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Sample records for natural lithospheric radioxenon

  1. Categorization of Radioxenon

    SciTech Connect

    Keller, Paul E.

    2012-04-26

    This report summarizes a study into some false positive issues in the use of radioxenon as a method to verify a clandestine nuclear weapons explosion. False positives arise due to similarities between the radioxenon signature generated in medical isotope production and that generated in a nuclear weapon explosion. This report also discusses how to categorize the radioxenon by levels of urgency for manual analysis and interpretation and recommends applying machine learning and time series analysis techniques in the automation of radioxenon characterization. The literature indicates that medical isotope production is a major contributor to atmospheric radioxenon and is the main source of confusion in determining the source of radioxenon. While radioxenon emissions from nuclear power plants can be distinguished from that from nuclear weapon explosions, emissions from medical isotope production generate signatures similar to certain nuclide ratios found in nuclear weapons explosions. Different techniques for analyzing nuclide concentrations and ratios as well as including other sensing modalities via sensor fusion are discussed.

  2. 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

  3. Radioxenon Atmospheric Measurements in North Las Vegas

    SciTech Connect

    Milbrath, Brian D.; Cooper, Matthew W.; Lidey, Lance S.; Bowyer, Ted W.; Hayes, James C.; McIntyre, Justin I.; Karr, L.; Shafer, David S.; Tappen, J.

    2007-09-25

    Pacific Northwest National Laboratory (PNNL) deployed the Automated Radioxenon Sampler/Analyzer (ARSA) in North Las Vegas for two weeks in February and March 2006 for the purpose of measuring the radioxenon background at a level of sensitivity much higher than previously done in the vicinity of the Nevada Test Site (NTS). The measurements establish what might be expected if future measurements are taken at NTS itself and investigate improved methods of environmental monitoring of NTS for test site readiness. Also, such radioxenon measurements have not previously been performed in a United States location considered to be as remote from nuclear reactors. A second detector, the Portable Environmental Monitoring Station (PEMS), built and operated by the Desert Research Institute (DRI), was deployed in conjunction with the ARSA and contained a pressure ion chamber, aerosol collection filters, and meteorological sensors. Some of the radioxenon measurements detected 133Xe at levels up to 3 mBq/m3. This concentration of radioxenon is consistent with the observation of low levels of radioxenon emanating from distance nuclear reactors. Previous measurements in areas of high nuclear reactor concentration have shown similar results, but the western US, in general, does not have many nuclear reactors. Measurements of the wind direction indicate that the air carrying the radioxenon came from south of the detector and not from the NTS.

  4. Environmental Applications of Stable Xenon and Radioxenon Monitoring

    SciTech Connect

    Dresel, P. Evan; Olsen, Khris B.; Hayes, James C.; McIntyre, Justin I.; Waichler, Scott R.; Kennedy, B. M.

    2008-06-01

    Improved detection capabilities are needed at several Department of Energy sites to make remedial decisions about facilities and landfill cleanup. For facility monitoring air samples can be collected from within a facility and analyzed for short lived radioxenons to estimate inventories of residual plutonium holdup within the facility. For landfill cleanup activities soil gas sampling for xenon isotopes can be used to define the locations of spent fuel and transuranic wastes. Short-lived radioxenon isotopes are continuously produced by spontaneous fission of plutonium-240 in transuranic wastes. Large volume soil-gas samples provide extremely sensitive measurement of radioxenon in the subsurface; a characteristic of transuranic waste. The analysis employs a modified Automated Radioxenon Sampling and Analysis (ARSA) system. Proof of principle measurements at a Hanford Site liquid waste disposal site showed xenon-133 at levels in soil gas are approximately 16,000 times the detection limit and lower levels of xenon-135 from the spontaneous fission of plutonium-240 were also measured. Stable xenon isotopes are also produced by spontaneous fission but are subject to background concentrations in ambient air samples (facilities) but less so in soil gas where free exchange with ambient air is restricted. Rare gas mass spectrometry is used for highly precise stable xenon isotopic measurements. Stable xenon isotopic ratios from fission are distinct from natural xenon background ratios. Neutron capture on xenon-135 produces an excess of xenon-136 above fission ratios and thus provides a means of distinguishing reactor sources (e.g. spent fuel) from separated transuranic materials (plutonium).

  5. Progress in Advanced Spectral Analysis of Radioxenon

    SciTech Connect

    Haas, Derek A.; Schrom, Brian T.; Cooper, Matthew W.; Ely, James H.; Flory, Adam E.; Hayes, James C.; Heimbigner, Tom R.; McIntyre, Justin I.; Saunders, Danielle L.; Suckow, Thomas J.

    2010-09-21

    Improvements to a Java based software package developed at Pacific Northwest National Laboratory (PNNL) for display and analysis of radioxenon spectra acquired by the International Monitoring System (IMS) are described here. The current version of the Radioxenon JavaViewer implements the region of interest (ROI) method for analysis of beta-gamma coincidence data. Upgrades to the Radioxenon JavaViewer will include routines to analyze high-purity germanium detector (HPGe) data, Standard Spectrum Method to analyze beta-gamma coincidence data and calibration routines to characterize beta-gamma coincidence detectors. These upgrades are currently under development; the status and initial results will be presented. Implementation of these routines into the JavaViewer and subsequent release is planned for FY 2011-2012.

  6. Automatic radioxenon analyzer for CTBT monitoring

    SciTech Connect

    Bowyer, T.W.; Abel, K.H.; Hensley, W.K.

    1996-12-01

    Over the past 3 years, with support from US DOE`s NN-20 Comprehensive Test Ban Treaty (CTBT) R&D program, PNNL has developed and demonstrated a fully automatic analyzer for collecting and measuring the four Xe radionuclides, {sup 131m}Xe(11.9 d), {sup 133m}Xe(2.19 d), {sup 133}Xe (5.24 d), and {sup 135}Xe(9.10 h), in the atmosphere. These radionuclides are important signatures in monitoring for compliance to a CTBT. Activity ratios permit discriminating radioxenon from nuclear detonation and that from nuclear reactor operations, nuclear fuel reprocessing, or medical isotope production and usage. In the analyzer, Xe is continuously and automatically separated from the atmosphere at flow rates of about 7 m{sup 3}/h on sorption bed. Aliquots collected for 6-12 h are automatically analyzed by electron-photon coincidence spectrometry to produce sensitivities in the range of 20-100 {mu}Bq/m{sup 3} of air, about 100-fold better than with reported laboratory-based procedures for short time collection intervals. Spectral data are automatically analyzed and the calculated radioxenon concentrations and raw gamma- ray spectra automatically transmitted to data centers.

  7. Measurements of Worldwide Radioxenon Backgrounds - The "EU" Project

    SciTech Connect

    Bowyer, Ted W.; Cooper, Matthew W.; Hayes, James C.; Forrester, Joel B.; Haas, Derek A.; Hansen, Randy R.; Keller, Paul E.; Kirkham, Randy R.; Lidey, Lance S.; McIntyre, Justin I.; Miley, Harry S.; Payne, Rosara F.; Saey, Paul R.; Thompson, Robert C.; Woods, Vincent T.; Williams, Richard M.

    2009-09-24

    Under the Comprehensive Nuclear-Test-Ban Treaty (CTBT), radioactive xenon (radioxenon) measurements are one of the principle techniques used to detect nuclear underground nuclear explosions, and specifically, the presence of one or more radioxenon isotopes allows one to determine whether a suspected event was a nuclear explosion or originated from an innocent source. During the design of the International Monitoring System (IMS), which was designed as the verification mechanism for the Treaty, it was determined that radioxenon measurements should be performed at 40 or more stations worldwide. At the time of the design of the IMS, however, very few details about the background of the xenon isotopes was known and it is now recognized that the backgrounds were probably evolving anyhow. This paper lays out the beginning of a study of the worldwide concentrations of xenon isotopes that can be used to detect nuclear explosions and several sources that also release radioxenons, and will have to be accounted for during analysis of atmospheric levels. Although the global concentrations of the xenon isotopes are the scope of a much larger activity that could span over several years, this study measures radioxenon concentrations in locations where there was either very little information or there was a unique opportunity to learn more about emissions from known sources. The locations where radioxenon levels were measured and reported are included.

  8. Atmospheric Radioxenon Measurements in North Las Vegas, NV

    SciTech Connect

    Milbrath, Brian D.; Cooper, Matthew W.; Lidey, Lance S.; Bowyer, Ted W.; Hayes, James C.; McIntyre, Justin I.; Karr, L.; Shafer, D.; Tappen, J.

    2006-07-31

    PNNL deployed the ARSA radioxenon measurement system in North Las Vegas for two weeks in February and March 2006 for the purpose of measuring the radioxenon background at a level of sensitivity much higher than previously done in the vicinity of the NTS. The measurements establish what might be expected if future measurements are taken at NTS itself. The measurements are also relevant to test site readiness. A second detector, the PEMS, built and operated by DRI, was deployed in conjunction with the ARSA and contained a PIC, aerosol collection filters, and meteorological sensors. Originally, measurements were also to be performed at Mercury, NV on the NTS, but these were canceled due to initial equipment problems with the ARSA detector. Some of the radioxenon measurements detected 133Xe at levels up to 3 mBq/m3. This concentration of radioxenon is consistent with the observation of low levels of radioxenon emanating from distance nuclear reactors. Previous measurements in areas of high nuclear reactor concentration have shown similar results, but the western US, in general, does not have many nuclear reactors. Measurements of the wind direction indicate that the air carrying the radioxenon came from south of the detector and not from the NTS.

  9. On the nature and origin of highly-refractory Archean lithosphere: Petrological and geophysical constraints from the Tanzanian craton

    NASA Astrophysics Data System (ADS)

    Gibson, S. A.; McMahon, S. C.; Day, J. A.; Dawson, J. B.

    2012-12-01

    The nature and timescales of garnet formation are important to understanding how subcontinental lithospheric mantle (SCLM) has evolved since the Archean, and also to mantle dynamics, because the presence of garnet greatly influences the density of the lower lithosphere and hence the long-term stability of thick (150 to 220 km) subcratonic lithosphere. Nevertheless, the widespread occurrence of garnet in the SCLM remains one of the 'holy grails' of mantle petrology. Garnets found in mantle xenoliths from the eastern margin of the Tanzanian Craton (Lashaine) have diverse compositions and provide major constraints on how the underlying deep (120 to 160 km) mantle evolved during the last 3 billion years. Certain harzburgite members of the xenolith suite contain the first reported occurrence of pyrope garnets with rare-earth element patterns similar to hypothetical garnets proposed to have formed in the Earth's SCLM during the Archean, prior to metasomatism [Stachel et al., 2004]. These rare ultradepleted low-Cr garnets occur in low temperature (~1050 oC) xenoliths derived from depths of ~120 km and coexist in chemical and textural equilibrium with highly-refractory olivine (Fo95.4) and orthopyroxene (Mg#=96.4). These phases are all more magnesian than generally encountered in global mantle harzburgites and diamond inclusions. The ultradepleted garnets form interconnecting networks around grains of orthopyroxene which give the rocks a banded appearance: we propose that the increase in pressure associated with cratonization may have caused isochemical exsolution of ultradepleted garnet from orthopyroxene. These unique garnets have not previously been identified in global suites of mantle xenoliths or diamond inclusions. We believe they are rare because their low concentrations of trace elements make them readily susceptible to geochemical overprinting. This highly-refractory low-density peridotite may be common in the 'shallow' SCLM but not normally brought to the

  10. Nature and timing of multiple metasomatic events in the sub-cratonic lithosphere beneath Labait, Tanzania

    NASA Astrophysics Data System (ADS)

    Koornneef, Janne M.; Davies, Gareth R.; Döpp, Sonja P.; Vukmanovic, Zoja; Nikogosian, Igor K.; Mason, Paul R. D.

    2009-11-01

    Petrography, mineral major- and trace element analyses and Rb-Sr and Sm-Nd systematics of xenoliths from Labait volcano, north-central Tanzania, document multiple metasomatic events after initial depletion of the Archaean sub-lithospheric mantle. Four distinct metasomatic phases occurred during the 2.8-3.2 Ga history of the mantle section of the Tanzanian craton. 1) Garnet and Cr-diopside in two depleted lherzolites record LREE enrichment in an early cryptic metasomatic event (~ 2 Ga) resulting in unradiogenic ɛ Nd (- 6.6) and relatively radiogenic Sr signature ( 87Sr/ 86Sr = 0.7049); 2) Four texturally equilibrated peridotites contain phlogopite and Cr-diopside inferred to be introduced by a hydrous melt/fluid that produced LREE enrichment related to the subduction and collision during the 650 Ma Pan-African Orogeny; 3) Fe-enrichment is observed in many garnet-free wehrlites and dunites having low Mg# olivines. Timing of this enrichment event remains poorly defined; and 4) One spinel lherzolite records orthopyroxene replacing clinopyroxene due to recent infiltration of a rift-related H 2O poor, K-alkaline silicate melt. This ongoing metasomatic reaction caused by rift-related magmatism would result in the conversion of lherzolite to orthopyroxene-rich harzburgite. The reaction possibly represents the mechanism involved in the formation of orthopyroxene-rich sub-continental lithospheric mantle below the Kaapvaal and Siberian cratons. Generally, the rift-related metasomatism beneath Tanzania has caused formation of interstitial clinopyroxene, melt veins and melt pockets and new rims of phlogopite, all of which are in chemical disequilibrium with the original xenolith mineralogy.

  11. Polar Regions: Natural Laboratories for Understanding the Dynamic Lithosphere-Cryosphere-Climate System

    NASA Astrophysics Data System (ADS)

    Wilson, T. J.; Antec Group,.

    2003-12-01

    The polar regions have unique geodynamic environments where the solid earth, the cryosphere, the oceans, the atmosphere and the global climate system are intimately linked. It is increasingly clear that deciphering the feedbacks and interactions between these spheres is required to comprehend the earth system as a whole. The ANTEC group has developed a vision for a major interdisciplinary program to investigate neotectonic processes in Antarctica, focused on understanding 1) how changing ice mass loads influence lithospheric stress/strain regimes; 2) how glacial isostatic adjustment and the tectono-thermal structure of the lithosphere control modern ice sheet dynamics; 3) the history of inception and growth of Antarctic ice sheets and how it's fluctuations are linked with tectonic motions and magmatism; and 4) how evolving continental-scale paleogeography, volcanism, erosion/sedimentation act to force climate change. We are poised at a critical threshold where developing technologies will enable experiments at unprecedented scales across the continent. Autonomous vehicles will allow exploration over unmapped ice sheets and under the ice shelves and sea ice that shroud immense portions of polar terrain. Unattended remote geodetic and geophysical observatories are beginning to operate through the polar night due to advances in power sources and data storage capacity. Drilling technologies are being adapted to the harsh polar environment and will allow us to sample the vast swaths of virtually unknown subglacial terrain. Spaceborne instruments now have an unprecedented capability to capture change in the polar environment. These emerging technologies will allow us to make the next `great leap forward' in exploring the polar frontier of the earth system with immensely more ease than during the IGY 50 years ago. If we can meet the logistical and funding challenges of instrumenting the polar regions on a grand scale, we will reap a wealth data that will serve as the

  12. Radioxenons: Their role in monitoring a Comprehensive Test Ban Treaty

    SciTech Connect

    Perkins, R. W.; Casey, L. A.

    1996-06-01

    Monitoring for xenon radionuclides which are produced in a nuclear detonation can provide a strong deterrent to the violation of a Comprehensive nuclear Test Ban Treaty (CTBT). There are 18 known radioactive xenon isotopes produced in nuclear fission with half-lives ranging from less than one second to 11.9 days. However, only four of these remain in significant amounts more than a day after a detonation. In order for radioxenon monitoring to be practical, it was necessary to develop an automated measurement system which could operate unattended for periods of months, measure the entire spectrum of radioxenons, and provide hundreds of times better sensitivities than current laboratory procedures. This capability was developed at the US Department of Energy`s Pacific Northwest National Laboratory based on rapid separation of atmospheric xenon coupled with a unique high sensitivity measurement device for the radioxenons. A fieldable prototype analyzer is scheduled for testing in August 1996 with commercial availability planned by 1998.

  13. On the nature and origin of garnet in highly-refractory Archean lithosphere: implications for continent stabilisation

    NASA Astrophysics Data System (ADS)

    Gibson, Sally

    2014-05-01

    The nature and timescales of garnet formation in the Earth's subcontinental lithospheric mantle (SCLM) are important to our understanding of how this rigid outer shell has evolved and stabilised since the Archean. Nevertheless, the widespread occurrence of pyrope garnet in the sub-cratonic mantle remains one of the 'holy grails' of mantle petrology. The paradox is that garnet often occurs in mantle lithologies (dunites and harzburgites) which represent residues of major melting events (up to 40 %) whereas experimental studies on fertile peridotite suggest this phase should be exhausted by <20 % melting. Furthermore, garnets commonly found in mantle peridotite suites have diverse compositions that are typically in equilibrium with high-pressure, small-fraction, mantle melts suggesting they formed as a result of enrichment of the lithospheric mantle following cratonisation. This refertilisation -- which typically involves addition of Fe, incompatible trace elements and volatiles -- affects the lower 30 km of the lithosphere and potentially leads to negative buoyancy and destabilisation. Pyrope garnets found in mantle xenoliths from the eastern margin of the Tanzanian Craton (Lashaine) have diverse compositions and provide major constraints on how the underlying deep (120 to 160 km) mantle stabilised and evolved during the last 3 billion years. The garnets display systematic trends from ultra-depleted to enriched compositions that have not been recognised in peridotite suites from elsewhere (Gibson et al., 2013). Certain harzburgite members of the xenolith suite contain the first reported occurrence of pyrope garnets with rare-earth element (REE) patterns similar to hypothetical garnets proposed by Stachel et al. (2004) to have formed in the Earth's SCLM during the Archean, prior to metasomatism. These rare ultra-depleted low-Cr garnets occur in low temperature (~1050 oC) xenoliths derived from depths of ~120 km and coexist in chemical and textural equilibrium with

  14. Nature and melting processes of the lithosphere beneath the North-East Qiangqtang terrane, Central Tibet, during Eocene times.

    NASA Astrophysics Data System (ADS)

    Goussin, Fanny; Guillot, Stéphane; Schulmann, Karel; Cordier, Carole; Oliot, Emilien; Replumaz, Anne; Roperch, Pierrick; Dupont-Nivet, Guillaume

    2016-04-01

    pyroxene xenocrysts, suggest that enrichment occurred at the source by metasomatism of the lithospheric mantle. Further work will precise which mechanisms could have accounted for such a metasomatism of the mantle beneath the Northern Qiangtang terrane during Eocene, and whether the lower crust had an autochtonous or allochtonous nature.

  15. 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.

  16. A phoswich well detector for radioxenon monitoring

    NASA Astrophysics Data System (ADS)

    Hennig, Wolfgang; Tan, Hui; Fallu-Labruyere, Anthony; Warburton, William K.; McIntyre, Justin I.; Gleyzer, Anshel

    2007-08-01

    One of several methods used to detect nuclear weapons testing is the monitoring of radioactive xenon in the atmosphere. For high sensitivity, monitoring stations use a complex system of separate beta and gamma detectors to detect beta-gamma coincidences from characteristic radioxenon isotopes in small amounts of xenon extracted from large volumes of air. We report a simpler approach that uses a single phoswich detector, comprising optically coupled plastic and CsI scintillators to absorb beta particles and gamma rays, respectively, and then detect coincidences by pulse shape analysis of the detector signal. Previous studies with a planar prototype detector have shown that the technique can clearly separate beta only, gamma only and coincidence events, does not degrade the energy resolution, and has an error rate for detecting coincidences of less than 0.1%. In this paper, we will present a new phoswich well detector design, consisting of a 1'' diameter plastic cell enclosed in a 3'' CsI crystal. Based on Monte Carlo modeling and experimental results, the design will be characterized in terms of energy resolution and its ability to separate beta and gamma only, and coincidence events.

  17. Radioxenon production through neutron irradiation of stable xenon gas

    SciTech Connect

    Haas, Derek A.; Biegalski, Steven R.; Foltz Biegalski, Kendra M.

    2009-12-01

    The Spectral Deconvolution Analysis Tool (SDAT) software was developed to improve counting statistics and detection limits for nuclear explosion radionuclide measurements. SDAT utilizes spectral deconvolution spectroscopy techniques and can analyze both β-γ coincidence spectra for radioxenon isotopes and high-resolution HPGe spectra from aerosol monitors. The deconvolution algorithm of the SDAT requires a library of β-γ coincidence spectra of individual radioxenon isotopes to determine isotopic ratios in a sample. In order to get experimentally produced spectra of the individual isotopes we have irradiated enriched samples of 130Xe, 132Xe, and 134Xe gas with a neutron beam from the TRIGA reactor at The University of Texas. The samples produced were counted in an Automated Radioxenon Sampler/Analyzer (ARSA) style β-γ coincidence detector. The spectra produced show that this method of radioxenon production yields samples with very high purity of the individual isotopes for 131mXe and 135Xe and a sample with a substantial 133mXe to 133Xe ratio.

  18. Radioxenon retention in the skeleton on a routine ventilation study.

    PubMed

    Kramer, E L; Tiu, S; Sanger, J J; Benjamin, D D

    1983-07-01

    Marked retention of radioxenon by the skeletal structures during a routine ventilation scan is described. Xenon uptake by bones occurs largely in the intraosseous fat. Augmented uptake in this case may be related to the patient's prolonged steroid therapy. PMID:6617033

  19. Observational Strategy of ACROSS towards the Time-evolving Natures in the Lithosphere

    NASA Astrophysics Data System (ADS)

    Kumazawa, M.; Fujii, N.; Kasahara, J.

    2005-12-01

    ACROSS (Accurately Controlled, Routinely Operated Signal System) is aiming at the detection of very small changes in physical states in the lithosphere, particularly for the focal region of the anticipated huge earthquakes as demanded socially. Our technical challenge is to device an ideal methodology to enable us to acquire the ideal observation data towards the real understanding of the EarthOs interiors even under the inherent noise and physical limitations. We need light to illuminate the dark EarthOs interiors, eyes to observe them and a brain to interpret the result: The light should be designed well to be really coherent, the eyes with high fidelity should be accurately synchronized to the light transmission and the brain should be smart enough to evolve by itself. In order for the whole system to be robust against noise, we have to devise all that can be done. In addition, a significant demand is imposed onto us; non-destructiveness against our environment. The recent progress of technology makes it possible what was impossible several years, so that we try to find out the ideal way to go. We have spent about 10 years for developmental works, which started a moment before the disastrous Kobe earthquake of 1995. Now we believe that the background theory has been known in addition to some of the basic technology elements, whereas the user-friendly hardware and other auxiliary tools including practical theory and software have not been acquired yet. The examples of the field observation have started to accumulate for demonstration as reported by companion papers. The data acquired by ACROSS in seismology is not seismogram but tensor transfer function (Green function) in frequency domain. The data carry substantially new information with high quality and rigorous estimate of reliability. The availability of ACROSS would change the strategy for underground study in the coming years. We would like to call for your attention and discussion to the next way to go

  20. Constraints on the Nature of the Lithosphere-Asthenosphere Boundary: Comparison of Observed Textural Evolution to Measured Seismic Anisotropy

    NASA Astrophysics Data System (ADS)

    Hansen, L. N.; Qi, C.; Kumamoto, K.; Warren, J. M.; Katz, R. F.; Kohlstedt, D. L.

    2014-12-01

    The nature of the lithosphere-asthenosphere boundary (LAB) determines the mechanical and compositional coupling between rigid plates and underlying convecting mantle. Seismological studies reveal distinct reflectors in the uppermost oceanic mantle that are sometimes interpreted as the LAB. These reflectors roughly correlate with the location of vertical gradients in radial seismic anisotropy. However, these proxies for the LAB 1) do not exhibit the depth-age relationship predicted by thermal models and 2) are not detected consistently throughout the major ocean basins. In contrast, gradients in azimuthal anisotropy in oceanic upper mantle do appear to coincide with the predicted thermal structure. This overall seismic signature is suggested to arise both from melt-related processes at mid-ocean ridges and from "freezing in" anisotropy during changes in the direction of plate motion. We test recent interpretations of this upper-mantle seismic structure with measurements of crystallographic textures in experimentally and naturally deformed peridotites. Key observations: 1) Experimental deformation of nominally melt-free olivine aggregates reveals a protracted increase in texture strength and, therefore, in magnitude of elastic anisotropy with progressive deformation, 2) melt-rich systems attain a steady state at very low strain (<~1) with fast seismic directions normal to those in the melt-free case, and 3) textures measured in samples from peridotite massifs exhibit a cm- to m-scale compositional heterogeneity associated with melt production and extraction that could be a significant source of radial anisotropy. In conjunction with constraints on finite strain and melting extent from geodynamic simulations, we use textural observations to predict seismic structure in an oceanic setting. Comparison of our predictions to observed seismic anisotropy provide insight into the role of composition, melting, and rheology in defining the LAB.

  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. Inverse modeling of April 2013 radioxenon detections

    NASA Astrophysics Data System (ADS)

    Hofman, Radek; Seibert, Petra; Philipp, Anne

    2014-05-01

    Significant concentrations of radioactive xenon isotopes (radioxenon) were detected by the International Monitoring System (IMS) for verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) in April 2013 in Japan. Particularly, three detections of Xe-133 made between 2013-04-07 18:00 UTC and 2013-04-09 06:00 UTC at the station JPX38 are quite notable with respect to the measurement history of the station. Our goal is to analyze the data and perform inverse modeling under different assumptions. This work is useful with respect to nuclear test monitoring as well as for the analysis of and response to nuclear emergencies. Two main scenarios will be pursued: (i) Source location is assumed to be known (DPRK test site). (ii) Source location is considered unknown. We attempt to estimate the source strength and the source strength along with its plausible location compatible with the data in scenario (i) and (ii), respectively. We are considering also the possibility of a vertically distributed source. Calculations of source-receptor sensitivity (SRS) fields and the subsequent inversion are aimed at going beyond routine calculations performed by the CTBTO. For SRS calculations, we employ the Lagrangian particle dispersion model FLEXPART with high resolution ECMWF meteorological data (grid cell sizes of 0.5, 0.25 and ca. 0.125 deg). This is important in situations where receptors or sources are located in complex terrain which is the case of the likely source of detections-the DPRK test site. SRS will be calculated with convection enabled in FLEXPART which will also increase model accuracy. In the variational inversion procedure attention will be paid not only to all significant detections and their uncertainties but also to non-detections which can have a large impact on inversion quality. We try to develop and implement an objective algorithm for inclusion of relevant data where samples from temporal and spatial vicinity of significant detections are added in an

  3. 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

  4. Radioxenon monitoring in Beijing following the Fukushima Daiichi NPP accident.

    PubMed

    Shilian, Wang; Qi, Li; Qinghua, Meng; Zhanying, Chen; Yungang, Zhao; Huijuan, Li; Huaimao, Jia; Yinzhong, Chang; Shujiang, Liu; Xinjun, Zhang; Yuanqing, Fan; Ling, Wan; Yun, Lou

    2013-11-01

    This paper reports the brief process and results of radioxenon monitoring and analysis in Beijing following the Fukushima Daiichi nuclear power plant accident. The accident and release of volatile radionuclides were caused by 9.0 magnitude earthquake and tsunami on March 11, 2011. The maximum concentrations of (133)Xe and (131 m)Xe were in excess of 0.90 Bq.m(-3) and 0.047 Bq.m(-3), respectively. The activity ratio of (131 m)Xe to (133)Xe and the dynamic trend of (133)Xe activity concentration were analyzed. PMID:23601858

  5. Impact of Monthly Radioxenon Source Time-Resolution on Atmospheric Concentration Predictions

    NASA Astrophysics Data System (ADS)

    Schöppner, Michael; Kalinowski, Martin; Plastino, Wolfango; Budano, Antonio; de Vincenzi, Mario; Ringbom, Anders; Ruggieri, Federico; Schlosser, Clemens

    2014-03-01

    The general characterisation of the global radioxenon background is of interest for the verification of the Comprehensive Nuclear-Test-Ban Treaty. Since the major background sources are only a few isotope production facilities, their source term has an emphasized influence on the worldwide monitoring process of radioxenon. In this work, two different datasets of source terms are applied through atmospheric transport modelling, to estimate the concentration at two radioxenon detection stations in Germany and Sweden. One dataset relies on estimated average annual emissions; the other includes monthly resolved measurements from an isotope production facility in Fleurus, Belgium. The quality of the estimations is then validated by comparing them to the radioxenon concentrations that have been sampled at two monitoring stations over the course of 1 year.

  6. Improved β-γ Coincidence Detector For Radioxenon Detection

    SciTech Connect

    Cooper, Matthew W; Carman, April J; Hayes, James C; Heimbigner, Tom R; Hubbard, Charles W; Litke, Kevin E; McIntyre, Justin I; Morris, Scott J; Ripplinger, Michael D; Suarez, Reynold

    2005-08-31

    The Automated Radio-xenon Analyzer/Sampler (ARSA), built by Pacific Northwest National Laboratory (PNNL), can collect and detect several radioxenon isotopes. ARSA is very sensitive to 133Xe, 131mXe, 133mXe and 135Xe due to the compact high efficiency coincidence detector it uses. For this reason it is an excellent treaty monitoring and environmental sampling device. Although the system is shown to be both robust and reliable, based on several field tests, it is also complex due to a detailed photomultiplier tube gain matching regime. This complexity is a problem from a maintenance and quality assurance/quality control (QA/QC) standpoint. To reduce these issues a simplified coincident detector has been developed. A comparison of three different well detectors has been completed. In addition, a new plastic scintillator gas cell was constructed. The new simplified detector system has been demonstrated to equal or better performance compared with the original ARSA design in spectral resolution and efficiency and significantly easier to setup and calibrate.

  7. Testing of the KRI-developed Silicon PIN Radioxenon Detector

    SciTech Connect

    Foxe, Michael P.; McIntyre, Justin I.

    2015-01-23

    Radioxenon detectors are used for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) in a network of detectors throughout the world called the International Monitoring System (IMS). The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) Provisional Technical Secretariat (PTS) has tasked Pacific Northwest National Laboratory (PNNL) with testing a V.G. Khlopin Radium Institute (KRI) and Lares Ltd-developed Silicon PIN detector for radioxenon detection. PNNL measured radioxenon with the silicon PIN detector and determined its potential compared to current plastic scintillator beta cells. While the PNNL tested Si detector experienced noise issues, a second detector was tested in Russia at Lares Ltd, which did not exhibit the noise issues. Without the noise issues, the Si detector produces much better energy resolution and isomer peak separation than a conventional plastic scintillator cell used in the SAUNA systems in the IMS. Under the assumption of 1 cm3 of Xe in laboratory-like conditions, 24-hr count time (12-hr count time for the SAUNA), with the respective shielding the minimum detectable concentrations for the Si detector tested by Lares Ltd (and a conventional SAUNA system) were calculated to be: 131mXe – 0.12 mBq/m3 (0.12 mBq/m3); 133Xe – 0.18 mBq/m3 (0.21 mBq/m3); 133mXe – 0.07 mBq/m3 (0.15 mBq/m3); 135Xe – 0.45 mBq/m3 (0.67 mBq/m3). Detection limits, which are one of the important factors in choosing the best detection technique for radioxenon in field conditions, are significantly better than for SAUNA-like detection systems for 131mXe and 133mXe, but similar for 133Xe and 135Xe. Another important factor is the amount of “memory effect” or carry over signal from one radioxenon measurement to the subsequent sample. The memory effect is

  8. Environmental characterisation of a major radioxenon source in Europe

    NASA Astrophysics Data System (ADS)

    Saey, P. R. J.; Ringbom, A.; Becker, A.; Camps, J.; Paquet, N.; Sonck, M.; Taffary, T.; van der Meer, K.; Verboomen, B.; Zähringer, M.

    2009-04-01

    In the framework of the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) the atmospheric background of environmental radioxenon has been studied. It was recently shown that radiopharmaceutical facilities (RPF) have a major contribution to the general background of 133Xe and other xenon isotopes both in the northern and southern hemisphere. The daily International Monitoring System (IMS) noble gas measurements around the globe are influenced from such anthropogenic sources that could hide relevant radioxenon signals. To distinguish a nuclear explosion from releases from civil nuclear facilities, not only the activity concentration but also the ratio of different radioxenon isotopes (131mXe, 133mXe, 133Xe and 135Xe) plays a crucial role, since the ratios can be used to discriminate source types. Theoretical release and ratio studies were recently published, but no high-sensitive measurements in and close to radiopharmaceutical facilities have ever been performed. During the summer of 2008, a three week field campaign was carried out in the region around the Belgian radiopharmaceutical facility IRE in Fleurus, the world third largest one. The scope was to obtain the activity concentration of the releases and the isotopic composition. Two 6-hour noble gas measurements, using mobile SAUNA sampling equipment were collected each day at different distances from the facility (1 - 100 km). The sampling locations were guided by atmospheric dispersion model results. Three samples from the stack itself were also collected. All 38 samples were shipped after collection to and measured with a SAUNA at the laboratory in Stockholm, Sweden. The environmental concentrations of 133Xe were measured and found to be in the range between 0.7 - 4 105 mBq/m3. Nine samples contained all four CTBT relevant radioxenon isotopes. The concentrations of the stack spike samples were in the range 2 109 - 4 1010 mBq/m3. This corresponds to a daily release of around 1 TBq. This is

  9. Nature of the Lithosphere-Asthenosphere Boundary Beneath Normal Oceans from High-Resolution Anisotropic Receiver Functions

    NASA Astrophysics Data System (ADS)

    Olugboji, T. M.; Park, J. J.; Karato, S. I.; Kawakatsu, H.; Shinohara, M.

    2014-12-01

    The oceanic lithosphere-asthenosphere boundary (LAB) has a seismological expression that is characterized by a sharp velocity reduction located at relatively shallow depths (40-100 km). Observations suggest that the LAB deepens with age for relatively young oceans, and may be coincident with anisotropic gradients. Models to explain these features include partial melting, sub-solidus physical dispersion caused by grain-boundary sliding, and a change in anisotropic rock texture. To assess these models, we provide new seismological constraints using frequency-dependent harmonic decomposition of receiver functions. We analyze data from 17 ocean-bottom stations and 2 borehole stations in the Philippine Sea and Northwest Pacific ocean. Underneath young oceans, we observe that the depth of the LAB follows the ~1300°K thermal contour. For old oceans the LAB lies at a constant depth ~70 km. These results are consistent with the predictions of the grain-boundary sliding model. Preliminary analysis of the width of the velocity gradient, shows that at some young oceans, the velocity reduction is gradational, while at old oceans a sharper velocity reduction is required to fit the receiver function data. This behavior is also consistent with the prediction of the grain-boundary sliding model, where thermal relaxation in the young oceans leads to a gradational velocity reduction, while the water-dependent relaxation leads to a sharp velocity reduction at old oceans. Additionally, harmonic decomposition of the receiver functions suggests that strong anisotropic gradients do not coincide with the velocity reduction. At undisturbed ocean lithosphere, therefore, we argue that anisotropy is not a causal mechanism for the LAB. We also fail to observe oppositely-polarized Ps conversions from the top and bottom boundary of a putative melt-layer, except underneath stations above the subducting Pacific-plate slab.

  10. Nature of the seismic lithosphere-asthenosphere boundary within normal oceanic mantle from high-resolution receiver functions

    NASA Astrophysics Data System (ADS)

    Olugboji, Tolulope Morayo; Park, Jeffrey; Karato, Shun-ichiro; Shinohara, Masanao

    2016-04-01

    Receiver function observations in the oceanic upper mantle can test causal mechanisms for the depth, sharpness, and age dependence of the seismic wave speed decrease thought to mark the lithosphere-asthenosphere boundary (LAB). We use a combination of frequency-dependent harmonic decomposition of receiver functions and synthetic forward modeling to provide new seismological constraints on this "seismic LAB" from 17 ocean-bottom stations and 2 borehole stations in the Philippine Sea and northwest Pacific Ocean. Underneath young oceanic crust, the seismic LAB depth follows the ˜1300 K isotherm but a lower isotherm (˜1000 K) is suggested in the Daito ridge, the Izu-Bonin-Mariana trench, and the northern Shikoku basin. Underneath old oceanic crust, the seismic LAB lies at a constant depth ˜70 km. The age dependence of the seismic LAB depth is consistent with either a transition to partial-melt conditions or a subsolidus rheological change as the causative factor. The age dependence of interface sharpness provides critical information to distinguish these two models. Underneath young oceanic crust, the velocity gradient is gradational, while for old oceanic crust, a sharper velocity gradient is suggested by the receiver functions. This behavior is consistent with the prediction of the subsolidus model invoking anelastic relaxation mediated by temperature and water content, but is not readily explained by a partial-melt model. The Ps conversions display negligible two-lobed or four-lobed back azimuth dependence in harmonic stacks, suggesting that a sharp change in azimuthal anisotropy with depth is not responsible for them. We conclude that these ocean-bottom observations indicate a subsolidus elastically accommodated grain-boundary sliding (EAGBS) model for the seismic LAB. Because EAGBS does not facilitate long-term ductile deformation, the seismic LAB may not coincide with the conventional transition from lithosphere to asthenosphere corresponding to a change in

  11. Examining Changes in Radioxenon Isotope Activity Ratios during Subsurface Transport

    NASA Astrophysics Data System (ADS)

    Annewandter, R.

    2013-12-01

    The Non-Proliferation Experiment (NPE) has demonstrated and modelled the usefulness of barometric pumping induced soil gas sampling during On-Site inspections. Gas transport has been widely studied with different numerical codes. However, gas transport of all radioxenons in the post-detonation regime and their possible fractionation is still neglected in the open literature. Atmospheric concentrations of the radioxenons Xe-135, Xe-133m, Xe-133 and Xe-131m can be used to discriminate between civilian releases (nuclear power plants or medical isotope facilities), and nuclear explosion sources. It is based on the isotopic activity ratio method. Yet it is not clear whether subsurface migration of the radioxenons, with eventual release into the atmosphere, can affect the activity ratios due to fractionation. Fractionation can be caused by different diffusivities due to mass differences between the radioxenons. A previous study showed surface arrival time of a chemically inert gaseous tracer is affected by its diffusivity. They observed detectable amount for SF6 50 days after detonation and 375 days for He-3. They predict 50 and 80 days for Xe-133 and Ar-37 respectively. Cyclical changes in atmospheric pressure can drive subsurface gas transport. This barometric pumping phenomenon causes an oscillatoric flow in upward trending fractures which, combined with diffusion into the porous matrix, leads to a net transport of gaseous components - a ratcheting effect. We use a general purpose reservoir simulator (Complex System Modelling Platform, CSMP++) which has been applied in a range of fields such as deep geothermal systems, three-phase black oil simulations , fracture propagation in fractured, porous media, Navier-Stokes pore-scale modelling among others. It is specifically designed to account for structurally complex geologic situation of fractured, porous media. Parabolic differential equations are solved by a continuous Galerkin finite-element method, hyperbolic

  12. Measurement and modelling of radioxenon plumes in the Ottawa Valley.

    PubMed

    Stocki, T J; Armand, P; Heinrich, Ph; Ungar, R K; D'Amours, R; Korpach, E P; Bellivier, A; Taffary, T; Malo, A; Bean, M; Hoffman, I; Jean, M

    2008-11-01

    Since 2001 a real-time radiation monitoring network of Canadian nuclear facilities and major population centres has been implemented for response to nuclear incidents including a possible terrorist attack. Unshielded NaI(Tl) spectroscopic detectors are employed to measure gamma radiation from airborne radioactivity and radioactivity deposited on the ground. These detectors are composed of a standard 3''x3'' cylindrical NaI(Tl) spectrometers with data storage and integrated telemetry. Some of the detectors have been deployed in the Ottawa Valley near Chalk River Laboratories and Ottawa, which has a complex radioxenon environment due to the proximity of nuclear power reactors, and medical isotope facilities. Although not a health threat, these releases have provided an opportunity for the Canadian Meteorological Centre and the Commissariat à l'Energie Atomique to validate their meteorological models. The meteorological models of the two organizations are in good agreement on the origin and the source terms of these releases. PMID:18799247

  13. DESIGN OF A PHOSWICH WELL DETECTOR FOR RADIOXENON MONITORING

    SciTech Connect

    Hennig, Wolfgang; Tan, Hui; Fallu-Labruyere, A; Warburton, William K.; McIntyre, Justin I.; Gleyzer, A

    2006-09-19

    The network of monitoring stations established through the Comprehensive Nuclear-Test-Ban Treaty includes systems to detect radioactive xenon released into the atmosphere from nuclear weapons testing. One such monitoring system is the Automated Radio-xenon Sampler/Analyzer (ARSA) developed at Pacific Northwest National Laboratory. For high sensitivity, the ARSA system currently uses a complex arrangement of separate beta and gamma detectors to detect beta-gamma coincidences from characteristic radioxenon isotopes in small samples of xenon extracted from large volumes of air. The coincidence measurement is very sensitive, but the large number of detectors and photomultiplier tubes requires careful calibration. A simplified approach is to use a single phoswich detector, consisting of optically coupled plastic and CsI scintillators. In the phoswich detector, most beta particles are absorbed in the plastic scintillator and most gamma rays are absorbed in the CsI, and pulse shape analysis of the detector signal is used to detect coincidences. As only a single detector and electronics readout channel is used, the complexity of the system is greatly reduced. Previous studies with a planar detector have shown that the technique can clearly separate beta only, gamma only and coincidence events, does not degrade the energy resolution, and has an error rate for detecting coincidences of less than 0.1%. In this paper, we will present the design of a phoswich well detector, consisting of a 1'' diameter plastic cell enclosed in a 3'' CsI crystal. Several variations of the well detector geometry have been studied using Monte Carlo modeling and evaluated for detection efficiency, effects on energy resolution, and ease of manufacturing. One prototype detector has been built and we will present here some preliminary experimental results characterizing the detector in terms of energy resolution and its ability to separate beta only, gamma only, and coincidence events.

  14. Application of thermodynamic modelling to natural mantle xenoliths: examples of density variations and pressure-temperature evolution of the lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Ziberna, L.; Klemme, S.

    2016-02-01

    In this paper, we show how the results of phase equilibria calculations in different mantle compositions can be reconciled with the evidence from natural mantle samples. We present data on the response of bulk rock density to pressure (P), temperature (T) and compositional changes in the lithospheric mantle and obtain constraints on the P-T evolution recorded by mantle xenoliths. To do this, we examine the mantle xenolith suite from the Quaternary alkali basalts of Pali-Aike, Patagonia, using phase equilibria calculation in six representative compositions. The calculations were done subsolidus and in volatile-free conditions. Our results show that the density change related to the spinel peridotite to garnet peridotite transition is not sharp and strongly depends on the bulk composition. In a depleted mantle composition, this transition is not reflected in the density profile, while in a fertile mantle it leads to a relative increase in density with respect to more depleted compositions. In mantle sections characterized by hot geothermal gradients (~70 mW/m2), the spinel-garnet transition may overlap with the lithosphere-asthenosphere boundary. Phase equilibria calculations in peridotitic compositions representative of the Pali-Aike mantle were also used to constrain the origin and evolution of the mantle xenoliths. Our results indicate that the mineral modes and compositions, and the mineral zonation reported for the low-temperature peridotites (spinel and spinel + garnet harzburgites and lherzolites), are linked to a cooling event in the mantle which occurred long before the eruption of the host basalts. In addition, our phase equilibria calculations show that kelyphitic rims around garnets, as those observed in the high-temperature garnet peridotites from Pali-Aike, can be explained simply by decompression and do not require additional metasomatic fluid or melt.

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

  16. Understanding radioxenon isotopical ratios originating from radiopharmaceutical facilities

    NASA Astrophysics Data System (ADS)

    Saey, P. R. J.; Ringbom, A.; Bowyer, T. W.; Becker, A.; de Geer, L.-E.; Nikkinen, M.; Payne, R. F.

    2009-04-01

    It was recently shown that radiopharmaceutical facilities (RPF) are major contributors to the general background of 133Xe and other xenon isotopes both in the northern and southern hemisphere. To distinguish a nuclear explosion signal from releases from civil nuclear facilities, not only the activity concentrations but also the ratios of the four different CTBT relevant radioxenon isotopes (131mXe, 133mXe, 133Xe and 135Xe) have to be well understood. First measurements taken recently in and around two of the world's largest RPF's: NTP at Pelindaba, South Africa and IRE at Fleurus, Belgium have been presented. At both sites, also stack samples were taken in close cooperation with the facility operators. The radioxenon in Belgium could be classified in four classes: the normal European background (133Xe activity between 0 - 5 mBq/m3) on one hand and then the samples where all four isotopes were detected with 133mXe/131mXe > 1. In northern South Africa the Pelindaba RPF is in practice the sole source of radioxenon. It generated a background of 133Xe at the measurement site some 230 km to the west of the RPF of 0 - 5 mBq/m3. In the cases where the air from the Pelindaba facility reached the measurement site directly and in a short time period, the 133Xe was higher, also 135Xe was present and in some samples 133mXe as well. The ratios of the activity concentrations of 135Xe/133Xe vs. 133mXe/131mXe (Multiple Isotope Ratio Plot - MIRC) have been analysed. For both facilities, the possible theoretical ratio's for different scenarios were calculated with the information available and compared with the measurements. It was found that there is an excess of 131mXe present in the European samples compared to theoretical calculations. A similar excess has also been seen in samples measured in northern America. In South Africa, neither the environmental samples nor the stack ones contained 131mXe at measurable levels. This can probably be explained by different processes and

  17. Field test of the PNNL Automated Radioxenon Sampler/Analyzer (ARSA)

    SciTech Connect

    Lagomarsino, R.J.; Ku, E.; Latner, N.; Sanderson, C.G.

    1998-07-01

    As part of the requirements of the Comprehensive Test Ban Treaty (CTBT), the Automated Radioxenon/Sampler Analyzer (ARSA) was designed and engineered by the Pacific Northwest National Laboratory (PNNL). The instrument is to provide near real-time detection and measurement of the radioxenons released into the atmosphere after a nuclear test. Forty-six field tests, designed to determine the performance of the ARSA prototype under simulated field conditions, were conducted at EML from March to December 1997. This final report contains detailed results of the tests with recommendations for improvements in instrument performance.

  18. Examining Changes in Radioxenon Isotope Activity Ratios during Subsurface Transport

    NASA Astrophysics Data System (ADS)

    Annewandter, Robert

    2014-05-01

    The Non-Proliferation Experiment (NPE) has demonstrated and modelled the usefulness of barometric pumping induced gas transport and subsequent soil gas sampling during On-Site inspections. Generally, gas transport has been widely studied with different numerical codes. However, gas transport of radioxenons and radioiodines in the post-detonation regime and their possible fractionation is still neglected in the open peer-reviewed literature. Atmospheric concentrations of the radioxenons Xe-135, Xe-133m, Xe-133 and Xe-131m can be used to discriminate between civilian releases (nuclear power plants or medical isotope facilities), and nuclear explosion sources. It is based on the multiple isotopic activity ratio method. Yet it is not clear whether subsurface migration of the radionuclides, with eventual release into the atmosphere, can affect the activity ratios due to fractionation. Fractionation can be caused by different mass diffusivities due to mass differences between the radionuclides. Cyclical changes in atmospheric pressure can drive subsurface gas transport. This barometric pumping phenomenon causes an oscillatoric flow in upward trending fractures or highly conductive faults which, combined with diffusion into the porous matrix, leads to a net transport of gaseous components - a so-called ratcheting effect. We use a general purpose reservoir simulator (Complex System Modelling Platform, CSMP++) which is recognized by the oil industry as leading in Discrete Fracture-Matrix (DFM) simulations. It has been applied in a range of fields such as deep geothermal systems, three-phase black oil simulations, fracture propagation in fractured, porous media, and Navier-Stokes pore-scale modelling among others. It is specifically designed to account for structurally complex geologic situation of fractured, porous media. Parabolic differential equations are solved by a continuous Galerkin finite-element method, hyperbolic differential equations by a complementary finite

  19. Validation studies for brain blood flow assessment by radioxenon tomography

    SciTech Connect

    Rezai, K.; Kirchner, P.T.; Armstrong, C.; Ehrhardt, J.C.; Heistad, D.

    1988-03-01

    A tomographic technique has been used recently for cerebral blood flow measurements with inhaled radioxenon. Based on experiments in a specially developed dynamic phantom and on studies in primates in vivo, we have analyzed the validity of this method for measurements of both regional and total blood flow in the brain. We have also examined the errors introduced into flow computations as a function of changes in such parameters as: rate of xenon input, size of region of interest, magnitude of regional flow rates, and inter-regional flow differences. Our findings indicate a reasonable degree of accuracy for flow measurements in gray matter regions that are 3 cm in diameter or larger, while white matter blood flow is generally overestimated. The accuracy for regional flow assessments degrades as a function of: diminishing region size, increasing inter-regional flow differences, and flow rates in excess of 100 ml/100 g/min. Measurements for brain regions 2 cm or smaller in diameter can be in error by 25-50% as a result of partial volume averaging. Although the technique is not ideal for accurate flow measurements in small regions of the brain, it nevertheless provides a convenient means of assessing perfusion in major vascular territories of the brain in routine clinical applications.

  20. Production of Samples of Individual Radioxenon Isotopes Through Neutron Irradiation of Stable Xenon Gas

    SciTech Connect

    Haas, Derek A.; Biegalski, Steven R.; Foltz Biegalski, Kendra M.

    2008-09-23

    The Spectral Deconvolution Analysis Tool (SDAT) software was developed to improve counting statistics and detection limits for nuclear explosion radionuclide measurements. SDAT utilizes spectral deconvolution spectroscopy techniques and can analyze both β-γ coincidence spectra for radioxenon isotopes and high-resolution HPGe spectra from aerosol monitors. The deconvolution algorithm of the SDAT requires a library of β-γ coincidence spectra of individual radioxenon isotopes to determine isotopic ratios in a sample. In order to get experimentally produced spectra of the individual isotopes we have irradiated enriched samples of 130Xe, 132Xe, and 134Xe gas with a neutron beam from the TRIGA reactor at The University of Texas. The samples produced were counted in an Automated Radioxenon Sampler/Analyzer (ARSA) style β-γ coincidence detector. The spectra produced show that this method of radioxenon production yields samples with very high purity of the individual isotopes for 131mXe and 135Xe and a sample with a substantial 133mXe to 133Xe ratio.

  1. Gain calibration of a β/γ coincidence spectrometer for automated radioxenon analysis

    NASA Astrophysics Data System (ADS)

    Reeder, P. L.; Bowyer, T. W.; McIntyre, J. I.; Pitts, W. K.; Ringbom, A.; Johansson, C.

    2004-04-01

    Detection and measurement of atmospheric radioxenon is an important component of international monitoring systems for nuclear weapons testing. Monitoring stations separate xenon from air and perform isotopic analysis of the radioxenon. In one such radioxenon measurement scheme, the isotopes of interest are identified by coincident spectroscopy of electrons and photons in a β/γ coincidence spectrometer (BGCS). The β spectrometer is a plastic scintillator, manufactured as a cylindrical cell containing the separated xenon sample. This cell is surrounded by the NaI(Tl) γ spectrometer. We report here the development of a calibration process for the BGCS suitable for use in remote sampling systems. This procedure is based upon γ-ray Compton scattering, resulting in a true coincident signal in both detectors, generation of electrons over a wide energy range that matches the energy distribution of electrons from radioxenon decay, and a relative insensitivity to source location. In addition to gain calibration, this procedure determines the resolution of the β detector as a function of energy.

  2. 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.

  3. 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.

  4. 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.

  5. Metastable Radioxenon Verification Laboratory (MRVL) Year-End Report

    SciTech Connect

    Cooper, Matthew W.; Hayes, James C.; Lidey, Lance S.

    2014-11-07

    This is the year end report that is due to the client. The MRVL system is designed to measure multiple radioxenon isotopes (135Xe, 133Xe, 133mXe and 133mXe) simultaneously. The system has 12 channels to load samples and make nuclear measurements. Although the MRVL system has demonstrated excellent stability in measurements of Xe-133 and Xe-135 over the year of evaluation prior to delivery, there has been concern about system stability over measurements performed on samples with orders of magnitude different radioactivity, and samples containing multiple isotopes. To address these concerns, a series of evaluation test have been performed at the end-user laboratory. The evaluation was performed in two separate phases. Phase 1 made measurements on isotopically pure Xe-133 from high radioactivity down to the system background levels of activity, addressing the potential count rate dependencies when activities change from extreme high to very low. The second phase performed measurements on samples containing multiple isotopes (Xe-135, Xe-133 and Xe-133m), and addressed concerns about the dependence of isotopic concentrations on the presence of additional isotopes. The MRVL showed a concentration dependence on the Xe-133 due to the amount of Xe-133m that was in the sample. The dependency is due to the decay of Xe-133m into Xe-133. This document focuses on the second phase and will address the analysis used to account for ingrowth of Xe-133 from Xe-133m.

  6. Comparison of Phoswich and ARSA-type detectors for Radioxenon Measurements

    SciTech Connect

    Ward, Rebecca; Biegalski, Steven R.; Haas, Derek A.; Hennig, Wolfgang

    2009-12-01

    The monitoring of atmospheric radioxenon to ensure compliance with the Comprehensive Nuclear Test Ban Treaty has driven the development of improved detectors for measuring xenon, including the development of a phoswich detector. This detector uses only one PMT to detect beta-gamma coincidence, thus greatly reducing the bulk and electronics of the detector in comparison to the ARSA-type detector. In this experiment, 135Xe was produced through neutron activation and a phoswich detector was used to attain spectra from the gas. These results were compared to similar results from an ARSA-type beta-gamma coincidence spectrum. The spectral characteristics and resolution were compared for the coincidence and beta spectra. Using these metrics, the overall performance of the phoswich detector for beta-gamma coincidence of radioxenon was evaluated.

  7. Nature of the lithospheric mantle beneath the Arabian Shield and genesis of Al-spinel micropods: Evidence from the mantle xenoliths of Harrat Kishb, Western Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Ahmed, Ahmed H.; Moghazi, Abdel Kader M.; Moufti, Mohamed R.; Dawood, Yehia H.; Ali, Kamal A.

    2016-01-01

    The Harrat Kishb area of western Saudi Arabia is part of the Cenozoic volcanic fields in the western margin of the Arabian Shield. Numerous fresh ultramafic xenoliths are entrained in the basanite lava of Harrat Kishb, providing an opportunity to study the nature and petrogenetic processes involved in the evolution of the lithospheric mantle beneath the Arabian Shield. Based on the petrological characteristics and mineralogical compositions, the majority of the mantle xenoliths (~ 92%) are peridotites (lherzolites and pyroxene-bearing harzburgites); the remaining xenoliths (~ 8%) are unusual spinel-rich wehrlites containing black Al-spinel micropods. The two types of mantle xenoliths display magmatic protogranular texture. The peridotite xenoliths have high bulk-rock Mg#, high forsterite (Fo90-Fo92) and NiO (0.24-0.46 wt.%) contents of olivine, high clinopyroxene Mg# (0.91-0.93), variable spinel Cr# (0.10-0.49, atomic ratio), and approximately flat chondrite-normalized REE patterns. These features indicate that the peridotite xenoliths represent residues after variable degrees of melt extraction from fertile mantle. The estimated P (9-16 kbar) and T (877-1227 °C) as well as the oxidation state (∆logfO2 = - 3.38 to - 0.22) under which these peridotite xenoliths originated are consistent with formation conditions similar to most sub-arc abyssal-type peridotites worldwide. The spinel-rich wehrlite xenoliths have an unusual amount (~ 30 vol.%) of Al-spinel as peculiar micropods with very minor Cr2O3 content (< 1 wt.%). Olivines of the spinel-rich wehrlites have low-average Fo (Fo81) and NiO (0.18 wt.%) contents, low-average cpx Mg# (0.79), high average cpx Al2O3 content (8.46 wt.%), and very low-average spinel Cr# (0.01). These features characterize early mantle cumulates from a picritic melt fraction produced by low degrees of partial melting of a garnet-bearing mantle source. The relatively high Na2O and Al2O3 contents of cpx suggest that the spinel-rich wehrlites

  8. Nature and evolution of the lithospheric mantle beneath the eastern Central Asian Orogenic Belt: Constraints from peridotite xenoliths in the central part of the Great Xing'an Range, NE China

    NASA Astrophysics Data System (ADS)

    Pan, Shaokui; Zheng, Jianping; Griffin, W. L.; Xu, Yixian; Li, Xiyao

    2015-12-01

    Our knowledge of the lithospheric mantle beneath the Central Asian Orogenic Belt is still sparse. Petrologic, major- and trace-element studies on the peridotite xenoliths from the Cenozoic volcanic fields in the Aershan area, the central part of the Great Xing'an Range, NE China, provide insights into the nature and evolution of the lithospheric mantle beneath the eastern part of the belt. According to the REE patterns of clinopyroxene, these peridotites can be divided into three groups which show clear differences in microstructure, geochemistry and equilibration temperature. Group 1 xenoliths (LREE-depleted patterns of Cpx) are lherzolites, with protogranular microstructure and high modal Cpx (8-13 wt.%), low Cr# in spinel (< 21.1), high whole-rock CaO and Al2O3 contents and estimated temperatures of 834-849 °C. Group 2 xenoliths (flattened REE patterns of Cpx) are harzburgites, with microstructures transitional between mosaic and tabular and low Cpx content (2-3 wt.%); they have high Cr# in spinel (41.1-49.6), low whole-rock CaO and Al2O3 levels and equilibration temperatures of 1183-1244 °C. Group 1 peridotites represent the newly accreted fertile mantle which was not significantly affected by post-melting enrichment; while Group 2 xenoliths may be older relics of moderately refractory mantle that underwent H2O bearing silicate-melt metasomatism (Ti/Eu > 3300, (La/Yb)N < 1 and occurrence of amphibole). Group 3 peridotites (convex-up REE patterns of Cpx) comprise both lherzolite and harzburgite; they have porphyroclastic microstructures, and show a broad range of Cpx modes (0-7 wt.%), spinel-Cr# (26.5-71.1), bulk rock CaO and Al2O3 contents and temperatures (941-1239 °C). The high TiO2 contents (up to 1.42 wt.%) in spinels of Group 3 imply the involvement of melt/rock reactions. We suggest that the upwelling of asthenospheric material played a key role in modifying the lithospheric mantle underneath the eastern Central Asian Orogenic Belt, and resulted in

  9. 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.

  10. Evaluation of environmental radioxenon isotopical signals from a singular large source emitter

    NASA Astrophysics Data System (ADS)

    Saey, P. R. J.; Bowyer, T. W.; Aldener, M.; Becker, A.; Cooper, M. W.; Elmgren, K.; Faanhof, A.; Hayes, J. C.; Hosticka, B.; Lidey, L. S.

    2009-04-01

    In the framework of the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) the atmospheric background of environmental radioxenon is been studied near areas that could be affected by man-made sources. It was recently shown that radiopharmaceutical facilities (RPF) make a major contribution to the general background of 133Xe and other xenon isotopes both in the northern and southern hemisphere. The daily IMS noble gas measurements around the globe are influenced from such anthropogenic sources that could mask radioxenon signals from a nuclear explosion. To distinguish a nuclear explosion signal from releases from civil nuclear facilities, not only the activity concentration but also the ratio of different radioxenon isotopes (131mXe, 133mXe, 133Xe and 135Xe) plays a crucial role, since the ratios can be used to discriminate source types. Theoretical release and ratio studies were recently published, but no measurements close to radiopharmaceutical facilities have ever been performed. The world's fourth largest radiopharmaceutical facility, NTP Radioisotopes Ltd, is located in Pelindaba, South Africa. Other than a small nuclear power plant, located 1300 km southwest, near Cape Town and a small research reactor in the DR of Congo, located 2700 km northwest, this is the only facility that is known to emit any radioxenon on the African continent south of the Equator. This source is likely very dominant with respect to xenon emission. This makes it a point source, which is a unique situation, as all other worldwide large radiopharmaceutical facilities are situated in regions surrounded by many other nuclear facilities. Between 10 November and 22 December 2008, radioxenon was measured continuously with a radioactive xenon measurement system, at the North-West University, Mafikeng, South Africa, which is situated 250 km northwest of Pelindaba. Fifty-six 12-hour samples were measured with a beta-gamma coincidence detector, of which 55 contained 133Xe with

  11. Radioxenon Production and Transport from an Underground Nuclear Detonation to Ground Surface

    NASA Astrophysics Data System (ADS)

    Sun, Yunwei; Carrigan, Charles R.; Hao, Yue

    2015-02-01

    Radioxenon isotopes are considered as possible indicators for detecting and discriminating underground nuclear explosions. To monitor and sample the release of radioxenon isotopes, both independent and chain-reaction yields need to be considered together with multiphase transport in geological systems from the detonation point to the ground surface. For the sake of simplicity, modeling of radioxenon isotopic radioactivities has typically been focused either on chain reactions in a batch reactor without considering multiphase transport or on radionuclide transport with simplified reactions. Although numerical methods are available for integrating coupled differential equations of complex decay networks, the stiffness of ordinary differential equations due to greatly differing decay rates may require substantial additional effort to obtain solutions for the fully coupled system. For this reason, closed-form solutions for sequential reactions and numerical solutions for multiparent converging and multidaughter branching reactions were previously developed and used to simulate xenon isotopic radioactivities in the batch reactor mode. In this paper, we develop a fully coupled numerical model, which involves tracking 24 components (i.e., 22 radionuclide components plus air and water) in two phases to enhance model predictability of simultaneously simulating xenon isotopic transport and fully coupled chain reactions. To validate the numerical model and verify the corresponding computer code, we derived closed-form solutions for first-order xenon reactions in a batch reactor mode and for single-gas phase transport coupled with the xenon reactions in a one-dimensional column. Finally, cylindrical 3-D simulations of two-phase flow within a dual permeability fracture-matrix medium, simulating the geohydrologic regime of an underground nuclear explosion, indicate the existence of both a strong temporal and spatial dependence of xenon isotopic ratios sampled at the surface. In

  12. Neural Network Based State of Health Diagnostics for an Automated Radioxenon Sampler/Analyzer

    SciTech Connect

    Keller, Paul E.; Kangas, Lars J.; Hayes, James C.; Schrom, Brian T.; Suarez, Reynold; Hubbard, Charles W.; Heimbigner, Tom R.; McIntyre, Justin I.

    2009-05-13

    Artificial neural networks (ANNs) are used to determine the state-of-health (SOH) of the Automated Radioxenon Analyzer/Sampler (ARSA). ARSA is a gas collection and analysis system used for non-proliferation monitoring in detecting radioxenon released during nuclear tests. SOH diagnostics are important for automated, unmanned sensing systems so that remote detection and identification of problems can be made without onsite staff. Both recurrent and feed-forward ANNs are presented. The recurrent ANN is trained to predict sensor values based on current valve states, which control air flow, so that with only valve states the normal SOH sensor values can be predicted. Deviation between modeled value and actual is an indication of a potential problem. The feed-forward ANN acts as a nonlinear version of principal components analysis (PCA) and is trained to replicate the normal SOH sensor values. Because of ARSA’s complexity, this nonlinear PCA is better able to capture the relationships among the sensors than standard linear PCA and is applicable to both sensor validation and recognizing off-normal operating conditions. Both models provide valuable information to detect impending malfunctions before they occur to avoid unscheduled shutdown. Finally, the ability of ANN methods to predict the system state is presented.

  13. Neural-network-based state of health diagnostics for an automated radioxenon sampler/analyzer

    NASA Astrophysics Data System (ADS)

    Keller, Paul E.; Kangas, Lars J.; Hayes, James C.; Schrom, Brian T.; Suarez, Reynold; Hubbard, Charles W.; Heimbigner, Tom R.; McIntyre, Justin I.

    2009-05-01

    Artificial neural networks (ANNs) are used to determine the state-of-health (SOH) of the Automated Radioxenon Analyzer/Sampler (ARSA). ARSA is a gas collection and analysis system used for non-proliferation monitoring in detecting radioxenon released during nuclear tests. SOH diagnostics are important for automated, unmanned sensing systems so that remote detection and identification of problems can be made without onsite staff. Both recurrent and feed-forward ANNs are presented. The recurrent ANN is trained to predict sensor values based on current valve states, which control air flow, so that with only valve states the normal SOH sensor values can be predicted. Deviation between modeled value and actual is an indication of a potential problem. The feed-forward ANN acts as a nonlinear version of principal components analysis (PCA) and is trained to replicate the normal SOH sensor values. Because of ARSA's complexity, this nonlinear PCA is better able to capture the relationships among the sensors than standard linear PCA and is applicable to both sensor validation and recognizing off-normal operating conditions. Both models provide valuable information to detect impending malfunctions before they occur to avoid unscheduled shutdown. Finally, the ability of ANN methods to predict the system state is presented.

  14. 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.

  15. 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.

  16. 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

  17. 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.

  18. 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

  19. 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.

  20. 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.

  1. 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.

  2. 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.

  3. Subsurface mass transport affects the radioxenon signatures that are used to identify clandestine nuclear tests

    NASA Astrophysics Data System (ADS)

    Deinert, M. R.

    2012-12-01

    Underground nuclear tests produce anthropogenic isotopes that provide the only definitive means by which to determine whether a nuclear explosion has taken place. Verification of a suspected test under the Comprehensive Nuclear-Test-Ban Treaty often relies on ratios of radioxenon isotopes. Gas samples are gathered either on-site or off-site with certain ranges of xenon isotope ratios considered to be a signature of a weapons test. It is well established that below ground transport can affect the rate at which Noble gasses will reach the surface. However, the relative abundance of anthropogenic isotopes is has long been assumed to rely solely on fission yield and decay rate. By including in subsurface transport models the effects of mass dependent diffusion, and a time dependent source term for the decay of radioiodine precursors, we show here that this assumption is not true. In fact, certain combinations of geology and atmospheric conditions can alter xenon isotope ratios sufficiently for a weapons test going unconfirmed under the current standards.

  4. 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).

  5. 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

  6. 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.

  7. 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.

  8. Measurements of radioxenon in ground level air in South Korea following the claimed nuclear test in North Korea on October 9, 2006

    SciTech Connect

    Ringbom, Anders; Elmgren, K.; Lindh, Karin; Peterson, Jenny; Bowyer, Ted W.; Hayes, James C.; McIntyre, Justin I.; Panisko, Mark E.; Williams, Richard M.

    2009-12-03

    Abstract Following the claimed nuclear test in the Democratic People’s Republic of Korea (DPRK) on October 9, 2006, and a reported seismic event, a mobile system for sampling of atmospheric xenon was transported to the Republic of South Korea (ROK) in an attempt to detect possible emissions of radioxenon in the region from a presumed test. Five samples were collected in the ROK during October 11–14, 2006 near the ROK–DPRK border, and thereafter transported to the Swedish Defense Research Agency (FOI) in Stockholm, Sweden, for analysis. Following the initial measurements, an automatic radioxenon sampling and analysis system was installed at the same location in the ROK, and measurements on the ambient atmospheric radioxenon background in the region were performed during November 2006 to February 2007. The measured radioxenon concentrations strongly indicate that the explosion in October 9, 2006 was a nuclear test. The conclusion is further strengthened by atmospheric transport models. Radioactive xenon measurement was the only independent confirmation that the supposed test was in fact a nuclear explosion and not a conventional (chemical) explosive.

  9. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    energy required through filtering of the geoid. We evaluate the role of revised ridge push forces with plate-scale stress models. Our revised estimate of the ridge push force based on the lithospheric geoid, and corresponding stress models, further emphasizes the "active" rather than "passive" nature of the plates themselves in plate tectonics.

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

    NASA Astrophysics Data System (ADS)

    Houseman, Gregory A.

    2015-04-01

    relationship between faulting and earthquake activity and the continuous deformation field below the seismogenic layer continues to be poorly understood. Prominent surface faults may be a natural consequence of the localization of strain caused by processes within the ductile layer such as shear heating, grain-size reduction, or simply the interaction of non-Newtonian constitutive law and irregular geometry. Where intra-plate convergence occurs the lithosphere must thicken, and the question naturally arises as to whether the thickened lithosphere will remain stable or somehow be removed by convective overturn with underlying asthenophere. Such overturn is expected of a viscous lithospheric layer that is denser than the asthenosphere; it will be denser because it is colder, unless there is some compositional contrast which makes it intrinsically buoyant. A relatively low viscosity is required, however, in order that the instability can grow at a sufficiently fast rate to overcome diffusive stabilisation of the temperature field. The high stresses created by plate convergence may provide the mechanism that activates the viscosity (and explains why the lithosphere elsewhere is generally stable). High-resolution tomographic investigations find convincing evidence of small-scale mantle drips occurring beneath recently active orogenic zones such as the western USA and the SE Carpathians. However, seismic observations of thickened lithosphere remaining beneath Tibet apparently contradict the interpretation of mantle overturn suggested by recent volcanism and uplift. Although the Tibetan mantle lithosphere may be relatively buoyant, the possibility that this layer has overturned internally may allow these conflicting interpretations to be reconciled.

  12. 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.

  13. 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.

  14. International challenge to predict the impact of radioxenon releases from medical isotope production on a comprehensive nuclear test ban treaty sampling station.

    PubMed

    Eslinger, Paul W; Bowyer, Ted W; Achim, Pascal; Chai, Tianfeng; Deconninck, Benoit; Freeman, Katie; Generoso, Sylvia; Hayes, Philip; Heidmann, Verena; Hoffman, Ian; Kijima, Yuichi; Krysta, Monika; Malo, Alain; Maurer, Christian; Ngan, Fantine; Robins, Peter; Ross, J Ole; Saunier, Olivier; Schlosser, Clemens; Schöppner, Michael; Schrom, Brian T; Seibert, Petra; Stein, Ariel F; Ungar, Kurt; Yi, Jing

    2016-06-01

    The International Monitoring System (IMS) is part of the verification regime for the Comprehensive Nuclear-Test-Ban-Treaty Organization (CTBTO). At entry-into-force, half of the 80 radionuclide stations will be able to measure concentrations of several radioactive xenon isotopes produced in nuclear explosions, and then the full network may be populated with xenon monitoring afterward. An understanding of natural and man-made radionuclide backgrounds can be used in accordance with the provisions of the treaty (such as event screening criteria in Annex 2 to the Protocol of the Treaty) for the effective implementation of the verification regime. Fission-based production of (99)Mo for medical purposes also generates nuisance radioxenon isotopes that are usually vented to the atmosphere. One of the ways to account for the effect emissions from medical isotope production has on radionuclide samples from the IMS is to use stack monitoring data, if they are available, and atmospheric transport modeling. Recently, individuals from seven nations participated in a challenge exercise that used atmospheric transport modeling to predict the time-history of (133)Xe concentration measurements at the IMS radionuclide station in Germany using stack monitoring data from a medical isotope production facility in Belgium. Participants received only stack monitoring data and used the atmospheric transport model and meteorological data of their choice. Some of the models predicted the highest measured concentrations quite well. A model comparison rank and ensemble analysis suggests that combining multiple models may provide more accurate predicted concentrations than any single model. None of the submissions based only on the stack monitoring data predicted the small measured concentrations very well. Modeling of sources by other nuclear facilities with smaller releases than medical isotope production facilities may be important in understanding how to discriminate those releases from

  15. 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.

  16. 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).

  17. 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.

  18. 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).

  19. Remarkable isotopic and trace element trends in potassic through sodic Cretaceous plutons of the Yukon-Koyukuk Basin, Alaska, and the nature of the lithosphere beneath the Koyukuk terrane

    USGS Publications Warehouse

    Arth, Joseph G.; Criss, Robert E.; Zmuda, Clara C.; Foley, Nora K.; Patton, W.W., Jr.; Miller, T.P.

    1989-01-01

    During the period from 110 to 80 m.y. ago, a 450-km-long magmatic belt was active along the northern margin of Yukon-Koyukuk basin and on eastern Seward Peninsula. The plutons intruded Upper Jurassic(?) and Lower Cretaceous volcanic arc rocks and Cretaceous sedimentary rocks in Yukon-Koyukuk basin and Proterozoic and lower Paleozoic continental rocks in Seward Peninsula. Within Yukon-Koyukuk basin, the plutons vary in composition from calc-alkalic plutons on the east to potassic and ultrapotassic alkalic plutons on the west. Plutons within Yukon-Koyukuk basin were analyzed for trace element and isotopic compositions in order to discern their origin and the nature of the underling lithosphere. Farthest to the east, the calc-alkalic rocks of Indian Mountain pluton are largely tonalite and sodic granodiorite, and have low Rb (average 82 ppm), high Sr (>600 ppm), high chondrite-normalized (cn) Ce/Yb (16–37), low δ18O (+6.5 to +7.1), low initial 87Sr/86Sr (SIR) (0.704), and high initial 143Nd/144Nd (NIR) (0.5126). These rocks resemble those modelled elsewhere as partial melts and subsequent fractionates of basaltic or gabbroic metaigneous rocks, and may be products of melting in the deeper parts of the Late Jurassic(?) and Early Cretaceous volcanic arc. Farthest to the west, the two ultrapotassic bodies of Selawik and Inland Lake are high in Cs (up to 93 ppm), Rb (up to 997 ppm), Sr, Ba, Th, and light rare earth elements, have high (Ce/Yb)cn (30, 27), moderate to low δ18O (+8.4, +6.9), high SIR (0.712, 0.710), and moderate NIR (0.5121–0.5122). These rocks resemble rocks of Australia and elsewhere that were modelled as melts of continental mantle that had been previously enriched in large cations. This mantle may be Paleozoic or older. The farthest west alkalic pluton of Selawik Hills is largely monzonite, quartz monzonite, and granite; has moderate Rb (average 284 ppm), high Sr (>600 ppm), high (Ce/Yb)cn (15–25), moderate δ18O (+8.3 to +8.6), high SIR (0.708

  20. 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.

  1. 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

  2. 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

  3. 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

  4. 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

  5. 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.

  6. 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.

  7. 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

  8. 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.

  9. A sharp lithosphere-asthenosphere boundary imaged beneath eastern North America.

    PubMed

    Rychert, Catherine A; Fischer, Karen M; Rondenay, Stéphane

    2005-07-28

    Plate tectonic theory hinges on the concept of a relatively rigid lithosphere moving over a weaker asthenosphere, yet the nature of the lithosphere-asthenosphere boundary remains poorly understood. The gradient in seismic velocity that occurs at this boundary is central to constraining the physical and chemical properties that create differences in mechanical strength between the two layers. For example, if the lithosphere is simply a thermal boundary layer that is more rigid owing to colder temperatures, mantle flow models indicate that the velocity gradient at its base would occur over tens of kilometres. In contrast, if the asthenosphere is weak owing to volatile enrichment or the presence of partial melt, the lithosphere-asthenosphere boundary could occur over a much smaller depth range. Here we use converted seismic phases in eastern North America to image a very sharp seismic velocity gradient at the base of the lithosphere-a 3-11 per cent drop in shear-wave velocity over a depth range of 11 km or less at 90-110 km depth. Such a strong, sharp boundary cannot be reconciled with a purely thermal gradient, but could be explained by an asthenosphere that contains a few per cent partial melt or that is enriched in volatiles relative to the lithosphere. PMID:16049485

  10. 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.

  11. 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.

  12. 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

  13. Seismic evidence for the layered mantle lithosphere: a comparsion between Zagros and South Africa

    NASA Astrophysics Data System (ADS)

    Sodoudi, Forough; Kind, Rainer

    2014-05-01

    Recent S receiver function studies present evidence for the existence of the layered mantle lithosphere beneath ancient cratons. However, the nature of these layers is still unclear. They can be attributed to the presence of accumulated melts, remnants of subduction interfaces, changes in anisotropic properties or fluids. Further characterization of these layers is needed to provide more insights into the assembly and evolution of cratons. Here we compare the mantle lithosphere of the ancient Kalahari craton with the relatively young mantle lithosphere of Zagros, which is assumed as the location of the future craton. We applied the S receiver function method to map the internal layering of the lithosphere and to image its lower limit. For this aim, we used teleseismic events recorded at 97 seismic stations within the Kalahari craton and those recorded at 61 permanent seismic stations in Iran. Our results reveal a thick and stratified mantle lithosphere beneath the Kalahari craton containing three significant negative velocity contrasts at 85, 150-200, and 260-280 km depth. Moreover, they imply that frozen-in anisotropy as well as notable compositional variations can lead to sharp Mid-Lithospheric Discontinuities (MLD) that can be clearly observed in the SRF data. We show that a 50 km thick anisotropic layer just below the Moho boundary with 3% S wave anisotropy may be responsible for producing a MLD at 85 km depth. The horizontal anisotropy in the upper lithosphere may be attributed to processes during the formation of the Kalahari Craton. Furthermore, significant correlation between the depths of an apparent boundary separating the depleted and metasomatised lithosphere, as inferred from chemical tomography, and those of our second layer led us to characterize it as a compositional boundary, most likely due to the modification of the cratonic mantle lithosphere by magma infiltration. The largest velocity contrast (3.6-4.7%) is observed at a boundary located at

  14. 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.

  15. 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

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

    NASA Astrophysics Data System (ADS)

    Wittig, Nadine

    2014-05-01

    of Re-Os isotope dating of cratonic peridotites. Empiric mineralogical and geochemical data of continental and oceanic lithospheric mantle has been examined extensively and metasomatism has been studied in great detail. I will present a numerical modelling approach generating a comprehensive catalogue of variously depleted plagioclase-, spinel- and garnet-peridotite major and trace element compositions. In addition primary Pb, Sr, Nd, Hf and Os isotope data will approximate refractory mantle generated during Earth's major episodes of depletion and continental crust formation (1.2, 1.8, 2.9, 3.8 Ga). These hypothetical compositions will be compared to natural peridotites from on- and off-cratonic xenoliths, abyssal and orogenic peridotites to identify those rare samples least altered by interaction with silicate, hydrous and carbonatitic melts. Extremely depleted mantle has the potential to harbour Pb, Sr, Nd, Hf and Os isotope compositions that would be easily recognized if silicate melts were generated from this type of pristine mantle and the record of volcanic rocks will be examined to identify potential lithospheric melts.

  17. 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.

  18. 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

  19. Continental growth by successive accretion of oceanic lithosphere: Evidence from tilted seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Babuska, V.; Plomerova, J.; Karato, S. I.

    2012-04-01

    Although many studies indicate that subduction-related accretion, subduction-driven magmatism and tectonic stacking are major crustal-growth mechanisms, how the mantle lithosphere forms remains enigmatic. Cook (AGU Geod. Series 1986) published a model of continental 'shingling' based on seismic reflection data indicating dipping structures in the deep crust of accreted terranes. Helmstaedt and Gurney (J. Geoch. Explor. 1995) and Hart et al. (Geology 1997) suggest that the Archean continental lithosphere consists of alternating layers of basalt and peridotite derived from subducted and obducted Archean oceanic lithosphere. Peridotite xenoliths from the Mojavian mantle lithosphere (Luffi et al., JGR 2009), as well as xenoliths of eclogites underlying the Sierra Nevada batholith in California (Horodynskij et al., EPSL 2007), are representative for oceanic slab fragments successively attached to the continent. Recent seismological findings also 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. Aust. 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 almost a half century ago (Hess, Nature 1964). Though it is difficult to determine seismic anisotropy within an active subducting slab (e.g., Healy et al., EPSL 2009; Eberhart-Phillips and Reyners, JGR 2009), field observations and laboratory experiments indicate the oceanic olivine fabric might be preserved there to a depth of at least 200-300 km. Dipping anisotropic fabrics in domains of the European mantle lithosphere were interpreted as systems of 'frozen' paleosubductions (Babuska and Plomerova, PEPI 2006), and the lithosphere base as a boundary between a fossil anisotropy in the

  20. 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.

  1. 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.

  2. 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.

  3. 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

  4. 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.

  5. Estimates of Radioxenon Released from Southern Hemisphere Medical isotope Production Facilities Using Measured Air Concentrations and Atmospheric Transport Modeling

    SciTech Connect

    Eslinger, Paul W.; Friese, Judah I.; Lowrey, Justin D.; McIntyre, Justin I.; Miley, Harry S.; Schrom, Brian T.

    2014-09-01

    Abstract The International Monitoring System (IMS) of the Comprehensive-Nuclear-Test-Ban-Treaty monitors the atmosphere for radioactive xenon leaking from underground nuclear explosions. Emissions from medical isotope production represent a challenging background signal when determining whether measured radioxenon in the atmosphere is associated with a nuclear explosion prohibited by the treaty. The Australian Nuclear Science and Technology Organisation (ANSTO) operates a reactor and medical isotope production facility in Lucas Heights, Australia. This study uses two years of release data from the ANSTO medical isotope production facility and Xe-133 data from three IMS sampling locations to estimate the annual releases of Xe-133 from medical isotope production facilities in Argentina, South Africa, and Indonesia. Atmospheric dilution factors derived from a global atmospheric transport model were used in an optimization scheme to estimate annual release values by facility. The annual releases of about 6.8×1014 Bq from the ANSTO medical isotope production facility are in good agreement with the sampled concentrations at these three IMS sampling locations. Annual release estimates for the facility in South Africa vary from 1.2×1016 to 2.5×1016 Bq and estimates for the facility in Indonesia vary from 6.1×1013 to 3.6×1014 Bq. Although some releases from the facility in Argentina may reach these IMS sampling locations, the solution to the objective function is insensitive to the magnitude of those releases.

  6. Inelastic models of lithospheric stress - I. Theory and application to outer-rise plate deformation

    USGS Publications Warehouse

    Mueller, S.; Choy, G.L.; Spence, W.

    1996-01-01

    Outer-rise stress distributions determined in the manner that mechanical engineers evaluate inelastic stress distributions within conventional materials are contrasted with those predicted using simple elastic-plate models that are frequently encountered in studies of outer-rise seismicity. This comparison indicates that the latter are inherently inappropriate for studies of intraplate earthquakes, which are a direct manifestation of lithospheric inelasticity. We demonstrate that the common practice of truncating elastically superimposed stress profiles so that they are not permitted to exceed laboratory-based estimates of lithospheric yield strength will result in an accurate characterization of lithospheric stress only under relatively restrictive circumstances. In contrast to elastic-plate models, which predict that lithospheric stress distributions depend exclusively upon the current load, inelastic plate models predict that stress distributions are also significantly influenced by the plate-loading history, and, in many cases, this influence is the dominant factor in determining the style of potential seismicity (e.g. thrust versus normal faulting). Numerous 'intuitive' interpretations of outer-rise earthquakes have been founded upon the implicit assumption that a unique relationship exists between a specified combination of plate curvature and in-plane force, and the resulting lithospheric stress distribution. We demonstrate that the profound influence of deformation history often invalidates such interpretations. Finally, we examine the reliability of 'yield envelope' representations of lithospheric strength that are constructed on the basis of empirically determined frictional sliding relationships and silicate plastic-flow laws. Although representations of this nature underestimate the strength of some major interplate faults, such as the San Andreas, they appear to represent a reliable characterization of the strength of intraplate oceanic lithosphere.

  7. Can compaction, caused by melt extraction and intrusion, generate tectonically effective stresses in the lithosphere?

    NASA Astrophysics Data System (ADS)

    Wallner, Herbert; Schmeling, Harro

    2016-04-01

    Aim of our study is to deepen understanding the role of melt processes while the lithospheric evolution by means of numerical modeling. In the sense of plate tectonics, on the one hand, stresses are transferred by stiff lithospheric plates, on the other, lithosphere is deformed, broken, or modified in various ways. Melting often plays an important role but is not easy to model numerically due to all the interactions of physics, phase changes, non-linearities, time scales, petrology, heterogeneities and chemical reactions. Here we restrict on a thermo-mechanical model of visco-plastic two phase flow with partial melting. Viscosity is temperature-, stress- and depth-dependent. Freezing and melting are determined by a simplified linear binary solid solution model. The fast melt transport through and into the lithosphere, acting on a short time scale, is replaced by melt extraction and intrusion in a given emplacement level. Numerical approximation is done in 2D with Finite Differences with markers in an Eulerian formulation. A scenario of continental rifting serves for a model of lithosphere above asthenosphere under extensional conditions. An anomaly of increased temperature at the bottom produces a low fraction of melt initially in the asthenosphere. Above a porosity limit melt is extracted and leads to compaction at its origin which induces under-pressure attracting ambient melt and contracting the depleted matrix. In a higher, colder lithospheric level the emplaced melt extends the matrix, immediately freezes; an increase of enrichment and heating takes place. The dilatation of the rock matrix generates relative high compaction pressures if it's viscosity is high as in the uppermost mantle lithosphere. Local and temporary varying stresses provide deviatoric components which sometimes may be the origin of tectonic activity in nature. Divergence terms of the full compaction formulation, responsible for viscous stress, are tested and reviewed. Quality and stability

  8. Application of Artificial Neural Network Modeling to the Analysis of the Automated Radioxenon Sampler-Analyzer State Of Health Sensors OF HEALTH SENSORS

    SciTech Connect

    Hayes, James C.; Doctor, Pam G.; Heimbigner, Tom R.; Hubbard, Charles W.; Kangas, Lars J.; Keller, Paul E.; McIntyre, Justin I.; Schrom, Brian T.; Suarez, Reynold

    2006-09-19

    The Automated Radioxenon Analyzer/Sampler (ARSA) is a radioxenon gas collection and analysis system operating autonomously under computer control. The ARSA systems are deployed as part of an international network of sensors, with individual stations feeding radioxenon concentration data to a central data center. Because the ARSA instrument is complex and is often deployed in remote areas, it requires constant self-monitoring to verify that it is operating according to specifications. System performance monitoring is accomplished by over 200 internal sensors, with some values reported to the data center. Several sensors are designated as safety sensors that can automatically shut down the ARSA when unsafe conditions arise. In this case, the data center is advised of the shutdown and the cause, so that repairs may be initiated. The other sensors, called state of health (SOH) sensors, also provide valuable information on the functioning of the ARSA and are particularly useful for detecting impending malfunctions before they occur to avoid unscheduled shutdowns. Any of the sensor readings can be displayed by an ARSA Data Viewer, but interpretation of the data is difficult without specialized technical knowledge not routinely available at the data center. Therefore it would be advantageous to have sensor data automatically evaluated for the precursors of malfunctions and the results transmitted to the data center. Artificial Neural Networks (ANN) are a class of data analysis methods that have shown wide application to monitoring systems with large numbers of information inputs, such as the ARSA. In this work supervised and unsupervised ANN methods were applied to ARSA SOH data recording during normal operation of the instrument, and the ability of ANN methods to predict system state is presented.

  9. 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

  10. 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.

  11. 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

  12. 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.

  13. Assessing thermo-mechanical properties of the lithospheric mantle in Asia

    NASA Astrophysics Data System (ADS)

    Stolk, W.; Kaban, M. K.; Beekman, F.; Tesauro, M.; Cloetingh, S.

    2012-12-01

    Asia is a key natural laboratory for the study of active intra-continental deformation in response to the ongoing far-field collision of India and Eurasia. The resulting tectonic processes strongly depend on the thermo-mechanical structure of the lithosphere. However, the problem of the thermo-mechanical properties of the lithospheric mantle is complex and still not well resolved. While seismic studies give an indication of the heterogeneity of the mantle lithosphere it alone is insufficient to attribute these anomalies to thermal differences, since compositional difference may have a significant effect on observed wave velocities. Using solely gravity field analysis one cannot distinguish between e.g. stacked density anomalies or lateral density anomalies. Combining both datasets allows for a better insight into the mantle lithosphere, though the solution to the problem at hand remains non-unique. This study, of which the preliminary results will be presented here, attempts to gain insight into both compositional and thermal aspects of the mantle lithosphere in Asia. By combining a recent high resolution tomographic inversion with gravity field data, but without the assumption of a steady state mantle, a trade off between compositional and thermal effects can be made. Furthermore, susceptibility of the resulting model to small changes in parameter space can be obtained thus creating a 'playing field' for possible solutions to the thermo-mechanical problem. This 'playing field' can be further constrained by additional data from other sources, such as xenolith studies.

  14. The prediction of oceanic lithospheric magnetic anomalies from magnetisation estimates, using vector spherical harmonics

    NASA Astrophysics Data System (ADS)

    Masterton, S.; Gubbins, D.; Ivers, D.; Müller, D.; Winch, D.

    2009-12-01

    High resolution lithospheric magnetic field anomaly maps derived from satellite data now offer immense opportunities to interpret crustal magnetic properties such as susceptibility, depth to Curie isotherm, magnetisation type and intensity. We present a method in which a vector spherical harmonic formulation allows the natural separation of 3 types of lithospheric magnetisation: one responsible for the observed potential field external to the crust, one responsible for the field inside the Earth that is not observed, and a toroidal magnetisation associated with a radial electric current responsible for a non-potential field. The latter two constitute the annihilator in the inverse problem for magnetisation using magnetic field data. Starting from a model of vertically integrated lithospheric magnetisation based on geology, we compute all 3 types of magnetisation and discuss implications of the 2 annihilators for inversion studies. We adopt a forward-modelling approach in which lithospheric magnetisation is estimated independently of satellite data, with particular emphasis on the oceans. Induced and remanent contributions are determined separately. Remanent magnetisation is derived from a combination of magnetic crustal thickness, a remanence intensity-age profile superimposed onto a geomagnetic polarity timescale and a digital age grid of the ocean floor, and magnetisation directions derived from the implementation of updated plate reconstruction models. Induced magnetisation is derived from a combination of magnetic crustal thickness and standard magnetic susceptibilities associated with major geological units. We present comparisons between magnetic anomalies predicted from magnetisation estimates and lithospheric magnetic field models.

  15. 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

  16. 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.

  17. Erosion of the continental lithosphere at the cusps of the Calabrian arc: Evidence from S receiver functions analysis

    NASA Astrophysics Data System (ADS)

    Miller, Meghan S.; Piana Agostinetti, Nicola

    2011-12-01

    Mediterranean tectonics has been characterized by an irregular, complex temporal evolution with episodic rollback and retreat of the subducted plate followed by period of slow trench-migration. To provide insight into the geodynamics of the Calabrian arc, we image the characteristics and lithospheric structure of the convergent, Apulian and Hyblean forelands at the cusps of the arc. Specifically we investigate the crustal and lithospheric thicknesses using teleseismic S-to-p converted phases, applied to the Adria-Africa plate margin for the first time. We find that the Moho in the Apulian foreland is nearly flat at ˜30 km depth, consistent with previous P receiver functions results, and that the Hyblean crustal thickness is more complex, which can be understood in terms of the nature of the individual pieces of carbonate platform and pelagic sediments that make up the Hyblean platform. The lithospheric thicknesses range between 70-120 km beneath Apulia and 70-90 km beneath Sicily. The lithosphere of the forelands at each end of the Calabrian arc are continental in nature, buoyant compared to the subducting oceanic lithosphere and have previously been interpreted as mostly undeformed carbonate platforms. Our receiver function images also show evidence of lithospheric erosion and thinning close to Mt. Etna and Mt. Vulture, two volcanoes which have been associated with asthenospheric upwelling and mantle flow around of the sides the slab. We suggest that as the continental lithosphere resists being subducted it is being thermo-mechanically modified by toroidal flow around the edges of the subducting oceanic lithosphere of the Calabrian arc.

  18. 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

  19. Thermal erosion of cratonic lithosphere as a potential trigger for mass-extinction

    NASA Astrophysics Data System (ADS)

    Pilet, Sebastien; Guex, Jean; Muntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Schaltegger, Urs

    2016-04-01

    studies of the composition of the Kaapvaal craton have shown that sulfide minerals are enclosed in the basal part of the cratonic lithosphere. The formation of these sulfide minerals are linked to multiple refertilization/metasomatic events, which affected the base of the subcontinental lithospheric mantle from the Archean to the Proterozoic. We suggest that the transitions from an initial cool period to greenhouse conditions recorded by T-J and Pl-To sedimentary sections result of changing gas species emitted during the progressive thermal erosion of cratonic lithosphere by plume activity or thermal internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere causing global cooling and eustatic regression, which was followed by warming/transgression associated with the progressive increase of CO2 in the atmosphere associated to LIPs emission and metamorphic reactions in sedimentary basins. We suggest that the nature of the underlying lithosphere during large LIP eruption potentially exerts an important control on the consequences at the Earth's surface. This model offers an explanation for why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere.

  20. Geodynamic inversion to constrain the non-linear rheology of the lithosphere

    NASA Astrophysics Data System (ADS)

    Baumann, T. S.; Kaus, Boris J. P.

    2015-08-01

    One of the main methods to determine the strength of the lithosphere is by estimating it's effective elastic thickness. This method assumes that the lithosphere is a thin elastic plate that floats on the mantle and uses both topography and gravity anomalies to estimate the plate thickness. Whereas this seems to work well for oceanic plates, it has given controversial results in continental collision zones. For most of these locations, additional geophysical data sets such as receiver functions and seismic tomography exist that constrain the geometry of the lithosphere and often show that it is rather complex. Yet, lithospheric geometry by itself is insufficient to understand the dynamics of the lithosphere as this also requires knowledge of the rheology of the lithosphere. Laboratory experiments suggest that rocks deform in a viscous manner if temperatures are high and stresses low, or in a plastic/brittle manner if the yield stress is exceeded. Yet, the experimental results show significant variability between various rock types and there are large uncertainties in extrapolating laboratory values to nature, which leaves room for speculation. An independent method is thus required to better understand the rheology and dynamics of the lithosphere in collision zones. The goal of this paper is to discuss such an approach. Our method relies on performing numerical thermomechanical forward models of the present-day lithosphere with an initial geometry that is constructed from geophysical data sets. We employ experimentally determined creep-laws for the various parts of the lithosphere, but assume that the parameters of these creep-laws as well as the temperature structure of the lithosphere are uncertain. This is used as a priori information to formulate a Bayesian inverse problem that employs topography, gravity, horizontal and vertical surface velocities to invert for the unknown material parameters and temperature structure. In order to test the general methodology

  1. A Multi-Layer Phoswich Radioxenon Detection System, Reporting Period 07/01/07 - 09/30/07

    SciTech Connect

    David M. Hamby

    2007-10-25

    previous simulations, to confirm convergence of the solutions provided by our software methods. We have initiated our investigation into the radon signature expected in our XEPHWICH system. We intend to utilize this signature to confirm earth movement, in the event of an underground nuclear explosion, by continuously monitoring radon levels and noting increases in radon concentration in conjunction with increased levels of radioxenons. The research group is also designing and constructing a fission chamber to be used for the collection of radioxenon gases following neutron bombardment of HEU in the Oregon State University TRIGA reactor. To this point, we have completed milling the aluminum housing and have modeled fission product nuclide production associated with the fissioning of HEU. Additionally, the students have been busy compiling the appropriate information in preparation for irradiation approvals. Using beta spectra of three initial nuclides collected on the prototype phoswich detector, spectral identification by a preliminary neural network was compared to that of solvers of a linear system of equations. Pre-processing in areas such as smoothing and endpoint identification is also being investigated as a means of improving spectral identification.

  2. 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.

  3. 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.

  4. 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.

  5. The lithosphere-asthenosphere system beneath Ireland from integrated geophysical-petrological modeling II: 3D thermal and compositional structure

    NASA Astrophysics Data System (ADS)

    Fullea, J.; Muller, M. R.; Jones, A. G.; Afonso, J. C.

    2014-02-01

    The lithosphere-asthenosphere boundary (LAB) depth represents a fundamental parameter in any quantitative lithospheric model, controlling to a large extent the temperature distribution within the crust and the uppermost mantle. The tectonic history of Ireland includes early Paleozoic closure of the Iapetus Ocean across the Iapetus Suture Zone (ISZ), and in northeastern Ireland late Paleozoic to early Mesozoic crustal extension, during which thick Permo-Triassic sedimentary successions were deposited, followed by early Cenozoic extrusion of large scale flood basalts. Although the crustal structure in Ireland and neighboring offshore areas is fairly well constrained, with the notable exception of the crust beneath Northern Ireland, the Irish uppermost mantle remains to date relatively unknown. In particular, the nature and extent of a hypothetical interaction between a putative proto Icelandic mantle plume and the Irish and Scottish lithosphere during the Tertiary opening of the North Atlantic has long been discussed in the literature with diverging conclusions. In this work, the present-day thermal and compositional structure of the lithosphere in Ireland is modeled based on a geophysical-petrological approach (LitMod3D) that combines comprehensively a large variety of data (namely elevation, surface heat flow, potential fields, xenoliths and seismic tomography models), reducing the inherent uncertainties and trade-offs associated with classical modeling of those individual data sets. The preferred 3D lithospheric models show moderate lateral density variations in Ireland characterized by a slightly thickened lithosphere along the SW-NE trending ISZ, and a progressive lithospheric thinning from southern Ireland towards the north. The mantle composition in the southern half of Ireland (East Avalonia) is relatively and uniformly fertile (i.e., typical Phanerozoic mantle), whereas the lithospheric composition in the northern half of Ireland (Laurentia) seems to vary

  6. 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.

  7. 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.

  8. 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.

  9. Sub-lithospheric small scale convection - a process for continental collision magmatism

    NASA Astrophysics Data System (ADS)

    Kaislaniemi, Lars; van Hunen, Jeroen; Allen, Mark; Neill, Iain

    2014-05-01

    lithosphere, and advection of hot material into contact with the cold lithosphere. The degree of melting might be strongly controlled by the lithosphere thickness and the extent of its hydration during the past Tethyan subduction. Mantle melting leaves behind depleted, more viscous and more buoyant layers at the lithosphere-asthenosphere boundary which may take millions of years before sinking back to deeper mantle. Average volcanic layer thicknesses of hundreds of meters can be reached via the SSC process, corresponding to observations from the Turkish-Iranian plateau. By its random nature, SSC can explain why the continental collision magmatism on the Turkish-Iranian plateau does not seem to have clearly recognisable spatial or temporal patterns. The potential of the SSC to effectively mix the asthenosphere-lithosphere close to their boundary appears to offer an explanation for the geochemical heterogeneity of the observed volcanism. SSC may be related to whole mantle delamination (offering zones of weaknesses for its initiation) or slab break-off (SSC being enhanced by the break-off), but neither is a prerequisite for SSC magmatism.

  10. 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.

  11. 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.

  12. Metasomatic Enrichment of Oceanic Lithospheric Mantle Documented by Petit-Spot Xenoliths

    NASA Astrophysics Data System (ADS)

    Pilet, S.; Abe, N.; Rochat, L.; Hirano, N.; Machida, S.; Kaczmarek, M. A.; Muntener, O.

    2015-12-01

    Oceanic lithosphere is generally interpreted as mantle residue after MORB extraction. It has been proposed, however, that metasomatism could take place at the interface between the low-velocity zone and the cooling and thickening oceanic lithosphere or by the percolation of low-degree melts produced in periphery of Mid Ocean Ridges. This later process is observed in slow spreading ridges and ophiolites where shallow oceanic lithospheric mantle could be metasomatized/refertilized during incomplete MORB melt extraction. Nevertheless, direct evidence for metasomatic refertilization of the deep part of the oceanic lithospheric mantle is still missing. Xenoliths and xenocrysts sampled by petit-spot volcanoes interpreted as low-degree melts extracted from the base of the lithosphere in response to plate flexure, provide important new information about the nature and the processes associated with the evolution of oceanic lithospheric mantle. Here, we report, first, the presence of a garnet xenocryst in petit-spot lavas from Japan characterized by low-Cr, low-Ti content and mostly flat MREE-HREE pattern. This garnet is interpreted as formed during subsolidus cooling of pyroxenitic or gabbroic cumulates formed at ~1 GPa during the incomplete melt extraction at the periphery of the Pacific mid-ocean ridge. It is the first time that such processes are documented in fast spreading context. Second, we report petit-spot mantle xenoliths with cpx trace element "signatures" characterized by high U, Th, relative depletion in Nb, Pb, Ti and high but variable LREE/HREE ratio suggesting equilibration depth closed to the Gt/Sp transition zone. Such "signatures" are unknown from oceanic settings and show unexpected similarity to melt-metasomatized gt-peridotites sampled by kimberlites. This similarity suggests that metasomatic processes are not restricted to continental setting, but could correspond to a global mechanism at the lithosphere-asthenosphere boundary. As plate flexure

  13. Paleoproterozoic Collisional Structures in the Hudson Bay Lithosphere Constrained by Multi-Observable Probabilistic Inversion

    NASA Astrophysics Data System (ADS)

    Darbyshire, F. A.; Afonso, J. C.; Porritt, R. W.

    2015-12-01

    The Paleozoic Hudson Bay intracratonic basin conceals a Paleoproterozoic Himalayan-scale continental collision, the Trans-Hudson Orogen (THO), which marks an important milestone in the assembly of the Canadian Shield. The geometry of the THO is complex due to the double-indentor geometry of the collision between the Archean Superior and Western Churchill cratons. Seismic observations at regional scale show a thick, seismically fast lithospheric keel beneath the entire region; an intriguing feature of recent models is a 'curtain' of slightly lower wavespeeds trending NE-SW beneath the Bay, which may represent the remnants of more juvenile material trapped between the two Archean continental cores. The seismic models alone, however, cannot constrain the nature of this anomaly. We investigate the thermal and compositional structure of the Hudson Bay lithosphere using a multi-observable probabilistic inversion technique. This joint inversion uses Rayleigh wave phase velocity data from teleseismic earthquakes and ambient noise, geoid anomalies, surface elevation and heat flow to construct a pseudo-3D model of the crust and upper mantle. Initially a wide range of possible mantle compositions is permitted, and tests are carried out to ascertain whether the lithosphere is stratified with depth. Across the entire Hudson Bay region, low temperatures and a high degree of chemical depletion characterise the mantle lithosphere. Temperature anomalies within the lithosphere are modest, as may be expected from a tectonically-stable region. The base of the thermal lithosphere lies at depths of >250 km, reaching to ~300 km depth in the centre of the Bay. Lithospheric stratification, with a more-depleted upper layer, is best able to explain the geophysical data sets and surface observables. Some regions, where intermediate-period phase velocities are high, require stronger mid-lithospheric depletion. In addition, a narrow region of less-depleted material extends NE-SW across the Bay

  14. 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

  15. 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.

  16. 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.

  17. 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

  18. Role of orthopyroxene in rheological weakening of the lithosphere via dynamic recrystallization.

    PubMed

    Farla, Robert J M; Karato, Shun-Ichiro; Cai, Zhengyu

    2013-10-01

    For plate tectonics to operate on a terrestrial planet, the surface layer (the lithosphere) must have a modest strength (Earth, ≤ 200 MPa), but a standard strength profile based on olivine far exceeds this threshold value. Consequently, it is essential to identify mechanisms that reduce the strength of the lithosphere on Earth. Here we report results of high-strain laboratory deformation experiments on a representative olivine-orthopyroxene composition that show the addition of orthopyroxene substantially reduces the strength in the ductile regime within a certain temperature window. The reduction in strength is associated with the formation of small orthopyroxene and olivine grains. Our samples show heterogeneous microstructures similar to those observed in natural peridotites in shear zones: fine-grained regions containing both orthopyroxene and olivine that form interconnected bands where a large fraction of strain is accommodated. A model is developed to apply these results to geological conditions. Such a model, combined with our experimental observations, suggests that orthopyroxene may play a key role in the plastic deformation of the lithosphere in a critical temperature range, leading to long-term weakening associated with strain localization in the lithosphere. PMID:24067645

  19. Role of orthopyroxene in rheological weakening of the lithosphere via dynamic recrystallization

    PubMed Central

    Farla, Robert J. M.; Karato, Shun-ichiro; Cai, Zhengyu

    2013-01-01

    For plate tectonics to operate on a terrestrial planet, the surface layer (the lithosphere) must have a modest strength (Earth, ≤200 MPa), but a standard strength profile based on olivine far exceeds this threshold value. Consequently, it is essential to identify mechanisms that reduce the strength of the lithosphere on Earth. Here we report results of high-strain laboratory deformation experiments on a representative olivine–orthopyroxene composition that show the addition of orthopyroxene substantially reduces the strength in the ductile regime within a certain temperature window. The reduction in strength is associated with the formation of small orthopyroxene and olivine grains. Our samples show heterogeneous microstructures similar to those observed in natural peridotites in shear zones: fine-grained regions containing both orthopyroxene and olivine that form interconnected bands where a large fraction of strain is accommodated. A model is developed to apply these results to geological conditions. Such a model, combined with our experimental observations, suggests that orthopyroxene may play a key role in the plastic deformation of the lithosphere in a critical temperature range, leading to long-term weakening associated with strain localization in the lithosphere. PMID:24067645

  20. 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.

  1. 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

  2. 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

  3. Theory and detection scheme of seismic EM signals transferred into the atmosphere from the oceanic and continental lithosphere

    NASA Astrophysics Data System (ADS)

    Novik, Oleg; Ershov, Sergey; Ruzhin, Yuri; Smirnov, Fedor; Volgin, Maxim

    2014-07-01

    Due to the compound structure of the medium and large portions of energy transferred, a seismic excitation in the oceanic or continental lithosphere disturbs all types of geophysical fields. To investigate the problem of electromagnetic (EM) disturbances in the atmosphere from the seismically activated lithosphere, we have formulated two mathematical models of interaction of fields of different physical nature resulting in arising of the low-frequency (from 0.1 to 10 Hz by amplitude of a few hundreds of pT) EM signals in the atmosphere. First we have considered the EM field generation in the moving oceanic lithosphere and then in the moving continental one. For both cases, the main physical principles and geological data were applied for formulation of the model and characteristics of the computed signals of different nature agree with measurements of other authors. On the basis of the 2D model of the seismo-hydro-EM-temperature interaction in the lithosphere-Ocean-atmosphere domain, a block-scheme of a multisensory vertically distributed (from a seafloor up to the ionosphere) tsunami precursors' detection system is described. On the basis of the 3D model of the seismo-EM interaction in a lithosphere-atmosphere domain, we explain why Prof. Kopytenko (Inst. IZMIRAN of Russian Acad. Sci.) and co-authors were able to estimate location of the future seismic epicenter area from their magnetic field measurements in the atmosphere near the earth's surface.

  4. 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.

  5. Radioxenon detections in the CTBT International Monitoring System likely related to the announced nuclear test in North Korea conducted on February 12, 2013

    SciTech Connect

    Ringbom, Anders; Axelssson, A.; Aldener, M.; Auer, M.; Bowyer, Ted W.; Fritioff, T.; Hoffman, Ian; Khrustalev, Kirill; Nikkinen, Mika; Popov, Vladimir Y.; Popov, Y.; Ungar, R. Kurt; Wotawa, G.

    2014-02-01

    Abstract: Observations of the radioxenon isotopes 133Xe and 131mXe collected at the IMS stations RN38 and RN58 on April 7-8, and April 12-13 2013, respectively, are unique with respect to the measurement history of these stations. Comparison of measured data with calculated isotopic ratios as well as analysis using atmospheric transport modeling indicate that it is likely that the xenon measured was created in the underground nuclear test conducted by North Korea on February 12, 2013, and released 7 weeks later. More than one release is required to explain all observations. The 131mXe source terms for each release were calculated to 7x1011 Bq, corresponding to about 1-10% of the total xenon inventory for a 10-kt explosion, depending on fractionation and release scenario. The observed ratios could not be used to obtain any information regarding the fissile material that was used in the test.

  6. 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

  7. 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

  8. 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.

  9. Apparent Susceptibility Contrast Distribution of Continental Lithosphere in China and Its Surroundings: Implications to Regional Tectonics

    NASA Astrophysics Data System (ADS)

    Du, J.; Chen, C.; Sun, S.; Zhang, Y.; Liang, Q.

    2015-12-01

    Lithospheric magnetic field characterizes response of magnetic properties of rocks, which are mainly dependent on mineral and temperature variations. Hence, lithospheric magnetic structure brings important information to understand tectonic and thermal processes in the crust and uppermost mantle. In particular, the reliable global geomagnetic field models with large-scales based on satellite magnetic measurements provide regional view of the lithospheric magnetic structure. Here, with smallest and flattest constraints we use the inversion method based on the single layer model to calculate the spatial distribution of apparent susceptibility of continental lithosphere in China and its surroundings. It should be noted that: (1) magnetic anomaly data we used has removed the effect of global oceanic remanent magnetization, (2) the error of magnetic anomaly data is estimated from statistical analysis among MF7, GRIMM_L120, CHAOS5 and CM5 models, (3) the magnetic layer is bounded by the bottom of sediment and the Moho from CRUST1.0 model and is discretized into ellipsoidal prisms with equal angles in latitude and longitude, and (4) an adaptive subdivision & Gauss-Legendre quadrature with fixed order is adopted to solve the forward problem and IGRF11 is utilized as inducing field model. Since the missing longest wavelength components in the lithospheric magnetic field models and the so-called magnetic annihilators, the Apparent Susceptibility Contrast (ASC) distribution is obtained. The ASC distribution has obvious variations and illustrates the mosaic continent with old blocks, orogenic belts, rework fragments and also earthquake regions/zones. Moreover, the ASC distribution provides new insights and evidences of the destruction of North China Craton and geodynamic processes of Tibetan plateau and Baikal rift etc. This study is supported by China Postdoctoral Science Foundation (Grant No.: 2015M572217) and Natural Science Fund of Hubei Province (Grant No.: 2015CFB361).

  10. Imaging the Subduction of Continental Lithosphere in the Banda Sea Region

    NASA Astrophysics Data System (ADS)

    Fichtner, A.; De Wit, M.; van Bergen, M.

    2014-12-01

    We present a 3D tomographic model of Australasia obtained by full seismic waveform inversion. Our model features a sharp lateral velocity contrast extending to >200 km depth, coincident with the abrupt transition from low to high Helium ratios in arc volcanics near 123°E (see figure). The joint analysis of the tomographic model and isotope data (for He, Pb, Nd, Sr) suggests that the North Australian craton subducted beneath the Banda Sea to around 100 km depth. The continuous increase of computing power combined with advances in numerical seismology allow us to develop full waveform inversion techniques that translate complete seismograms into 3D Earth models. The natural incorporation of any type of body and surface waves in full waveform inversion improves tomographic resolution in terms of both resolution length and amplitude recovery. We applied full waveform inversion to Australasia, including the Sunda and Banda arcs. The correlation of the tomographic model with isotope signatures of arc volcanics supports the shallow-angle subduction of North Australian lithosphere. The integrated data suggest that the late Jurassic ocean lithosphere north of the North Australian craton was capable of entraining large volumes of continental lithosphere. A plausible explanation involves delamination within the continental crust, separating upper from lower crustal units. This interpretation is consistent with the existence of a massive accretionary complex on Timor island, with evidence from Pb isotope analysis for lower-crust involvement in arc volcanism; and with the approximate gravitational stability of the subducted lithosphere as inferred from the tomographic images. The Banda arc example demonstrates that continental lithosphere in arc-continent collisions may not generally be preserved, thus increasing the complexity of tectonic reconstructions and models of recycling continental crust.

  11. The Heterogeneous Hawaiian Lithosphere: New Isotope Data From Kauai and Oahu Peridotites

    NASA Astrophysics Data System (ADS)

    Bizimis, M.; Salters, V. J.; Sen, G.; Keshav, S.; Ducea, M.

    2005-12-01

    The Hf-Os isotope systematics of peridotites from Oahu suggest that the Hawaiian lithosphere is highly heterogeneous. We found that many Salt Lake Crater (SLC) peridotites vent are fragments of > 500Ma old recycled depleted lithosphere brought to the surface by the Hawaiian plume. In contrast, peridotites from the Pali vent have Hf-Os compositions appropriate for the 90 Ma old Pacific lithosphere. Here we present additional Sr-Nd isotope, major and trace element data from peridotites from the island of Kauai and from Salt Lake Crater and Pali vents on Oahu, in order to further constrain the extent and origin of the heterogeneity observed in the Hawaiian lithosphere. Two Kauai peridotites overlap the Koloa Volcanics post erosional lava compositions in Sr-Nd isotope space. A third Kauai peridotite has higher 87Sr/86Sr (0.70422) and lower 143Nd/144Nd (0.51291) than any published Kauai lavas and extends towards more radiogenic Sr (for a given Nd) than the Hawaiian lavas in general. This sample has depleted characteristics with high Mg# (0.925) and Cr# (0.311), and low HREE in cpx. It also has the highest La/Yb ratio and largest Ti depletions we have yet determined in Hawaiian peridotites. In contrast, three peridotites from SLC have less radiogenic 87Sr/86Sr (0.70316-0.70334), for a given 143Nd/144Nd (0.51285-0.51278) than all other analyzed peridotites and Hawaiian lavas. One peridotite straddles the proposed LoNd array thought to represent mixing between HIMU and EM-I mantle endmember components. In Hf-Nd space, this sample falls below the Hawaiian lava array having lower 176Hf/177Hf (0.28287) than even the most isotopically enriched Koolau lavas. Our new peridotite data show a much greater Sr and Nd isotope variability than previously observed, and support our conclusions based on the Hf-Os systematics for a highly heterogeneous lithosphere. The incompatible nature of Sr and Nd in peridotites makes them very susceptible to metasomatic overprint but the observed

  12. Seismic imaging of the lithosphere beneath Hudson Bay: Episodic growth of the Laurentian mantle keel

    NASA Astrophysics Data System (ADS)

    Darbyshire, Fiona A.; Eaton, David W.; Bastow, Ian D.

    2013-07-01

    The Hudson Bay basin in northern Canada conceals one of the major collisional zones of the Canadian Shield, the Trans-Hudson Orogen (THO), which marks the Paleoproterozoic collision between the Archean Superior and Western Churchill cratons at ˜1.9-1.8Ga. Improved knowledge of upper mantle structure beneath the region is essential to establish the nature of the THO, specifically whether Himalayan-style plate tectonics operated in Paleoproterozoic times. Detailed seismological constraints on lithospheric architecture are also required to advance our understanding of the mechanism and timing of keel formation. We use surface wave tomography to illuminate new details of the lithospheric architecture of the Hudson Bay region, resolving both seismic wavespeed and azimuthal anisotropy. Phase velocity maps are calculated from fundamental-mode Rayleigh wave dispersion curves, then used to construct a 3D model exploring upper mantle structure to depths of ˜300km. Fast shear wavespeeds suggest a lithospheric thickness varying from ˜180km to almost 280 km beneath the Hudson Bay region. The new study confirms previous inferences that there is no correlation between crustal ages and lithospheric thickness. Patterns of shear wavespeed and azimuthal anisotropy indicate a layered lithosphere. In the uppermost mantle, both the highest velocities and the anisotropic fast directions wrap around the Bay. This structure is likely related to the formation processes of the Paleozoic intracratonic basin. At greater depth (˜70-150km) we resolve two high-wavespeed cores separated by a relatively narrow near-vertical lower-velocity curtain. This internal architecture is suggested to result from the terminal phase of a modern-style plate-tectonic collision between the Archean Superior and Churchill cratons during the Trans-Hudson orogeny, entrapping juvenile Proterozoic material. The lower lithosphere (≥160km depth) has a relatively homogeneous wavespeed structure across the region

  13. Interference of lithospheric folding in Central Asia by simultaneous Indian and Arabian plate indentation

    NASA Astrophysics Data System (ADS)

    Smit, J. H. W.; Cloetingh, S. A. P. L.; Burov, E.; Sokoutis, D.; Kaban, M.; Tesauro, M.; Burg, J.-P.

    2012-04-01

    Although large-scale folding of the crust and the lithosphere in Central Asia as a result of the indentation of India has been extensively documented, the impact of continental collision between Arabia and Eurasia has been largely overlooked. The resulting Neogene shortening and uplift of the Zagros, Albors, Kopet Dagh and Kaukasus mountain belts in Iran and surrounding areas is characterised by a simultaneous onset of major topography growth at ca. 5 Ma. At the same time, the adjacent Caspian, Turan and Amu Darya basins underwent an acceleration in subsidence. It is common knowledge that waves with different orientations will interfere with each other. Folding, by its nature similar to a standing wave, is not likely to be an exception. We demonstrate that collision of the Eurasian plate with the Arabian and Indian plates generates folding of the Eurasian lithosphere in two different directions and that interaction between both generates characteristic interference patterns that can be recognised from the regional gravity signal. We present evidence for interference of lithospheric folding patterns induced by Arabian and Indian collision with Eurasia. Wavelengths (from 50 to 250 km) and spatial patterns are inferred from satellite-derived topography and gravity models and attest for rheologically stratified lithosphere with relatively strong mantle rheology (thickness of strong mechanical core on the order of 40-50 km) and less competent crust (thickness of the mechanical core on the order of 10-15 km). The observations are compared with inferences from numerical and analogue tectonic experiments for a quantitative assessment of factors such as lithosphere rheology and stratification, lateral variations in lithosphere strength, thermo-mechanical age and distance to the plate boundary on the activity of folding as a mechanism of intra-plate deformation in this area. The observed interference of the patterns of folding appears to be primarily the result of spatial

  14. Melt infiltration of the lower lithosphere beneath the Tanzania craton and the Albertine rift inferred from S receiver functions

    NASA Astrophysics Data System (ADS)

    WöLbern, Ingo; Rümpker, Georg; Link, Klemens; Sodoudi, Forough

    2012-08-01

    The transition between the lithosphere and the asthenosphere is subject to numerous contemporary studies as its nature is still poorly understood. The thickest lithosphere is associated with old cratons and platforms and it has been shown that seismic investigations may fail to image the lithosphere-asthenosphere boundary in these areas. Instead, several recent studies have proposed a mid-lithospheric discontinuity of unknown origin existing under several cratons. In this study we investigate the Tanzania craton in East Africa which is enclosed by the eastern and western branches of the East African Rift System. We present evidence from S receiver functions for two consecutive discontinuities at depths of 50-100 km and 140-200 km, which correspond to significant S wave velocity reductions under the Tanzania craton and the Albert and Edward rift segments. By comparison with synthetic waveforms we show that the lower discontinuity coincides with the LAB exhibiting velocity reductions of 6-9%. The shallower interface reveals a velocity drop that varies from 12% beneath the craton to 24% below the Albert-Edward rift. It is interpreted as an infiltration front marking the upper boundary of altered lithosphere due to ascending asthenospheric melts. This is corroborated by computing S velocity variations based on xenolith samples which exhibit a dense system of crystallized veins acting as pathways of the infiltrating melt. Mineral assemblages in these veins are rich in phlogopite and pyroxenite which can explain the reduced shear wave velocities. Melt infiltration represents a suitable mechanism to form a mid-lithospheric discontinuity within cratonic lithosphere that is underlain by anomalously hot mantle.

  15. Understanding lithospheric stresses: systematic analysis of controlling mechanisms with applications to the African Plate

    NASA Astrophysics Data System (ADS)

    Medvedev, Sergei

    2016-06-01

    gives us confidence that our methodology appropriately models the stress pattern of Africa, and that it may be further applied to other plates on Earth. Our modelling approach allows us to quantify several important features controlling the lithospheric stress pattern. Even though the initial oversimplified model does not fit the observations satisfactorily, it shows how ridge push may create significant compressive stresses in the lithosphere. More complex models show the importance of the density structure of the lithosphere, specifically in the subcrustal lithosphere. The stress regime within the TAP mainly results from a global balance of masses and mass moments between continental and oceanic parts of the plate. The orientation of stresses, in turn, is influenced more by local features expressed by topographic and crustal density variations, whereas existent subcrustal density variations appear to be smoothed by the crust above. The models show that accounting separately for either basal tractions or rheological heterogeneities brings moderate improvement, but the combination of these two mechanisms results in a substantially better match between model and observations. The bending stresses caused by isostatical re-adjustment improve the model match, but they have to be analyzed with caution because of their depth- dependent nature.

  16. 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

    plateau, Tarim Basin, Tian Shan Mountains, Zungarian Basin and Altai Mountains correspond successively to the downward flow, the upward flow, the downward flow and the upward flow in the mantle. It shows that the basic tectonic pattern of the interphase basin-and-range structure corresponds to the convective pattern of the interphase downward-and-upward flows. In China-Mongolian Continent, the convective pattern in the eastern area of 105°E structural zone is entirely different from the western area, weak in the East and intensive in the West. West Pacific island arc-underthrust belt is located in the intensive convective areas and along this northeastern belt, there exist a series of interphase downward-and-upward flows. In summary, our results suggest that small-scale mantle convection in the upper mantle is one of the main driving forces of lithospheric dynamics of the Eurasian region. This work was supported by the Sinoprobe-02 project, the National Natural Science Foundation of China (40874067) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (20070491520)

  17. Microcrystalline diamonds in the oceanic lithosphere and their nature

    NASA Astrophysics Data System (ADS)

    Galimov, E. M.; Sevastyanov, V. S.; Karpov, G. A.; Shilobreeva, S. N.; Maksimov, A. P.

    2016-07-01

    The carbon isotope composition of microdiamonds found in products of the Tolbachik Volcano eruption, Kamchatka (porous lavas and ash), was studied. The isotope composition of microdiamonds (with an average value of δ13C =-25.05‰) is close to that of microsized carbon particles in lavas (from-28.9 to-25.3‰). The general peculiarities of the diamond-forming environment include (1) no evidence for high pressure in the medium; (2) a reduced environment; and (3) mineralogical evidence for the presence of a fluid. The geochemical data characterizing the type of diamonds studied allow us to suggest that they were formed in accordance with the mechanism of diamond synthesis during cavitation in a rapidly migrating fluid, which was suggested by E.M. Galimov.

  18. 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.

  19. 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.

  20. 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

  1. 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.

  2. 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.

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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”).

  9. 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").

  10. 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.

  11. A lithosphere-dynamics constraint on mantle flow: Analysis of the Eurasian plate

    NASA Astrophysics Data System (ADS)

    Warners-Ruckstuhl, K. N.; Meijer, P. Th.; Govers, R.; Wortel, M. J. R.

    2010-09-01

    We present a method to estimate the poorly understood mechanical coupling between lithosphere and underlying mantle, and apply it to the Eurasian plate. Mechanical equilibrium of tectonic plates requires the torque from mantle tractions ($\\overline{TM) to be balanced by the torques from edge forces ($\\overline{TE) and lithospheric body forces ($\\overline{TB). The direction of $\\overline{TE proves tightly constrained by plate boundary nature but $\\overline{TB is affected uncertainties in the density structure of continents. We consistently find that the non-zero torque required from mantle tractions does not agree with the orientation of any published absolute motion model. We conclude that mechanical balance of the Eurasian plate requires an actively convecting mantle, which should result in a torque on the Eurasian plate located in the southwest Pacific.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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

  17. 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

  18. Stagnant-lid Convection with Lateral Variations in Lithospheric Thickness: Application to Mars

    NASA Astrophysics Data System (ADS)

    Sramek, O.; Zhong, S.

    2009-12-01

    The hemispheric dichotomy and Tharsis volcanic province are dominant planetary scale features on Mars. The formation mechanism of the dichotomy remains unclear and arguments have been made for both exogenic (i.e., giant impact) and endogenic (i.e., related to internal dynamics) origin. A recently proposed model by Zhong (2009) aims to link the formation of the dichotomy and the subsequent emplacement of the Tharsis region. This model requires a large-scale (spherical harmonic degree 1) flow in the mantle and involves a differential rotation of Martian lithosphere with respect to the sublithospheric mantle. It has been shown previously that a long-wavelength pattern of mantle flow naturally arises in convection models when realistic viscosity profiles are used. The lithosphere-mantle differential motion is excited by lateral variations in lithospheric thickness, assumed to represent the melt residue left after dichotomy formation by an endogenic process that involves melting. This motion could explain the inferred past migration of the Tharsis volcanic center from the current southern hemisphere to the dichotomy boundary. We present a suite of 3-D spherical-shell models to investigate the effect of various parameters on crucial ingredients of this model. First we look at the effect of varying asthenospheric channel thickness on the wavelength of convection for uniform lithospheric thickness, thus extending results of previous studies. We find a trade-off between the channel thickness and viscosity reduction in the channel, where a thinner asthenosphere requires a larger decrease in viscosity in order to generate degree-1 flow pattern. We then incorporate into our models a rigid lithospheric keel of hemispheric extent and observe the following. i) The position of the keel controls the orientation of the thermal structure, such that in the early stages of evolution the thermal upwelling is focused below the center of the keel. ii) Differential movement between the

  19. 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

  20. 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

  1. Hydration of Archean lithosphere: A chemico-physical case study of the lherzolitic upper mantle below the Kaapvaal Craton

    NASA Astrophysics Data System (ADS)

    Gauert, C. D.; Globig, J.; Sommer, H.

    2013-12-01

    Since its formation in the Archean the subcratonic upper mantle of the Kaapvaal in southern Africa has undergone several processes of modification. Detailed analysis of Kaapvaal xenoliths from kimberlites show clear differences in age, origin, mineralogy, fertility and degree and type of alteration illustrating a period of complex interaction between asthenospheric and lithospheric mantle domains. The evolution of the cratonic lithosphere through time involved several metasomatic events leading to chemical and thermal anomalies. Global and regional 3-D shear wave velocity models are imaging a low velocity zone for the lower Kaapvaal lithosphere. However, regardless the resolution and significance of the lithospheric low velocity zone its origin is a matter of debate and is discussed to be either of thermal or chemical nature. Petrological evidence points to a rather chemical origin caused by refertilization and/or hydration of lithospheric mantle by metasomatizing fluids. Here we present a chemico-physical study of the lherzolitic lithosphere below South Africa using a recalculated bulk composition based on analyses of the rock forming minerals from lherzolites from the Roberts Victor Mine. The thermo-chemical calculations were carried out for a water saturated lherzolite representative of published compositions of garnet lherzolites from the Kaapvaal Craton in order to estimate the distribution of hydrous phases and the combined influence on physical properties as density and P- and S- wave velocities. Our results confirm the existence of a zone with slightly lower S-wave velocities and are supporting the idea of chemically layered lherzolitic mantle that has been repeatedly hydrated by slab released volatiles in a two sided subduction model.

  2. A Multi-Layer Phoswich Radioxenon Detection System (7th Qtr Report), Reporting Period 10/01/07 - 12/31/07

    SciTech Connect

    David M. Hamby

    2008-01-29

    Description of activities conducted this report period: (1) Electronics Development--To improve the overall performance of the two-channel digital pulse processor (DPP2), the PCB has been redesigned and the new printed board is now under assembly. The system is enhanced with two new fast ADCs from Analog Devices (AD9230-250), each with a sampling rate of 250 MHz and a resolution of 12 bits. The data bus uses a high performance Low Voltage Differential Signaling (LVDS) standard. The offset and gain of each channel are separately controlled digitally by the GUI software. (2) GUI Software Development--A GUI is being developed using the Python programming language. All functions from the preceding MATLAB code have been re-implemented including basic waveform readout, pulse shape discrimination, and plotting of energy spectra. In addition, the GUI can be used to control sampling runs based on the number of pulses captured, either in real or live time. Calibration coefficients and pulse shape discrimination boundaries can be changed on the fly so that the detector may be characterized experimentally. Plots generated by the GUI can be exported as graphic data. At present, the software has only been tested using one channel, pending availability of the new DPP board (DPP2). However, the functions have been written to allow easy expansion to two channels. (3) Light Collection Modeling--The XEPHWICH design has been modeled to determine its light capture efficiency. Research in the 7th quarter includes additional simulations representing significant increase in data resolution, well over an order of magnitude greater than previous simulations. The final data set represents approximately 11 billion visible photons divided equally among 110 thousand data points. A laboratory experiment is being designed and executed to experimentally determine light capture efficiency as a function of position within the scintillators. (4) Radioxenon Fission Source--We have designed and

  3. Radioxenon spiked air.

    PubMed

    Watrous, Matthew G; Delmore, James E; Hague, Robert K; Houghton, Tracy P; Jenson, Douglas D; Mann, Nick R

    2015-12-01

    Four of the radioactive xenon isotopes ((131m)Xe, (133m)Xe, (133)Xe and (135)Xe) with half-lives ranging from 9 h to 12 days are produced from nuclear fission and can be detected from days to weeks following their production and release. Being inert gases, they are readily transported through the atmosphere. Sources for release of radioactive xenon isotopes include operating nuclear reactors via leaks in fuel rods, medical isotope production facilities, and nuclear weapons' detonations. They are not normally released from fuel reprocessing due to the short half-lives. The Comprehensive Nuclear-Test-Ban Treaty has led to creation of the International Monitoring System. The International Monitoring System, when fully implemented, will consist of one component with 40 stations monitoring radioactive xenon around the globe. Monitoring these radioactive xenon isotopes is important to the Comprehensive Nuclear-Test-Ban Treaty in determining whether a seismically detected event is or is not a nuclear detonation. A variety of radioactive xenon quality control check standards, quantitatively spiked into various gas matrices, could be used to demonstrate that these stations are operating on the same basis in order to bolster defensibility of data across the International Monitoring System. This paper focuses on Idaho National Laboratory's capability to produce three of the xenon isotopes in pure form and the use of the four xenon isotopes in various combinations to produce radioactive xenon spiked air samples that could be subsequently distributed to participating facilities. PMID:26318775

  4. Radioxenon spiked air

    DOE PAGESBeta

    Watrous, Matthew G.; Delmore, James E.; Hague, Robert K.; Houghton, Tracy P.; Jenson, Douglas D.; Mann, Nick R.

    2015-08-27

    Four of the radioactive xenon isotopes (131mXe, 133mXe, 133Xe and 135Xe) with half-lives ranging from 9 h to 12 days are produced from nuclear fission and can be detected from days to weeks following their production and release. Being inert gases, they are readily transported through the atmosphere. Sources for release of radioactive xenon isotopes include operating nuclear reactors via leaks in fuel rods, medical isotope production facilities, and nuclear weapons' detonations. They are not normally released from fuel reprocessing due to the short half-lives. The Comprehensive Nuclear-Test-Ban Treaty has led to creation of the International Monitoring System. The Internationalmore » Monitoring System, when fully implemented, will consist of one component with 40 stations monitoring radioactive xenon around the globe. Monitoring these radioactive xenon isotopes is important to the Comprehensive Nuclear-Test-Ban Treaty in determining whether a seismically detected event is or is not a nuclear detonation. A variety of radioactive xenon quality control check standards, quantitatively spiked into various gas matrices, could be used to demonstrate that these stations are operating on the same basis in order to bolster defensibility of data across the International Monitoring System. This study focuses on Idaho National Laboratory's capability to produce three of the xenon isotopes in pure form and the use of the four xenon isotopes in various combinations to produce radioactive xenon spiked air samples that could be subsequently distributed to participating facilities.« less

  5. Radioxenon spiked air

    SciTech Connect

    Watrous, Matthew G.; Delmore, James E.; Hague, Robert K.; Houghton, Tracy P.; Jenson, Douglas D.; Mann, Nick R.

    2015-08-27

    Four of the radioactive xenon isotopes (131mXe, 133mXe, 133Xe and 135Xe) with half-lives ranging from 9 h to 12 days are produced from nuclear fission and can be detected from days to weeks following their production and release. Being inert gases, they are readily transported through the atmosphere. Sources for release of radioactive xenon isotopes include operating nuclear reactors via leaks in fuel rods, medical isotope production facilities, and nuclear weapons' detonations. They are not normally released from fuel reprocessing due to the short half-lives. The Comprehensive Nuclear-Test-Ban Treaty has led to creation of the International Monitoring System. The International Monitoring System, when fully implemented, will consist of one component with 40 stations monitoring radioactive xenon around the globe. Monitoring these radioactive xenon isotopes is important to the Comprehensive Nuclear-Test-Ban Treaty in determining whether a seismically detected event is or is not a nuclear detonation. A variety of radioactive xenon quality control check standards, quantitatively spiked into various gas matrices, could be used to demonstrate that these stations are operating on the same basis in order to bolster defensibility of data across the International Monitoring System. This study focuses on Idaho National Laboratory's capability to produce three of the xenon isotopes in pure form and the use of the four xenon isotopes in various combinations to produce radioactive xenon spiked air samples that could be subsequently distributed to participating facilities.

  6. Introduction of sub-lithospheric component into melted lithospheric base by propagating crack: Case study of migrated Quaternary volcanoes in Wudalianchi, China

    NASA Astrophysics Data System (ADS)

    Chuvashova, Irina; Sun, Yi-min

    2016-04-01

    mantle beneath the northern Songliao basin and that admixture of the common sub-lithospheric component was locally introduced into the melted region by mechanism of propagating crack. This study is based on analytical data obtained for volcanic rocks in the Chinese-Russian Wudalianchi-Baikal Research Center on recent volcanism and environment. Major oxides were determined by "wet chemistry" at the Institute of the Earth's Crust SB RAS, Irkutsk. Trace-elements were measured by ICP-MS technique using mass-spectrometer Agilent 7500ce of the Center for collective use "Microanalysis" (Limnological Institute of SB RAS, Irkutsk) and Nd, Pb, and Sr isotopes by TIMS technique using mass-spectrometer Finnigan MAT 262 of the Center for collective use "Geodynamics and geochronology" (Institute of the Earth's Crust SB RAS). The work was supported by the RFBR grant № 16-05-00774. References Chuvashova, I.S., Rasskazov, S.V., Liu, J., Meng, F., Yasnygina, T.A., Fefelov, N.N., Saranina, E.V., 2009. Isotopically-enriched components in evolution of Late Cenozoic potassic magmatism in Heilongjiang province, northeast China, Proceedings of the Irkutsk State University. Series of Earth Sciences, 2 (2), pp. 181-198. Guide book for field mission to Wudalianchi National Park, China, 2010. Prepared by Wudalianchi National Park and Nature Management Committee Heilongjiang province, 50 p. Foulger, G.R., 2010. Plates vs. plumes: a geological controversy. Wiley-Blackwell, 328 p. Rasskazov, S.V., Yasnygina, T.A., Chuvashova, I.S. Mantle sources of the Cenozoic volcanic rocks of East Asia: Derivatives of slabs, the sub-lithospheric convection, and the lithosphere. Russian Journal of Pacific Geology. 2014. V. 8 (5), 355-371. Wang, Y., Chen, H., 2005. Tectonic controls on the Pleistocene-Holocene Wudalianchi volcanic field (northeastern China), Journal of Asian Earth Sciences, 24, pp. 419-431.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. Grain size in lithospheric-scale shear zones: Chicken or Egg?

    NASA Astrophysics Data System (ADS)

    Thielmann, M.; Rozel, A.; Kaus, B. J. P.; Ricard, Y.

    2012-04-01

    Lithospheric-scale shear zones are commonly defined as regions inhomogeneous and localized deformation. Strain softening has been demonstrated to be necessary for localization in those shear zones, but there is still debate about the physical cause of this softening. As natural shear zones typically have a significantly reduced grain size, it has been proposed that grain size reduction provides the necessary strain softening to localize deformation. As grain size reduces, the dominant deformation mechanism switches from dislocation to diffusion creep, thus requiring less stress to deform the rock. Until recently, the equilibrium grain size has been thought to follow a piezometric relationship, thus indicating the stress under which a shear zone deformed. More recent work (Austin and Evans (2007), Rozel et. al. (2011)) suggests that the equilibrium grain size is not dependent on stress, but rather on the deformational work. Using this relationship, we use numerical models to investigate the effect of grain size evolution on lithospheric deformation. We focus on the question if grain size provides sufficient weakening to effectively localize deformation under lithospheric conditions or if it's effect is rather passive and as such a marker for the deformational work done in a shear zone. We then compare the localization potential of grain size reduction to shear heating and investigate the interplay between the two weakening mechanisms.

  13. 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

  14. 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

  15. The Changing Lithosphere: formation of minerals and dissapearance of rocks

    NASA Astrophysics Data System (ADS)

    Vignola, Teresa; Floriano, Michele A.

    2014-05-01

    dissolve under acid conditions;as such substances can be present in rain, for example. A clear example in nature of calcium carbonate dissolution by acid rain is the karst.The phenomenon was discussed with the aid ofimages depicting karst landscapes wherethe erosion phenomena produced by acid precipitations were evident. Therefore, acidity is one of the causes of erosion triggered by atmospheric precipitation and increasing acidity produced by volcanic eruptions or anthropic activities will enhance lithosphere erosionphenomena.

  16. 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.

  17. 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).

  18. Lithospheric discontinuities beneath Australia: interaction of large-scale and fine scale structure

    NASA Astrophysics Data System (ADS)

    Kennett, Brian L. N.; Yoshizawa, Kazunori

    2016-04-01

    Understanding the complex heterogeneity of the continental lithosphere involves a wide variety of spatial scales and the synthesis of multiple classes of information. Seismic surface waves and multiply reflected body waves provide the main information on broad-scale structure, and bounds on the extent of the lithosphere-asthenosphere transition (LAT) can be found from the vertical gradients of S wavespeed. Information on finer scale structures comes though body wave studies, including detailed seismic tomography and P wave reflectivity extracted from stacked autocorrelograms of continuous component records. With the inclusion of deterministic large-scale structure and realistic medium-scale stochastic features there is not a need for strong fine-scale variations. The resulting multi-scale heterogeneity model for the Australian region gives a good representation of the character of observed seismograms and their geographic variations and matches the observations of P wave reflectivity. The presence of reflections in the 0.5-3.0 Hz band in the uppermost mantle suggests variations on vertical scales of a few hundred metres with amplitudes of the order of 1%. There are some indications of a change of reflection character in the lower part of the lithosphere in the transition to the asthenosphere. In some parts of central Australia there is a reasonable tie between a change in reflectivity and other information on mid-lithospheric discontinuities. Individual seismic probes illuminate different aspects of the heterogeneity, but the full spectrum has to be taken into account to understand the properties of apparent discontinuities and their geodynamic implications. Once fine-scale structure is taken into consideration it becomes apparent that wave interference plays a very important role in determining the nature of apparent discontinuities seen with lower frequency probes such as S wave receiver functions. Changes in the character of fine-scale heterogeneity can

  19. Tearing of the Indian lithospheric slab beneath southern Tibet revealed by SKS-wave splitting measurements

    NASA Astrophysics Data System (ADS)

    Chen, Yun; Li, Wei; Yuan, Xiaohui; Badal, José; Teng, Jiwen

    2015-03-01

    Shear wave birefringence is a direct diagnostic of seismic anisotropy. It is often used to infer the northern limit of the underthrusting Indian lithosphere, based on the seismic anisotropy contrast between the Indian and Eurasian plates. Most studies have been made through several near north-south trending passive-source seismic experiments in southern Tibet. To investigate the geometry and the nature of the underthrusting Indian lithosphere, an east-west trending seismic array consisting of 48 seismographs was operated in the central Lhasa block from September 2009 to November 2010. Splitting of SKS waves was measured and verified with different methods. Along the profile, the direction of fast wave polarization is about 60° in average with small fluctuations. The delay time generally increases from east to west between 0.2 s and 1.0 s, and its variation correlates spatially with north-south oriented rifts in southern Tibet. The SKS wave arrives 1.0-2.0 s later at stations in the eastern part of the profile than in the west. The source of the anisotropy, estimated by non-overlapped parts of the Fresnel zones at stations with different splitting parameters, is concentrated above ca. 195 km depth. All the first-order features suggest that the geometry of the underthrusting Indian lithospheric slab in the Himalayan-Tibetan collision zone beneath southern Tibet is characterized by systematic lateral variations. A slab tearing and/or breakoff model of Indian lithosphere with different subduction angles is likely a good candidate to explain the observations.

  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. Radioxenon detections in the CTBT international monitoring system likely related to the announced nuclear test in North Korea on February 12, 2013.

    PubMed

    Ringbom, A; Axelsson, A; Aldener, M; Auer, M; Bowyer, T W; Fritioff, T; Hoffman, I; Khrustalev, K; Nikkinen, M; Popov, V; Popov, Y; Ungar, K; Wotawa, G

    2014-02-01

    Observations made in April 2013 of the radioxenon isotopes (133)Xe and (131m)Xe at measurement stations in Japan and Russia, belonging to the International Monitoring System for verification of the Comprehensive Nuclear-Test-Ban Treaty, are unique with respect to the measurement history of these stations. Comparison of measured data with calculated isotopic ratios as well as analysis using atmospheric transport modeling indicate that it is likely that the xenon measured was created in the underground nuclear test conducted by North Korea on February 12, 2013, and released 7-8 weeks later. More than one release is required to explain all observations. The (131m)Xe source terms for each release were calculated to 0.7 TBq, corresponding to about 1-10% of the total xenon inventory for a 10 kt explosion, depending on fractionation and release scenario. The observed ratios could not be used to obtain any information regarding the fissile material that was used in the test. PMID:24316684

  5. Lithosphere - asthenosphere boundary (LAB) around the Trans-European Suture Zone (TESZ)

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

    platform in the west and the East European Craton in the east. Lateral variations of anisotropic parameters across the zone indicate differences in mantle lithosphere fabrics on both sides. Distinct deepening of the LAB towards the east across the TESZ is detected by different approaches of the LAB modelling, but on the other hand, it is the region where European LAB models differ substantially (Jones et al., Lithos 2010). This finding re-opens a question to which extent different methods identify the same discontinuity, though discrepancies may also reflect differences in resolution and accuracy of individual methods. On the other hand, if there are differences in a nature of the LAB, we should try to understand which physical parameters are relevant to the most important interface in the upper mantle from the plate-tectonic point of view.

  6. Interaction of Sublithospheric Mantle with a Complex Continental Lithosphere: Radiogenic Isotope Constraints

    NASA Astrophysics Data System (ADS)

    Hanan, B. B.; Jean, M. M.; Shervais, J. W.; Graham, D. W.; Vetter, S.

    2012-12-01

    The Yellowstone-Snake River Plain (YSRP) consists of an 800 km swath of bimodal volcanic centers in southern Idaho and western Wyoming formed as the North American continent overrode the Yellowstone hotspot since ˜17 Ma. The rhyolitic centers show a time transgressive relationship with plate motion, but basalt volcanism persisted long after the locus of rhyolitic volcanism moved to the NE. The hotspot track is underlain by a 10-km-thick mafic sill complex that contains much of the basaltic melt produced. Seismic tomography, the age progressive nature, its relationship the Columbia River Basalts, and the isotopic signature of 3He/4He in the basalts suggest presence of a mantle hotspot originating in the sublithospheric mantle. Basalt major and trace element, and He isotope systematics are consistent with a deep mantle source, similar to ocean island basalt (OIB). In contrast, the Pb, Sr, and Nd isotopes are indistinguishable from xenoliths and melts from sub-continental lithospheric mantle (SCLM) underlying the YSRP. The SCLM stabilized in the Late Archean to Early Proterozoic, and was subsequently rejuvenated/enriched during subduction related metasomatism. Initial Pb and Sr isotope ratios are higher, and Nd lower than expected for a depleted upper mantle source of Late Archean age. Incompatible element concentrations in OIB-plume sources are more than 10X lower than found in the SCLM. Assimilation of small percentage partial melts of continental lithosphere into larger degree partial melts derived from the sublithospheric mantle source produces hybrid magmas whose Pb (Nd,Sr,Hf) isotopic compositions are controlled by the isotopic composition of the continental component, while the deeper mantle source dominates the 3He/4He signature. We tested this prediction with analyses of 75 basalts from the YSRP. The Pb isotope results are consistent with mixing between an OIB-like plume component with 1% to 4% melt derived from an enriched SCLM source and show that the

  7. 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

  8. 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

  9. 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.

  10. 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.

  11. 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

  12. 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.

  13. 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.

  14. Are the geoelectric structures of the Betics lithosphere anisotropic? Insights from a complete dimensionality analysis of magnetotelluric data

    NASA Astrophysics Data System (ADS)

    Martí, A.; Rosell, O.; Queralt, P.; Ledo, J.; Marcuello, A.; Roca, E.

    2012-04-01

    Magnetotelluric (MT) exploration in orogenic zones has provided valuable information on the sructures and processes that can be correlated with other geophysical observations. In the Betic Chain (SE Spain), an extensive broadband and long period MT dataset allowed the charaterization of the geoelectric structure of the lithosphere using 3D modelling, in agreement with the 3D character of the data. The model revealed the presence of a low conductivity zone at lithospheric mantle depths that was identified as asthenospheric intruding material related to the westwards roll-back of a lithospheric subduction. As an advancing step in the understanding of the Betics lithospheric structure we present the results of a new dimensionality analysis of the magnetotelluric data with the goal of identifying the possible presence of electrical anisotropy. In the last years, electrical anisotropy within the Earth has been included in geoelectric models and has been related to tectonic structures and geodynamic processes. However, given the amount of information contained in the magnetotelluric impedance tensor and induction arrows, it is not a routine task to uniquely identify the presence and nature of anisotropy, given that it can occur within different types of structures and at different scales. Thus, our analysis is based on a previous methodological work, which established the relationship between the dimensionality pattern obtained at different sites and periods and the possible presence of anisotropy. Additionally, we use the information contained in the induction arrows to constrain the results and compare them with other geophysical data.

  15. Growth of lithospheric-scale fault system in NE Tibet: numerical modeling constrained by high-resolution seismic reflection data

    NASA Astrophysics Data System (ADS)

    Fu, Zhen; Zhang, Haiming

    2016-04-01

    The growth of lithospheric-scale fault system is strongly coupled with the deformation of continental lithosphere in Tibetan Plateau. Therefore, prediction of fault growth is important to understand the tectonic history of continental deformation with fault system. Recently, high-resolution seismic reflection profiling across the Kunlun fault in northeasten Tibet reveals several fault systems at the scale of lithosphere. A 2D mid-crustal strain-transfer model, which emphasized on the lateral heterogeneity of crust, was proposed to explain the seismic reflection profiling under the condition of compression. In order to understand the dynamic process of lithospheric deformation, an elastic-plastic constitutive relationship in finite element modeling is used to investigate the mechanism of the fault growth in the section under the condition of compression by allowing permanent strains to develop in response to the applied loads. The vertical and lateral heterogeneity of material, effect of plastic parameters and geometry of models from nature structure are all discussed in this study. The results compared with high-resolution seismic image show that well-designed geomechanical modeling can produce overall process of fault growth for both continuum without preexisting fault and discontinuous deformation with a peexisting fault. But the model of the Kunlun fault cutting down the Moho is not supported by the results compared with the seismic data.

  16. Global Lithospheric Apparent Susceptibility Distribution Converted from Geomagnetic Models by CHAMP and Swarm Satellite Magnetic Measurements

    NASA Astrophysics Data System (ADS)

    Du, Jinsong; Chen, Chao; Xiong, Xiong; Li, Yongdong; Liang, Qing

    2016-04-01

    magnetic measurements and obtained global lithospheric apparent susceptibility distribution models. Finally, we compared these deduced models with previous results in the literature and some other geophysical, geodetic and geologic datum. Both tests and applications suggest, indeed, that the improved AS85 method can be adopted as a fast and effective interpretation tool of global induced large-scale magnetic anomaly field models in form of spherical harmonics. Arkani-Hamed, J. & Srangway, D.W., 1985. Lateral variations of apparent magnetic susceptibility of lithosphere deduced from Magsat data, J. Geophys. Res., 90(B3), 2655-2664. Gubbins, D., Ivers, D., Masterton, S.M. & Winch, D.E., 2011. Analysis of lithospheric magnetization in vector spherical harmonics, Geophys. J. Int., 187(1), 99-117. Hemant, K. & Maus, S., 2005. Geological modeling of the new CHAMP magnetic anomaly maps using a geographical information system technique, J. Geophys. Res., 110, B12103, doi: 10.1029/2005JB003837. Masterton, S.M., Gubbins, D., Müller, R.D. & Singh, K.H., 2013. Forward modeling of oceanic lithospheric magnetization, Geophys. J. Int., 192(3), 951-962. Nolte, H.J. & Siebert, M., 1987. An analytical approach to the magnetic field of the Earth's crust, J. Geophys., 61, 69-76. This study is supported by State Key Laboratory of Geodesy and Earth's Dynamics (Institute of Geodesy and Geophysics, Chinese Academy of Sciences) (SKLGED2015-5-5-EZ), Natural Science Fund of Hubei Province (2015CFB361), International Cooperation Project in Science and Technology of China (2010DFA24580), China Postdoctoral Science Foundation (2015M572217 and 2014T70753), Hubei Subsurface Multi-scale Imaging Key Laboratory (Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan) (SMIL-2015-06) and National Natural Science Foundation of China (41574070, 41104048 and 41504065).

  17. A Preliminary Study on the Lithosphere-Asthenosphere Boundary beneath the South China Sea

    NASA Astrophysics Data System (ADS)

    Lee, T. T. Y.; Chen, C. W.

    2014-12-01

    The lithosphere-asthenosphere boundary (LAB) is an important boundary at which the rigid lithosphere translates coherently upon the viscous asthenosphere. New observations have been made on LAB through detailed seismic analysis, especially that from receiver functions. Previous studies have found LAB depth varies significantly, systematically getting shallower from continental to oceanic lithosphere. In smaller scale, the depth and sharpness of LAB also differ from region to region, suggesting the effects of a combination of thermal and compositional origins. In this study, we investigate the LAB beneath the South China Sea, a region poorly instrumented that conventional seismological are less effective and impractical. The South China Sea is on the Sunda Plate, which is considered to be once the southeastern part of the Eurasia Plate before separating with a distinct moving direction from that of India-Eurasia continental collision. The South China Sea is Phanerozoic in age and continental in nature, but the striped magnetic anomalies observed from the sea floor have suggested multiple spreading events since early Miocene, indicating the presence of latter formed oceanic lithosphere. Previous seismic studies of this region focused mainly on shallow basin structure pertaining to petroleum exploration. The lithospheric structure, particularly the LAB, remains elusive, while it provides important insight into the complex tectonic history in this region. To image the LAB, we use the precursor of SS phase. The precursor bounces at the LAB discontinuity at depth would appear before the SS and presents a signal amenable to analysis for depth and properties. We collect seismic waveform data recorded mainly at Japan and Cocos Islands of corresponding teleseismic events from Southern Sumatera and Japan, with SS and potential precursors bouncing beneath the South China Sea. We employ an analysis technique, velocity spectral analysis (vespagrams), to identify precursory

  18. Wrinkle ridges, reverse faulting, and the depth penetration of lithospheric stress in lunae planum, Mars

    NASA Technical Reports Server (NTRS)

    Zuber, M. T.

    1993-01-01

    Tectonic features on a planetary surface are commonly used as constraints on models to determine the state of stress at the time the features formed. Quantitative global stress models applied to understand the formation of the Tharsis province on Mars constrained by observed tectonics have calculated stresses at the surface of a thin elastic shell and have neglected the role of vertical structure in influencing the predicted pattern of surface deformation. Wrinkle ridges in the Lunae Planum region of Mars form a conentric pattern of regularly spaced features in the eastern and southeastern part of Tharsis; they are formed due to compressional stresses related to the response of the Martian lithosphere to the Tharsis bulge. As observed in the exposures of valley walls in areas such as the Kasei Valles, the surface plains unit is underlain by an unconsolidated impact-generated megaregolith that grades with depth into structurally competent lithospheric basement. The ridges have alternatively been hypothesized to reflect deformation restricted to the surface plains unit ('thin skinned deformation') and deformation that includes the surface unit, megaregolith and basement lithosphere ('thick skinned deformation'). We have adopted a finite element approach to quantify the nature of deformation associated with the development of wrinkle ridges in a vertically stratified elastic lithosphere. We used the program TECTON, which contains a slippery node capability that allowed us to explicitly take into account the presence of reverse faults believed to be associated with the ridges. In this study we focused on the strain field in the vicinity of a single ridge when slip occurs along the fault. We considered two initial model geometries. In the first, the reverse fault was assumed to be in the surface plains unit, and in the second the initial fault was located in lithospheric basement, immediately beneath the weak megaregolith. We are interested in the conditions underwhich

  19. 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

  20. 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.

  1. 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.

  2. 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.

  3. 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

  4. 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).

  5. Helium isotopes of the Siberian sub-continental lithospheric mantle: Insights from eclogite xenoliths

    NASA Astrophysics Data System (ADS)

    Barry, P. H.; Hilton, D. R.; Day, J. M.; Pernet-Fisher, J.; Howarth, G. H.; Taylor, L. A.

    2014-12-01

    Helium isotopes (3He/4He) have been extensively used to define distinct segments of Earth's mantle and characterize its chemical structure. Specifically, they have been used to illustrate the long-term isolation and preservation of high-3He/4He (≥50 RA; [1]) plume-derived materials from the well-mixed and more-extensively degassed depleted MORB mantle (DMM) (8 RA; [2]). However, the He-isotope signature of the sub-continental lithospheric mantle (SCLM) remains relatively poorly characterized (6.1 RA; [3]). The Siberian craton hosts >1000 kimberlite intrusions, which carry mantle-derived xenoliths - of varying compositions (i.e., peridotites, dunites, and eclogites) - to the Earth's surface, making it an ideal setting for investigating the chemical evolution of the SCLM. Here, we report new He-isotope and concentration data for a suite of eclogitic xenoliths (n=10) from the Udachnaya pipe, Siberia. He-isotopes and [He] contents were determined by crushing garnet and pyroxene mineral separates from 2.7-3.1 Ga Siberian eclogites. 3He/4He values ranged from 0.11 to 1.0 RA, displaying predominantly radiogenic (i.e., low 3He/4He) He-isotope values. In contrast, Siberian flood basalt values extend up to ~13 RA [4]. Helium concentrations span ~4 orders of magnitude from 60 to 569,000 [4He]C ncm3STP/g. The radiogenic nature of Udachnaya eclogites indicate that they have been largely isolated from basaltic metasomatic fluxes over geological time due to position within the lithosphere and/or lithospheric age. Further, low 3He/4He values may reflect the addition of high U-Th material into the lithosphere by accretion of ancient island-arc terrains. These new data add to the growing He-isotope database [5,6] for the Siberian SCLM, and reveal the heterogeneous nature of this region with respect to He-isotopes, as well as the potential importance of crustal recycling and metasomatic processes. [1] Stuart et al., 2003. Nature. [2] Graham, 2002. Reviews in Mineralogy and

  6. 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

  7. 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.

  8. 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.

  9. 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

  10. 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.

  11. 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.

  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. 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

  14. 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.

  15. 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

  16. 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

  17. 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

  18. 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.

  19. 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

  20. 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

  1. 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.

  2. Parameters driving strain localization in the lithosphere are highly scale-dependent

    NASA Astrophysics Data System (ADS)

    Jolivet, Laurent

    2016-04-01

    Modelling lithospheric deformation requires specifying mechanisms that promote strain localization. This can be done in different ways, such as the inclusion of weaker zones in the model setup (to initiate subduction or slab tearing, for instance) or using various sorts of weakening processes depending upon temperature, grain-size, fluid content or metamorphic reactions, among others. In most cases, this choice is ad hoc because the relevant parameters are largely unknown, especially at the scale of geodynamic models. Two lines of research have been developed, a traditional one which seeks to determine the rheological parameters of natural or synthetic rocks experimentally, and a more recent one, promoted by the development of fast computing, which aims at reproducing a natural tectonic or rheological evolution through time, not only geometries. The latter requires that the parameters allowing this reproduction are significant at the scale of the model, and which may be different from those obtained in the experimental lab, thus questioning the extrapolation through a wide range of scales of experimental parameters. This apparent discrepancy is due to the intrinsic complexity of the lithosphere, and even more so for the continental lithosphere with its highly heterogeneous crust and its long tectonic history, which implies the co-existence of many different parameters active in nature. In this presentation, we review the main localizing factors and look to the range of scales in which they are significant. Small-scale processes such as grain-size reduction, coexistence of several mineralogical phases with different strength and rheological behaviour, fluid-rock interactions and/or metamorphic reactions, often cannot initiate strain localization in nature but are all efficient to locally reduce the strength of rock material once localization has started. Some exceptions to this rule, however, exist, such as the mixing of ductile and brittle behaviour in the same

  3. Deformation Processes In SE Tibet: How Coupled Are The Surface And The Deeper Lithosphere? (Invited)

    NASA Astrophysics Data System (ADS)

    Zeitler, P. K.; Meltzer, A.

    2010-12-01

    We all like to cite the Himalayan collision as a type example of continent-continent collision, and the region has been used as a natural laboratory by a considerable number of diverse investigations. Southeastern Tibet and the Lhasa Block provide an interesting case to consider in this context. Surrounding portions of the Himalayan-Tibet system have been and are being intensely deformed, whereas the Andean-arc lithosphere of the Lhasa Block has remained enigmatically unscathed. High elevations throughout much of the terrane are fairly uniform but the eastern and western portions of block have experienced very different degrees of exhumation. Regions that experienced more exhumation have thinner crustal thicknesses, with the results that that Moho is warped up with respect to the surface. Thicker, less-exhumed portions of the Lhasa Block currently are underlain by what is inferred to be eclogitized lower crust, but this eclogitization is not seen where exhumation is significant. Beneath SE Tibet, subduction of the Indian lithosphere has been complicated, with tomographic imaging showing variations in mantle structure that do not register with the strike of surface features. Adjacent to the Lhasa Block, the Namche Barwa-Gyala metamorphic massif demonstrates a strong coupling between shallower crustal flow and localized erosion that is significant for the evolution of the Lhasa Block in the way that this feature controls base level for the upper Tsangpo drainage and thus the erosional driver for the system. More broadly, a weak lower crust and lower-crustal flow have been invoked by many workers to explain aspects of the region’s deformation patterns and topography. Thus it would seem that in SE Tibet, mid-to-upper crustal, lower-crustal, and whole-lithosphere processes all have the potential to either impact Earth-surface dynamics or be impacted by them. This leads to a number of questions about the 4D nature and scale of compensation, controls on the evolution of

  4. Lithospheric structure at the leading edge of the North American craton: Imaging the Shield-Cordillera transition in western Canada by teleseismic Rayleigh-wave analysis

    NASA Astrophysics Data System (ADS)

    Bao, X.; Eaton, D. W.

    2013-12-01

    The tectonics of southwestern Canada is characterized by a transition from the Mesozoic-Cenozoic Cordilleran orogen to the Archean-Proterozoic craton, making this region an excellent natural laboratory to study episodic growth of continents. Here, we explore regional lithospheric structure using fundamental Rayleigh waves recorded by broadband seismometers from CNSN and various temporary networks (ATSN, CRANE, USArray) from 2006 to the present. Using a two-station cross-correlation technique, we extract phase velocities at periods ranging from 20 s to more than 200 s. Phase velocities for the region west of the deformation front are significantly lower than those of the region to the east, especially at periods of 30-200 s, reflecting strong lateral variations in lithosphere structure due to thermal and compositional differences. Our analysis is particularly focused on characterizing the lithosphere-asthenosphere boundary (LAB) across the transition, and identifying small-scale convection associated with the edge of the craton. Detailed 3-D lithospheric structural models developed in the near feature will shed more new lights on the transition from backarc to craton lithosphere and their geodynamical interactions.

  5. 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

  6. An equivalent source method for modelling the global lithospheric magnetic field

    NASA Astrophysics Data System (ADS)

    Kother, Livia; Hammer, Magnus D.; Finlay, Christopher C.; Olsen, Nils

    2015-10-01

    We present a new technique for modelling the global lithospheric magnetic field at Earth's surface based on the estimation of equivalent potential field sources. As a demonstration we show an application to magnetic field measurements made by the CHAMP satellite during the period 2009-2010 when it was at its lowest altitude and solar activity was quiet. All three components of the vector field data are utilized at all available latitudes. Estimates of core and large-scale magnetospheric sources are removed from the measurements using the CHAOS-4 model. Quiet-time and night-side data selection criteria are also employed to minimize the influence of the ionospheric field. The model for the remaining lithospheric magnetic field consists of magnetic equivalent potential field sources (monopoles) arranged in an icosahedron grid at a depth of 100 km below the surface. The corresponding model parameters are estimated using an iteratively reweighted least-squares algorithm that includes model regularization (either quadratic or maximum entropy) and Huber weighting. Data error covariance matrices are implemented, accounting for the dependence of data variances on quasi-dipole latitude. The resulting equivalent source lithospheric field models show a degree correlation to MF7 greater than 0.7 out to spherical harmonic degree 100. Compared to the quadratic regularization approach, the entropy regularized model possesses notably lower power above degree 70 and a lower number of degrees of freedom despite fitting the observations to a very similar level. Advantages of our equivalent source method include its local nature, the possibility for regional grid refinement and the production of local power spectra, the ability to implement constraints and regularization depending on geographical position, and the ease of transforming the equivalent source values into spherical harmonics.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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.

  19. 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.

  20. 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

  1. Strike-slip earthquakes in the oceanic lithosphere: Observations of exceptionally high apparent stress

    USGS Publications Warehouse

    Choy, G.L.; McGarr, A.

    2002-01-01

    The radiated energies, Es, and seismic moments, Mo, for 942 globally distributed earthquakes that occurred between 1987 to 1998 are examined to find the earthquakes with the highest apparent stresses (??a = ?? Es/Mo, where ?? is the modulus of rigidity). The globally averaged ??a for shallow earthquakes in all tectonic environments and seismic regions is 0.3 MPa. However, the subset of 49 earthquakes with the highest apparent stresses (??a greater than about 5.0 MPa) is dominated almost exclusively by strike-slip earthquakes that occur in oceanic environments. These earthquakes are all located in the depth range 7-29 km in the upper mantle of the young oceanic lithosphere. Many of these events occur near plate-boundary triple junctions where there appear to be high rates of intraplate deformation. Indeed, the small rapidly deforming Gorda Plate accounts for 10 of the 49 high-??a events. The depth distribution of ??a, which shows peak values somewhat greater than 25 MPa in the depth range 20-25 km, suggests that upper bounds on this parameter are a result of the strength of the oceanic lithosphere. A recently proposed envelope for apparent stress, derived by taking 6 per cent of the strength inferred from laboratory experiments for young (less than 30 Ma) deforming oceanic lithosphere, agrees well with the upper-bound envelope of apparent stresses over the depth range 5-30 km. The corresponding depth-dependent shear strength for young oceanic lithosphere attains a peak value of about 575 MPa at a depth of 21 km and then diminishes rapidly as the depth increases. In addition to their high apparent stresses, which suggest that the strength of the young oceanic lithosphere is highest in the depth range 10-30 km, our set of high-??a earthquakes show other features that constrain the nature of the forces that cause interplate motion. First, our set of events is divided roughly equally between intraplate and transform faulting with similar depth distributions of ??a for

  2. 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

  3. 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

  4. 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.

  5. Garnet: a key to unraveling Earth's dynamic lithosphere

    NASA Astrophysics Data System (ADS)

    Smit, M. A.; Scherer, E. E.; Mezger, K.; Lee, J.; Ratschbacher, L.; Kooijman, E.; Stearns, M. A.

    2015-12-01

    Garnet enables constraints on all parameters relevant to lithosphere studies: pressure, temperature, strain, and time. This aspect, in combination with its widespread occurrence in metamorphic rocks and ability to resist retrogression, make the mineral a prime target in research into the dynamics of mountain belts. Garnet-based petrological and geochemical tools have diversified and improved as a result of recent advancements in spatial and analytical resolution. In particular, our ability to obtain precise age constraints using garnet Lu-Hf and Sm-Nd geochronology, and interpret these in a geological context has greatly improved. This contribution highlights a series of recent enhancements to the garnet toolkit and demonstrates its versatility in two case studies set in an archetypal collisional orogen: the Pamir-Himalaya-Tibet mountain chain. To enable a more effective use of garnet geochronology, we investigated the retentiveness of Lu-Hf and Sm-Nd isotope signatures in naturally metamorphosed garnet. Diffusive re-equilibration of these signatures is shown to occur to a minor, if not insignificant, extent during crustal metamorphism, thus firmly establishing these methods as reliable geochronometers. Diffusive major-element zoning analysis of the same garnet led to the development of a new thermometric tool, which was shown to provide reliable temperature estimates for a wide variety of rocks and terranes. We used Lu-Hf garnet geochronology to show that mid-crustal flow and 'Barrovian-type' metamorphism of rocks now exposed in the North Himalayan Gneiss Domes in Central Tibet commenced in the early Eocene. This result is the first to confirm that crustal thickening and contraction in the Tibetan Himalaya was broadly synchronous with the collision between Greater India and Eurasia. Garnet dating and thermometry, and rutile U-Pb thermochronology in the Pamir revealed a history of heating to 750-830 °C, commencing at 37 Ma in the South Pamir and occurring

  6. Applying modern measurements of Pleistocene loads to model lithospheric rheology

    NASA Astrophysics Data System (ADS)

    Beard, E. P.; Hoggan, J. R.; Lowry, A. R.

    2011-12-01

    The remnant shorelines of Pleistocene Lake Bonneville provide a unique opportunity for building a dataset from which to infer rheological properties of the lower crust and upper mantle. Multiple lakeshores developed over a period of around 30 kyr which record the lithosphere's isostatic response to a well-constrained load history. Bills et al. (1994) utilized a shoreline elevation dataset compiled by Currey (1982) in an attempt to model linear (Maxwell) viscosity as a function of depth beneath the basin. They estimated an effective elastic thickness (Te) for the basin of 20-25 km which differs significantly from the 5-15 km estimates derived from models of loading on geologic timescales (e.g., Lowry and Pérez-Gussinyé, 2011). We propose that the discrepancy in Te modeled by these two approaches may be resolved with dynamical modeling of a common rheology, using a more complete shoreline elevation dataset applied to a spherical Earth model. Where Currey's (1982) dataset was compiled largely from observations of depositional shoreline features, we are developing an algorithm for estimating elevation variations in erosional shorelines based on cross-correlation and stacking techniques similar to those used to automate picking of seismic phase arrival times. Application of this method to digital elevation models (DEMs) will increase the size and accuracy of the shoreline elevation dataset, enabling more robust modeling of the rheological properties driving isostatic response to unloading of Lake Bonneville. Our plan is to model these data and invert for a relatively small number of parameters describing depth- and temperature-dependent power-law rheology of the lower crust and upper mantle. These same parameters also will be used to model topographic and Moho response to estimates of regional mass variation on the longer loading timescales to test for inconsistencies. Bills, B.G., D.R. Currey, and G.A. Marshall, 1994, Viscosity estimates for the crust and upper

  7. Peculiarities of the deep structure of the lithosphere in the northeastern part of the Indian Ocean

    NASA Astrophysics Data System (ADS)

    Mazo, E. L.; Shreider, A. A.; Kulikova, M. P.; Gilod, D. A.; Shreider, Al. A.

    2009-12-01

    The seismicity of the northeastern part of the Indian Ocean in connection with the general structural peculiarities of the main tectonic structures of the bottom is presented. The three main ranges of higher seismic activity at the depths of 0-17, 20-27, and 32-35 km divided by aseismic layers are revealed. The seismic activity at depths of more than 35 km is almost not detected both for the ocean and for the Indian peninsula. The nature of the distribution of the seismicity as such in the lithosphere is discussed. Using the results of anomalous au]gravitational field transformations, the prolongation of the East Indian Ridge structure is revealed to 19 degrees north, while the relationship of the Afanasy Nikitin Rise and the 85th Degree Ridge is not reflected at the tranforms. In the Cocos Basin, the mutually perpendicular disturbance zones of northeastern and northwestern strike, as well as the point of their crossing, where the maximal number of earthquake foci are concentrated, are distinguished. A conclusion concerning the substantial disturbance of the strength properties of the lithosphere in this zone as a consequence of the geodynamical processes, which are accompanied by fracture tectonics, is reached.

  8. Serpentinization of the Martian early lithosphere to produce magnetite, CO2 and H2

    NASA Astrophysics Data System (ADS)

    Quesnel, Y.; Sotin, C.; Langlais, B.; Mandea, M.

    2007-08-01

    The magnetic measurements made on board the Mars Global Surveyor (MGS) probe revealed the intense nature of the lithospheric magnetic field at Mars. Since most of the strong anomalies at satellite altitudes (between 100 and 400 km) were measured over Noachian surfaces, crustal magnetization and / or demagnetization in the presence of an early axial, global and dipolar magnetic field is the most likely scenario to explain the anomaly distribution. Martian lithospheric magnetization contrasts derived by numerous studies range from 10 to more than 60 A/m, much larger than classical terrestrial values. In this study, we show that the chemical remanent magnetization (CRM) through the serpentinization can produce such large values. This produces magnetite as well as CO2 and H2. The Noachian period gathers all the necessary ingredients for this process to be emplaced: a strong magnetic field, a basaltic crust, large amounts of water causing hydrothermalism, and local heating due to the internal mantle convection. Predicted magnetization values are computed according to different hypotheses on the intensity of the main paleofield and the serpentinization process. This latter aspect is investigated with respect to the initial iron content of the mafic mineral, the water abundance and the reacting rock volume. The amounts of released H2 and CO2, possibly combining in CH4, are also studied.

  9. Continental scale body wave tomography of India: Evidence for attrition and preservation of lithospheric roots

    NASA Astrophysics Data System (ADS)

    Singh, Arun; Mercier, J.-P.; Ravi Kumar, M.; Srinagesh, D.; Chadha, R. K.

    2014-03-01

    assemble P and S waveforms of 2301 teleseismic earthquakes registered at 413 broadband seismic stations spanning the Indian plate from the southern tip of India to the Himalayan collision belt and generate an accurate data set of 52,050 P and 30,423 S arrival times through the multichannel cross-correlation approach. These traveltimes are then inverted to obtain 3-D P and S velocity structures of the subcontinent at a 2° × 2° lateral resolution. The heterogeneous nature of the Indian lithospheric mantle revealed in this study suggests that the lithospheric roots are not uniformly thick on a regional scale. The key cratonic segments of the Indian shield are characterized by pockets of high velocity anomalies (˜3%) at shallow depths (<300 km), with the diamondiferous regions like Wajrakarur revealing high shear wave anomalies down to ˜300 km. In contrast to the southern Deccan volcanic province (DVP), the northwestern DVP is underlain by low velocity anomalies at similar depths suggesting that the upper mantle retains imprints of Deccan volcanism which was facilitated by the reactivation of the rift systems.

  10. Lithospheric deformation and mantle/crust coupling related to slab roll-back and tearing processes: the role of magma-related rheological weakening highlighted by 3D numerical modeling

    NASA Astrophysics Data System (ADS)

    Menant, Armel; Jolivet, Laurent; Guillou-Frottier, Laurent; Sternai, Pietro; Gerya, Taras

    2016-04-01

    Active convergent margins are the locus of various large-scale lithospheric processes including subduction, back-arc opening, lithospheric delamination, slab tearing and break-off. Coexistence of such processes results in a complex lithospheric deformation pattern through the rheological stratification of the overriding lithosphere. In this context, another major feature is the development of an intense arc- and back-arc-related magmatism whose effects on lithospheric deformation by rheological weakening are largely unknown. Quantifying this magma-related weakening effect and integrating the three-dimensional (3D) natural complexity of subduction system is however challenging because of the large number of physico-chemical processes involved (e.g. heat advection, dehydration of subducted material, partial melting of the mantle wedge). We present here a set of 3D high-resolution petrological and thermo-mechanical numerical experiments to assess the role of low-viscosity magmatic phases on lithospheric deformation associated with coeval oceanic and continental subduction, followed by slab retreat and tearing processes. Results in terms of crustal kinematics, patterns of lithospheric deformation and distribution and composition of magmatic phases are then compared to a natural example displaying a similar geodynamical evolution: the eastern Mediterranean subduction zone. Our modeling results suggest that the asthenospheric flow controls the ascending trajectories of mantle-derived magmatic sources developed in the mantle wedge in response to dehydration of oceanic slab. Once stored at the base of the overriding continental crust, low-viscosity mantle- and crustal-derived magmatic phases allow to decrease the lithospheric strength. This weakening then enhances the propagation of localized extensional and strike-slip deformation in response to slab roll-back and extrusion tectonics respectively. In addition, we show that storage of large amounts of low-viscosity magmas

  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. 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

  16. HuBLE-UK: the Hudson Bay Lithospheric Experiment: Insights into the Formation of the Canadian Shield From Broadband Seismology

    NASA Astrophysics Data System (ADS)

    Bastow, I. D.; Thompson, D. A.; Kendall, J.-M.; Helffrich, G.; Wookey, J.; Snyder, D.; Eaton, D.; Darbyshire, F.

    2012-04-01

    The Canadian Shield is one of the largest exposures of Precambrian rocks on Earth. It is a mosaic of several Archean terranes that were brought together during a series of Paleoproterozoic orogens culminating in the so-called Trans-Hudson orogen, which is thought to have been similar to the Himalayan orogen in scale and nature. The tectonic evolution and lithospheric subdivisions of this region are poorly understood, but new seismic networks in northern Hudson Bay provide fresh opportunity to place constraints on the Preachbrian processes that formed and shaped it. Using a combination of seismic tomography, anisotropy and receiver function analysis we show that the lithosphere of the northern Hudson Bay region retains a strong signature of Archean-Paleoproterozoic tectonics. We map the boundary between the upper (Churchill) and lower (Superior) plates that collided ca. 1.8 Ga and identify backazimuth dependent shear-wave splitting parameters (phi, dt) on Baffin Island that indicate complex anisotropy (e.g., dipping fabric) beneath the region. Our results support the view that significant lithospheric deformation occurred during the Paleoproterozoic and that modern-day plate tectonic processes were thus in operation by at least ca. 1.8 Ga.

  17. 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

  18. 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.

  19. 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

  20. 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

  1. 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.

  2. 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.

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

    NASA Astrophysics Data System (ADS)

    Gerya, Taras; Burov, Evgenii

    2016-04-01

    Triple junctions are among the most remarkable features of global plate tectonics but their nucleation and evolution remains debatable. Divergent (R-R-R) triple junctions (at 120o and T junctions) are particular ones since their stability depends on the exact values of the relative velocities of multi-directional plate motions and hence is strongly affected by plate rheology and processes of crustal and lithospheric accretion. It is commonly accepted (although not quantitatively tested) that the geometry and stability of R-R-R triple junctions should be related to the intuitive geometric considerations that 3-branch configurations should be more "stable" compared to >3-branch configurations (e.g. quadruple junctions) under conditions of long-term multi-directional extension on a 3D Earth surface. Indeed, it has been long-time suggested that triple junctions result from evolution of short-lived quadruple junctions, yet, without providing a consistent mechanical explanation or experimental demonstration of this process, due to the rheological complexity of the breaking lithosphere subsequently subjected to complex oceanic crustal and lithospheric accretion processes. Therefore, a complete 3D thermo-mechanically consistent approach is needed to understand the processes of formation of multi-branch junctions. Here, we study numerically the processes of multi-branch junctions formation under condition of multi-directional lithospheric extension. We use high-resolution 3D numerical magmatic-thermo-mechanical experiments that take into account realistic thermo-rheological structure and rheology of the lithosphere and account for crustal and lithospheric accretion processes. We find that two major types of quadruple and triple junctions are formed under bi-directional or multidirectional far-field stress field: (1) plate rifting junctions are formed by the initial plate fragmentation and can be subsequently re-arranged into (2) oceanic spreading junctions controlled by the

  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. Structure of the deep oceanic lithosphere in the Northwestern Pacific ocean basin derived from active-source seismic data

    NASA Astrophysics Data System (ADS)

    Ohira, A.; Kodaira, S.; Nakamura, Y.; Fujie, G.; Arai, R.; Miura, S.

    2015-12-01

    Many seismological studies have detected the sharp seismic discontinuities in the upper mantle, some of which are interpreted the lithosphere-asthenosphere boundary (LAB). However there are few data at the old Pacific plate, in particular at ocean basin, which is critical information for understanding nature of the oceanic LAB. In 2014 we conducted an active-source refraction/reflection survey along a 1130-km-long line in southeast of the Shatsky Rise. Five ocean bottom seismometers (OBSs) were deployed and recovered by R/V Kairei of JAMSTEC. We used an airgun array with a total volume of 7,800 cubic inches with firing at intervals of 200 m. Multi-channel seismic reflection (MCS) data were also collected with a 444-channel, 6,000-m-long streamer cable. In OBS records the apparent velocity of the refraction waves from the uppermost mantle was high (< 8.6 km/sec), and considered to be caused by preferred orientation of olivine (e.g., Kodaira et al., 2014). Another remarkable feature is wide-angle reflection waves from the deep lithosphere at large (150-500 km) offsets. We applied the traveltime mapping method (Fujie et al., 2006), forward analysis (Zelt and Smith, 1992) and the amplitude modeling (Larsen and Grieger, 1998) to the OBS data. The results show that deep mantle reflectors exist at the depths from 35 to 60 km, and one possible explanation is that these reflectors correspond to patched low velocity zones around the base of the lithosphere. On MCS sections the clear and sharp Moho was imaged only at the southwestern end of the profile, but Moho was ambiguous or even not imaged in the most part of the profile. Since our seismic line covers the oceanic lithosphere with different ages that correspond to different stages of the Shatsky activity, the Moho appearance may reflect the variation of the Shatsky activity.

  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. 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).

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  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

    2016-04-01

    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.

  19. 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

  20. 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

  1. The lithosphere-asthenosphere system beneath Ireland from integrated geophysical-petrological modeling - I: Observations, 1D and 2D hypothesis testing and modeling

    NASA Astrophysics Data System (ADS)

    Jones, Alan G.; Afonso, Juan Carlos; Fullea, Javier; Salajegheh, Farshad

    2014-02-01

    Modeling the continental lithosphere's physical properties, especially its depth extent, must be done within a self-consistent petrological-geophysical framework; modeling using only one or two data types may easily lead to inconsistencies and erroneous interpretations. Using the LitMod approach for hypothesis testing and first-order modeling, we show how assumptions made about crustal information and the probable compositions of the lithospheric and sub-lithospheric mantle affect particular observables, particularly especially surface topographic elevation. The critical crustal parameter is density, leading to ca. 600 m error in topography for 50 kg m- 3 imprecision. The next key parameter is crustal thickness, and uncertainties in its definition lead to around ca. 4 km uncertainty in LAB for every 1 km of variation in Moho depth. Possible errors in the other assumed crustal parameters introduce a few kilometers of uncertainty in the depth to the LAB. We use Ireland as a natural laboratory to demonstrate the approach. From first-order arguments and given reasonable assumptions, a topographic elevation in the range of 50-100 m, which is the average across Ireland, requires that the lithosphere-asthenosphere boundary (LAB) beneath most of Ireland must lie in the range 90-115 km. A somewhat shallower (to 85 km) LAB is permitted, but the crust must be thinned (< 29 km) to compensate. The observations, especially topography, are inconsistent with suggestions, based on interpretation of S-to-P receiver functions, that the LAB thins from 85 km in southern Ireland to 55 km in central northern Ireland over a distance of < 150 km. Such a thin lithosphere would result in over 1000 m of uplift, and such rapid thinning by 30 km over less than 150 km would yield significant north-south variations in topographic elevation, Bouguer anomaly, and geoid height, none of which are observed. Even juxtaposing the most extreme probable depleted composition for the lithospheric mantle

  2. Lithosphere-Atmosphere-Ionosphere Coupling System

    NASA Astrophysics Data System (ADS)

    Kachakhidze, Manana; Kachakhidze, Nino; Kaladze, Tamaz

    2014-06-01

    Modern ground-based and satellite methods of viewing enables to reveal those multiple anomalous geophysical phenomena which become evident in the period preceding earthquake and are directly connected with the process of its preparation. Lately special attention is attributed to the electromagnetic emissions fixed during large earthquake, and has already been successfully detected in Japan, America and Europe. Unfortunately there is no electromagnetic emissions detection network in Georgia, but the offered work, based on experimental data of foreign researchers and electrodynamics, presents an important theory about the electromagnetic emissions generation fixed in the earthquake preparation period. The extremely interesting methodology of possible prediction of earthquake is created and all anomalous geophysical phenomena are interpreted which take place some months, days or hours before earthquake in the lithosphereatmosphere-ionosphere coupling system. Most interesting is the idea of the authors to consider the electromagnetic radiation as the main earthquake precursor for the purpose of earthquake prediction, because of its informative nature and to consider all other anomalous geophysical phenomena which accompany the process of earthquake preparation as earthquake indicators. The offered work is the completely novel approach in earthquake problem searching with the view of earthquake prediction. It can form the base for creation of principally new trend in seismology, to be called conditionally "Earthquake Predictology".

  3. Numerical Simulation of the Variation of Schumann Resonance Associated with Seismogenic Processe in the Lithosphere-Atmosphere-Ionosphere system

    NASA Astrophysics Data System (ADS)

    Liu, L.; Huang, Q.; Wang, Y.

    2012-12-01

    The variations in the strength and frequency shift of the Schumann resonance (SR) of the electromagnetic (EM) field prior to some significance earthquakes were reported by a number of researchers. As a robust physical phenomenon constantly exists in the resonant cavity formed by the lithosphere-atmosphere-ionosphere system, irregular variations in SR parameters can be naturally attributed to be the potential precursory observables for forecasting earthquake occurrences. Schumann resonance (SR) of the EM field between the lithosphere and the ionosphere occurs because the space between the surface of the Earth and the conductive ionosphere acts as a closed waveguide. The cavity is naturally excited by electric currents generated by lightning. SR is the principal background in the electromagnetic spectrum at extremely low frequencies (ELF) between 3-69 Hz. We simulated the EM field in the lithosphere-ionosphere waveguide with a 2-dimensional (2D), cylindrical whole-earth model by the hybrid pseudo-spectral and finite difference time domain method. Considering the seismogensis as a fully coupled seismoelectric process, we simulate the seismic wave and EM wave in this 2D model. The excitation of SR in the background EM field are generated by the electric-current impulses due to lightning thunderstorms within the lowest 10 kilometers of the atmosphere . The diurnal variation and the latitude-dependence in ion concentration in the ionosphere are included in the model. After the SR has reached the steady state, the impulse generated by the seismogenic process (pre-, co- and post-seismic) in the crust is introduced to assess the possible precursory effects on SR strength and frequency. The modeling results explain the observed fact of why SR has a much more sensitive response to continental earthquakes, and much less response to oceanic events; the reason is simply due to the shielding effect of the conductive ocean that prevents effective radiation of the seismoelectric

  4. Lithosphere and Asthenosphere Properties beneath Oceans and Continents and their Relationship with Domains of Partial Melt Stability in the Mantle

    NASA Astrophysics Data System (ADS)

    Dasgupta, R.

    2014-12-01

    The depth of the lithosphere-asthenosphere boundary (LAB) and the change in properties across the lithosphere, asthenosphere, and LAB in various tectonic settings are captured in a variety of geophysical data, including seismic velocities and electrical conductivity. A sharp drop in shear wave velocity and increase in electrical conductivity can potentially be caused by the appearance of partial melt at or below the LAB but the chemical and dynamic stability of partial melt across lithosphere and at LAB remain debated. Here I apply the recent models of mantle melting in the presence of water and carbon [1, 2] to evaluate the domains of stability of partial melt both beneath continents and oceans. The model allows prediction of the possible presence, the fraction, and composition of partial melt as a function of depth, bulk C and H2O content, and fO2 [3] in various geologic/tectonic settings. The results show that while a hydrous, carbonated melt is stable only beneath LAB and in the asthenospheric mantle beneath oceans, continental mantle can contain a carbonate-rich melt within the lithosphere. For geotherms corresponding to surface heat flux (SHF) of 40-50 mW m-2, which also match P-T estimates beneath cratons based on thermo-barometry of peridotite xenoliths [4], the solidus of fertile peridotite with trace amount of CO2 and H2O is crossed at depths as shallow as 80-120 km [5]. If elevated geotherms of the Proterozoic and Phanerozoic terrains are applied, carbonatitic melt becomes stable somewhat shallower. These depths are similar to those argued for a mid-lithospheric discontinuity (MLD) where a negative velocity gradient has been detected much shallower than the proposed depth of LAB in many places. With a drop in oxygen fugacity with depth, a freezing of carbonatitic melt may be expected at intermediate depths (~150-200 km). At 200-250 km a hydrous, carbonated silicate melt may reappear owing to the interplay of fO2 and freezing point depression effect of CO

  5. 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

  6. 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

  7. From Oceanic Lithosphere Subduction To Continental Collision: Influence Of The Plate Contact

    NASA Astrophysics Data System (ADS)

    de Franco, R.; Govers, R.; Wortel, R.

    2004-12-01

    We showed recently that the overall dynamics of oceanic subduction differ depending on whether the plate contact is a fault or a channel (De Franco & al., 2007. GJI, doi: 10.1111/j.1365-246X.2006.03498.x). Here we investigate how the plate contact affects the transition from oceanic lithosphere subduction to continental collision. We use a finite element method to solve the heat and the time dependent momentum equations for elastic, (power law) viscous and plastic rheologies. For the same rheological properties and driving forces , varying the nature of the plate contact leads to three types of responses: subduction of the entire continental lithosphere, shear delamination of the continental crust or slab break-off. We make the following observations from our numerical experiments. The presence of a subduction channel promotes coherent and, when the boundary conditions allow it, plate-like subduction of the continental margin. In models with a subduction fault, coherent subduction of the incoming continental lithosphere occurs when the colliding passive margin has a gentle ocean-continent transition. The approaching continental sliver starts to subduct and the subduction is characterized by a non-plate-like behavior, slower subduction velocity than in channel models and strong slab deformation. If the continental margin is steep and the strength of the incoming continental crust is high, fault models result in locking of the trench, eventually leading to slab break-off. If the crustal strength is relatively low, shear delamination of the upper crust is expected. In the channel model this type of delamination never occurs. The tectonic setting does not significantly affect the nature of the model response. We conclude that the plate contact type, together with the geometrical and rheological properties of the incoming continental fragment, is a crucial subduction characteristic controlling the response of continental collision during the transition from oceanic

  8. From Oceanic Lithosphere Subduction To Continental Collision: Influence Of The Plate Contact

    NASA Astrophysics Data System (ADS)

    de Franco, R.; Govers, R.; Wortel, R.

    2007-12-01

    We showed recently that the overall dynamics of oceanic subduction differ depending on whether the plate contact is a fault or a channel (De Franco & al., 2007. GJI, doi: 10.1111/j.1365-246X.2006.03498.x). Here we investigate how the plate contact affects the transition from oceanic lithosphere subduction to continental collision. We use a finite element method to solve the heat and the time dependent momentum equations for elastic, (power law) viscous and plastic rheologies. For the same rheological properties and driving forces , varying the nature of the plate contact leads to three types of responses: subduction of the entire continental lithosphere, shear delamination of the continental crust or slab break-off. We make the following observations from our numerical experiments. The presence of a subduction channel promotes coherent and, when the boundary conditions allow it, plate-like subduction of the continental margin. In models with a subduction fault, coherent subduction of the incoming continental lithosphere occurs when the colliding passive margin has a gentle ocean-continent transition. The approaching continental sliver starts to subduct and the subduction is characterized by a non-plate-like behavior, slower subduction velocity than in channel models and strong slab deformation. If the continental margin is steep and the strength of the incoming continental crust is high, fault models result in locking of the trench, eventually leading to slab break-off. If the crustal strength is relatively low, shear delamination of the upper crust is expected. In the channel model this type of delamination never occurs. The tectonic setting does not significantly affect the nature of the model response. We conclude that the plate contact type, together with the geometrical and rheological properties of the incoming continental fragment, is a crucial subduction characteristic controlling the response of continental collision during the transition from oceanic

  9. 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

  10. 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

  11. Lithosphere-Surfacesphere-Atmosphere-Ionosphere coupling model for Vrancea seismic zone in Romania

    NASA Astrophysics Data System (ADS)

    Zoran, Maria; Savastru, Roxana; Savastru, Dan

    2016-03-01

    This paper presents a complex multidisciplinary approach concept to explain the nature of short-term earthquake precursors observed in land surface, atmosphere, ionosphere and magnetosphere for strong intermediate depth earthquakes recorded in Vrancea region in Romania. A developed Lithosphere-Surfacesphere-Atmosphere-Ionosphere (LSAI) coupling model can explain most of these presignals as a synergy between different anomalies of geophysical/geochemical parameters. These anomalies prior to medium to strong earthquakes are attributed to the thermodynamic, degassing and ionization processes in the Earth-Atmosphere system and micro-fracturing in the rocks especially along area's active faults. The main outcome of this paper is an unified concept for systematic validation of different types of earthquake precursors of which Land Surface Temperature (LST), outgoing Long wave Radiation (OLR), Surface Latent Heat Flux (SLHF), Air Temperature (AT), radon gas concentration, ionospheric Total Electron Content (TEC) are the most reliable parameters within the chain of the processes described by LSAI model.

  12. Events related to lithosphere-atmosphere-ionosphere-magnetosphere coupling observed by DEMETER

    NASA Astrophysics Data System (ADS)

    Parrot, Michel; Hattori, Katsumi; Liu, Tiger; Namgaladze, Alexander; Ouzounov, Dimitar; Pulinets, Sergey; Tramutoli, Valerio

    2015-04-01

    There are several models of Lithosphere-Atmosphere-Ionosphere-Magnetosphere (LAIM) coupling to explain ionospheric perturbations which are observed prior to earthquakes. In 2013 an ISSI Team led by S. Pulinets (RU) and D. Ouzounov (US) started to work with the following aim: "Multi-instrument Space-Borne Observations and Validation of the Physical Model of the LAIM Coupling" (see http://www.issibern.ch/teams/spaceborneobserve/). In the frame of this model validation several events have been studied with the DEMETER satellite data. It concerns the effects of (i) the ancient natural nuclear reactor located at Oklo (Gabon), (ii) the sand storms in Sahara, (iii) the volcanic activity, (iv) the lightning activity, and (v) the hurricanes. The main signature of these events in the ionosphere will be shown in this presentation.

  13. Electromagnetic, seismic and petro-physical investigations of the lithosphere-asthenosphere boundary in central Tibet

    NASA Astrophysics Data System (ADS)

    Vozar, J.; Fullea, J.; Jones, A. G.; Agius, M. R.; Lebedev, S.

    2011-12-01

    Combined seismological and electromagnetic investigations of the lithosphere and underlying asthenosphere have the potential to yield superior inferences than using either one on its own. Central Tibet offers an excellent natural laboratory for testing such approaches, given the high quality seismological and magnetotelluric (MT) data available as a consequence of INDEPTH studies. In particular, the presence and lateral and vertical extent of the Indian lithosphere beneath Tibet is highly debated. Integrated petrological-geophysical modeling of MT and surface-wave data, which are differently sensitive to temperature and composition, allows us to reduce the uncertainties associated with modeling these two data sets independently, as commonly undertaken. For the MT data, we use selected distortion-corrected MT transfer functions, from INDEPTH Phase III line 500 across central Tibet for 1D modeling. The selected data fit well the 1D assumption and exhibit large penetration depth. Our deep resistivity models can be classified into two different groups: i) the Lhasa Terrane and ii) the Qiangtang Terrane. For the Lhasa Terrane group, the models show the existence of two high conductive layers localized at depths of 60-80 km and more than 200 km, whereas for the Qiangtang Terrane these conductive layers appears to be occur at shallower depths, namely 30-50 km and 120 km depth respectively. Our dispersion curves for Rayleigh and Love surface waves were measured using seismograms recorded by stations of INDEPTH and PASSCAL experiments. Dispersion curves for central Lhasa and Qiangtang terranes show similarly low phase velocities at periods sampling the thick crust beneath the regions, but differ at periods sampling the mantle. Inverting the dispersion data for 1D, radially-anisotropic Vs profiles, we find that beneath central Qiangtang terrane shear velocity is lower than the global average down to 75 km below the Moho, indicating relatively high temperatures, whereas

  14. 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

  15. The buffering capacity of lithospheric mantle: implications for diamond formation

    NASA Astrophysics Data System (ADS)

    Luth, Robert W.; Stachel, Thomas

    2014-11-01

    Current models for the formation of natural diamond involve either oxidation of a methane-bearing fluid by reaction with oxidized mantle, or reduction of a carbonate-bearing fluid (or melt) by reaction with reduced mantle. Implicit in both models is the ability of the mantle with which the fluid equilibrates to act as an oxidizing or reducing agent, or more simply, to act as a source or sink of O2. If only redox reactions involving iron are operating, the ability of mantle peridotite to fulfill this role in diamond formation may not be sufficient for either model to be viable. Using the recent experimental recalibration of olivine-orthopyroxene-garnet oxybarometers of Stagno et al. (2013), we re-evaluated the global database of ~200 garnet peridotite samples for which the requisite Fe3+/Fe2+ data for garnet exist. Relative to the previous calibration of Gudmundsson and Wood (1995), the new calibration yields somewhat more oxidized values of Δlog fO2 (FMQ), with the divergence increasing from <0.5 units of log fO2 at ~3 GPa to as much as 1.5 units at 5-6.5 GPa. Globally, there is a range of ~4 log units fO2 for samples from the diamond stability field at any given pressure. Most samples are sufficiently reduced such that diamond, rather than carbonate, would be stable, and CHO fluids at these conditions would be H2O-rich (>60 mol%), with CH4 being the next most abundant species. To ascertain the capacity for mantle peridotite to act as a source or sink of O2, we developed a new model to calculate the fO2 for a peridotite at a given P, T, and Fe3+/Fe2+. The results from this model predict 50 ppm or less O2 is required to shift a depleted mantle peridotite the observed four log units of fO2. Coupled with the observed distribution of samples at values of fO2 intermediate between the most reduced (metal-saturated) and most oxidized (carbonate-saturated) possible values for diamond stability, these results demonstrate that peridotites are very poor sinks or sources of O

  16. 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.

  17. 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

  18. 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.

  19. Using subsidence and P-T-t history on the Alpine Tethys margin to constrain lithosphere deformation modes during continental breakup

    NASA Astrophysics Data System (ADS)

    Jeanniot, Ludovic; Kusznir, Nick; Manatschal, Gianreto; Mohn, Geoffroy; Beltrando, Marco

    2014-05-01

    Mantle exhumation and hyper-extended crust, as observed on the Iberia-Newfoundland conjugate margins, are key components of both present-day and fossil analogue magma-poor rifted margins. Conceptual models of the Alpine Tethys paleogeography evolution show a complex subsidence history, determined by the nature and composition of sedimentary, crustal and mantle rocks in the Alpine domains (Mohn et al., 2010). The relative timing of crustal rupture and decompressional melt initiation and inherited mantle composition control whether mantle exhumation may occur; the presence or absence of exhumed mantle therefore provides useful information on the timing of these events and constraints on lithosphere deformation modes and composition. A single mode of lithosphere deformation leading to continental breakup and sea-floor spreading cannot explain observations. We have determined the sequence of lithosphere deformation modes for the fossil Alpine Tethys margin using a numerical model of the temporal and spatial evolution of lithosphere deformation; the model has been calibrated against observations of subsidence and P-T-t history for the Alpine Tethys margin. A 2D finite element viscous flow model (FeMargin) is used to generate flow fields for a sequence of lithosphere deformation modes, which are used to advect lithosphere and asthenosphere temperature and material. FeMargin is kinematically driven by divergent deformation in the topmost 15-20 km of the lithosphere inducing passive upwelling beneath that layer; the upper lithosphere is assumed to deform by extensional faulting and magmatic intrusions, consistent with observations of deformation processes occurring at slow spreading ocean ridges (Cannat, 1996). We also include buoyancy enhanced upwelling in the kinematic model as proposed by Braun et al. (2000). We generate melt by decompressional melting using the parameterization and methodology of Katz et al. (2003). In the modelling of the Alpine Tethys margin

  20. 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

  1. 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

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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

  7. 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

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. Imaging Lithospheric-scale Structure Beneath Northern Altiplano in Southern Peru and Northern Bolivia

    NASA Astrophysics Data System (ADS)

    Kumar, A.; Wagner, L. S.; Beck, S. L.; Zandt, G.; Long, M. D.

    2014-12-01

    The northern Altiplano plateau of southern Peru and northern Bolivia is one of the highest topographic features on the Earth, flanked by Western and Eastern Cordillera along its margin. It has strongly influenced the local and far field lithospheric deformation since the early Miocene (Masek et al., 1994). Previous studies have emphasized the importance of both the crust and upper mantle in the evolution of Altiplano plateau (McQuarrie et al., 2005). Early tomographic and receiver function studies, south of 16° S, show significant variations in the crust and upper mantle properties in both perpendicular and along strike direction of the Altiplano plateau (Dorbath et. al., 1993; Myers et al., 1998; Beck and Zandt, 2002). In order to investigate the nature of subsurface lithospheric structure below the northern Altiplano, between 15-18° S, we have determined three-dimensional seismic tomography models for Vp and Vs using P and S-wave travel time data from two recently deployed local seismic networks of CAUGHT and PULSE. We also used data from 8 stations from the PERUSE network (PERU Subduction Experiment). Our preliminary tomographic models show a complex variation in the upper mantle velocity structure with depth, northwest and southeast of lake Titicaca. We see the following trend, at ~85 km depth, northwest of lake Titicaca: low Vp and Vs beneath the Western Cordillera, high Vs beneath the Altiplano and low Vp and Vs beneath the Eastern Cordillera. This low velocity anomaly, beneath Eastern Cordillera, seems to coincide with Kimsachata, a Holocene volcano in southern Peru. At depth greater than ~85 km: we find high velocity anomaly beneath the Western Cordillera and low Vs beneath the Altiplano. This high velocity anomaly, beneath Western Cordillera, coincides with the well-located Wadati-Benioff zone seismicity and perhaps represents the subducting Nazca slab. On the southeast of lake Titicaca, in northern Bolivia, we see a consistently high velocity anomaly

  13. Pseudotachylites and Earthquakes: New Evidence for the "Jelly Sandwich" Rheology of Continental Lithosphere (Invited)

    NASA Astrophysics Data System (ADS)

    Chen, W.; Yang, Z.

    2009-12-01

    The occurrence of pseudotachylite, an often-used proxy for brittle, seismogenic deformation, in mafic granulite facies has been cited as key evidence for the lower continental crust being stronger than the underlying uppermost mantle (“crème brûlée” model). Such reasoning seems unsound in that spectacular examples of pseudotachylite, exceeding 100 meters in length, occur in outcrops of the upper mantle. So if pseudotachylites indicate high mechanical strength, then the mantle lithosphere must be strong, supporting the “jelly sandwich” model of rheology. Moreover, pseudotachylites do occur in rocks of amphibolite facies where hydrous minerals are abundant, ruling out the notion that pseudotachylite implies dry conditions in the crust. Recent results from laboratory experiments also indicate that in general, mafic granulite is weaker than peridotite (Wang et al. [2008] and H. Green, personal communication). Perhaps the only stone left unturned is the pathological case where absolute-dry, mafic granulite were to juxtapose with hydrous peridotite - a hypothetical situation not observed in nature and yet to be linked with any specific, known geological processes. Meanwhile, cases of well-established, large- to moderate-sized earthquakes in the sub-continental mantle lithosphere (SCML) have been steadily accumulating, including events that generated clear underside reflections off the Moho above the hypocenters. Furthermore, a continent-wide analysis of precisely determined focal depths along and near the East African rift system (EARS) shows that different segments of the EARS exhibit three distinct patterns in focal depths, with a clear bimodal distribution beneath well-known but amagmatic rift valleys. The peaks of seismic moment release occur in the upper to mid-crust and near and below the Moho - a pattern established in several regions more than 25 years ago that implies a similar vertical distribution in limiting stress of the continental lithosphere

  14. Inelastic models of lithospheric stress - II. Implications for outer-rise seismicity and dynamics

    USGS Publications Warehouse

    Mueller, S.; Spence, W.; Choy, G.L.

    1996-01-01

    Outer-rise seismicity and dynamics are examined using inelastic models of lithospheric deformation, which allow a more realistic characterization of stress distributions and failure behaviour. We conclude that thrust- and normal-faulting outer-rise earthquakes represent substantially different states of stress within the oceanic lithosphere. Specifically, the normal-faulting events occur in response to downward plate bending, which establishes the 'standard', bending-dominated state of outer-rise stress, and the thrust-faulting events occur in response to an elevated level of in-plane compression, which develops only in response to exceptional circumstances. This interpretation accounts for the observation that normal-faulting outer-rise earthquakes occur more frequently and are more widely distributed than their thrust-faulting counterparts, an observation for which the simple bending model offers no explanation. In addition, attributing both thrust- and normal-faulting outer-rise earthquakes to plate bending implies that both classes of events should occur within relatively close lateral proximity to one another because both are allegedly a manifestation of the same bending-dominated stress distribution, whereas, in reality, this is not observed. We propose that the tendency for thrust-faulting outer-rise earthquakes to exhibit greater source depths than their normal-faulting counterparts (an observation that is frequently cited in support of the bending interpretation of the former) is merely a consequence of the fact that bending-induced tension is confined to the upper lithosphere. Our model predicts that outer-rise in-plane-force variations may promote thrust-faulting outer-rise activity prior to an underthrusting interplate subduction earthquake and normal-faulting outer-rise activity following such an earthquake, but that both forms of outer-rise activity are unlikely to be associated with the same subduction earthquake. A corollary implication of our model

  15. Imaging the LAB and other major lithospheric discontinuities beneath South Africa

    NASA Astrophysics Data System (ADS)

    Sodoudi, Forough; Kind, Rainer; Kaestle, Emanuel

    2013-04-01

    Investigation of the thickness of continental roots, which migrate coherently with plates belongs to the most systematic keys in order to understand the continental evolution. South Africa's lithosphere preserves a nearly un-interrupted geological history of more than 3.5 billion years. It was formed during the break-up of the supercontinent Gondwana over a period of 80 million years and therefore is the longest, best-preserved geological record of the planet Earth. To estimate the LAB and other major lithospheric discontinuities beneath South Africa, we used the novel technique of S receiver function, which employs S-to-P conversions and appears promising for detecting the LAB. This technique has already proven its power for mapping the LAB in the tectonically different regions. Although the South African craton has been extensively studied in recent years, especially by SRFs, the depth extent of the lithosphere and its nature varies somewhat between studies. It seems that several differences in methodology and data selection criteria leading to variations in the SRFs obtained. Some authors observed S-to-P conversions at 25-30 s (~250-300 km) and interpreted them as being from the LAB. In contrast, some other authors found these conversions at shallower depths (~150 km). We calculated SRFs for the data of more than 120 stations within South Africa. Such a huge amount of data has not been yet applied for SRF studies in South Africa. Our results clearly shows 3 different LVZs at about 100 (~10s), 150-220 (15-22 s) km and 300-330 km (30-33 s), which were not seen in any of the previous studies. Based on our preliminary results, the deep and sharp LAB phase at 300-330 km is significantly imaged beneath the cratons (Kaapvaal and Zimbabwe cratons; > 2.7 Ga) and the oldest belt (Limpopo belt ~ 2.7 Ga). This phase may not be visible northward and southward beneath the much younger mobile belts (Mozambique and Namaqua-Natal belt; ~ 1.1 Ga). Instead, a shallower and less

  16. 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

  17. 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.

  18. 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

  19. Characterising East Antarctic Lithosphere and its Rift Systems using Gravity Inversion

    NASA Astrophysics Data System (ADS)

    Vaughan, Alan P. M.; Kusznir, Nick J.; Ferraccioli, Fausto; Leat, Phil T.; Jordan, Tom A. R. M.; Purucker, Michael E.; Golynsky, A. V. Sasha; Rogozhina, Irina

    2013-04-01

    Since the International Geophysical Year (1957), a view has prevailed that East Antarctica has a relatively homogeneous lithospheric structure, consisting of a craton-like mosaic of Precambrian terranes, stable since the Pan-African orogeny ~500 million years ago (e.g. Ferracioli et al. 2011). Recent recognition of a continental-scale rift system cutting the East Antarctic interior has crystallised an alternative view of much more recent geological activity with important implications. The newly defined East Antarctic Rift System (EARS) (Ferraccioli et al. 2011) appears to extend from at least the South Pole to the continental margin at the Lambert Rift, a distance of 2500 km. This is comparable in scale to the well-studied East African rift system. New analysis of RadarSat data by Golynsky & Golynsky (2009) indicates that further rift zones may form widely distributed extension zones within the continent. A pilot study (Vaughan et al. 2012), using a newly developed gravity inversion technique (Chappell & Kusznir 2008) with existing public domain satellite data, shows distinct crustal thickness provinces with overall high average thickness separated by thinner, possibly rifted, crust. Understanding the nature of crustal thickness in East Antarctica is critical because: 1) this is poorly known along the ocean-continent transition, but is necessary to improve the plate reconstruction fit between Antarctica, Australia and India in Gondwana, which will also better define how and when these continents separated; 2) lateral variation in crustal thickness can be used to test supercontinent reconstructions and assess the effects of crystalline basement architecture and mechanical properties on rifting; 3) rift zone trajectories through East Antarctica will define the geometry of zones of crustal and lithospheric thinning at plate-scale; 4) it is not clear why or when the crust of East Antarctica became so thick and elevated, but knowing this can be used to test models of

  20. 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

  1. Low water contents in diamond mineral inclusions: Proto-genetic origin in a dry cratonic lithosphere

    NASA Astrophysics Data System (ADS)

    Taylor, Lawrence A.; Logvinova, Alla M.; Howarth, Geoffrey H.; Liu, Yang; Peslier, Anne H.; Rossman, George R.; Guan, Yunbin; Chen, Yang; Sobolev, Nikolay V.

    2016-01-01

    The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., Bell and Rossman, 1992; Peslier et al., 2010; Peslier, 2010; Nestola and Smyth, 2015), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.

  2. 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.

  3. Application of MAGSAT to Lithospheric Modeling in South America. Part 2: Synthesis of Geologic and Seismic Data for Development of Integrated Crustal Models

    NASA Technical Reports Server (NTRS)

    Keller, G. R.; Lidiak, E. G.; Hinze, W. J.; Braile, L. W.; Vonfrese, R. R. B. (Principal Investigator)

    1984-01-01

    Research activities performed on MAGSAT scalar data over South America, Central America, and the adjacent marine areas are summarized. The geologic utility of magnetic anomalies detected by satellite is demonstrated by focusing on the spherical-Earth interpretation of scalar MAGSAT data in combination with ancillary geological and geophysical data to obtain lithospheric models for these regions related to their contemporary crustal dynamics processes, geologic history, current volcanism seismicity and natural resources.

  4. 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

  5. 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.

  6. 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

  7. 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.

  8. 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.

  9. 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.

  10. 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

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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

  17. 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

  18. 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

  19. 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.

  20. 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.

  1. 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.

  2. 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

  3. Lithosphere stress changes due to groundwater unloading in North China Plain

    NASA Astrophysics Data System (ADS)

    Pang, Yajin; Zhang, Huai; Shi, Yaolin

    2015-04-01

    find that the earthquake events between1978-2014 are obviously less than that during the former 35-year period. There are 11 earthquakes with magnitude greater than M6.0 occurred during 1942-1977 and none in the period of 1978-2014. Although the seismicity may have natural fluctuations, the human effects cannot be excluded, as earthquake events are controlled by regional tectonic stress, which is significantly perturbed by groundwater overpumping in North China Plain. This study also suggests that earth is a complex system, in which each part interacts with others. Human activities on the earth surface may not only affect the atmosphere and the hydrosphere as well known, but also affect the lithosphere.

  4. Deformation of the lithosphere and what microstructures can tell us about it (Stephan Mueller Medal Lecture)

    NASA Astrophysics Data System (ADS)

    Heilbronner, Renée

    2016-04-01

    The lithosphere is a roughly stratified and heterogeneous rock body that constitutes the outer layer of our planet. It is subdivided into irregularly shaped stiff plates that move with respect to one another deforming each other along their margins. At the large scale the lithosphere is usually modeled as a flat-lying multi-layer, its rheological profile being based on flow laws determined experimentally for key minerals of the crust and upper mantle. At the somewhat smaller scale of field observations, geometrical and physical complexities become apparent: rocks are folded, sheared and fractured, and - in general - quite heterogeneously deformed. And finally, at the even smaller scale of mechanical testing and microscopic investigations, rocks are seen as polycrystalline aggregates or granular composites whose bulk properties depends both on the composition and shape of the individual grains and the spatial arrangement of the crystals with respect to one another. In other words, the physical properties of the lithosphere and the inferred style or type of deformation depend very much on the scale of observation. Microstructures and textures (crystallographic preferred orientations) of deformed rocks provide a wealth of information: when used as archives of the deformation history, they allow us to unravel the tectonic evolution of the lithosphere at plate boundaries. At the same time, they enable us to assess past and/or present geophysical properties. By comparing the microstructures of experimentally and naturally deformed rocks it is possible to infer the active deformation mechanisms and thus to extrapolate flow laws to geological time scales. With the advent of digital image processing, microstructure and texture analysis have taken a great leap forward. By amalgamating methods from neighbouring disciplines such as mathematical morphometry, stereology, geostatistics, material sciences, etc., microstructure and texture analysis have come a long way since the

  5. Birth of the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL)

    NASA Astrophysics Data System (ADS)

    James, D. E.; Sacks, I. S.

    2002-05-01

    As recently as 1984 institutions doing portable seismology depended upon their own complement of instruments, almost all designed and built in-house, and all of limited recording capability and flexibility. No data standards existed. Around 1980 the National Research Council (NRC) of the National Academy of Sciences (NAS), with National Science Foundation (NSF) support, empanelled a committee to study a major new initiative in Seismic Studies of the Continental Lithosphere (SSCL). The SSCL report in 1983 recommended that substantial numbers (1000 or more) of new generation digital seismographs be acquired for 3-D high resolution imaging of the continental lithosphere. Recommendations of the SSCL committee dovetailed with other NRC/NAS and NSF reports that highlighted imaging of the continental lithosphere as an area of highest priority. For the first time in the history of portable seismology the question asked was "What do seismologists need to do the job right?" A grassroots effort was undertaken to define instrumentation and data standards for a powerful new set of modern seismic research tools to serve the national seismological community. In the spring and fall of 1983 NSF and IASPEI sponsored workshops were convened to develop specifications for the design of a new generation of portable instrumentation. PASSCAL was the outgrowth of these seminal studies and workshops. The first step toward the formal formation of PASSCAL began with an ad-hoc organizing committee, comprised largely of the members of the NAS lithospheric seismology panel, convened by the authors at Carnegie Institution in Washington in November 1983. From that meeting emerged plans and promises of NSF support for an open organizational meeting to be held in January 1984, in Madison, Wisconsin. By the end of the two-day Madison meeting PASSCAL and an official consortium of seismological institutions for portable seismology were realities. Shortly after, PASSCAL merged with the complementary

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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

  13. 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.

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. Constraining the dynamic response of subcontinental lithospheric mantle to rifting using Re-Os model ages in the Western Ross Sea, Antarctica

    NASA Astrophysics Data System (ADS)

    Doherty, C.; Class, C.; Goldstein, S. L.; Shirey, S. B.; Martin, A. P.; Cooper, A. F.; Berg, J. H.; Gamble, J. A.

    2012-12-01

    lithosphere extends beneath the TAM. With further analyses we hope to determine if there is lateral flow of cratonic lithosphere into the rift. Huismans, R., Beaumount, C., 2011. Depth-dependent extension, two stage breakup and cratonic underplating at rifted margins. Nature 473, 74-78. Reisberg, L.C., Lorand, J.P., 1995. Longevity of sub-continental mantle lithosphere from osmium isotope systematics in orogenic peridotite massifs. Nature 376, 159-162. Rudnick, R.L., Walker, R.J., 2009. Interpreting ages from Re-Os isotopes in peridotites. Lithos 1125, 1083-1095.

  20. Diffuse Deformation Across the Southern Mariana Margin: Possible Effects of Water on Large-Scale Lithospheric Rheology

    NASA Astrophysics Data System (ADS)

    Martinez, F.; Sleeper, J. D.

    2012-12-01

    The southern Mariana margin is extending in an approximately E-W direction at about 45 mm/yr above the subducting Pacific slab. We use earthquake locations, shallow- and new deep-towed side-scan sonar imagery together with compiled multibeam bathymetry to document the nature of the tectonic and volcanic deformation. A well-defined and magmatically robust spreading center takes up part of this extension along the northern part of the southern margin. However, it does not intersect the trench. Instead the spreading center curves westward and becomes a diffuse zone of volcanism extending to the western end of the back-arc basin. The approximately E-W oriented fabric of this diffuse volcanic zone suggest it is taking up a southerly-directed extensional component associated with southward trench rollback in this area. The main E-W oriented extension in the southern margin appears to be distributed broadly across the margin to the south, as indicated by ~N-S-oriented seafloor fabric and the distribution of earthquakes in this area. Near the outer margin, deep-towed side-scan sonar data to near the 6000 m isobath image possible isolated volcanic emplacements within an otherwise tectonized terrain. Possible ~E-W oriented mullion structures are also imaged suggesting low angle tectonic deformation within the margin. The transition of the organized spreading center to a diffuse volcano-tectonic zone and the broadly distributed deformation in the entire southern margin may be consequences of the high water content in the mantle wedge and overlying lithosphere predicted here. Hydrous and therefore weak subduction margin lithosphere may not be able to localize the narrow plate boundary zones characteristic of extension in oceanic lithosphere.

  1. 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

  2. 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

  3. 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

  4. 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.

  5. 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

  6. 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

  7. 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.

  8. 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].

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. Lithospheric structure beneath NW Iran using regional and teleseismic travel-time tomography

    NASA Astrophysics Data System (ADS)

    Bavali, K.; Motaghi, K.; Sobouti, F.; Ghods, A.; Abbasi, M.; Priestley, K.; Mortezanejad, G.; Rezaeian, M.

    2016-04-01

    We compute a 2-D tomogram using the P wave arrival time readings from a temporary seismic experiment to study the seismic structure of the crust and upper mantle in NW Iran. The study area includes the western margins of the South Caspian Basin (SCB), and the Sahand and Sabalan post-collisional volcanoes in NW Iran. We invert 2780 regional and teleseismic relative P wave arrival times recorded by 23 stations along the seismic profile extending from the western shoreline of the Caspian Sea to Lake Urumieh. Our tomographic results show a higher-velocity region beneath the SCB. The observed higher velocities strongly correlate with the observed positive gravity anomalies over the southwestern margins of the Caspian Sea, suggesting an oceanic like nature for the SCB lithosphere. The tomographic results also show several lower-velocity anomalies in the crust. The Sabalan volcano is underlain by a low-velocity zone in the lower crust, which is most likely thermal in nature. In the Sahand region, the lower velocities are considerably shallower in depth and might be controlled by shallow sedimentary structures, as well as an anomalously warm upper crust. The shallow low-velocity regions are connected with deeper low-velocity zones 60-100 km deep in the upper mantle. This pattern points to a possible mantle source of post-collisional volcanism in NW Iran, i.e. the melting of a subducted slab.

  14. 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

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. Heterogeneous lithospheric mantle metasomatism in the eastern North China Craton: He-Ar isotopes in peridotite xenoliths from Cenozoic basalts

    NASA Astrophysics Data System (ADS)

    Tang, Huayun; Matsumoto, Takuya; Zheng, Jianping; Czuppon, György; Yu, Chunmei; Miyakawa, Chie; Ping, Xianquan

    2014-02-01

    The abundances and isotopic compositions of Helium and Argon have been analyzed in a suite of fresh spinel peridotite xenoliths in Cenozoic basalts from the eastern North China Craton (NCC) by step-wise heating experiments, to investigate the nature of noble gas reservoirs in the subcontinental lithospheric mantle beneath this region. The xenoliths include one harzburgite collected from Hebi in the interior of the NCC, two lherzolites from Hannuoba at the northern margin of the craton, and three lherzolites from Shanwang and Nushan on the eastern margin. 3He/4He ratios in most of the xenoliths are similar to those of mid-ocean ridge basalts (MORB) or slightly lower (2-10.5 Ra, where Ra is the 3He/4He ratio of the atmosphere), suggesting mixing of MORB-like and radiogenic components. One olivine separate from Nushan has a helium value of 25.3 Ra, probably suggesting cosmogenic 3He addition. The 40Ar/36Ar ratios vary from atmospheric value (296) to 1625, significantly lower than the MORB value. Available data of the peridotite xenoliths indicate the He and Ar isotopic systematics of the mantle reservoirs beneath the NCC can be interpreted as mixtures of at least three end-members including MORB-like, radiogenic and atmospheric components. We suggest that the MORB-like noble gases were derived from the underlying asthenosphere during mantle upwelling, whereas the radiogenic and recycled components probably were incorporated into the lithospheric mantle during circum-craton subduction of oceanic crust. Available data suggest that the MORB-like fluids are better preserved in the interior of the NCC, whereas the radiogenic ones are more prevalent at the margins. The Paleo-Asian ocean subduction system probably was responsible for the enriched and recycled noble gas signatures on the northern margin of the craton, while the Pacific subduction system could account for the observed He-Ar isotopic signatures beneath the eastern part. Therefore, integration of helium and

  20. 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.

  1. 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.

  2. 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

  3. 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

  4. 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

  5. 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.

  6. 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.

  7. 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

  8. 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.

  9. 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.

  10. Constraining the dynamical behavior of the western US lithosphere through computational modeling: Evidence that lower crustal flow plays a significant role in load accommodation

    NASA Astrophysics Data System (ADS)

    Seunarine, L.; Lowry, A. R.

    2011-12-01

    The rheology of the continental lithosphere at depth remains an unresolved issue in geodynamics. Debate persists over whether lithospheric deformation includes flow of the lower crust between strong upper crustal and upper mantle layers (behavior analogous to a "jelly sandwich") or if flow is predominantly in a weak uppermost mantle beneath a strong crust (analogous to a "crème brulee"). The degree of Moho deflection accompanying lithospheric flexure provides an important clue as to which dynamical behavior dominates: If the lower crust does not flow significantly, then the Moho can be expected to conform, in part or in whole, to the lithosphere's flexural response to surface and internal mass loads. Here we examine where strength resides in the lithosphere using new estimates of western US Moho, along with estimates of internal mass variation associated with crustal composition and lithospheric temperature, derived from receiver function analysis of Earthscope Transportable Array seismic data (Lowry & Perez-Gussinye, 2011). We consider two end-member models of elevation response to the mass fields. One model assumes that the Moho will deflect identically to the surface elevation in response to each mass field (an expected feature of a crème brulee scenario) while the other treats the Moho as an independent mass field with no flexural stress coupling to the surface (the extreme end-member for a jelly-sandwich). The crème brulee model is significantly less successful at reproducing observed western US elevations than the jelly sandwich model. The modeling approach allows density parameters relating the seismic and thermal fields to mass to vary, and estimates of optimal density parameters associated with each mass field are also very different between the two models. The jelly sandwich model yields density parameters similar to those derived from laboratory and geophysical analyses, while the crème brulee model minimizes density parameters related to composition

  11. The Driving Forces of the Large-Scale Deformation in the India-Eurasia Collision Region: Joint Modeling of Lithosphere and Mantle Dynamics

    NASA Astrophysics Data System (ADS)

    Wang, X.; Holt, W. E.; Ghosh, A.

    2014-12-01

    The origin of the large-scale deformation of the India-Eurasia collision zone has been pursued for more than 4 decades. However, the driving forces for the largest area of continental deformation zone on earth have not been entirely resolved; the source of such driving forces remains enigmatic. One reason could be that the driving forces have to be sufficiently large to overcome the resistance of the Tibetan Plateau, created by excess gravitational potential energy (GPE) over a long time span. Another reason is that seismic experiments carried out in the Tibetan Plateau, due to the harsh natural conditions, are fewer, making it challenging to resolve high-resolution seismic structure beneath Tibet. We address this issue of driving forces in this deformation zone by quantifying the primary contributions to the lithospheric stress field. We take into account effects of topography and shallow lithosphere structure, as well as tractions originating from deeper mantle convection, in order to calculate model estimates of the total lithosphere stresses. We evaluate recent published global seismic tomographic models (P-wave, S-wave, and geodynamic models) and select a tomographic model which, when used in the semi-analytical mantle circulation model HC (Hager and O'Connell, 1981; Milner et al., 2009), provides a best fit to observations of geoid, surface motions, strain rates, and stress orientations. We use the joint modeling of lithosphere and mantle dynamics approach of Ghosh and Holt (2012) to compute the full lithosphere stresses, except that we use HC for the circulation model, which can only handle radial viscosity variations. After using the selected seismic tomographic model of SAW642AN (Panning and Romanowicz, 2006) to compute the global lithosphere stresses, we refine the calculated stresses in the India-Eurasia collision zone. Our results show that both the driving stresses from mantle convection and GPE differences contribute to the deviatoric stress field in

  12. 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

  13. 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 of ophiolite complexes. Historically, the stratiform ophiolite architecture has been the basis for inferences of the interaction between tectonism and magmatism at mid-ocean ridge spreading centers. A growing number of constraints on geological relations along seafloor escarpments hint at much broader range of interactions between tectonic deformation and magmatic construction. Along slow-spreading ridges, the magma budget (volume of magma per unit plate separation) is highly variable, giving rise to a wide range of morphologic and geologic features along wide rift valleys. The diversity of crustal architectures and internal structures seen in tectonic windows is correspondingly large. Although it is possible that a relatively simple, layered ophiolite-like crust develops in regions of relatively high magma budget (Reykjanes Ridge, Azores region, etc.), more complex structures that differ from those of stratiform ophiolites are present where lower magma budgets prevail. Significant deviations from a simple

  14. Melt Migration in the Mantle Lithosphere: Evidence From Ophiolitic Peridotites

    NASA Astrophysics Data System (ADS)

    Spagnolo, G.; Piccardo, G. B.; Poggi, E.

    2006-12-01

    Records of diffuse porous flow migration of asthenospheric melts through the lithospheric mantle are evident in mantle peridotites deriving from the oceanic lithosphere of the Jurassic Ligurian Tethys, exposed in the Alpine- Apennine orogenic system of Northern Italy. The migrating melts caused structural and chemical modifications, as a consequence of melt/peridotite interaction. Microstructures indicating pyroxene(Px)-dissolving/olivine(Ol)-forming reactions suggest that early percolating melts were Px(-silica)-undersaturated and their intergranular flow through the peridotite enhanced melt/peridotite interaction. Px dissolution modified: 1) the peridotite composition: in fact, the reacted peridotites changed their bulk rock characteristics to significantly SiO2-depleted, MgO-enriched compositions, and their mineral modal contents to significantly Ol-enriched compositions, with respect to any refractory residua after any kink of mantle partial melting; 2) the melt composition: in fact, the melt composition progressively attained Px(-silica)-saturation at the end of the reactive percolation, as evidenced by late Px interstitial crystallization. Depending on the degree of Px dissolution, the reacted peridotites from the same peridotite body have highly variable Px contents but their clinopyroxenes(Cpx) have closely similar trace element contents. This decoupling between mineral modal content and geochemical composition strongly suggests that these peridotites cannot have been originated by partial melting but it supports the evidence of melt/peridotite interaction. Thus, Cpx trace element composition depends on the geochemical equilibration with the percolating melt; it indicates, moreover, the MORB affinity of the percolating melt. Significant evidences of melt migration through lithospheric peridotites are represented by the plagioclase(Plg)- enriched peridotites, which are frequently present within the ophiolitic peridotites and particularly abundant in those

  15. Dynamics of the Pacific Northwest Lithosphere and Asthenosphere

    NASA Astrophysics Data System (ADS)

    Humphreys, E.

    2013-12-01

    Seismic imaging resolves a complex structure beneath the Pacific Northwest (PNW) that is interpreted as: an high-velocity piece of accreted (~50 Ma) Farallon lithosphere that deepens from being exposed (at coast, where it is called Siletzia) to lower crust in SE Washington and then descending vertically to ~600 km as a 'curtain' beneath central Idaho; a stubby Juan de Fuca slab (to <250 km in most places, and with a gap beneath much of Oregon); and very slow 'fingers' above the slab with an especially anomalous volume beneath central Oregon. The evacuation of asthenosphere with the descending Juan de Fuca slab presents an interesting problem. Typical corner-flow supply is prevented because the mantle wind blows eastward. A stagnation line between mantle drawn west into the mantle wedge and that flowing east in the far field may exist near the Farallon curtain. Other sources of supply to the Cascadia mantle wedge include: mantle flowing north beneath California, entrained with the Pacific Plate; flow from below the Juan de Fuca slab, perhaps especially through the Oregon gap; toroidal flow around the southern edge of the slab; and possible up-flow beneath the backarc. Anisotropy studies suggest the importance of toroidal flow. PNW lithosphere stress is dominated by N-S compression, a result of the Sierra Nevada driving the PNW into British Columbia; this push, and oblique subduction, are responsible for PNW clockwise rotation. Modeled (in 2D, map view) north-directed tractions on the Cascadia mega-thrust average ~4 TN per meter of along-strike fault length, or probably a shear stress of ~40 MPa over much of the locked mega-thrust (i.e., much more shear stress than the typical earthquake stress drop of 1-10 MPa). Normal to the coast, southern Cascadia is relatively tensional (where margin-normal compression is less than typical ridge push by ~4 TN/m of along-strike fault length) whereas northern Cascadia is compressional. This indicates that the southern Cascadia

  16. Widespread refertilization of cratonic and circum-cratonic lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Tang, Yan-Jie; Zhang, Hong-Fu; Ying, Ji-Feng; Su, Ben-Xun

    2013-03-01

    Studies of mantle xenoliths have confirmed that Archean subcontinental lithospheric mantle (SCLM) is highly depleted in basaltic components (such as Al, Ca and Na) due to high-degree extraction of mafic and ultramafic melts and thus is refractory and buoyant, which made it chronically stable as tectonically independent units. However, increasing studies show that ancient SCLM can be refertilized by episodic rejuvenation events like infiltration of upwelling fertile material. The North China Craton is one of the most typical cases for relatively complete destruction of its Archean keel since the eruption of Paleozoic kimberlites, as is evidenced by a dramatic change in the compositions of mantle xenoliths sampled by Paleozoic to Cenozoic magmas, reflecting significant lithospheric thinning and the change in the character of the SCLM. The compositional change has been interpreted as the result of refertilization of Archean SCLM via multiple-stage peridotite-melt reactions, suggested by linear correlations between MgO and indices of fertility, covariations of Al2O3 with CaO, La/Yb, 87Sr/86Sr, 143Nd/144Nd, 187Os/188Os and Re-depletion ages (TRD), high Re abundances, scatter in Re-Os isotopic plot, variable in situ TRD ages of sulfides, and correlation between TRD ages and olivine Fo of peridotite xenoliths in Paleozoic kimberlites and Cenozoic basalts on the craton. By integrating major and trace element, Sr, Nd and Os isotopic compositions of peridotite xenoliths and orogenic massif peridotites from the continents of Europe, Asia, America, Africa and Australia, together with previous studies of petrology and geochemistry of global peridotites, we suggest that (1) refertilization of cratonic and circum-cratonic lithospheric mantle is widespread; (2) Archean SCLM worldwide has experienced a multi-stage history of melt depletion and refertilization since segregation from the convecting mantle; (3) cratonic SCLM may be more susceptible to compositional change caused by

  17. Development of a Package for Modeling Stress in the Lithosphere

    NASA Astrophysics Data System (ADS)

    Williams, C. A.

    2006-05-01

    One of the primary difficulties when modeling stresses in the Earth's lithosphere is finding a numerical code with the necessary capabilities. The lithosphere represents a unique challenge due to structural complexity, the presence of faults, complex materials with large spatial variations in their properties, a wide range of pertinent spatial and temporal scales, and interactions with other processes (such as mantle convection), leading to complex boundary conditions. To address such problems, a modeling package should have a number of features that are not generally found in combination. The code should be able to use a number of different element types, allowing the geometry to be represented using any desired meshing package. The code should be able to accurately represent fault behavior, allowing both kinematic specification of fault slip as well as fault behavior defined by a constitutive relationship. The code should include a number of different material models (various combinations of elastic, viscous, and plastic behavior) and should also provide an easy mechanism for adding new material models. The code should be parallel and scalable, allowing the simulation of problems over a wide range of spatial scales and resolutions. The code should also be able to easily interact with other modeling codes, which could address some of the issues related to representing multiple time scales, as well as aiding in the determination of appropriate boundary conditions. Finally, the code should be easy to use, modular, and easily adaptable to different needs. We describe the current status, development plans and example usage of a finite element code with the above features as design goals. The current quasi-static finite element code (LithoMop) is being merged with the EqSim dynamic rupture code to form a new modeling package to be named PyLith. The code makes use of the Pyre simulation framework, with top-level code written in Python. This provides a number of useful

  18. A spherical model of lithospheric dynamics and seismicity

    NASA Astrophysics Data System (ADS)

    Rozenberg, Valerii; Ismail-Zadeh, Alik; Melnikova, Lidiya; Soloviev, Alexander

    2010-05-01

    The dynamics of lithospheric plates is approximated by an interaction of rigid blocks of the modeled lithosphere (spherical segments) and faults separating them. The spherical block-and-fault dynamics (BAFD) model consists of fifteens blocks approximating tectonic plates and several additional blocks representing parts of the plates, where deep seismicity is observed. The blocks move as a consequence of the prescribed underlying mantle motion. The block displacements at any time are defined so that the system of the blocks is in a quasi-static equilibrium state. Because of the block rigidity, all deformations take place in the fault zones. The interaction between the spherical blocks is visco-elastic (a state of stress accumulation), so long as the ratio of the stress to the pressure is below a certain strength level. When this level is exceeded in some part of a fault, a stress-drop (a synthetic earthquake) occurs in accordance with the dry friction law. Immediately following the earthquake and for some period of time, the corresponding parts of the faults are in a state of creep. Catalogs of synthetic earthquakes are produced as results of numerical simulations. Using the catalogs of synthetic events we study frequency-magnitude relationships, clustering of the events, long-range interaction of earthquakes, earthquake mechanisms, and fault slips. The model catalogs obtained reflect important features of global seismicity: (i) two large seismic belts, the circum-Pacific and Alpine-Himalayan; (ii) extensive, but less pronounced, seismicity at mid-oceanic ridges; and (iii) increased seismic activity associated with triple junctions of plate boundaries. The model results are consistent with the observations: Nazca/South America, Cocos/Caribbean, India/Eurasia, California region, Arabia/Eurasia, northern Australia, and the Philippine plate margin are marked in the model as the regions prone to strong earthquakes. The modeled seismic activity is moderate at the

  19. Structure, seismicity, and instrumentation of stable North American lithosphere

    NASA Astrophysics Data System (ADS)

    Wolin, Emily Lynne Gregonis

    The work in this thesis covers a number of different fields in seismology. These can be divided into three parts. In the first, I discuss earthquakes that occur along the North American passive margin. In the second, I investigate the noise characteristics of Superior Rifting Earthscope Experiment (SPREE) seismic stations. In the third, I evaluate the predictions of pre-existing tomographic models, expand raypath coverage in eastern North America by fitting waveforms, and produce an updated tomographic model focusing on the structure of stable North American lithosphere. First, I explore the rare, but moderate-to-large earthquakes that occur along the passive margin of North America, such as the August 2011 Mineral, Virginia earthquake. I discuss the tectonic setting, possible causes, and challenges of studying such earthquakes and propose directions for their future study. Examining the seismicity of the regions surrounding the 1929 Grand Banks and 1933 Baffin Bay earthquakes, I conclude that the aftershocks of these events continue today due to low strain rates along the margin. Second, I characterize the long-period noise characteristics of SPREE stations. At periods greater than 20 seconds, horizontal noise levels at SPREE stations vary seasonally and diurnally. I find that SPREE stations in sandy soil have the most consistent noise levels. Stations in fine-grained soil become extremely noisy during summer days, but very quiet when the surrounding soil freezes solid in winter. Finally, I evaluate previous generations of tomographic models and develop my own. I calculate synthetic seismograms for three tomographic models and compare them to new observed seismograms from earthquakes in stable North America. I find that adding data to a tomographic model does not necessarily improve predictions of regional S and Rayleigh waveforms. With this in mind, I apply the method of partitioned waveform inversion to derive constraints on S-velocity structure. I add these

  20. Lithospheric Signature of Paleorifts in Cratonal Europe and North America

    NASA Astrophysics Data System (ADS)

    Keller, G. R.; Stephenson, R. A.; Mickus, K. L.

    2002-12-01

    Southwestern North America and Central Europe share many aspects of their Neoproterozoic and Phanerozoic tectonic evolution. In particular, the break-up of late Precambrian supercontinent created the continental blocks called Laurentia and Baltica. Passive margins developed during and after the rifting that formed these continents, and these margins were deformed by Paleozoic orogenies (Appalachian-Ouachita; Caledonian-Variscan respectively). Our group has studied the Ouachita orogeny that affected southern Laurentia for many years and has recently had the opportunity to study Central Europe by participating in several large seismic experiments. The results of the POLONAISE' 97 experiment delineated the rifted margin of Baltica, and we interpret it to be quite similar to crustal models we have developed for the Ouachita margin. For example, the Holy Cross Mountains in southern Poland exposed a crustal block that is similar to the Devil's River uplift in west Texas. The Polish basin contains a thick pre-Permian section similar to that observed along the Ouachita orogenic belt that is overlain by a Permian and younger sequence that is analogous to the Gulf Coast sequence. Both the Variscan orogeny in Central Europe and the Ouachita orogeny appear to be the result of soft collisions that have left the pre-orogenic rifted margins largely intact. In terms of continental tectonics, rifts that do not succeed in breaking a continent apart are sometimes referred to as having "failed". These failed rifts usually are the sites of post-rift sedimentation that contain important records of continental evolution and prolific petroleum resources. During the past 15 years, a number of studies have shown that the modification of the lithosphere (most evidence is actually for the crust) in failed rifts takes on many forms, is highly variable, and is often very substantial. Although semantic arguments are seldom productive, these results call into question any perception that these

  1. Seafloor morphology and the thermal evolution of oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Stein, Carol

    2010-05-01

    Standard reference models for the cooling of oceanic lithosphere, on average, predict the observed temperature-dependent properties, such as depth and heat flow with age. However, for all generally accepted models a significant discrepancy exists between measured and expected heat flow for ages less than about 65 million years. Traditionally lower measured heat flow average values are explained by significant hydrothermal circulation through the uppermost oceanic crust. In this approach, it is assumed that some of the heat transferred by conduction from lithospheric depths is removed by the water flow and transferred between the oceans and crust at locations where the seafloor is bare or poorly sedimented. This component of heat transfer would not be detected by the measurements, which record only conductive transfer. However, recently Hofmeister and Criss [2005] have suggested that hydrothermal circulation is not a significant factor, so the measured marine heat flow results should be used instead of the thermal models in calculating total heat loss for the earth. This approach lowers the loss by about 25%. This hypothesis is tested by examining whether the discrepancy between the predicted and measured heat flows varies between sites such that the highest heat flow is observed where hydrothermal flux should be least due to the regional topography of the igneous basement and sediment thickness. This appears to be the case. Sites with ages less than 65 million years in areas with smooth basement and thick sediment cover have average heat flow equal to that expected from thermal cooling models, except at the very youngest ages. In contrast, sites in areas with thinly sedimented basement outcrops have significantly lower heat flow. These sites make up a progressively lower fraction of the total at older ages as sedinent cover increases. Moreover, for all site types, the measured heat flow approaches the cooling model's predictions at older ages. As a result, by 65

  2. Evolution of planetary lithospheres - Evidence from multiringed structures on Ganymede and Callisto

    NASA Technical Reports Server (NTRS)

    Mckinnon, W. B.; Melosh, H. J.

    1980-01-01

    The thickness and viscosity of a planetary lithosphere increase with time as the mantle cools, with a thicker lithosphere leading to the formation of one (or very few) irregular normal faults concentric to the crater. Since a gravity wave or tsunami induced by impact into a liquid mantle would result in both radial and concentric extension features, which are not observed in the case of the large impact structures on Ganymede and Callisto, an alternative mechanism is proposed in which the varying ice/silicate ratios, tectonic histories, and erosional mechanisms of the two bodies are considered to explain the subtle differences in thin lithosphere ring morphology between Ganymede and Callisto. It is concluded that the present lithosphere thickness of Ganymede is too great to permit the development of any rings.

  3. Dramatic effect of elasticity on thermal softening and strain localization during lithospheric shortening

    NASA Astrophysics Data System (ADS)

    Jaquet, Yoann; Duretz, Thibault; Schmalholz, Stefan M.

    2016-02-01

    We present two-dimensional numerical simulations for shortening a viscoelastoplastic lithosphere to quantify the impact of elasticity on strain localization due to thermal softening. The model conserves energy and mechanical work is converted into heat or stored as elastic strain energy. For a shear modulus G = 1010 Pa, a prominent lithospheric shear zone forms and elastic energy release increases the localization intensity (strain rate amplification). For G = 5 × 1010 Pa shear zones still form but deformation is less localized. For G = 1012 Pa, the lithosphere behaves effectively viscoplastic and no shear zones form during homogeneous thickening. Maximal shearing-related increase of surface heat flux is 15-25 mW m-2 and of temperature at lower crustal depth is ˜150 °C, whereby these peak values are transient (0.1-1 My). Elasticity and related energy release can significantly contribute to strain localization and plate-like behaviour of the lithosphere required for plate tectonics.

  4. Nappe stacking resulting from subduction of oceanic and continental lithosphere below Greece

    NASA Astrophysics Data System (ADS)

    van Hinsbergen, Douwe Jacob Jan; Hafkenscheid, E.; Spakman, Wim; Meulenkamp, J. E.; Wortel, Rinus

    2005-04-01

    We quantitatively investigate the relation between nappe stacking and subduction in the Aegean region. If nappe stacking is the result of the decoupling of upper-crustal parts (5 10 km thick) from subducting lithosphere, then the amount of convergence estimated from balancing the nappe stack provides a lower limit to the amount of convergence accommodated by subduction. The balanced nappe stack combined with the estimated amount of completely subducted lithosphere indicates 700 km of Jurassic and 2400 km of post-Jurassic convergence. From seismic tomographic images of the underlying mantle, we estimate 2100 2500 km of post-Jurassic convergence. We conclude that (1) the imaged slab represents the subducted lithosphere that originally underlay the nappes, (2) since the Early Cretaceous, subduction in the Aegean has occurred in one single subduction zone, and (3) the composition of the original basement of the nappes indicates that at least 900 km of sub-upper-crust continental lithosphere subducted in the Aegean.

  5. Seismic structure of the lithosphere beneath the ocean islands near the mid-oceanic ridges

    NASA Astrophysics Data System (ADS)

    Haldar, C.; Kumar, P.; Kumar, M. Ravi

    2013-10-01

    Deciphering the seismic character of the young lithosphere near the mid-oceanic ridges (MOR) is a challenging endeavor. In this study, we determine the seismic structure of the oceanic plate near the MORs, using the P-to-s conversions isolated from good quality data recorded at 5 broadband seismological stations situated on the ocean Islands in their vicinity. Estimates of the crustal and lithospheric thickness values from waveform modeling of the P receiver function stacks reveal that the crustal thickness varies between 6 and 8 km with the corresponding depths to the lithosphere asthenosphere boundary (LAB) varying between 43 and 68 km. However, the depth to the LAB at Macquire Island is intriguing in view of the observation of a thick (~ 87 km) lithosphere beneath a relatively young crust. At three other stations i.e., Ascension Island, Sao Jorge and Easter Island, we find evidence for an additional deeper low velocity layer probably related to the presence of a hotspot.

  6. Geodynamic constraints on stress and strength of the continental lithosphere during India-Asia collision.

    NASA Astrophysics Data System (ADS)

    Kaus, B. J. P.; Schmalholz, S. M.; Lebedev, S.; Deschamps, F.

    2009-04-01

    There has been quite some debate in recent years on what the long-term strength of the continental lithosphere is and how it is related to the occurrence of earthquakes. One of the best studied areas in this respect is the India-Asia collision zone, where -in some profiles- the Moho depth is known to within a few km's. A relocation of earthquake source locations revealed that in India earthquakes occur throughout the whole lithosphere whereas in Tibet, earthquakes are restricted to the upper 10-15 km of the crust with few exceptions slightly above or below the Moho. The lack of substantial earthquake activity in the sub-Moho mantle lithosphere seems puzzling since (1D) strength envelop models for the continental lithosphere predict large differential stresses (and brittle failure) in these locations. A way out of this paradox is to assume that the rheology of the mantle lithosphere (i.e. the effective viscosity) is significantly smaller than usually assumed, either because of the effects of hydration, or because of increased Moho temperatures. As a consequence, the strength of the lithosphere resides in the crust and not in the upper mantle as previously assumed. This conclusion gets some support from spectral-based inverse models of the effective elastic thickness (using topography and gravity as input data), which is typically smaller than the seismogenic thickness. Even though this explanation might appear appealing at first, there are at least two major problems with it: (1) Estimations of the effective elastic thickness (EET) of the lithosphere are non-unique and model-dependent. Others, using a direct (non-spectral) modelling approach, find significantly larger values of the EET in the same locations (again using gravity & topography as constraints). (2) Long term geodynamic models indicate that if the mantle lithosphere would indeed be as weak as suggested, it would be very difficult to generate plate-tectonics like behavior: Subducting slabs behave more

  7. Lithospheric age dependence of off-ridge volcano production in the North Pacific

    SciTech Connect

    Batiza, R.

    1981-08-01

    Data for numbers of seamounts on North Pacific Ocean crust of different age indicate that the production rate of new off-ridge seamounts (volcanoes) is proportional to the inverse of the square root of the age of the lithosphere. This observation is consistent with several hypotheses which have been offered to explain the origin of off-ridge oceanic volcanism and, in combination with petrologic and paleomagnetic evidence for small oceanic volcanoes, leads to a new self-consistent model for off-ridge volcanism. In this model, the production rate of off-ridge volcanoes is controlled primarily by the availability of fracture-zone conduit systems in the thickening lithosphere. In contrast, mantle plume or ''hotspot'' volcanoes may punch through the oceanic lithosphere. Decreasing production rate of off-ridge volcanoes on old oceanic lithosphere may be associated with decreasing extent of partial melting of chemically and isotopically heterogeneous mantle material.

  8. Lithosphere thickness and rupture mode variation along the Gulf of California, Mexico

    NASA Astrophysics Data System (ADS)

    Perez-Campos, X.; Fernández, A.

    2014-12-01

    We study the lithosphere thickness along the Baja California peninsula and continental northwestern Mexico in order to identify the different rupture modes along the Gulf of California. We use receiver functions (RFs) of 29 seismological stations from three networks: NARS-Baja (Network of Autonomously Recording Seismographs), RESBAN (Broadband seismic network from the Gulf of California), and SSN (National Seismological Service, Mexico). Thickness of the lithosphere varies from 43 to 89 km. The largest values are found at stations located east of Sierra Madre Occidental, and are consistent with a continental lithosphere. The thinner lithosphere is found at station NE70 (Mexicali, Mexico), consistent with a pre-rift region that has experimented serveral extension episodes. From RFs retroprojected along various profiles, we can distinguish the different rupture modes along the Gulf and identify a region under the center of the peninsula where the listhosphere has been removed by the former subduction of the Guadalupe plate.

  9. Formation of Bright Terrain Bands on Ganymede by Unstable Lithosphere Extension

    NASA Technical Reports Server (NTRS)

    Zuber, M. T.; Parmentier, E. M.

    1985-01-01

    The surface of Ganymede, as seen in Voyager images, was greatly modified by tectonic activity. Geologic evidence is consistent with the formation of younger bright terrain by the emplacement of icy material into rift zones formed in the older dark terrain. To better understand the formation of both terrestrial rift zones and bright terrain bands on Ganymede, the mechanics of rift initiation by the growth of necking instabilities in an extending lithosphere are studied. In particular, how an initial, small perturbation in lithosphere thickness at the lithosphere/asthenosphere boundary is amplified by uniform horizontal extension is studied. The lithosphere is treated as a layer of power-law viscous fluid of thickness H overlying a weaker, viscous mantle substrate. In order to assess the relative importance of buoyant upwelling and mechanical instability in rift formation, both density and strength stratification were incorporated into the model.

  10. Lithospheric detachment of India and Tibet inferred from thickening of the mantle transition zone

    NASA Astrophysics Data System (ADS)

    Duan, Yaohui; Tian, Xiaobo; Liu, Zhen; Zhu, Gaohua; Nie, Shitan

    2016-07-01

    To spatially and temporally interpret eruptive volcanic activity and plateau uplift, the dynamic model of the Himalayan-Tibetan orogen requires several scenarios in which the deep part of the lithosphere is removed. The removed cold, dense material sank deeply and may rest in the mantle transition zone, which is considered as the graveyard for descending mantle lithosphere. Beneath the Himalayas and southern Tibet, stacking teleseismic P-wave receiver functions reveal thickening of the mantle transition zone (MTZ), which is caused by decreasing temperatures. We interpret the MTZ thickening beneath southern Tibet as being a result of a remnant of detached thickened Tibet mantle lithosphere, whereas the other thickening is most likely caused by a lithospheric slab that detached from the Indian plate and is sinking into the MTZ beneath the Himalayas.

  11. Support for a Uniformitarian Model of Continental Mantle Lithosphere Formation from the "Near-Cratonic" Composition of Proterozoic Southern African Mantle Lithosphere

    NASA Astrophysics Data System (ADS)

    Janney, P. E.

    2014-12-01

    The transition at the end of the Archean between the generation of cratonic and mobile belt continental lithosphere is regarded as a first-order change in the mode of generation of continental lithosphere. It is widely debated whether this transition represented a fundamental change in the process by which the lithospheric mantle was generated (i.e., as melting residues of deep-seated mantle upwellings to residues of relatively shallow mantle melting at subduction zones), or whether it primarily reflected a more gradual change in the conditions (i.e., temperatures, depths and degrees of melting) of lithosphere generation in a suprasubduction zone setting. The marked contrast, in many cases, between the major element compositions of peridotite xenoliths from Archean cratons and those from adjacent post-Archean mobile belts has accentuated the significance of this transition. Peridotite xenoliths from the post-Archean mobile belt terranes surrounding the Kaapvaal craton in southern Africa are clearly Proterozoic in age from Re-Os isotope constraints, but they are unusual in that they share several key similarities in composition and mineralogy with Archean Kaapvaal peridotites (e.g., low bulk-rock Al2O3, relatively low modal olivine and high modal orthopyroxene). Although they lack the low FeO and high olivine Mg# values of the most extreme Kaapvaal samples, they show a very large degree of overlap (extending to olivine Mg# values of greater than 93 for example). These similarities support a common mode of origin for cratonic and post-cratonic lithosphere in southern Africa (although varying somewhat in the degrees and depths of melt extraction) and a similar history of post-formation modification. A comparison of the conditions of melt extraction for cratonic and post-cratonic lithosphere inferred from compatible and mildly incompatible trace elements will be presented.

  12. Numerical Simulation of Electromagnetic Field Variation in the Lithosphere-Atmosphere-Ionosphere Associated with Seismogenic Process in a Curvature Coordinate System

    NASA Astrophysics Data System (ADS)

    Liu, L.; Zhao, Z.; Wang, Y.; Huang, Q.

    2013-12-01

    The lithosphere-atmosphere- ionosphere (LAI) system formed an electromagnetic (EM) cavity that hosts the EM field excited by electric currents generated by lightning and other natural sources. There have also been numerous reports on variations of the EM field existing in LAI system prior to some significance earthquakes. We simulated the EM field in the lithosphere-ionosphere waveguide with a whole-earth model using a curvature coordinate by the hybrid pseudo-spectral and finite difference time domain method. Considering the seismogensis as a fully coupled seismoelectric process, we simulate the seismic wave and the EM wave in this 2D model. In the model we have observed the excitation of the Schumann Resonance (SR) as the background EM field generated by randomly placed electric-current impulses within the lowest 10 kilometers of the atmosphere. The diurnal variation and the latitude-dependence in ion concentration in the ionosphere are included in the model. After the SR reaching a steady state, an electric impulse is introduced in the shallow lithosphere to mimic the seismogenic process (pre-, co- and post-seismic) to assess the possible precursory effects on SR strength and frequency. The modeling results can explain the observed fact of why SR has a much more sensitive response to continental earthquakes, and much less response to oceanic events. The fundamental reason is simply due to the shielding effect of the conductive ocean that prevents effective radiation of the seismoelectric signals from oceanic earthquake events into the LAI waveguide.

  13. Seismic evidence for stratification in composition and anisotropic fabric within the thick lithosphere of Kalahari Craton

    NASA Astrophysics Data System (ADS)

    Sodoudi, F.; Yuan, X.; Kind, R.; Lebedev, S.; Tilmann, F. J.

    2013-12-01

    S receiver functions obtained from the data of 97 seismic stations present evidence for the existence of a layered and thick lithosphere beneath the Kalahari Craton. We identified three negative discontinuities within the lithosphere of the Archean cratons and Proterozoic mobile belts of southern Africa. We also employed a novel combination of SRFs and surface-wave analysis to constrain the anisotropic properties of the lithosphere and its internal layering. Our results show that frozen-in anisotropy and compositional changes can generate sharp Mid-Lithospheric Discontinuities (MLD) at depths of 85 and 150-200 km, respectively. We found that a 50 km thick anisotropic layer containing 3% S wave anisotropy and with a fast-velocity axis different from that in the layer beneath can account for the first MLD at about 85 km depth. This depth is largely consistent with that of 8° discontinuity suggested as a global characteristic of cratonic lithosphere. Significant correlation between the depths of an apparent boundary separating the depleted and metasomatic refertilized lithosphere, as inferred from chemical tomography, and those of our second MLD (at 150-200 km depth) led us to characterize this negative discontinuity as a compositional boundary, most likely due to the modification of the cratonic mantle lithosphere by magma infiltration. We detected this MLD at a depth of about 150 km beneath the Zimbabwe Craton and Limpopo belt with a steep deepening to about 200 km underneath the Kaapvaal Craton and its passive margin. The deepening of this boundary is spatially correlated with the surficial expression of the ancient Thabazimbi-Murchison Lineament (TML). This may imply that the translithospheric TML isolates the lithospheric block of the relatively younger Limpopo terrane from that of the ancient Kaapvaal terrane. Finally, the largest velocity contrast (3.6-4.7%) is observed at a boundary located at depths of 260-280 km beneath the Archean domains and the older

  14. Lithosphere erosion and breakup due to the interaction between extension and plume upwelling

    NASA Astrophysics Data System (ADS)

    Lavecchia, Alessio; Thieulot, Cedric; Beekman, Fred; Cloetingh, Sierd; Clark, Stuart

    2016-04-01

    We have built up 2D numerical models of coupled crust - lithospheric mantle - upper mantle systems. The reconstructed sections are subjected to external velocity fields and mantle plume impingement beneath the lithosphere, both acting simultaneously. The models are designed to simulate the interaction between plumes and lithosphere in an extensional setting with the main purpose to contribute to address the following questions: 1) Are plumes capable of weakening certain lithospheric regions? Where and when are the main effects observed? 2a) Can a plume really cause a plate break-off and drifting with no external contribution; 2b) if yes, are there any particularly favorable conditions required? In our models a novel aspect is melt generation due to plume, upper mantle and lithospheric mantle partial melting. Produced melts are capable to ascend across the reconstructed sections due to buoyancy. Furthermore, heat transport related to melt movement is taken into account and leads to a significant heating of host rocks at the melt neutral buoyancy depth. In absence of external stress or velocity fields, the effects of plume impingement beneath the lithosphere are negligible at surface. Here the main observed feature is the production of doming at various length scales, depending on the adopted rheology for the crust. At depth, the main effect is a thermo-mechanical erosion of the lithospheric mantle with production of melts and subsequent underplating of the crust. The heat flux due to plume impingement and crust underplating determines a weakening of crust and lithosphere. However, the strength drop is not followed by an appreciable deformation. When external stress or velocity fields are applied, the coupled effects with plume presence and melt production lead to great modifications of the lithospheric structure. Topography profiles are characterized by the presence of a horst and graben structure, and extensive erosion of the lithosphere always occurs. The presence

  15. Controls of Lithospheric Mechanical Strength on the Deformation Pattern of Tien Shan

    NASA Astrophysics Data System (ADS)

    Li, Y.; Xiong, X.; Zheng, Y.; Hu, X.; Zhang, Y.

    2015-12-01

    The Tien Shan is an outstanding example of intracontinental mountain belt, which was built rapidly and formed far away from plate boundaries. It exhibits 300~500 km in width and extends ~2000 km EW, located in central Asia. The Tien Shan is a key area for solution of the problems relating to intracontinental geodynamics. During last decades, despite a large amount of results based on various geological, geophysical and geodetic data about the Tien Shan, however, deformation mechanism remains controversial and other several principal problems related to its structure and evolution also have not been completely resolved. As for patterns of continental deformation, they are always controlled by both the forces applied to the lithosphere and by lithospheric resistance to the forces. The latter is often measured by the mechanical strength of lithosphere. The lateral variation of strength of lithosphere has been recognized to be an important factor controlling the spatial construction and temporal evolution of continent. In this study, we investigate the mechanical strength (Te) of lithosphere in the Tien Shan using wavelet coherency between Bouguer anomaly and topography. The patterns of Te variations are closely related to major tectonic boundaries and blocks. Mechanical strength exhibits a weak zone (Te~5-20km) beneath the Tien Shan while its surrounding blocks including Tarim Basin, Junggar Basin and Kazakh platform are characterized by a strong lithosphere (Te>40km). The lateral variations in mechanical strength and velocity field of horizontal movement with GPS demonstrate that strain localization appears at the margins of Tarim Basin, which is also the strong lithospheric domain. It is suggested that the weak lithosphere allows the crustal stress accumulation and the strong lithosphere helps to stress transfer. There is also a good agreement between mechanical strength and shear wave velocity structure in upper mantle. It indicates a strong domain located in the

  16. Grainsize evolution in ductile shear zones: Implications for strain localization and the strength of the lithosphere

    NASA Astrophysics Data System (ADS)

    Platt, J. P.; Behr, W. M.

    2011-04-01

    At high stresses and low temperatures, grainsize reduction by dynamic recrystallization profoundly modifies rock rheology. Strain energy driven grain-boundary migration (ρGBM) is involved both in the nucleation of new grains by the grain-boundary bulging mechanism (BLG), and in the subsequent evolution of the microstructure. Above the Dmin line, which is a line in stress/grainsize space that defines the minimum size of nucleus that can form by BLG, ρGBM dominates the microstructure, and grain growth by surface energy driven grain-boundary migration (γGBM) is inhibited. The recrystallized grainsize is therefore dominated by the nucleation process, possibly controlled by the size of subgrains or dislocation cells within the old grains. This provides a first-order explanation for the experimentally observed grainsize-stress relationship. ρGBM is an important agent of recovery in rocks deformed by dislocation creep, sweeping out dislocations and counteracting work-hardening. We have derived a new flow law (DRX-assisted dislocation creep) based on this process, which exhibits grainsize sensitivity as a result of the role of ρGBM. If grainsize obeys the empirically-determined grainsize-stress relationship, DRX creep has an effective stress exponent of a little over 4, consistent with experimental observations and inferences from naturally deformed rocks. DRX creep may be an important agent in weakening quartz at low temperatures, whereas current flow law data suggest it may not be important in olivine. Rocks deformed and dynamically recrystallized above the Dmin line may switch from climb-assisted dislocation creep to grainsize-sensitive creep (Coble creep, DRX creep, or creep dominated by grain-boundary sliding), resulting in weakening. Lithospheric-scale shear zones are likely to evolve at approximately constant stress; under these conditions weakening results in an increase in strain rate, not a stress drop. The rate of dislocation motion, the dislocation density

  17. Geophysical Exploration Technologies for the Deep Lithosphere Research: An Education Materials for High School Students

    NASA Astrophysics Data System (ADS)

    Xu, H.; Xu, C.; Luo, S.; Chen, H.; Qin, R.

    2012-12-01

    The science of Geophysics applies the principles of physics to study of the earth. Geophysical exploration technologies include the earthquake seismology, the seismic reflection and refraction methods, the gravity method, the magnetic method and the magnetotelluric method, which are used to measure the interior material distribution, their structure and the tectonics in the lithosphere of the earth. Part of the research project in SinoProbe-02-06 is to develop suitable education materials for carton movies targeting the high school students and public. The carton movies include five parts. The first part includes the structures of the earth's interior and variation in their physical properties that include density, p-wave, s-wave and so on, which are the fundamentals of the geophysical exploration technologies. The second part includes the seismology that uses the propagation of elastic waves through the earth to study the structure and the material distribution of the earth interior. It can be divided into earthquake seismology and artifice seismics commonly using reflection and refraction. The third part includes the magnetic method. Earth's magnetic field (also known as the geomagnetic field)extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun. The aim of magnetic survey is to investigate subsurface geology on the basis of anomalies in the Earth's magnetic field resulting from the magnetic properties of the underlying rocks. The magnetic method in the lithosphere attempts to use magnetic disturbance to analyse the regional geological structure and the magnetic boundaries of the crust. The fourth part includes the gravity method. A gravity anomaly results from the inhomogeneous distribution of density of the Earth. Usually gravity anomalies contain superposed anomalies from several sources. The long wave length anomalies due to deep density contrasts are called regional anomalies. They are

  18. Lithosphere structure of the west Qinling orogenic belt revealed by deep seismic reflection profile