50 and 100 km as well as Moho boundary are presented for the eastern China continent, and the 600 °C and 1100 °C isotherm depth maps are also given. Meanwhile, the thermal thickness of lithosphere is calculated as the depth of 1350 °C isotherm. The rheological strength of lithosphere is calculated based on a four-layer model, which consists of the upper, lower, lowermost crust layers and a wet lithospheric mantle layer. The results show that, the lithosphere of 160-180 km thick exists under Sichuan basin in the western part of Yangtze Craton, but keel is absent beneath the entire North China Craton. It is notable that the lithospheric thickness is only 130-140 km beneath Ordos basin in the western part of North China Craton; accordingly, there is no lithospheric keel beneath Ordos basin. These geothermal modeling results are in good agreement with those of seismic tomography and other geothermal studies. The temperatures in the lower crust of the North China Craton estimated are in range of 500-600 °C. Consequently, the composition of lower crust of the North China Craton should be more mafic than that of previous estimation based on much cooler geotherms. The spatial variation of lithospheric rheology in the eastern China is influenced by local geotherms and crustal compositions. The "crème brûlée" model, which is represent by a strong crust portion but a weak lithospheric mantle portion in vertical strength profile, approximates the lithospheric rheological layering for the regions of the western half of North China Craton and the northern part of NE China, which has thicker crust or moderate-to-high geotherm. On the other hand, the "jelly sandwich" model demonstrates the rheological layering of the western part of the Yangtze Craton and

  17. Lithospheric Stress and Deformation (Paper 7R0323)

    NASA Astrophysics Data System (ADS)

    McNutt, Marcia

    1987-07-01

    The study of the mechanisms, magnitudes, and modes of lithospheric stress and deformation occupies a central position in the discipline of Tectonophysics. It is therefore difficult in assembling a comprehensive review to place limits on what should be included. For example, one cannot discuss stress and deformation apart from considerations of the rheological models of the lithosphere which link the two and ultimately control the behavior of plates at their margins and in their interiors. Once admitting to the importance of rheological considerations in discussing stress and deformation, one is then compelled to include information relating to the thermal state of the lithosphere, since temperature is one of the foremost environmental variables controlling rheology. Furthermore, lithospheric processes cannot be considered in isolation from those affecting the asthenosphere, since surely there exists some coupling between the motion of the lithosphere and convection in the Earth's interior. In fact, an understanding of lithospheric stress and deformation can be considered a prerequisite to solving problems in geodynamics because it is through the lithospheric filter that one detects the consequences of thermal and compositional anomalies at greater depth. The topic of this report cannot even be cleanly separated from reviews in other disciplines such as Seismology and Geodesy. The magnitude and orientation of lithospheric stress is often indicated by earthquakes, while lithospheric strain can be observed geodetically. The philosophy I have adopted here is that some overlap with other review papers in this series is unavoidable, and probably desirable from the standpoint of emphasizing how recent work n other areas of geophysics has contributed to the advancement in our understanding of the lithosphere's state-of-stress and mechanisms of deformation. I have, however, resisted the temptation to include extraterrestrial studies, presumably covered in the Planetary

  18. The effects of strain heating in lithospheric stretching models

    NASA Technical Reports Server (NTRS)

    Stanton, M.; Hodge, D.; Cozzarelli, F.

    1985-01-01

    The deformation by stretching of a continental type lithosphere has been formulated so that the problem can be solved by a continuum mechanical approach. The deformation, stress state, and temperature distribution are constrained to satisfy the physical laws of conservation of mass, energy, momentum, and an experimentally defined rheological response. The conservation of energy equation including a term of strain energy dissipation is given. The continental lithosphere is assumed to have the rheology of an isotropic, incompressible, nonlinear viscous, two layered solid.

  19. Solar Insolation Driven Variations of Mercury's Lithospheric Strength

    NASA Astrophysics Data System (ADS)

    Williams, Jean-pierre; Ruiz, J.; Rosenburg, M. A.; Aharonson, O.; Phillips, R. J.

    2010-10-01

    Mercury's coupled 3:2 spin-orbit resonance in conjunction with its relatively high eccentricity of 0.2 results in a surface variation in annual average solar insolation and thus equatorial hot and cold regions. This results in an asymmetric temperature distribution in the lithosphere and a long wavelength lateral variation in lithosphere structure and strength that mirrors the insolation pattern. We employ a thermal evolution model for Mercury generating strength envelopes of the lithosphere to demonstrate and quantify the possible effects the insolation pattern has on Mercury's lithosphere. We find the heterogeneity in lithosphere strength is substantial, increases with time, and is accentuated by the differential timing of the mantle contribution to the lithosphere strength. For example, by the end of late heavy bombardment ( 4 Ga) we find a difference in brittle-ductile transition depth of 6 km between the hot and cold equatorial thermal poles and 24 km between the hot equatorial pole and the latitudes ±90°. We also find that a crust thicker than that of the Moon or Mars and dry rheologies for the crust and mantle are favorable when compared with estimates of brittle-ductile transition depths derived from lobate scarps. Regions of stronger and weaker compressive strength imply that the accommodation of radial contraction of Mercury as its interior cooled, manifest as lobate scarps, may not be isotropic, imparting a preferential orientation and distribution to the lobate scarps. Although many of the parameters of the model are poorly constrained for Mercury, the overall lithospheric heterogeneity remains regardless of the choice of parameters. The latitudinal surface temperature variation experienced by Mercury is not unlike that of the Earth's Moon presently and thus one should expect an analogous latitude dependence on lithospheric strength to have developed over time on the Moon as well. Funded by the NSF Astrophysics Research Grants program (AST-0709151).

  20. Fertile Lithospheric Mantle beneath Far East Russia; evidence for Lithospheric delamination

    NASA Astrophysics Data System (ADS)

    Ntaflos, T.; Koutsovitis, P.; Aschchepkov, I.; Hauzenberger, C. A.; Prikhodko, V.; Barkar, A.

    2012-12-01

    In the back-arc environment of Far East Russia, mantle xenoliths from Sikhoti-Alin( Komku area, KO) and Primorie (Sviyaginsky area, SV), Far East Russia are fertile spinel lherzolites with traces of amphibole, phlogopite and hyalophane in some of the studied samples. Though samples from both localities are fertile there is a systematic difference in their fertility. The KO samples have mg# varying from 0.891 to 0.899 and are slightly more fertile than the SV samples that have mg# ranging from 0.898 to 0.904. LA-ICP-MS analyses on clinopyroxenes confirm this trend as the (La/Yb)N in KO samples range from 1.49 to 5.4 and in SV samples from 0.15 to 1.73. The estimated equilibration temperatures for the KO suite range from 940 °C to 1035 °C and for the SV suite from 770 to 945. The differences in the estimated equilibrium temperatures between the KO and SV suites suggest that the less fertile SV suite originated in shallower depths than the more fertile KO suite. Kaersutite, and extremely Ti-rich phlogopite, up to 14 wt% TiO2, are associated with intergranular glass indicating clearly metasomatism of undersaturated alkaline melts. Pargasitic amphibole occurs as inclusion in clinopyroxene. Incompatible element abundances, besides Ba, Sr and Ti that are slightly enriched in the amphibole, are similar in both phases suggesting minor metasomatism due to percolation of small amounts of water-rich fluids. The lithospheric mantle beneath the studied area represents the residue after partial melting of up to 2 % of a primitive mantle and is comparable to that of Mongolia. Despite the fact that the studied area experienced several subducting episodes, the lithospheric mantle appears to be unaffected from the upwelling fluids/melts of the subducted slab(s). Since there is no indication for plume activity, and/or evidence for refertilization, it is likely that the lithospheric mantle has been delaminated as the result of tectonic events (lithospheric attenuation, inverse

  1. Swarm magnetic gradients for lithospheric modelling (SLIM)

    NASA Astrophysics Data System (ADS)

    Bouman, Johannes; Ebbing, Jörg; Kotsiaros, Stavros; Brönner, Marco; Haagmans, Roger; Fuchs, Martin; Holzrichter, Nils; Olsen, Nils; Baykiev, Eldar

    2016-04-01

    We present first results of a feasibility study to use magnetic gradient information derived from Swarm data for crustal field modelling. The study is part of ESA's Support To Science Element (STSE) Swarm+ Innovations. In a first step, magnetic gradients have been derived from the observations taken by the three Swarm satellites, with emphasis on the two side-by-side flying spacecraft. Next, these gradients are used to compute magnetic gradient grids at 450 km altitude (the present mean altitude of the lower Swarm satellites) for one example region, North-West Europe. The suggested area comprise both exposed basement geology in southern Sweden and Norway with crustal scale magnetic anomalies and the Sorgenfrei-Tornquist Zone, a well-studied large scale tectonic fault system. With sensitivity analysis we studied the added benefit of the information from the gradient grids for lithospheric magnetic field modelling. A wealth of aeromagnetic data and additional constraining information for the example area allows us to validate our modelling results in great detail.

  2. Lithospheric imaging via teleseismic scattering tomography

    NASA Astrophysics Data System (ADS)

    Frederiksen, A. W.; Revenaugh, J.

    2004-12-01

    The coda of the teleseismic P phase consists largely of energy scattered by small inhomogeneities in the receiver-side lithosphere. Given large collections of teleseismic data from dense permanent networks, previous workers have successfully back-propagated coda energy back to scattering source points using various kinematic migration schemes, as well as by inverting using an inverse scattering/radon transform approach. Under the Born approximation, seismic scattering is a linear process; therefore it is possible to approach coda scattering as a linear waveform inversion problem, mathematically similar to transmission-based tomography. Assuming ray-theoretical propagation and Rayleigh scattering, we pose the inverse scattering problem in tomographic form, and recover perturbations in density and P and S velocities from Pp and Ps scattered data. The method is applied to data from the Southern California Seismic Network (SCSN) covering the San Jacinto-Anza region. The results show a considerable correlation between seismicity and velocity perturbation structure, particularly in the region between the Mission Creek and Banning fault branches. Features connecting the Coyote Creek and Elsinore faults at right angles are correlated with seismicity lineations and may represent conjugate faulting with no surface expression.

  3. Influence of sediment deposition on deep lithospheric tectonics

    NASA Astrophysics Data System (ADS)

    Gray, R.; Pysklywec, R. N.

    2012-06-01

    Previous geodynamic models of continental collision show that the behavior of the lithosphere can be strongly influenced by the presence of surface erosion. That said, absent from these investigations are the effects of sediment deposition. We quantitatively investigate this process using thermal-mechanical numerical experiments of the coupled processes of tectonic deformation and crustal mass flux. The models demonstrate that the inclusion of the effects of sediment deposition can change the style of deformation of the crust and consequently, the evolution of the underlying deforming mantle lithosphere. In the absence of sediment deposition, the early stages of collision are accommodated by subduction of lower crust and mantle lithosphere along a discrete shear zone beneath the overriding plate. Following this initial stage of subduction, the subducting lower crust and mantle lithosphere retreat from the collision zone, permitting the sub-lithospheric mantle to upwell and come into contact with the thickened upper crust. When sediment deposition is imposed subduction-like consumption of the subducting plate remains stable. The presence of sediment deposition introduces a negative Vy-component in the overriding plate in the area adjacent to the collisional zone. The negative Vy-component leads to a greater degree of coupling between the colliding continental plates and decoupling of the overriding upper crust and lower crust/mantle lithosphere. The results demonstrate the first quantitative insights into the feedback between surface deposition and tectonics.

  4. Thermal thickness and evolution of Precambrian lithosphere: A global study

    USGS Publications Warehouse

    Artemieva, I.M.; Mooney, W.D.

    2001-01-01

    The thermal thickness of Precambrian lithosphere is modeled and compared with estimates from seismic tomography and xenolith data. We use the steady state thermal conductivity equation with the same geothermal constraints for all of the Precambrian cratons (except Antarctica) to calculate the temperature distribution in the stable continental lithosphere. The modeling is based on the global compilation of heat flow data by Pollack et al. [1993] and more recent data. The depth distribution of heat-producing elements is estimated using regional models for ???300 blocks with sizes varying from 1?? ?? 1?? to about 5?? ?? 5?? in latitude and longitude and is constrained by laboratory, seismic and petrologic data and, where applicable, empirical heat flow/heat production relationships. Maps of the lateral temperature distribution at depths 50, 100, and 150 km are presented for all continents except Antarctica. The thermal thickness of the lithosphere is calculated assuming a conductive layer overlying the mantle with an adiabat of 1300??C. The Archean and early Proterozoic lithosphere is found to have two typical thicknesses, 200-220 km and 300-350 km. In general, thin (???220 km) roots are found for Archean and early Proterozoic cratons in the Southern Hemisphere (South Africa, Western Australia, South America, and India) and thicker (>300 km) roots are found in the Northern Hemisphere (Baltic Shield, Siberian Platform, West Africa, and possibly the Canadian Shield). We find that the thickness of continental lithosphere generally decreases with age from >200 km beneath Archean cratons to intermediate values of 200 ?? 50 km in early Proterozoic lithosphere, to about 140 ?? 50 km in middle and late Proterozoic cratons. Using known crustal thickness, our calculated geotherms, and assuming that isostatic balance is achieved at the base of the lithosphere, we find that Archean and early Proterozoic mantle lithosphere is 1.5% less dense (chemically depleted) than the

  5. Lithosphere-Atmosphere-Ionosphere coupling model

    NASA Astrophysics Data System (ADS)

    Kachakhidze, M. K., III

    2015-12-01

    The present work offers interpretation of a mechanism of formation of hypothetic ideal electromagnetic contour, creation of which is envisaged in incoming earthquake focal zone. Model of generation of EM emissions detected before earthquake is based on physical analogues of distributed and conservative systems and focal zones. According to the model the process of earthquake preparation from the moment of appearance of cracks in the system, including completion of series of foreshocks, earthquake and aftershocks, are entirely explained by oscillating systems.According to the authors of the work electromagnetic emissions in radio diapason is more universal and reliable than other anomalous variations of various geophysical phenomena in earthquake preparation period; Besides, VLF/LF electromagnetic emissions might be declared as the main precursor of earthquake because it might turn out very useful with the view of prediction of large (M5) inland earthquakes and to govern processes going on in lithosphere-atmosphere-ionosphere coupling (LAIC) system. Based on this model, in case of electromagnetic emissions spectrum monitoring in the period that precedes earthquake it is possible to determine, with certain accuracy, the time, location and magnitude of an incoming earthquake simultaneously.The present item considers possible physical mechanisms of the geophysical phenomena, which may accompany earthquake preparation process and expose themselves several months, weeks or days prior to earthquakes. Such as: Changing of intensity of electro-telluric current in focal area; Perturbations of geomagnetic field in forms of irregular pulsations or regular short-period pulsations; Perturbations of atmospheric electric field; Irregular changing of characteristic parameters of the lower ionosphere (plasma frequency, electron concentration, height of D layer, etc.); Irregular perturbations reaching the upper ionosphere, namely F2-layer, for 2-3 days before the earthquake

  6. Integrating local pastoral knowledge, participatory mapping, and species distribution modeling for risk assessment of invasive rubber vine (Cryptostegia grandiflora) in Ethiopia’s Afar region

    USGS Publications Warehouse

    Luizza, Matthew; Wakie, Tewodros; Evangelista, Paul; Jarnevich, Catherine S.

    2016-01-01

    The threats posed by invasive plants span ecosystems and economies worldwide. Local knowledge of biological invasions has proven beneficial for invasive species research, but to date no work has integrated this knowledge with species distribution modeling for invasion risk assessments. In this study, we integrated pastoral knowledge with Maxent modeling to assess the suitable habitat and potential impacts of invasive Cryptostegia grandiflora Robx. Ex R.Br. (rubber vine) in Ethiopia’s Afar region. We conducted focus groups with seven villages across the Amibara and Awash-Fentale districts. Pastoral knowledge revealed the growing threat of rubber vine, which to date has received limited attention in Ethiopia, and whose presence in Afar was previously unknown to our team. Rubber vine occurrence points were collected in the field with pastoralists and processed in Maxent with MODIS-derived vegetation indices, topographic data, and anthropogenic variables. We tested model fit using a jackknife procedure and validated the final model with an independent occurrence data set collected through participatory mapping activities with pastoralists. A Multivariate Environmental Similarity Surface analysis revealed areas with novel environmental conditions for future targeted surveys. Model performance was evaluated using area under the receiver-operating characteristic curve (AUC) and showed good fit across the jackknife models (average AUC = 0.80) and the final model (test AUC = 0.96). Our results reveal the growing threat rubber vine poses to Afar, with suitable habitat extending downstream of its current known location in the middle Awash River basin. Local pastoral knowledge provided important context for its rapid expansion due to acute changes in seasonality and habitat alteration, in addition to threats posed to numerous endemic tree species that provide critical provisioning ecosystem services. This work demonstrates the utility of integrating local ecological

  7. Lithospheric thermal and strength model of the Arctic region

    NASA Astrophysics Data System (ADS)

    Struijk, Maartje; Tesauro, Magdala; Lebedeva-Ivanova, Nina; Beekman, Fred; Gaina, Carmen; Cloetingh, Sierd

    2016-04-01

    We estimate the lithospheric strength distribution in the Arctic region. With this aim, we use the most recently updated models of the Arctic's crust of Lebedeva-Ivanova et al. (in preparation), based on seismic and gravity data. These models include the thickness and density of the crust and sediments, the boundaries between the continental and oceanic crust, and the age of the oceanic lithosphere. We estimate the temperature variation in the continental lithosphere by using the one-dimensional steady-state heat conductive equation, assuming a ratio between the upper and lower crust of 0.5 and 0.7 and a constant surface heat flow of 50 and 65 mWm ^ 2, respectively. We take also into account the temperature dependence of the the thermal conductivity in the lithospheric mantle. We adopt the cooling plate model of McKenzie (1976) to estimate the temperature in the oceanic domain. At a depth of 50 km, the resulting thermal models show a stronger lateral variations in the oceanic (~550 °C) than in the continental lithosphere (~100°C). Within the continental domain, the increase of a surface heat flow from 50 to 65mWm ^ 2 raises the temperatures of ~300 °C. This is translated in a significant lithospheric strength reduction (from 3x10 ^ 13 Pa to ~ 0.5x10 ^ 13 Pa) and decoupling between the crust and mantle lithosphere. Other parameters, such as the crustal rheology and thickness cause second order strength variations. Continental strength variations reflect the different tectonic evolution of the Artic basins and ridges.

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

    USGS Publications Warehouse

    Thatcher, W.; Pollitz, F.F.

    2008-01-01

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

  9. Global model for the lithospheric strength and effective elastic thickness

    NASA Astrophysics Data System (ADS)

    Tesauro, Magdala; Kaban, Mikhail K.; Cloetingh, Sierd A. P. L.

    2013-08-01

    Global distribution of the strength and effective elastic thickness (Te) of the lithosphere are estimated using physical parameters from recent crustal and lithospheric models. For the Te estimation we apply a new approach, which provides a possibility to take into account variations of Young modulus (E) within the lithosphere. In view of the large uncertainties affecting strength estimates, we evaluate global strength and Te distributions for possible end-member 'hard' (HRM) and a 'soft' (SRM) rheology models of the continental crust. Temperature within the lithosphere has been estimated using a recent tomography model of Ritsema et al. (2011), which has much higher horizontal resolution than previous global models. Most of the strength is localized in the crust for the HRM and in the mantle for the SRM. These results contribute to the long debates on applicability of the "crème brulée" or "jelly-sandwich" model for the lithosphere structure. Changing from the SRM to HRM turns most of the continental areas from the totally decoupled mode to the fully coupled mode of the lithospheric layers. However, in the areas characterized by a high thermal regime and thick crust, the layers remain decoupled even for the HRM. At the same time, for the inner part of the cratons the lithospheric layers are coupled in both models. Therefore, rheological variations lead to large changes in the integrated strength and Te distribution in the regions characterized by intermediate thermal conditions. In these areas temperature uncertainties have a greater effect, since this parameter principally determines rheological behavior. Comparison of the Te estimates for both models with those determined from the flexural loading and spectral analysis shows that the 'hard' rheology is likely applicable for cratonic areas, whereas the 'soft' rheology is more representative for young orogens.

  10. Chemical Evolution of Dynamic Mantle Models with Strong, Mobile Lithosphere.

    NASA Astrophysics Data System (ADS)

    Brandenburg, J.; van Keken, P.; Ballentine, C.; Hauri, E.

    2005-12-01

    In recent years, a number of models that examine the chemical evolution of the mantle have been put forth by the geodynamics community. Important criteria such as heat flow and degassing rate are reconciled by these models, but the full range of isotopic heterogeneity as observed in oceanic basalts is not reproduced. The inherent numerical difficulty of representing extreme variations in mantle viscosity while maintaining a mobile, segmented lithosphere may be reflected in this problem. Thick, stagnant lithosphere is the inevitable consequence of realistic temperature dependent rheology. Special numerical techniques are needed to break the lithosphere into plates. However, some choices for tuning parameters are inherent in all such methods. The variability of modeled isotopic heterogeneity as a function of the numerical treatment of the lithosphere is explored. Mantle convection is simulated by the numerical solution of the time dependent Boussinesq equations on a two dimensional finite element mesh. Two related techniques for maintaining a mobile lithosphere, the kinematic plate and force balance method, are used. In the kinematic plate method (Christensen and Hofmann, 1994) an arbitrary plate velocity field is applied to the surface boundary. The force balance method (Gable, 1989) is functionally the same, except that the plate velocities are computed to minimize the shear stress on the base of the lithosphere. Isotopic inventories are discretized to a large number of passive tracers. Mixing properties and isotopic evolution of the Rb/Sr, U/Pb, Sm/Nd, U/He, and K/Ar systems are compared. A solidus model for peridotite melting is then introduced. Given these features, we examine the sensitivity of the geochemical evolution to the different methods of modeling the lithosphere.

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  12. Block Rotation Linked to Southward Right-Stepping Propagation and Overlap of the Red Sea Rift Segments Afar Depression, Ethiopia

    NASA Astrophysics Data System (ADS)

    Birke, T. K.; Atnafu, A.; Rowland, J. V.; Bachtadse, V.

    2012-12-01

    Thirty-four cooling units from the Pleistocene extrusive volcanic rocks in the northwestern central Afar depression were sampled for paleomagnetic study. The samples are collected from localities bounded to the west by the active magmatic segments of Dabbahu and Northern Manda Hararo segment; to the northwest by the Alayta segment and to the northeast by the Tat'ale magmatic segment. Some samples were collected from south of Afdera, located on the southern continuation of the Tat'ale magmatic segment. Seven to twelve samples were collected from each cooling unit and on average 5 samples per unit were treated with Alternating Field (AF) demagnetizations while one sample was treated by Thermal (TH) demagnetization technique for a check, all at the department of Earth Sciences of Addis Ababa University. The Natural Remanent Magnetization (NRM) direction reveals two simple and straightforward components of magnetizations. Generally, the first and low stability component is isolated by heating to 100°C-300°C or by AF of 10-20mT. The magnetization directions after these steps have defined straight lines directed towards the origin, and interpreted as primary NRM or ChRM (Characteristic Remanent Magnetization). Results of the magnetization decay curve plots and rock magnetic analyses using Variable Field Translation Balance (VFTB) at the paleomagnetic laboratory of LMU, Germany indicate magnetic mineralogy is dominated by titanium poor titano-magnetite and magnetite with magnetic grain sizes with in the pseudo single domain range. The paleomagnetic site mean directional analyses reveal, 2 reversed, 32 normal, and 1 anomalous magnetic polarity directions. When an overall mean direction calculated for the entire sampled region; D=356.9°, I=11.7°, N=35, K=26.9, α95=4.8°, and a separate overall mean direction excluding the anomalous direction and results of seven sites located along the axes; D=354.4°, I=13.2°, N=26, K=43.5, α95=4.3° is obtained, which when compared

  13. Tephrostratigraphy of the Waki-Mille area of the Woranso-Mille paleoanthropological research project, Afar, Ethiopia.

    PubMed

    Saylor, Beverly Z; Angelini, Joshua; Deino, Alan; Alene, Mulugeta; Fournelle, John H; Haile-Selassie, Yohannes

    2016-04-01

    Tephra geochemistry and (40)Ar/(39)Ar geochronology are reported for the Waki-Mille area in the northwestern part of the Woranso-Mille paleoanthropological project area in the west central Afar region of Ethiopia. Previous studies documented dentognathic fossils that are morphologically intermediate between Australopithecus anamensis and Australopithecus afarensis and some that are attributed to Australopithecus afarensis. Additional dentognathic remains from the study area were assigned to the newly identified species Australopithecus deyiremeda. These fossil hominin taxa were recovered from volcanic and sedimentary strata containing tuffs ranging in age from more than 3.77 million years ago (Ma) to less than 3.469 Ma. One of the tuffs was correlated based on geochemistry, feldspar mineralogy, and age to the Lokochot Tuff of the Omo-Turkana Basin of southern Ethiopia and Kenya. Variations in major and minor element abundances in volcanic glass demarcate ten geochemically distinct tuffs and tuff sequences, including three that are geochemically similar to widespread regional tuffs, specifically the Lomogol, Lokochot, and β- Tulu Bor/Sidi Hakoma tuffs. A new (40)Ar/(39)Ar age for the Waki Tuff, which is geochemically similar to the Lomogol Tuff, is 3.664 ± 0.016 Ma. Other tuffs in the Waki-Mille area are geochemically dissimilar to regional tuffs documented to date. Identification of tuffs based on character, stratigraphic position, and geochemistry refines local stratigraphic correlations and delineates the geographic distributions of precisely dated fossiliferous levels within the Waki-Mille area. PMID:27086054

  14. Camelus dromedarius brucellosis and its public health associated risks in the Afar National Regional State in northeastern Ethiopia

    PubMed Central

    2013-01-01

    Background A cross-sectional study was carried out in four districts of the Afar region in Ethiopia to determine the prevalence of brucellosis in camels, and to identify risky practices that would facilitate the transmission of zoonoses to humans. This study involved testing 461 camels and interviewing 120 livestock owners. The modified Rose Bengal plate test (mRBPT) and complement fixation test (CFT) were used as screening and confirmatory tests, respectively. SPSS 16 was used to analyze the overall prevalence and potential risk factors for seropositivity, using a multivariable logistic regression analysis. Results In the camel herds tested, 5.4% had antibodies against Brucella species, and the district level seroprevalence ranged from 11.7% to 15.5% in camels. The logistic regression model for camels in a herd size > 20 animals (OR = 2.8; 95% CI: 1.16-6.62) and greater than four years of age (OR = 4.9; 95% CI: 1.45-16.82) showed a higher risk of infection when compared to small herds and those ≤ 4 years old. The questionnaire survey revealed that most respondents did not know about the transmission of zoonotic diseases, and that their practices could potentially facilitate the transmission of zoonotic pathogens. Conclusions The results of this study revealed that camel brucellosis is prevalent in the study areas. Therefore, there is a need for implementing control measures and increasing public awareness in the prevention methods of brucellosis. PMID:24344729

  15. Lithospheric structure and compensation mechanisms of the Galapagos Archipelago

    NASA Astrophysics Data System (ADS)

    Feighner, Mark A.; Richards, Mark A.

    1994-04-01

    Volcanic islands of the Galapagos Archipelago are the most recent subaerial expression of the Galapagos hotspot. These islands and numerous seamounts are constructed mainly upon a broad volcanic platform that overlies very young (less than 10 m.y.) oceanic lithosphere just south of the active Galapagos Spreading Center. The 91 deg W fracture zone crosses the platform and creates an estimated 5-m.y. age discontinuity in the lithosphere. Major tectonic features of the Galapagos include an unusually broad distribution of volcanic centers, pronounced structural trends such as the NW-SE Wolf-Darwin Lineament (WDL), and a steep escarpment along the western and southern margins of the archipelago. We use shipboard gravity and bathymetry data along with Geosat geoid data to explain the tectonic and structural evolution of the Galapagos region. We model the gravity anomalies using a variety of compensation models, including Airy isostasy, continuous elastic flexure of the lithosphere, and an elastic plate with embedded weaknesses, and we infer significant lithospheric strength variations across the archipelago. The outboard parts of the southern and western escarpment are flexurally supported with an effective elastic thickness of approximately 12 km. This area includes the large shield volcanoes of Fernandina and Isabela Islands, where the lithosphere regionally supports these volcanic loads. The central platform is weaker, with an elastic thickness of 6 km or less, and close to Airy isostasy. The greatest depths to the Moho are located beneath eastern Isabela Island and the central platform. Thinner lithosphere in this region may account for the broad distribution of volcanoes, the extended period of eruption of the central volcanoes, and their reduced size. The transition from strong to weak lithosphere along the southern escarpment appears to be abrupt, within the resolution of our models, and can be best represented by a free end or faultlike discontinuity. Also

  16. Thermal stresses due to cooling of a viscoelastic oceanic lithosphere

    SciTech Connect

    Denlinger, R.P. ); Savage, W.Z. )

    1989-01-10

    Theories based upon thermal contraction of cooling oceanic lithosphere provide a successful basis for correlating seafloor bathymetry and heat flow. The horizontal components of the contraction of the lithosphere as it cools potentially give rise to large thermal stresses. Current methods to calculate these stresses assume that on the time scales of cooling, the lithosphere initially behaves as an inviscid fluid and instantly freezes into an elastic solid at some critical temperature. These instant-freezing methods inaccurately predict transient thermal stresses in rapidly cooling silicate glass plates because of the temperature dependent rheology of the material. The temperature dependent rheology of the lithosphere may affect the transient thermal stress distribution in a similar way, and for this reason the authors use a thermoviscoelastic model to estimate thermal stresses in young oceanic lithosphere. This theory is formulated here for linear creep processes that have an Arrhenius rate dependence on temperature. Results show that the stress differences between instant freezing and linear thermoviscoelastic theory are most pronounced at early times (0-20 m.y.) when the instant freezing stresses may be twice as large. The solutions for the two methods asymptotically approach the same solution with time. A comparison with intraplate seismicity shows that both methods underestimate the depth of compressional stresses inferred from the seismicity in a systematic way.

  17. Thermal stresses due to cooling of a viscoelastic oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Denlinger, Roger P.; Savage, William Z.

    1989-01-01

    Theories based upon thermal contraction of cooling oceanic lithosphere provide a successful basis for correlating seafloor bathymetry and heat flow. The horizontal components of the contraction of the lithosphere as it cools potentially give rise to large thermal stresses. Current methods to calculate these stresses assume that on the time scales of cooling, the lithosphere initially behaves as an inviscid fluid and instantly freezes into an elastic solid at some critical temperature. These instant-freezing methods inaccurately predict transient thermal stresses in rapidly cooling silicate glass plates because of the temperature dependent rheology of the material. The temperature dependent rheology of the lithosphere may affect the transient thermal stress distribution in a similar way, and for this reason we use a thermoviscoelastic model to estimate thermal stresses in young oceanic lithosphere. This theory is formulated here for linear creep processes that have an Arrhenius rate dependence on temperature. Our results show that the stress differences between instant freezing and linear thermoviscoelastic theory are most pronounced at early times (0-20 m.y.) when the instant freezing stresses may be twice as large. The solutions for the two methods asymptotically approach the same solution with time. A comparison with intraplate seismicity shows that both methods underestimate the depth of compressional stresses inferred from the seismicity in a systematic way.

  18. Comprehensive plate models for the thermal evolution of oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Grose, Christopher J.; Afonso, Juan Carlos

    2013-09-01

    Seafloor spreading and the cooling of oceanic lithosphere is a fundamental feature of plate tectonics in the Earth, the details of which are unveiled by modeling with constraints from mineral physics and geophysical observations. To work toward a more complete model of the thermal evolution of oceanic lithosphere, we investigate the contributions of axial hydrothermal circulation, oceanic crust, and temperature-pressure-dependent thermal properties. We find that models with only temperature-dependent properties disagree with geophysical observations unless properties are artificially modified. On the other hand, more comprehensive models are in better agreement with geophysical observations. Our preferred model requires a thermal expansivity reduction of 15% from a mineral physics estimate, and predicts a plate thickness of about 110-130 km. A principal result of our analysis is that the oceanic crust is a major contributor to the cooling of oceanic lithosphere. The oceanic crust acts as an insulating lid on the mantle, causing the rate of lithospheric cooling to increase from "crustal" values near the ridge to higher mantle values at old-age. Major consequences of this insulation effect are: (a) low seafloor subsidence rate in proximity to ridge axes (<5 Ma), (b) the thermal structure of oceanic lithosphere is significantly warmer than previous models, (c) seafloor heat flow is significantly lower over young (<35 Ma) seafloor compared to simple models, (d) a low net seafloor heat flux (˜27 TW), and (e) temperature at the base of the seismogenic zone extends to 700-800°C mantle.

  19. Circum-Arctic lithospheric transects from onshore to offshore

    NASA Astrophysics Data System (ADS)

    Pease, V.; Coakley, B.; Faleide, J. I.; Jokat, W.; Miller, E. L.; Stephenson, R.; Meisling, K. E.

    2015-12-01

    Understanding the evolution of the lithosphere over time involves the integration and interpretation of geological and geophysical data, combined with good knowledge of the physical processes at work in the lithosphere giving rise to past and present structures. Tectonic activity related to the rifting process created the present-day structure of today's Arctic basins and bathymetric highs, and in the process modified older structures and architecture of the crust and lithosphere. The correlation of circum-Arctic terranes and orogens help to not only reconstruct paleogeography but to also define the role and determine the nature of the lithospheric processes that were active in the complex tectonic evolution of the Arctic. CALE (Circum Arctic Lithosphere Evolution), an international and multidisciplinary effort involving c. 35 geologists and geophysicists from ten different countries working to link the onshore and offshore regions across the circum-Arctic region, is a scientific network in it's last year of a 5-year program. Sedimentary cover and crust to mantle cross-sections from onshore to offshore have been created integrating the latest scientific knowledge and data sets available for the Arctic. The project's principal Arctic transects include: Ellesmere-Canada Basin, Pacific Ocean-Lomonosov Ridge through the Bering Strait, across the Laptev Sea rift to the DeLong Islands, Barents and Kara regions across Timan-Pechora and Taimyr. These sections, the culmination of the CALE project, and their principle findings will be presented for the first time with discussion of outstanding issues yet to be resolved.

  20. Tracing lithosphere amalgamation through time: chemical geodynamics of sub-continental lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Wittig, Nadine

    2014-05-01

    The theory of plate tectonics is a relatively young concept in the Earth Sciences and describes the surface expression of planetary cooling via magmatism and reconciles mantle convection and plate movement with orogenesis, earthquakes and volcanism. Detailed observation of current tectonic plate movement has purported a relatively clear picture of the planet's geodynamics. Modern oceanic basins are the predominant sites of thermal equilibration of Earth interior resulting from decompressional, convective melting of peridotites. This magmatism generates mid-ocean ridge mafic crust and depleted upper mantle and in this model, oceanic crust becomes associated with buoyant mantle to form oceanic lithosphere. Subduction zones return this material together with sediments into the deeper mantle and presumably aid the formation of continental crust via arc magmatism. The mechanisms of continental crust amalgamation with buoyant mantle are less clear, and distinctly more difficult to trace back in time because metamorphism and metasomatism render the processes associating convecting mantle with continental crust elusive. Paramount in assessing these mechanisms is understanding the timing of crust and mantle formation so that the onset of plate tectonics and potential changes in modi operandi with respect to convection, mantle composition and melting pressure and temperature may be traced from the early Hadean to the present day. Typically the formation age of continental crust is more easily determined from felsic samples that contain accessory and relatively robust phases such as zircon and monazite that render a geochronological approach feasible. The lack of equally robust minerals and pervasive and ubiquitous metasomatism afflicting obducted orogenic peridotites and mantle xenoliths obliterates primary mineralogical and geochemical information. Hence it has proven difficult to acquire mantle depletion ages from continental lithospheric mantle, perhaps with the exception

  1. Majorite Garnet and Lithosphere Evolution: Kaapvaal Craton

    NASA Astrophysics Data System (ADS)

    Griffin, W. L.; Tessalina, S.; O'Reilly, S. Y.

    2013-12-01

    The uppermost 50-70 km of the subcontinental lithospheric mantle (SCLM) beneath the Kaapvaal Craton (S. Africa) consists largely of highly-depleted chromite harzburgites. These rocks are understudied, mainly because of their uniformity and their lack of indicator minerals such as garnet and clinopyroxene (cpx). Kimberlite-borne xenoliths of these rocks contain rare volumes of cpx-spinel (modal 76/24) symplectite, with smooth grain boundaries; many studies have suggested that these might represent low-pressure breakdown products of garnet (majorite + olivine → cpx + spinel). Our reconstruction of a suite of these grains, using element mapping and EMP analysis of constituent minerals, gives a majoritic garnet with mean composition 21.8% CaO, 15.8% Cr2O3, 9.22% Al2O3, Si=3.118, mg#=0.93. The majorite contents suggest formation at depths of 250-280 km. Ni contents imply temperatures ≥1500 °C, but have large uncertainties related to the subtraction of olivine (ca 20%) during the reconstruction calculation. LAM-ICPMS analyses show strongly sinuous REE patterns with CN Dy/Lu <0.1 and Ce/Dy >100. Most analyses have negative Eu anomalies, consistent with chromite compositions that indicate strongly reducing conditions (ΔfO2(FMQ) = -4 to -5). Melt modeling suggests that the harzburgites are products of 30-40% melting of asthenospheric mantle at 250 km depth, leaving residues of ol+opx+chromite. The presence of the majorites and their overall LREE enrichment are ascribed to the introduction of carbonatitic metasomatic fluids, similar to those recorded by diamond-inclusions (subcalcic garnets), shortly after the depletion. We suggest that the melting, the metasomatism and the ultimate breakdown of the majorite track a process of mantle upwelling, with melt-extraction at depth providing the buoyancy that allowed the residual harzburgites to rise to shallow levels and stabilize the SCLM. Os-isotope analyses of sulfides associated with the majorites give TRD = 2.5-3.4 Ga

  2. Lithosphere formation in the central Slave Craton (Canada): plume subcretion or lithosphere accretion?

    NASA Astrophysics Data System (ADS)

    Karlsen, Stein Rune; Solheim, Inger; Beck, Pieter S. A.; Høgda, Kjell Arild; Wielgolaski, Frans Emil; Tømmervik, Hans

    2007-10-01

    Major-element compositions of minerals in peridotite xenoliths from the Lac de Gras kimberlites provide constraints on the mode of lithosphere formation beneath the central Slave Craton, Canada. Magnesia contents of reconstructed whole rocks correlate positively with NiO and negatively with CaO contents, consistent with variable partial melt extraction. Alumina and Cr2O3 contents are broadly positively correlated, suggestive of melt depletion in the absence of a Cr Al phase. Garnet modes are high at a given Al2O3 content (a proxy for melt depletion), falling about a 7 GPa melt depletion model. These observations, combined with high olivine Mg# and major-element relationships of FeO-poor peridotites (<7.5 wt%) indicative of melt loss at pressures >3 GPa (residual FeO content being a sensitive indicator of melt extraction pressure), and similar high pressures of last equilibration (˜4.2 to 5.8 GPa), provide multiple lines of evidence that the mantle beneath the central Slave Craton has originated as a residue from high-pressure melting, possibly during plume subcretion. Apparent low melt depletion pressures for high-FeO peridotites (>7.5 wt%) could suggest formation in an oceanic setting, followed by subduction to their depth of entrainment. However, these rocks, which are characterised by low SiO2 contents (<43 wt%), are more likely to be the result of post-melting FeO-addition, leading to spuriously low estimates of melt extraction pressures. They may have reacted with a silica-undersaturated melt that dissolved orthopyroxene, or experienced olivine injection by crystallising melts. A secular FeO-enrichment of parts of the deep mantle lithosphere is supported by lower average Mg# in xenolithic olivine (91.7) compared to olivine inclusions in diamond (92.6).

  3. Constraints on Composition, Structure and Evolution of the Lithosphere

    NASA Astrophysics Data System (ADS)

    Bianchini, Gianluca; Bonadiman, Costanza; Aulbach, Sonja; Schutt, Derek

    2015-05-01

    The idea for this special issue was triggered at the Goldschmidt Conference held in Florence (August 25-30, 2013), where we convened a session titled "Integrated Geophysical-Geochemical Constraints on Composition and Structure of the Lithosphere". The invitation to contribute was extended not only to the session participants but also to a wider spectrum of colleagues working on related topics. Consequently, a diverse group of Earth scientists encompassing geophysicists, geodynamicists, geochemists and petrologists contributed to this Volume, providing a comprehensive overview on the nature and evolution of lithospheric mantle by combining studies that exploit different types of data and interpretative approaches. The integration of geochemical and geodynamic datasets and their interpretation represents the state of the art in our knowledge of the lithosphere and beyond, and could serve as a blueprint for future strategies in concept and methodology to advance our knowledge of this and other terrestrial reservoirs.

  4. A global view of the lithosphere-asthenosphere boundary.

    PubMed

    Rychert, Catherine A; Shearer, Peter M

    2009-04-24

    The lithosphere-asthenosphere boundary divides the rigid lid from the weaker mantle and is fundamental in plate tectonics. However, its depth and defining mechanism are not well known. We analyzed 15 years of global seismic data using P-to-S (Ps) converted phases and imaged an interface that correlates with tectonic environment, varying from 95 +/- 4 kilometers beneath Precambrian shields and platforms to 81 +/- 2 kilometers beneath tectonically altered regions and 70 +/- 4 kilometers at oceanic island stations. High-frequency Ps observations require a sharp discontinuity; therefore, this interface likely represents a boundary in composition, melting, or anisotropy, not temperature alone. It likely represents the lithosphere-asthenosphere boundary under oceans and tectonically altered regions, but it may constitute another boundary in cratonic regions where the lithosphere-asthenosphere boundary is thought to be much deeper. PMID:19390041

  5. Gravity gradient grids at GOCE satellite altitude for lithospheric modelling

    NASA Astrophysics Data System (ADS)

    Bouman, Johannes; Ebbing, Jörg; Sebera, Josef; Fuchs, Martin; Lieb, Verena; Holzrichter, Nils; Novak, Pavel; Haagmans, Roger

    2015-04-01

    We explore how GOCE gravity gradient data can improve modeling of the Earth's lithosphere and thereby contribute to a better understanding of the Earth's dynamic processes. We study the use of gravity gradient grids to provide improved information about the lithosphere and upper mantle in the well-surveyed North-East Atlantic Margin. In particular, we present the computation of gravity gradient grids at GOCE satellite altitude combining GOCE with GRACE gravity information. It is shown that regional solutions based on a tesseroid approach may contain more signal content than global gravity field models do. The patchwork of regional grids is presented as well as the subsequent error reduction through iterative downward and upward continuation using the Poisson integral equation. The promises and pitfalls are discussed of using grids at nominal altitude of 255 km and a lower altitude of 225 km for lithospheric modeling.

  6. Oceanic lithosphere and asthenosphere: The thermal and mechanical structure

    NASA Technical Reports Server (NTRS)

    Schubert, G.; Froidevaux, C.; Yuen, D. A.

    1976-01-01

    A coupled thermal and mechanical solid state model of the oceanic lithosphere and asthenosphere is presented. The model includes vertical conduction of heat with a temperature dependent thermal conductivity, horizontal and vertical advection of heat, viscous dissipation or shear heating, and linear or nonlinear deformation mechanisms with temperature and pressure dependent constitutive relations between shear stress and strain rate. A constant horizontal velocity u sub 0 and temperature t sub 0 at the surface and zero horizontal velocity and constant temperature t sub infinity at great depth are required. In addition to numerical values of the thermal and mechanical properties of the medium, only the values of u sub 0, t sub 0 and t sub infinity are specified. The model determines the depth and age dependent temperature horizontal and vertical velocity, and viscosity structures of the lithosphere and asthenosphere. In particular, ocean floor topography, oceanic heat flow, and lithosphere thickness are deduced as functions of the age of the ocean floor.

  7. Magmatism & lithospheric destruction along the Colorado Plateau margin

    NASA Astrophysics Data System (ADS)

    Bendersky, C.; Plank, T.; Forsyth, D. W.; Hauri, E. H.; Lee, C.; Holtzman, B. K.

    2012-12-01

    The process of cratonic lithosphere deformation remains mysterious. The Colorado Plateau (CP), including its underlying lithosphere, has persisted for over a 1Ga, while in parts of the adjacent Basin and Range (B&R) Cenozoic extension has thinned the lithosphere by half. Today, extensional processes are focused in the transition zone between these two regions, which is defined by a region of volcanism and active faulting. We combine seismic tomography models from EarthScope data with melt thermobarometry from mafic scoria erupted in three volcanic fields since 100ka to investigate lithospheric deformation in this actively rifting area. Our sample locations lie along the western and southwestern margin of the CP and record different evolutionary stages in the process of lithosphere destruction via melt infiltration. For each volcanic area studied we use seismic profiles of shear wave velocities (Vs) with pressures and temperatures of mantle-melt equilibrium calculated using the Si and Mg thermobarometer (Lee et al 2009). The thermobarometric results depend highly on the water and Fe3+ content of the melts, which were constrained separately for each volcanic field. Magmatic water contents have been determined by ion-microprobe measurements of olivine hosted melt inclusions. Fe/ΣFe+3 ratios were estimated for each volcanic field via LA-ICP-MS analysis of V in olivine and whole rock compositions (Canil 2002). In the northernmost volcanic field, Black Rock (BRVF), Utah, melts are hot (consistent with mantle potential temperature (Tp) >1400°C), dry (≤1 wt% H2O), and have equilibrated at shallow depths (<70 km), within the seismic lid. Shear velocities in this lid, however, are anomalously slow (4.1 km/s), and the mantle beneath (Vs ~ 3.9 km/s), is the slowest in the B&R, coincident with the highest and most focused extension rates (Wasatch Fault Zone). Together, these observations support high mantle temperatures, inefficient melt extraction, and a weak lithosphere

  8. Lithospheric records of orogeny within the continental U.S.

    NASA Astrophysics Data System (ADS)

    Porter, Ryan; Liu, Yuanyuan; Holt, William E.

    2016-01-01

    In order to better understand the tectonic evolution of the North American continent, we utilize data from the EarthScope Transportable Array network to calculate a three-dimensional shear velocity model for the continental United States. This model was produced through the inversion of Rayleigh wave phase velocities calculated using ambient noise tomography and wave gradiometry, which allows for sensitivity to a broad depth range. Shear velocities within this model highlight the influence of orogenic and postorogenic events on the evolution of the lithosphere. Most notable is the contrast in crustal and upper mantle structure between the relatively slow western and relatively fast eastern North America. These differences are unlikely to stem solely from thermal variations within the lithosphere and highlight both the complexities in lithospheric structure across the continental U.S. and the varying impacts that orogeny can have on the crust and upper mantle.

  9. Plume-lithosphere interaction beneath a fast moving plate

    NASA Astrophysics Data System (ADS)

    Thoraval, Catherine; Tommasi, Andréa; Doin, Marie-Pierre

    2006-01-01

    Two-dimensional numerical simulations of mantle convection with temperature and pressure dependent viscosity are used to study plume-lithosphere interaction beneath a fast moving plate. Plumes behavior and, hence, their erosional and melting potential, depend on the Rayleigh number and plume buoyancy flux. Analysis of the balance between large-scale and plume-induced flow and of the ability of the plume to melt allows to put bounds on the upper mantle viscosity (1020, 1021 Pas), on the plumes diameter (<200 km) and temperature anomaly (200-400°C). Within this range of parameters, strongly time-dependent small-scale instabilities form in the plume-lithosphere boundary layer. They lead to thermal rejuvenation of the lithosphere downstream from the plume. The 1200°C isotherm is raised by up to 30km, but the 800°C isotherm is hardly moved, leading to a steep transient geotherm at the base of the plate.

  10. Preliminary results of ERTS-investigations by W-German investigations. [multidisciplinary geoscientific experiments in central Germany and hydrogeology of Argentina Pampas

    NASA Technical Reports Server (NTRS)

    Muehlfeld, R.

    1974-01-01

    Results are presented of West German investigations into multidisciplinary geoscientific experiments in central Germany and the Alps, and hydrogeological investigations in the Pampa of Argentina based on ERTS-1 data. The main goals of the investigation were achieved. The studies have given a good idea of the possibilities and limitations of ERTS imagery depending on the objectives in question and on the geographical conditions of the areas under investigation. Even in the well known region of central Europe, ERTS has proven its ability of improving present knowledge. In fields such as pollution monitoring and regional planning the satellite techniques should have distinct practical value. For any regional study of less known areas, the value of ERTS imagery can hardly be overestimated.

  11. Understanding lithospheric stresses in Arctic: constraints and models

    NASA Astrophysics Data System (ADS)

    Medvedev, Sergei; Minakov, Alexander; Lebedeva-Ivanova, Nina; Gaina, Carmen

    2016-04-01

    This pilot project aims to model stress patterns and analyze factors controlling lithospheric stresses in Arctic. The project aims to understand the modern stresses in Arctic as well as to define the ways to test recent hypotheses about Cenozoic evolution of the region. The regions around Lomonosov Ridge and Barents Sea are of particular interest driven by recent acquisition of high-resolution potential field and seismic data. Naturally, the major contributor to the lithospheric stress distribution is the gravitational potential energy (GPE). The study tries to incorporate available geological and geophysical data to build reliable GPE. In particular, we use the recently developed integrated gravity inversion for crustal thickness which incorporates up-to-date compilations of gravity anomalies, bathymetry, and sedimentary thickness. The modelled lithosphere thermal structure assumes a pure shear extension and the ocean age model constrained by global plate kinematics for the last ca. 120 Ma. The results of this approach are juxtaposed with estimates of the density variation inferred from the upper mantle S-wave velocity models based on previous surface wave tomography studies. Although new data and interpretations of the Arctic lithosphere structure become available now, there are areas of low accuracy or even lack of data. To compensate for this, we compare two approaches to constrain GPE: (1) one that directly integrates density of modelled lithosphere and (2) one that uses geoid anomalies which are filtered to account for density variations down to the base of the lithosphere only. The two versions of GPE compared to each other and the stresses calculated numerically are compared with observations. That allows us to optimize GPE and understand density structure, stress pattern, and factors controlling the stresses in Arctic.

  12. A Sharp Edge of the Cratonic Lithosphere of North America

    NASA Astrophysics Data System (ADS)

    Harper, T. B.; Skryzalin, P. A.; Menke, W. H.; Levin, V. L.; Darbyshire, F. A.

    2015-12-01

    Using teleseismic travel time delays, we develop a tomographic model of the lithosphere beneath northeastern North America, from the shore of James Bay in Quebec to the Atlantic coast of New England and to a depth of 300 km. Three major terranes lie within this cratonic margin: the 2.7 Ga Superior province, the 1 Ga Grenville orogenic belt and the 0.3-0.4 Ga Appalachian terranes, which are bounded by the Grenville Front (GF) and Appalachian Front (AF), respectively. Additionally, the 0.8 Ga Avalon terrain was accreted to coastal New England by strike-skip faulting during the Appalachian orogeny. Our tomographic model uses earthquake seismograms recorded by permanent US and Canadian stations, the Transportable Array and the temporary QMIII deployment. All data were corrected for instrument response and record sections were examined visually to identify gross errors in response and timing. Differential arrival times of P and PKP waves were determined by cross-correlation and have a maximum amplitude of about ±1 second. In our model, lithospheric boundaries do not correlate well with geological boundaries, nor do they strike parallel to them. The seismically-fast (by 5% relative to AK135) cratonic lithosphere of North America is much thicker than that of the younger terranes, extending to 200 km or more depth but with a sharp east-dipping eastern edge located (at Moho depths) 100-250 km northwest of the GF. The lithosphere beneath the Grenville and Appalachian terranes, which were affected by subduction during the Grenville and Appalachian orogenies, is slower (by 4%). A sliver of seismically-fast lithosphere, extending to ~150 km depth, occurs along the Atlantic coast and is interpreted as the Avalonian lithosphere.

  13. Feed intake, digestibility, body weight and carcass parameters of Afar rams fed tef (Eragrostis tef) straw supplemented with graded levels of concentrate mix.

    PubMed

    Hagos, Tesfay; Melaku, Solomon

    2009-04-01

    The experiment was conducted at Alamata Agricultural Research Center, Ethiopia using 20 Afar rams with an initial body weight (BW) of 18.2 +/- 1.76 (mean +/- SD) kg. The objectives were to study the effect of supplementation with concentrate mix consisting of wheat bran (WB), noug seed cake (NSC) and sesame seed cake (SSC) at the ratio of 2:1:1 on dry matter (DM) basis, respectively on feed intake, digestibility, BW gain and carcass parameters of Afar rams fed tef (Eragrostis tef) straw basal diet. The experiment was arranged with four treatments and five replications in a randomized complete block design. The treatments included feeding sole tef straw (T1, control), and daily supplementation with the concentrate mix offered at 150 (T2, low), 250 (T3, medium) and 350 (T4, high) g DM per head. Total DM intake, crude protein (CP) digestibility, daily BW gain (P < 0.001), DM and organic matter (OM) digestibility, and carcass parameters (P < 0.05) were higher in the supplemented than in the control treatment. Intake of tef straw reduced as the level of supplementation increased, whereas the contrary was true for CP intake. Performance in carcass parameters was better for the medium compared to the low level of concentrate mix supplementation. Moreover, the medium level of supplementation did not substitute tef straw intake. Therefore, it is concluded that the medium level of concentrate mix supplement maintained the utilization of the roughage feed and resulted in better carcass parameters. PMID:18777140

  14. Remobilization in the cratonic lithosphere recorded in polycrystalline diamond

    PubMed

    Jacob; Viljoen; Grassineau; Jagoutz

    2000-08-18

    Polycrystalline diamonds (framesites) from the Venetia kimberlite in South Africa contain silicate minerals whose isotopic and trace element characteristics document remobilization of older carbon and silicate components to form the framesites shortly before kimberlite eruption. Chemical variations within the garnets correlate with carbon isotopes in the diamonds, indicating contemporaneous formation. Trace element, radiogenic, and stable isotope variations can be explained by the interaction of eclogites with a carbonatitic melt, derived by remobilization of material that had been stored for a considerable time in the lithosphere. These results indicate more recent formation of diamonds from older materials within the cratonic lithosphere. PMID:10947983

  15. Extensional and compressional instabilities in icy satellite lithospheres

    SciTech Connect

    Herrick, D.L.; Stevenson, D.J. )

    1990-05-01

    The plausibility of invoking a lithospheric instability mechanism to account for the grooved terrains on Ganymede, Encedalus, and Miranda is presently evaluated in light of the combination of a simple mechanical model of planetary lithospheres and asthenospheres with recent experimental data for the brittle and ductile deformation of ice. For Ganymede, high surface gravity and warm temperatures render the achievement of an instability sufficiently great for the observed topographic relief virtually impossible; an instability of sufficient strength, however, may be able to develop on such smaller, colder bodies as Encedalus and Miranda. 15 refs.

  16. An elastoviscoplastic finite element model of lithospheric deformation

    NASA Astrophysics Data System (ADS)

    Albert, Richard Alan

    1998-12-01

    Via the finite element method, the stress state and deformation of the lithosphere were investigated with topographic loading. An elastoviscoplastic (EVP) rheology governed the mechanical response in a 40 km thick lithospheric plate of wet olivine. The viscous aspect of the rheology utilized a steady-state dislocation creep constitutive relation. The plastic part of the rheology treated frictional slip on faults within pervasively fractured rock via Byerlee's rule. The first studies use a time-invariant, steady-state conduction temperature distribution in the plate. These studies involved topographic loading with different maximum loads and different load growth rates. The EVP results were compared to elastic perfectly-plastic (EPP) solutions for plate bending as constrained by the Yield Strength Envelope (YSE) formulation for lithospheric mechanics. The EVP results included the often-overlooked effect that the load has on strengthening underlying rock against brittle deformation. The creep strain rate in the EVP models varied with time, depth, and lateral location, unlike the EPP/YSE models that constrain all creep to a single a priori creep strain rate. At the transition from frictional slip to creep, the EVP models showed a three km zone with contributions from both mechanisms, relative to the EPP/YSE's artificially sharp and immediate transition. The last study incorporated two variations on the temperature distribution in the EVP lithosphere. The "whole-lithosphere cooling model" cooled the plate during and after the load growth period, following the half-space cooling model for oceanic lithosphere. Unlike its non-cooling counterpart, the whole-lithosphere cooling model showed no further frictional slip after the load growth period. The "magma conduit model" used an initial temperature distribution that had a high temperature along a vertical symmetry axis during loading to approximate temperature effects from a magma conduit. After loading, its temperature

  17. Revisiting the Ridge-Push Force Using the Lithospheric Geoid

    NASA Astrophysics Data System (ADS)

    Richardson, R. M.; Coblentz, D. D.

    2014-12-01

    The geoid anomaly and driving force associated with the cooling oceanic lithosphere ("ridge push") are both proportional to dipole moment of the density-depth distribution, and allow a reevaluation of the ridge push force using the geoid. The challenge with this approach is to isolate the "lithospheric geoid" from the full geoid signal. Our approach is to use a band-pass spherical harmonic filter on the full geoid (e.g., EGM2008-WGS84, complete to spherical harmonic degree and order 2159) between orders 6 and 80. However, even this "lithospheric geoid" is noisy, and thus we average over 100 profiles evenly spaced along the global ridge system to obtain an average geoid step associated with the mid-ocean ridges. Because the positive ridge geoid signal is largest near the ridge (and to capture fast-spreading ridges), we evaluate symmetrical profiles extending ±45 m.y. about the ridge. We find an average ridge geoid anomaly of 4.5m, which is equivalent to a 10m anomaly for 100 m.y. old oceanic lithosphere. This geoid step corresponds to a ridge push force of ~2.4 x1012N/m for old oceanic lithosphere of 100 m.y., very similar to earlier estimates of ~2.5 x1012N/m based on simple half-space models. This simple half-space model also predicts constant geoid slopes of about 0.15 m/m.y. for cooling oceanic lithosphere. Our observed geoid slopes are consistent with this value for ages up to 40-50 m.y., but drop off to lower values at greater ages. We model this using a plate cooling model (with a thickness of the order of 125km) to fit the observation that the geoid anomaly and ridge driving force only increase slowly for ages greater than 40 m.y. (in contrast to the half-space model where the linear dependence on age holds for all ages). This reduction of the geoid slope results in a 20% decrease in the predicted ridge push force. This decrease is due to the combined effects of treating the oceanic lithosphere as a cooling plate (vs. a half-space), and the loss of geoidal

  18. Seismic Tomography of the Arctic Lithosphere and Asthenosphere

    NASA Astrophysics Data System (ADS)

    Schaeffer, Andrew; Lebedev, Sergei

    2015-04-01

    Lateral variations in seismic velocities in the upper mantle, mapped by seismic tomography, primarily reflect variations in the temperature of the rocks at depth. Seismic tomography thus provides a proxy for lateral changes in the temperature and thickness of the lithosphere, in addition to delineating the deep boundaries between tectonic blocks with different properties and age of the lithosphere. Our new, 3D tomographic model of the upper mantle and the crust of the Arctic region is constrained by an unprecedentedly large global dataset of broadband waveform fits (over one million seismograms) and provides improved resolution of the lithosphere, compared to other available models. The most prominent high-velocity anomalies, seen down to 150-200 km depths, indicate the cold, thick, stable mantle lithosphere beneath Precambrian cratons. The northern boundaries of the Canadian Shield's and Greenland's cratonic lithosphere closely follow the coastlines, with the Greenland and North American cratons clearly separated from each other. Sharp velocity gradients in western Canada indicate that the craton boundary at depth closely follows the Rocky Mountain Front. High velocities between the Great Bear Arc and Beaufort Sea provide convincing evidence for the recently proposed 'MacKenzie Craton', unexposed at the surface. In Eurasia, cratonic continental lithosphere extends northwards beneath the Barents and eastern Kara Seas. The boundaries of the Archean cratons and intervening Proterozoic belts mapped by tomography indicate the likely offshore extensions of major Phanerozoic sutures and deformation fronts. The old oceanic lithosphere of the Canada Basin is much colder and thicker than the younger lithosphere beneath the adjacent Amundsen Basin, north of the Gakkel Ridge. Beneath the slow-spreading Gakkel Ridge, we detect the expected low-velocity anomaly associated with partial melting in the uppermost mantle; the anomaly is weaker, however, than beneath faster

  19. Satellite Gravity Gradients Complementing Seismology for Imaging of the Lithosphere

    NASA Astrophysics Data System (ADS)

    Ebbing, J.

    2014-12-01

    Satellite gravity gradients as for example derived from the recent GOCE satellite mission can be used to improve modeling of the Earth's lithosphere and thereby contribute to a better understanding of the Earth's dynamic processes. In general, gravity gradient data are sensitive to shallower structures than the gravity field itself and provide information about the variations in both the horizontal and vertical plane. Validation of satellite data in different orbit heights show that the gradients in different heights have a significantly different sensitivity, which can be exploited to construct the most reasonable lithospheric setting. To explore the benefit by using gravity gradients in addition to conventional gravity data, a case example from the well-explored and understood North-East Atlantic Margin will be shown. Here, a 3D model of the lithosphere preexisted that incorporates a wealth of geophysical data sets, e.g. seismics, magnetics and borehole information. The model is initially optimized for near-surface gravity data. However, in the NE Atlantic the gravity field is affected by a regional trend, which is reflected as well in the geoid, and associated to sub-lithospheric density domains. Using this model for sensitivity analysis shows that the satellite gravity gradients are little affected by the sub-lithospheric field, but are especially sensitive to the density contrasts from the lower crust to 100 km depth. Another important observation is that modeling of the gravity gradients requires depth-dependent crustal densities and temperature dependent upper mantle densities. A too simplified use of average densities leads to clear misfit to the observed data. Satellite gravity gradients are as well sensitive to compositional changes in the upper mantle, but for to decipher the thermal structure and composition integration with information from seismic tomography is needed. In summary, the sensitivity analysis in the NE Atlantic region shows, that

  20. Extensional and compressional instabilities in icy satellite lithospheres

    NASA Technical Reports Server (NTRS)

    Herrick, David L.; Stevenson, David J.

    1990-01-01

    The plausibility of invoking a lithospheric instability mechanism to account for the grooved terrains on Ganymede, Encedalus, and Miranda is presently evaluated in light of the combination of a simple mechanical model of planetary lithospheres and asthenospheres with recent experimental data for the brittle and ductile deformation of ice. For Ganymede, high surface gravity and warm temperatures render the achievement of an instability sufficiently great for the observed topographic relief virtually impossible; an instability of sufficient strength, however, may be able to develop on such smaller, colder bodies as Encedalus and Miranda.

  1. Seismic imaging of the downwelling Indian lithosphere beneath central Tibet.

    PubMed

    Tilmann, Frederik; Ni, James

    2003-05-30

    A tomographic image of the upper mantle beneath central Tibet from INDEPTH data has revealed a subvertical high-velocity zone from approximately 100- to approximately 400-kilometers depth, located approximately south of the Bangong-Nujiang Suture. We interpret this zone to be downwelling Indian mantle lithosphere. This additional lithosphere would account for the total amount of shortening in the Himalayas and Tibet. A consequence of this downwelling would be a deficit of asthenosphere, which should be balanced by an upwelling counterflow, and thus could explain the presence of warm mantle beneath north-central Tibet. PMID:12775838

  2. Tibetan Apples and Oranges: Surficial Sutures and Overlapping Lithospheres

    NASA Astrophysics Data System (ADS)

    Chen, W. P.; Hung, S. H.; Wang, C. Y.; Tseng, T. L.

    2014-12-01

    Multi-scale, finite-frequency tomography across the Himalayas-Tibet shows a clear, subhorizontal anomaly of high VP and VSin the upper mantle that can be traced from under N. India all the way to beneath central Tibet. This configuration of the "Greater India" (GI), or the submerged, northern portion of the Indian shield, is combined with other constraints to reconstruct position of the Indian lithospheric mantle (ILM) relative to Asia back to about 15 Ma ago, or the onset of the latest magmatic activity in Tibet. By then, the leading edge of the ILM (Indian mantle front, IMF) has advanced subhorizontally past the entire Lhasa terrane and also probably have caused the lithospheric mantle beneath the Qiangtang terrane to thicken. The thickening is likely to have led to Rayleigh-Taylor instability, causing widespread but small volume of magmatic activity in northern Tibet. Meanwhile, the detached lithospheric mantle foundered through the upper mantle to rest at the bottom of the mantle transition zone (MTZ), just above the lower mantle. This detached lithospheric mantle manifests itself as a large-scale seismic anomaly of high compressional wave speed (VP) but curiously is undetectable through shear-waves. Based on laboratory data for nominally anhydrous olivine and its high-pressure polymorphs (NAO), the discordant results between P- and S-waves is explained by abundant hydroxyls in the foundered lithospheric mantle, a hypothesis supported by other evidences as well. Since NAO can hold ~1 wt% of water throughout the upper mantle and the MTZ, foundering of thickened lithospheric mantle caused by continental collision is an under-appreciated but effective pathway for water to enter the deep mantle. Currently, the Indus-Yarlung suture between northern India and the Lhasa terrane appears to be an inactive, crustal feature, as the GI continues to pass beneath it. On the other hand, even though the IMF has now advanced northward beyond the Bangong-Nujiang suture (BNS

  3. Project Skippy explores lithosphere and mantle beneath Australia

    NASA Astrophysics Data System (ADS)

    van der Hilst, Rob; Kennett, Brian; Christie, Doug; Grant, John

    A new project is probing the seismic structure of the lithosphere and mantle beneath Australia. The Skippy Project, named after the bush kangaroo, exploits Australia's regional seismicity and makes use of recent advances in digital recording technology to collect three-component broadband seismic data from over 60 sites across the continent (Figure 1).The main goal of the Skippy Project, which is run by Australian National University's Research School of Earth Sciences (RSES), is to delineate the three-dimensional seismic structure of the lithosphere and mantle beneath the continent.

  4. Witnessing the birth of a new ocean? The first 6 years of the Dabbahu rifting episode, and other activity in Afar

    NASA Astrophysics Data System (ADS)

    Wright, T.; Ayele, A.; Barnie, T.; Belachew, M.; Calais, E.; Field, L.; Hamling, I.; Hammond, J.; Keir, D.

    2012-04-01

    Intense earthquake activity and a small rhyolitic eruption in September 2005 heralded the onset of an unprecedented period of geological activity in the Afar Depression. The seismic activity accompanied dyke intrusion in the upper 10 km of crust along 60 km of the Dabbahu (northern Manda-Hararo) Magmatic Segment (DMS) of the Nubia-Arabia plate boundary, a nascent seafloor spreading centre. InSAR observations of the resulting deformation showed that the initial dyke was up to 8 m thick, with a total volume of 2-2.5 km3. Urgency funding from the UK Natural Environmental Research Council (NERC) and US National Science Foundation (NSF) enabled us to deploy a local array of seismometers in October 2005, continuous GPS instruments in January 2006, and to acquire a dense time series of satellite radar images. The medium-term viability of these instruments was secured with major follow-on funding from NSF and NERC; these projects supported the collection and analysis of additional unique data sets, including data from a broader array of seismic and GPS instruments, magneto-telluric transects of the rift, airborne LiDAR, petrological sampling and micro-gravity work. The combination of these data has allowed us to quantify the processes associated with crustal growth at divergent plate boundaries for the first time. Here, we present a broad overview of geological activity in the Afar depression in the hyperactive 21st century. Activity in the DMS began after September 2000, when Gabho volcano at the north of the segment began uplifting, as its magma chamber, ~3 km below the surface, was replenished. It is likely that the inflation at Gabho ultimately triggered the onset of the Dabbahu rifting episode. The rifting episode began with intense seismicity at the northern end of the DMS, before jumping to the Ado Ale Volcanic Complex at the segment centre. This initial dyking was fed from shallow (~3 km) chambers at Gabho and Dabbahu as well as a deeper (~10 km) source at Ado Ale

  5. Some Problems of the Lithosphere (Augustus Love Medal Lecture)

    NASA Astrophysics Data System (ADS)

    Houseman, Gregory A.

    2015-04-01

    In 1911 Augustus Love published a monograph: Some Problems of Geodynamics which in part dealt with the problem of isostasy and the support of mountain belts. In doing so he was one of the first authors to use the concept of the lithosphere. Although his analysis used the framework of linear elasticity, he clearly recognised that the evident structural heterogeneity of the Earth's crust could not simply be interpreted in terms of elastic displacement, and he had no simple explanation for what processes had produced the major topographic features of the Earth: continents, oceans and mountain belts. Today we have a far more complete understanding of those processes, but there are still unresolved problems. In this presentation I will focus on two of those problems that are of particular interest in understanding the geological evolution of the continents: the relationship of near-surface faults and ductile deformation in the lithosphere, and the stability of continental lithosphere in actively deforming zones. While the lithosphere certainly manifests elastic strain, most notably in the context of earthquakes and seismic waves, the large strains that have shaped the continents result from diffuse ductile strain at the deeper levels, coupled with movement on fault planes in the upper crust. Although plates in many regions move coherently with little internal deformation, the stresses that act on different parts of a plate may cause broad deformation zones to develop within a plate interior. Plate boundaries that cross continental regions also typically involve broadly distributed deformation. In recent years the distribution of deformation in such regions is measured accurately using GPS, and in general is explained well by a model in which the lithosphere behaves as a thin viscous sheet, albeit with a non-linear temperature-dependent viscosity law. Such models are broadly consistent with laboratory deformation experiments on small rock samples. However, the

  6. Effective Elastic Thickness of the Lithosphere in Continental China from Heat Flow: Implications for the Lithospheric Rheology

    NASA Astrophysics Data System (ADS)

    Liu, S.; Wang, L.

    2006-12-01

    The effective elastic thickness (Te) of lithosphere is one parameter describing the responses of the lithosphere to long term forces, and is still controversial in estimation by different methods. Here we present the effective elastic thickness of the lithosphere in continental China from heat flow data by the method proposed by Burov et al, J.G.R., 1995,100(B3):3905-3927. Our results show that Te varies much in different sub-areas in continental China due to different geological evolution and associated thermal regimes. Te is much greater than the crustal thickness in the area where the heat flow is really low and the lithosphere is really thick, indicating much more contribution from the lithospheric mantle and the dominative control of the mantle with olivine on the rheology of the lithosphere, and the major basins (Tarim, Junggar, Ordos and Sichuan basins) in central-western China share this characteristic. For instance, the Te of the Tarim basin is 66km with crustal thickness of 45km. Te is less than the crustal thickness in the region where the heat flow is relatively high, and approximates to the crustal brittle-ductile transition depth, suggesting more contribution from the crust and the dominative control of the felsic crust on the rheology of the lithosphere, and this phenomenon is obvious in the SE coastal China, eastern North China and the orogenic belts. Compared the estimated Te with the seismogenic layer thickness (Ts) available in China, it is also found that the Te is much greater than Ts in the major basins with low heat flow, and is similar to Ts in the active zones with high heat flow, which is inconsistent with that Te is usually smaller than Ts proposed by Maggi et al., Geology,2000,28(6):495-498. Generally, two end elements rheological modes for continental lithosphere of the strong crust-weak mantle and the weak crust-strong mantle are all available in continental China considering different thermal regime, composition and geological

  7. Testing models of dike intrusion during rifting episodes: The role of heating in triggering earthquakes in Afar

    NASA Astrophysics Data System (ADS)

    Kulpinski, K.; Cote, D. M.; Ebinger, C. J.; Keir, D.; Ayele, A.

    2009-12-01

    In September 2005, a major rifting episode occurred in the western Afar depression. Following the initial intrusion of an approximately 60 km-long, up to 8m-wide dike, seismic stations were deployed to the surrounding region. Since the 2005 deployment, discrete magma intrusions have been observed propagating along the rift axis as migrating swarms of moderate magnitude earthquakes that occur over times of vertical and horizontal deformation imaged in satellite radar interferometry. Between these discrete diking events, however, persistent, moderate magnitude (mb > 2 -4) earthquakes occur singly and in swarms along the length of the 2005 dike intrusion zone. More intense zones of seismicity correspond to areas of greater strain in the 2005 and subsequent dikes. Is the persistent along-axis seismicity indicative of ongoing, small volume dike intrusions at depth, tectonic faulting in response to the original dike intrusion, or is it caused by thermal stresses of dike solidification and cooling? Differentiating between mechanisms provides insights into rock failure and magma intrusion processes. A simple one-dimensional dike model is presented as an instantaneous, purely thermal (i.e. neglecting volumetric changes due to fluid emplacement) intrusion that cools by conduction to the surrounding host rock. The associated thermal stresses with a large volume intrusion (~0.05km3 or greater) are on the order of tens of MPa for several months. We compare seismogenic layer thickness variations with yield stress envelopes at the dike wall at a range of times after dike emplacement. The yield stress for the depth of the earthquakes is of the same order or less, meaning the thermal stresses are large enough to contribute to causing the mb 2-4 earthquakes observed in the region. This is evidence that earthquakes not only correspond to the rock breaking in front of the dike, but also mark the dike's path by the generation of thermally induced earthquakes in the wallrock. A second

  8. Low geomagnetic field intensity in the Matuyama Chron: palaeomagnetic study of a lava sequence from Afar depression, East Africa

    NASA Astrophysics Data System (ADS)

    Ahn, Hyeon-Seon; Kidane, Tesfaye; Yamamoto, Yuhji; Otofuji, Yo-ichiro

    2016-01-01

    Palaeointensity variation is investigated for an inferred time period spanning from 2.34 to 1.96 Ma. Twenty-nine consecutive lava flows are sampled along cliffs 350 m high generated by normal faulting on the Dobi section of Afar depression, Ethiopia. Magnetostratigraphy and K-Ar measurements indicate a lava sequence of R-N-R-N geomagnetic field polarities in ascending order; the lower normal polarity is identified as the Réunion Subchron. Reliability of palaeomagnetic data is ascertained through careful thermal demagnetization and by the reversal test. The Tsunakawa-Shaw method yielded 70 successful palaeointensity results from 24 lava flows and gave 11 acceptable mean palaeointensities. Reliability in palaeointensity data is ascertained by the similar values obtained by the IZZI-Thellier method and thus 11 reliable mean values are obtained from our combined results. After the older reverse polarity with the field intensity of 19.6 ± 7.8 μT, an extremely low palaeointensity period with an average of 6.4 μT is shown to occur prior to the Réunion Subchron. During the Réunion Subchron, the dipole field strength is shown to have returned to an average of 19.5 μT, followed by second extreme low of 3.6 μT and rejuvenation with 17.1 ± 5.3 μT in the younger reverse polarity. This `W-shape' palaeointensity variation is characterized by occurrences of two extremely weak fields lower than 8 μT prior to and during the Réunion Subchron and a relatively weak time-averaged field of approximately 15 μT. This feature is also found in sedimentary cores from the Ontong Java Plateau and the north Atlantic, indicative of a possibly global geomagnetic field phenomenon rather than a local effect on Ethiopia. Furthermore, we estimate a weak virtual axial dipole moment of 3.66 (±1.85) × 1022 Am2 during early stage of the Matuyama Chron (inferred time period of 2.34-1.96 Ma).

  9. InSAR analysis of diking and incipient oceanic spreading in the Danakil depression of the northern Afar rift

    NASA Astrophysics Data System (ADS)

    Gomez, F. G.; Tesfaye, S.

    2011-12-01

    The Danakil depression in the northern part of the Afar rift is a location of incipient oceanic spreading. Deformation associated with recent magmatic events provides a view of the incremental stages in rifting. Using Interferometric Synthetic Aperture Radar (InSAR) and geodetic modeling, this study assesses magmatic events that occurred in October 2004 and November 2008 in the axial graben of the Danakil depression. The study utilized C-band Envisat ASAR imagery and L-band ALOS PALSAR imagery. Both events involve deflation of magma chambers and intrusion of NNW-SSE striking dikes. The 2004 event took place in the Dallol area and involved ~5 km deep magmatic source and a SSE-propagating dike that opens approximately 80 - 120 cm - this contrasts with seismological results suggesting a 20 km depth with primarily normal faulting. The 2008 event occurred on the northern segment of the Erta Ale volcanic range between Alu and Dalafillal volcanic centers. Available image data permitted assessing pre- and post-event deformation, as well as the main eruption. Interferograms spanning the 2008 eruption image the deflation of a 3 - 4 km deep magmatic source, as well as dilatation of a SSE-striking dike. Additionally, the erupted lava flow is evident as a decorrelated region along the northeast flank of the fissure event. Prior to the eruption, the inflation of the magmatic chamber is imaged, and after the eruption, resurgence of the magma chamber is also documented. Furthermore, post-eruption interferograms also document the contraction and cooling of the lava flow, which facilitates 1-dimensional modeling of the thickness of the flow and calculation of the volume of the effusive component of the eruption. In both cases, aseismic deformation dominates - i.e., the seismologically measured moment is significantly less than the geodetically observed moment: ~25% for the 2004 event and 10% for the 2008 event. Magnitudes of incremental extension are then compared with regional

  10. Two-dimensional surface velocity field across the Asal Rift (Afar Depression) from 11 years of InSAR data

    NASA Astrophysics Data System (ADS)

    Tomic, J.; Peltzer, G.; Doubre, C.

    2010-12-01

    We analyze two-dimensional surface velocity maps of the 200x400 km2 region covering the Asal Rift located at the western tip of the Aden Ridge, using the 1997-2008 archive of InSAR data from the RADARSAT satellite. The large phase signal due to turbulent tropospheric conditions over the Afar region is mostly removed from the 11-year average line of sight (LOS) velocity maps, revealing a clear deformation signal across the rift. Assuming the horizontal velocity to be parallel to the direction predicted by the Arabia/Somalia rotation pole (Vigny et al., 2007), we compute the fields of the vertical and horizontal components of the velocity from the ascending and descending line of sight (LOS) velocity maps. The horizontal velocity field shows the divergence between the Arabia and Somalia plates concentrated along the Asal rift, and veering toward the south-west, into the Derella-Gaggade basin system. The Asal rift shoulders open at a rate of ~15 mm/yr, while the horizontal velocity decreases away from the rift down to the plate motion rate of ~11-12 mm/yr. The vertical velocity field shows a ~60 km wide zone of doming centered over the rift associated with shoulder uplift and subsidence of the rift inner floor. The differential movement between the shoulders and the rift floor is accommodated by two main antithetic faults: the south-dipping Fault γ well developed in the topography and the recent north-dipping Fault E with a small topographic scarp. We explain the observed velocity field with 2D-forward and 3D-inverse models combining dislocations of rectangular elements in an elastic half-space. The forward model allows us to estimate the overall geometry and rates of an inflating body at 5 km depth (represented by a combination of a dike and a horizontal sill) and creep on two faults. The least-squares inverse model shows an inflating body located under the Fieale volcano expanding at 2 106 m3/yr. Faults bordering the rift show down-dip and opening motion especially

  11. Horizontal stress in planetary lithospheres from vertical processes

    NASA Technical Reports Server (NTRS)

    Banerdt, W. B.

    1991-01-01

    Understanding the stress states in a lithosphere is of fundamental importance for planetary geophysics. It is closely linked to the processes which form and modify tectonic features on the surface and reflects the behavior of the planet's interior, providing a constraint for the difficult problem of determining interior structure and processes. The tectonics on many extraterrestrial bodies (Moon, Mars, and most of the outer planet satellites) appears to be mostly vertical, and the horizontal stresses induced by vertical motions and loads are expected to dominate the deformation of their lithospheres. Herein, only changes are examined in the state of stress induced by processes such as sedimentary and volcanic deposition, erosional denudation, and changes in the thermal gradient that induce uplift or subsidence. This analysis is important both for evaluating stresses for specific regions in which the vertical stress history can be estimated, as well as for applying the proper loading conditions to global stress models. All references to lithosphere herein should be understood to refer to the elastic lithosphere, that layer which deforms elastically or brittlely when subjected to geologically scaled stresses.

  12. Seismological constraints on lithospheric structure beneath rifted margins

    NASA Astrophysics Data System (ADS)

    Fishwick, Stewart

    2014-05-01

    There is considerable variation in both topography and crustal architecture along passive margins worldwide. However, the variations in lithospheric mantle structure are less well studied. This is, perhaps, in part due to the technical challenge for offshore-onshore passive seismology and also the lower resolution obtained in most mantle studies, particularly when compared to detailed imaging of the crust available from reflection seismology. The available large scale observations of mantle structure (predominately from surface waves), and crustal structure (from receiver functions) for the continental region adjacent to the margins are reviewed. Results for Africa and Australia show clear correlations between the mantle structure and the present day topography of the margin, and this relationship is explored from a worldwide perspective. Seismic studies can also provide information on lithospheric thickness at the margin, which can be used as an additional constraint for the thermal modelling of basin structure. In this case the limitations include the depth resolution of the method, and the particular proxy used to extract a lithospheric thickness estimate from a seismic model. Perhaps most importantly, is to remember that these seismological observations tell us only the existing structure. The challenge remains how to decipher whether the present structures relate to inherited pre-rift architecture; to alteration of lithospheric mantle during rifting, or to much later post rift changes associated with separate tectonic events.

  13. The lithosphere in central Europe—seismological and petrological aspects

    NASA Astrophysics Data System (ADS)

    Babuška, V.; Plomerová, J.

    1992-06-01

    The lithosphere thickness in the Variscan belt of central Europe varies between about 60 and 150 km with typical values of 100-120 km. Our estimates, derived from directionally independent representative P-wave residuals, are in good agreement with magnetotelluric determinations of a layer with increased conductivity in the upper mantle. The large-scale anisotropies of the subcrustal lithosphere beneath four seismological stations determined from spatial variations of relative P residuals vary between 6.5 and 15.2% for P velocities; the S-wave anisotropies determined from SKS polarizations vary between 2.2 and 6.7%. These values are in reasonable agreement with the anisotropies of peridotites determined in laboratory. Systematic spatial variations of the directional terms of relative residuals in dependence on azimuths and incidence angles suggest the existence of large dipping anisotropic structures in the subcrustal lithosphere. The residual patterns at most stations in the Saxothuringicum, Rhenohercynicum and in the Massif Central imply northwesterly orientated dips of the anisotropic structures while stations in the Moldanubicum, the Alpine Foredeep and most of the Alps north of the Insubric line, suggest southeasterly orientated dips. In our interpretation the dipping anisotropic structures may represent paleosubductions which retain olivine preferred orientations originating from an ancient oceanic lithosphere. The Variscides of central Europe may thus represent a collision zone characterized by two systems of paleosubductions divergent relative to the suture between the Moldanubicum and the Saxothuringicum.

  14. Heat flow, crustal differentiation and lithospheric strength in North America

    NASA Astrophysics Data System (ADS)

    Perry, C.; Mareschal, J.; Jaupart, C.

    2008-12-01

    In stable North America, thermal models based on heat flow and heat production measurements suggest that the mechanical resistance of the lithosphere on a regional scale is greater in provinces of elevated heat flow. This is contrary to the general belief that higher surface heat flow means less stable lithosphere. We show that crustal differentiation is equally important to determine lithospheric strength. The degree of crustal radiogenic differentiation may be described using the average surface and crustal heat flows, and is quantified through the differentiation index. This index is obtained as the ratio between regional average values of heat production at the surface and in the bulk crust. The differentiation index is calculated with the bulk average heat production, suggesting that crustal differentiation processes are largely driven by internal radiogenic heat. We show that the most stable of lithospheres may be characterized by relatively high surface heat flow, simply a result of the distribution of heat sources through the crust. This may have important implications for the thermo-mechanical evolution of stable continental interiors, and for the vertical distribution of crustal heat sources through time.

  15. Lithospheric and atmospheric interaction on the planet Venus

    NASA Technical Reports Server (NTRS)

    Volkov, Vladislav P.

    1991-01-01

    Lithospheric and atmospheric interaction in the planet Venus are discussed. The following subject areas are covered: (1) manifestation of exogenic processes using photogeological data; (2) the chemical composition and a chemical model of the troposphere of Venus; (3) the mineral composition of surface rock on Venus; and (4) the cycles of volatile components.

  16. Linking lithosphere deformation and sedimentary basin formation over multiple scales

    NASA Astrophysics Data System (ADS)

    Huismans, Ritske S.

    2014-05-01

    In the spirit of Peter Ziegler we are interested in and explore the relationships between tectonic deformation and sedimentary basin formation. Resolving the interaction and feedback between tectonic crust-lithosphere scale deformation and surface processes through erosion of elevated areas and formation of sedimentary basins over multiple scales has been a long-standing challenge. While forward process based models have been successful at showing that a feedback is expected between tectonic deformation and redistribution of mass at the earth's surface by erosion, transport, and deposition, demonstrating this coupling for natural systems has been an even greater challenge and is strongly debated. Observational constraints on crust-lithosphere deformation and surface processes are typically collected at highly varying spatial and temporal scales, while forward process based models are typically run at either very large lithosphere-mantle scale, or at the scale of the sedimentary basin making it difficult to investigate and explore the detailed interaction and feedback between these systems. Here I will report on recent advances in forward modelling linking crust-lithosphere deformation with surface processes over a large range of scales resolving tectonic plate scale deformation and sedimentary basin formation at stratigraphic scales. The forward numerical models indicate a linkage and interaction between the structural style of thick-skinned large-scale mountain belt and rift-passive margin formation, erosion-transport-deposition processes operating at the surface, and the thin-skinned deformation occurring in the associated sedimentary basins.

  17. Lithospheric Architecture, Heterogenities, Instabilities, Melting - insight form numerical modelling

    NASA Astrophysics Data System (ADS)

    Gorczyk, Weronika; Hobbs, Bruce; Ord, Alison; Gessner, Klaus; Gerya, Taras V.

    2010-05-01

    The seismological structure of the Earth's lithosphere is identified to be strongly heterogeneous in terms of thermal and rheological structures. Lithospheric discontinuities (sharp changes in the thermal and/or compositional structure) are thought to be long lived and are mostly correlated with major tectonic boundaries that commonly have been reactivated and which subsequently are the foci of magma intrusion and major mineralization. Resent studies have shown that mantle metasomatism is also controlled by such boundaries. This paper explores the control that lithospheric heterogeneity exerts on the thermal and chemical evolution during deformation subsequent to the development of the heterogeneity. We explore the behaviour of the rheological heterogeneous lithosphere in a compressional regime. The occurrence of such variations may be caused for instance by amalgamation of micro-continents such as is thought to be characteristic of the Yilgarn, Western Australia or South Africa. Theses micro-continents, due to diverse histories may be characterised by various thermal and rheological structures. The models are simplistic but illustrate the basic principles. The code used in this study is based on a conservative finite-difference, multi-grid, marker in cell method. Devolatilisation reactions and melting can affect the physical properties of rocks and are incorporated in a self-consistent manner. We use a petrological-thermomechanical modelling approach with all rock properties including mechanical properties calculated in the Lagrangian scheme for rock markers at every time step based on Gibbs free energy minimization as a function of the local pressure, temperature and rock composition. The results illustrate that initial structural complexity is necessary for and has a dramatic effect on fault and development, the growth of deep basins, core complex formation, melting and devolatilisation within the lithosphere. The horizontal and vertical variation in plastic

  18. Anorogenic plateau formation induced by a heated lithosphere

    NASA Astrophysics Data System (ADS)

    Duesterhoeft, Erik; Oberhänsli, Roland; Wichura, Henry; Bousquet, Romain

    2013-04-01

    Plateau formation processes in geodynamic settings outside of orogens have not been unambiguously established. Those anorogenic plateaus are topographic barriers that reach medium elevations of approximately 1500 m, e.g. South African-, East African- or Mongolian Plateau. They are inferred to be closely link to mantle plumes away from plate boundaries. Such a heat source at the lithosphere-asthenosphere boundary (LAB) have an influence to the density structure of the crust and lithosphere, due to thermal expansion and mineralogical changes. Consequently, these density variations affect topography and thus we hypothesize topographic variations by lithospheric thermal expansion, due to heating processes at the LAB. Based on this hypothesis, we calculated the density distribution along a one-dimensional depth-profile using two different approaches - with and without mineral reactions. Therefore, we present a new petrologic aspect for plateau uplift, because models on plateau uplift generally do not take into account the effects of metamorphic phase transitions and ignore the fact that chemical reactions influence both, the stability of mineral assemblages and rock density. Our model underscores how metamorphic density of the lithosphere varies with depth and reveals how combination of chemical composition of rocks, mineralogy, and geothermal gradient all have significant effects on the density distribution within the lithosphere and ultimately the evolution of anorogenic plateaus. Furthermore, in order to better understand the temporal characteristics of mantle plume related topography we calculated the timing to generate significant topographic uplift. Our results suggest uplift rates of >20 m/Myr within the first 20 Myr after the onset of heating and considerable primary thermal uplift of approximately 700 m after 20 Myr as a viable mechanism for anorogenic plateau formation. In this way, our model may help to explain pre-rift topography of the East-African Plateau

  19. Chromatographic metasomatism of the Arabian—Nubian lithosphere

    NASA Astrophysics Data System (ADS)

    Stein, Mordechai; Navon, Oded; Kessel, Ronit

    1997-11-01

    Trace elements and isotopic ratios of calc-alkaline and tholeiitic dikes from the very last stage of the late Proterozoic, Pan-African orogeny in the northern Arabian-Nubian Shield (ANS), and alkali basalts from the overlying Phanerozoic section are used to constrain the composition and model the evolution of the lithospheric mantle in this region. The dikes and basalts are interpreted as lithospheric melts formed during the post-orogenic (and post-subduction) history of the shield. While the mafic member of all suites share a primitive La/Th ratio, the Nb/Th and Ce/Pb are distinct for each suite. The (Nb/Th) PM (primitive mantle normalized) is ˜0.2 in the calc-alkaline dikes and 1.4 in the tholeiitic dikes and the Phanerozoic alkali basalts. The (Ce/Pb) PM ratios are low in the dikes (0.4 in the calc-alkaline and 0.3 in the tholeiitic) and high in the Phanerozoic basalts (2.8). We suggest that the variations in the trace element ratios reflect sampling of different zones in the lithospheric mantle, which were formed by subduction related metasomatism of the mantle wedge. We constructed a chromatographic model to explain this zonation. In this model a plume-derived oceanic lithosphere is subducted and dehydrates at depth. Fluids released from the dehydrating slab metasomatize the overlying wedge and form amphibole-rich channels. Nb is preferentially taken by the amphibole and is enriched only in the lower zones of the column. The other elements (U, Th, REE and especially Pb and Rb) behave incompatibly. They are enriched in the fluid and transported efficiently to the melting zone in the centre of the wedge. Dehydration of the base of the wedge as it descends below the amphibole stability field depletes this region in Pb and Rb. After the end of subduction, the wedge is fossilized and forms the lithospheric mantle. The zone above the Nb concentration front is sampled by the calc-alkaline magmas. The tholeiitic magmas sample the zone below the Nb front. The

  20. Chromatographic metasomatism of the Arabian-Nubian lithosphere

    NASA Astrophysics Data System (ADS)

    Kessel, R.; Navon, O.; Stein, M.

    1997-11-01

    Trace elements and isotopic ratios of calc-alkaline and tholeiitic dikes from the very last stage of the late Proterozoic, Pan-African orogeny in the northern Arabian-Nubian Shield (ANS), and alkali basalts from the overlying Phanerozoic section are used to constrain the composition and model the evolution of the lithospheric mantle in this region. The dikes and basalts are interpreted as lithospheric melts formed during the post-orogenic (and post-subduction) history of the shield. While the mafic member of all suites share a primitive La/Th ratio, the Nb/Th and Ce/Pb are distinct for each suite. The (Nb/Th)PM (primitive mantle normalized) is ~0.2 in the calc-alkaline dikes and 1.4 in the tholeiitic dikes and the Phanerozoic alkali basalts. The (Ce/Pb)PM ratios are low in the dikes (0.4 in the calc-alkaline and 0.3 in the tholeiitic) and high in the Phanerozoic basalts (2.8). We suggest that the variations in the trace element ratios reflect sampling of different zones in the lithospheric mantle, which were formed by subduction related metasomatism of the mantle wedge. We constructed a chromatographic model to explain this zonation. In this model a plume-derived oceanic lithosphere is subducted and dehydrates at depth. Fluids released from the dehydrating slab metasomatize the overlying wedge and form amphibole-rich channels. Nb is preferentially taken by the amphibole and is enriched only in the lower zones of the column. The other elements (U, Th, REE and especially Pb and Rb) behave incompatibly. They are enriched in the fluid and transported efficiently to the melting zone in the centre of the wedge. Dehydration of the base of the wedge as it descends below the amphibole stability field depletes this region in Pb and Rb. After the end of subduction, the wedge is fossilized and forms the lithospheric mantle. The zone above the Nb concentration front is sampled by the calc-alkaline magmas. The tholeiitic magmas sample the zone below the Nb front. The Phanerozoic

  1. Rheological implications of the seismological lithosphere-asthenosphere boundary

    NASA Astrophysics Data System (ADS)

    Fischer, K. M.; Ford, H. A.; Lekic, V.; Hirth, G.

    2012-12-01

    Evidence from scattered and reflected seismic waves indicates that the transition between the seismologically-defined lithosphere and asthenosphere is relatively rapid in depth in many tectonic settings with the exception of cratons. For example, in non-cratonic regions of Australia and North America Sp receiver functions typically image a downward deccrease in shear-wave velocity whose depth ranges from ~50 to 130 km. Modeling of these Sp phases with synthetic receiver functions indicates significant (4-10%) shear velocity drops over depth ranges of ~30 km or less. Because these Sp phases originate at depths within the transition from high velocity mantle lid to low velocity zone in surface wave tomography models, and because the Sp phases account for most or all of the lid to low velocity zone contrast, they are interpretable as the seismological lithosphere-asthenosphere boundary. Thermal gradients in a wide range of geodynamical models for comparable tectonic environments are typically distributed over much larger depth ranges. The Sp phases are thus difficult to reconcile with an LAB that is governed solely by temperature. Rather, they suggest the presence of other factors that weaken the top of the asthenosphere, for example small amounts of partial melt (0.5-1.5%) or greater volatile content. Beneath cratons, a drop in shear velocity and rotation of azimuthal anisotropy at the base of the lithosphere in surface wave tomography argue for widespread sub-cratonic asthenosphere. However, Sp imaging that we conducted in cratonic regions of Australia and North America did not find coherent converted phases at potential LAB depths, arguing for LAB velocity gradients that are distributed over more than ~60 km in depth. Such gradual velocity gradients are consistent with a purely thermal origin, although slow vertical increases in melt or volatile content cannot be ruled out. To more directly assess the rheological implications of the seismological LAB, we estimated

  2. Hyperextension of continental lithospheric mantle to oceanic-like lithosphere: the record of late gabbros in the Ronda subcontinental lithospheric mantle section (Betic Cordillera, S-Spain)

    NASA Astrophysics Data System (ADS)

    Hidas, Karoly; Garrido, Carlos; Targuisti, Kamal; Padron-Navarta, Jose Alberto; Tommasi, Andrea; Marchesi, Claudio; Konc, Zoltan; Varas-Reus, Maria Isabel; Acosta Vigil, Antonio

    2014-05-01

    Rupturing continents is a primary player in plate tectonic cycle thus longevity, stability, evolution and breakup of subcontinental lithosphere belongs for a long time to a class of basic geological problems among processes that shape the view of our Earth. An emerging body of evidences - based on mainly geophysical and structural studies - demonstrates that the western Mediterranean and its back-arc basins, such as the Alborán Domain, are hyperextended to an oceanic-like lithosphere. Formation of gabbroic melts in the late ductile history of the Ronda Peridotite (S-Spain) - the largest (ca. 300 km2) outcrop of subcontinental lithospheric mantle massifs on Earth - also attests for the extreme thinning of the continental lithosphere that started in early Miocene times. In the Ronda Peridotite, discordant gabbroic veins and their host plagioclase lherzolite, as well as gabbroic patches in dunite were collected in the youngest plagioclase tectonite domains of the Ojén and Ronda massifs, respectively. In Ojén, gabbro occurs as 1-3 centimeter wide discordant veins and dikes that crosscut the plagioclase tectonite foliation at high angle (60°). Within the veins cm-scale igneous plagioclase and clinopyroxene grains show a shape preferred orientation and grow oriented, subparallel to the trace of high temperature host peridotite foliation and oblique to the trend of the vein. In contrast to Ojén, mafic melts in the Ronda massif crystallized along subcentimeter wide anastomozing veins and they often form segregated interstitial melt accumulations in the host dunite composed of plagioclase, clinopyroxene and amphibole. Despite the differences in petrography and major element composition, the identical shape of calculated REE patterns of liquid in equilibrium with clinopyroxenes indicates that the percolating melt in Ronda and Ojén shares a common source. However, unlike gabbros from the oceanic lithosphere that shows clinopyroxene in equilibrium with LREE-depleted MORB

  3. Lithospheric-scale centrifuge models of pull-apart basins

    NASA Astrophysics Data System (ADS)

    Corti, Giacomo; Dooley, Tim P.

    2015-11-01

    We present here the results of the first lithospheric-scale centrifuge models of pull-apart basins. The experiments simulate relative displacement of two lithospheric blocks along two offset master faults, with the presence of a weak zone in the offset area localising deformation during strike-slip displacement. Reproducing the entire lithosphere-asthenosphere system provides boundary conditions that are more realistic than the horizontal detachment in traditional 1 g experiments and thus provide a better approximation of the dynamic evolution of natural pull-apart basins. Model results show that local extension in the pull-apart basins is accommodated through development of oblique-slip faulting at the basin margins and cross-basin faults obliquely cutting the rift depression. As observed in previous modelling studies, our centrifuge experiments suggest that the angle of offset between the master fault segments is one of the most important parameters controlling the architecture of pull-apart basins: the basins are lozenge shaped in the case of underlapping master faults, lazy-Z shaped in case of neutral offset and rhomboidal shaped for overlapping master faults. Model cross sections show significant along-strike variations in basin morphology, with transition from narrow V- and U-shaped grabens to a more symmetric, boxlike geometry passing from the basin terminations to the basin centre; a flip in the dominance of the sidewall faults from one end of the basin to the other is observed in all models. These geometries are also typical of 1 g models and characterise several pull-apart basins worldwide. Our models show that the complex faulting in the upper brittle layer corresponds at depth to strong thinning of the ductile layer in the weak zone; a rise of the base of the lithosphere occurs beneath the basin, and maximum lithospheric thinning roughly corresponds to the areas of maximum surface subsidence (i.e., the basin depocentre).

  4. Support Of The Lunar Fossil Figure By The Elastic Lithosphere

    NASA Astrophysics Data System (ADS)

    Matsuyama, Isamu

    2010-10-01

    The figure of the Moon is triaxial, with three different principal moments of inertia, as expected. However, the moment differences are significantly larger than those predicted assuming hydrostatic equilibrium. This has been explained as due to a fossil bulge that retains a figure for prior rotational and tidal deformation, at a time when the Moon was closer to Earth (Jeffeys, 1915; Lambeck & Pullan, 1980; Garrick-Bethell et al.,2006). Garrick-Bethell et al. (2006) illustrated that a fossil figure can entirely account for the moment differences if it is established at a time when the orbital eccentricity was high. They approximate the Moon as a strengthless homogeneous body; however, a strengthless Moon cannot support a fossil figure over billions of years. We extend the analysis of Garrick-Bethell et al. (2006) by taking into the presence of an elastic lithosphere capable of supporting a fossil figure. The fossil figure is established when the elastic lithosphere forms. For a 50 km thick elastic lithosphere, the moment differences can be explained by a lunar orbit with an initial semimajor axis a=17.1 Earth radii and eccentricity e=0.49 if the Moon remains locked in synchronous rotation. If the fossil figure is established during a 3:2 spin-orbit resonance, a=18.1 Earth radii and e=0.16, or a=20.0 Earth radii and e=0.60. The initial semimajor axis decreases with decreasing elastic lithospheric thickness, as expected. The initial orbital eccentricity is not sensitive to the elastic lithospheric thickness. As Lambeck & Pullan (1980) noted, it is unlikely that the moment differences are due to a fossil figure alone. Therefore, we also consider the effect of including other contributions to the moment differences. This work is supported by the Miller Institute for Basic Research.

  5. Support of the lunar fossil figure by the elastic lithosphere

    NASA Astrophysics Data System (ADS)

    Matsuyama, I.

    2010-12-01

    The figure of the Moon is triaxial, with three different principal moments of inertia, as expected. However, the moment differences are significantly larger than those predicted assuming hydrostatic equilibrium. This has been explained as due to a fossil bulge that retains a figure for prior rotational and tidal deformation, at a time when the Moon was closer to Earth (Jeffeys, 1915; Lambeck & Pullan, 1980; Garrick-Bethell et al.,2006). Garrick-Bethell et al. (2006) illustrated that a fossil figure can entirely account for the moment differences if it is established at a time when the orbital eccentricity was high. They approximate the Moon as a strengthless homogeneous body; however, a strengthless Moon cannot support a fossil figure over billions of years. We extend the analysis of Garrick-Bethell et al. (2006) by taking into the presence of an elastic lithosphere capable of supporting a fossil figure. The fossil figure is established when the elastic lithosphere forms. For a 50 km thick elastic lithosphere, the moment differences can be explained by a lunar orbit with an initial semimajor axis a=17.1 Earth radii and eccentricity e=0.49 if the Moon remains locked in synchronous rotation. If the fossil figure is established during a 3:2 spin-orbit resonance, a=18.1 Earth radii and e=0.16, or a=20.0 Earth radii and e=0.60. The initial semimajor axis decreases with decreasing elastic lithospheric thickness, as expected. The initial orbital eccentricity is not sensitive to the elastic lithospheric thickness. As Lambeck & Pullan (1980) noted, it is unlikely that the moment differences are due to a fossil figure alone. Therefore, we also consider the effect of including other contributions to the moment differences. This work is supported by the Miller Institute for Basic Research.

  6. Preseismic Lithosphere-Atmosphere-Ionosphere Coupling

    NASA Astrophysics Data System (ADS)

    Kamogawa, Masashi

    hardly verified so far, a statistical approach has been unique way to promote the research. After the 2000s, several papers showing robust statistical results have arisen. In this paper, we focus on publications satisfying the following identification criteria: 1) A candidate of precursor, namely anomaly, is quantitatively defied. 2) Two time-series of anomalies and earthquake are constructed within the fixed thresholds such as a minimum magnitude, a region, and a lead-time. 3) To obtain a statistical correlation, a statistical process which includes four relations considering all combination among earthquake - no earthquake versus anomaly and no anomalies is applied, e. g., phi correlation. 4) For correlations under various thresholds the results keep consistency. 5) Large anomalies appear before large earthquakes. One of papers based on the identification criteria, which concerns preseismic geoelectrically anomalies, is introduced as an educative example. VAN method in Greece, i. e., Geo-electric potential difference measurement for precursor study in Greece, has been often discussed in the point of view of success and failure performance for practical prediction [Varotsos et al, Springer, 2011] to show a correlation and then less number of papers shows the statistical correlation with satisfying the identification criteria [Geller (ed.), GRL, 1996], so that the phenomena had been controversial. However, recent related study in Kozu-Island, Japan which satisfied the criteria showed the robust correlation [Orihara and Kamogawa et al., PNAS, 2012]. Therefore, the preseismic geoelectric anomalies are expected to be a precursor. Preseismic lithosphere-atmosphere-ionosphere coupling has been intensively discussed [Kamogawa, Eos, 2006]. According to review based on the identification criteria with considering recent publications, plausible precursors have been found, which are tropospheric anomaly [Fujiwara and Kamogawa, GRL, 2004], daytime electron depletion in F region

  7. Petrological constraints on evolution of continental lithospheric mantle beneath the northwestern Ethiopian plateau: Insight from mantle xenoliths from the Gundeweyn area, East Gojam, Ethiopia

    NASA Astrophysics Data System (ADS)

    Alemayehu, Melesse; Zhang, Hong-Fu; Zhu, Bin; Fentie, Birhanu; Abraham, Samuel; Haji, Muhammed

    2016-01-01

    Detailed petrographical observations and in-situ major- and trace-element data for minerals from ten spinel peridotite xenoliths from a new locality in Gundeweyn area, East Gojam, have been examined in order to understand the composition, equilibrium temperature and pressure conditions as well as depletion and enrichment processes of continental lithospheric mantle beneath the Ethiopian plateau. The peridotite samples are very fresh and, with the exception of one spinel harzburgite, are all spinel lherzolites. Texturally, the xenoliths can be divided into two groups as primary and secondary textures. Primary textures are protogranular and porphyroclastic while secondary ones include reaction, spongy and lamellae textures. The Fo content of olivine and Cr# of spinel ranges from 86.5 to 90.5 and 7.7 to 14.1 in the lherzolites, respectively and are 89.8 and 49.8, respectively, in the harzburgite. All of the lherzolites fall into the lower Cr# and Fo region in the olivine-spinel mantle array than the harzburgite, which indicates that they are fertile peridotites that experienced low degrees of partial melting and melt extraction. Orthopyroxene and clinopyroxene show variable Cr2O3 and Al2O3 contents regardless of their lithology. The Mg# of orthopyroxene and clinopyroxene are 87.3 to 90.1 and 85.8 to 90.5 for lherzolite and 90.4 and 91.2 for harzburgite, respectively. The peridotites have been equilibrated at a temperature and pressure ranging from 850 to 1100 °C and 10.2 to 30 kbar, respectively, with the highest pressure record from the harzburgite. They record high mantle heat flow between 60 and 150 mW/m2, which is not typical for continental environments (40 mW/m2). Such a high geotherm in continental area shows the presence of active mantle upwelling beneath the Ethiopian plateau, which is consistent with the tectonic setting of nearby area of the Afar plume. Clinopyroxene of five lherzolites and one harzburgite samples have a LREE enriched pattern and the rest

  8. Lithospheric structure and deformation of the North American continent

    NASA Astrophysics Data System (ADS)

    Tesauro, Magdala; Kaban, Mikhail; Cloetingh, Sierd; Mooney, Walter

    2013-04-01

    We estimate the integrated strength and elastic thickness (Te) of the North American lithosphere based on thermal, density and structural (seismic) models of the crust and upper mantle. The temperature distribution in the lithosphere is estimated considering for the first time the effect of composition as a result of the integrative approach based on a joint analysis of seismic and gravity data. We do this via an iterative adjustment of the model. The upper mantle temperatures are initially estimated from the NA07 tomography model of Bedle and Van der Lee (2009) using mineral physics equations. This thermal model, obtained for a uniform composition, is used to estimate the gravity effect and to remove it from the total mantle gravity anomalies, which are controlled by both temperature and compositional variations. Therefore, we can predict compositional variations from the residual gravity anomalies and use them to correct the initial thermal model. The corrected thermal model is employed again in the gravity calculations. The loop is repeated until convergence is reached. The results demonstrate that the lithospheric mantle is characterized by strong compositional heterogeneity, which is consistent with xenolith data. Seismic data from the USGS database allow to define P-wave velocity and thickness of each crustal layer of the North American geological provinces. The use of these seismic data and of the new compositional and thermal models gives us the chance to estimate lateral variation of rheology of the main lithospheric layers and to evaluate coupling-decoupling conditions at the layers' boundaries. In the North American Cordillera the strength is mainly localized in the crust, which is decoupled from the mantle lithosphere. In the cratons the strength is chiefly controlled by the mantle lithosphere and all the layers are generally coupled. These results contribute to the long debates on applicability of the "crème brulée" or "jelly-sandwich" models for the

  9. Impact of lithosphere rheology on the dynamic topography

    NASA Astrophysics Data System (ADS)

    Burov, Evgueni; Gerya, Taras; Koptev, Alexander

    2014-05-01

    Dynamic topography is a key observable signature of the Earth's and planetary (e.g. Venus) mantle dynamics. In general view, it reflects complex mantle flow patterns, and hence is supposed to correlate at different extent with seismic tomography, SKS fast orientations, geodetic velocity fields and geoid anomalies. However, identification of dynamic topography had no systematic success, specifically in the Earth's continents. Here we argue that lithosphere rheology, in particular, rheological stratification of continents, results in modulation of dynamic topography, converting commonly expected long-wavelength/small amplitude undulations into short-wavelength surface undulations with wide amplitude spectrum, superimposed onto "tectonic" topography. These ideas are explored in 3D using unprecedentedly high resolution numerical experiments (grid step size 2-3 km for 1500x1500x600 km computational area) incorporating realistic rheologically stratified lithosphere. Such high resolution is actually needed to resolve small-scale crustal faulting and inter-layer coupling/uncoupling that shape surface topography. The results reveal strikingly discordant, counterintuitive features of 3D dynamic topography, going far beyond the inferences from previous models. In particular, even weak anisotropic tectonic stress field results both in large-scale small-amplitude dynamic topography and in strongly anisotropic short-wavelength (at least in one direction) dynamic topography with wide amplitude range (from 100 to 2000-3000 m), including basins and ranges and large-scale linear normal and strike-slip faults. Even very slightly pre-stressed strong lithosphere yields and localizes deformation much easier , than un-prestressed one, in response to plume impact and mantle flow. The results shed new light on the importance of lithosphere rheology and active role of lithosphere in mantle-lithosphere interactions as well as on the role of mantle flow and far-field stresses in tectonic

  10. Mechanisms and geologic significance of the mid-lithosphere discontinuity in the continents

    NASA Astrophysics Data System (ADS)

    Karato, Shun-Ichiro; Olugboji, Tolulope; Park, Jeffrey

    2015-07-01

    The stable continents have a puzzling structure. Recent seismological studies have revealed a marked drop in seismic velocity at middle-lithosphere depths, but a generally small velocity drop at the lithosphere-asthenosphere boundary. The mid-lithosphere discontinuity has previously been attributed to changes in composition and/or crystal alignment (anisotropy) caused by metasomatic alteration, as well as to partial melting and/or accretion of intruded materials that occurred after the formation of the continents. We show that these models cannot easily explain the global presence of a large seismic velocity drop in the middle lithosphere and a small velocity change at the lithosphere-asthenosphere boundary. These models are also difficult to reconcile with long-term continental stability and, in particular, observations of nearly depth-invariant ages of rocks in the continental lithosphere that do not support the notion of late alteration events. Instead, we propose an elastically accommodated grain-boundary sliding model that predicts a substantial velocity drop at the mid-lithosphere discontinuity and a weak seismic signal at the lithosphere-asthenosphere boundary, as observed, without invoking late-stage modifications to the lithosphere. In this model, the mid-lithosphere discontinuity is a general feature of the stable continents, the precise depth of which depends primarily on temperature and water content. Consequently, the depth of the mid-lithosphere discontinuity may provide clues to the evolution of continents.

  11. Lithosphere-asthenosphere Structure and Active Tectonics In Central Italy

    NASA Astrophysics Data System (ADS)

    Chimera, G.; Aoudia, A.; Saraò, A.; Panza, G. F.

    We investigate the lithosphere-asthenosphere structure and the active tectonics along a stripe from the Tyrrhenian to the Adriatic with emphasis on the Umbria-Marche area by means of surface-wave tomography, and inversion studies for structure and seismic moment tensor retrieval. The data include seismic waveforms, a large compilation of local group velocities (0.8-4s) and regional phase and group velocity (10-100s) measurements. The local group velocity maps cover the area reactivated by the 1997 Umbria-Marche earthquake sequence. These maps suggest a relation between the lat- eral heterogeneity and distribution of the active faults and related basins. Such relation is confirmed by the non-linear inversion of the local dispersion curves. To image the deeper structure from the Tyrrhenian to the Adriatic coast, we fix the uppermost part of the crust using the Umbria-Marche models along with the CROP03 profile and related shear wave velocity, and invert the additional long period dispersion measure- ments. The results of the inversion show the geometry and lateral heterogeneity of the lithosphere-asthenosphere system. The retrieved models for the Umbria-Marche up- per crust reveal the importance of the inherited compression on the ongoing extension and related seismic activity. The reactivated 1997 normal fault zone displays a thrust fault geometry as evidenced by the lateral extent of the faulted Late Triassic evap- orites that did not yet balance the cumulative normal faulting deformation attesting therefore recent extensional tectonics within the thrust belt. Our data are in favor of a listric geometry of faulting at depth. Source inversion studies of the two main crustal events of September 26 and October 14, 1997 show the dominance of normal faulting mechanisms, whereas selected aftershocks between the reactivated fault segments re- veal that the prevailing deformation at the step-over is of strike-slip faulting type. The rupture of the three distinct and

  12. Matching Lithosphere velocity changes to the GOCE gravity signal

    NASA Astrophysics Data System (ADS)

    Braitenberg, Carla

    2016-07-01

    Authors: Carla Braitenberg, Patrizia Mariani, Alberto Pastorutti Department of Mathematics and Geosciences, University of Trieste Via Weiss 1, 34100 Trieste Seismic tomography models result in 3D velocity models of lithosphere and sublithospheric mantle, which are due to mineralogic compositional changes and variations in the thermal gradient. The assignment of density is non-univocal and can lead to inverted density changes with respect to velocity changes, depending on composition and temperature. Velocity changes due to temperature result in a proportional density change, whereas changes due to compositional changes and age of the lithosphere can lead to density changes of inverted sign. The relation between velocity and density implies changes in the lithosphere rigidity. We analyze the GOCE gradient fields and the velocity models jointly, making simulations on thermal and compositional density changes, using the velocity models as constraint on lithosphere geometry. The correlations are enhanced by applying geodynamic plate reconstructions to the GOCE gravity field and the tomography models which places today's observed fields at the Gondwana pre-breakup position. We find that the lithosphere geometry is a controlling factor on the overlying geologic elements, defining the regions where rifting and collision alternate and repeat through time. The study is carried out globally, with focus on the conjugate margins of the African and South American continents. The background for the study can be found in the following publications where the techniques which have been used are described: Braitenberg, C., Mariani, P. and De Min, A. (2013). The European Alps and nearby orogenic belts sensed by GOCE, Boll. Bollettino di Geofisica Teorica ed Applicata, 54(4), 321-334. doi:10.4430/bgta0105---- Braitenberg, C. and Mariani, P. (2015). Geological implications from complete Gondwana GOCE-products reconstructions and link to lithospheric roots. Proceedings of 5th

  13. Mesozoic thermal evolution of the southern African mantle lithosphere

    NASA Astrophysics Data System (ADS)

    Bell, David R.; Schmitz, Mark D.; Janney, Philip E.

    2003-12-01

    The thermal structure of Archean and Proterozoic lithospheric terranes in southern Africa during the Mesozoic was evaluated by thermobarometry of mantle peridotite xenoliths erupted in alkaline magmas between 180 and 60 Ma. For cratonic xenoliths, the presence of a 150-200 °C isobaric temperature range at 5-6 GPa confirms original interpretations of a conductive geotherm, which is perturbed at depth, and therefore does not record steady state lithospheric mantle structure. Xenoliths from both Archean and Proterozoic terranes record conductive limb temperatures characteristic of a "cratonic" geotherm (˜40 mW m -2), indicating cooling of Proterozoic mantle following the last major tectonothermal event in the region at ˜1 Ga and the probability of thick off-craton lithosphere capable of hosting diamond. This inference is supported by U-Pb thermochronology of lower crustal xenoliths [Schmitz and Bowring, 2003. Contrib. Mineral. Petrol. 144, 592-618]. The entire region then suffered a protracted regional heating event in the Mesozoic, affecting both mantle and lower crust. In the mantle, the event is recorded at ˜150 Ma to the southeast of the craton, propagating to the west by 108-74 Ma, the craton interior by 85-90 Ma and the far southwest and northwest by 65-70 Ma. The heating penetrated to shallower levels in the off-craton areas than on the craton, and is more apparent on the southern margin of the craton than in its western interior. The focus and spatial progression mimic inferred patterns of plume activity and supercontinent breakup 30-100 Ma earlier and are probably connected. Contrasting thermal profiles from Archean and Proterozoic mantle result from penetration to shallower levels of the Proterozoic lithosphere by heat transporting magmas. Extent of penetration is related not to original lithospheric thickness, but to its more fertile character and the presence of structurally weak zones of old tectonism. The present day distribution of surface heat flow

  14. Viscoelastic Lithosphere Response and Stress Memory of Tectonic Force History (Invited)

    NASA Astrophysics Data System (ADS)

    Kusznir, N. J.

    2009-12-01

    While great attention is often paid to the details of creep deformation mechanisms, brittle failure and their compositional controls when predicting the response of lithosphere to tectonic forces, the lithosphere’s elastic properties are usually neglected; a viscous rheology alone is often used to predict the resulting distribution of stress with depth or to determine lithosphere strength. While this may simplify geodynamic modelling of lithosphere response to tectonic processes, the omission of the elastic properties can often give misleading or false predictions. The addition of the elastic properties of lithosphere material in the form of a visco-elastic rheology results is a fundamentally different lithosphere response. This difference can be illustrated by examining the application of horizontal tectonic force to a section of lithosphere incorporating the brittle-visco-elastic response of each infinitesimal lithosphere layer with temperature and stress dependent viscous rheology. The transient response of a visco-elastic lithosphere to a constant applied tectonic force and the resulting distribution of stress with depth are substantially different from that predicted by a viscous lithosphere model, with the same lithosphere composition and temperature structure, subjected to a constant lateral strain rate. For visco-elastic lithosphere subject to an applied horizontal tectonic force, viscous creep in the lower crust and mantle leads to stress decay in these regions and to stress amplification in the upper lithosphere through stress redistribution. Cooling of lithosphere with a visco-elastic rheology results in thermal stresses which, as a consequence of stress dissipation by creep and brittle failure, results in a complex and sometimes counter-intuitive distribution of stress with depth. This can be most clearly illustrated for the cooling of oceanic lithosphere, however similar or more complex behaviour can be expected to occur for continental lithosphere

  15. A quantitative geomorphological approach to constraining the volcanic and tectonic evolution of the active Dabbahu rift segment, Afar, Ethiopia.

    NASA Astrophysics Data System (ADS)

    Medynski, Sarah; Pik, Raphaël; Burnard, Peter; Vye-Brown, Charlotte; Blard, Pierre-Henri; France, Lydéric; Dumont, Stéphanie; Grandin, Raphaël; Schimmelpfennig, Irene; Benedetti, Lucilla; Ayalew, Dereje; Yirgu, Gezahegn

    2013-04-01

    In the Afar depression (Ethiopia), extension is organised along rift segments that morphologically resemble oceanic rifts. Segmentation results from interactions between dyke injection and volcanism, as observed during the well-documented 2005 rifting event on the Dabbahu rift segment. This tectono-volcanic crisis was observed in detail via remote sensing techniques, providing invaluable information on the present-day tectonic - magmatic interplay during a sequence of dyke intrusions. However, lack of data remains on timescales of 1 to 100 kyr, the period over which the main morphology of the rift is acquired. The Dabbahu rift segment represents an ideal natural laboratory to study the evolution of rift morphology as a response to volcanic and tectonic influences. We use cosmogenic nuclides (3He and 36Cl) to determine the ages of young (<100 kyr) lava flows and to date the initiation and movement of fault scarps, which cut the lavas. Where possible, we analysed vertical profiles along fault scarps, in an attempt to distinguish individual tectonic events that offset the scarp, estimate their amplitudes and date the recurrence intervals. These geochronological constraints, combined with major & trace element compositions, field mapping and digital mapping (Landsat, ASTER and SPOT imagery), provide valuable insights on the magmatic and tectonic history of the segment. The results show that over the last 100 ka, the northern part of the Dabbahu segment was supplied by at least two different magma reservoirs, which can be identified from their distinctive chemistries. The main reservoir is located beneath Dabbahu volcano at the northern tip of the rift segment, and has been supplied with magma for at least 72 ka. The second reservoir is located further south on the rift axis and corresponds to the current mid-segment magma chamber, which was responsible for the 2005 rifting episode. Two magmatic cycles linked to the Dabbahu magma chamber were recorded, lasting 20-30 kyr

  16. Earthquake relocations and InSAR analysis following the June 12th 2011 eruption of Nabro volcano, Afar

    NASA Astrophysics Data System (ADS)

    Hamlyn, Joanna; Wright, Tim; Keir, Derek; Neuberg, Jurgen; Grandin, Raphael; Goitom, Berhe; Hammond, James; Kibreab, Alem; Ogubazghi, Ghebrebrhan; Pagli, Carolina; Sansosti, Eugenio

    2014-05-01

    Nabro volcano sits on the southern part of Danakil block to the east of the Afar depression, on the Arabian plate. On the 12th June 2011, Nabro volcano suddenly erupted after being inactive for 10,000 years. The eruption caused a 17-km-long lava flow, a 15-km-high ash cloud, and ranks as one of the largest emissions of SO2 since the Mt. Pinatubo (1991) event. This eruption creates an important opportunity to use seismicity and surface deformation measurements to understand the subsurface magmatic system and deformation of a hazardous, off axis caldera during continental rupture. We installed a network of 8 seismometers around Nabro caldera which began recording on the 31st August and tasked SAR acquisitions from TerraSAR-X (TSX) and Cosmo-SkyMed (CSK) satellites. The SAR images used for this study post date the eruption. We used TSX stripmap mode images from ascending and descending orbits. Using a small baseline approach, we used 25 images acquired between the 1st July 2011 to the 5th October 2012 on descending orbit 046, to create 34 interferograms. We complemented these with 19 images from ascending orbit 130 spanning the 6th July 2011 to the 10th October 2012 from ascending orbit 130, which we used to create 21 interferograms. We produced a velocity ratemap and timeseries using π-RATE showing subsidence of up to 25cm/yr centred on Nabro. We used a Monte-Carlo hybrid downhill simplex technique to invert the dataset and found the best fitting solution as a mogi source at 6.9 ±1.1 km depth, and located at a 13.35 (lat) and 41.69 (long). The time dependence observed is consistent with a viscoelastic relaxation around the magma chamber, following depletion. Concurrent with the TSX acquisitions, CSK imaged the volcano on a descending track between 26th June 2011 and 18th July 2012 within the ASI project SAR4Volcanoes, and 64 images were used to produce 171 interferograms which were inverted to form a timeseries using a SBAS approach. This dataset has an overall

  17. South China Sea crustal thickness and lithosphere thinning from satellite gravity inversion incorporating a lithospheric thermal gravity anomaly correction

    NASA Astrophysics Data System (ADS)

    Kusznir, Nick; Gozzard, Simon; Alvey, Andy

    2016-04-01

    The distribution of ocean crust and lithosphere within the South China Sea (SCS) are controversial. Sea-floor spreading re-orientation and ridge jumps during the Oligocene-Miocene formation of the South China Sea led to the present complex distribution of oceanic crust, thinned continental crust, micro-continents and volcanic ridges. We determine Moho depth, crustal thickness and continental lithosphere thinning (1- 1/beta) for the South China Sea using a gravity inversion method which incorporates a lithosphere thermal gravity anomaly correction (Chappell & Kusznir, 2008). The gravity inversion method provides a prediction of ocean-continent transition structure and continent-ocean boundary location which is independent of ocean isochron information. A correction is required for the lithosphere thermal gravity anomaly in order to determine Moho depth accurately from gravity inversion; the elevated lithosphere geotherm of the young oceanic and rifted continental margin lithosphere of the South China Sea produces a large lithosphere thermal gravity anomaly which in places exceeds -150 mGal. The gravity anomaly inversion is carried out in the 3D spectral domain (using Parker 1972) to determine 3D Moho geometry and invokes Smith's uniqueness theorem. The gravity anomaly contribution from sediments assumes a compaction controlled sediment density increase with depth. The gravity inversion includes a parameterization of the decompression melting model of White & McKenzie (1999) to predict volcanic addition generated during continental breakup lithosphere thinning and seafloor spreading. Public domain free air gravity anomaly, bathymetry and sediment thickness data are used in this gravity inversion. Using crustal thickness and continental lithosphere thinning factor maps with superimposed shaded-relief free-air gravity anomaly, we improve the determination of pre-breakup rifted margin conjugacy, rift orientation and sea-floor spreading trajectory. SCS conjugate margins

  18. Physico-chemical constraints on cratonic lithosphere discontinuities

    NASA Astrophysics Data System (ADS)

    Aulbach, Sonja; Rondenay, Stéphane; Huismans, Ritske

    2014-05-01

    The origins of the mid-lithospheric discontinuity (MLD) and lithosphere-asthenosphere boundary (LAB) have received much attention over the recent years. Peculiarities of cratonic lithosphere construction - compositional and rheological stratification due to thickening in collisional settings or by plume subcretion, multiple metasomatic overprints due to longevity - offer a variety of possibilities for the generation of discontinuities. Interconnected small degrees of conductive partial melt (carbonate-rich melts, such as carbonatites and kimberlites, or highly alkaline melts) at the cratonic LAB, which produce seismic discontinuities, may be generated in the presence of volatiles. These depress the peridotite solidus sufficiently to intersect the mantle adiabat at depths near the cratonic LAB at ~160-220 km, i.e. above the depth of metal saturation where carbonatite becomes unstable. The absence of agreement between the different seismic and magnetotelluric estimates for the depth of the LAB beneath Kaapvaal may be due to impingement of a plume, leading to a pervasively, but heterogeneously metasomatised ('asthenospherised') hot and deep root. Such a root and hot sublithosphere may yield conflicting seismic-thermal-geochemical depths for the LAB. The question arises whether the chemical boundary layer should be defined as above or below the asthenospherised part of the SCLM, which has preserved isotopic, compositional (non-primitive olivine forsterite content) and physical evidence (e.g. from teleseismic tomography and receiver functions) for a cratonic heritage and which therefore is still distinguishable from the asthenospheric mantle. If cratonic lithosphere overlies anomalously hot mantle for extended periods of time, the LAB may be significantly thinned, aided by penetration of relatively high-degree Fe-rich partial melts, as has occurred beneath the Tanzanian craton. Xenoliths from the deep Slave craton show little evidence for 'asthenospherisation'. Its root

  19. Swarm magnetic and GOCE gravity gradient grids for lithospheric modelling

    NASA Astrophysics Data System (ADS)

    Bouman, Johannes; Haagmans, Roger; Olsen, Nils; Ebbing, Jörg; Baykiev, Eldar; Novak, Pavel; Kotsiaros, Stavros; Sebera, Josef; Brönner, Marco; Fuchs, Martin; Holzrichter, Nils

    2016-07-01

    We explore how Swarm magnetic gradient and GOCE gravity gradient data can improve modelling of the Earth's lithosphere and thereby contribute to a better understanding of Earth's dynamic processes. We study the use of gradient grids to provide improved information about the lithosphere and upper mantle in the well-surveyed North-East Atlantic Margin. In particular, we present the computation of magnetic and gravity gradient grids at satellite altitude (roughly 450 km and 250 km above the Earth for Swarm and GOCE respectively). It is shown that regional solutions based on a tesseroid approach may contain more signal content than global models do. The patchwork of regional grids is presented as well as the subsequent error reduction through iterative downward and upward continuation using the Poisson integral equation. The promises and pitfalls are discussed of using grids at mean satellite altitude.

  20. On geoid heights and flexure of the lithosphere at seamounts

    NASA Astrophysics Data System (ADS)

    Watts, A. B.; Ribe, N. M.

    1984-12-01

    The sea surface height has now been mapped to an accuracy of better than ±1 m by using radar altimeters on board orbiting satellites. The major influence on the mean sea surface height is the marine geoid which is an equipotential surface. We have carried out preliminary studies of how oceanic volcanoes, which rise above the ocean floor as isolated seamounts and oceanic islands or linear ridges, contribute to the marine geoid. Simple one- and two-dimensional models have been constructed in which it is assumed that the oceanic lithosphere responds to volcanic loads as a thin elastic plate overlying a weak fluid substratum. Previous studies based on gravity and bathymetry data and uplift/subsidence patterns show that the effective flexural rigidity of oceanic lithosphere and the equivalent elastic thickness Te increase with the age of the lithosphere at the time of loading. The models predict that isolated seamounts emplaced on relatively young lithosphere on or near a mid-ocean ridge crest will be associated with relatively low amplitude geoid anomalies (about 0.4-0.5 m/km of height), while seamounts formed on relatively old lithosphere, on ridge flanks, will be associated with much higher amplitude anomalies (1.4-1.5 m/km). Studies of the Seasat altimetric geoid prepared by NASA's Jet Propulsion Laboratory support these model predictions; geoid amplitudes are relatively low over the Mid-Pacific Mountains and Line Islands, which formed on or near a mid-ocean ridge crest, and relatively high over the Magellan Seamounts and Wake Guyots, which formed off ridge. Direct modeling of the altimetric geoid over these features is complicated, however, by the wide spacing of the satellite tracks (which can exceed 100 km) and poor bathymetric control beneath individual satellite tracks. In regions where multibeam bathymetric surveys are available, models can be constructed that fit the altimetric geoid to better than ±1 m. Studies of geoid anomalies over the Emperor seamount

  1. Magmatic expression of lithospheric thinning across continental rifts

    NASA Astrophysics Data System (ADS)

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

    1994-05-01

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

  2. Thick plate flexure. [for lithospheric models of Mars and earth

    NASA Technical Reports Server (NTRS)

    Comer, R. P.

    1983-01-01

    Analytical expressions are derived for the displacements and stresses due to loading of a floating, uniform, elastic plate of arbitrary thickness by a plane or axisymmetric harmonic load. The solution is exact except for assumptions of small strains and linear boundary conditions, and gravitation within the plate is neglected. For typical earth parameters its predictions are comparable to those of the usual thin plate theory frequently assumed in studies of lithospheric flexure, gravity and regional isostasy. Even for a very thick lithosphere, which may exist in some regions of Mars, the thin plate theory is a better approximation to the thick plate solution than the elastic half-space limit, except for short-wavelength loads.

  3. The elastic thickness of the lithosphere in the Pacific Ocean

    NASA Astrophysics Data System (ADS)

    Calmant, Stephane

    1987-09-01

    The effective elastic thickness T(e) of the oceanic lithosphere along the Hawaiian-Emperor, the Marquesas, the Pitcairn-Mururoa-Gloucester (PMG) chains, the Tuamotu archipelago, and the Samoa islands was determined by computing the deflection of a continuous elastic plate under the load of volcanoes and was constrained by the geoid heights over the oceans provided by Seasat. The prediction by Watts (1978) according to which the value of the T(e) should increase with the square root of crustal age of the lithosphere at the time of volcano emplacement was not confirmed; while the T(e) estimate of the Hawaiian-Emperor chain and an isolated estimate in the Samoan group agree with the empirical trend found by Watts, the Marquesas and the PMG chains, as well as the previously analyzed Cook-Austral and Society chains, present anomalously low values which increase only slightly with age.

  4. The Gutenberg discontinuity: melt at the lithosphere-asthenosphere boundary.

    PubMed

    Schmerr, Nicholas

    2012-03-23

    The lithosphere-asthenosphere boundary (LAB) beneath ocean basins separates the upper thermal boundary layer of rigid, conductively cooling plates from the underlying ductile, convecting mantle. The origin of a seismic discontinuity associated with this interface, known as the Gutenberg discontinuity (G), remains enigmatic. High-frequency SS precursors sampling below the Pacific plate intermittently detect the G as a sharp, negative velocity contrast at 40- to 75-kilometer depth. These observations lie near the depth of the LAB in regions associated with recent surface volcanism and mantle melt production and are consistent with an intermittent layer of asthenospheric partial melt residing at the lithospheric base. I propose that the G reflectivity is regionally enhanced by dynamical processes that produce melt, including hot mantle upwellings, small-scale convection, and fluid release during subduction. PMID:22442480

  5. Linking petrology and seismology of the southwest Greenland lithosphere

    NASA Astrophysics Data System (ADS)

    Lesher, C. E.; Vestergaard, C.; Brown, E.; Schutt, D.

    2015-12-01

    Mantle xenoliths from late-Proterozoic diamond-bearing kimberlitic dikes in the Kangerlussuaq, Sarfartoq and Maniitsoq areas of southwestern Greenland provide constraints on the composition and thermal state of lithospheric mantle beneath Greenland to depths of ~200 km [1]. Similarly, surface wave tomography studies carried out as part of the GLATIS project use a range of Rayleigh wave periods sensitive to structures at a similar depth interval within southwestern Greenland lithospheric mantle [2]. Here we link petrologic and seismologic constraints on the mantle lithosphere beneath Greenland utilizing methods of [3] that show that inferred chemical and mineralogical stratification inferred from petrology, showing mantle peridotite transitioning from garnet-free harzburgite to garnet lherzolite between ~70 and 180 km, cannot readily be resolved with fundamental mode Rayleigh waves. On the other hand, comparing phase velocities predicted from xenolith compositions, mineralogy and last equilibration temperatures and pressures, defining the continental geotherm during late-Proterozoic time, with those for the present-day mantle lithosphere suggest significant cooling of the cratonic mantle to a modern geotherm characterized by a heat flux of 30 mW/m2 and average crustal heat production of 0.3 mW/m3 [4]. These preliminary findings point to the weak dependence of shear wave velocities on mantle peridotite composition and mineralogy, and further illustrate its strong temperature dependence. Comparison of ancient and modern continental geotherms made possible by combining petrologic and seismological data, as shown here for southwest Greenland, provide additional constraints on secular cooling of cratonic regions linked to large-scale tectonic processes. [1] Bizzarro et al., 2003, CMP, 146; Sand et al., Lithos, 112. [2] Darbyshire et al., 2004, GJI, 158. [3] Schutt and Lesher, 2006, JGR, 111. [4] Meirerbachtol et al., 2015, JGR/ES, 120.

  6. Analysis of lithospheric magnetization in vector spherical harmonics

    NASA Astrophysics Data System (ADS)

    Gubbins, D.; Ivers, D.; Masterton, S. M.; Winch, D. E.

    2011-10-01

    The lithospheric contribution to the geomagnetic field arises from magnetized rocks in a thin shell at the Earth's surface. The lithospheric field can be calculated as an integral of the distribution of magnetization using standard results from potential theory. Inversion of the magnetic field for the magnetization suffers from a fundamental non-uniqueness: many important distributions of magnetization yield no potential magnetic field outside the shell. We represent the vertically integrated magnetization (VIM) in terms of vector spherical harmonics that are new to geomagnetism. These vector functions are orthogonal and complete over the sphere: one subset (?) represents the part of the magnetization that produces a potential field outside the shell, the observed field; another subset (?) produces a potential field exclusively inside the shell; and a third, toroidal, subset (?) produces no potential field at all. ? and ? together span the null space of the inverse problem for magnetization with perfect, complete data. We apply the theory to a recent global model of VIM, give an efficient algorithm for finding the lithospheric field, and show that our model of magnetization is dominated by ?, the part producing a potential field inside the shell. This is largely because, to a first approximation, the model was formed by magnetizing a shell with a substantial uniform component by an potential field originating inside the shell. The null space for inversion of lithospheric magnetic anomaly data for VIM is therefore huge. It can be reduced if the magnetization is assumed to be induced by a known inducing field, but the null space for susceptibility is not so easily recovered.

  7. Variations in effective elastic thickness of the North American lithosphere

    NASA Technical Reports Server (NTRS)

    Bechtel, Timothy D.; Forsyth, Donald W.; Sharpton, Virgil L.; Grieve, Richard A. F.

    1990-01-01

    A technique for estimating flexural rigidity that is not limited to sedimentary basins is used here to map variations in the effective elastic thickness of the North American lithosphere. The effective elastic thickness ranges from a minimum of about 4 km in the Basin and Range Province to more than 100 km in the Precambrian core of the continent. This finding supports the idea that flexural rigidity has increased with time since the last thermal event.

  8. Marine electromagnetic constraints on lithosphere/asthenosphere structure

    NASA Astrophysics Data System (ADS)

    Constable, Steven; Key, Kerry; Naif, Samer

    2014-05-01

    Marine controlled-source electromagnetic (CSEM) experiments long ago showed that the oceanic lithosphere is highly resistive, but deeper, conductive asthenospheric structure is beyond the resolution of this method. The development of a "broadband" marine magnetotelluric (MT) instrument allowed deepwater MT data collection down to about 20 second periods, overlapping in sensitivity with CSEM data and capturing the peak sensitivity of the asthenosphere in MT data at around 100 seconds. In two end-member experiments, one at the Pacific mid-ocean ridge at 9.5 degrees north, and one where the same, now 23 Ma, Cocos plate subducts beneath Nicaragua, we carried out joint CSEM and broadband MT data collection. At the mid-ocean ridge, symmetric melting above the wet solidus is consistent with passive upwelling of hydrated mantle. Deeper, carbonate-induced melting shows asymmetry that is consistent with upwelling due to viscous coupling across the nearby Clipperton transform offset. At 100 km off-axis, a 70 km thick resistive layer is consistent with melt-depleted lithosphere and asthenosphere. By the time the plate has migrated to the subduction zone, an anisotropic and conductive asthenosphere 25 km thick has developed at a depth of 45-70 km, again inferred to be melting of hydrated mantle. The nature of the anisotropy is consistent with shearing and alignment in the plate motion direction, suggesting viscous decoupling of the lithosphere and asthenosphere. We observe conductivities consistent with a smaller fraction of isotropic melt in the deeper mantle, suggesting that the melt at the lithosphere-asthenosphere boundary (LAB) is a result of upward migrating melt accumulating beneath a thermal and/or dehydration boundary. At both the ridge and subduction zone we estimate several hundred ppm water in the mantle, but this is dependent on laboratory data and any additional impact from carbon dioxide on the solidus.

  9. Rheology of the Northern Apennines: Lateral variations of lithospheric strength

    NASA Astrophysics Data System (ADS)

    Pauselli, Cristina; Ranalli, Giorgio; Federico, Costanzo

    2010-03-01

    A thermorheological WSW-ENE profile along the Northern Apennines is presented. The variations in rheology are estimated using two different models: the conventional model in which only frictional sliding and power-law creep are taken into account, and a new model in which in addition to the previous two mechanisms, a high-pressure brittle fracture mechanism is included. Upper and lower bounds for the rheology are considered: a hard case where the lower crust is mafic granulite and the lithospheric mantle is dry peridotite, and a soft case where the lower crust is felsic granulite and the lithospheric mantle is wet peridotite. Important differences in lithospheric rheological structure are inferred between the western and the eastern sectors of the chain. In the former a "crème brulée" structure generally applies; in the latter a "jelly-sandwich" structure applies. These rheological models, therefore, are not alternative but end members of a continuous spectrum of rheological behaviours.

  10. Volcanism and Volatile Recycling on Venus from Lithospheric Delamination

    NASA Technical Reports Server (NTRS)

    Elkins-Tanton, L. T.; Hess, P. C.; Smrekar, S. E.; Parmentier, E. M.

    2005-01-01

    Venus has an unmoving lithosphere, a young surface indicative of volcanic resurfacing, and a wide variety of volcanic and tectonic features. The planet s ubiquitous magmatic features include 100,000 small shield volcanoes as well as the descriptively named pancakes, ticks, and arachnoids [1]. Coronae, volcanic and tectonic features up to 2,600 km in diameter, have been attributed to lithospheric interactions with upwelling plumes [e.g., 2], but more recently to delamination of the lower lithosphere with [3] or without [4] a central upwelling. Lavas issuing from different volcanic features appear to have a range of compositions, as evidenced by their apparent viscosities and by data from Soviet landers. Steep-sided or "pancake" domes [e.g., 5] appear to consist of more viscous magma [6], perhaps silicic compositions created by remelting basaltic crust [7]. These steep-sided domes are associated with coronae and with shield volcanoes effusing basaltic magmas [7,8] with apparently low viscosities (low enough to allow fluid flow for hundreds of km, creating channels reminiscent of water rivers on Earth). Pancake domes, in contrast, can be up to 3 km in height and have volumes from 30 to approx.3,000 km3 [calculated from data in 8], and hundreds dot the planet [6-8].

  11. Structural regularities in the lithosphere of continents and plate tectonics

    NASA Astrophysics Data System (ADS)

    Pavlenkova, N. I.

    1995-03-01

    Two fundamental, but competing earth science concepts have been under discussion in Russia. The first one, that of endogenous regimes, is based on the assumption that permanent vertical relationships or long-term interactions between the crust and upper mantle control crustal evolution. Significant horizontal movements of the lithosphere, as required by the second concept, that of global plate tectonics, would destroy these crust-mantle interactions. Certain regular features of the crust and upper mantle support the endogenous regime concept and are difficult to explain in terms of conventional plate tectonics. In particular, the close correlation between near-surface features and deep (> 400 km) mantle inhomogeneities suggests that many geological structures are deeply rooted in the mantle. Moreover, geophysical studies have failed to reveal a well-defined and continuous asthenosphere at relatively shallow depths (˜ 100 km) that would allow lithospheric plates to be transported over large distances, and the rheology of the lithosphere itself is found to be sufficiently inhomogeneous as to cast doubt on the principle of thin rigid plates. In contrast, palaeomagnetic and other data require that horizontal movements of many near-surface geological structures must have taken place. To explain this apparent contradiction, it is suggested here that the crust and its connected deep root are capable of gliding along one of the deep mantle phase transition zones with respect to the inner Earth.

  12. Horizontal stresses induced by vertical processes in planetary lithospheres

    NASA Technical Reports Server (NTRS)

    Banerdt, W. B.

    1993-01-01

    Understanding the state of stress in the elastic lithosphere is of fundamental importance for planetary geophysics, as it is the link between the observed geologic structures on the surface and the processes which form and modify these structures. As such, it can provide valuable constraints for the difficult problem of determining interior structure and processes. On the Earth, most large scale, organized deformation can be related to lateral tectonics associated with plate dynamics; however, the tectonics on many extraterrestrial bodies (such as the Moon, Mars, and most of the outer-planet satellites) appears to be primarily vertical in nature, and the horizontal stresses induced by vertical motions and loads are expected to dominate the deformation of their lithospheres. The largest stress contributions from vertical loading come from the flexure of the lithosphere, which induces both bending moments and membrane stresses. We are concerned here only with nonflexural changes in the state of stress induced by processes such as sedimentary and volcanic deposition, erosional denudation, and changes in the thermal gradient that induce uplift or subsidence. This analysis is important both for evaluating stresses for specific regions in which the vertical stress history can be estimated, as well as for applying the proper loading conditions to global stress models. It is also of interest for providing a reference state of stress for interpreting stress measurements in the crust of the Earth.

  13. Improved determination of vector lithospheric magnetic anomalies from MAGSAT data

    NASA Technical Reports Server (NTRS)

    Ravat, Dhananjay

    1993-01-01

    Scientific contributions made in developing new methods to isolate and map vector magnetic anomalies from measurements made by Magsat are described. In addition to the objective of the proposal, the isolation and mapping of equatorial vector lithospheric Magsat anomalies, isolation of polar ionospheric fields during the period were also studied. Significant progress was also made in isolation of polar delta(Z) component and scalar anomalies as well as integration and synthesis of various techniques of removing equatorial and polar ionospheric effects. The significant contributions of this research are: (1) development of empirical/analytical techniques in modeling ionospheric fields in Magsat data and their removal from uncorrected anomalies to obtain better estimates of lithospheric anomalies (this task was accomplished for equatorial delta(X), delta(Z), and delta(B) component and polar delta(Z) and delta(B) component measurements; (2) integration of important processing techniques developed during the last decade with the newly developed technologies of ionospheric field modeling into an optimum processing scheme; and (3) implementation of the above processing scheme to map the most robust magnetic anomalies of the lithosphere (components as well as scalar).

  14. Observations of flexure and the rheology of the oceanic lithosphere

    SciTech Connect

    Bodine, J.H.; Steckler, M.S.; Bodine, J.H.; Watts, A.B.

    1981-05-10

    Observations of flexure indicate that the effective flexural rigidity of oceanic lithosphere is a function of the age of the lithosphere at the time of loading, and hence temperature. We have used a yield stress envelope model constrained by data from experimental rock mechanics to determine how the flexure parameters and rheologic properties of oceanic lithosphere are related. The results of our model for seamounts and oceanic island loads in the interior of plates suggest that following loading, rapid stress relaxation occurs as the plate 'thins' from its short-term to its long-term (>10/sup 6/ years) mechanical thickness. The mechanical thickness, which determines the effective flexural rigidity of the plate, is strongly dependent on temperature and weakly dependent on load size and duration (>1-10 m.y.). The results of our model for convergent plate boundaries suggest that changes in the shape of the Outer Rise along an individual trench system may be due to variations in the horizontal load acting across the boundary (<1 kbar). The model predicts a narrow zone of high strain accumulation seaward of a trench which is in agreement with variations in crustal velocities and seismicity patterns observed along some trench systems.

  15. A Top to Bottom Lithospheric Study of Africa and Arabia

    SciTech Connect

    Pasyanos, M

    2006-10-31

    We study the lithospheric structure of Africa, Arabia and adjacent oceanic regions with fundamental-mode surface waves over a wide period range. Including short period group velocities allows us to examine shallower features than previous studies of the whole continent. In the process, we have developed a crustal thickness map of Africa. Main features include crustal thickness increases under the West African, Congo, and Kalahari cratons. We find crustal thinning under Mesozoic and Cenozoic rifts, including the Benue Trough, Red Sea, and East, Central, and West African rift systems. Crustal shear wave velocities are generally faster in oceanic regions and cratons, and slower in more recent crust and in active and formerly active orogenic regions. Deeper structure, related to the thickness of cratons and modern rifting, is generally consistent with previous work. Under cratons we find thick lithosphere and fast upper mantle velocities, while under rifts we find thinned lithosphere and slower upper mantle velocities. There are no consistent effects in areas classified as hotspots, indicating that there seem to be numerous origins for these features. Finally, it appears that the African Superswell has had a significantly different impact in the north and the south, indicating specifics of the feature (temperature, time of influence, etc.) to be dissimilar between the two regions. Factoring in other information, it is likely that the southern portion has been active in the past, but that shallow activity is currently limited to the northern portion of the superswell.

  16. Towards Full-Waveform Tomography of the Italian Lithosphere

    NASA Astrophysics Data System (ADS)

    Casarotti, E.; Magnoni, F.; Komatitsch, D.; Melini, D.; Michelini, A.; Piersanti, A.; Tape, C.; Tromp, J.

    2015-12-01

    Within the framework of our PRACE project IMAGINE_IT (3D full-wave tomographic IMAGINg of the Entire ITalian lithosphere) we iteratively improved an initial 3D tomographic model of the Italian lithospheric structure. Our goal was to build a new reference 3D seismic velocity model for the region at unprecedented high resolution, constrained by a large number of observed full seismic waveforms. To this purpose, we used recorded data of dense seismological networks together with extremely efficient numerical techniques and an enormous computational power provided by European Tier-0 systems. We exploited the powerful combination of a spectral-element method (code SPECFEM3D), for high-resolution numerical simulations of seismic wave fields, and an adjoint method, for tomographic inversion and imaging based on misfit reduction between observed and synthetic full waveforms. The earthquakes and stations considered in the inversion procedure homogeneously cover the Italian peninsula and neighbouring zones. All the 3D heterogeneities that characterize the region are implemented in the simulations, also accounting for wave attenuation. We expect that the results of the study will have an important impact in increasing our knowledge of geophysical processes and in addressing societal issues. Creating a refined geological model of the lithosphere in Italy will enhance the capability of analysing seismic effects. This has consequences for the assessment of seismic hazard, for engineering purposes and for planning effective measures based on rapid scenarios.

  17. Oceanic lithosphere and asthenosphere - Thermal and mechanical structure

    NASA Technical Reports Server (NTRS)

    Schubert, G.; Yuen, D. A.; Froidevaux, C.

    1976-01-01

    A coupled thermomechanical subsolidus model of the oceanic lithosphere and asthenosphere is developed which includes vertical heat conduction, a temperature-dependent thermal conductivity, heat advection by a horizontal and vertical mass flow that depends on depth and age, contributions of viscous dissipation or shear heating, a linear or nonlinear deformation law relating shear stress and strain rate, as well as a temperature- and pressure-dependent viscosity. The model requires a constant horizontal velocity and temperature at the surface, but zero horizontal velocity and constant temperature at great depths. The depth- and age-dependent temperature, horizontal and vertical velocities, and viscosity structure of the lithosphere and asthenosphere are determined along with the age-dependent shear stress in those two zones. The ocean-floor topography, oceanic heat flow, and lithosphere thickness are deduced as functions of ocean-floor age; seismic velocity profiles which exhibit a marked low-velocity zone are constructed from the age-dependent geotherms and assumed values of the elastic parameters. It is found that simple boundary-layer cooling determines the thermal structure at young ages, while effects of viscous dissipation become more important at older ages.

  18. Lithospheric Structure of the Northeastern North China Craton Imaged by S Receiver Functions

    NASA Astrophysics Data System (ADS)

    Wang, Xingchen; Ding, Zhifeng; Zhu, Lupei

    2016-08-01

    Lithosphere thickness variation is important for understanding the significant tectonic reactivation of the North China Craton (NCC) in the Mesozoic and Cenozoic time. Here, we determined the lithospheric structure in the northeastern NCC using S receiver functions from 305 teleseismic events recorded by 223 seismic stations. The Moho and lithosphere-asthenosphere boundary (LAB) are imaged clearly beneath the region. The Moho depth decreases from ~45 km beneath the western NCC to ~25 km beneath the eastern NCC. We found that the lithospheric thickness varies from 60 to 80 km beneath the Trans-North China Orogen (TNCO) and eastern NCC with no significant change of the LAB depth. The lithosphere thickness beneath the northwestern Ordos plateau is 100-130 km. In addition, there is a mid-lithosphere discontinuity at a depth of 80 km beneath the plateau that is connected to the base of thinned lithosphere in TNCO and eastern NCC. We suggest that the mid-lithosphere discontinuity represents a mechanically weak zone in the original cratonic lithosphere of the NCC. The material in the lower lithosphere of the craton, when warmed and hydrated by water released from the subducting slab of Western Pacific, became weak due to decrease in viscosity and/or partial melting and was subsequently removed through small-scale mantle convections.

  19. Lithospheric Structure of the Northeastern North China Craton Imaged by S Receiver Functions

    NASA Astrophysics Data System (ADS)

    Wang, Xingchen; Ding, Zhifeng; Zhu, Lupei

    2016-04-01

    Lithosphere thickness variation is important for understanding the significant tectonic reactivation of the North China Craton (NCC) in the Mesozoic and Cenozoic time. Here, we determined the lithospheric structure in the northeastern NCC using S receiver functions from 305 teleseismic events recorded by 223 seismic stations. The Moho and lithosphere-asthenosphere boundary (LAB) are imaged clearly beneath the region. The Moho depth decreases from ~45 km beneath the western NCC to ~25 km beneath the eastern NCC. We found that the lithospheric thickness varies from 60 to 80 km beneath the Trans-North China Orogen (TNCO) and eastern NCC with no significant change of the LAB depth. The lithosphere thickness beneath the northwestern Ordos plateau is 100-130 km. In addition, there is a mid-lithosphere discontinuity at a depth of 80 km beneath the plateau that is connected to the base of thinned lithosphere in TNCO and eastern NCC. We suggest that the mid-lithosphere discontinuity represents a mechanically weak zone in the original cratonic lithosphere of the NCC. The material in the lower lithosphere of the craton, when warmed and hydrated by water released from the subducting slab of Western Pacific, became weak due to decrease in viscosity and/or partial melting and was subsequently removed through small-scale mantle convections.

  20. Origin of the lithosphere-asthenosphere boundary (LAB) and the mid-lithosphere discontinuity (MLD)

    NASA Astrophysics Data System (ADS)

    Karato, Shun-ichiro

    2014-05-01

    Sub-solidus origin of the lithosphere-asthenosphere boundary (LAB) has been discussed based on recent laboratory data (e.g., Karato, 2012). The principal message in such a model is that in these sub-solidus models, the observed geophysical anomalies (low seismic wave velocities and high electrical conductivity) can be explained without ad hoc assumptions. In contrast, if one were to explain these anomalies by partial melting, a number of parameters such as melt fraction and melt geometry must be chosen, making it less attractive model because the choice of these parameters is not easy to justify. In this presentation, I will discuss two issues related to the origin of LAB and MLD. (1) One of the strongest arguments for the partial melt origin of the LAB was the geophysically observed high and highly anisotropic electrical conductivity. Previous lab studies on olivine conductivity showed small anisotropy, and explaining high conductivity (~0.1 S/m) was also difficult. We have recently measured the electrical conductivity of hydrated olivine single crystals to high temperature (to 1373 K). The results show that the degree of anisotropy becomes large at temperature above ~1000 K, and both the average values and the anisotropy of conductivity of the asthenosphere are consistent with the lab data on hydrogen-assisted conductivity if the water content of ~0.01 wt % that is consistent with geochemical inferences is assumed. (2) One of the challenges in sub-solidus models for the LAB is to explain geophysically observed sharp and large velocity drop at the LAB. Through the analysis of lab data on anelasticity obtained in Jackson's lab at ANU, I showed that a large and sharp velocity drop at the LAB can be explained by a subsolidus model assuming a sharp increase in water (hydrogen) content at ~70 km depth that enhances elastically accommodated grain-boundary sliding (EAGBS) (Karato, 2012). I will present a compilation of geophysical observations to show that the same

  1. New insights on lithospheric foundering from thermo-mechanically coupled numerical modelling

    NASA Astrophysics Data System (ADS)

    Pastor-Galán, Daniel; Thieulot, Cedric

    2015-04-01

    Earth's lithosphere is recycled into the mantle as required by global mass considerations. At least during the latest 1 G.y. the main mechanism of lithospheric foundering into the mantle has been subduction. Yet other mechanisms of mantle removal such as Rayleigh-Taylor-type instability or delamination have significant influence at present as revealed by mantle anomalies, and are considered to be likely candidates for the main recycling mechanisms of lithospheric during the Archean. Although lithospheric mantle removal has been geophysically imaged, e.g. Carpathians, Colorado Plateau, at many other locations geophysical and geological observations also seem to indicate that mantle lithosphere is anomalously thin or absent. Potential places where lithospheric mantle foundering processes took place are The Urals, the Variscides, underneath the Ibero Armorican Orocline in western Europe, and the Tibetan, Puna and Anatolian Plateaus. Lithospheric foundering has been blamed for, among others, cratonization processes, rapid surface uplift, generation of voluminous magmatism, changes in crustal stress from compression to extension and a long etc. However, its triggering mechanisms are not well studied, and a variety of possible explanations have been given for lithospheric foundering processes, including convective instability following orogenic thickening or some other perturbation of thermal boundary layers, development of eclogitic roots, erosion of the lithosphere by a flat-subducting slab or partial melting of the asthenosphere, and partial intruding pyroxenites into the base of lithosphere. To understand the mechanisms, causes and consequences of lithospheric foundering, we explored lithospheric foundering in an assortment of scenarios using the numerical code, ELEFANT, an user-friendly multipurpose geodynamics code. Preliminary results indicate that changes in geometry, thermal state and composition of the lithosphere, associated with mantle flow, can have a first

  2. Structure of the Lithosphere in Central Europe: Integrated Density Modelling

    NASA Astrophysics Data System (ADS)

    Bielik, M.; Grinč, M.; Zeyen, H. J.; Plašienka, D.; Pasteka, R.; Krajňák, M.; Bošanský, M.; Mikuška, J.

    2014-12-01

    Firstly, we present new results related to the lithospheric structure and tectonics of the Central Europe and the Western Carpathians. For geophysical study of the lithosphere in Central Europe we calculated four original 2D lithosphere-scales transects crossing this area from the West European Platform in the North to the Aegean Sea in the South and from the Adriatic Sea in the West to the East European Platform in the East. Modelling is based on the joint interpretation of gravity, geoid, topography and surface heat flow data with temperature-dependent density. Wherever possible, crustal structure is constrained by seismic data. The thickness of the lithosphere decreases from the older and colder platforms to the younger and hotter Pannonian Basin with a maximum thickness under the Eastern and Southern Carpathians. The thickness of the Carpathian arc lithosphere varies between 150 km in the North (the Western Carpathians) and about 300 km in the Vrancea zone (the Eastern and Southern Carpathian junction). In the Platform areas it is between 120 and 150 km and in the Pannonian Basin it is about 70 km. The models show that the Moesian Platform is overthrust from the North by the Southern Carpathians and from the South by the Balkanides and characterized by bending of this platform. In all transects, the thickest crust is found underneath the Carpathian Mountains or, as in the case of the Vrancea area, under their immediate foreland. The thickest crust outside the orogens is modelled for the Moesian Platform with Moho depths of up to 45 km. The thinnest crust is located under the Pannonian Basin with about 26-27 km. Secondly, our presentation deals with construction of the stripped gravity map in the Turiec Basin, which represents typical intramontane Neogene depression of the Western Carpathians. Based on this new and original gravity map corrected by regional gravity effect we were able to interpret the geological structure and tectonics of this sedimentary basin

  3. Deep Continental Crustal Earthquakes and Lithospheric Structure: A Global Synthesis

    NASA Astrophysics Data System (ADS)

    Devlin, S.; Isacks, B. L.

    2007-12-01

    The distribution of earthquake depths within the continental crust defines the seismogenic thickness (TS), over which at least some part of crustal deformation is accommodated by rapid release of stored elastic strains. Intraplate continental seismicity is often thought to be restricted to the upper crust where TS is within the range of 15 to 20 km. This appears consistent with a lithospheric strength profile involving a weak, ductile lower crust located beneath a stronger, brittle upper crust. With the assumption of a strong uppermost mantle lid, this is often referred to the Jelly Sandwich model of lithosphere rheology. Studies in many places, however, document lower crustal earthquakes beneath continents in apparent disagreement with the model. We explore this and related issues through a survey of where and in what tectonic settings deep intraplate earthquakes are well documented in the continental crust. TS reaches Moho depth in many intraplate regions \\--- Sierra Nevada, Colorado Plateau, East African and Baikal Rift Systems, North Island New Zealand, Tien Shan, and the Andean and Alpine forelands. A review of possible deformation mechanisms which could control continental earthquake depth and facilitate seismicity beneath the brittle-ductile transition suggests that the influence of fluids is the only mechanism capable of encouraging earthquake occurrence throughout the continental crust at any tectonic setting. Surface derived fluids can induce pore fluid pressure changes to depths of 25 km and melt-reactions can induce earthquakes at depths throughout continental crust. On a global scale, fluid-enhanced embrittlement is not limited by depth or tectonic environment. We find that deep crustal earthquakes occur where the lithosphere is in a transitional state between primarily stable (e.g., shields) and highly deformed (e.g., U.S. Basin and Range or Southern California). Observations of relative intensity of tectonic deformation and regional percent strain

  4. Australia's lithospheric density field, and its isostatic equilibration

    NASA Astrophysics Data System (ADS)

    Aitken, A. R. A.; Altinay, C.; Gross, L.

    2015-12-01

    Density is a key driver of tectonic processes, but it is a difficult property to define well in the lithosphere because the gravity method is non-unique, and because converting to density from seismic velocity models, themselves non-unique, is also highly uncertain. Here we use a new approach to define the lithospheric density field of Australia, covering from 100°E to 165°E, from 5°N to 55°S and from the crust surface to 300 km depth. A reference model was derived primarily from the recently released Australian Seismological Reference Model, and refined further using additional models of sedimentary basin thickness and crustal thickness. A novel form of finite-element method based deterministic gravity inversion was applied in geodetic coordinates, implemented within the open-source escript modelling environment. Three spatial resolutions were modelled: half-, quarter- and eighth-degree in latitude and longitude, with vertical resolutions of 5, 2.5 and 1.25 km, respectively. Parameter sweeps for the key inversion regularization parameters show that parameter selection is not scale dependent. The sweep results also show that finer resolutions are more sensitive to the uppermost crust, but less sensitive to the mid- to lower-crust and uppermost mantle than lower resolutions. All resolutions show similar sensitivity below about 100 km depth. The final density model shows that Australia's lithospheric density field is strongly layered but also has large lateral density contrasts at all depths. Within the continental crust, the structure of the middle and lower crust differs significantly from the crystalline upper crust, suggesting that the tectonic processes or events preserved in the deep crust differ from those preserved in the shallower crust. The lithospheric mantle structure is not extensively modified from the reference model, but the results reinforce the systematic difference between the density of the oceanic and continental domains, and help identify

  5. Using natural laboratories and modeling to decipher lithospheric rheology

    NASA Astrophysics Data System (ADS)

    Sobolev, Stephan

    2013-04-01

    Rheology is obviously important for geodynamic modeling but at the same time rheological parameters appear to be least constrained. Laboratory experiments give rather large ranges of rheological parameters and their scaling to nature is not entirely clear. Therefore finding rheological proxies in nature is very important. One way to do that is finding appropriate values of rheological parameter by fitting models to the lithospheric structure in the highly deformed regions where lithospheric structure and geologic evolution is well constrained. Here I will present two examples of such studies at plate boundaries. One case is the Dead Sea Transform (DST) that comprises a boundary between African and Arabian plates. During the last 15- 20 Myr more than 100 km of left lateral transform displacement has been accumulated on the DST and about 10 km thick Dead Sea Basin (DSB) was formed in the central part of the DST. Lithospheric structure and geological evolution of DST and DSB is rather well constrained by a number of interdisciplinary projects including DESERT and DESIRE projects leaded by the GFZ Potsdam. Detailed observations reveal apparently contradictory picture. From one hand widespread igneous activity, especially in the last 5 Myr, thin (60-80 km) lithosphere constrained from seismic data and absence of seismicity below the Moho, seem to be quite natural for this tectonically active plate boundary. However, surface heat flow of less than 50-60mW/m2 and deep seismicity in the lower crust ( deeper than 20 km) reported for this region are apparently inconsistent with the tectonic settings specific for an active continental plate boundary and with the crustal structure of the DSB. To address these inconsistencies which comprise what I call the "DST heat-flow paradox", a 3D numerical thermo-mechanical model was developed operating with non-linear elasto-visco-plastic rheology of the lithosphere. Results of the numerical experiments show that the entire set of

  6. Lithosphere extension and magmatism at volcanic passive margins

    NASA Astrophysics Data System (ADS)

    Geoffroy, Laurent; Gernigon, Laurent; Werner, Philippe

    2014-05-01

    We present onshore and offshore evidences suggesting that volcanic passive margins are distinct in origin and evolution from non-volcanic hyper-extended margins. Consecutively, they should not be integrated in a single evolutionary process and do not necessarily represent the ultimate stage of an hyper-extension with or without mantle exhumation. Volcanic passive margins usually form in mobile areas between cratonic areas which may have been submitted to long-term periods of divergence and convergence or strike-slip tectonics. In the NE-Atlantic, for example, a complete illustration of a Wilson cycle is illustrated between Greenland and Baltica cratonic areas. From the Devonian to the end of the Jurassic, the Caledonian orogenic crust has suffered from a number of wrench and extensional tectonic stretching episodes. The late-Jurassic/Early Cretaceous extension was severe, leading to extreme crustal thinning (e.g. Rockall Through, Vøring Basin, Lofoten Basin) and was followed by a long-term regional thermal subsidence of the NE-Atlantic lithosphere. Meanwhile, pre-thinning lithospheric thickness was restored progressively during ~80 Myr, in spite of some tectonic reactivation occurring in Late Cretaceous (e.g. Outer Vøring Basin) resulting in little coeval stretching and thinning. During the Paleocene (or even earlier, especially in the Rockall area) a regional mantle melting event occurred. The mantle melted in specific locations but led ultimately to a large igneous province formation during the onset of breakup. The NE-Atlantic continental crust was at this time extremely heterogeneous due to its tectonic inheritance but we think that generally the lithosphere was much thicker than during the Jurassic-Cretaceous event, and thus much stronger. Although we must consider the existence of some extension during the latest Cretaceous and Paleocene, the main stretching and thinning event leading to volcanic passive margins formations and successful break-up occurred

  7. The Neotectonic crustal uplift and lithospheric softening in plate interiors caused by infiltration of mantle fluids into the lithosphere

    NASA Astrophysics Data System (ADS)

    Artyushkov, Eugene

    2013-04-01

    Large-scale crustal uplifts on the continents are commonly attributed to plate collision. Within the continents convergent boundaries now exist only in some regions, e.g., between the Eurasian and Indian plates. A predominant part of continental lithosphere refers to intraplate areas. Thus, the Precambrian crust where shortening terminated half a billion years ago or earlier covers about 70% of the continental areas. However, during the Pliocene and Pleistocene most of the Precambrian crust underwent the uplifts from 100-200 m to 1-2 km. They occurred over most of the African continent, in Greenland and East Siberia, and in many other regions. Neotectonic crustal uplift widely occurred on the Phanerozoic lithosphere. In most regions, e.g., in the Central and Northeastern Asia, the uplift by 1-2 km or more took place long after strong shortening of the crust in the Mesozoic and Paleozoic. It was accompanied by extension or compression of only a few per cent. In the absence of strong crustal thickening, the Neotectonic uplift in intraplate areas required a density decrease in the lithosphere which was caused by two main processes. The first one is expansion of previously metamorphosed dense mafic rocks within the crust due to a secondary metamorphism, diaphtoresis, under the temperature T = 350-400 °C. This mechanism is evidenced by a strong heterogeneity of the uplift in space. Thus in the Archean East Siberia in many places the uplift varies by 300-500 m in regions, only 20 km wide. Rock expansion from diaphtoresis required an inflow into the crust of large volumes of fluid from the mantle. The second process is a convective replacement by the asthenosphere of a denser mantle lithosphere whose viscosity was reduced by several orders of magnitude due to infiltration of fluids from the mantle. In many areas, e.g. in Central Asia and western North America this gave rise to a rise of the top of the asthenospheric layer by ~100 km. Over most of the continental areas

  8. Shape evolution and finite deformation pattern in analog experiments of lithosphere necking

    NASA Astrophysics Data System (ADS)

    Nestola, Yago; Storti, Fabrizio; Bedogni, Enrico; Cavozzi, Cristian

    2013-10-01

    necking evolution determines the 3-D architecture of crustal and upper mantle thinning and related basins, and the heat flow distribution in rifted regions. Despite a large number of studies, lithosphere necking evolution is still a matter of debate. We present the result from lithospheric-scale analog models designed for investigating the necking shape during extension and the vertical distribution of finite deformation in the mechanical lithosphere. In our experiments, lithosphere necking is asymmetric and, in particular, the 3-D distribution of thinning is cylindrical in the crust and very heterogeneous in the mantle. Overall, the evolution of rifting and necking progresses from delocalized to localized deformation.

  9. Arctic Crustal Thickness and Ocean-Continent Transition from Gravity Inversion Incorporating a Lithosphere Thermal Correction

    NASA Astrophysics Data System (ADS)

    Greenhalgh, E.; Kusznir, N. J.; Lebedeva-Ivanova, N.; Alvey, A.; Gaina, C.; Torsvik, T. H.

    2007-12-01

    Crustal thickness and continental lithosphere thinning factors have been determined for the High Arctic using a gravity inversion method which incorporates a lithosphere thermal gravity anomaly correction. Continental lithosphere thinning factor maps, determined by the inversion of the NGA (U) Arctic Gravity Project data have been used to predict the distribution of oceanic lithosphere and ocean-continent transition (OCT) location for the Amerasia Basin. Thin crust and high lithosphere thinning factors are predicted in the Makarov, Podvodnikov and Canada Basins consistent with these basins being oceanic. Larger crustal thicknesses, in the range 20 - 30 km, are predicted for the Lomonosov, Alpha and Mendeleev Ridges. Moho depths predicted by gravity inversion have been compared with seismic estimates for the TransArctica and Arctica profiles with seismically observed sediment thickness included in the gravity inversion. Agreement between gravity and seismic Moho depths is generally good. The largest differences between gravity and seismic Moho depths occur where lower crustal seismic velocities, Vp, are in excess of ~ 7.3km/s. Gravity inversion to determine Moho depth and crustal thickness variation is carried out in the 3D spectral domain. A correction for the large negative residual thermal gravity anomaly within oceanic and stretched continental margin lithosphere is made and requires a lithosphere thermal model to predict the present day lithosphere thermal anomaly. For continental margin lithosphere, the lithosphere thermal perturbation is calculated from the lithosphere thinning factor (1-1/beta) obtained from crustal thinning determined by gravity inversion and breakup age for thermal re-equilibration time. A correction is made for crustal volcanic addition due to decompression melting during breakup and sea-floor spreading. For the Amerasia Basin, where ocean isochrons are uncertain, all lithosphere is assumed to be initially continental, and a lithosphere

  10. Lithospheric evolution of the Northern Arabian Shield: Chemical and isotopic evidence from basalts, xenoliths and granites

    NASA Technical Reports Server (NTRS)

    Stein, M.

    1988-01-01

    The evolution of the upper-mantle and the lower-crust (the conteinental lithosphere), is the area of Israel and Sinai was studied, using the chemical composition and the Nd-Sr isotopic systematics from mantle and crustal nodules, their host basalts, and granites. The magmatism and the metasomatism making the lithosphere are related to uprise of mantle diapirs in the uppermost mantle of the area. These diapirs heated the base of the lithosphere, eroded, and replaced it with new hot material. It caused a domal uplift of the lithosphere (and the crust). The doming resulted in tensional stresses that in turn might develop transport channels for the basalt.

  11. Linking plate reconstructions with deforming lithosphere to geodynamic models

    NASA Astrophysics Data System (ADS)

    Müller, R. D.; Gurnis, M.; Flament, N.; Seton, M.; Spasojevic, S.; Williams, S.; Zahirovic, S.

    2011-12-01

    While global computational models are rapidly advancing in terms of their capabilities, there is an increasing need for assimilating observations into these models and/or ground-truthing model outputs. The open-source and platform independent GPlates software fills this gap. It was originally conceived as a tool to interactively visualize and manipulate classical rigid plate reconstructions and represent them as time-dependent topological networks of editable plate boundaries. The user can export time-dependent plate velocity meshes that can be used either to define initial surface boundary conditions for geodynamic models or alternatively impose plate motions throughout a geodynamic model run. However, tectonic plates are not rigid, and neglecting plate deformation, especially that of the edges of overriding plates, can result in significant misplacing of plate boundaries through time. A new, substantially re-engineered version of GPlates is now being developed that allows an embedding of deforming plates into topological plate boundary networks. We use geophysical and geological data to define the limit between rigid and deforming areas, and the deformation history of non-rigid blocks. The velocity field predicted by these reconstructions can then be used as a time-dependent surface boundary condition in regional or global 3-D geodynamic models, or alternatively as an initial boundary condition for a particular plate configuration at a given time. For time-dependent models with imposed plate motions (e.g. using CitcomS) we incorporate the continental lithosphere by embedding compositionally distinct crust and continental lithosphere within the thermal lithosphere. We define three isostatic columns of different thickness and buoyancy based on the tectonothermal age of the continents: Archean, Proterozoic and Phanerozoic. In the fourth isostatic column, the oceans, the thickness of the thermal lithosphere is assimilated using a half-space cooling model. We also

  12. Lithospheric scale model of Merida Andes, Venezuela (GIAME Project)

    NASA Astrophysics Data System (ADS)

    Schmitz, M.; Orihuela, N. D.; Klarica, S.; Gil, E.; Levander, A.; Audemard, F. A.; Mazuera, F.; Avila, J.

    2013-05-01

    Merida Andes (MA) is one of the most important orogenic belt in Venezuela and represents the northern culmination of South America Andes. During the last 60 years, several models have been proposed to explain the shallow and deep structure, using different geological, geophysical, seismological, geochemical and petrologic concepts; nevertheless, most of them have applied local observation windows, and do not represent the major structure of MA. Therefore, a multidisciplinary research group, coordinated by FUNVISIS, in close cooperation with UCV, ULA and PDVSA, is proposed in order to get the outlined goals in the project entitled GIAME ("Geociencia Integral de los Andes de MErida") was established, which aims to generate a lithospheric scale model and the development of a temporal dynamic model for the MA. As a base for lithospheric investigations of the Merida Andes, we are proposing three wide angle seismic profiles across the orogen on three representative sites, in order to determine the inner structure and its relation with the orogen's gravimetric root. To the date, there are no seismic studies at lithospheric scale which cross MA. The wide angle seismic will be complemented with the re-processing and re-interpretation of existing reflection seismic data, which will allow to establish a relationship between MA and its associated flexural basins (Maracaibo and Barinas-Apure basins). Depending on the results of the VENCORP Project (VENezuelan COntinental Reflection Profiling), which might show some reliable results about crustal features and Moho reflectors along three long seismic profiles at Caribbean Moutain system, a reflection seismic profile across the central portion of MA is proposed. Additional tasks, consisting in MA quaternary deformation studies, using research methods like neotectonics and paleoseismology, georadar, numerical modeling, cinematic GPS, SAR interferometry, thermocronology, detailed studies on regional geology, flexural modeling

  13. Crustal and Lithospheric Structure at Isidis Planitia, Mars

    NASA Astrophysics Data System (ADS)

    Ritzer, J. A.; Hauck, S. A.

    2005-12-01

    Isidis Planitia is the site of a large free air gravity anomaly consistent with modification and sedimentary or magmatic filling of an impact basin. Mars Global Surveyor gravity and topography data of the Isidis basin on Mars are analyzed to elucidate the crustal and lithospheric structure near the dichotomy boundary in the Eastern hemisphere. Global gravity and topography datasets are inverted using an extension of Banerdt's [1986] model for deformation of a thin elastic shell and a nominal assumption that Isidis was isostatically compensated prior to modification by infilling. Using this model we attempt to constrain potential variations in (and trade-offs among) local crustal thickness, density and thickness of basin fill material, and lithospheric deformation. The permissible parameter space is limited by assuming that the local crustal thickness of the basin cannot be less than zero at the time of infilling. Our results suggest that the density of the fill inside Isidis must be more than 2500 kg/m3 and higher densities are probable. Recent work of Wieczorek and Zuber [2004] indicates that the most likely average crustal thickness of Mars is between 38 and 62 km. On the basis of this range of values for crustal thickness, we infer that the average density of the fill is more than 2900 kg/m3. A high fill density suggests that the material inside the basin is predominantly of igneous rather than sedimentary origin. Under the assumption of an average crustal density of 2900 kg/m3 the inferred thickness of the fill layer is at least 5 km, and could be significantly larger depending upon the degree of compensation of the basin before it was loaded by fill material. A comparison of the faulting observed at Nili Fossae to the predicted zone of extensional strain northwest of Isidis may further constrain the thickness of the elastic lithosphere beneath the basin at the time of loading.

  14. Modeling Plume-Triggered, Melt-Enabled Lithospheric Delamination

    NASA Astrophysics Data System (ADS)

    Perry-Houts, J.; Humphreys, G.

    2015-12-01

    It has been suggested that arrival of the Yellowstone plume below North America triggered a lithospheric foundering event which aided the eruption of the Columbia River flood basalts. This hypothesis potentially accounts for some of the biggest mysteries related to the CRB's including their location as "off-track" plume volcanism; and the anomalous chemical signatures of the most voluminous units. The foundered lithosphere appears to be a remnant chunk of Farallon slab, which had been stranded beneath the Blue Mountains terrain since the accretion of Siletzia. If this is the case then the mechanisms by which this slab stayed metastable between Siletzia accretion and CRB time, and then so suddenly broke loose, is unclear. The addition of heat and mantle buoyancy supplied by the Yellowstone plume provides a clue, but the geodynamic process by which the slab was able to detach remains unclear.Efforts to model numerically the underlying processes behind delamination events have been gaining popularity. Typically, such models have relied on drastically weakened regions within the crust, or highly non-linear rheologies to enable initiation and propagation of lithosphere removal. Rather than impose such a weak region a priori, we investigated the role of mantle and crustal melt, generated by the addition of plume heat, as the source of such a rheologic boundary.We track melt generation and migration though geodynamic models using the Eulerian finite element code, ASPECT. Melt moves relative to the permeable, compacting, and viscously-deforming mantle using the approach of (Keller, et al. 2013) with the notable exception that ASPECT currently cannot model elasticity. Dike and sill emplacement is therefore still a work in progress. This work is still in the preliminary stages and results are yet inconclusive.

  15. Switching between alternative responses of the lithosphere to continental collision

    NASA Astrophysics Data System (ADS)

    Baes, Marzieh; Govers, Rob; Wortel, Rinus

    2011-12-01

    We study possible responses to arc-continent or continent-continent collision using numerical models. Our short-term integration models show that the initial stage of deformation following continental collision is governed by the competition between three potential weakness zones: (1) mantle wedge, (2) plate interface and (3) lower continental crust. Depending on which of these is the weakest zone in the system, three different responses can be recognized: (1) subduction polarity reversal, (2) continuation of subduction and (3) delamination and back stepping. Subduction polarity reversal occurs if the mantle wedge is the weakest zone in the system. This happens only if the viscosity of the mantle wedge is at least one order of magnitude lower than the average viscosity of the lithosphere. In continent-continent collision, one additional condition needs to be satisfied for subduction polarity reversal to occur: for the subducting lithosphere the ratio of the viscosity of the lower continental crust to the viscosity of the upper lithospheric mantle must be equal to or higher than 0.006. The time required for polarity reversal depends on several parameters: the convergence rate, the sinking velocity of the detached slab and the relative strength of the mantle wedge, arc and backarc. The response to collision is continued subduction if the plate interface is the weakest zone, and is delamination and back stepping if the lower continental crust is the weakest area in the system. Our finding that a low-viscosity wedge is a prerequisite for a reversal of subduction polarity agrees with inferences about regions for which subduction polarity reversal has been proposed.

  16. Deformation of olivine single crystals under lithospheric conditions

    NASA Astrophysics Data System (ADS)

    Demouchy, S.; Tommasi, A.; Cordier, P.

    2012-12-01

    The rheology of mantle rocks at lithospheric temperatures (<1000°C) remains poorly constrained, in contrast to the extensive experimental data on creep of olivine single crystals and polycrystalline aggregates at high temperature (T > 1200°C). Consequently, we have performed tri-axial compression experiments on oriented single crystals and polycrystalline aggregates of San Carlos olivine at temperatures ranging from 800° to 1090°C. The experiments were carried out at a confining pressure of 300 MPa in a high-resolution gas-medium mechanical testing apparatus at constant strain rates ranging from 7 × 10-6 s-1 to 1 × 10-4 s-1 . Compression was applied along three different crystallographic directions: [101]c, [110]c and [011]c, to activate the several slip systems. Yield differential stresses range from 88 to 1076 MPa. To constrain hardening, stick-and-slip, or strain localization behaviors, all samples were deformed at constant displacement rate for finite strains between 4 to 23 %. Hardening was observed in all experiments and the maximum differential stress often overcame the confining pressure. EBSD mapping highlights macroscale bending of the crystalline network in three crystals. TEM observations on several samples show dislocations with [100] and [001] Burgers vectors in all samples, but dislocation arrangements vary. The results from the present study permit to refining the power-law expressing the strain rate dependence on stress and temperature for olivine, allowing its application to the lithospheric mantle. Our experiments confirm that previous published high-temperature power flow laws overestimate the strength of lithospheric mantle and that the transition to low-temperature creep occurs at higher temperatures than it has previously been established.

  17. Rethinking the problem of ionosphere-lithosphere coupling

    NASA Astrophysics Data System (ADS)

    Ruzhin, Yuri; Novikov, Victor

    2014-05-01

    An overview of research of possible relations between variations of geomagnetic field and seismicity is presented, including Sq-variations and geomagnetic storms. There are many papers demonstrating positive correlations between geomagnetic field variations and subsequent earthquake occurrence that allows to authors to talk about earthquake triggering impact provided by ionospheric processes on lithosphere. Nevertheless, there is another opinion on negligible impact of geomagnetic disturbances on the earthquake source supported by statistical analysis of correlation between variations of geomagnetic field and global and regional seismicity. Mainly, the both points of view on this problem are based on statistical research without detailed consideration of possible physical mechanisms which may be involved into the supposed earthquake triggering, or very rough estimations of possible increase of stresses in the faults under critical (near to failure) state were made. Recently it was shown that the fluids may play very important role in the electromagnetic earthquake triggering, and the secondary triggering mechanism should be considered when the fluid migrating into the fault under electromagnetic action may provide fault weakening up to earthquake triggering threshold. At the same time, depending on fault orientation, local hydrological structure of the crust around the fault, location of fluid reservoirs, etc. it may be possible that fluid migration from the fault may provide the fault strengthening, and in this case the impact of variation of geomagnetic field may provide an opposite effect. In so doing, it is useless to apply only statistical approach for the problem of ionosphere-lithosphere coupling, and in each case the possible behavior of fluids should be considered under electromagnetic impact on lithosphere. Experimental results supporting this idea and obtained at the spring-block model simulating the seismic cycle (slow accumulation and sharp drop of

  18. Melt-Enabled Lithospheric Delamination in the Western US

    NASA Astrophysics Data System (ADS)

    Perry-Houts, J.; Humphreys, E.

    2014-12-01

    Signs of convective downwelling in non-subduction environments have been increasingly prevalent in recent studies. From seismic images of the "Isabella Anomaly" beneath the southern Sierra Nevada, to studies on the origin of the Columbia River flood basalts, to long standing speculation on the state of the lithosphere beneath the Colorado Plateau, the lithospheric delamination hypothesis potentially accounts for a substantial amount of otherwise anomalous topographic uplift and magmatism in western North America and elsewhere. Efforts to model numerically the underlying processes behind such events have also been gaining popularity. Typically, such models have relied on drastically weakened lithosphere or highly non-linear rheologies at the boundaries of dense regions to allow the propagation of delamination in reasonable timescales. However, these low viscosities are rarely justified by a systematic investigation of mantle rheology.Based on suspected cases of delamination, the process seems to be commonly associated with localized magmatism. For instance, the Columbia River flood basalts, the renewed magmatism in the southern Sierras, and the radially converging magmatism propagating though the Colorado Plateau, all match well temporally with suspected delamination cases. Geochemical signatures are consistent with delamination in all of these cases.Because of the spatial and temporal correlations between delamination and magmatism, we are interested in investigating the rheological role that melt might play in the initiation and propagation of delamination. Moderate melt fraction in the asthenosphere is a potential source for the low viscosity necessary to appropriately predict timescales of these events. To that end, we are working on tracking melt generation and migration though geodynamic models using the Eulerian finite element code, ASPECT. Preliminary results predict melt being generated during delamination events but the magnitudes and rheological

  19. Lithospheric Structure and Seismotectonics of Central East Antarctica

    NASA Astrophysics Data System (ADS)

    Reading, A. M.

    2006-12-01

    The lithosphere of central East Antarctica, the sector of the continent between 30°E - 120°E, is investigated using seismic methods including receiver function and shear-wave splitting analysis. Data from the broadband stations of the temporary SSCUA deployment (in the continental interior) are used together with records from the permanent GSN stations (on the coast) to carry out the first studies of crustal depth and structure, and patterns of seismic anisotropy across this region. The depth of the Moho is found to be 42 km (+/- 2 km) beneath Mawson station with similar structures extending southward across the Rayner province as far south as Beaver Lake. The Fisher Terrane is characterised by a crustal shear wavespeed profile showing few discontinuties with the Moho at a similar depth to the Rayner. South of Fisher, the crust becomes much shallower, with the Moho at 32 km depth. This shallow crust extends across the Lambert glacier to the Prydz coast and the Lambert Terrane. The characteristic crustal wavespeed profiles provide baseline structure for mapping the extent of the terrance beneath the Antarctic Ice Sheet in future deployments. Observations of seismic anisotropy are less well- defined but, at a reconnaissance level, show fast directions parallel to the present day coastline. This may be controlled by rift-related influences on the lithosphere associated with the breakup of East Gondwana. The seismicity is confirmed to be extremely low. The only seismogenic forces on the Antarctic plate in this region are acting at the boundary between the continental and oceanic lithosphere west of 50°E and east of 100°E and represent a superposition of tectonic and glaciogenic controls. The Lambert Glacier region shows little or no seismotectonic activity in the continental interior or on the oceanic margin.

  20. Lithospheric strength variations in Mainland China: tectonic implications

    NASA Astrophysics Data System (ADS)

    Deng, Y.; Tesauro, M.

    2015-12-01

    We present new thermal and strength models of Mainland China. We integrate thermal model for the crust, using a 3D steady-state heat conduction equation, with estimates for the upper mantle thermal structure obtained by inverting an S-wave tomography model. Using the new thermal model and attributing to the lithospheric layers a 'soft' and 'hard' rheology, respectively, we estimate the integrated strength of the lithosphere. In the Ordos and the Sichuan basins, characterized by intermediate temperatures, strength is primarily concentrated in the crust, when the rheology is 'soft', and in both the crust and upper mantle, when the rheology is 'hard'. In turn, the Tibetan Plateau and the Tarim basin have a weak/strong lithosphere mainly on account of their high/low temperatures. Deep earthquakes releasing high seismic energy, occurring beneath Tien Shan orogen, may be related to the brittle failure of anhydrous granulite-faciesrocks composing its lower crust. In contrast, the fluids released by the Indian slab favor the triggering of earthquakes located in the deep crust of south Tibet. Comparison of temperatures, strength and effective viscosity variations with the earthquakes distribution and their seismic energy released indicates that both the deep part of the crust and the upper mantle of the Tibetan Plateau are weak and prone to flow towards the adjacent areas. On account of the high strength of some of the tectonic domains surrounding Tibet, the flow is directed northward beneath the Qaidam basin and turns south of the Sichuan basin, moving toward the weak South China block.

  1. Anomalous variations of lithosphere magnetic field before several earthquakes

    NASA Astrophysics Data System (ADS)

    Ni, Z.; Chen, B.

    2015-12-01

    Based on the geomagnetic vector data measured each year since 2011 at more than 500 sites with a mean spatial interval of ~70km.we observed anomalous variations of lithospheric magnetic field before and after over 15 earthquakes having magnitude > 5. We find that the field in near proximity (about 50km) to the epicenter of large earthquakes shows high spatial and temporal gradients before the earthquake. Due to the low frequency of repeat measurements it is unclear when these variations occurred and how do them evolve. We point out anomalous magnetic filed using some circles with radius of 50km usually in June of each year, and then we would check whether quake will locat in our circles during one year after that time (June to next June). Now we caught 10 earthquakes of 15 main shocks having magnitude > 5, most of them located at less than10km away from our circles and some of them were in our circles. Most results show that the variations of lithosphere magnetic filed at the epicenter are different with surrending backgroud usually. When we figure out horizontal variations (vector) of lithosphere magnetic field and epicenter during one year after each June, we found half of them show that the earthquakes will locat at "the inlands in a flowing river", that means earthquakes may occur at "quiet"regions while the backgroud show character as"flow" as liquid. When we compared with GPS results, it appears that these variations of lithospere magnetic field may also correlate with displacement of earth's surface. However we do not compared with GPS results for each earthquake, we are not clear whether these anomalous variations of lithospere magnetic field may also correlate with anomalous displacement of earth's surface. Future work will include developing an automated method for identifying this type of anomalous field behavior and trying to short repeat measurement period to 6 month to try to find when these variations occur.

  2. A top to bottom lithospheric study of Africa and Arabia

    NASA Astrophysics Data System (ADS)

    Pasyanos, Michael E.; Nyblade, Andrew A.

    2007-11-01

    We study the lithospheric structure of Africa, Arabia and adjacent oceanic regions with fundamental-mode surface waves over a broad period range. Including group velocities with periods shorter than 35 s allows us to examine shallower features than previous studies of the whole continent. In the process, we have developed a crustal thickness map of Africa. Main features include crustal thickness increases under the West African, Congo, and Kalahari cratons. We find crustal thinning under Mesozoic and Cenozoic rifts, including the Benue Trough, Red Sea, and East, Central, and West African rift systems, along with less abrupt crustal thickness changes at passive continental margins. We also find crustal thickness differences in North Africa between the West African Craton and East Saharan Shield. Crustal shear wave velocities are generally faster in oceanic regions and cratons, and slower in more recent crust and in active and remnant orogenic regions. Deeper structure, related to the thickness of cratons and modern rifting, is generally consistent with previous work. Under cratons we find thick lithosphere and fast upper mantle velocities, while under rifts we find thinned lithosphere and slower upper mantle velocities. However, we also find the lack of a thick cratonic keel beneath the central portion of the Congo Craton. There are no consistent effects in areas classified as hotspots, indicating that there seem to be numerous origins for these features. Finally, it appears that the African Superswell, which is responsible for high elevation and uplift over large portions of Africa, has had a significantly different impact (as indicated by features such as temperature, time of influence, etc.) in the north and the south. This is consistent with episodic activity at shallow depths, which is well-expressed in northeastern Africa and Arabia today.

  3. Lithospheric Mantle Contribution to High Topography in Central Mongolia

    NASA Astrophysics Data System (ADS)

    Carlson, R. W.; Ionov, D. A.

    2014-12-01

    Over 110 spinel peridotite xenoliths collected from four localities in the Tariat region, central Mongolia, show a predominance (over 90%) of fertile lherzolites with subordinant harzburgite and peridotites veined with pyroxenite. Equilibration temperatures are high (~900°C at 1.5 GPa [1]). Major element compositions of the fertile samples are consistent with them being the residues of 0-6% partial melt removal at shallow depths [2]. The clinopyroxenes in the lherzolites are moderately LREE depleted (average chondrite normalized La/Sm = 0.45) and most whole rocks show small, if any, depletions in Re and Pd compared to the other HSE. These data point to minimal metasomatic overprinting of these fertile lherzolites. 187Os/188Os for samples with more than 3.2% Al2O3 range only from 0.126 to 0.131, within the range of modern fertile asthenospheric mantle. In contrast to the indicators of fertility in most samples, Sr, Nd and Hf isotopic composition of acid-leached clinopyroxene separates from the lherzolites plot within the range of modern MORB with 87Sr/86Sr from 0.7021 to 0.7026, eNd from +7.7 to +9.8 and eHf from +13.3 to +18.5. The lherzolites thus appear to sample a section of mantle that has compositional and isotope characteristics consistent with modern fertile asthenosphere. The isotopic composition of the Tariat lherzolites are distinct from that of Cenozoic Mongolian basaltic volcanism pointing to limited involvement of the lithospheric mantle in magma generation in this area. The implied asthenospheric provenance of the mantle lithosphere suggests that it either could be the replacement for recently delaminated lithosphere or, more likely, a section of fertile mantle accreted to the base of the crust earlier, e.g. during construction of the Central Asian Orogenic Belt in the Mesozoic/Paleozoic. Although fertile, and hence compositionally dense, the high temperatures of the shallow lithospheric mantle under this section of Mongolia likely contribute to the

  4. Lithosphere structure in Northern Canada from receiver function (RF)

    NASA Astrophysics Data System (ADS)

    Barantseva, Olga; Vinnik, Lev; Artemieva, Irina

    2016-04-01

    We present preliminary results of seismic data analysis for Northern Canada (around the Slave craton and NE of the Hudson Bay) in order to infer the lithosphere and asthenosphere structure beneath various Precambrian terrains of the North American craton. Seismic analysis includes data processing for the several stations of the Canadian National Seismic Network, for which P and S-velocity profiles are calculated through the simultaneous inversion of receiver functions. We report variations in the Moho depth and sharpness, as well as the depth to the LAB. The results are compared with regional petrological data for xenoliths.

  5. On searching applicants for mechanism of solar-lithosphere relations

    NASA Astrophysics Data System (ADS)

    Kairatkyzy, Dina; Andreyev, Alexey; Zhumabayeva, Saltanat; Seraliyev, Alibek

    2016-04-01

    It is actively discussed at present a question on possible influence of solar activity (high-speed solar wind streams bearing the "frozen" magnetic field lines of the Sun) on the stress status of the lithosphere and, consequently, on the Earth's seismic activity (e.g. Zhang, 1998, Acta Seismologica Sinica; Khachikyan et al., EGU2016-2754-1; IUGG2015-3132). There are at least two ideas on possible applicants for physical mechanism of solar-lithosphere relations: (i) - the muons of cosmic rays, which can penetrate the Earth's crust to a depth of at least the first ten kilometers and in tense seismic environment generate nuclear-electromagnetic cascade which energy can be a trigger of earthquake (Tsarev and Chechin, 1988, Preprint № 179, Physical Institute after Lebedev, Moscow); (ii) - the geomagnetic storms (Sobolev et al., 1998, Physics of the Earth #7) when the high-frequency oscillations of the geomagnetic field during the main phase of the storm generate significant induction currents which electric energy entering into the crust can be converted into mechanical energy increasing the stress status of the lithosphere (Sobolev and Demin, Mechano-electric phenomena into the Earth. M . Nauka. 1980). Besides this, among the possible cosmogenic factors changing the stress state of the lithosphere, could be the variation of the angular velocity of rotation of the Earth (e.g. Bostrom, 2000. Tectonic consequence of the Earth's Rotation), if it depends on solar activity variations. More of 50 years ago, Munk and Donald (The Rotation of the Earth, Cambrige University Press, 1960) suggested that the interaction between solar wind and geomagnetic field would probably influence the short period variation of angular velocity of the Earth. In this work, we check up this suggestion on the base of very precise data on the length of day (LOD) from 1986 to the present, which are presented by the International Earth Rotation and Reference Systems Service (IERS). Using the methods

  6. Three-dimensional density distributions in the Asian lithosphere

    NASA Astrophysics Data System (ADS)

    Zhang, G.; Li, C.; Wang, X.; Wang, Z.; Fang, J.; Sino-probe-cugb

    2011-12-01

    We have inversed the residual Bouguer gravity anomalies to study the three-dimensional density distributions of the Asian lithosphere (60°~150°E and 15°~60°N). Firstly, we have collected the free-air gravity anomalies (30'×30') and topography data of GTOP030 with 5'×5' grid spacing, and then calculated the Bougouer gravity anomalies by terrain correction and Bougouer correction. We have also collected the depth data of the Moho discontinuity (30'×30') and the discontinuity of sedimentary layer. By using the Oldenburg-Parker formula (Parker, 1972) and the forward modeling method, we calculated the theoretical gravity anomalies which mainly are caused by the Moho discontinuity and the sedimentary layer discontinuity. In our study, the average depths of Moho discontinuity and sedimentary layer discontinuity are 33 km and 4 km, and the density differences are 0.42 g/cm3 and 0.2 g/cm3, respectively. In addition, we have simulated the gravity anomalies of the spherical harmonics with the 2-6 order for the lower mantle by using the formula of Bowin (1983) which represented the relation between the depth of field source and the order of the geopotential spherical harmonics. Using all data mentioned above, we have calculated the residual Bougouer gravity anomalies, which may be caused by anomalous density bodies in the lithosphere. Secondly, we used the calculated residual Bougouer gravity anomalies to inverse the three-dimensional density differences in the Asian lithosphere by using the Algebra Reconstruction Techniques (ART). During the inversion, the densities converted from the P-wave velocity data (with grid spacing of 2°×2°) according to the Birch Law are considered as the initial density model. The grid spacing is set as 2°×2° in the horizontal direction, and it is 25 km, 55 km and 100 km in the vertical direction, respectively. Comparing the density anomalies at the three depths, we can conclude that (1) the density in the lithosphere beneath Asian

  7. Counterclockwise block rotation linked to southward propagation and overlap of sub-aerial Red Sea Rift segments, Afar Depression: Insight from paleomagnetism

    NASA Astrophysics Data System (ADS)

    Muluneh, Ameha A.; Kidane, Tesfaye; Rowland, Julie; Bachtadse, Valerian

    2013-05-01

    Thirty-four cooling units from the Pleistocene extrusive volcanic rocks exposed in regions bordered by the active Red Sea magmatic segments, in the northwestern central Afar Depression, were sampled for paleomagnetic study. Six to 12 samples were collected from each paleomagnetic site. Samples were demagnetized using Alternating Field (AF) and Thermal (TH) demagnetization techniques. The samples were then measured using the JR-6A spinner magnetometer available at the department of Earth Sciences of Addis Ababa University. The Natural Remanent Magnetization (NRM) direction reveals mostly one or two simple and straightforward components of magnetizations; the first, low-stability component is isolated by heating to 100 °C-300 °C or by AF of 10-30 mT. The magnetization directions after these steps generally define straight lines that are directed towards the origin, and interpreted as primary NRM or ChRM (Characteristic Remanent Magnetization) and the direction of magnetization was determined. Results of the magnetization decay curve plots and rock magnetic analyses using a Variable Field Translation Balance (VFTB) indicate that magnetic mineralogy is dominated by Ti-poor titano-magnetite with magnetic grain sizes in the pseudo-single domain (PSD) range, with minor presence of goethite and maghemite in a few cases. An overall mean direction calculated for the 26 sites located within the overlap zone is obtained (D = 354.4°, I = 13.2°, N = 26, K = 43.5, α95 = 4.3°), and compared to the expected Geomagnetic Axial Dipole (GAD) field, based on the Apparent Polar Wander Path (APWP) Curve of the African plate (Besse and Courtillot, 1991, 2003), a difference in declination ΔD = - 6.5° ± 4.0° is obtained. This declination difference is interpreted as counterclockwise rotation about a vertical axis, in agreement with rift propagation and right stepping overlap geometry of the Alayta-Dabbahu magmatic segments. The difference in inclination ΔI = 7.8° ± 3.9° is

  8. Fossilized Dipping Fabrics in Continental Mantle Lithosphere as Possible Remnants of Stacked Oceanic Paleosubductions

    NASA Astrophysics Data System (ADS)

    Babuska, V.; Plomerova, J.; Vecsey, L.; Munzarova, H.

    2015-12-01

    We have examined seismic anisotropy within the mantle lithosphere of Archean, Proterozoic and Phanerozoic provinces of Europe by means of shear-wave splitting and P-wave travel-time deviations of teleseismic waves observed at dense arrays of seismic stations (e.g., Vecsey et al., Tectonophys. 2007). Lateral variations of seismic-wave anisotropy delimit domains of the mantle lithosphere, each of them having a consistent fabric. The domains, modeled in 3D by olivine aggregates with dipping lineation a, or foliation (a,c), represent microplates or their fragments that preserved their pre-assembly fossil fabrics in the mantle lithosphere. Evaluating seismic anisotropy in 3D, as well as mapping boundaries of the domains helps to decipher processes of the lithosphere formation. Systematically dipping mantle fabrics and other seismological findings seem to support a model of continental lithosphere built from systems of paleosubductions of plates of ancient oceanic lithosphere (Babuska and Plomerova, AGU Geoph. Monograph 1989), or by stacking of the plates (Helmstaedt and Schulze, Geol. Soc. Spec. Publ. 1989). Seismic anisotropy in the oceanic mantle lithosphere, explained mainly by the olivine A- or D-type fabric (Karato et al., Annu. Rev. Earth Planet. Sci. 2008), was discovered a half century ago (Hess, Nature 1964). Field observations and laboratory experiments indicate the oceanic olivine fabric might be preserved in the subducting lithosphere to a depth of at least 200-300 km. We thus interpret the dipping anisotropic fabrics in domains of the European mantle lithosphere as systems of "frozen" paleosubductions (Babuska and Plomerova, PEPI 2006), and the lithosphere base as a boundary between a fossil anisotropy in the lithospheric mantle and an underlying seismic anisotropy related to present-day flow in the asthenosphere (Plomerova and Babuska, Lithos 2010).

  9. Constraining Lithosphere Deformation Modes during Continental Breakup for the Iberia-Newfoundland Conjugate Margins

    NASA Astrophysics Data System (ADS)

    Jeanniot, L.; Kusznir, N. J.; Mohn, G.; Manatschal, G.

    2014-12-01

    How the lithosphere and asthenosphere deforms during continental rifting leading to breakup and sea-floor spreading initiation is poorly understood. Observations at present-day and fossil analogue rifted margins show a complex OCT architecture which cannot be explained by a single simplistic lithosphere deformation modes. This OCT complexity includes hyper-extended continental crust and lithosphere, detachments faults, exhumed mantle, continental slivers and scattered embryonic oceanic crust. We use a coupled kinematic-dynamic model of lithosphere and asthenosphere deformation to determine the sequence of lithosphere deformation modes leading to continental breakup for Iberia-Newfoundland conjugate margin profiles. We quantitatively calibrate the models using observed present-day water loaded subsidence and crustal thickness, together with subsidence history and the age of melt generation. Flow fields, representing a sequence of lithosphere deformation modes, are generated by a 2D finite element viscous flow model (FE-Margin), and used to advect lithosphere and asthenosphere temperature and material. FE-Margin is kinematically driven by divergent deformation in the upper 15-20 km of the lithosphere inducing passive upwelling below. Buoyancy enhanced upwelling (Braun et al. 2000) is also kinematically included. Melt generation by decompressional melting is predicted using the methodology of Katz et al., 2003. The extension magnitudes used in the lithosphere deformation models are taken from Sutra et al (2013). The best fit calibrated models of lithosphere deformation evolution for the Iberia-Newfoundland conjugate margins require (i) an initial broad region of lithosphere deformation and passive upwelling, (ii) lateral migration of deformation, (iii) an increase in extension rate with time, (iv) focussing of deformation and (v) buoyancy induced upwelling. The preferred calibrated models predict faster extension rates and earlier continental crustal rupture and

  10. Lithosphere-Asthenosphere Transition, Mid-Lithosphere Discontinuity and Radial Anisotropy from Multi-mode Surface Wave Tomography

    NASA Astrophysics Data System (ADS)

    Yoshizawa, K.; Kennett, B. L. N.

    2015-12-01

    Seismic surface waves are the major source of information to delineate the lateral heterogeneity and anisotropy in the upper mantle. S-wave radial anisotropy representing the difference between SV and SH velocities is often discussed with the seismic properties of the lithosphere-asthenosphere transition (LAT) and the mid-lithosphere discontinuity (MLD). Such boundaries have been studied well by body-wave receiver functions, which have a good sensitivity to the sharpness of boundaries. Surface waves are rather insensitive to the boundary sharpness, but can delineate the spatial distributions of shear wave speeds and radial anisotropy in the upper mantle, which can be alternative constraints on LAT and MLD. We have recently constructed a radially anisotropic 3-D S wave speed model of the Australian continent from multi-mode Love and Rayleigh waves with enhanced ray coverage. In the inversions for S-wave radial anisotropy, we can use either parameterizations for SH and SV velocities, or for dimensionless radially anisotropic parameter ξ=(Vsh/Vsv)2. Both are theoretically the same, but this difference causes non-negligible effects on the estimated radial anisotropy, mainly due to the different sensitivities of Love-wave phase speeds to the structural parameters. Synthetic experiments and data variance reductions suggest the former is the better choice. The LAT throughout the continent can be estimated by the vertical velocity gradient of the isotropic S-wave model. The radial anisotropy with the suitable model parameterization shows strong anisotropy with faster SH velocity in the asthenosphere, suggesting the influence of strong shear beneath the fast drifting Australian continent. We can also identify the clear vertical changes in the radial anisotropy profiles at the MLD depth estimated from earlier receiver function studies in cratonic regions, which can be a key to elucidate the enigmatic MLD in the continental lithosphere.

  11. Impact of far-field stress distributions and thermo-rheological structure of continental lithosphere on mantle-lithosphere interactions.

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

    We implement fully-coupled high resolution 3D thermo-mechanical numerical models to investigate the impact of the laterally heterogeneous structure and rheological stratification of the continental lithosphere on the plume-activated rifting and continental break-up processes in presence of preexisting far-field tectonic stresses. In our experiments, "mantle plumes" represent short-lived diapiric upwellings that have no continuous feeding at depth. Such upwellings may be associated with "true" plumes but also with various instabilities in the convective mantle. Numerical models demonstrate strong dependence of crustal strain distributions and surface topography on the rheological composition of the lower crust and the initial thermal structure of the lithosphere. In contrast to the usual inferences from passive rifting models, distributed wide rifting takes place in case of cold (500 °C at Moho depth) initial isotherm and mafic composition of the lower crust, whereas hotter geotherms and weaker (wet quartzite) lower crustal rheology lead to strong localization of rifting. Moreover, it appears that the prerequisite of strongly anisotropic strain localization during plume-lithosphere interaction (linear rift structures instead of axisymmetric radial faulting) refers to simultaneous presence of a mantle upwelling and of (even extremely weak) directional stress field produced by far-field tectonic forces (i.e. ultra-slow far field extension at < 3 mm/y). Higher (than 1.5-3 mm/y) velocities of far-field extension lead to enlargement of the active fault zone for the same lapse of time. Yet, simultaneous rise of the lithospheric geotherm associated with active rifting has an opposite effect leading to the narrowing of the rift zone. Presence of heterogeneities (cratonic blocks) leads to splitting of the plume head onto initially nearly symmetrical parts, each of which flows towards beneath the craton borders. This craton-controlled distribution of plume material causes

  12. Compositional vs. thermal buoyancy and the evolution of subducted lithosphere

    NASA Technical Reports Server (NTRS)

    Gaherty, James B.; Hager, Bradford H.

    1994-01-01

    We formulate 2-D Cartesian finite element models that explore the fate of compositionally defined lithosphere as it encounters a viscosity increase at the boundary between the upper and lower mantle. Subducted lithosphere is represented as a cold, stiff, layered composite of denser eclogite underlain by more buoyant harzburgite. Slabs impinging on a lower mantle 30 and 100 times more viscous than the upper mantle thicken and fold strongly as they penetrate the lower mantle. Approximately a factor of two thickening occurs via pure shear just above the discontinuity, with additional enhancement due to folding by over a factor of two. No separation of the individual slab components occurs at the discontinuity, and direct comparison with models in which compositional buoyancy is explicitly ignored indicates that slab evolution is largely controlled by the thermal buoyancy. These results are at odds with hypotheses about slab evolution in which the compositional buoyancy contributions lead to component separation and the formation of slab megaliths or a compositionally layered upper mantle.

  13. Colorado Plateau magmatism and uplift by warming of heterogeneous lithosphere.

    PubMed

    Roy, Mousumi; Jordan, Thomas H; Pederson, Joel

    2009-06-18

    The forces that drove rock uplift of the low-relief, high-elevation, tectonically stable Colorado Plateau are the subject of long-standing debate. While the adjacent Basin and Range province and Rio Grande rift province underwent Cenozoic shortening followed by extension, the plateau experienced approximately 2 km of rock uplift without significant internal deformation. Here we propose that warming of the thicker, more iron-depleted Colorado Plateau lithosphere over 35-40 Myr following mid-Cenozoic removal of the Farallon plate from beneath North America is the primary mechanism driving rock uplift. In our model, conductive re-equilibration not only explains the rock uplift of the plateau, but also provides a robust geodynamic interpretation of observed contrasts between the Colorado Plateau margins and the plateau interior. In particular, the model matches the encroachment of Cenozoic magmatism from the margins towards the plateau interior at rates of 3-6 km Myr(-1) and is consistent with lower seismic velocities and more negative Bouguer gravity at the margins than in the plateau interior. We suggest that warming of heterogeneous lithosphere is a powerful mechanism for driving epeirogenic rock uplift of the Colorado Plateau and may be of general importance in plate-interior settings. PMID:19536263

  14. Lithosphere-Asthenosphere interactions near the San Andreas fault.

    NASA Astrophysics Data System (ADS)

    Houlie, N.

    2015-12-01

    We decipher the strain history of the upper mantle in California through the comparison of the long-term finite strain field in the mantle and the surface strain-rate field, respectively inferred from fast polarization directions of seismic phases (SKS and SKKS), and Global Positioning System (GPS) surface velocity fields. We show that mantle strain and surface strain-rate fields are consistent in the vicinity of San Andreas Fault (SAF) in California. Such an agreement suggests that the lithosphere and strong asthenosphere have been deformed coherently and steadily since >1 Ma. We find that the crustal stress field rotates (up to 40 degrees of rotation across a 50 km distance from 50 degrees relative to the strike of the SAF, in the near-field of SAF) from San Francisco to the Central Valley. Both observations suggest that the SAF extends to depth, likely through the entire lithosphere. From Central Valley towards the Basin and Range, the orientations of GPS strain-rates, shear wave splitting measurements and seismic stress fields diverge indicating reduced coupling or/and shallow crustal extension and/or presence of frozen anisotropy.

  15. The Cascadia Subduction Zone: two contrasting models of lithospheric structure

    USGS Publications Warehouse

    Romanyuk, T.V.; Blakely, R.; Mooney, W.D.

    1998-01-01

    The Pacific margin of North America is one of the most complicated regions in the world in terms of its structure and present day geodynamic regime. The aim of this work is to develop a better understanding of lithospheric structure of the Pacific Northwest, in particular the Cascadia subduction zone of Southwest Canada and Northwest USA. The goal is to compare and contrast the lithospheric density structure along two profiles across the subduction zone and to interpet the differences in terms of active processes. The subduction of the Juan de Fuca plate beneath North America changes markedly along the length of the subduction zone, notably in the angle of subduction, distribution of earthquakes and volcanism, goelogic and seismic structure of the upper plate, and regional horizontal stress. To investigate these characteristics, we conducted detailed density modeling of the crust and mantle along two transects across the Cascadia subduction zone. One crosses Vancouver Island and the Canadian margin, the other crosses the margin of central Oregon.

  16. Lithospheric thinning associated with rifting in East Africa

    NASA Technical Reports Server (NTRS)

    Wendlandt, R. F.; Morgan, R.

    1982-01-01

    The rate of ascent of the thermal perturbation of eruptive rock types in the Kenya Dome region can be calculated for depths of origin ranging from greater than 170 km to the lower crust (25-30 km) as combined with dated occurrences. An equation is derived and solved iteratively for ascent velocity at 10 km depth intervals from 150-30 km, assuming a thermal diffusivity of 1 sq mm/sec. For radii of 50 and 100 km and constant gradient increases within the range 55-60 C/km over the lithosphere-asthenosphere boundary (LAB) ascent depth range, the calculated rate of upwelling corresponds well with the results of a simple thermal model of lithospheric thinning. A 1 C/km change in gradient results approximately in a 0.55 Myr change in ascent time from 150 to 30 km, due to the increasing differential between the geotherm and the solidus with decreasing depths and the increasing rate of heat loss to the surface. It is suggested by the temporal-spatial-compositional relations of eruptives in the Kenya Dome region that there is a shallowing of magma source regions with time, implying encroachment of a thermal anomaly.

  17. Thermal structure of the lithosphere: a petrologic model.

    PubMed

    Macgregor, I D; Basu, A R

    1974-09-20

    A preliminary evaluation of the thermal history of the upper mantle as determined by petrologic techniques indicates a general correspondence with theoretically derived models. The petrologic data supply direct information which may be used as an independent calibration of calculated models, serve as a base for evaluating the assumptions of the theoretical approach, and allow more careful selection of the variables describing mantle thermal properties and processes. Like the theoretical counterpart, the petrological approach indicates that the lithosphere is dominated by two thermal regimes: first, there is a continental regime which cools at rates of the order of 10(9) years and represents the longterm cooling of the earth. Secondly, superimposed on the continental evolution is the thermal event associated with the formation of an oceanic basin, and which may be thought of as a 10(8) year convective perturbation on the continental cycle. Of special interest is petrologic evidence for a sudden steepening of the thermal gradients across the lithosphere-asthenosphere boundary not seen in the theoretical models. The unexpected change of slope points to the need for a critical reevaluation of the thermal processes and properties extant in the asthenosphere. The potential of the petrologic contribution has yet to be fully realized. For a start, this article points to an important body of independent evidence critical to our understanding of the earth's thermal history. PMID:17738235

  18. Renewal: Continential lithosphere evolution as a function of tectonic environment

    SciTech Connect

    McMillan, N.J.; Baldridge, W.S.

    1995-06-01

    The Cenozoic tectonic environment and stress regime of the southwestern United States have changed dramatically from compression during shallow-angle subduction during the Laramide orogeny in the early Cenozoic to the current mode of Basin and Range extension. Questions remain unresolved concerning the causes of this transition, including the timing of the initiation of extension (estimates range from 36 to 25 Ma), and is the Basin and Range simply an mega-example of back-arc extension, or is extension related to the subduction of an oceanic spreading center about 30 Ma? We have examined the patterns of magmagenesis and geochemical composition through Cenozoic time in southern New Mexico. We have defined four magma sources that have contributed to Cenozoic magmas. Immediately following the Laramide, magmas contain substantial contributions from the lower crust. Mid-Tertiary extension is related to the eruption of rhyolitic ash-flow tuffs and basalts. The basalts were generated by melting of the lithospheric mantle; intercalated rhyolites have a strong upper crustal signature. Eruption of basalts and andesites with sources in the lithospheric mantle and lower crust continued for several million years after rhyolitic volcanism ceased. The region was nearly void of volcanic activity for 16 million years despite continued extension, but at 10 Ma, basalts derived from the asthenosphere began to erupt.

  19. Continents as lithological icebergs: The importance of buoyant lithospheric roots

    USGS Publications Warehouse

    Abbott, D.H.; Drury, R.; Mooney, W.D.

    1997-01-01

    An understanding of the formation of new continental crust provides an important guide to locating the oldest terrestrial rocks and minerals. We evaluated the crustal thicknesses of the thinnest stable continental crust and of an unsubductable oceanic plateau and used the resulting data to estimate the amount of mantle melting which produces permanent continental crust. The lithospheric mantle is sufficiently depleted to produce permanent buoyancy (i.e., the crust is unsubductable) at crustal thicknesses greater than 25-27 km. These unsubductable oceanic plateaus and hotspot island chains are important sources of new continental crust. The newest continental crust (e.g., the Ontong Java plateau) has a basaltic composition, not a granitic one. The observed structure and geochemistry of continents are the result of convergent margin magmatism and metamorphism which modify the nascent basaltic crust into a lowermost basaltic layer overlain by a more silicic upper crust. The definition of a continent should imply only that the lithosphere is unsubductable over ??? 0.25 Ga time periods. Therefore, the search for the oldest crustal rocks should include rocks from lower to mid-crustal levels.

  20. Renewal: Continental lithosphere evolution as a function of tectonic environment

    NASA Astrophysics Data System (ADS)

    McMillan, N. J.; Baldridge, W. S.

    1995-04-01

    The Cenozoic tectonic environment and stress regime of the southwestern United States have changed dramatically from compression during shallow-angle subduction during the Laramide orogeny in the early Cenozoic to the current mode of Basin and Range extension. Questions remain unresolved concerning the causes of this transition, including the timing of the initiation of extension (estimates range from 36 to 25 Ma), and is the Basin and Range simply an mega-example of back-arc extension, or is extension related to the subduction of an oceanic spreading center about 30 Ma? We have examined the patterns of magmagenesis and geochemical composition through Cenozoic time in southern New Mexico. We have defined four magma sources that have contributed to Cenozoic magmas. Immediately following the Laramide, magmas contain substantial contributions from the lower crust. Mid-Tertiary extension is related to the eruption of rhyolitic ash-flow tuffs and basalts. The basalts were generated by melting of the lithospheric mantle; intercalated rhyolites have a strong upper crustal signature. Eruption of basalts and andesites with sources in the lithospheric mantle and lower crust continued for several million years after rhyolitic volcanism ceased. The region was nearly void of volcanic activity for 16 million years despite continued extension, but at 10 Ma, basalts derived from the asthenosphere began to erupt.

  1. Groundwater flow as a cooling agent of the continental lithosphere

    NASA Astrophysics Data System (ADS)

    Kooi, Henk

    2016-03-01

    Groundwater that flows through the outer shell of the Earth as part of the hydrologic cycle influences the distribution of heat and, thereby, the temperature field in the Earth’s crust. Downward groundwater flow in recharge areas lowers crustal temperatures, whereas upward flow in discharge areas tends to raise temperatures relative to a purely conductive geothermal regime. Here I present numerical simulations of generalized topography-driven groundwater flow. The simulations suggest that groundwater-driven convective cooling exceeds groundwater-driven warming of the Earth’s crust, and hence that groundwater flow systems cause net temperature reductions of groundwater basins. Moreover, the simulations demonstrate that this cooling extends into the underlying crust and lithosphere. I find that horizontal components of groundwater flow play a central role in this net subsurface cooling by conveying relatively cold water to zones of upward groundwater flow. The model calculations suggest that the crust and lithosphere beneath groundwater basins can cool by several tens of degrees Celsius where groundwater flows over large distances in basins that consist of crustal rock. In contrast, groundwater-induced cooling is small in unconsolidated sedimentary settings, such as deltas.

  2. Why intracontinental basins subside longer: 3-D feedback effects of lithospheric cooling and sedimentation on the flexural strength of the lithosphere

    NASA Astrophysics Data System (ADS)

    Cacace, M.; Scheck-Wenderoth, M.

    2016-05-01

    The aim of this study is to reevaluate the character and evolution of the large-scale subsidence of intracontinental basins using 3-D thermomechanical numerical simulations accounting for the coupling between sedimentation, rheology-dependent lithospheric flexure, and thermal contraction by lithospheric cooling. The flexural rigidity of the lithospheric plate is controlled by elastic-brittle-plastic rheology, enabling the computation of thermal and mechanical feedback processes occurring during basin subsidence. Numerical results show that depending on the sediment loading history, a rheological stratified lithosphere can subside over geological time scales without imposition of ad hoc geometric and kinematic initial conditions. Three-dimensional feedback effects of sedimentation on the thermomechanical structure of the plate result in a weakened lower crust mechanically decoupled from the underlying mantle and therefore easily reactivated even under low background stresses. Our results explain the first-order characteristics of the subsidence in intracontinental basins and reconcile basic observations of their deformation history.

  3. Numerical modelling of lithospheric extension: doming vs. thermal condition

    NASA Astrophysics Data System (ADS)

    Schenker, Filippo Luca; Gerya, Taras; Kaus, Boris; Burg, Jean-Pierre

    2010-05-01

    Structural aspects of extensional doming have been modelled numerically using simplified 2D visco-plastic models (e.g. Huismans et al. 2005, Buiter et al. 2008) concentrating mainly on symmetric/asymmetric doming, fault tectonics and deformation of domes and surrounding rocks. Recent works focus their attention to the influence of geotherms on the rheology (Tirel et al. 2008), even taking into account melting (Rey et al., 2009). However, thermal aspects remain difficult to compute because of the coupled interaction between mechanical forces and temperature. This coupling is fundamental, because it provides a link between modelling and thermochronometry. Indeed, cooling ages of extensional dome flanks can constrain time, size, and patterns of metamorphic overprints simulated in thermo-mechanical models. We treat mechanical and thermal aspects together (including modelling of metamorphic P-T-time paths of crustal rocks), using a visco-elasto-plastic rheology in a four layer setup (upper crust, lower crust, lithospheric mantle and asthenospheric mantle). The asthenospheric mantle is considered in order to predict the bending effect of the lithosphere. We employed I2ELVIS, a numerical 2D computer code designed for conservative finite differences method. The model domain is 300 km wide and 160 km deep. We observed two modes of dome development and geometry, depending on first order parameters such as temperature at the Moho and thickness of the crust: (i) Lower crustal doming: with a hot Moho (TMOHO > 700 °C) and/or a thick crust, strain is localized in the upper crust and distributed in the mantle. At these conditions partial melting in the lower crust forms the core of the dome and maintains a flat Moho. (ii) Asthenospheric-triggered doming: with a cold Moho (TMOHO < 700 °C), strain is distributed in the crust and localized in the lithospheric mantle, which allows upwelling of the asthenosphere. The migmatite "core complexes" develop after the upwelling of the

  4. Satellite-derived geoid for the estimation of lithospheric cooling and basal heat flux anomalies over the northern Indian Ocean lithosphere

    NASA Astrophysics Data System (ADS)

    Rajesh, S.; Majumdar, T. J.

    2015-12-01

    The northern Indian Ocean consists of older Bay of Bengal (BOB) oceanic lithosphere with numerous intra-plate loads; whereas, contrasting elements like active Mid-Ocean ridge divergence and slow spreading ridges are present in the relatively younger (<60 Ma) Arabian Sea oceanic lithosphere. The mechanism of lithospheric cooling of young age oceanic lithosphere from the moderately active and slow spreading Carlsberg Ridge is analysed by considering the hypothesis of near lithospheric convective action or whole upper mantle convection. We addressed these issues by studying the marine geoid at different spatial wavelengths and retrieved and compared their lithospheric cooling signatures, plate spreading and distribution of mass and heat anomalies along with seismicity, bathymetry, gravity and isochron age data. Results show that progressive cooling of young-aged oceanic lithosphere from the Mid-Ocean Carlsberg Ridge is because of conductive cooling and those signals are retrieved in the shorter wavelength band (111 < λ< 1900 km) of constrained residual geoid with mass anomaly sources near to sublithospheric. This shows steadiness in the geoid anomaly decay rate (˜-0.1 m/Ma), consistency in the growth of thermal boundary layer and progressive fall of basal temperature and heat flux (900- 300 K and 100-18 mW m-2) with increase of lithospheric age. The above observations are attributed to the fact that the advective-convective action beneath the Mid-Ocean Carlsberg Ridge is driven by the basal temperature gradient between the lithosphere and the near lithospheric low viscose thin layer. But, for the case of old-aged oceanic lithosphere in the BOB, the residual geoid anomaly cooling signals are not prominently seen in the same band as that of the Arabian Sea because of the Ninetyeast Ridge magmatism. However, its cooling anomaly signatures are retrieved at relatively higher band (1335 ≤ λ≤ 3081 km) having erratic geoid decay rates (-0.3 to 0.2 m/Ma) owing to

  5. Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere.

    PubMed

    Schlindwein, Vera; Schmid, Florian

    2016-07-14

    Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges. However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere-asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation. PMID:27362231

  6. Convective Removal of Continental Margin Lithosphere at the Edges of Subducting Oceanic Plates

    NASA Astrophysics Data System (ADS)

    Levander, A.; Bezada, M. J.; Palomeras, I.; Masy, J.; Humphreys, E.; Niu, F.

    2013-12-01

    Although oceanic lithosphere is continuously recycled to the deeper mantle by subduction, the rates and manner in which different types of continental lithospheric mantle are recycled is unclear. Cratonic mantle can be chemically reworked and essentially decratonized, although the frequency of decratonization is unclear. Lithospheric mantle under or adjacent to orogenic belts can be lost to the deeper mantle by convective downwellings and delamination phenomena. Here we describe how subduction related processes at the edges of oceanic plates adjacent to passive continental margins removes the mantle lithosphere from beneath the margin and from the continental interior. This appears to be a widespread means of recycling non-cratonic continental mantle. Lithospheric removal requires the edge of a subducting oceanic plate to be at a relatively high angle to an adjacent passive continental margin. From Rayleigh wave and body wave tomography, and receiver function images from the BOLIVAR and PICASSO experiments, we infer large-scale removal of continental margin lithospheric mantle from beneath 1) the northern South American plate margin due to Atlantic subduction, and 2) the Iberian and North African margins due to Alboran plate subduction. In both cases lithospheric mantle appears to have been removed several hundred kilometers inland from the subduction zones. This type of ';plate-edge' tectonics either accompanies or pre-conditions continental margins for orogenic activity by thinning and weakening the lithosphere. These processes show the importance of relatively small convective structures, i.e. small subducting plates, in formation of orogenic belts.

  7. Uplift of the Colorado Plateau due to lithosphere attenuation during Laramide low-angle subduction

    USGS Publications Warehouse

    Spencer, J.E.

    1996-01-01

    The Colorado Plateau is blanketed by Phanerozoic marine and nonmarine strata as young as Cretaceous that are now exposed at elevations of about 2 km. Crustal thickening due to magmatism and horizontal crustal shortening was far less than necessary to cause this uplift, which is commonly attributed to the consequences of mantle lithosphere thinning and heating. The Colorado Plateau and the midcontinent region around Iowa consist of Precambrian bedrock overlain by a similar amount of Paleozoic platformal strata, and thus both regions once had similar lithospheric buoyancy. Mesozoic sedimentation increased the crustal thickness and lithospheric buoyancy of the Colorado Plateau relative to the midcontinent region. Backstripping calculations yield elevation without these sediments and lead to a calculated elevation difference between the two areas of about 1200 m, which represents unexplained plateau uplift. Review of constraints on uplift timing finds little support for a late Cenozoic uplift age and allows early to middle Cenozoic uplift, which is consistent with uplift mechanisms related to low-angle subduction that ended in the middle Cenozoic. Finite element heat flow calculations of low-angle subduction and lithosphere attenuation, using a range of initial lithosphere thicknesses and degree of attenuation, indicate that required uplift can result from tectonic removal of about 120 km of mantle lithosphere from an initially 200-km-thick lithosphere. This allows for partial preservation of North American mantle lithosphere with its distinctive isotopic signature in some late Cenozoic volcanic rocks and is consistent with normal Pn velocities in the uppermost mantle beneath the plateau.

  8. Trace element characteristics of lithospheric and asthenospheric mantle in the Rio Grande rift region

    SciTech Connect

    Perry, F.V.

    1994-06-01

    Trace element analyses of 10 mafic volcanic rocks from the Colorado Plateau transition zone, Colorado Plateau, Rio Grande rift, and Great Plains were obtained to characterize the trace element characteristics of asthenospheric and lithospheric mantle beneath these regions. Characterization of these mantle reservoirs using the trace element contents of basalts allows one to track the response of the lithosphere to continental rifting and extension.

  9. (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. PMID:20034653

  10. Evaluating methods used for fission track dating of tephras: examples from the Afar Depression, Ethiopia, and the Denali fault zone, Alaska

    NASA Astrophysics Data System (ADS)

    Blythe, A. E.; Warfel, T. S.; Phillips, D. J.

    2015-12-01

    Although fission track geochronology has been successfully used to date volcanic glasses and tephras in several studies, a variety of approaches have been used (see Westgate et al., 2013), and no consensus for a standardized methodology has emerged. As a result, this technique is rarely employed, despite having the potential to date tephras and glasses that cannot be dated by other methods, such as K-Ar dating. We have been evaluating the various approaches used to address the technical issues in fission track dating of tephras, by applying them to standards of known ages, including Moldavite tektite, and Huckleberry and Bishop Tuffs. Some of these issues include track etching and counting protocol, and corrections for the effects of track fading at low temperatures. Track etching is generally done in 24% HF for 75 or more seconds, but the time necessary for optimal etching appears to vary according to sample composition and grain size. To correct for track fading, we are using the diameter correction technique of Sandhu and Westgate (1995). We have obtained tephra samples from two regions, the Afar Depression in Ethiopia, an area with significant early hominid fossils, and the Denali fault zone in Alaska, an area with a complicated tectonic evolution. For both of these regions, we have samples that have been dated by other methods for calibration purposes, and we will explore the application of a Zeta correction to the technique. This underutilized technique can provide powerful constraints on studies of timing in diverse geologic environments.

  11. Melt-induced weakening of the lithosphere: theory and geodynamic implications

    NASA Astrophysics Data System (ADS)

    Gerya, T.

    2015-12-01

    Melt-induced weakening can play critical role for enabling lithospheric deformation in the areas of intense mantle-derived magmatism, such as mid-ocean ridges, rift zones and hot spots. It implies significant reduction in the long-term brittle strength of the deforming lithosphere subjected to frequent melt percolation episodes. Such weakening corresponds to conditions when shear stress reaches the tensile yield strength of rocks at nearly equal melt and lithostatic pressures. The dominant features of melt transport in this regime are planar, sharply localized zones (dykes) in which melt is transported though the lithosphere from the source region. Mechanical energy dissipation balance shows that the long-term effective strength of the melt-weakened lithosphere is a strain-averaged rather than a time-averaged quantity. Its magnitude is mainly defined by the ratio between melt pressure and lithostatic pressure along dykes during short dyke emplacement episodes, which control most of the lithospheric deformation and mechanical energy dissipation. We quantified the range of expected values of the lithospheric strength by performing 2D numerical hydro-mechanical experiments on melt-bearing rock deformation as well as seismo-mechanical experiments on long-term lithospheric deformation assisted by frequent short-term dyke propagation episodes. These numerical experiments showed that the long-term lithospheric strength in the areas of intense magmatism can be as low as few MPa and is critically dependent on the availability of melt for enabling frequent episodes of dyke propagation through the lithosphere. Short-lived viscous-plastic deformation is localized along propagating weak dykes whereas bulk of the lithosphere only deforms elastically and is subjected to large deviatoric stresses. The experiments suggest that it is not the high strength of the elastically deforming strong lithospheric blocks but the low strength of visco-plastically deforming dykes that define the

  12. Early thermal profiles and lithospheric strength of Ganymede from extensional tectonic features

    SciTech Connect

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

    1986-11-01

    The early thermal profiles and the lithospheric stability and strength of Ganymede are quantitatively determined on the basis of brittle lithosphere thickness estimates derived from the width and spacing of extensional tectonic features, together with lithospheric strength envelopes for ice. Plots of the brittle and ductile yield stress vs. depth for the icy lithosphere of Ganymede exhibit a linear increase in brittle strength with depth to a maximum at the brittle-ductile transition that is followed by an exponential decrease in ductile yield stress with depth. The results obtained imply that the thermal gradient and lithospheric strength have varied laterally by factor as great as 5, and that Ganymede underwent cooling in a highly inhomogeneous fashion with lateral thermal anomalies. The present analysis furnishes reasons for the stability of large cratered terrain remnants. 47 references.

  13. Layered structure of the lithospheric mantle changes dynamics of craton extension

    NASA Astrophysics Data System (ADS)

    Liao, J.; Gerya, T.; Wang, Q.

    2013-11-01

    Although presence of weak layers due to hydration and/or metasomatism in the lithospheric mantle of cratons has been detected by both geophysical and geochemical studies, its influence on craton evolution remains elusive. Using a 2‒D thermomechanical viscoelastoplastic numerical model, we studied the craton extension of a heterogeneous lithospheric mantle with a rheologically weak layer. Our results demonstrate that the effect of the weak mantle layer is twofold: (1) enhances deformation of the overlying lithosphere and (2) inhibits deformation of the underlying lithospheric mantle. Depending on the weak‒layer depth, the Moho temperature and extension rate, three extension patterns are found (1) localized mantle necking with exposed weak layer, (2) widespread mantle necking with exposed weak layer, and (3) widespread mantle necking without exposed weak layer. The presence of the weak mantle layer reduces long‒term acting boundary forces required to sustain extensional deformation of the lithosphere.

  14. Actual data of the allocation of oxygen from the Earth's lithosphere

    NASA Astrophysics Data System (ADS)

    Kapochkin, Borys; Mikhaylov, Valery; Kucherenko, Nataliya; Kapochkina, Anastasiya

    2010-05-01

    We present observational data of the allocation of oxygen from the lithosphere to the oceans and seas. Observational data include marine hydrochemical observations in the Pacific Ocean, performed in 1991-1998. Studied dissolved gases in seawater. Measurements of dissolved oxygen were carried out by different methods. The observations were made from the scientific ship every 4 hours. Measurements were made from surface to bottom. We also present data of the allocation of oxygen from the lithosphere to the deep layers of the hydrogen sulphide zone of the Black Sea. Nature of the oxygen of the lithosphere - is the destruction of minerals, that containing oxygen. The destruction of minerals occurs in conditions of discontinuous deformation in the Earth's lithosphere. The more intense geodeformations, than more oxygen is released from the minerals of the lithosphere.

  15. Convective thinning of the lithosphere - A mechanism for the initiation of continental rifting

    NASA Technical Reports Server (NTRS)

    Spohn, T.; Schubert, G.

    1982-01-01

    A model of lithospheric thinning, in which heat is convected to the base and conducted within the lithosphere, is presented. An analytical equation for determinining the amount of thinning attainable on increasing the heat flux from the asthenosphere is derived, and a formula for lithosphere thickness approximations as a function of time is given. Initial and final equilibrium thicknesses, thermal diffusivity, transition temperature profile, and plume temperature profile are all factors considered for performing rate of thinning determinations. In addition, between initial and final equilibrium states, lithospheric thinning occurs at a rate which is inversely proportional to the square root of the time. Finally, uplift resulting from thermal expansion upon lithospheric thinning is on the order of 10 to the 2nd to 10 to the 3rd m.

  16. Growth, stabilization, and reactivation of Proterozoic lithosphere in the southwestern United States

    SciTech Connect

    Bowring, S.A. ); Karlstrom, K.E. )

    1990-12-01

    Growth of Proterozoic continental lithosphere in the southwestern United States involved assembly of tectonostratigraphic terranes during several pulses of convergent tectonism ca. 1.74, 1.70, and 1.65-1.60 Ga. Prograde metamorphism accompanied orogenic assembly, and peak metamorphic conditions outlasted deformation. Regions now characterized by the highest metamorphic grades underwent slow isobaric cooling and were not uplifted until more than 200 m.y. after assembly. Regions of low metamorphic grade were not uplifted substantially after assembly. The authors suggest that (1) relatively thin lithospheric fragments were assembled into isostatically stable, normal thickness continental lithosphere; (2) assembly did not erase lithospheric-scale heterogeneities; (3) the present juxtaposition of different crustal levels reflects differential uplift related to 1.4-1.1 Ga tectonomagmatic activity; and (4) the boundaries between different lithospheric blocks were repeatedly reactivated from Precambrian through Tertiary time.

  17. Io: Generation of Silicate Magma by Shear Melting at the Base of a Basaltic Lithosphere

    NASA Technical Reports Server (NTRS)

    Carr, M. H.

    1985-01-01

    Tidal theory and observational evidence indicates that about 1 w/sq. m. of energy is released at the surface of Io. In order to place limits on how much tidal energy can be dissipated within a rigid lithosphere, depth-temperature profiles were calculated for different lithosphere thickness assuming that the tidal energy was dissipated uniformly throughout the lithosphere. Thus a thick lithosphere implies that a significant fraction of the tidal energy is dissipated below the depth where solidus temperatures are reached. One possibility is that Io has a crust consisting of a low melting temperature fraction such as basalt, overlying a mantle of a high melting temperature fraction such as peridotite. Thus, if the lithosphere of Io is thicker than 30 km, as appears probable, then high rates of silicate volcanism are implied and a significant fraction of the tidal energy must be dissipated by viscous deformation rather than rigid flexure.

  18. Is the Asian lithosphere underthrusting beneath northeastern Tibetan Plateau? Insights from seismic receiver functions

    NASA Astrophysics Data System (ADS)

    Shen, Xuzhang; Yuan, Xiaohui; Liu, Mian

    2015-10-01

    Whether or not the Asian lithosphere has underthrusted beneath the Tibetan Plateau is important for understanding the mechanisms of the plateau's growth. Using data from the permanent seismic stations in northeastern Tibetan Plateau, we studied seismic structures of the lithosphere and upper mantle across the plateau's northeastern margin using P and S receiver functions. The migrated P- and S-receiver function images reveal a thick crust and a diffuse lithosphere-asthenosphere boundary (LAB) beneath the Tibetan Plateau, contrasting sharply with the relatively thin crust and clear, sharp LAB under the bounding Asian blocks. The well-defined LAB under the Asian blocks tilts toward but does not extend significantly under the Tibetan Plateau; this is inconsistent with the model of Asian mantle lithosphere underthrusting beneath the Tibet Plateau. Instead, our results indicate limited, passive deformation of the bounding Asian lithosphere as it encounters the growing Tibetan Plateau.

  19. Revealing the Fine Structures of the Lithosphere Asthenosphere Boundary

    NASA Astrophysics Data System (ADS)

    Olugboji, Tolulope Morayo

    Earth's near surface layer is made of relatively strong materials and often referred to as tectonic plates (or the lithosphere). Below this layer is a softer layer called the asthenosphere. The transition from strong lithosphere to weak asthenosphere is caused by temperature. However, recent high-resolution seismological observations suggest that the transition from the lithosphere to the asthenosphere cannot be attributed to the temperature alone: the change in seismic wave velocity at the boundary is too sharp and too large to be attributed solely to the gradual increase in temperature. There have been hot debates on what causes the observed sharp transition between two layers. In this dissertation, I synthesize insights from seismological observation, in particular receiver functions, with experiments and theory of anelasticity caused by grain-boundary sliding, to test and assess candidate models for the oceanic lithosphere asthenosphere boundary (LAB) as well as the Mid-lithospheric discontinues (MLD) observed in stable continental regions. I conduct new statistical analysis of the results of mineral physics experiments, providing a description of the uncertainties in the parameters of an elastically accommodated grain boundary sliding model (EAGBS). I extend the EAGBS model originally proposed by Karato (2012) to describe and explain the seismological signatures at the oceanic LAB, showing that this hypothesis suitably explains both the oceanic LAB and MLD in the continents within the limits of uncertainties in both mineral physics studies and seismological models. I then describe new supporting evidence for the specific predictions of the EAGBS model using novel seismological technique and data from stations in the oceanic regions. High-resolution receiver function (RF) stacking techniques can provide robust characterization of the age-dependence in the sharpness, depth, and anisotropic fabric within the normal oceanic LAB and underneath 'anomalous' Pacific

  20. Asymmetric vs. symmetric deep lithospheric architecture of intra-plate continental orogens

    NASA Astrophysics Data System (ADS)

    Calignano, Elisa; Sokoutis, Dimitrios; Willingshofer, Ernst; Gueydan, Frédéric; Cloetingh, Sierd

    2015-08-01

    The initiation and subsequent evolution of intra-plate orogens, resulting from continental plate interior deformation due to transmission of stresses over large distances from the active plate boundaries, is controlled by lateral and vertical strength contrasts in the lithosphere. We present lithospheric-scale analogue models combining 1) lateral strength variations in the continental lithosphere, and 2) different vertical rheological stratifications. The experimental continental lithosphere has a four-layer brittle-ductile rheological stratification. Lateral heterogeneity is implemented in all models by increased crustal strength in a central narrow block. The main investigated parameters are strain rate and strength of the lithospheric mantle, both playing an important role in crust-mantle coupling. The experiments show that the presence of a strong crustal domain is effective in localizing deformation along its boundaries. After deformation is localized, the evolution of the orogenic system is governed by the mechanical properties of the lithosphere such that the final geometry of the intra-plate mountain depends on the interplay between crust-mantle coupling and folding versus fracturing of the lithospheric mantle. Underthrusting is the main deformation mode in case of high convergence velocity and/or thick brittle mantle with a final asymmetric architecture of the deep lithosphere. In contrast, lithospheric folding is dominant in case of low convergence velocity and low strength brittle mantle, leading to the development of a symmetric lithospheric root. The presented analogue modelling results provide novel insights for 1) strain localization and 2) the development of the asymmetric architecture of the Pyrenees.

  1. Dipping fossil fabrics of continental mantle lithosphere as tectonic heritage of oceanic paleosubductions

    NASA Astrophysics Data System (ADS)

    Babuska, Vladislav; Plomerova, Jaroslava; Vecsey, Ludek; Munzarova, Helena

    2016-04-01

    Subduction and orogenesis require a strong mantle layer (Burov, Tectonophys. 2010) and our findings confirm the leading role of the mantle lithosphere. We have examined seismic anisotropy of Archean, Proterozoic and Phanerozoic provinces of Europe by means of shear-wave splitting and P-wave travel-time deviations of teleseismic waves observed at dense arrays of seismic stations (e.g., Vecsey et al., Tectonophys. 2007). Lateral variations of seismic-velocity anisotropy delimit domains of the mantle lithosphere, each of them having its own consistent fabric. The domains, modeled in 3D by olivine aggregates with dipping lineation a, or foliation (a,c), represent microplates or their fragments that preserved their pre-assembly fossil fabrics. Evaluating seismic anisotropy in 3D, as well as mapping boundaries of the domains helps to decipher processes of the lithosphere formation. Systematically dipping mantle fabrics and other seismological findings seem to support a model of continental lithosphere built from systems of paleosubductions of plates of ancient oceanic lithosphere (Babuska and Plomerova, AGU Geoph. Monograph 1989), or from stacking of the plates (Helmstaedt and Schulze, Geol. Soc. Spec. Publ. 1989). Seismic anisotropy in the oceanic mantle lithosphere, explained mainly by the olivine A- or D-type fabric (Karato et al., Annu. Rev. Earth Planet. Sci. 2008), was discovered a half century ago (Hess, Nature 1964). Field observations and laboratory experiments indicate the oceanic olivine fabric might be preserved in the subducting lithosphere to a depth of at least 200-300 km. We thus interpret the dipping anisotropic fabrics in domains of the European mantle lithosphere as systems of "frozen" paleosubductions (Babuska and Plomerova, PEPI 2006) and the lithosphere base as a boundary between the fossil anisotropy in the lithospheric mantle and an underlying seismic anisotropy related to present-day flow in the asthenosphere (Plomerova and Babuska, Lithos 2010).

  2. Slow Mid-Lithosphere Beneath the Mid-continent, USA

    NASA Astrophysics Data System (ADS)

    Bedle, H.; van der Lee, S.

    2005-12-01

    The Illinois basin is one of several basins within the North American craton. Despite years of study, the formational mechanisms of the basin remain largely unknown. While the crustal structure of the basin is well-defined from seismic studies, it remains unclear whether the basin is expressed in the sub-crustal structure; and, if the mantle structure provides clues towards an understanding the basin. We study the S-velocity structure of the upper-mantle using seismic tomography beneath the Illinois basin and its surrounding area. Since this area of the North American craton has been relatively stable since 1.4 Gy, significant deviations in S-velocities within the uppermost mantle are not expected. Nevertheless, a slower S-velocity in the upper-mantle of the Illinois basin was previously modeled in the 3D model NA04 (van der Lee and Frederiksen, 2005). To improve resolution and verify the existence of this seismically slow region, we fit seismic waveforms from 11 mid-continent events, using the methodology of Partitioned Waveform Inversion (Nolet, 1990). In doing so, we also incorporate constraints that are representative of the Illinois basin's crustal and sedimentary layering. The resultant 3D model, IL05, confirms the existence of a slow S-velocity structure in the uppermost mantle beneath this region. This anomalously slow region exists from the base of the crust to depths of ~100 km, and is slower than a cratonic average by about 200 m/s. The slow uppermost-mantle beneath the Illinois basin is then underlain by a faster lithosphere typical of the North American craton to depths of ~200 km. The coherency of this deeper cratonic lithosphere rules out a lithospheric delamination origin of the anomaly. The model IL05 points to a heterogeneous mantle beneath the mid-continent. Our imaging agrees with evidence of a heterogeneous mantle recently found in the form of sub-Moho reflectors from reprocessed seismic reflection surveys (McBride and Okure, 2004). The

  3. Water and its influence on the lithosphere-asthenosphere boundary.

    PubMed

    Green, David H; Hibberson, William O; Kovács, István; Rosenthal, Anja

    2010-09-23

    The Earth has distinctive convective behaviour, described by the plate tectonics model, in which lateral motion of the oceanic lithosphere of basaltic crust and peridotitic uppermost mantle is decoupled from the underlying mechanically weaker upper mantle (asthenosphere). The reason for differentiation at the lithosphere-asthenosphere boundary is currently being debated with relevant observations from geophysics (including seismology) and geochemistry (including experimental petrology). Water is thought to have an important effect on mantle rheology, either by weakening the crystal structure of olivine and pyroxenes by dilute solid solution, or by causing low-temperature partial melting. Here we present a novel experimental approach to clarify the role of water in the uppermost mantle at pressures up to 6 GPa, equivalent to a depth of 190 km. We found that for lherzolite in which a water-rich vapour is present, the temperature at which a silicate melt first appears (the vapour-saturated solidus) increases from a minimum of 970 °C at 1.5 GPa to 1,350 °C at 6 GPa. We have measured the water content in lherzolite to be approximately 180 parts per million, retained in nominally anhydrous minerals at 2.5 and 4 GPa at temperatures above and below the vapour-saturated solidus. The hydrous mineral pargasite is the main water-storage site in the uppermost mantle, and the instability of pargasite at pressures greater than 3 GPa (equivalent to more than about 90 km depth) causes a sharp drop in both the water-storage capacity and the solidus temperature of fertile upper-mantle lherzolite. The presence of interstitial melt in mantle with more than 180 parts per million of water at pressures greater than 3 GPa alters mantle rheology and defines the lithosphere-asthenosphere boundary. Modern asthenospheric mantle acting as the source for mid-oceanic ridge basalts has a water content of 50-200 parts per million (refs 3-5). We show that this matches the

  4. Lithospheric structure in southern Sweden—results from FENNOLORA

    NASA Astrophysics Data System (ADS)

    Clowes, R. M.; Gens-Lenartowicz, E.; Demartin, M.; Saxov, S.

    1987-10-01

    The major objective of FENNOLORA, the Fennoscandian Long Range Seismic Project of 1979, was the determination of lower lithospheric and upper mantle structure down to depths in excess of 400 km below the Fennoscandian Shield. In the component represented by this study, data recorded at stations in southern Sweden from three shotpoints, one in northern Germany (profile WN) and two separated by 300 km in southern Sweden (profiles BN and CS), are used to derive a two-dimensional lithosphere structure model extending over 600 km. From north to south, the profile crosses the Svecofennides, the Småland-Värmland Granite Belt, Paleozoic cover rocks lying unconformably on the Fennoscandian Shield, the Baltic Sea (where there were no stations) and into the Caledonian tectonic province of northern Germany where one shotpoint but no stations were located. Interpretation of the three record sections was based on a two-dimensional ray tracing procedure which included the calculation of asymptotic ray theory synthetic seismograms. Lack of data from 0-150 km, for the shotpoint in Germany to the first station in southern Sweden, precluded derivation of any detailed crustal structure for this region. Crustal thickness was determined to be 32 km. A rapid increase in velocity from 8.0 to 8.35 km/s at about a depth of 50 km is underlain by a low velocity zone extending to a depth of 70 km. The reversed profiles BN and CS in southern Sweden are fundamentally different. Interpretation shows a difference in crustal thickness of about 12 km, with the northern segment having a deeper Moho, a feature resulting from a thicker lower crust and a crust-mantle transition zone rather than a rapid increase in velocity. In the southern segment of the reversed profile, a crustal low velocity zone of limited extent is indicated. The most significant aspect of the interpretation is the suggestion that the change in crustal structure between the two shotpoints in southern Sweden occurs within a

  5. A lithospheric 3D temperature study from the South Atlantic

    NASA Astrophysics Data System (ADS)

    Hirsch, K. K.; Scheck-Wenderoth, M.; Maystrenko, Y.; Sippel, J.

    2009-04-01

    The East African continental margin is a passive volcanic margin that experienced a long post-rifting history after break up in Early Cretaceous times. The break up resulted in the formation of a number of basins along the margin. The by far largest depocentre in the South Atlantic, the Orange Basin, was the location of previously performed studies. These studies of the Orange Basin have been performed to investigate the crustal structure and the temperature evolution of the basin. In this way, they gave way to new insights and to a number of questions. With 3D gravity modelling we found the crust to include high density bodies. Furthermore, a rifting model was developed which explained both the geometry and the thermal constraints of the basin. Now, this study has been extended spatially to cover a larger area and into depth to include the deep lithosphere. The main goal is to combine information on the geometry and properties of the sedimentary part of the system with data on the geometry and physical properties of the deep crust. It was also aimed to integrate both the continental and the oceanic parts of the margin into a consistent 3D structural model on a lithospheric scale. A 3D temperature model was evaluated for the passive continental margin of the South Atlantic including the lithospheric structure of the margin. We evaluate a case study for different scenarios to estimate the influence of sediments and crustal structures on the thermal field. The calculated conductive field is constrained by temperature measurements and 3D gravity modelling. At the Norwegian continental margin it has been found that a differentiation of the physical properties of the lower crust and the mantle is needed between the oceanic and continental domains to explain the observations. We aim to compare the younger setting of the Norwegian continental margin with the old passive margin in the South Atlantic. In particular, the South Atlantic is interesting since the southern half

  6. Recent progress in modelling 3D lithospheric deformation

    NASA Astrophysics Data System (ADS)

    Kaus, B. J. P.; Popov, A.; May, D. A.

    2012-04-01

    Modelling 3D lithospheric deformation remains a challenging task, predominantly because the variations in rock types, as well as nonlinearities due to for example plastic deformation result in sharp and very large jumps in effective viscosity contrast. As a result, there are only a limited number of 3D codes available, most of which are using direct solvers which are computationally and memory-wise very demanding. As a result, the resolutions for typical model runs are quite modest, despite the use of hundreds of processors (and using much larger computers is unlikely to bring much improvement in this situation). For this reason we recently developed a new 3D deformation code,called LaMEM: Lithosphere and Mantle Evolution Model. LaMEM is written on top of PETSc, and as a result it runs on massive parallel machines and we have a large number of iterative solvers available (including geometric and algebraic multigrid methods). As it remains unclear which solver combinations work best under which conditions, we have implemented most currently suggested methods (such as schur complement reduction or Fully coupled iterations). In addition, we can use either a finite element discretization (with Q1P0, stabilized Q1Q1 or Q2P-1 elements) or a staggered finite difference discretization for the same input geometry, which is based on a marker and cell technique). This gives us he flexibility to test various solver methodologies on the same model setup, in terms of accuracy, speed, memory usage etc. Here, we will report on some features of LaMEM, on recent code additions, as well as on some lessons we learned which are important for modelling 3D lithospheric deformation. Specifically we will discuss: 1) How we combine a particle-and-cell method to make it work with both a finite difference and a (lagrangian, eulerian or ALE) finite element formulation, with only minor code modifications code 2) How finite difference and finite element discretizations compare in terms of

  7. Deformation of island-arc lithosphere due to steady plate subduction

    NASA Astrophysics Data System (ADS)

    Fukahata, Yukitoshi; Matsu'ura, Mitsuhiro

    2016-02-01

    Steady plate subduction elastically brings about permanent lithospheric deformation in island arcs, though this effect has been neglected in most studies based on elastic dislocation theory. We investigate the characteristics of the permanent lithospheric deformation using a kinematic model, in which steady slip motion is given along a plate interface in the elastic lithosphere overlying the viscoelastic asthenosphere under gravity. As a rule of thumb, long-term lithospheric deformation can be understood as a bending of an elastic plate floating on non-viscous fluid, because the asthenosphere behaves like water on the long term. The steady slip below the lithosphere-asthenosphere boundary does not contribute to long-term lithospheric deformation. Hence, the key parameters that control the lithospheric deformation are only the thickness of the lithosphere and the geometry of the plate interface. Slip on a plate interface generally causes substantial vertical displacement, and gravity always tries to retrieve the original gravitational equilibrium. For a curved plate interface gravity causes convex upward bending of the island-arc lithosphere, while for a planar plate interface gravity causes convex downward bending. Larger curvature and thicker lithosphere generally results in larger deformation. When the curvature changes along the plate interface, internal deformation is also involved intrinsically, which modifies the deformation field due to gravity. Because the plate interface generally has some curvature, at least near the trench, convex upward bending of the island-arc lithosphere, which involves uplift of island-arc and subsidence around the trench, is always realized. On the other hand, the deformation field of the island-arc lithosphere sensitively depends on lithospheric thickness and plate interface geometry. These characteristics obtained by the numerical simulation are consistent with observed topography and free-air gravity anomalies in subduction

  8. Lithosphere Structure and Mantle Characterization of the Alpine-Himalayan Belt: Atlas, Zagros and Tibet

    NASA Astrophysics Data System (ADS)

    Jiménez-Munt, I.; Tunini, L.; Fernandez, M.; Verges, J.; Garcia-Castellanos, D.

    2015-12-01

    By combining geophysical and petrological information, we investigate the crust and upper mantle of three orogens of the Alpine-Himalayan Belt (Atlas, Zagros and Tibet), characterizing the lithosphere from the thermal, compositional and seismological viewpoint. The modeling is based on an integrated geophysical-petrological methodology combining elevation, gravity, geoid, surface heat flow, seismic and geochemical data.The results show prominent lithospheric mantle thickening beneath the Moroccan margin followed by thinning beneath the Atlas Mountains. Different convergence accommodation between the crust and lithospheric mantle suggests a decoupled crustal-mantle mechanical response. In the northern Zagros the lithosphere-asthenosphere boundary rises sharply below the Sanandaj Sirjan Zone in a narrow region, whereas in the central Zagros the thinning is smoother and affects a wider region. The transition from the Arabian to the Eurasian lithospheric domain is located beneath the Zagros range, and it is marked by a change in the mantle velocity anomaly and in the lithospheric mantle composition. In the western Himalaya-Tibetan orogen, the lithosphere thickening is gradual reaching the maximum below the northern edge of the Plateau. The Indian lithospheric mantle underlies the whole Tibetan Plateau up to the boundary with the Tarim Basin. In the eastern sector, the thickening generates sharp steps beneath the Himalaya Range, and it thins abruptly beneath the Qiangtang and the Songpan Ganzi terrains. The Indian underthrusting is restricted to the southern Plateau. Different Eurasian domains have been also identified beneath the Tarim Basin, the Altaids region and NE Plateau by means of different lithospheric mantle compositions. The lithospheric models crossing Zagros and Tibetan Plateau show that the present-day lithosphere mantle structure of the Arabia-Eurasia and India-Eurasia collision zones are laterally-varying along the strike of both orogens, not just in

  9. On the relations between cratonic lithosphere thickness, plate motions, and basal drag

    USGS Publications Warehouse

    Artemieva, I.M.; Mooney, W.D.

    2002-01-01

    An overview of seismic, thermal, and petrological evidence on the structure of Precambrian lithosphere suggests that its local maximum thickness is highly variable (140-350 km), with a bimodal distribution for Archean cratons (200-220 km and 300-350 km). We discuss the origin of such large differences in lithospheric thickness, and propose that the lithospheric base can have large depth variations over short distances. The topography of Bryce Canyon (western USA) is proposed as an inverted analog of the base of the lithosphere. The horizontal and vertical dimensions of Archean cratons are strongly correlated: larger cratons have thicker lithosphere. Analysis of the bimodal distribution of lithospheric thickness in Archean cratons shows that the "critical" surface area for cratons to have thick (>300 km) keels is >6-8 ?? 106 km2 . Extrapolation of the linear trend between Archean lithospheric thickness and cratonic area to zero area yields a thickness of 180 km. This implies that the reworking of Archean crust should be accompanied by thinning and reworking of the entire lithospheric column to a thickness of 180 km in accord with thickness estimates for Proterozoic lithosphere. Likewise, extrapolation of the same trend to the size equal to the total area of all Archean cratons implies that the lithospheric thickness of a hypothesized early Archean supercontinent could have been 350-450 km decreasing to 280-400 km for Gondwanaland. We evaluate the basal drag model as a possible mechanism that may thin the cratonic lithosphere. Inverse correlations are found between lithospheric thickness and (a) fractional subduction length and (b) the effective ridge length. In agreement with theoretical predictions, lithospheric thickness of Archean keels is proportional to the square root of the ratio of the craton length (along the direction of plate motion) to the plate velocity. Large cratons with thick keels and low plate velocities are less eroded by basal drag than small

  10. On the use of boussinesq approximation for lithospheric scale problem.

    NASA Astrophysics Data System (ADS)

    Le Pourhiet, Laetitia

    2016-04-01

    Boussinesq approximation is widely use to model mantle convection with success. Yet mantle convection problem are not long term tectonic problems. First of all, it is very difficult to define a reference temperature profile, second when you model a long term lithospheric problem it is necessary to use a free surface. The main difference between a free surface and using a freeslip boundary condition is that in the later case the dynamics is driven only by density variations at a given depth and true density of rock is not important. Using a free surface true density becomes very important for the dynamics of the flow because it interplays with topography. My poster will present the spurious effect of boussinesq approximation with free surface based mainly on subduction set up and my failed and less failed attempts to fix it.

  11. Rheology, tectonics, and the structure of the Venus lithosphere

    NASA Technical Reports Server (NTRS)

    Zuber, M. T.

    1994-01-01

    Given the absence of ground truth information on seismic structure, heat flow, and rock strength, or short wavelength gravity or magnetic data for Venus, information on the thermal, mechanical and compositional nature of the shallow interior must be obtained by indirect methods. Using pre-Magellan data, theoretical models constrained by the depths of impact craters and the length scales of tectonic features yielded estimates on the thickness of Venus' brittle-elastic lithosphere and the allowable range of crustal thickness and surface thermal gradient. The purpose of this study is to revisit the question of the shallow structure of Venus based on Magellan observations of the surface and recent experiments that address Venus' crustal rheology.

  12. Upper mantle flow and lithospheric dynamics beneath the Eurasian region

    NASA Astrophysics Data System (ADS)

    Zhang, G.; Jiang, G.; Jia, Z.; Gao, R.; Fu, R.

    2010-12-01

    Evidence from seismic tomography, geothermal and short wavelength geoid anomalies reveals the existence of small-scale convective systems in the upper mantle, with scales ranging from 500 km to 700 km. It is reasonable to suggest that these small-scale convective systems probably control the regional tectonic structure and the dynamical processes of the lithosphere. Here we have calculated the patterns of small-scale convection in the upper mantle for the Eurasian region (20°E~170°E,15°N~75°N), using the anomaly of isostatic gravity. The results show that the regional lithospheric tectonics is strongly correlated with the upper mantle flow in the Eurasian region. Two intensive convective belts against the weak background convection can be recognized from convection patterns in this region: Alpine-Himalayan collision belt and West Pacific island arc-underthrust belt. Alpine-Himalayan belt is caused by the collision between the northern plate (Eurasian plate) and the southern plates (African plate and Indian plate). West Pacific island arc-underthrust belt is caused by the subduction of the Pacific plate beneath the Eurasian plate. Both of them are also seismotectonic belts. The collision and the subduction are two important geological events occurred since Mesozoic era and Cenozoic era in the Eurasian region. Therefore, the mantle flows may be one of the main driving forces of two events. In addition, most plate boundaries in this region can be recognized and the characteristics of upper mantle convection are different completely between the Eurasian plate and the plates around it (African plate, Arabian plate, Indian plate, Philippine Sea plate and Pacific plate). Main structures and geodynamic characteristics of the Eurasian can also be explained by our model results. The Tibet plateau is located in the intensive convective belt. Around the belt, the upwelling materials push the lithosphere to lift unitarily and form the plateau. Towards the north of the Tibet

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  14. Fault models and constitutive laws across the lithosphere

    NASA Astrophysics Data System (ADS)

    Shimamoto, T.

    2011-12-01

    Establishment of fault model is important not only for modeling earthquake cycles (stress accumulation, earthquake generation and afterslip), but also for analyzing tectonics of lithosphere. Since Sibson (1977, J. Geol. Soc. London) proposed a famous fault model, several fault models have been proposed (Scholz, 1988, Geol. Rundschau; Shimamoto, 1989, J. Struct. Geol.; Kawamoto and Shimamoto, 1998, Tectonophy). There has not been much progress in fault models since then, and even those fault models had limited applications in the modeling earthquakes and tectonics because no constitutive laws describing brittle to high-temperature ductile deformation across the lithosphere have been proposed. Moreover there was no additional experimental data reported to cover the brittle-ductile transition under large shearing deformation. However, the situation has been changing since Shimamoto (2004, JpGU) and Shimamoto and Noda, 2010, AGU) proposed an empirical friction to flow law which describes the transition from friction to fully plastic flow under shear for halite. Only frictional constitutive parameters and parameters in flow law are used and properties in the transitional regime can be predicted once those parameters are known. Thus this law provides a working model for reanalyzing diverse fault properties such as clay-bearing faults, for planning experimental researches to produce friction to flow behavior for realistic rocks, and for modeling fault and plate-boundary behaviors including generation of large earthquakes. I will summarize the current status on fault models and friction to flow constitutive laws across the lithosphere focusing the following aspects. (1) Friction to flow transition for important rocks. Existing friction and flows laws will be combined to propose constitutive property across the lithosphere (e.g., rate and state friction law combined with flow law for diabase). I will show how such a law can be used in the modeling using 2D modeling of

  15. Electromagnetic induction studies. [of earth lithosphere and asthenosphere

    NASA Technical Reports Server (NTRS)

    Hermance, J. F.

    1983-01-01

    Recent developments in electromagnetic induction studies of the lithosphere and the asthenosphere are reviewed. Attention is given to geoelectrical studies of active tectonic areas in terms of the major zones of crustal extension, the basin and range province along western regions of North America, and the Rio Grande rift. Studies have also been performed of tectonic activity around Iceland, the Salton Trough and Cerro Prieto, and the subduction zones of the Cascade Mountains volcanic belt, where magnetotelluric and geomagnetic variation studies have been done. Geomagnetic variations experiments have been reported in the Central Appalachians, and submarine electromagnetic studies along the Juan de Fuca ridge. Controlled source electromagnetic and dc resistivity investigations have been carried out in Nevada, Hawaii, and in the Adirondacks Mountains. Laboratory examinations on the conductivity of representative materials over a broad range of temperature, pressure, and chemistry are described.

  16. Experimental Constraints on the Rheology of the Lithospheric Mantle

    NASA Astrophysics Data System (ADS)

    Mei, S.; Suzuki, A. M.; Kohlstedt, D. L.; Dixon, N. A.; Durham, W. B.

    2009-12-01

    To provide a better understanding of rheological properties of mantle rocks under lithospheric conditions, we carried out a series of experiments on the steady-state creep behavior of polycrystalline olivine, the most abundant mineral of the upper mantle, at high pressures (~4 to 9 GPa), relatively low temperatures (673 ≤ T ≤ 1273 K), and anhydrous conditions using a deformation-DIA. Differential stress and sample displacement were monitored in-situ using synchrotron x-ray diffraction and radiography, respectively. The low-temperature plasticity of olivine is well constrained by our data with a Peierls stress of 6.2 ± 0.3 GPa and an activation energy of 290 ± 60 kJ/mol. The flow stress in the low-temperature plasticity regime characterized in this study is less temperature sensitive than reported in earlier studies using micro-indentation and load relaxation techniques, in which samples were deformed in a transient rather than steady-state fashion. A transition from low-T plasticity to high-T creep occurs at ~1300 K for a laboratory strain rate of ~10-5 s-1. Low-T plasticity dominates deformation of olivine-rich rocks at depths in the lithospheric mantle where pressure is high enough to suppress frictional sliding while temperature is low enough not to activate dislocation climb. Extrapolation of our low-temperature flow law to a strain rate of 10-14s-1 and a temperature of 873 K, the cut-off temperature for earthquakes in the mantle, yields a strength of ~500 MPa. This value is similar to that obtained from the flow law of Evans and Goetze [1979] and a factor of five large than that calculated from the flow law of Raterron et al. [2004].

  17. Lithospheric architecture of the Hudson Bay region (Invited)

    NASA Astrophysics Data System (ADS)

    Eaton, D. W.; Darbyshire, F. A.

    2009-12-01

    Hudson Bay conceals several fundamental tectonic elements of North America, including most of the ca. 1.8 Ga Trans Hudson orogen (THO) and the Paleozoic Hudson Bay basin. Formed due to a collision between the Superior and Churchill Provinces of the Canadian Shield, the THO is similar in scale and tectonic style to the modern Himalayan-Karakorum orogen. During collision, the lobate shape of the indentor (Superior Province) formed an orogenic template that, along with the Sask craton of central North America, exerted a persistent influence on the tectonic evolution of the region resulting in anomalous preservation of juvenile crustal material. Juvenile crust in the southeastern part of Hudson Bay is interpreted to have formed within an island-arc setting proximal to the Superior Province, in contrast to the Reindeer Zone of Saskatchewan and Manitoba, which accreted first to the Churchill Province. Thick, cold and refractory lithosphere that underlies the Bay is well imaged by surface-wave and S-Receiver function studies and comprises a large component of the cratonic mantle keel that forms the nucleus of the North American continent. The existence of an unusually thick mantle root beneath Hudson Bay indicates that subduction and plate collision during the Trans-Hudson orogeny were ‘root-preserving’ (if not ‘root-forming’) processes. Although the Hudson Bay basin is the largest by surface area of four major intracratonic basins in North America, it is also the shallowest. Crustal thinning imaged from ambient-noise tomography is consistent with previous models of basin subsidence caused by extension. Compared to other basins of similar age in North America, however, relatively stiff Early Paleozoic lithosphere may have inhibited subsidence of the Hudson Bay basin.

  18. Experimental Constraints on the Strength of the Lithospheric Mantle

    SciTech Connect

    Mei, S.; Suzuki, A; Kohlstedt, D; Dixon, N; Durham, W

    2010-01-01

    To provide a better understanding of rheological properties of mantle rocks under lithospheric conditions, we carried out a series of experiments on the creep behavior of polycrystalline olivine at high pressures ({approx}4-9 GPa), relatively low temperatures (673 {le} T {le} 1273 K), and anhydrous conditions, using a deformation-DIA. Differential stress and sample displacement were monitored in situ using synchrotron X-ray diffraction and radiography, respectively. Experimental results were fit to the low-temperature plasticity flow law. On the basis of this analysis, the low-temperature plasticity of olivine deformed under anhydrous conditions is well constrained by our data with a Peierls stress of {sigma}{sub P} = 5.9 {+-} 0.2 GPa, a zero-stress activation energy of E{sub k}(0) = 320 {+-} 50 kJ mol{sup -1}, and A{sub P} = 1.4 x 10{sup -7} s{sup -1} MPa{sup -2}. Compared with published results for high-temperature creep of olivine, a transition from low-temperature plasticity to high-temperature creep occurs at {approx}1300 K for a strain rate of {approx}10{sup -5} s{sup -1}. For a geological strain rate of 10{sup -14} s{sup -1}, extrapolation of our low-temperature flow law to 873 K, the cutoff temperature for earthquakes in the mantle, yields a strength of {approx}600 MPa. The low-temperature, high-stress flow law for olivine in this study provides a solid basis for modeling tectonic processes occurring within Earth's lithosphere.

  19. Experimental study of lithosphere-troposphere-ionosphere coupling

    NASA Astrophysics Data System (ADS)

    Korepanov, Valery; Fedorov, Oleh; Lizunov, Georgy

    The ionosphere is the closest to the Earth area of near-Earth space and because of this the powerful natural and man-made processes, such as hurricanes, earthquakes, tsunami, big explosions and starts of heavy rockets to name a few, have to create corresponding disturbances there. Numerous experimental observations evidence the existence of such "terragenic" variations of ionospheric parameters, created by the sources below the ionosphere, in the troposphere and even in the lithosphere. Probably the most important question here is whether it is possible to detect enough reliably the ionospheric disturbances, related to the earthquake preparation state, at the background of the much stronger influences "from above" - of solar and galactic sources. So, the seismo-ionospheric coupling is one of the hottest topics of modern scientific research. The importance to shed more light to this process is connected with the study of possible earthquakes precursors in the ionosphere, monitoring of which could be a further step to the scientifically substantiated solution of the problem of earthquakes warning. The processing results of the data collected at Ukrainian Antarctic Station "Academician Vernadsky" are discussed. Basing on these data the theoretical study was performed and the energy transmission mechanism in the lithosphere-atmosphere-ionosphere system is analyzed. The atmospheric gravity waves are proposed as the most probable energy carriers for troposphere-ionosphere coupling. Basing on this study, as well as on the experience of preparation of several dedicated satellite mission (e. g., WARNING, INTERBALL-PROGNOZ) a new idea of spatial experiment IONOSAT is proposed to study in details the mechanism of seismo-ionospheric coupling and its realization peculiarities are discussed. This study was supported by NSAU contract No 1-02/03.

  20. Lithospheric flexure and gravity spreading of Olympus Mons volcano, Mars

    NASA Astrophysics Data System (ADS)

    Musiol, S.; Holohan, E. P.; Cailleau, B.; Platz, T.; Dumke, A.; Walter, T. R.; Williams, D. A.; Gasselt, S.

    2016-03-01

    The structural architecture of large volcanoes is governed substantially by gravity-driven deformation that is manifest as distinct processes such as basement flexure or volcanic spreading. Temporal effects and the mutual interplay of these processes have been investigated only to a limited extent, and so we present novel numerical models of the time-dependent deformation associated with them. The models simulate the combined effects of lithospheric flexure and volcanic spreading during growth increments of an elastoplastic volcanic cone. Different spreading scenarios are considered by a variable coupling decoupling behavior at the interface between volcano and basement. We apply our models to Olympus Mons on Mars, which is characterized by upper to middle flank terraces on the shield, is encircled by a basal scarp that has an average slope of 30° and is surrounded by distant deposits that resemble large-scale slumping features on Earth. Our results are consistent with the interpretation that terraces on Olympus Mons' flanks form by thrust faulting that results from lithospheric flexure. The presence and expression of terraces depend on the coupling of volcano and basement, on the time of volcano growth relative to mantle relaxation, and on the cohesion of the edifice. The encircling scarp may be related to a very low friction detachment at the edifice base, which leads to a normal fault regime on the lowermost flanks. With time and volcano growth, predicted stress and faulting regimes migrate only slightly, indicating that the structural architecture of volcanoes is largely set in the very early stages of formation.

  1. Lithospheric dynamics in eastern Mediterranean: Insights from seismic structure and anisotropy

    NASA Astrophysics Data System (ADS)

    Lebedev, S.; Neenan, E.; Knapmeyer-Endrun, B.; Meier, T.; Agius, M. R.; Schaeffer, A. J.; Tirel, C.; Friederich, W.

    2012-04-01

    The slow convergence of the African, Arabian and Eurasian Plates in eastern Mediterranean has been accompanied by widespread deformation of continental lithosphere within the region. The continuing southward retreat of the Hellenic Trench, where oceanic lithosphere is subducting northward, is accommodated by lateral movements and stretching of continental blocks of the overriding plate. Both the boundary conditions and lateral variations in the mechanical strength of the lithosphere determine the distribution and character of the localised and diffuse deformation that is taking place. Seismic velocities within the lithosphere depend on its temperature and can be used to infer its strength. Azimuthal seismic anisotropy indicates fabrics within the crust, mantle lithosphere and asthenosphere that are a record of their deformation and flow. Seismic observations can thus provide key information on the flow patterns and underlying mechanisms of the complex lithospheric deformation. Here we present the results of the analysis of surface waves recorded at seismic stations in eastern Mediterranean. Frequency-dependent surface-wave anisotropy reveals depth-dependent orientations of anisotropic fabrics in the crust and mantle. Isotropic-average shear-velocity profiles and surface-wave tomography indicate variability in temperature and, therefore, in mechanical strength of the lithosphere. Published shear-wave splitting measurements show remarkably uniform, NNE-SSW fast-propagation orientations across much of eastern Mediterranean, suggesting large-scale flow within the asthenosphere towards the retreating Hellenic Trench. The motion and deformation of lithospheric blocks are also driven by the trench retreat but are very different from those of the asthenosphere beneath them. The character of deformation within the lithosphere varies laterally and, also, vertically, from the brittle upper crust to the ductile lower crust and lithospheric mantle. The shear associated with the

  2. Utilizing thermal isostasy to estimate sub-lithospheric heat flow and anomalous crustal radioactivity

    NASA Astrophysics Data System (ADS)

    Hasterok, D.; Gard, M.

    2016-09-01

    While surface heat flow relates to the heat loss through the lithosphere, it can be difficult to quantify and separate the heat produced internally through radiogenic decay from the heat transferred across the base of the lithosphere by mantle convection. In this study, we apply a thermo-isostatic analysis to Australia and estimate the sub-lithospheric and radiogenic heat flow components by employing a simple 1-D conservation of energy model. We estimate an anomalous radiogenic heat production across much of eastern Australia generally accounting for >50 mW m-2, while western Australia appears to have high crustal compositionally corrected elevation, possibly related to chemical buoyancy of the mantle lithosphere. A moderately high sub-lithospheric heat flow (∼40 mW m-2) along the eastern and southeastern coast, including Tasmania, is coincident with locations of Cenozoic volcanism and supports an edge-driven convection hypothesis. However, the pattern of sub-lithospheric heat flow along the margin does not support the existence of hotspot tracks. Thermo-isostatic models such as these improve our ability to identify and quantify crustal from mantle sources of heat loss and add valuable constraints on tectonic and geodynamic models of the continental lithosphere's physical state and evolution.

  3. Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere

    NASA Astrophysics Data System (ADS)

    Schlindwein, Vera; Schmid, Florian

    2016-07-01

    Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges. However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere–asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation.

  4. Strain localization at the margins of strong lithospheric domains: Insights from analog models

    NASA Astrophysics Data System (ADS)

    Calignano, Elisa; Sokoutis, Dimitrios; Willingshofer, Ernst; Gueydan, Frédéric; Cloetingh, Sierd

    2015-03-01

    The lateral variation of the mechanical properties of continental lithosphere is an important factor controlling the localization of deformation and thus the deformation history and geometry of intraplate mountain belts. A series of three-layer lithospheric-scale analog models, with a strong domain (SD) embedded at various depths, are presented to investigate the development of topography and deformation patterns by having lateral heterogeneities within a weak continental lithosphere. The experiments, performed at a constant velocity and under normal gravity, indicate that the presence or absence of the SD controls whether deformation is localized or distributed at a lithospheric scale. Deformation and topography localize above the edges of the SD, while the SD region itself is characterized by minor amounts of surficial deformation and topography. The depth of the SD (within the ductile crust, ductile mantle lithosphere, or both) controls the pattern of deformation and thus the topography. The presence of a SD in the ductile crust or in the mantle results in limited surficial topographic effects but large variations in the Moho topography. Strong Moho deflection occurs when the SD is in the ductile crust, while the Moho remains almost flat when the SD is in the mantle. When the SD occupies the ductile lithosphere, the SD is tilted. These analog experiments provide insights into intraplate strain localization and could in particular explain the topography around the Tarim Basin, a lithospheric-scale heterogeneity north of the India-Asia collision zone.

  5. Temperature, lithosphere-asthenosphere boundary, and heat flux beneath the Antarctic Plate inferred from seismic velocities

    NASA Astrophysics Data System (ADS)

    An, Meijian; Wiens, Douglas A.; Zhao, Yue; Feng, Mei; Nyblade, Andrew; Kanao, Masaki; Li, Yuansheng; Maggi, Alessia; Lévêque, Jean-Jacques

    2015-12-01

    We estimate the upper mantle temperature of the Antarctic Plate based on the thermoelastic properties of mantle minerals and S velocities using a new 3-D shear velocity model, AN1-S. Crustal temperatures and surface heat fluxes are then calculated from the upper mantle temperature assuming steady state thermal conduction. The temperature at the top of the asthenosphere beneath the oceanic region and West Antarctica is higher than the dry mantle solidus, indicating the presence of melt. From the temperature values, we generate depth maps of the lithosphere-asthenosphere boundary and the Curie temperature isotherm. The maps show that East Antarctica has a thick lithosphere similar to that of other stable cratons, with the thickest lithosphere (~250 km) between Domes A and C. The thin crust and lithosphere beneath West Antarctica are similar to those of modern subduction-related rift systems in East Asia. A cold region beneath the Antarctic Peninsula is similar in spatial extent to that of a flat-subducted slab beneath the southern Andes, indicating a possible remnant of the Phoenix Plate, which was subducted prior to 10 Ma. The oceanic lithosphere generally thickens with increasing age, and the age-thickness correlation depends on the spreading rate of the ridge that formed the lithosphere. Significant flattening of the age-thickness curves is not observed for the mature oceanic lithosphere of the Antarctic Plate.

  6. Lithospheric discontinuity structure in Alaska, thickness variations determined by Sp receiver functions

    NASA Astrophysics Data System (ADS)

    O'Driscoll, Leland J.; Miller, Meghan S.

    2015-04-01

    We present the first broad-scale image of lithospheric thickness across the major tectonic domains of Alaska based on S wave receiver functions and joint interpretation with the potential field, seismic velocity, and heat flow measurements. Thus, we provide context for the distribution of strain throughout the Alaskan orocline. In the north, below the Brooks Range, a 130 km thick lithosphere is resolved, consistent with the presence of strong lithosphere that deflects strain to the south into central and southern Alaska. In southern Alaska beneath the Chugach and St. Elias Mountains, multiple interfaces are present, and we interpret a thinner (80-90 km) North American lithosphere above a deeper interface that represents the base of the Yakutat microplate, thereby extending it to the area below the Wrangell Volcanic Field and St. Elias Mountains. Immediately north of the E-W striking Denali Fault, shallow negative conversions (80 km) denote thin lithosphere in the greater back-arc region where heat flow is observed to be high. Thin lithosphere in eastern Alaska and adjacent Yukon Territory coincides with the occurrence of inboard crustal seismicity and may be indicative of transmitted compression caused by the collision of the Yakutat microplate. Relatively thin lithosphere (<90 km) south of the Arctic Alaska domain that is deforming throughout the Alaskan orocline may result from lithospheric thinning associated with guided deformation. Expansion of this model using the upcoming Transportable Array will be critical to establish lateral continuity (or lack thereof) of lithospheric structure and directly discriminate between existing regional deformation models.

  7. Imaging Canary Island hotspot material beneath the lithosphere of Morocco and southern Spain

    NASA Astrophysics Data System (ADS)

    Miller, Meghan S.; O'Driscoll, Leland J.; Butcher, Amber J.; Thomas, Christine

    2015-12-01

    The westernmost Mediterranean has developed into its present day tectonic configuration as a result of complex interactions between late stage subduction of the Neo-Tethys Ocean, continental collision of Africa and Eurasia, and the Canary Island mantle plume. This study utilizes S receiver functions (SRFs) from over 360 broadband seismic stations to seismically image the lithosphere and uppermost mantle from southern Spain through Morocco and the Canary Islands. The lithospheric thickness ranges from ∼65 km beneath the Atlas Mountains and the active volcanic islands to over ∼210 km beneath the cratonic lithosphere in southern Morocco. The common conversion point (CCP) volume of the SRFs indicates that thinned lithosphere extends from beneath the Canary Islands offshore southwestern Morocco, to beneath the continental lithosphere of the Atlas Mountains, and then thickens abruptly at the West African craton. Beneath thin lithosphere between the Canary hot spot and southern Spain, including below the Atlas Mountains and the Alboran Sea, there are distinct pockets of low velocity material, as inferred from high amplitude positive, sub-lithospheric conversions in the SRFs. These regions of low seismic velocity at the base of the lithosphere extend beneath the areas of Pliocene-Quaternary magmatism, which has been linked to a Canary hotspot source via geochemical signatures. However, we find that this volume of low velocity material is discontinuous along strike and occurs only in areas of recent volcanism and where asthenospheric mantle flow is identified with shear wave splitting analyses. We propose that the low velocity structure beneath the lithosphere is material flowing sub-horizontally northeastwards beneath Morocco from the tilted Canary Island plume, and the small, localized volcanoes are the result of small-scale upwellings from this material.

  8. Global map of lithosphere thermal thickness on a 1 deg x 1 deg grid - digitally available

    NASA Astrophysics Data System (ADS)

    Artemieva, Irina

    2014-05-01

    This presentation reports a 1 deg ×1 deg global thermal model for the continental lithosphere (TC1). The model is digitally available from the author's web-site: www.lithosphere.info. Geotherms for continental terranes of different ages (early Archean to present) are constrained by reliable data on borehole heat flow measurements (Artemieva and Mooney, 2001), checked with the original publications for data quality, and corrected for paleo-temperature effects where needed. These data are supplemented by cratonic geotherms based on xenolith data. Since heat flow measurements cover not more than half of the continents, the remaining areas (ca. 60% of the continents) are filled by the statistical numbers derived from the thermal model constrained by borehole data. Continental geotherms are statistically analyzed as a function of age and are used to estimate lithospheric temperatures in continental regions with no or low quality heat flow data. This analysis requires knowledge of lithosphere age globally. A compilation of tectono-thermal ages of lithospheric terranes on a 1 deg × 1 deg grid forms the basis for the statistical analysis. It shows that, statistically, lithospheric thermal thickness z (in km) depends on tectono-thermal age t (in Ma) as: z=0.04t+93.6. This relationship formed the basis for a global thermal model of the continental lithosphere (TC1). Statistical analysis of continental geotherms also reveals that this relationship holds for the Archean cratons in general, but not in detail. Particularly, thick (more than 250 km) lithosphere is restricted solely to young Archean terranes (3.0-2.6 Ga), while in old Archean cratons (3.6-3.0 Ga) lithospheric roots do not extend deeper than 200-220 km. The TC1 model is presented by a set of maps, which show significant thermal heterogeneity within continental upper mantle. The strongest lateral temperature variations (as large as 800 deg C) are typical of the shallow mantle (depth less than 100 km). A map of the

  9. Continuous deformation versus faulting through the continental lithosphere of new zealand

    PubMed

    Molnar; Anderson; Audoine; Eberhart-Phillips; Gledhill; Klosko; McEvilly; Okaya; Savage; Stern; Wu

    1999-10-15

    Seismic anisotropy and P-wave delays in New Zealand imply widespread deformation in the underlying mantle, not slip on a narrow fault zone, which is characteristic of plate boundaries in oceanic regions. Large magnitudes of shear-wave splitting and orientations of fast polarization parallel to the Alpine fault show that pervasive simple shear of the mantle lithosphere has accommodated the cumulative strike-slip plate motion. Variations in P-wave residuals across the Southern Alps rule out underthrusting of one slab of mantle lithosphere beneath another but permit continuous deformation of lithosphere shortened by about 100 kilometers since 6 to 7 million years ago. PMID:10521344

  10. Mantle exhumation and OCT architecture dependency on lithosphere deformation modes during continental breakup: Numerical experiments

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

    The initiation of sea-floor spreading, during the continental breakup process, requires both the rupture of the continental crust and the initiation of decompression melting. This process results in mantle upwelling and at some point decompressional melting which creates new oceanic crust. Using numerical experiments, we investigate how the deformation mode of continental lithosphere thinning and stretching controls the rupture of continental crust and lithospheric mantle, the onset of decompression melting, their relative timing, and the circumstances under which mantle exhumation may occur. We assume that the topmost continental and ocean lithosphere, corresponding to the cooler brittle seismogenic layer, deforms by extensional faulting (pure-shear deformation) and magmatic intrusion, consistent with the observations of deformation processes occurring at slow spreading ocean ridges (Cannat, 1996). We assume that deformation beneath this topmost lithosphere layer (approximately 15-20 km thick) occurs in response to passive upwelling and thermal and melt buoyancy driven small-scale convection. We use a 2D finite element viscous flow model (FeMargin) to describe lithosphere and asthenosphere deformation. This flow field is used to advect lithosphere and asthenosphere temperature and material. The finite element model is kinematically driven by Vx for the topmost upper crust inducing passive upwelling beneath that layer. A vertical velocity Vz is defined for buoyancy enhanced upwelling as predicted by Braun et al. (2000). Melt generation is predicted by decompression melting using the parameterization and methodology of Katz et al. (2003). Numerical experiments have been used to investigate the dependency of continental crust and lithosphere rupture, decompression melt initiation, rifted margin ocean-continent transition architecture and subsidence history on the half-spreading rate Vx, buoyancy driven upwelling rate Vz, the relative contribution of these deformation

  11. Convective thinning of the lithosphere: a model constrained by geoid observations

    NASA Astrophysics Data System (ADS)

    Dalloubeix, C.; Fleitout, L.

    1989-11-01

    Geoid observations in the oceans suggest that lithospheric thinning is effected by small-scale convection within hot-spot material trapped in dimples a few tens of kilometers deep at the base of the lithosphere. Significant thinning can occur within 5 Ma if the viscosity of the convecting material is 1016 Pa s. Partial melting can enhance considerably the vigour of convection and the same rate of lithospheric thinning is obtained for a viscosity about five times higher. These results are derived from convection models with a realistic temperature-dependent rheology using the mean field approximation.

  12. Magma explains low estimates of lithospheric strength based on flexure of ocean island loads

    NASA Astrophysics Data System (ADS)

    Buck, W. Roger; Lavier, Luc L.; Choi, Eunseo

    2015-04-01

    One of the best ways to constrain the strength of the Earth's lithosphere is to measure the deformation caused by large, well-defined loads. The largest, simple vertical load is that of the Hawaiian volcanic island chain. An impressively detailed recent analysis of the 3D response to that load by Zhong and Watts (2013) considers the depth range of seismicity below Hawaii and the seismically determined geometry of lithospheric deflection. These authors find that the friction coefficient for the lithosphere must be in the normal range measured for rocks, but conclude that the ductile flow strength has to be far weaker than laboratory measurements suggest. Specifically, Zhong and Watts (2013) find that stress differences in the mantle lithosphere below the island chain are less than about 200 MPa. Standard rheologic models suggest that for the ~50 km thick lithosphere inferred to exist below Hawaii yielding will occur at stress differences of about 1 GPa. Here we suggest that magmatic accommodation of flexural extension may explain Hawaiian lithospheric deflection even with standard mantle flow laws. Flexural stresses are extensional in the deeper part of the lithosphere below a linear island load (i.e. horizontal stresses orthogonal to the line load are lower than vertical stresses). Magma can accommodate lithospheric extension at smaller stress differences than brittle and ductile rock yielding. Dikes opening parallel to an island chain would allow easier downflexing than a continuous plate, but wound not produce a freely broken plate. The extensional stress needed to open dikes at depth depends on the density contrast between magma and lithosphere, assuming magma has an open pathway to the surface. For a uniform lithospheric density ρL and magma density ρM the stress difference to allow dikes to accommodate extension is: Δσxx (z) = g z (ρM - gρL), where g is the acceleration of gravity and z is depth below the surface. For reasonable density values (i.e.

  13. Osmium isotopic evidence for ancient subcontinental lithospheric mantle beneath the kerguelen islands, southern indian ocean

    PubMed

    Hassler; Shimizu

    1998-04-17

    Upper mantle xenoliths found in ocean island basalts are an important window through which the oceanic mantle lithosphere may be viewed directly. Osmium isotopic data on peridotite xenoliths from the Kerguelen Islands, an archipelago that is located on the northern Kerguelen Plateau in the southern Indian Ocean, demonstrate that pieces of mantle of diverse provenance are present beneath the Islands. In particular, peridotites with unradiogenic osmium and ancient rhenium-depletion ages (to 1.36 x 10(9) years old) may be pieces of the Gondwanaland subcontinental lithosphere that were incorporated into the Indian Ocean lithosphere as a result of the rifting process. PMID:9545216

  14. Report of the panel on lithospheric structure and evolution, section 3

    NASA Technical Reports Server (NTRS)

    Chase, Clement G.; Lang, Harold; Mcnutt, Marcia K.; Paylor, Earnest D.; Sandwell, David T.; Stern, Robert J.

    1991-01-01

    The panel concluded that NASA can contribute to developing a refined understanding of the compositional, structural, and thermal differences between continental and oceanic lithosphere through a vigorous program in solid Earth science with the following objectives: determine the most fundamental geophysical property of the planet; determine the global gravity field to an accuracy of a few milliGals at wavelengths of 100 km or less; determine the global lithospheric magnetic field to a few nanoTeslas at a wavelength of 100 km; determine how the lithosphere has evolved to its present state via acquiring geologic remote sensing data over all the continents.

  15. Evidence for a Late Pliocene faunal transition based on a new rodent assemblage from Oldowan locality Hadar A.L. 894, Afar Region, Ethiopia.

    PubMed

    Reed, Denné N; Geraads, Denis

    2012-03-01

    The time interval between 3 Ma and 2 Ma marks several important transitions in human evolution, including the extinction of Australopithecus afarensis, the origin of the genus Homo, and the appearance of concentrated stone tool assemblages forming recognizable archaeological sites. The period also marks important changes in Earth's climatic history, with the onset of northern hemisphere glaciation starting sometime between 2.8 Ma and 2.5 Ma, and it remains an unresolved question in paleoanthropology whether or not the global climatic events influenced in whole or in part, local terrestrial paleoenvironments in Africa and, through this, the course of human evolution. Changes in the terrestrial mammalian faunas of East Africa during this time interval are an important source of data about terrestrial paleoenvironments, and it has been argued that during this time period the mammalian faunas of Africa experienced a sudden pulse in the extinction and origination of taxa. The data corroborating this Turnover Pulse Hypothesis derive from both large mammal and micromammal data, though the fossil record of the former is much more abundant in this interval. New micromammal fossils recovered from ca. 2.4 Ma deposits at locality A.L. 894, low in the Busidima Formation in the Hadar study area of the Afar region, Ethiopia, reveal a significant faunal turnover when compared with previously published material from older 3.2 Ma micromammal assemblages from the Hadar Formation deposits. The results support the hypothesis of a major faunal transition, but larger sample sizes and more extensive temporal sampling are needed to refine the time and rate of change within this interval at Hadar. PMID:21514622

  16. The electrical resistivity structure of lithosphere across the Dharwar craton nucleus and Coorg block of South Indian shield: Evidence of collision and modified and preserved lithosphere

    NASA Astrophysics Data System (ADS)

    Abdul Azeez, K. K.; Veeraswamy, K.; Gupta, Arvind K.; Babu, Narendra; Chandrapuri, Sateesh; Harinarayana, T.

    2015-10-01

    Magnetotelluric-derived two-dimensional lithospheric resistivity structure of the western Dharwar craton (WDC) and adjoining Coorg block indicates isolated low-resistivity zones in the crust and three striking upper mantle conductive features within the highly resistive Archean lithosphere. The crustal conductors in the WDC show good spatial correlation with the exposed supracrustal rocks conformable with the relic schist belt channels having conductive mineral grains. Conductive zones within the Coorg crust might be related to the relatively young (933 Ma) metamorphic processes in the area and/or possible fluids derived from the Cretaceous passage of Reunion plume in the proximity of Coorg area. A near-vertical conductive structure extending from the lower crust into the upper mantle coincides with the transition zone between Coorg and WDC. This is interpreted as the suture zone between the two tectonic blocks and provides evidence for the individuality of the two Archean terrains. An anomalous upper mantle conductive zone found beneath the craton nucleus may indicate a modified cratonic lithosphere. This could have been derived due to the collision between Coorg and WDC and possibly survived by the subsequent multiple episodes of melt and fluid infiltration processes experienced in the region. Thick (~190 km) and preserved lithosphere is mapped at the eastern segment of WDC. Resistive lithosphere of ~125 km thickness is imaged for the Coorg block.

  17. Geodynamic inversion to constrain the rheology of the lithosphere: What is the effect of elasticity?

    NASA Astrophysics Data System (ADS)

    Baumann, Tobias; Kaus, Boris; Thielmann, Marcel

    2016-04-01

    The concept of elastic thickness (T_e) is one of the main methods to describe the integrated strength of oceanic lithosphere (e.g. Watts, 2001). Observations of the Te are in general agreement with yield strength envelopes estimated from laboratory experiments (Burov, 2007, Goetze & Evans 1979). Yet, applying the same concept to the continental lithosphere has proven to be more difficult (Burov & Diament, 1995), which resulted in an ongoing discussion on the rheological structure of the lithosphere (e.g. Burov & Watts, 2006, Jackson, 2002; Maggi et al., 2000). Recently, we proposed a new approach, which constrains rheological properties of the lithosphere directly from geophysical observations such as GPS-velocity, topography and gravity (Baumann & Kaus, 2015). This approach has the advantage that available data sets (such as Moho depth) can be directly taken into account without making the a-priori assumption that the lithosphere is thin elastic plate floating on the mantle. Our results show that a Bayesian inversion method combined with numerical thermo-mechanical models can be used as independent tool to constrain non-linear viscous and plastic parameters of the lithosphere. As the rheology of the lithosphere is strongly temperature dependent, it is even possible to add a temperature parameterisation to the inversion method and constrain the thermal structure of the lithosphere in this manner. Results for the India-Asia collision zone show that existing geophysical data require India to have a quite high effective viscosity. Yet, the rheological structure of Tibet less well constrained and a number of scenarios give a nearly equally good fit to the data. Yet, one of the assumptions that we make while doing this geodynamic inversion is that the rheology is viscoplastic, and that elastic effects do not significantly alter the large-scale dynamics of the lithosphere. Here, we test the validity of this assumption by performing synthetic forward models and retrieving

  18. Internal structure of oceanic lithosphere: A perspective from tectonic windows

    NASA Astrophysics Data System (ADS)

    Karson, Jeffrey A.

    Major faulted escarpments on the seafloor provide "tectonic windows" into oceanic crust and upper mantle. Direct observations in these settings reveal that the spatial arrangement, internal structure, and contacts between major rock units are significantly more complex than commonly anticipated on the basis of seismic studies and ophiolite analogs. From this perspective, a stratiform, ophiolite-like sequence of rock units, including basaltic volcanic rocks, sheeted diabase dike complex, isotropic and layered gabbroic and ultramafic rocks over upper mantle peridotites—all separated by generally horizontal contacts, may be much less common in the oceanic lithosphere than generally thought. Conversely, documented examples of large outcrop areas (tens of kilometers across) that lack the ophiolite-like sequence or that contain structures that do not conform to the ophiolite model call into question the basic assumptions made in the reconstruction and interpretation