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Sample records for african oceanic lithosphere

  1. 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.

  2. Oceanic earthquakes and the tectonic evolution of oceanic lithosphere

    NASA Technical Reports Server (NTRS)

    Solomon, Sean C.

    1988-01-01

    The body waveform inversion method of Nabelek (1984) is used to study the centroid depths and source properties of oceanic earthquakes. The source parameters for 50 earthquakes which occurred along slowly spreading midocean ridges between 1962 and 1983 are used to examine the mechanical characteristics of the median valley, including the water depth in the epicentral region, the depth range of seismic faulting, the centroid depth and seismic moment versus spreading rate, and the seismic moment budget. The locations and source characteristics of oceanic intraplate earthquakes are discussed, including near-ridge earthquakes, lithospheric stress, and earthquakes in older oceanic lithosphere. The results suggest that the median valley form by the necking of a strong layer. The properties of near-ridge earthquakes support the hypothesis that thermal stress generated by diferential cooling of the plate can be stored and accumulated over millions of years. Earthquakes in older oceanic lithosphere are most likely to reflect stresses generated by plate driving forces.

  3. Deformation of Indian Ocean Lithosphere Implies Highly Non-linear Rheological Law for Oceanic Lithosphere

    NASA Astrophysics Data System (ADS)

    Gordon, Richard; Houseman, Gregory

    2015-04-01

    The width of diffuse oceanic plate boundaries is determined by the rheology of oceanic lithosphere. Here we apply thin viscous sheet models, which have been successfully applied to deformation in several continental deforming zones, to investigate the deformation of oceanic lithosphere in the diffuse oceanic plate boundaries between the India, Capricorn, and Australia plates. We apply kinematic boundary conditions based on the current motion between these plates. We neglect buoyancy forces due to plate thinning or thickening and assume that the thin viscous sheet has the same depth-integrated non-linear viscosity coefficient everywhere. Our initial models have only one adjustable parameter, n, the power-law exponent, with n=1, 3, 10, 30, 100. The predicted width of the deforming zone decreases with increasing n, with n ≥ 30 explaining the observations. This n-value is higher than has been estimated for continental lithosphere, and suggests that more of the strength of oceanic lithosphere lies in layers deforming by faulting or by dislocation glide than for continental lithosphere. To obtain a stress field that better fits the distribution and type of earthquake focal mechanisms in the diffuse oceanic plate boundary, we add a second adjustable parameter, representing the effect of slab-pull stretching the oceanic plate near the Sumatra trench. We show that an average velocity increment on this boundary segment of 5 mm/a (relative to the average velocity of the India and Australia plates) fits the observed distribution of fault types better than velocities of 3.3 mm/a or 10 mm/a.

  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. 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.

  6. Lithospheric strength across the ocean-continent transition in the NW of the Iberian Peninsula

    NASA Astrophysics Data System (ADS)

    Martín-Velázquez, Silvia; Martín-González, Fidel

    2014-05-01

    The main objective of this work is to investigate the relation between the strength of the lithosphere and the observed pattern of seismicity across the ocean-continent transition in the NW margin of the Iberian Peninsula. The seismicity is diffuse in this intraplate area, far from the seismically active margin of the plate: the Eurasia-African plate boundary, where convergence occurs at a rate of 4-5mm/year. The earthquake epicentres are mainly limited to an E-W trending zone (onshore seismicity is more abundant than offshore), and most earthquakes occur at depths less than 30 km, however, offshore depths are up to 150 km). Moreover, one of the problems to unravel in this area is that the seismotectonic interpretations of the anomalous seismicity in the NW peninsular are contradictory. The temperature and strength profiles have been modelled in three domains along the non-volcanic rifted West Iberian Margin: 1) the oceanic lithosphere of the Iberian Abyssal Plain, 2) the oceanic lithosphere near the ocean-continent transition of the Galicia Bank, and 3) the continental lithosphere of the NW Iberian Massif. The average bathymetry and topography have been used to fit the thermal structures of the three types of lithospheres, given that the heat flow and heat production values show a varied range. The geotherms, together with the brittle and ductile rheological laws, have been used to calculate the strength envelopes in different stress regimes (compression, shear and tensile). The continental lithosphere-asthenosphere boundary is located at 123 km and several brittle-ductile transitions appear in the crust and the mantle. However, the oceanic lithospheres are thinner (110 km near the Galicia Bank and 87 km in the Iberian Abbysal Plain) and more simple (brittle behaviour in the crust and upper mantle). The earthquake distribution is best explained by lithospheres with dry compositions and shear or tensile stress regimes. These results are similar can be compared to

  7. 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.

  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. Formation of Oceanic Lithosphere by Basal Magma Accretion

    NASA Astrophysics Data System (ADS)

    Hamza, V. M.; Cardoso, R. R.; Alexandrino, C. H.

    2009-12-01

    The thermal models of the lithosphere proposed to date have failed to provide satisfactory accounts of some of the important features in large-scale variations of ocean floor bathymetry and heat flow. The systematic difference between model calculations and observational data have given rise to the so-called “oceanic heat flow paradox”, for which no satisfactory solution has been found for over the last forty years. In the present work, we point out that this paradox is a consequence of the assumption that lateral temperature variations are absent in the sub-lithospheric mantle. In the present work we propose a simple magma accretion model and examine its implications for understanding the thermal field of oceanic lithosphere. The new model (designated VBA) assumes existence of lateral variations in magma accretion rates and temperatures at the boundary zone between the lithosphere and the asthenosphere, similar in character to those observed in magma solidification processes in the upper crust. However, unlike the previous thermal models of the lithosphere, the ratio of advection to conduction heat transfer (the Peclet number) is considered a space dependent variable. The solution to the problem of variable basal heat input has been obtained by the method of integral transform. The results of VBA model simulations reveal that the thickness of the young lithosphere increases with distance from the ridge axis, at rates faster than those predicted by Half-Space Cooling and Plate models. Another noteworthy feature of the new model is its ability to account for the main observational features in the thermal behavior of both young and old oceanic lithosphere. Thus, heat flow and bathymetry variations calculated on the basis of the VBA model provide vastly improved fits to respective observational datasets. More importantly, the improved fits to bathymetry and heat flow have been achieved for the entire age range of oceanic lithosphere and without the need to invoke

  10. 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.

  11. Satellite tidal magnetic signals constrain oceanic lithosphere-asthenosphere boundary

    PubMed Central

    Grayver, Alexander V.; Schnepf, Neesha R.; Kuvshinov, Alexey V.; Sabaka, Terence J.; Manoj, Chandrasekharan; Olsen, Nils

    2016-01-01

    The tidal flow of electrically conductive oceans through the geomagnetic field results in the generation of secondary magnetic signals, which provide information on the subsurface structure. Data from the new generation of satellites were shown to contain magnetic signals due to tidal flow; however, there are no reports that these signals have been used to infer subsurface structure. We use satellite-detected tidal magnetic fields to image the global electrical structure of the oceanic lithosphere and upper mantle down to a depth of about 250 km. The model derived from more than 12 years of satellite data reveals a ≈72-km-thick upper resistive layer followed by a sharp increase in electrical conductivity likely associated with the lithosphere-asthenosphere boundary, which separates colder rigid oceanic plates from the ductile and hotter asthenosphere. PMID:27704045

  12. Satellite tidal magnetic signals constrain oceanic lithosphere-asthenosphere boundary.

    PubMed

    Grayver, Alexander V; Schnepf, Neesha R; Kuvshinov, Alexey V; Sabaka, Terence J; Manoj, Chandrasekharan; Olsen, Nils

    2016-09-01

    The tidal flow of electrically conductive oceans through the geomagnetic field results in the generation of secondary magnetic signals, which provide information on the subsurface structure. Data from the new generation of satellites were shown to contain magnetic signals due to tidal flow; however, there are no reports that these signals have been used to infer subsurface structure. We use satellite-detected tidal magnetic fields to image the global electrical structure of the oceanic lithosphere and upper mantle down to a depth of about 250 km. The model derived from more than 12 years of satellite data reveals a ≈72-km-thick upper resistive layer followed by a sharp increase in electrical conductivity likely associated with the lithosphere-asthenosphere boundary, which separates colder rigid oceanic plates from the ductile and hotter asthenosphere.

  13. Thermoelastic stress in oceanic lithosphere due to hotspot reheating

    NASA Technical Reports Server (NTRS)

    Zhu, Anning; Wiens, Douglas A.

    1991-01-01

    The effect of hotspot reheating on the intraplate stress field is investigated by modeling the three-dimensional thermal stress field produced by nonuniform temperature changes in an elastic plate. Temperature perturbations are calculated assuming that the lithosphere is heated by a source in the lower part of the thermal lithosphere. A thermal stress model for the elastic lithosphere is calculated by superposing the stress fields resulting from temperature changes in small individual elements. The stress in an elastic plate resulting from a temperature change in each small element is expressed as an infinite series, wherein each term is a source or an image modified from a closed-from half-space solution. The thermal stress solution is applied to midplate swells in oceanic lithosphere with various thermal structures and plate velocities. The results predict a stress field with a maximum deviatoric stress on the order of 100 MPa covering a broad area around the hotspot plume. The predicted principal stress orientations show a complicated geographical pattern, with horizontal extension perpendicular to the hotspot track at shallow depths and compression along the track near the bottom of the elastic lithosphere.

  14. 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.

  15. 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.

  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. The base of the seismogenic zone in the oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Greg, H.; Behn, M.; McGuire, J.

    2008-12-01

    Geophysical observations indicate that seismicity in the oceanic lithosphere is generally limited to depths above the 600°C isotherm. This relationship is in good agreement with extrapolation of experimental data on the frictional behavior of olivine (Boettcher et al., 2007). Under laboratory conditions, a transition from unstable to stable frictional sliding is observed at a temperature of approximately 1000°C. By accounting for the rate-dependence of crystal plasticity at asperities, the same transition is predicted to occur at a temperature of approximately 600°C in the Earth. While this agreement is encouraging, several issues remain poorly constrained - resolution of which may provide important insights into understanding the dynamics of earthquakes in general. A unique aspect of many oceanic earthquakes is that they likely occur in what was previously undamaged rock. Owing to upwelling and corner flow, the mantle rocks cool below the 600°C isotherm prior to any brittle deformation. Thus, rocks in the source regions for these earthquakes are likely intact at relatively high pressure with no pore fluids present. In other words, almost all the mechanisms hypothesized to produce weakening along faults in continental settings are unlikely to be active prior to an earthquake in the oceanic lithosphere. These rocks could thus be capable of supporting shear stresses in the range of 500 MPa at depths of 20 to 30 km. We will review these rheological constraints, discuss the evidence (or lack thereof) for high stresses based on earthquake seismology, and investigate alternate mechanisms that could be responsible for weakening the oceanic lithosphere - such as penetration of fluid from the surface to the greatest depths of lithospheric seismicity.

  18. 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.

  19. 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.

  20. Geoid data and thermal structure of the oceanic lithosphere

    SciTech Connect

    Richardson, W.P.; Stein, S.; Stein, C.A.

    1995-07-15

    A long-standing question is whether old oceanic lithosphere continues cooling as the boundary layer of a halfspace or approaches thermal equilibrium as modeled by a finite thickness plate. Although the latter is the most direct inference from seafloor depths and heat flow, other explanations have been proposed. We investigate this issue using published results for the derivative of the oceanic geoid with age estimated from geoid offsets across fracture zones. Such data have not been used extensively in analyses of the thermal evolution of the lithosphere, primarily because they are inconsistent with two commonly used thermal models; a halfspace or a 125-km-thick plate. Recent studies, however, find that depth and heat flow data are better fit by a thinner (95 km) plate model. We thus compile published geoid slope results, and find that these data, though scattered, can discriminate between the models. Geoid slope changes with age, rather than being constant as predicted for a cooling halfspace. This variation is greater than predicted for a thick plate and is better fit by a thin plate. Geoid data should thus be useful for improving thermal models of the lithosphere. 30 refs., 4 figs., 1 tab.

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

    USGS Publications Warehouse

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

    1989-01-01

    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. -from Authors

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

    NASA Astrophysics Data System (ADS)

    Medvedev, Sergei

    2016-10-01

    Many mechanisms control the state of stress within Earth plates. First-order well-known mechanisms include stresses induced by lateral variations of lithospheric density structure, sublithospheric tractions, ridge push and subduction pull. In this study, we attempt to quantify the influence of these mechanisms to understand the origin of stresses in the lithosphere, choosing the African plate (TAP) as an example. A finite-element based suite, Proshell, was developed to combine several data sets, to estimate the gravitational potential energy (GPE) of the lithosphere and to calculate stresses acting on the real (non-planar) geometry of TAP. We introduce several quantitative parameters to measure the degree of fit between the model and observations. Our modelling strategy involves nine series of numerical experiments. We start with the simplest possible model and then, step by step, build it up to be a more physically realistic model, all the while discussing the influence of each additional component. The starting (oversimplified) model series (1) is based on the CRUST2 data set for the crust and a half-space-cooling approximation of the lithospheric mantle. We then describe models (series 2-5) that account for lithospheric mantle density heterogeneities to build a more reliable GPE model. The consecutive series involve basal traction from the convective mantle (series A, C) and the rheological heterogeneity of the TAP via variations in its effective elastic thickness (series B, C). The model quality reflects the increase in complexity between series with an improving match to observed stress regimes and directions. The most complex model (series D) also accounts for the bending stresses in the elastic lithosphere and achieves a remarkably good fit to observations. All of our experiments were based on the iteration of controlling parameters in order to achieve the best fit between modelled and observed stresses, always considering physically feasible values. This

  3. Thermal cooling of the oceanic lithosphere from geoid height data

    NASA Technical Reports Server (NTRS)

    Cazenae, A.

    1985-01-01

    Another type of geophysical observation has proved to be very useful in the study of thermal cooling of the oceanic lithosphere. It is the geoid height derivative with respect to plate age, a quantity computed from the short wavelength geoid step across fracture zones measured by altimeter satellites. Two categories of simples models are proposed to describe cooling and contraction of the oceanic lithosphere with age. Both plate model and half space model, give almost similar results up to ages of 50 to 70 ma, but predict quite distinct behavior of seafloor depth, heat flow and other parameters in old basins. Tests of thermal models are based on heat flow and topography data. However, heat flow is not very sensitive to the form of the thermal model. Large areas of the ocean floor are particularly shallow, and as a result topography data may not be very appropriate to discriminate between plate and half space models, and no clear concensus on a preferred model yet exists.

  4. Factors controlling the location of compressional deformation of oceanic lithosphere in the Central Indian Ocean

    NASA Technical Reports Server (NTRS)

    Karner, Garry D.; Weissel, Jeffrey K.

    1990-01-01

    One- and two-dimensional models for the deformation by horizontal compression of an elastic plate containing a preexisting deflection were developed and analyzed in order to explain why the compressionally deformed oceanic lithosphere in the Central Indian-Ocean basin is not located where maximum levels of compressive stress in the Indo-Australian plate were predicted by Cloetingh and Wortel (1985, 1986). It is concluded from the results that the location of the deformed region is controlled by an earlier lithospheric deformation that is attributed to the emplacement of the Afanazy-Nikitin seamount group in Late Cretaceous or early Tertiary time.

  5. Formation of the Oceanic Lithosphere from the Upper Asthenosphere

    NASA Astrophysics Data System (ADS)

    Presnall, D. C.; Gudfinnsson, G. H.

    2007-12-01

    In a global examination of the chemistry of MORBs, we find that Na8-Fe8-axial depth data do not support large variations in the temperature and pressure of MORB extraction from the mantle. Instead, the complete absence of high-pressure (> ~1.5 GPa) olivine-controlled crystallization of MORBs combined with solidus phase relations in the CaO-MgO-Al2O3-SiO2-Na2O-FeO system indicate that the inverse and positive Na8-Fe8 variations are produced from a heterogeneous source by melt extraction over a very narrow range of P and T (~1.2-1.5 GPa and 1250-1280°C) at the plagioclase-spinel lherzolite transition. This is inconsistent with the existence of hot mantle plumes (Easter, Galapagos, Iceland, Azores, St. Helena, Tristan, Afar) on or close to ridges. However, it is consistent with the very flat 410 km discontinuity beneath the East Pacific Rise, which does not permit the existence of even a single hot plume (Easter) beneath the ridge (Melbourne and Helmberger, 2002, JGR, 107, doi:10.1029/2001B000332). The global absence of MORBs with a high-pressure major-element signature implies that the isolation of the East Pacific Rise from the deeper mantle applies to all ridges. A new model is developed (Presnall and Gudfinnsson, in press, Origin of the Oceanic Lithosphere, J. Petrol.) that explains the formation of new seismic lithosphere (~70 km thickness) by lateral and upward migration of the slightly melted upper part (~70-140 km depth) of the low-velocity zone toward the ridge. Although decompression melting occurs over a large pressure range, melt extraction is constrained to the very narrow P-T range given above by the maximum T in the mantle at which CO2 vapor can be extracted. This condition occurs at a pressure just below that of the abrupt 280°C temperature decrease of the carbonated lherzolite solidus at the base of the seismic lithosphere. The constant association of strombolian and effusive eruptions at ridges (Clague, 2007, Geophys. Res. Abstr., 9, EUG, 02096

  6. Seismic structure of the oceanic lithosphere inferred from guided wave

    NASA Astrophysics Data System (ADS)

    Shito, A.; Suetsugu, D.; Furumura, T.; Sugioka, H.; Ito, A.

    2012-12-01

    Characteristic seismic waves are observed by seismological experiment using Broad-Band Ocean Bottom Seismometers (BBOBSs) conducted in the northwestern Pacific from 2007 to 2008 and from 2010 to 2011. The seismic waves have low frequency onset (< 1 Hz) followed by high frequency later phases (2.5-10 Hz). The high frequency later phases have large amplitude and long duration for both P and S waves. The seismic waves are observed commonly at the BBOBS array from events in the subducting Pacific plate. To investigate generation and propagation mechanisms of the seismic wave will help us to understand the seismic structure and the origin of the oceanic lithosphere. High frequency phases travelling efficiently through the oceanic lithosphere more than 3000 km are well known phenomenon. These phases were previously called as Po/So waves. Po/So waves were observed as early as 1935, and were studied actively from the 1970s to 1990s. However, the mechanism of generation and propagation of the phases are still controversial. The guided waves propagating in subducting plate are also common phenomenon in the subduction zone. The waves are generally characterized by separation of low frequency and high frequency components. In order to explain the separation, Martin and Rietbrock [2003] considered the trapping of waves in the waveguide formed by thin low velocity former oceanic crust at the top of the plate. However, large amplitude and long duration of the high frequency component cannot be achieved by the model. From the analysis of waveform observed at the eastern seaboard of northern Japan and numerical simulation of seismic wave propagation, Furumura and Kennet [2005] demonstrate that the guided wave travelling in the subducting plate is produced by multiple forward scattering of high-frequency seismic waves due to small-scale random heterogeneity in the plate structure. We apply the method proposed by Furumura and Kennett [2005] to reproduce the seismograms recorded by

  7. Relict Oceanic Lithosphere in Cuba: Types and Emplacement Ages

    NASA Astrophysics Data System (ADS)

    CobiellaReguera, J. L.

    2001-12-01

    According to their composition and tectonic position, three different types of relict oceanic lithosphere are present in Cuba: (1) the northern ophiolitic belt, a complex melange that extents more than 1000 km along the island, (2) the basement of the Cretaceous volcanic arc terrane: high temperature/low pressure amphibolites with some serpentinites and, (3) tectonic slices of serpentinite melanges (with eclogites and blueschists) and high pressure amphibolites, in the metamorphic Escambray massif (tectonostratigraphic terrane, microcontinent?) of southcentral Cuba. Available age constrains (paleontological and geochronological) indicate that relicts of oceanic lithosphere in Cuba are upper Mesozoic in age. Geochemical, petrological, and regional geology data suggest that such oceanic relicts probably originated in two different tectonic environments in the Proto-Caribbean basin; (1) a small oceanic basin of Upper Jurassic- Neocomian age, related to drift between North America and a southern continental mass and (2) a suprasubduction marginal basin, between the southeastern North American passive margin and an Aptian-Albian volcanic arc. Tectonic emplacement of the Cuban relict oceanic Proto-Caribbean lithosphere was likely related to several tectonic events and processes. Serpentinite melange slices and the high pressure amphibolites in the Jurassic and Cretaceous passive margin sequences of Escambray massif, characterized by low to moderate temperature and high pressure metamorphism, probably were emplaced from subduction and closure of the small oceanic depression located to the south (present geographic coordinates) of the volcanic arc in the Albian. The basement amphibolites of the volcanic arc terrane were derived from the Upper Jurassic-Neocomian oceanic crust, metamorphosed by the high temperatures and hot solutions related to the development on this crust of an Aptian-Albian volcanic arc with a north dipping subduction zone. These amphibolites were

  8. Thermoelastic stress - How important as a cause of earthquakes in young oceanic lithosphere?

    NASA Technical Reports Server (NTRS)

    Bratt, S. R.; Bergman, E. A.; Solomon, S. C.

    1985-01-01

    Thermoelastic or thermal stress is a potentially important contributor to the state of stress in the oceanic lithosphere. The present paper provides several simple models for the state of thermoelastic stress in a young oceanic lithosphere, taking into account a comparison of the predictions of these models with the characteristics of near-ridge earthquakes. Attention is given to the characteristics of near-ridge earthquakes, sources of stress in an oceanic lithosphere, previous models of thermal stress, the calculation of thermal stress, and thermal stress models. A test is conducted of the hypothesis that thermoelastic stress is a significant component of the stress field in a young oceanic lithosphere. The considered models support the hypothesis that thermoelastic stress is a significant component of the stress field in a young oceanic lithosphere.

  9. 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.

  10. Pre-subduction metasomatic enrichment of the oceanic lithosphere induced by plate flexure

    NASA Astrophysics Data System (ADS)

    Pilet, S.; Abe, N.; Rochat, L.; Kaczmarek, M.-A.; Hirano, N.; Machida, S.; Buchs, D. M.; Baumgartner, P. O.; Müntener, O.

    2016-12-01

    Oceanic lithospheric mantle is generally interpreted as depleted mantle residue after mid-ocean ridge basalt extraction. Several models have suggested that metasomatic processes can refertilize portions of the lithospheric mantle before subduction. Here, we report mantle xenocrysts and xenoliths in petit-spot lavas that provide direct evidence that the lower oceanic lithosphere is affected by metasomatic processes. We find a chemical similarity between clinopyroxene observed in petit-spot mantle xenoliths and clinopyroxene from melt-metasomatized garnet or spinel peridotites, which are sampled by kimberlites and intracontinental basalts respectively. We suggest that extensional stresses in oceanic lithosphere, such as plate bending in front of subduction zones, allow low-degree melts from the seismic low-velocity zone to percolate, interact and weaken the oceanic lithospheric mantle. Thus, metasomatism is not limited to mantle upwelling zones such as mid-ocean ridges or mantle plumes, but could be initiated by tectonic processes. Since plate flexure is a global mechanism in subduction zones, a significant portion of oceanic lithospheric mantle is likely to be metasomatized. Recycling of metasomatic domains into the convecting mantle is fundamental to understanding the generation of small-scale mantle isotopic and volatile heterogeneities sampled by oceanic island and mid-ocean ridge basalts.

  11. Factors controlling the location of compressional deformation of oceanic lithosphere in the central Indian Ocean

    SciTech Connect

    Karner, G.D.; Weissel, J.K. )

    1990-11-10

    The compression of oceanic lithosphere in the Central Indian Ocean does not occur where recent models for the state of stress in the Indo-Australian plate predict maximum horizontal compressive stress. The Afanazy-Nikitin seamount group, which was erupted in Late Cretaceous or Early Tertiary time, is centrally located in the region where deformation is best developed. The authors suggest that critical wavelength components in the deflection caused by the emplacement of these seamounts were preferentially amplified when north-south directed compression was applied to the northern part of the Indo-Australian plate in the late Miocene. To test this hypothesis, they develop simple one- and two-dimensional models for compression of a thin elastic plate overlying an inviscid fluid, where the plate contains a preexisting deflection. The {le} 2 km peak-to-trough amplitude and 200 km average wavelength characteristics of the broad-scale crustal deformation and the observed east-west trending pattern of free-air gravity anomalies are best matched in the modeling with an applied horizontal compression of 1.5-2.0 {times} 10{sup 13} N/m, and a plate with an effective elastic thickness of 10-15 km at the time of compression. In addition, the lithosphere is particularly susceptible to deformation by horizontal compression if seawater initially filled the deflection due to seamount emplacement, but Bengal Fan sediment fills the additional deflection caused by compression. The value of effective elastic thickness determined for the deformed lithosphere is about a factor of 2 less than values obtained from flexure of comparably aged lithosphere beneath lithosphere in response to horizontal loading. They determine a north-south shortening rate of {approx}1 mm/yr from the amplitude of horizontal compression, the width of the deformed region, and the time interval over which the deformation has occurred.

  12. 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).

  13. Gentle Africanized bees on an oceanic island.

    PubMed

    Rivera-Marchand, Bert; Oskay, Devrim; Giray, Tugrul

    2012-11-01

    Oceanic islands have reduced resources and natural enemies and potentially affect life history traits of arriving organisms. Among the most spectacular invasions in the Western hemisphere is that of the Africanized honeybee. We hypothesized that in the oceanic island Puerto Rico, Africanized bees will exhibit differences from the mainland population such as for defensiveness and other linked traits. We evaluated the extent of Africanization through three typical Africanized traits: wing size, defensive behavior, and resistance to Varroa destructor mites. All sampled colonies were Africanized by maternal descent, with over 65% presence of European alleles at the S-3 nuclear locus. In two assays evaluating defense, Puerto Rican bees showed low defensiveness similar to European bees. In morphology and resistance to mites, Africanized bees from Puerto Rico are similar to other Africanized bees. In behavioral assays on mechanisms of resistance to Varroa, we directly observed that Puerto Rican Africanized bees groomed-off and bit the mites as been observed in other studies. In no other location, Africanized bees have reduced defensiveness while retaining typical traits such as wing size and mite resistance. This mosaic of traits that has resulted during the invasion of an oceanic island has implications for behavior, evolution, and agriculture.

  14. Gentle Africanized bees on an oceanic island

    PubMed Central

    Rivera-Marchand, Bert; Oskay, Devrim; Giray, Tugrul

    2012-01-01

    Oceanic islands have reduced resources and natural enemies and potentially affect life history traits of arriving organisms. Among the most spectacular invasions in the Western hemisphere is that of the Africanized honeybee. We hypothesized that in the oceanic island Puerto Rico, Africanized bees will exhibit differences from the mainland population such as for defensiveness and other linked traits. We evaluated the extent of Africanization through three typical Africanized traits: wing size, defensive behavior, and resistance to Varroa destructor mites. All sampled colonies were Africanized by maternal descent, with over 65% presence of European alleles at the S-3 nuclear locus. In two assays evaluating defense, Puerto Rican bees showed low defensiveness similar to European bees. In morphology and resistance to mites, Africanized bees from Puerto Rico are similar to other Africanized bees. In behavioral assays on mechanisms of resistance to Varroa, we directly observed that Puerto Rican Africanized bees groomed-off and bit the mites as been observed in other studies. In no other location, Africanized bees have reduced defensiveness while retaining typical traits such as wing size and mite resistance. This mosaic of traits that has resulted during the invasion of an oceanic island has implications for behavior, evolution, and agriculture. PMID:23144660

  15. Metasomatism in the oceanic lithosphere beneath La Palma, Canary Islands

    NASA Astrophysics Data System (ADS)

    Janisch, Astrid; Ntaflos, Theodoros

    2016-04-01

    La Palma is the most active island within the Canary archipelago with historical eruption along the Cumbre Vieja Rift. Mantle peridotite xenoliths brought to the surface during the eruption 1677/78 at the site of San Antonio Volcano, close to Fuencaliente in the south of the island, gives us an excellent opportunity to study an old oceanic lithosphere. The collection of xenoliths comprises sp-harzburgites, sp-lherzolites, sp-dunites and pyroxenites but only the first three were used for this work. Metasomatic processes are evident in all samples. A common feature is a variable channelling of melt flow through the mantle xenoliths displayed in variations from pervasively metasomatized, through veined to dyke intruded peridotites. Orthopyroxene breakdown into olivine, clinopyroxene and glass is evidence for anhydrous melt percolation. Furthermore, fine-grained veins in various thicknesses consisting of olivine, pyroxene as well as amphibole with apatite and phlogopite reveal additional anhydrous and hydrous metasomatic processes, respectively. Peridotites mainly influenced by anhydrous metasomatism exhibit locally phlogopite and/or amphibole around spinel or in glass-veinlets. Pentlandite has been found in all veined samples. Amphiboles are mostly pargasites but kaersutites are also present in the amphibole-bearing veins. Two different types of amphibole veins have been recognized. The first type is an amphibole-apatite-glass-bearing amphibolite, forming a cross-cutting vein that propagates through the xenolith. The amphiboles in this vein are coarse-grained while the disseminated amphiboles are fine-grained. Clinopyroxene always occurs in association with amphibole and in textural equilibrium suggesting that both minerals have grown together. The glass is of tephritic/basanitic to trachy-basaltic composition. The second amphibole-vein contains phlogopite and traces of apatite. Textural evidence (cross-cutting olivine grains and the absence of hydrous minerals in the

  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. Empirical model of the gravitational field generated by the oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Tenzer, Robert; Chen, Wenjin; Ye, Zhourun

    2015-01-01

    We present an empirical model of the gravitational field generated by the oceanic lithosphere computed over the world's oceans with a spectral resolution complete to a spherical harmonic degree of 180. This gravity model is compiled based on applying methods for a spherical harmonic analysis and synthesis of the global gravity and crustal structure models. The in situ seawater densities and the density samples from ocean-floor drilling sites are utilized in the gravimetric forward modeling of bathymetry and marine sediments. The gravitational signal attributed to the oceanic lithosphere density structure is described empirically in terms of the ocean-floor age and depth. The former is explained by the increasing density with age due to conductive cooling of the oceanic lithosphere. The latter describes the gravitational signature of thermal lithospheric contraction, which is isostatically compensated by ocean deepening. The long-wavelength gravity spectrum reflects mainly the compositional and thermal structures within the sub-lithospheric mantle. We demonstrate that this empirical gravity model reproduces realistically most of the long-to-medium wavelength features of the actual gravity field, except for some systematic discrepancies, especially along continental slopes and large sedimentary accumulations, which cannot be described accurately by applied empirical models.

  18. Recycled dehydrated lithosphere observed in plume-influenced mid-ocean-ridge basalt.

    PubMed

    Dixon, Jacqueline Eaby; Leist, Loretta; Langmuir, Charles; Schilling, Jean-Guy

    2002-11-28

    A substantial uncertainty in the Earth's global geochemical water cycle is the amount of water that enters the deep mantle through the subduction and recycling of hydrated oceanic lithosphere. Here we address the question of recycling of water into the deep mantle by characterizing the volatile contents of different mantle components as sampled by ocean island basalts and mid-ocean-ridge basalts. Although all mantle plume (ocean island) basalts seem to contain more water than mid-ocean-ridge basalts, we demonstrate that basalts associated with mantle plume components containing subducted lithosphere--'enriched-mantle' or 'EM-type' basalts--contain less water than those associated with a common mantle source. We interpret this depletion as indicating that water is extracted from the lithosphere during the subduction process, with greater than 92 per cent efficiency.

  19. Vertical movements of the oceanic lithosphere above La Reunion hotspot

    NASA Astrophysics Data System (ADS)

    de Voogd, B.; Deplus, C.; Sisavath, E.; Depuiset, F.; Mercier, M.

    2009-04-01

    While La Reunion hot spot is often considered as a typical mantle plume, several studies have pointed out the complexity of its history, and documented major differences with the Hawaian archetype. We present new seismic data to discuss vertical movements related to La Reunion-Mauritius intraplate magmatic activity. High resolution seismic reflection data acquired during the FOREVER cruise in 2006 (for a presentation of the cruise, see Deplus et al., this meeting) document the evolution of lithospheric flexure related to Mauritius and Reunion loading. With a total length of 12200 km, the seismic profiles span a large area, basically the whole compartment between the Mahanoro and Mauritius fracture zones, at latitudes ranging from 19 to 25°S. Seismic penetration reaches the top of oceanic basement, i.e. up to 1.5 s of penetration. Maps of basement topography and sediment thickness have been derived from these and previous data (cruise REUSIS, 1993). The FOREVER data provide no information on seismic velocities, even in the shallowest layers, due to a 240 m long streamer and a waterdepth over 2000 m. Time-to-depth conversion of the seismic interpretation has been done for a range of likely velocities, based on published results from deep multichannel seismic reflexion and OBS data from the REUSIS cruise. The sedimentary and volcaniclastic deposits are much thicker to the North and East of La Reunion. Both tectono-magmatic deformation and gravitational processes disrupt the stratigraphic column, especially in the southern area of the survey. In several places, sedimentary reflectors are deformed or disrupted by basement highs. Correlation of seismic horizons indicates that magmatic activity occurred well after oceanic basement formation, in places quite recently, and with a spatial distribution not simply related to the known volcanic activity. Three major units are identified in the vicinity of Mauritius. The lower one, resting directly on oceanic basement, is

  20. Constraints on lithospheric thermal structure for the Indian Ocean from depth and heat flow data

    NASA Technical Reports Server (NTRS)

    Shoberg, Tom; Stein, Carol A.; Stein, Seth

    1993-01-01

    Models for the thermal evolution of oceanic lithosphere are primarily constrained by variations in seafloor depth and heat flow with age. These models have been largely based on data from the Pacific and Atlantic Ocean basins. We construct seafloor age relations for the Indian Ocean which we combine with bathymetric, sediment isopach and heat flow data to derive curves for depth and heat flow versus age. Comparison of these curves with predictions from three thermal models shows that they are better fit by the shallower depths and higher heat flow for the GDH1 model, which is characterized by a thinner and hotter lithosphere than previous models.

  1. Constraints on lithospheric thermal structure for the Indian Ocean from depth and heat flow data

    NASA Astrophysics Data System (ADS)

    Shoberg, Tom; Stein, Carol A.; Stein, Seth

    1993-06-01

    Models for the thermal evolution of oceanic lithosphere are primarily constrained by variations in seafloor depth and heat flow with age. These models have been largely based on data from the Pacific and Atlantic Ocean basins. We construct seafloor age relations for the Indian Ocean which we combine with bathymetric, sediment isopach and heat flow data to derive curves for depth and heat flow versus age. Comparison of these curves with predictions from three thermal models shows that they are better fit by the shallower depths and higher heat flow for the GDH1 model, which is characterized by a thinner and hotter lithosphere than previous models.

  2. Source mechanisms of earthquakes near mid-ocean ridges from body waveform inversion - Implications for the early evolution of oceanic lithosphere

    NASA Technical Reports Server (NTRS)

    Bergman, E. A.; Solomon, S. C.

    1984-01-01

    An evaluation is presented of the tectonics of the near-ridge environment based on the source mechanisms of earthquakes in young oceanic lithosphere. A catalog of near-ridge earthquakes is presented, and source parameters are determined from body waveform inversions. Source parameter-age relations are examined, and the near-ridge seismicity of the Indian Ocean is discussed. Deep normal faulting in young oceanic lithosphere is addressed, as is the relationship between thermoelastic stress and near-ridge earthquakes. The possibility of secondary convection beneath young oceanic lithosphere is considered. Finally, the broader tectonic implications of these results for the evolution of young oceanic lithosphere are discussed.

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

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

  4. 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

  5. Numerical Modeling of Gravity, Geoid, and the Thermal Structure of Oceanic Lithosphere

    NASA Astrophysics Data System (ADS)

    Davis, D.; Grose, C. J.

    2010-12-01

    In order to better understand links between the thermal and density structure of oceanic lithosphere with its surface manifestations, we implement numerical techniques to predict both the thermal structure of oceanic lithosphere and the consequent gravity field and geoid height. The two principal problems of our potential field numerical algorithm are the transposition of density data in two-dimensional Cartesian space into a three-dimensional spherical shell model, and accurately quantifying deformation in the Cartesian grid associated with contraction and isostatic adjustment. Using this model we investigate anomalies in gravity and the geoid due to density heterogeneities associated with varying properties and boundary conditions of the oceanic lithosphere. Such sources of thermal and density structure include the impact of variable heat transport properties, lattice preferred orientation, mantle potential temperature and adiabate, plate thickness, and the composite mineral assemblage. Variable transport properties are known to result in substantial deviations in the geotherm relative to that expected from models with constant thermal parameters. Since the near-surface lithosphere is cooler, the impact on geoid height should be smaller and this is important for constraining the effective diffusivity of the lithosphere. In addition, composite mineralogy is important due to phase changes. In particular, the spinel-garnet transition rapidly elevates in lithosphere around ages of 20 to 40 Mya. Such a density anomaly is likely to be observable in the geoid height, and this may account for unexplained features of geoid slope at young ages.

  6. 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

  7. 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

  8. 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

  9. Global rate and distribution of H2 gas produced by serpentinization within oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Worman, Stacey L.; Pratson, Lincoln F.; Karson, Jeffrey A.; Klein, Emily M.

    2016-06-01

    It has recently been estimated that serpentinization within continental lithosphere produces H2 at rates comparable to oceanic lithosphere (both are ~1011 mol H2/yr). Here we present a simple model that suggests that H2 production rates along the mid-oceanic ridge alone (i.e., excluding other marine settings) may exceed continental production by an order of magnitude (~1012 mol H2/yr). In our model, H2 production rates increase with spreading rate and the net thickness of serpentinizing peridotite (S-P) in a column of lithosphere. Lithosphere with a faster spreading rate therefore requires a relatively smaller net thickness of S-P to produce H2 at the same rate as lithosphere with a slower rate and greater thickness of S-P. We apply our model globally, incorporating an inverse relationship between spreading rate and net thickness of S-P to be consistent with observations that serpentinization is more common within lithosphere spreading at slower rates.

  10. Segmented African Lithosphere Beneath Anatolia Imaged by Teleseismic P-Wave Tomography

    NASA Astrophysics Data System (ADS)

    Biryol, Cemal; Zandt, George; Beck, Susan; Ozacar, Atilla

    2010-05-01

    Anatolia, a part of the Alpine-Himalayan orogenic belt, is shaped by a variety of complex tectonic processes that define the major tectonic provinces across which different deformation regimes exist. Collision related plateau formation dominates the present lithospheric deformation to the east and slab roll-back related back-arc extension takes place in the west. The two zones are connected at the northern part of the region by strike-slip faulting along the right-lateral North Anatolian Fault Zone. Recent seismological studies show that the Eastern Anatolian Plateau (EAP) is supported by hot asthenosphereric material that was emplaced beneath the plateau following the detachment of subducted Arabian lithosphere. The westward continuation of the deeper structure of Anatolia was previously less well constrained due to the lack of geophysical observations. In order to study the deeper lithosphere and mantle structure beneath Anatolia, we used teleseismic P-wave tomography and data from several temporary and permanent seismic networks deployed in the region. A major part of the data comes from the North Anatolian Fault passive seismic experiment (NAF) that consists of 39 broadband seismic stations operated at the north central part of Anatolia between 2005 and 2008. We also used data collected from permanent seismic stations of the National Earthquake Monitoring Center (NEMC) and stations from the Eastern Turkey Seismic Experiment (ETSE). Approximately 34,000 P-wave travel time residuals, measured in multiple frequency bands, are inverted using approximate finite-frequency sensitivity kernels. Our tomograms reveal a fast anomaly that corresponds to the subducted portion of the African lithosphere along the Cyprean Arc. This fast anomaly dips northward beneath central Anatolia with an angle of approximately 45 degrees. However, the anomaly disappears rather sharply to the east beneath the western margin of the EAP and to the west beneath the Isparta Angle. The western

  11. Macroscopic strength of oceanic lithosphere revealed by ubiquitous fracture-zone instabilities

    NASA Astrophysics Data System (ADS)

    Cadio, Cécilia; Korenaga, Jun

    2016-09-01

    The origin of plate tectonics is one of the most fundamental issues in earth and planetary sciences. Laboratory experiments indicate that the viscosity of silicate rocks is so strongly temperature-dependent that the entire surface of the Earth should be one immobile rigid plate. The rheology of oceanic lithosphere is, however, still poorly understood, and there exist few constraints on the temperature dependency of viscosity on the field scale. Here we report a new kind of observational constraint based on the geoid along oceanic fracture zones. We identify a large number of conspicuous small-scale geoid anomalies, which cannot be explained by the standard evolution model of oceanic lithosphere, and estimate their source density perturbations using a new Bayesian inversion method. Our results suggest that they are caused most likely by small-scale convection involving temperature perturbations of ∼ 300 K ± 100 K. Such thermal contrast requires the activation energy of mantle viscosity to be as low as 100 ± 50 kJmol-1 in case of diffusion creep, and 225 ± 112 kJmol-1 in case of dislocation creep, substantially reducing the thickness of the stiffest part of oceanic lithosphere. Oceanic lithosphere may thus be broken and bent much more easily than previously thought, facilitating the operation of plate tectonics.

  12. Arctic and N Atlantic Crustal Thickness and Oceanic Lithosphere Distribution from Gravity Inversion

    NASA Astrophysics Data System (ADS)

    Kusznir, Nick; Alvey, Andy

    2014-05-01

    The ocean basins of the Arctic and N. Atlantic formed during the Mesozoic and Cenozoic as a series of distinct ocean basins, both small and large, leading to a complex distribution of oceanic crust, thinned continental crust and rifted continental margins. The plate tectonic framework of this region was demonstrated by the pioneering work of Peter Ziegler in AAPG Memoir 43 " Evolution of the Arctic-North Atlantic and the Western Tethys" published in 1988. The spatial evolution of Arctic Ocean and N Atlantic ocean basin geometry and bathymetry are critical not only for hydrocarbon exploration but also for understanding regional palaeo-oceanography and ocean gateway connectivity, and its influence on global climate. Mapping crustal thickness and oceanic lithosphere distribution represents a substantial challenge for the Polar Regions. Using gravity anomaly inversion we have produced comprehensive maps of crustal thickness and oceanic lithosphere distribution for the Arctic and N Atlantic region, We determine Moho depth, crustal basement thickness, continental lithosphere thinning and ocean-continent transition location using a 3D spectral domain gravity inversion method, which incorporates a lithosphere thermal gravity anomaly correction (Chappell & Kusznir 2008). Gravity anomaly and bathymetry data used in the gravity inversion are from the NGA (U) Arctic Gravity Project and IBCAO respectively; sediment thickness is from a new regional compilation. The resulting maps of crustal thickness and continental lithosphere thinning factor are used to determine continent-ocean boundary location and the distribution of oceanic lithosphere. Crustal cross-sections using Moho depth from the gravity inversion allow continent-ocean transition structure to be determined and magmatic type (magma poor, "normal" or magma rich). Our gravity inversion predicts thin crust and high continental lithosphere thinning factors in the Eurasia, Canada, Makarov, Podvodnikov and Baffin Basins

  13. Deformation of Indian Ocean lithosphere: Evidence for a highly nonlinear rheological law

    NASA Astrophysics Data System (ADS)

    Gordon, Richard G.; Houseman, Gregory A.

    2015-06-01

    The width of diffuse oceanic plate boundaries is determined by the rheology of oceanic lithosphere. Here we apply thin viscous sheet models, which have been successfully applied to deformation in several continental deforming zones, to investigate the deformation of oceanic lithosphere in the diffuse oceanic plate boundaries between the India, Capricorn, and Australia Plates. We apply kinematic boundary conditions based on the current motion between these plates. We neglect buoyancy forces due to plate thinning or thickening and assume that the thin viscous sheet has the same depth-integrated nonlinear viscosity coefficient everywhere. Our initial models have only one adjustable parameter, n, the power-law exponent, with n = 1, 3, 10, 30, and 100. The predicted width of the deforming zone decreases with increasing n, with n ≥ 30 explaining the observations. This n value is higher than has been estimated for continental lithosphere and suggests that more of the strength of oceanic lithosphere lies in layers deforming by faulting or by dislocation glide than for continental lithosphere. To obtain a stress field that better fits the distribution and type of earthquake focal mechanisms in the diffuse oceanic plate boundary, we add a second adjustable parameter, representing the effect of slab pull stretching the oceanic plate near the Sumatra Trench. We show that an average velocity increment on this boundary segment of 5 mm a-1 (relative to the average velocity of the India and Australia Plates) fits the observed distribution of fault types better than velocities of 3.3 mm a-1 or 10 mm a-1.

  14. Geochemical evidence for pre- and syn-rifting lithospheric foundering in the East African Rift System

    NASA Astrophysics Data System (ADS)

    Nelson, W. R.; Furman, T.; Elkins-Tanton, L. T.

    2015-12-01

    The East African Rift System (EARS) is the archetypal active continental rift. The rift branches cut through the elevated Ethiopian and Kenyan domes and are accompanied by a >40 Myr volcanic record. This record is often used to understand changing mantle dynamics, but this approach is complicated by the diversity of spatio-temporally constrained, geochemically unique volcanic provinces. Various sources have been invoked to explain the geochemical variability across the EARS (e.g. mantle plume(s), both enriched and depleted mantle, metasomatized or pyroxenitic lithosphere, continental crust). Mantle contributions are often assessed assuming adiabatic melting of mostly peridotitic material due to extension or an upwelling thermal plume. However, metasomatized lithospheric mantle does not behave like fertile or depleted peridotite mantle, so this model must be modified. Metasomatic lithologies (e.g. pyroxenite) are unstable compared to neighboring peridotite and can founder into the underlying asthenosphere via ductile dripping. As such a drip descends, the easily fusible metasomatized lithospheric mantle heats conductively and melts at increasing T and P; the subsequent volcanic products in turn record this drip magmatism. We re-evaluated existing data of major mafic volcanic episodes throughout the EARS to investigate potential evidence for lithospheric drip foundering that may be an essential part of the rifting process. The data demonstrate clearly that lithospheric drip melting played an important role in pre-flood basalt volcanism in Turkana (>35 Ma), high-Ti "mantle plume-derived" flood basalts and picrites (HT2) from NW Ethiopia (~30 Ma), Miocene shield volcanism on the E Ethiopian Plateau and in Turkana (22-26 Ma), and Quaternary volcanism in Virunga (Western Rift) and Chyulu Hills (Eastern Rift). In contrast, there is no evidence for drip melting in "lithosphere-derived" flood basalts (LT) from NW Ethiopia, Miocene volcanism in S Ethiopia, or Quaternary

  15. Arctic Crustal Thickness and Oceanic Lithosphere Distribution from Gravity Inversion: Constraining Plate Reconstructions

    NASA Astrophysics Data System (ADS)

    Kusznir, N. J.; Alvey, A.; Roberts, A. M.

    2013-12-01

    Mapping crustal thickness, continental lithosphere thinning and oceanic lithosphere distribution represents a substantial challenge for the Polar Regions. Using gravity anomaly inversion, we have produced the first comprehensive maps of crustal thickness and oceanic lithosphere distribution for the Arctic. The Arctic region formed as a series of small distinct ocean basins leading to a complex distribution of oceanic crust, thinned continental crust, possible micro-continents and rifted continental margins. Mapping of continental lithosphere thinning factor and crustal thickness from gravity inversion provide predictions of ocean-continent transition structure and magmatic type and continent ocean boundary location independent of magnetic isochrons. Restoration of crustal thickness and continent-ocean boundary location from gravity inversion may be used to test plate tectonic reconstructions. 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 and sea-floor spreading trajectory within the Arctic basins. By restoring crustal thickness & continental lithosphere thinning maps of the Eurasia Basin & NE Atlantic to their initial post-breakup configuration we show the geometry and segmentation of the rifted continental margins at their time of breakup, together with the location of highly-stretched failed breakup basins and rifted micro-continents. Our gravity inversion predicts thin crust and high continental lithosphere thinning factors in the Makarov, Podvodnikov, Nautilus and Canada Basins consistent with these basins being underlain by oceanic or highly thinned continental crust. Larger crustal thicknesses, in the range 20 - 30 km, are predicted for the Lomonosov, Alpha and Mendeleev Ridges. Moho depths predicted compare well with seismic estimates. Predicted very thin continental or oceanic crust under the North Chuchki

  16. Evidence for recycled Archaean oceanic mantle lithosphere in the Azores plume.

    PubMed

    Schaefer, Bruce F; Turner, Simon; Parkinson, Ian; Rogers, Nick; Hawkesworth, Chris

    2002-11-21

    The compositional differences between mid-ocean-ridge and ocean-island basalts place important constraints on the form of mantle convection. Also, it is thought that the scale and nature of heterogeneities within plumes and the degree to which heterogeneous material endures within the mantle might be reflected in spatial variations of basalt composition observed at the Earth's surface. Here we report osmium isotope data on lavas from a transect across the Azores archipelago which vary in a symmetrical pattern across what is thought to be a mantle plume. Many of the lavas from the centre of the plume have lower 187Os/188Os ratios than most ocean-island basalts and some extend to subchondritic 187Os/188Os ratios-lower than any yet reported from ocean-island basalts. These low ratios require derivation from a depleted, harzburgitic mantle, consistent with the low-iron signature of the Azores plume. Rhenium-depletion model ages extend to 2.5 Gyr, and we infer that the osmium isotope signature is unlikely to be derived from Iberian subcontinental lithospheric mantle. Instead, we interpret the osmium isotope signature as having a deep origin and infer that it may be recycled, Archaean oceanic mantle lithosphere that has delaminated from its overlying oceanic crust. If correct, our data provide evidence for deep mantle subduction and storage of oceanic mantle lithosphere during the Archaean era.

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

    NASA Technical Reports Server (NTRS)

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

    2014-01-01

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

  18. An integrated geophysical study of north African and Mediterranean lithospheric structure

    NASA Astrophysics Data System (ADS)

    Dial, Paul Joseph

    1998-07-01

    This dissertation utilizes gravity and seismic waveform modeling techniques to: (1) determine models of lithospheric structure across northern African through gravity modeling and (2) determine lithospheric and crustal structure and seismic wave propagation characteristics across northern Africa and the Mediterranean region. The purpose of the gravity investigation was to construct models of lithospheric structure across northern Africa through the analysis of gravity data constrained by previous geological and geophysical studies. Three lithospheric models were constructed from Bouguer gravity data using computer modeling, and the gravity data was wavelength-filtered to investigate the relative depth and extent of the structures associated with the major anomalies. In the Atlas Mountains area, the resulting earth models showed slightly greater crustal thickness than those of previous studies if a low density mantle region is not included in the models. However, if a low density mantle region (density = 3.25 g/cm3) was included beneath the Atlas, the earth models showed little crustal thickening (38 km), in accord with previous seismic studies. The second portion of the research consisted of seismic waveform modeling of regional and teleseismic events to determine crustal and lithospheric structure across northern Africa and the Mediterranean. A total of 174 seismograms (145 at regional distances (200--1400 km) and 29 with epicentral distances exceeding 1900 km) were modeled using 1-D velocity models and a reflectivity code. At regional distances from four stations surrounding the western Mediterranean basin (MAL, TOL, PTO and AQU) and one station near the Red Sea (HLW), 1-D velocity models can satisfactorily model the relative amplitudes of both the Pnl and surface wave portions of the seismograms. Modeling of propagation paths greater than 1900 km was also conducted across northern Africa and the Mediterranean. The results indicate that the S-wave velocity model

  19. Seismic evidence of a two-layer lithospheric deformation in the Indian Ocean

    NASA Astrophysics Data System (ADS)

    Qin, Yanfang; Singh, Satish C.

    2015-09-01

    Intra-plate deformation and associated earthquakes are enigmatic features on the Earth. The Wharton Basin in the Indian Ocean is one of the most active intra-plate deformation zones, confirmed by the occurrence of the 2012 great earthquakes (Mw>=8.2). These earthquakes seem to have ruptured the whole lithosphere, but how this deformation is distributed at depth remains unknown. Here we present seismic reflection images that show faults down to 45 km depth. The amplitude of these reflections in the mantle first decreases with depth down to 25 km and then remains constant down to 45 km. The number of faults imaged along the profile and the number of earthquakes as a function of depth show a similar pattern, suggesting that the lithospheric mantle deformation can be divided into two layers: a highly fractured fluid-filled serpentinized upper layer and a pristine brittle lithospheric mantle where great earthquakes initiate and large stress drops occur.

  20. Estimating the stresses within the lithosphere: parameter check with applications to the African Plate

    NASA Astrophysics Data System (ADS)

    Medvedev, Sergei; Werner, Stephanie; Steinberger, Bernhard; "African Plate" Working Group

    2010-05-01

    Several mechanisms control the state of stress within plates on Earth. The list is rather long, but well-known and includes ridge push, mantle drag, stresses invoked by lateral variations of lithospheric density structure and subduction processes. We attempt to quantify the influence of these mechanisms and to construct a reliable model to understand modern and palaeo-stresses using the African plate (TAP) as an example. Previous studies explained stress patterns and their evolution solely by assigning different rheological properties to sub-domains and their boundaries. Such an approach often leads to unrealistically high variations of properties within a modeled plate. In our approach we find the best possible agreement with observations before differentiating between sub-domains of TAP. The finite-element based suite ProShell was utilized to calculate stresses on the real geometry of TAP (non-planar). The approach allows us to combine several data sets and to estimate stresses caused by lateral and vertical distribution of properties within the lithosphere, to quantify the in-plane and bending stresses, to account for forces due to ridge push and mantle heterogeneities and mantle flow. The modeled results are tested and iterated to match the observed stress pattern and potential fields as good as possible. The starting model is based on the CRUST2 data set to construct the model crust and half-space cooling model to approximate properties of the lithospheric mantle. The results however, are not satisfactory, and might be related to oversimplifications in the uniform model of lithosphere or/and to the unrealistic representation of the CRUST2 model in certain areas of TAP. The latter was also shown by simple evaluation using gravity forward modeling of the model boundaries. The model implementation of the crustal structure calculated from simple gravity inversion or derived through isostatical considerations agree better to today's observed stress pattern.

  1. Depth distribution of seismic slip in oceanic lithosphere: Observations following the April 2012 great earthquakes

    NASA Astrophysics Data System (ADS)

    Aderhold, K.; Abercrombie, R. E.

    2012-12-01

    We determine centroid depths and slip distributions of global oceanic strike-slip earthquakes to investigate the distribution of seismic and aseismic slip in oceanic lithosphere of different ages. It is unknown if seismic rupture is predominantly controlled by thermal or compositional differences on faults primarily due to the difficulty in resolving depth in seismic observations (Boettcher and Jordan, 2004; Sykes and Ekström, 2011). Continental earthquakes have better depth resolution, but continental lithosphere is exceptionally heterogeneous. Oceanic lithosphere is simple at first-order with well-determined age, thermal and compositional structure, but oceanic earthquakes often have poorly constrained or fixed depths. We use teleseismic, broadband body wave modeling to determine centroid depths and slip distribution of strike-slip earthquakes with magnitude 6 ≤ MW ≤ 8 in oceanic lithosphere ranging in age from 10My to 130My. The improved depth distribution of slip can be compared directly to thermal and compositional differences in the simple structure of oceanic strike-slip faults. On April 11th, 2012, two of the largest strike-slip earthquakes ever recorded occurred within hours of one another in the Wharton Basin, displaying unusual behavior by rupturing along multiple orthogonally oriented faults. These great intraplate earthquakes have Global CMT centroid depths well below the 600°C isotherm, the commonly assumed lower limit of seismicity in oceanic lithosphere. We examine smaller events as depth is difficult to resolve when modeling such complex rupture. Our centroid depths agree with a 600°C isotherm from a half-space cooling model as the lower limit to the seismogenic zone with seismic slip distributed throughout. Our preferred depth of 26km, ±5km, for the 2005 MW 7.2 earthquake is deeper than the Global CMT depth of 12km while most of the events prefer to be shallower. Complex rupture patterns are not limited to MW 8 events with the 2010 MW 7

  2. Interplay of variable thermal conductivity and expansivity on the thermal structure of oceanic lithosphere II

    NASA Astrophysics Data System (ADS)

    Honda, S.; Yuen, D. A.

    2004-04-01

    We have extended our previous analysis of the effects of constant vs. variable, i.e., pressure and temperature dependent thermal conductivity (k) and constant thermal expansivity (a) on the thermal structure of the oceanic lithosphere. We apply our analysis to the actual data set including information on the geoid slope. The heat flow and ocean floor depth data constrain the thermal expansivity (a ≍ 3 × 10-5 1/°C). Including geoid slope data may loosely constrain both the thermal expansivity and the thermal conductivity. The probable value of thermal conductivity is ≍ 3 W/m/°C for the constant k case and ≍ 4 W/m/°C (at ambient conditions) for the variable k case. These a and k are generally consistent with laboratory data of appropriate lithospheric materials. Our analysis supports the plate model with thin lithosphere and high bottom temperature, such as GDH1 (95 km; 1450°C). Variable k case requires slightly thinner and higher temperature lithosphere (≍ 85 km and ≍ 1500°C) and gives a slightly better fit to the geoid slope data.

  3. Seismic Tomography of the Arctic: Continental Cratons, Ancient Orogens, Oceanic Lithosphere and Convecting Mantle Beneath (Invited)

    NASA Astrophysics Data System (ADS)

    Lebedev, S.; Schaeffer, A. J.

    2013-12-01

    Lateral variations in seismic velocities in the upper mantle, mapped by seismic tomography, reflect primarily the variations in the temperature of the rock at depth. Seismic tomography thus reveals lateral changes in the temperature and thickness of the lithosphere; it maps deep boundaries between tectonic blocks with different properties and with different age of the lithosphere. Our new global, shear-wave tomographic model of the upper mantle and the crust is constrained by an unprecedentedly large number of broadband waveform fits (nearly one million seismograms, with both surface and S waves included) and provides improved resolution of the lithosphere across the whole of the Arctic region, 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. In Eurasia, in contrast, cratonic lithosphere extends hundreds of kilometres north of the coast of the continent, beneath the Barents and eastern Kara Seas. The boundaries of the Archean cratons mapped by tomography indicate the likely offshore extensions of major Phanerozoic sutures in northern Eurasia. 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-spreading ridges globally. South of the ridge, the Nansen Basin shows higher seismic velocities in the upper mantle beneath it, compared to the Amundsen Basin. At 150-250 km depth, most of the oceanic portions of the central Arctic (the

  4. Seismic evidence for sharp lithosphere-asthenosphere boundaries of oceanic plates.

    PubMed

    Kawakatsu, Hitoshi; Kumar, Prakash; Takei, Yasuko; Shinohara, Masanao; Kanazawa, Toshihiko; Araki, Eiichiro; Suyehiro, Kiyoshi

    2009-04-24

    The mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics, and thus the understanding of the processes at the lithosphere-asthenosphere boundary (LAB) is fundamental to understand how our planet works. It is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the LAB. Data from borehole broadband ocean bottom seismometers show that the LAB beneath the Pacific and Philippine Sea plates is sharp and age-dependent. The observed large shear wave velocity reduction at the LAB requires a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in meltless mantle, which accounts for the large viscosity contrast at the LAB that facilitates horizontal plate motions.

  5. Mass transfer in the oceanic lithosphere: Serpentinization is not isochemical

    NASA Astrophysics Data System (ADS)

    Malvoisin, Benjamin

    2015-11-01

    Whereas the serpentinization reaction leads to stark differences in the physical properties of mantle rocks at mid-ocean ridges, the chemical changes associated with this reaction are thought to be restricted to the addition of water and the generation of hydrogen ("isochemical" reaction). Here, I compile a geochemical dataset of serpentinized peridotites at mid-ocean ridges evidencing that a decrease by up to 11% of the MgO/SiO2 ratio is associated with serpentinization. This MgO/SiO2 decrease is consistent with the calculated distribution of Mg in the minerals since, during isochemical serpentinization, ∼10% of the Mg should be contained in brucite, an Mg-hydroxide not commonly observed in serpentinized peridotites, which are typically composed of serpentine (Mg3Si2O5(OH)4) and magnetite (Fe3O4). This latter mineralogical assemblage and a decrease of the MgO/SiO2 ratio were only reproduced in numerical models of peridotite reacting with fluids containing aqueous silica at fluid to rock (F/R) ratios greater than 20. At higher F/R ratios, talc (Mg3Si4O10(OH)2) was found to be stable, in agreement with observations in extremely altered samples found at mid-ocean ridges. The potential sources for aqueous silica in the fluid are the alteration of mafic units intruding mantle rocks at slow-spreading ridges. The mineralogical and chemical changes associated with SiO2 gain during serpentinization at mid-ocean ridges will have consequences on abiotic hydrogen production, contribute to a volume increase of 50% and decrease water incorporation during serpentinization by more than 10% compared to "isochemical" serpentinization. These changes will also increase the depth at which fluids are released by dehydration reactions in subduction zones by more than 20 km.

  6. Highly extended oceanic lithosphere: The basement and wallrocks for the Late Jurassic Rogue-Chetco oceanic arc, Oregon Klamath Mountains

    SciTech Connect

    Yule, J.D.; Saleeby, J.B.

    1993-04-01

    The superbly preserved, coeval Late Jurassic Rogue-Chetco oceanic arc and Josephine inter-arc basin exposed in the western Jurassic belt of the Oregon Klamath Mountains provide a unique opportunity to (1) directly observe the oceanic lithosphere upon which this oceanic arc was constructed, and (2) gain a better understanding of the pre-accretionary dynamic processes that shape oceanic arc and inter-arc basin lithosphere. Field relations exposed in the Roque, Illinois, and Chetco River areas show that (1) plutonic and volcanic rocks of the Rogue-Chetco arc both intruded and conformably overlapped fragmented composite blocks of oceanic crust and serpentinized, dike-filled depleted mantle rocks; and (2) arc growth occurred during regional oblique extension of the oceanic lithosphere resulting in the extreme fragmentation of oceanic crustal rocks and the local exposure of serpentinized mantle rocks on the sea floor. The Rogue-Chetco overlap sequence consists of rhythmically bedded volcanogenic turbidites, chert, argillite, and local deposits of polymict basal breccias. The clasts which comprise the distinctive basal breccias indicate derivation from a dominantly ophiolitic crust and serpentinized mantle source. Source materials for the basal breccias comprise the basement and wallrocks for the Roque-Chetco arc and consist of (1) rifted fragments of western Paleozoic and Triassic belt rocks (Yule and others, 1991) cut by heterogeneous mafic complexes inferred to represent early Josephine age rifting at approximately 165 Ma, (2) fault bounded blocks of massive gabbro, sheeted mafic dikes, pillow lava and breccia overlain by Callovian age chert, and (3) serpentinized depleted mantle peridotite cut by multiple generation of mafic and intermediate dikes. The basement rock types all share a pervasive brittle fragmentation and hydrothermal alteration history that is conspicuously absent in the arc volcanic and plutonic rocks.

  7. Rapid emplacement of young oceanic lithosphere: argon geochronology of the oman ophiolite.

    PubMed

    Hacker, B R

    1994-09-09

    (40)Ar/(39)Ar dates of emplacement-related metamorphic rocks beneath the Samail ophiolite in Oman show that cooling to <525 degrees C occurred within approximately 1 million years of igneous crystallization of the ophiolite. This unexpectedly short time span and rapid cooling means that old, cold continental or oceanic lithosphere must have been adjacent to the ophiolite during spreading and then been thrust beneath the ophiolite almost immediately afterward.

  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. Evidence for metasomatic enrichment in the oceanic lithosphere and implication for the generation of intraplate basalts

    NASA Astrophysics Data System (ADS)

    Pilet, S.; Buchs, D.; Cosca, M. A.; Baumgartner, P.

    2011-12-01

    Petrological studies play a significant role in the debate regarding the origin of intraplate magmas by providing unequivocal constraints about the source(s) composition and melting processes related to basalt formation. Two major hypotheses are currently in debate: first, intraplate magmas are produced at depth (i.e. within the asthenosphere) by low-degrees melting of an enriched peridotitic source in the presence of CO2 [1]; second, alkaline magmas are produced by the melting of metasomatic hydrous veins present within the lithospheric mantle [2]. If the existence of metasomatic veins in the continental lithospheric mantle is well documented, their existence and the mechanism of their formation in an oceanic setting are still mostly unconstrained. Here we report new petrological data demonstrating that metasomatic veins can be produced within the oceanic lithosphere by percolation and differentiation of low-degree melts initially located in the low velocity zone [3]. The existence of metasomatic veins in the oceanic lithosphere is documented by cpx xenocrysts in accreted basaltic sills from northern Costa Rica. New field observations, 40Ar-39Ar radiometric dating, biostratigraphic ages and geochemical analyses indicate that the sills represent a possible, ancient analogue of petit-spot volcanoes produced off Japan by oceanic plate flexure [4]. The cpx xenocrysts are interpreted as a relic of metasomatic veins based on their composition, which is similar to that of cpx from metasomatic veins observed in mantle outcrops and xenoliths. The major and trace element contents of the studied cpx xenocrysts indicate that they crystallized at high pressure in a differentiated liquid. This liquid represents the last stage of a fractional crystallization process that produced early anhydrous cumulates followed by later hydrous cumulates, a mechanism similar to that proposed by Harte et al. [5] for the formation of metasomatic veins in the continental lithosphere. Monte Carlo

  10. Seismic structure of the lithosphere and upper mantle beneath the ocean islands near mid-oceanic ridges

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    Deciphering the seismic character of the young lithosphere near mid-oceanic ridges (MORs) 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 quality data recorded at five broadband seismological stations situated on ocean islands in their vicinity. Estimates of the crustal and lithospheric thickness values from waveform inversion of the P-receiver function stacks at individual stations reveal that the Moho depth varies between ~ 10 ± 1 km and ~ 20 ± 1 km with the depths of the lithosphere-asthenosphere boundary (LAB) varying between ~ 40 ± 4 and ~ 65 ± 7 km. We found evidence for an additional low-velocity layer below the expected LAB depths at stations on Ascension, São Jorge and Easter islands. The layer probably relates to the presence of a hot spot corresponding to a magma chamber. Further, thinning of the upper mantle transition zone suggests a hotter mantle transition zone due to the possible presence of plumes in the mantle beneath the stations.

  11. Comparison of plate and asthenospheric flow models for the thermal evolution of oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Stein, Carol A.; Stein, Seth

    1994-04-01

    Although seafloor depth and heat flow for young oceanic lithosphere can be descibed by modeling the lithosphere as the boundary layer of a cooling halfspace, a long standing question has been why data at older ages deviate from those expected for a halfspace. Two classes of models have been proposed for these deviations. In one, heat added from below 'flattens' depth and heat flow. In the other, asthenospheric flow beneath the lithosphere perturbs the depths. We compare recent versions of the model classes: the GDH1 thin-lithosphere plate model (Stein and Stein, 1992) and an asthenospehric flow model (Phipps Morgan and Smith, 1992). The plate model fits heat flow data better than the flow model for all cases considered, and topographic data in all but one case. The flow model significantly overpredicts depths for the North Atlantic, because the assumed asthenospheric flow in the plate motion direction would yield deepening for old ages rather than the observed flattening. Overall, the GDH1 global average model does better than this flow model, whose parameters were fit to specific plates. Moreover, the plate models fit to specific plates do better than the flow model. Plate models thus appear more useful than this flow model, suggesting that deviations from a cooling halfspace are largely thermal in origin.

  12. Comparison of plate and asthenospheric flow models for the thermal evolution of oceanic lithosphere

    NASA Technical Reports Server (NTRS)

    Stein, Carol A.; Stein, Seth

    1994-01-01

    Although seafloor depth and heat flow for young oceanic lithosphere can be descibed by modeling the lithosphere as the boundary layer of a cooling halfspace, a long standing question has been why data at older ages deviate from those expected for a halfspace. Two classes of models have been proposed for these deviations. In one, heat added from below 'flattens' depth and heat flow. In the other, asthenospheric flow beneath the lithosphere perturbs the depths. We compare recent versions of the model classes: the GDH1 thin-lithosphere plate model (Stein and Stein, 1992) and an asthenospehric flow model (Phipps Morgan and Smith, 1992). The plate model fits heat flow data better than the flow model for all cases considered, and topographic data in all but one case. The flow model significantly overpredicts depths for the North Atlantic, because the assumed asthenospheric flow in the plate motion direction would yield deepening for old ages rather than the observed flattening. Overall, the GDH1 global average model does better than this flow model, whose parameters were fit to specific plates. Moreover, the plate models fit to specific plates do better than the flow model. Plate models thus appear more useful than this flow model, suggesting that deviations from a cooling halfspace are largely thermal in origin.

  13. Water in Hawaiian peridotite minerals: A case for a dry metasomatized oceanic mantle lithosphere

    NASA Astrophysics Data System (ADS)

    Peslier, Anne H.; Bizimis, Michael

    2015-04-01

    The distribution of water concentrations in the oceanic upper mantle has drastic influence on its melting, rheology, and electrical and thermal conductivities and yet is primarily known indirectly from analyses of OIB and MORB. Here, actual mantle samples, eight peridotite xenoliths from Salt Lake Crater (SLC) and one from Pali in Oahu in Hawaii were analyzed by FTIR. Water contents of orthopyroxene, clinopyroxene, and the highest measured in olivine are 116-222, 246-442, and 10-26 ppm weight H2O, respectively. Although pyroxene water contents correlate with indices of partial melting, they are too high to be explained by simple melting modeling. Mantle-melt interaction modeling reproduces best the SLC data. These peridotites represent depleted oceanic mantle older than the Pacific lithosphere that has been refertilized by nephelinite melts containing <5 weight % H2O. Metasomatism in the Hawaiian peridotites resulted in an apparent decoupling of water and LREE that can be reconciled via assimilation and fractional crystallization. Calculated bulk-rock water contents for SLC (50-96 ppm H2O) are on the low side of that of the MORB source (50-200 ppm H2O). Preceding metasomatism, the SLC peridotites must have been even drier, with a water content similar to that of the Pali peridotite (45 ppm H2O), a relatively unmetasomatized fragment of the Pacific lithosphere. Moreover, our data show that the oceanic mantle lithosphere above plumes is not necessarily enriched in water. Calculated viscosities using olivine water contents allow to estimate the depth of the lithosphere-asthenosphere boundary beneath Hawaii at ˜90 km.

  14. Global variations in gravity-derived oceanic crustal thickness: Implications on oceanic crustal accretion and hotspot-lithosphere interactions

    NASA Astrophysics Data System (ADS)

    Lin, J.; Zhu, J.

    2012-12-01

    We present a new global model of oceanic crustal thickness based on inversion of global oceanic gravity anomaly with constrains from seismic crustal thickness profiles. We first removed from the observed marine free-air gravity anomaly all gravitational effects that can be estimated and removed using independent constraints, including the effects of seafloor topography, marine sediment thickness, and the age-dependent thermal structure of the oceanic lithosphere. We then calculated models of gravity-derived crustal thickness through inversion of the residual mantle Bouguer anomaly using best-fitting gravity-modeling parameters obtained from comparison with seismically determined crustal thickness profiles. Modeling results show that about 5% of the global crustal volume (or 9% of the global oceanic surface area) is associated with model crustal thickness <5.2 km (designated as "thin" crust), while 56% of the crustal volume (or 65% of the surface area) is associated with crustal thickness of 5.2-8.6 km thick (designated as "normal" crust). The remaining 39% of the crustal volume (or 26% of the surface area) is associated with crustal thickness >8.6 km and is interpreted to have been affected by excess magmatism. The percentage of oceanic crustal volume that is associated with thick crustal thickness (>8.6 km) varies greatly among tectonic plates: Pacific (33%), Africa (50%), Antarctic (33%), Australia (30%), South America (34%), Nazca (23%), North America (47%), India (74%), Eurasia (68%), Cocos (20%), Philippine (26%), Scotia (41%), Caribbean (89%), Arabian (82%), and Juan de Fuca (21%). We also found that distribution of thickened oceanic crust (>8.6 km) seems to depend on spreading rate and lithospheric age: (1) On ocean basins younger than 5 Ma, regions of thickened crust are predominantly associated with slow and ultraslow spreading ridges. The relatively strong lithospheric plate at slow and ultraslow ridges might facilitate the loading of large magmatic

  15. New insights on the oceanic lithosphere at La Reunion hotspot volcano

    NASA Astrophysics Data System (ADS)

    Deplus, C.; de Voogd, B.; Dyment, J.; Bissessur, D.; Sisavath, E.; Depuiset, F.; Mercier, M.

    2009-04-01

    It is now clear that the structure and the mechanical properties of the lithosphere have to be taken into account to understand how mantle plumes are expressed by surface volcanism. La Reunion, a large volcanic system in the Indian Ocean, is widely considered as the most recent expression of a mantle plume. Previous studies have suggested that it could have developed on pre-existing structures of the oceanic lithosphere such as a fossil spreading centre or a fracture zone. Cruise FOREVER (FORmation and Evolution of the Volcanic Edifice of Reunion) of french R/V L Atalante has surveyed the oceanic plate around La Reunion Island in 2006 in order to investigate possible relationships between the structures of the plate and the emplacement of surface volcanism. The cruise collected swath bathymetry and back-scatter data, as well as magnetic, gravity, 3.5 kHz echosounder and 24-channel seismic reflection profiles. The coverage extends up to 250 km around the island. The new data confirm that the formation and evolution of the oceanic plate in La Reunion area is more complex than in the adjacent compartments. Oceanic magnetic lineations display various directions (discussed in Bissessur et al., this meeting) and do not support a fossil axis located close to the volcano. In addition to La Reunion large volcanic edifice, the high resolution bathymetry coverage reveals numerous volcanic structures on the surrounding oceanic plate: a series of elongated ridges regularly-spaced, several elongated volcanic structures and large isolated seamounts. The seismic data, complemented by older multi-channel seismic data (cruise REUSIS, 1993), allow investigations of the oceanic plate topography beneath the sedimentary cover. Results indicate that a fossil spreading axis is unlikely to underlie La Reunion, in agreement with magnetic data interpretation, but reveal, beneath the volcano, an EW alignment of topographic highs and a N35° E topographic structure, possibly a short fracture

  16. On the composition of ocean island basalts (OIB): The effects of lithospheric thickness variation and mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Humphreys, Emma R.; Niu, Yaoling

    2009-09-01

    We have examined island-averaged geochemical data for 115 volcanic islands with known eruption ages and ages of the underlain lithosphere from the Pacific, Atlantic and Indian Oceans. These age data allow calculation of the lithosphere thickness at the time of volcanism. After correcting the basalts (including alkalic types) (< 53% SiO2) for fractionation effect to Mg# = 0.72, we found that the island-averaged Si72 and Al72 decrease whereas Fe72, Mg72, Ti72 and P72 increase with increasing lithosphere thickness. The island-averaged [La/Sm]CN and [Sm/Yb]CN ratios also increase with increasing lithosphere thickness. These statistically significant trends are most consistent with the interpretation that the mean extent of melting decreases whereas the mean pressure of melting increases with increasing lithosphere thickness. This is physically consistent with the active role the lithosphere plays in limiting the final depth of intra-oceanic mantle melting. That is, beneath a thin lithosphere, a parcel of mantle rises to a shallow level, and thus melts more by decompression with the aggregated melt having the property of high extent and low pressure of melting. By contrast, a parcel of mantle beneath a thick lithosphere has restricted amount of upwelling, and thus melts less by decompression with the aggregated melt having the property of low extent and high pressure of melting. This demonstrates that oceanic lithosphere thickness variation exerts the first-order control on the geochemistry of ocean island basalts (OIB). Variation in initial depth of melting as a result of fertile mantle compositional variation and mantle potential temperature variation can influence OIB compositions, but these two variables must have secondary effects because they do not overshadow the effect of lithosphere thickness variation that is prominent on a global scale. The mantle potential temperature variation beneath ocean islands cannot be constrained with the existing data. Fertile

  17. Recycling of Oceanic Lithosphere: Water, fO2 and Fe-isotope Constraints

    NASA Technical Reports Server (NTRS)

    Bizmis, M.; Peslier, A. H.; McCammon, C. A.; Keshav, S.; Williams, H. M.

    2014-01-01

    Spinel peridotite and garnet pyroxenite xenoliths from Hawaii provide important clues about the composition of the oceanic lithosphere, and can be used to assess its contribution to mantle heterogeneity upon recycling. The peridotites have lower bulk H2O (approximately 70-114 ppm) than the MORB source, qualitatively consistent with melt depletion. The garnet pyroxenites (high pressure cumulates) have higher H2O (200-460 ppm, up to 550 ppm accounting for phlogopite) and low H2O/Ce ratios (less than 100). The peridotites have relatively light Fe-isotopes (delta Fe -57 = -0.34 to 0.13) that decrease with increasing depletion, while the pyroxenites are significantly heavier (delta Fe-57 up to 0.3). The observed xenolith, as well as MORB and OIB total Fe-isotope variability is larger that can be explained by existing melting models. The high H2O and low H2O/Ce ratios of pyroxenites are similar to estimates of EM-type OIB sources, while their heavy delta Fe-57 are similar to some Society and Cook-Austral basalts. Therefore, recycling of mineralogically enriched oceanic lithosphere (i.e. pyroxenites) may contribute to OIB sources and mantle heterogeneity. The Fe(3+)/Sigma? systematics of these xenoliths also suggest that there might be lateral redox gradients within the lithosphere, between juxtaposed oxidized spinel peridotites (deltaFMQ = -0.7 to 1.6, at 15 kb) and more reduced pyroxenites (deltaFMQ = -2 to -0.4, at 20-25kb). Such mineralogically and compositionally imposed fO2 gradients may generate local redox melting due to changes in fluid speciation (e.g. reduced fluids from pyroxenite encountering more oxidized peridotite). Formation of such incipient, small degree melts could further contribute to metasomatic features seen in peridotites, mantle heterogeneity, as well as the low velocity and high electrical conductivity structures near the base of the lithosphere and upper mantle.

  18. 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

  19. Breaking the oceanic lithosphere of a subducting slab: the 2013 Khash, Iran earthquake

    USGS Publications Warehouse

    Barnhart, William D.; Hayes, Gavin P.; Samsonov, S.; Fielding, E.; Seidman, L.

    2014-01-01

    [1] Large intermediate depth, intraslab normal faulting earthquakes are a common, dangerous, but poorly understood phenomenon in subduction zones owing to a paucity of near field geophysical observations. Seismological and high quality geodetic observations of the 2013 Mw7.7 Khash, Iran earthquake reveal that at least half of the oceanic lithosphere, including the mantle and entire crust, ruptured in a single earthquake, confirming with unprecedented resolution that large earthquakes can nucleate in and rupture through the oceanic mantle. A rupture width of at least 55 km is required to explain both InSAR observations and teleseismic waveforms, with the majority of slip occurring in the oceanic mantle. Combining our well-constrained earthquake slip distributions with the causative fault orientation and geometry of the local subduction zone, we hypothesize that the Khash earthquake likely occurred as the combined result of slab bending forces and dehydration of hydrous minerals along a preexisting fault formed prior to subduction.

  20. Geodynamic and Seismic Constraints on the Evolution of the Oceanic Lithosphere and Asthenosphere

    NASA Astrophysics Data System (ADS)

    Fahy, E. H.; Hall, P. S.; Dalton, C. A.; Faul, U.

    2011-12-01

    We report on a series of numerical geodynamic experiments undertaken to investigate the evolution the oceanic lithosphere and the characteristics of the underlying asthenosphere. In particular, we used the CitcomCU finite element package to model mantle flow beneath an oceanic plate. Experiments incorporated deformation by both diffusion creep and dislocation creep mechanisms, with experimentally constrained constants used for the relevant flow laws. We find that the use of flow laws appropriate for wet olivine aggregates leads to the formation of instabilities at the base of the thermal boundary layer corresponding to the lithosphere, which are not found in the experiments employing flow laws for dry olivine. These instabilities effectively thin the older portions of the thermal boundary layer, resulting in an average temperature structure closely resembling the GDH1 plate model [Stein and Stein, 1992] within the model domain. In contrast, the thermal structure of experiments in which instabilities do not form resembles resembles that of a half-space cooling model. Comparison of experimental results to seismic models of variations in shear wave velocity and shear attenuation with both depth and age within the oceanic upper mantle indicates that experiments in which instabilities occur provide a better match to seismic observations than do experiments without such instabilities.

  1. Seismic Reflection Imaging of the Lithosphere-asthenosphere Boundary Across the Atlantic Ocean

    NASA Astrophysics Data System (ADS)

    Singh, S. C.; Marjanovic, M.; Audhkhasi, P.; Mehouachi, F.

    2015-12-01

    Until now, the nature of the lithosphere-asthenosphere boundary (LAB) has been constrained by teleseismic data, which has resolution of tens of kilometres and sample the LAB sparsely. Seismic reflection imaging technique, in contrast, can provide both lateral and vertical resolution of a few hundred meters, but has not been used for imaging deep structures, thus so far. In March-April 2015, we acquired over 2,750 km of ultra-deep seismic reflection data in the Atlantic Ocean. To image LAB variations as a function of age one of our profiles extends continuously starting from 75 Ma old oceanic lithosphere off the margin of Africa, crosses the Mid-Atlantic Ridge at zero age, to up to 25 Ma old South America lithosphere. To image large differences in the LAB depth we also cross three major fracture zones in the equatorial Atlantic. For imaging deep structures, we used a very large energy source, 10,170 cubic inches, rich in low frequencies and a 12 km long multi-component streamer allowing to record low frequency energy reflected from deep earth and remove reverberation in the water column. Initial results show reflected seismic energy from 50-60 km depth. The seismic reflection experiment will be complemented by seismic refraction study to determine the crustal and upper mantle P-wave velocity, magnetotelluric study to determine resistivity, and broadband ocean bottom seismometer experiment for teleseismic study, collocated with our seismic reflection profiles. In this paper, we will present the design of the seismic reflection experiment and preliminary results from the onboard processed data.

  2. Softening of the subcontinental lithospheric mantle by asthenosphere melts and the continental extension/oceanic spreading transition

    NASA Astrophysics Data System (ADS)

    Ranalli, G.; Piccardo, G. B.; Corona-Chávez, P.

    2007-05-01

    The majority of ophiolitic peridotites in the Alpine-Apennine system show evidence of extensive interaction between subcontinental lithospheric mantle and fractional melts of asthenospheric origin. This interaction led to petrological, structural, and geochemical changes in the lithospheric mantle, and was accompanied by a temperature increase to near-asthenospheric values, resulting in the thermomechanical erosion of the lithosphere. We term the parts of mantle lithosphere thus affected the asthenospherized lithospheric mantle or ALM. The thermal and rheological consequences of thermomechanical erosion are explored by modelling the temperature and rheological properties of the thinned lithosphere as a function of thickness of ALM and time since asthenospherization (i.e., since the beginning of thermal relaxation). Results are given both in terms of rheological profiles (strength envelopes) and total lithospheric strength (TLS) for different lower crustal rheologies. The TLS decreases as a consequence of thermomechanical erosion. This decrease is a non-linear function of the thickness of the ALM. While practically negligible if less than 50% of lithospheric mantle is affected, it becomes significant (up to almost one order of magnitude) if thermomechanical erosion approaches the Moho. The maximum decrease in TLS is achieved within a short time span (˜1-2 Ma) after the end of the heating episode. As a working hypothesis, we propose that thermomechanical erosion of the lithospheric mantle, related to lithosphere/asthenospheric melts interaction, can be an important factor in a geologically rapid decrease in TLS. This softening could lead to whole lithospheric failure and consequently to a transition from continental extension to oceanic spreading.

  3. Origin and Distribution of Water Contents in Continental and Oceanic Lithospheric Mantle

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.

    2013-01-01

    The water content distribution of the upper mantle will be reviewed as based on the peridotite record. The amount of water in cratonic xenoliths appears controlled by metasomatism while that of the oceanic mantle retains in part the signature of melting events. In both cases, the water distribution is heterogeneous both with depth and laterally, depending on localized water re-enrichments next to melt/fluid channels. The consequence of the water distribution on the rheology of the upper mantle and the location of the lithosphere-asthenosphere boundary will also be discussed.

  4. Seismic evidence of a two-layer lithospheric deformation in the Indian Ocean.

    PubMed

    Qin, Yanfang; Singh, Satish C

    2015-09-14

    Intra-plate deformation and associated earthquakes are enigmatic features on the Earth. The Wharton Basin in the Indian Ocean is one of the most active intra-plate deformation zones, confirmed by the occurrence of the 2012 great earthquakes (Mw≥8.2). These earthquakes seem to have ruptured the whole lithosphere, but how this deformation is distributed at depth remains unknown. Here we present seismic reflection images that show faults down to 45 km depth. The amplitude of these reflections in the mantle first decreases with depth down to 25 km and then remains constant down to 45 km. The number of faults imaged along the profile and the number of earthquakes as a function of depth show a similar pattern, suggesting that the lithospheric mantle deformation can be divided into two layers: a highly fractured fluid-filled serpentinized upper layer and a pristine brittle lithospheric mantle where great earthquakes initiate and large stress drops occur.

  5. Flexure and gravity anomalies of the oceanic lithosphere beneath the Louisville seamount

    NASA Astrophysics Data System (ADS)

    Hwang, Gyuha; Kim, Seung-Sep

    2016-08-01

    We have calculated the elastic thickness (Te), flexural deflection, and gravity anomaly of the oceanic crust beneath the Louisville seamount (LSC-03), near the Kermadec trench. A regional-residual separation of the bathymetry was performed to remove the effect of other geologic features (e.g., the trench). We used the uniform density and dense core models to approximate the total mass of the seamount, which was defined as the surface load required for flexural deformation. From the flexure modeling results, we found that more flexural depression was predicted by the uniform density model than by the dense core model. However, the uniform density model predicted a significantly smaller gravity anomaly than observed, whereas the dense core model minimized the prediction misfits reasonably. The best flexure model was found with a Te of 16 km for the uniform density model and 6 km for the dense core model. The flexure computed with the dense core model was consistent with the seismically detected Moho. The flexure modeling for LSC-03, thus, indicates that the dense core model better approximates the inner structure of the LSC-03. Based on the crustal age and geochronology of the given seamount, the age of the oceanic crust at the time of seamount formation (Δt) is 20 Ma. If this is the case, however, the Te estimates from both flexure models require some degree of lithospheric reheating by Louisville hotspot activity. Alternatively, considering the tectonic plate motion of the Osbourn Trough, Δt becomes approximately 4 Ma. This younger lithosphere model is more consistent with the observed flexural deformation and the Te estimate from the dense core model. Therefore, the time that the seamount-induced lithospheric deformation occurred may be far earlier than the age-dated volcanism.

  6. Evolution of the oceanic lithosphere inferred from Po/So waves traveling in the Philippine Sea Plate

    NASA Astrophysics Data System (ADS)

    Shito, Azusa; Suetsugu, Daisuke; Furumura, Takashi

    2015-07-01

    Po/So waves are characterized by their high-frequency content and long-duration travel over great distances (up to 3000km) through the oceanic lithosphere. Po/So waves are developed by the multiple forward scattering of P and S waves due to small-scale stochastic random heterogeneities. To study the nature of these heterogeneities, Po/So waves are analyzed in the Philippine Sea Plate, which consists of three regions with different lithospheric ages. In the Philippine Sea Plate, Po/So waves propagate in the youngest region (15 Ma) and propagate more effectively in older regions. We investigate the mechanism of this propagation efficiency using numerical finite difference method simulations of 2-D seismic wave propagation. The results of this study demonstrate that the increase in propagation efficiency of Po/So waves depends on the age of the oceanic lithosphere, and this relationship can be qualitatively explained by thickening of the oceanic lithosphere including small-scale heterogeneities and a reduction in the intrinsic attenuation. These small-scale heterogeneities may form continuously in oceanic lithosphere from the time of its formation at a spreading ridge, via the solidification of melts distributed in the asthenosphere.

  7. The Oceanic Lithosphere as Reactive Filter: Implications for MORB and Abyssal Peridotite Compositions

    NASA Astrophysics Data System (ADS)

    Luffi, P. I.; Lee, C.; Antoshechkina, P. M.

    2010-12-01

    Melt-rock reaction in the lithosphere is, as suggested by textural observations and compositional data, a ubiquitous phenomenon capable of generating locally diverse peridotite series, such as those observed at oceanic spreading centers and transform faults, and may represent an important mechanism of creating compositional diversity in MORBs [1]. Whereas our understanding of the principles governing reactive melt transport is supported by basic theories and models, studies that attempt to quantify the physical conditions and mechanisms creating heterogeneities in the oceanic lithosphere are still limited in number [e.g. 2]. Using Adiabat_1ph 3.0 [3] in combination with the pMELTS algorithm [4], we have previously shown that reactive percolation of basaltic melts through depleted harzburgites can generate the dunite-(wehrlite)-harzburgite-lherzolite spectrum observed in the abyssal mantle and ophiolites, and that the amplitude of transformations is a function of thermal boundary layer thickness and amount of available melt [5]. To gain further insight into how melt-rock reactions shape the oceanic lithosphere, here we extend our study to show that the major and trace element variability in the oceanic mantle and rising melts are also significantly influenced by the mechanism of melt transport. If associated with cooling, distributed porous melt percolation (simulated by incremental addition of the same amount of melt) more efficiently converts harzburgites into fertile lherzolites and creates more pronounced compositional gradients in the abyssal mantle than imparted during channelized melt influx (simulated as batch addition of large amounts of melt) under otherwise identical circumstances. To remain within the tholeiitic trend observed in MORB, reacted melts must be released before clinopyroxene precipitation peaks. Further reaction with harzburgite causes liquids to evolve toward boninite-like compositions. As reaction progresses with decreasing temperature, the

  8. Mass independently fractionated sulfur isotopes reveal recycling of Archean lithosphere in modern oceanic hotspot lavas

    NASA Astrophysics Data System (ADS)

    Jackson, Matthew; Cabral, Rita; Rose-Koga, Estelle; Koga, Ken; Whitehouse, Martin; Antonelli, Michael; Farquhar, James; Day, James; Hauri, Erik

    2013-04-01

    Oceanic crust and sediments are introduced to the mantle at subduction zones, but the fate of this subducted material within the mantle, as well as the antiquity of this process, is unknown. The mantle is compositionally and isotopically heterogeneous, and it is thought that much of this heterogeneity derives from incorporation of diverse subducted components—both crustal and oceanic lithosphere—over geologic time. Basaltic lavas erupted at some oceanic hotspot volcanoes have long been considered to be melts of ancient subducted lithosphere. However, compelling evidence for the return of subducted materials in mantle plumes is lacking. We report mass independently fractionated (MIF) S-isotope signatures in olivine-hosted sulfides from 20-million-year-old ocean island basalts (OIBs) from Mangaia, Cook Islands (Polynesia). Terrestrial MIF S-isotope signatures were generated exclusively through atmospheric photochemical reactions until ~2.45 billion years ago. Therefore, the discovery of MIF-S in young OIBs indicates that sulfur—likely derived from hydrothermally-altered oceanic crust—was subducted into the mantle before 2.45 Ga and recycled into the mantle source of Mangaia lavas. These new data provide evidence for ancient materials, with MIF 33S depletions, in the mantle source for Mangaia lavas. An Archean age for recycled oceanic crust provides key constraints on the length of time that subducted crustal material can survive in the mantle and on the timescales of mantle convection from subduction to melting and eruption at plume-fed hotspots. The new S-isotope measurements confirm inferences about the cycling of sulfur between the major reservoirs from the Archean to the Phanerozoic, extending from the atmosphere and oceans to the crust and mantle, and ultimately through a return cycle to the surface that, here, is completed in Mangaia lavas. It remains to be seen whether hotspots lavas sampling different compositional mantle endmembers (e.g., EM1, EM2

  9. 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.

  10. Interplay of variable thermal conductivity and expansivity on the thermal structure of oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Honda, S.; Yuen, D. A.

    The sensitivity of pressure- and temperature-dependent thermal conductivity (k: W/m/K) and the thermal expansivity (α:1/K) on the thermal structure of the oceanic plate is investigated parametrically by comparing the ocean floor depth and heat flux calculated by one-dimensional conduction model with those of GDH1, a theoretical thermal model of the oceanic lithosphere. We find that an optimum fit is obtained, when the value of thermal expansivity is ˜ 3 × 10-5, while those associated with the thermal conductivity have many possibilities. The estimates, which give an equally good fit to the GDH1 model, of the plate thickness D (km) and the temperature at the base of the plate Tm (°C) may be given by Tm ˜ 1450-(k0-4.5) × 100-(α-3.0 × 10-5) × 105×100, D ˜ 90 + (k0-4.5) × 20 - (α-3.0 × 10-5) × 105 × 20 where k0 (W/m/K) is the lattice thermal conductivity at the ocean floor. A similar relation is obtained for constant thermal conductivity.

  11. Water in the Oceanic Lithosphere: Salt Lake Crater Xenoliths, Oahu, Hawaii

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.; Bizimis, Michael

    2010-01-01

    Water can be present in nominally anhydrous minerals of peridotites in the form of hydrogen bonded to structural oxygen. Such water in the oceanic upper mantle could have a significant effect on its physical and chemical properties. However, the water content of the MORB source has been inferred indirectly from the compositions of basalts. Direct determinations on abyssal peridotites are scarce because they have been heavily hydrothermally altered. Here we present the first water analyses of minerals from spinel peridotite xenoliths of Salt Lake Crater, Oahu, Hawaii, which are exceptionally fresh. These peridotites are thought to represent fragments of the Pacific oceanic lithosphere that was refertilized by alkalic Hawaiian melts. A few have unradiogenic Os and radiogenic Hf isotopes and may be fragments of an ancient (2 Ga) depleted and recycled lithosphere. Water contents in olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) were determined by FTIR spectrometry. Preliminary H_{2}O contents show ranges of 8-10 ppm for Ol, 151-277 ppm for Opx, and 337-603 ppm for Cpx. Reconstructed bulk rock H_{2}O contents range from 88-131 ppm overlapping estimates for the MORB source. Water contents between Ol minerals of the same xenolith are heterogeneous and individual OH infrared bands vary within a mineral with lower 3230 cm^{-1} and higher 3650-3400 cm^{-1} band heights from core to edge. This observation suggests disturbance of the hydrogen in Ol likely occurring during xenolith entrainment to the surface. Pyroxene water contents are higher than most water contents in pyroxenes from continental peridotite xenoliths and higher than those of abyssal peridotites. Cpx water contents decrease with increasing degree of depletion (e.g. increasing Fo in Ol and Cr# in spinel) consistent with an incompatible behavior of water. However Cpx water contents also show a positive correlation with LREE/HREE ratios and LREE concentrations consistent with refertilization. Opx water

  12. Experimental and modeling constraints on the seismic structure of oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Faul, Ulrich; Jackson, Ian; Hall, Paul

    2014-05-01

    Seismological techniques provide different views of the structure of the oceanic upper mantle. Surface waves can not resolve sharp changes in velocities with depth, but provide absolute velocities. Receiver functions are sensitive to sharp changes, but these steps in velocities have to be reconciled with absolute velocities. Experimental measurements of seismic properties can be used to translate velocities into a physical state and to explore possible mechanisms for sharp changes. Current experiments at elevated pressures and temperatures are designed to resolve the onset of the deviation from elastic behavior at temperatures above 600ºC. Initial experiments with copper jackets, which avoid the interference from phase transitions in mild steel, confirm the existence of a plateau in dissipation in olivine that is due to the grain-scale process of elastically accommodated grain boundary sliding. As this mechanism is solely due to temperature it occurs everywhere in the thermal boundary layer, although the sharpness will depend on the temperature gradient. Forward modeling of seismic velocities with the experimentally determined properties predict velocity gradients in the thermal boundary layer that are not sharp enough to generate receiver functions. However, particularly for younger oceanic lithosphere these gradients only need to be enhanced by a relatively small amount by other factors. Possible mechanisms include the presence of melt and/or volatiles. For older oceanic lithosphere small scale convection may steepen the temperature gradient beneath the lid. We are currently conducting numerical models with experimentally derived rheological parameters with the aim to determine the thermal structure and calculate seismic velocities from it. If operative, small scale convection may also lead to cooling of the whole upper mantle as a function of age. In the upper part of the lithosphere seismologic observations imply a constant, high velocity lid. This contrasts

  13. Water Content of the Oceanic Lithosphere at Hawaii from FTIR Analysis of Peridotite Xenoliths

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.; Bizmis, Michael

    2013-01-01

    Although water in the mantle is mostly present as trace H dissolved in minerals, it has a large influence on its melting and rheological properties. The water content of the mantle lithosphere beneath continents is better constrained by abundant mantle xenolith data than beneath oceans where it is mainly inferred from MORB glass analysis. Using Fourier transform infrared (FTIR) spectrometry, we determined the water content of olivine (Ol), clinopyroxene (Cpx) and orthopyroxene (Opx) in spinel peridotite xenoliths from Salt Lake Crater, Oahu, Hawaii, which are thought to represent fragments of the Pacific oceanic lithosphere that was refertilized by alkalic Hawaiian melts. Only Ol exhibits H diffusion profiles, evidence of limited H loss during xenolith transport to the surface. Water concentrations (Ol: 9-28 ppm H2O, Cpx: 246-566 ppm H2O, Opx: 116-224 ppm H2O) are within the range of those from continental settings but higher than those from Gakkel ridge abyssal peridotites. The Opx H2O contents are similar to those of abyssal peridotites from Atlantic ridge Leg 153 (170-230 ppm) but higher than those from Leg 209 (10- 14 ppm). The calculated bulk peridotite water contents (94 to 144 ppm H2O) are in agreement with MORB mantle source water estimates and lower than estimates for the source of Hawaiian rejuvenated volcanism (approx 540 ppm H2O) . The water content of Cpx and most Opx correlates negatively with spinel Cr#, and positively with pyroxene Al and HREE contents. This is qualitatively consistent with the partitioning of H into the melt during partial melting, but the water contents are too high for the degree of melting these peridotites experienced. Melts in equilibrium with xenolith minerals have H2O/Ce ratios similar to those of OIB

  14. Central role of detachment faults in accretion of slow-spreading oceanic lithosphere.

    PubMed

    Escartín, J; Smith, D K; Cann, J; Schouten, H; Langmuir, C H; Escrig, S

    2008-10-09

    The formation of oceanic detachment faults is well established from inactive, corrugated fault planes exposed on sea floor formed along ridges spreading at less than 80 km Myr(-1) (refs 1-4). These faults can accommodate extension for up to 1-3 Myr (ref. 5), and are associated with one of the two contrasting modes of accretion operating along the northern Mid-Atlantic Ridge. The first mode is asymmetrical accretion involving an active detachment fault along one ridge flank. The second mode is the well-known symmetrical accretion, dominated by magmatic processes with subsidiary high-angle faulting and the formation of abyssal hills on both flanks. Here we present an examination of approximately 2,500 km of the Mid-Atlantic Ridge between 12.5 and 35 degrees N, which reveals asymmetrical accretion along almost half of the ridge. Hydrothermal activity identified so far in the study region is closely associated with asymmetrical accretion, which also shows high levels of near-continuous hydroacoustically and teleseismically recorded seismicity. Increased seismicity is probably generated along detachment faults that accommodate a sizeable proportion of the total plate separation. In contrast, symmetrical segments have lower levels of seismicity, which occurs primarily at segment ends. Basalts erupted along asymmetrical segments have compositions that are consistent with crystallization at higher pressures than basalts from symmetrical segments, and with lower extents of partial melting of the mantle. Both seismic evidence and geochemical evidence indicate that the axial lithosphere is thicker and colder at asymmetrical sections of the ridge, either because associated hydrothermal circulation efficiently penetrates to greater depths or because the rising mantle is cooler. We suggest that much of the variability in sea-floor morphology, seismicity and basalt chemistry found along slow-spreading ridges can be thus attributed to the frequent involvement of detachment faults

  15. Results from SAMTEX: The Southern African lithospheric mantle - electrical structures and geometries and comparison with seismological information

    NASA Astrophysics Data System (ADS)

    G.. Jones, A.; Muller, M. P.; Miensopust, M. P.; Khosa, D.; Share, P.-E.

    2009-04-01

    The Southern African Magnetotelluric Experiment (SAMTEX) is imaging the electrical structures and geometries of the continental lithosphere below Botswana, Namibia and South Africa to depths of 200+ km. Primary geometrical information can readily be obtained from lithospheric-scale MT experiments about the three-dimensional variation in conductivity, and this information can be related to formation and deformation processes. In particular, one important piece of geometrical information easily and relatively precisely (to within 10%) obtained from MT data is the depth to the lithosphere-asthenosphere boundary (LAB), due to the sensitivity of conductivity to small fractions (<1%) of partial melt and/or increased water content. Over four phases of acquisition SAMTEX measurements have been made at a total of more than 700 MT sites in an area of greater than a million square kilometers, making it by far the largest-ever MT project undertaken. In particular, during Phase IV very challenging MT measurements were made in the highly-remote Central Kalahari Game Reserve, completing the coverage of Botswana. One of the most significant results from SAMTEX is the mapping of the LAB beneath the Archean cratons and bounding mobile belts of Southern Africa, particularly beneath Namibia and Botswana for which no prior lithospheric information exists. As would be expected, the electrically-defined LAB is generally shallow (150 km) beneath the mobile belts, deep (250 km) in the centres of the cratons, and transitional at the edges of cratons. Kimberlites are useful in also inferring lithospheric thickness, and diamondiferous kimberlites are located primarily where the electrical lithosphere is transitional in thickness, or where there is a change in its electrical anisotropy properties, both of which are craton edge effects. The electrical properties of the continental mantle derived from SAMTEX data can be compared with seismic ones derived from data from the South African Seismic

  16. Constraints on hydrothermal heat flux through the oceanic lithosphere from global heat flow

    NASA Technical Reports Server (NTRS)

    Stein, Carol A.; Stein, Seth

    1994-01-01

    A significant discrepancy exists between the heat flow measured at the seafloor and the higher values predicted by thermal models of the cooling lithosphere. This discrepancy is generally interpreted as indicating that the upper oceanic crust is cooled significantly by hydrothermal circulation. The magnitude of this heat flow discrepancy is the primary datum used to estimate the volume of hydrothermal flow, and the variation in the discrepancy with lithospheric age is the primary constraint on how the hydrothermal flux is divided between near-ridge and off-ridge environments. The resulting estimates are important for investigation of both the thermal structure of the lithosphere and the chemistry of the oceans. We reevaluate the magnitude and age variation of the discrepancy using a global heat flow data set substantially larger than in earlier studies, and the GDHI (Global Depth and Heat Flow) model that better predicts the heat flow. We estimate that of the predicted global oceanic heat flux of 32 x 10(exp 12) W, 34% (11 x 10(exp 12) W) occurs by hydrothermal flow. Approximately 30% of the hydrothermal heat flux occurs in crust younger than 1 Ma, so the majority of this flux is off-ridge. These hydrothermal heat flux estimates are upper bounds, because heat flow measurements require sediment at the site and so are made preferentially at topographic lows, where heat flow may be depressed. Because the water temperature for the near-ridge flow exceeds that for the off-ridge flow, the near-ridge water flow will be even a smaller fraction of the total water flow. As a result, in estimating fluxes from geochemical data, use of the high water temperatures appropriate for the ridge axis may significantly overestimate the heat flux for an assumed water flux or underestimate the water flux for an assumed heat flux. Our data also permit improved estimates of the 'sealing' age, defined as the age where the observed heat flow approximately equals that predicted, suggesting

  17. Further seismological consequences of millefeuille asthenosphere and evolution of oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Kawakatsu, H.; Song, T.

    2011-12-01

    Kawakatsu et al. (2009, Science) proposed a model of an oceanic lithosphere/asthenosphere system in which a lithosphere is underlain by a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in a background melt-free mantle. A simple version of this melt-lubricated ``millefeuille" asthenosphere model, i.e., both lithosphere and asthenosphere are composed of seismically isotropic background materials, results in a long-wavelength equivalent transversely isotropic (TI) medium that predicts peculiar behaviors of seismic waves in terms of the incident angle dependence of both P- and S-wave speeds (Kawakatsu et al., 2009, AGU). In particular, it predicts that Vsv}>V{sh for a S-wave incident angle range of 0-60 degrees, while Vsh}>V{sv for horizontally traveling S-waves (and surface waves), where we follow Aki?&Richards for a definition of SV and SH. This means that this model may have difficulty in explaining Vsh}>V{sv for the asthenosphere reported by Tan and Helmberger (2007, JGR) using multiply reflected SS-wave series, even though it may explain surface wave dispersion data. The effect of millefeuille structure in general is to reduce the value of a parameter ?eta = F/(A-2L) that affects the incident angle dependence of bodywave speeds in resulting TI (Dziewonski?&Anderson, 1981, PEPI), as well as making ?xi = N/L>1, where the notation follows that of Takeuchi?&Saito (1972) and for horizontally traveling S-waves, ?sqrt{N/L}=Vsh}/V{sv>1. It turns out that it is almost impossible to circumvent the aforementioned difficulty if we start from an isotropic background medium. On the other hand, it is possible to construct millefeuille models that show Vsh}>V{sv for a wide range of S-wave incident angles explainable wide range of seismic observations, if we start with a TI medium that show ?xi = N/L>1 and ?varphi = C/A <1. A range of background TI (or more generally orthorhombic) media that explain seismic observations are searched, and

  18. The African superswell

    NASA Technical Reports Server (NTRS)

    Nyblade, Andrew A.; Robinson, Scott W.

    1994-01-01

    Maps of residual bathymetry in the ocean basins around the African continent reveal a broad bathymetric swell in the southeastern Atlantic Ocean with an amplitude of about 500 m. We propose that this region of anomalously shallow bathymetry, together with the contiguous eastern and southern African plateaus, form a superswell which we refer to as the African superswell. The origin of the African superswell is uncertain. However, rifting and volcanism in eastern Africa, as well as heat flow measurements in southern Africa and the southeastern Atlantic Ocean, suggest that the superswell may be attributed, at least in part, to heating of the lithosphere.

  19. Mantle-crust differentiation of chalcophile elements in the oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Ciążela, J.; Dick, H. J.; Koepke, J.; Kuhn, T.; Muszynski, A.; Kubiak, M.

    2014-12-01

    The chalcophile elements, as associated with sulfides, are believed mainly from the study of ophiolites to be generally enriched in the upper mantle, but depleted by magmatic processes in the lower and upper ocean crust. However, studies of some orogenic lherzolites suggest a copper depletion of peridotites in relation to the primitive mantle, suggesting that a portion of the sulfides is melted during decompression and incorporated into the ascending magmas. The rarity of abyssal peridotites and the high degree of their alteration have not allowed these results to be verified in situ in the oceans.Here, we present the first complete study of chalcophile elements based on a suite of rocks from an oceanic core complex (OCC), the Kane Megamullion at 22°30'N at the Mid-Atlantic Ridge. OCCs provide large exposures of mantle and lower crustal rocks on the seafloor on detachment fault footwalls at slow and ultraslow spreading ridges. The Kane Megamullion is one of the best sampled OCCs in the world, with 1342 rocks from 28 dredge sites and 14 dives. We have made XRF, TD-MS and INAA analyses of 129 representative peridotites, gabbroic rocks, diabases and basalts. Our results suggest a depletion of some peridotites in relation to the primitive mantle (28 ppm Cu). Dunites, troctolites and olivine gabbros are relatively enriched in chalcophile elements. The amount of sulfides decreases gradually with progressive differentiation, reaching a minimum in gabbronorites and diabases. The highest bulk abundance of chalcophile elements in our sample suite was observed in dunites (up to ~ 300 ppm Cu in several samples) and a contact zone between residual peridotite and a mafic vein (294 ppm Cu). Plagioclase-bearing harzburgites, generally formed by late-stage melt impregnation in the mantle, are typically more enriched in Cu than unimpregnated residual peridotites. For these reasons, our initial results indicate sulfide melting during mantle melting, and their local precipitation in

  20. Spectral analysis of the gravity and elevation along the western Africa-Eurasia plate tectonic limit: Continental versus oceanic lithospheric folding signals

    NASA Astrophysics Data System (ADS)

    Muñoz-Martín, A.; De Vicente, G.; Fernández-Lozano, J.; Cloetingh, S.; Willingshofer, E.; Sokoutis, D.; Beekman, F.

    2010-12-01

    Large-scale folding is a key mechanism of lithospheric deformation and has been described in many parts of the Earth, both for the continental and oceanic lithospheres. Some aspects of this process such as the presence of coupling/decoupling between the crustal deformation and the mantle lithosphere, or between different lithospheres, make it necessary to accurately control the periodic characteristics of the elevation and of the gravity signal. 1D spectral analysis of gravity and topography profiles is sensitive to a series of factors: the location, length and orientation of the profiles, as well as the number of samples taken. We carry out a systematic analysis of the periodicities in the topography and gravity, both 1D and 2D, along the western border of the Africa-Eurasia plate tectonic boundary. We analyze the sensitivity of the 1D and 2D spectral analysis in order to compare the results along a plate boundary where oceanic and continental lithospheres are in contact with different tectonic, kinematic and rheological aspects. Our 1D spectral results indicate that the greater the profile length, the longer the wavelength peaks that are found. Nevertheless there are some periodic signals that appear in almost all the analyzed profiles: 100-250 km for the N-S profiles across oceanic plate boundary and 150-250 km where the plate boundary is developed over continental lithospheres. The 2D spectral analysis avoids the problems found in relation to the particular location of the profile but the resulting wavelengths are slightly higher than those obtained from the 1D spectral analysis. The wavelengths estimated for both oceanic and continental lithospheres at the Africa-Eurasia boundary (> 250 km) show low values of mean mantle strength (< 10 13 Pa m). The presence of lithospheric folds means that the continental and oceanic lithospheres are mechanically coupled. This had previously been suggested for Iberia but not for the limit between S Iberia and the Terceira

  1. Observed Oceanic and Terrestrial Drivers of North African Climate

    NASA Astrophysics Data System (ADS)

    Yu, Y.; Notaro, M.; Wang, F.; Mao, J.; Shi, X.; Wei, Y.

    2015-12-01

    Hydrologic variability can pose a serious threat to the poverty-stricken regions of North Africa. Yet, the current understanding of oceanic versus terrestrial drivers of North African droughts/pluvials is largely model-based, with vast disagreement among models. In order to identify the observed drivers of North African climate and develop a benchmark for model evaluations, the multivariate Generalized Equilibrium Feedback Assessment (GEFA) is applied to observations, remotely sensed data, and reanalysis products. The identified primary oceanic drivers of North African rainfall variability are the Atlantic, tropical Indian, and tropical Pacific Oceans and Mediterranean Sea. During the summer monsoon, positive tropical eastern Atlantic sea-surface temperature (SST) anomalies are associated with a southward shift of the Inter-Tropical Convergence Zone, enhanced ocean evaporation, and greater precipitable water across coastal West Africa, leading to increased West African monsoon (WAM) rainfall and decreased Sahel rainfall. During the short rains, positive SST anomalies in the western tropical Indian Ocean and negative anomalies in the eastern tropical Indian Ocean support greater easterly oceanic flow, evaporation over the western ocean, and moisture advection to East Africa, thereby enhancing rainfall. The sign, magnitude, and timing of observed vegetation forcing on rainfall vary across North Africa. The positive feedback of leaf area index (LAI) on rainfall is greatest during DJF for the Horn of Africa, while it peaks in autumn and is weakest during the summer monsoon for the Sahel. Across the WAM region, a positive LAI anomaly supports an earlier monsoon onset, increased rainfall during the pre-monsoon, and decreased rainfall during the wet season. Through unique mechanisms, positive LAI anomalies favor enhanced transpiration, precipitable water, and rainfall across the Sahel and Horn of Africa, and increased roughness, ascent, and rainfall across the WAM region

  2. Geochemical evidence for the melting of subducting oceanic lithosphere at plate edges.

    PubMed

    Yogodzinski, G M; Lees, J M; Churikova, T G; Dorendorf, F; Wöerner, G; Volynets, O N

    2001-01-25

    Most island-arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust. Volcanic rocks interpreted to contain a component of melt (not just a fluid) from the subducting slab itself are uncommon, but possible examples have been recognized in the Aleutian islands, Baja California, Patagonia and elsewhere. The geochemically distinctive rocks from these areas, termed 'adakites, are often associated with subducting plates that are young and warm, and therefore thought to be more prone to melting. But the subducting lithosphere in some adakite locations (such as the Aleutian islands) appears to be too old and hence too cold to melt. This implies either that our interpretation of adakite geochemistry is incorrect, or that our understanding of the tectonic context of adakites is incomplete. Here we present geochemical data from the Kamchatka peninsula and the Aleutian islands that reaffirms the slab-melt interpretation of adakites, but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate. We conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow. The use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction-zone environments.

  3. Oceanic lithospheric S-wave velocities from the analysis of P-wave polarization at the ocean floor

    NASA Astrophysics Data System (ADS)

    Hannemann, Katrin; Krüger, Frank; Dahm, Torsten; Lange, Dietrich

    2016-12-01

    Our knowledge of the absolute S-wave velocities of the oceanic lithosphere is mainly based on global surface wave tomography, local active seismic or compliance measurements using oceanic infragravity waves. The results of tomography give a rather smooth picture of the actual S-wave velocity structure and local measurements have limitations regarding the range of elastic parameters or the geometry of the measurement. Here, we use the P-wave polarization (apparent P-wave incidence angle) of teleseismic events to investigate the S-wave velocity structure of the oceanic crust and the upper tens of kilometres of the mantle beneath single stations. In this study, we present an up to our knowledge new relation of the apparent P-wave incidence angle at the ocean bottom dependent on the half-space S-wave velocity. We analyse the angle in different period ranges at ocean bottom stations (OBSs) to derive apparent S-wave velocity profiles. These profiles are dependent on the S-wave velocity as well as on the thickness of the layers in the subsurface. Consequently, their interpretation results in a set of equally valid models. We analyse the apparent P-wave incidence angles of an OBS data set which was collected in the Eastern Mid Atlantic. We are able to determine reasonable S-wave-velocity-depth models by a three-step quantitative modelling after a manual data quality control, although layer resonance sometimes influences the estimated apparent S-wave velocities. The apparent S-wave velocity profiles are well explained by an oceanic PREM model in which the upper part is replaced by four layers consisting of a water column, a sediment, a crust and a layer representing the uppermost mantle. The obtained sediment has a thickness between 0.3 and 0.9 km with S-wave velocities between 0.7 and 1.4 km s-1. The estimated total crustal thickness varies between 4 and 10 km with S-wave velocities between 3.5 and 4.3 km s-1. We find a slight increase of the total crustal thickness from

  4. Active Pacific North America Plate boundary tectonics as evidenced by seismicity in the oceanic lithosphere offshore Baja California, Mexico

    NASA Astrophysics Data System (ADS)

    Hauksson, Egill; Kanamori, Hiroo; Stock, Joann; Cormier, Marie-Helene; Legg, Mark

    2014-03-01

    Pacific Ocean crust west of southwest North America was formed by Cenozoic seafloor spreading between the large Pacific Plate and smaller microplates. The eastern limit of this seafloor, the continent-ocean boundary, is the fossil trench along which the microplates subducted and were mostly destroyed in Miocene time. The Pacific-North America Plate boundary motion today is concentrated on continental fault systems well to the east, and this region of oceanic crust is generally thought to be within the rigid Pacific Plate. Yet, the 2012 December 14 Mw 6.3 earthquake that occurred about 275 km west of Ensenada, Baja California, Mexico, is evidence for continued tectonism in this oceanic part of the Pacific Plate. The preferred main shock centroid depth of 20 km was located close to the bottom of the seismogenic thickness of the young oceanic lithosphere. The focal mechanism, derived from both teleseismic P-wave inversion and W-phase analysis of the main shock waveforms, and the 12 aftershocks of M ˜3-4 are consistent with normal faulting on northeast striking nodal planes, which align with surface mapped extensional tectonic trends such as volcanic features in the region. Previous Global Positioning System (GPS) measurements on offshore islands in the California Continental Borderland had detected some distributed Pacific and North America relative plate motion strain that could extend into the epicentral region. The release of this lithospheric strain along existing zones of weakness is a more likely cause of this seismicity than current thermal contraction of the oceanic lithosphere or volcanism. The main shock caused weak to moderate ground shaking in the coastal zones of southern California, USA, and Baja California, Mexico, but the tsunami was negligible.

  5. Implications of the diffuse deformation of the Indian Ocean lithosphere for slip partitioning of oblique plate convergence in Sumatra

    NASA Astrophysics Data System (ADS)

    Bradley, K. E.; Feng, L.; Hill, E. M.; Natawidjaja, D. H.; Sieh, K.

    2017-01-01

    Oblique plate convergence between Indian Ocean lithosphere and continental crust of the Sunda plate is distributed between subduction on the Sunda megathrust and upper plate strike-slip faulting on the Sumatran Fault Zone, in a classic example of slip partitioning. Over the last decade, a destructive series of great earthquakes has brought renewed attention to the mechanical properties of these faults and the intervening fore-arc crustal block. While observations of fore-arc deformation over the earthquake cycle indicate that the fore-arc crust is fundamentally elastic, the spatial pattern of slip vector azimuths for earthquakes sourced by rupture of the Sunda megathrust is strongly inconsistent with relative motion of two rigid plates. Permanent and distributed deformation therefore occurs in either the downgoing lithospheric slab or the overriding fore-arc crust. Previous studies have inferred from geodetic velocities and geological slip rates of the Sumatran Fault that the fore-arc crust is undergoing rapid trench-parallel stretching. Using new geological slip rates for the Sumatran Fault and an updated decadal GPS velocity field of Sumatra and the fore-arc islands, we instead show that permanent deformation within the fore-arc sliver is minor and that the Sumatran Fault is a plate boundary strike-slip fault. The kinematic data are best explained by diffuse deformation within the oceanic lithosphere of the Wharton Basin, which accommodates convergence between the Indian and Australian plates and has recently produced several large earthquakes well offshore of Sumatra. The slip partitioning system in Sumatra is fundamentally linked with the mechanical properties of the subducting oceanic lithosphere.

  6. Silicon Isotope Geochemistry of Ocean Island Basalts: Mantle Heterogeneities and Contribution of Recycled Oceanic Crust and Lithosphere

    NASA Astrophysics Data System (ADS)

    Pringle, E. A.; Moynier, F.; Savage, P. S.; Jackson, M. G.; Moreira, M. A.; Day, J. M.

    2015-12-01

    altered oceanic crust and lithosphere in the plume source. References: [1] Ziegler et al., GCA 2005 [2] Savage et al., GCA 2011 [3] Savage et al., EPSL 2010 [4] Day et al., Geology 2009 [5] Huang et al., GCA 2014

  7. The depth of the Lithosphere-Asthenophere boundary beneath the world oldest ocean: the Ionian plate case-study

    NASA Astrophysics Data System (ADS)

    Piana Agostinetti, N.; Bianchi, I.; Chiarabba, C.

    2015-12-01

    The depth of the Lithosphere-Asthenosphere boundary (LAB) beneath the ocean is widely accepted to increase with the age of the oceanic plate. In particular, in the first 30-50 My, the thickness of the Lithosphere increases almost linearly to about 60-70 km, for a number of models of the LAB formation. Observations from the Pacific plate generally support this hypothesis (e.g. Schmerr, 2012). However, measures of the LAB depth beneath old oceanic plates seem to indicate an almost constant depth of about 70-80 km for oceanic plate older than 50-60 Ma (e.g. Kumar and Kawakatsu, 2011), in dis-agreement with most of the earliest LAB model (e.g. half-space cooling). In this study, we present the first measurement of the LAB depth beneath a 250Myr old ocean, the Ionian plate, obtained from the analysis of teleseismic waveforms recorded at two Ocean Bottom Seismometers (OBS) deployed on the ocean floor. From such recordings, we compute two independent data-set of S receiver function (S-RF), a widely used tool for estimating the depth of the LAB. The S-RF data sets indicate the presence of a negative S-wave velocity discontinuity (i.e. where Vs decreases with depth) at 75 +/- 10 km depth, which is interpreted as the LAB. Insights into the evolution of the oceanic LAB during subduction are obtained by comparing the LAB depth found from the analysis of the S-RF data-sets, with the LAB depth beneath the Calabrian arc, obtained from the analysis of a huge P receiver function data-set.

  8. Lithospheric mantle heterogeneity across the continental-oceanic transition, northwest Ross Sea, Antarctica: new evidence from oxygen isotopes

    NASA Astrophysics Data System (ADS)

    Krans, S. R.; Panter, K. S.; Castillo, P.; Deering, C. D.; Kitajima, K.; Valley, J. W.; Hart, S. R.; Kyle, P. R.

    2013-12-01

    Oxygen isotopes and whole rock chemistry from alkali basalt and basanite in the northwest Ross Sea, Antarctica offer new insight on source heterogeneity across the transition from continental to oceanic lithosphere in a magma-poor rifted margin. In situ SIMS analysis of olivine (Fo 79-90) from the most primitive lavas (MgO ≥ 8 wt%, Mg# 53-70, Ni= 115-338 ppm, Cr= 244-540 ppm) yield an average δ18O = 5.18 × 0.60 ‰ (2σ, n=30) for alkali basalt and 5.25 × 0.44 ‰ (2σ, n=52) for basanite (× 0.28 ‰, 2σ precision on a homogeneous olivine standard). These are similar to the range for olivine from mantle peridotite and HIMU type oceanic basalts (δ18O= 5.0 to 5.4 ‰ and 4.9 to 5.2 ‰, respectively [1]), but with greater variability. Lavas in this region experienced little differentiation, have minimal evidence of crustal contamination (87Sr/86Sr < 0.7030, 143Nd/144Nd > 0.5129), and olivine show no correlation between δ18O and Fo content, further suggesting that the δ18O values are source related. Whole-rock chemistry of alkali basalt and basanite are spatially distributed. In general, alkali basalt is found in thicker continental lithosphere with lower Sr (477-672ppm) and Nb/Y (1.2-2.4) than basanite. Basanite is found in oceanic and thinned continental lithosphere with higher Sr (642-1131 ppm) and Nb/Y (2.4-3.6). Variation in degree of silica-undersaturation and Nb/Y can be explained by varying degree of partial melting. While alkali basalt and basanite can result from varying degrees of partial melting of similar source compositions, the presence of amphibole in mantle xenoliths have lead workers in this region to propose contributions from a metasomatic source [2, 3, 4] with variable 206Pb/204Pb ratios [5]. A negative correlation between Nb/Y and δ18O in both rock types suggests that varying degrees of partial melting are tapping sources with different δ18O values; lower degree melts have δ18O ≤ 5.0 ‰ and higher degree melts have δ18O > 5.3

  9. Wrangellia flood basalts in Alaska: A record of plume-lithosphere interaction in a Late Triassic accreted oceanic plateau

    NASA Astrophysics Data System (ADS)

    Greene, Andrew R.; Scoates, James S.; Weis, Dominique

    2008-12-01

    The Wrangellia flood basalts are part of one of the best exposed accreted oceanic plateaus on Earth. They provide important constraints on the construction of these vast submarine edifices and the source and temporal evolution of magmas for a plume head impinging beneath oceanic lithosphere. Wrangellia flood basalts (˜231-225 Ma) extend ˜450 km across southern Alaska (Wrangell Mountains and Alaska Range) where ˜3.5 km of mostly subaerial flows are bounded by late Paleozoic arc volcanics and Late Triassic limestone. The vast majority of the flood basalts are light rare earth element (LREE) -enriched high-Ti basalt (1.6-2.4 wt % TiO2) with uniform ocean island basalt (OIB) -type Pacific mantle isotopic compositions (ɛHf(t) = +9.7 to +10.7; ɛNd(t) = +6.0 to +8.1; t = 230 Ma). However, the lowest ˜400 m of stratigraphy in the Alaska Range is LREE-depleted low-Ti basalt (0.4-1.2 wt % TiO2) with pronounced negative high field strength element (HFSE) anomalies and Hf isotopic compositions (ɛHf(t) = +13.7 to +18.4) that are decoupled from Nd (ɛNd(t) = +4.6 to +5.4) and displaced well above the OIB mantle array (ΔɛHf = +4 to +8). The radiogenic Hf of the low-Ti basalts indicates involvement of a component that evolved with high Lu/Hf over time but not with a correspondingly high Sm/Nd. The radiogenic Hf and HFSE-depleted signature of the low-Ti basalts suggest pre-existing arc lithosphere was involved in the formation of flood basalts that erupted early in construction of part of the Wrangellia plateau in Alaska. Thermal and mechanical erosion of the base of the lithosphere by the impinging plume head may have led to melting of arc lithosphere or interaction of plume-derived melts and subduction-modified mantle. The high-Ti lavas dominate the main phase of construction of the plateau and were derived from a depleted mantle source distinct from the source of MORB and with compositional similarities to that of ocean islands (e.g., Hawaii) and plateaus (e.g., Ontong

  10. Release of Oceanic Intraplate Magmatic CO2, Carbonatization, and Decarbonatization Reactions in the Lower Oceanic Lithosphere and Subducting Slabs and Associated Intraplate and Intraslab Earthquakes

    NASA Astrophysics Data System (ADS)

    Kirby, S. H.

    2011-12-01

    Release of free CO2 from ascending mafic magmas is thought to be important for intraplate magmatic systems under the ocean basins and gives insight into the cause of deep mantle earthquakes such as those that occur under the Island of Hawai'i at depths of 20 to 60 km via pore-pressure effects. Moreover, this hypothesis is consistent with the occurrence of CO2-filled inclusions along healed fractures in mantle xenoliths in Hawai'ian in ultramafic xenoliths of presumed mantle or deep-crustal origin. The positive pressure effect on CO2 solubility in mafic melts implies that this volatile boils out of such magmas as they ascend and enables fracture and frictional sliding at mantle depths by reducing the effective normal stresses. It is likely that such CO2 is stored along such fractures and faults during the active stage of plume magmatic activity and that during cooling, this CO2 reacts with mantle silicates to form magnesite and dolomite as oceanic lithosphere cools. Such carbonates are much weaker than mantle peridotites (Holyoke and Kronenberg, this session) and are therefore expected to localize strain along such carbonated zones where the oceanic plate is under tectonic stresses. Such conditions are found in the zone of bending near trenches and within subducting slabs where double zones of seismicity are locally present. Localized plastic deformation and viscous heating leading to free CO2 release through decarbonatization and perhaps melting may enable seismogenesis at such depths in mantle lithosphere. This model for the lower zones of double seismic zones where the enabling fluid comes from below the plate from plume magmatic processes (Kirby, 1995; Seno and Yamanaka, 1996) is much more appealing than positing fluid penetration and serpentization downward through the entire oceanic lithosphere from the ocean floor followed by serpentinite dehydration upon subsequent heating during slab descent.

  11. The electrical conductivity of the upper mantle and lithosphere from satellite magnetic signal due to ocean tidal flow

    NASA Astrophysics Data System (ADS)

    Schnepf, N. R.; Kuvshinov, A. V.; Grayver, A.; Sabaka, T. J.; Olsen, N.

    2015-12-01

    Global electromagnetic (EM) studies provide information on mantle electrical conductivity with the ultimate aim of understanding the composition, structure, and dynamics of Earth's interior. There is great much interest in mapping the global conductivity of the lithosphere and upper mantle (i.e., depths of 10-400 km) because recent laboratory experiments demonstrate that the electrical conductivity of minerals in these regions are greatly affected by small amounts of water or by partial melt. For decades, studies of lithospheric/mantle conductivity were based on interpretation of magnetic data from a global network of observatories. The recent expansion in magnetic data from low-Earth orbiting satellite missions (Ørsted, CHAMP, SAC-C, and Swarm) has led to a rising interest in probing Earth from space. The largest benefit of using satellite data is much improved spatial coverage. Additionally, and in contrast to ground-based data, satellite data are overall uniform and very high quality. Probing the conductivity of the lithosphere and upper mantle requires EM variations with periods of a few hours. This is a challenging period range for global EM studies since the ionospheric (Sq) source dominates these periods and has a much more complex spatial structure compared to the magnetospheric ring current. Moreover, satellite-based EM induction studies in principle cannot use Sq data since the satellites fly above the Sq source causing the signals to be seen by the satellite as a purely internal source, thus precluding the separation of satellite Sq signals into internal and external parts. Lastly, magnetospheric and ionospheric sources interact inductively with Earth's conducting interior. Fortunately, there exists an alternative EM source in the Sq period range: electric currents generated by oceanic tides. Tides instead interact galvanically with the lithosphere (i.e. by direct coupling of the source currents in the ocean with the underlying substrate), enabling

  12. Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere.

    PubMed

    Bonatti, Enrico; Ligi, Marco; Brunelli, Daniele; Cipriani, Anna; Fabretti, Paola; Ferrante, Valentina; Gasperini, Luca; Ottolini, Luisa

    2003-05-29

    A 20-Myr record of creation of oceanic lithosphere is exposed along a segment of the central Mid-Atlantic Ridge on an uplifted sliver of lithosphere. The degree of melting of the mantle that is upwelling below the ridge, estimated from the chemistry of the exposed mantle rocks, as well as crustal thickness inferred from gravity measurements, show oscillations of approximately 3-4 Myr superimposed on a longer-term steady increase with time. The time lag between oscillations of mantle melting and crustal thickness indicates that the mantle is upwelling at an average rate of approximately 25 mm x yr(-1), but this appears to vary through time. Slow-spreading lithosphere seems to form through dynamic pulses of mantle upwelling and melting, leading not only to along-axis segmentation but also to across-axis structural variability. Also, the central Mid-Atlantic Ridge appears to have become steadily hotter over the past 20 Myr, possibly owing to north-south mantle flow.

  13. 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

  14. Compression of oceanic lithosphere - An analysis of intraplate deformation in the Central Indian Basin

    NASA Technical Reports Server (NTRS)

    Zuber, Maria T.

    1987-01-01

    The development of intraplate structure in the Central Indian Basin is examined using models in which deformation is due to flexural buckling and the hydrodynamic growth of instabilities. Comparison of the models reveal that in a strong viscous lithosphere, deformation of the layer occurs by flexural folding at a wavelength which corresponds to the flexural buckling theory; in a lithosphere of intermediate strength, the layer deforms by folding characterized by thickening which localizes beneath topographic heights; and in a relatively weak lithosphere, the layer incurs an even greater amount of localized thickening and deforms in the symmetric or pinch-and-swell mode by inverse boudinage. It is noted that the models in which the layer folds either flexurally or with periodic thickening correspond with observed depth distribution of seismicity in the Central Indian Basin and with experimental rock rheological data.

  15. Passive rifting of thick lithosphere in the southern East African Rift: Evidence from mantle transition zone discontinuity topography

    NASA Astrophysics Data System (ADS)

    Reed, Cory A.; Liu, Kelly H.; Chindandali, Patrick R. N.; Massingue, Belarmino; Mdala, Hassan; Mutamina, Daniel; Yu, Youqiang; Gao, Stephen S.

    2016-11-01

    To investigate the mechanisms for the initiation and early-stage evolution of the nonvolcanic southernmost segments of the East African Rift System (EARS), we installed and operated 35 broadband seismic stations across the Malawi and Luangwa rift zones over a 2 year period from mid-2012 to mid-2014. Stacking of over 1900 high-quality receiver functions provides the first regional-scale image of the 410 and 660 km seismic discontinuities bounding the mantle transition zone (MTZ) within the vicinity of the rift zones. When a 1-D standard Earth model is used for time-depth conversion, a normal MTZ thickness of 250 km is found beneath most of the study area. In addition, the apparent depths of both discontinuities are shallower than normal with a maximum apparent uplift of 20 km, suggesting widespread upper mantle high-velocity anomalies. These findings suggest that it is unlikely for a low-velocity province to reside within the upper mantle or MTZ beneath the nonvolcanic southern EARS. They also support the existence of relatively thick and strong lithosphere corresponding to the widest section of the Malawi rift zone, an observation that is consistent with strain localization models and fault polarity and geometry observations. We postulate that the Malawi rift is driven primarily by passive extension within the lithosphere attributed to the divergent rotation of the Rovuma microplate relative to the Nubian plate, and that contributions of thermal upwelling from the lower mantle are insignificant in the initiation and early-stage development of rift zones in southern Africa.

  16. 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.

  17. Tectonic Evolution of the Careón Ophiolite (Northwest Spain): A Remnant of Oceanic Lithosphere in the Variscan Belt.

    PubMed

    Díaz García F; Arenas; Martínez Catalán JR; González del Tánago J; Dunning

    1999-09-01

    Analysis of the Careón Unit in the Ordenes Complex (northwest Iberian Massif) has supplied relevant data concerning the existence of a Paleozoic oceanic lithosphere, probably related to the Rheic realm, and the early subduction-related events that were obscured along much of the Variscan belt by subsequent collision tectonics. The ophiolite consists of serpentinized harzburgite and dunite in the lower section and a crustal section made up of coarse-grained and pegmatitic gabbros. An Early Devonian zircon age (395+/-2 Ma, U-Pb) was obtained in a leucocratic gabbro. The whole section was intruded by numerous diabasic gabbro dikes. Convergence processes took place shortly afterward, giving rise to a mantle-rooted synthetic thrust system, with some coeval igneous activity. Garnet amphibolite, developed in metamorphic soles, was found discontinuously attached to the thrust fault. The soles graded downward to epidote-amphibolite facies metabasite and were partially retrogressed to greenschist facies conditions. Thermobarometric estimations carried out at a metamorphic sole (T approximately 650 degrees C; P approximately 11.5 kbar) suggested that imbrications developed in a subduction setting, and regional geology places this subduction in the context of an early Variscan accretionary wedge. Subduction and imbrication of oceanic lithosphere was followed by underthrusting of the Gondwana continental margin.

  18. West Indian Ocean variability and East African fish catch.

    PubMed

    Jury, M; McClanahan, T; Maina, J

    2010-08-01

    We describe marine climate variability off the east coast of Africa in the context of fish catch statistics for Tanzania and Kenya. The time series exhibits quasi-decadal cycles over the period 1964-2007. Fish catch is up when sea surface temperature (SST) and atmospheric humidity are below normal in the tropical West Indian Ocean. This pattern relates to an ocean Rossby wave in one phase of its east-west oscillation. Coastal-scale analyses indicate that northward currents and uplift on the shelf edge enhance productivity of East African shelf waters. Some of the changes are regulated by the south equatorial current that swings northward from Madagascar. The weather is drier and a salty layer develops in high catch years. While the large-scale West Indian Ocean has some impact on East African fish catch, coastal dynamics play a more significant role. Climatic changes are reviewed using 200 years of past and projected data. The observed warming trend continues to increase such that predicted SST may reach 30 degrees C by 2100 while SW monsoon winds gradually increase, according to a coupled general circulation model simulation with a gradual doubling of CO(2).

  19. Constraints on the Thermal and Compositional Nature of the Oceanic Lithosphere-Asthenosphere Boundary from Seismic Anisotropy

    NASA Astrophysics Data System (ADS)

    Beghein, C.; Yuan, K.; Schmerr, N. C.; Xing, Z.

    2014-12-01

    In this study we modeled S-wave velocities, radial and azimuthal anisotropy beneath the Pacific ocean, and compared our model with detections of the Gutenberg (G) discontinuity at 40-100 km depth to evaluate its context and relation to the lithosphere-asthenosphere boundary (LAB). The G is often associated with the LAB, but its sharpness and the low correlation between its depth and oceanic plate age suggest a compositional origin, in contradiction with tomographic models of isotropic wave velocities. Here, we inverted fundamental and higher mode anisotropic Rayleigh wave phase velocity maps to which we applied non-linear crustal corrections. Our model defines three layers within the upper 250km of the mantle. The bottom layer is characterized by relatively low velocities, strong (3%) azimuthal anisotropy, fast seismic directions that follow the absolute plate motion (APM), and strong (5%) radial anisotropy with VSH>VSV. This suggests alignment of olivine fast axes with mantle flow direction in the asthenosphere. The middle layer has fast axes aligned with the paleospreading directions, and the boundary between the bottom and middle layers follows a half-space cooling model. This suggests a thermal origin of the LAB if we use the change in alignment of the fast axes with the APM as a proxy for the LAB. Remarkably, a change in azimuthal anisotropy is found between the two top layers at a roughly constant depth that coincides with the location of the G. The G is therefore located within the thermal lithosphere and is primarily associated with a vertical gradient in azimuthal anisotropy, which may result from compositional changes. Dehydration of the mantle underlying mid-ocean ridges offers a possible explanation for our results. It could generate a chemically depleted, viscous layer that becomes overprinted by lowered temperatures as the plate cools and migrates away from the ridge. The olivine fast axes would align with the spreading direction at the ridge in the

  20. Sensing the Electrical Conductivity of the Upper Mantle and Lithosphere Using Satellite Magnetic Signal Due to Ocean Tidal Flow

    NASA Astrophysics Data System (ADS)

    Schnepf, N. R.; Kuvshinov, A. V.; Sabaka, T. J.; Olsen, N.

    2014-12-01

    A few scientific groups convincingly demonstrated that the magnetic fields induced by the lunar semidiurnal (M2) ocean flow can be identified in magnetic satellite observations. These results support the idea to recover M2 magnetic signals from Swarm data, and to use these data for constraining lithosphere and upper mantle electrical conductivity in oceanic regions. Induction studies using ionospheric and magnetospheric primary sources with periods of about one day are sensitive to mantle conductivity at a few hundred kilometers depth because of the inductive coupling between primary and induced sources. In contrast, using oceanic tides as a signal allows studying shallower regions since the coupling is galvanic. This corresponds to global electric sounding. In this study we perform global 3-D EM numerical simulations in order to investigate the sensitivity of M2 signals to conductivity distributions at different depths. The results of sensitivity analysis are discussed, and comparison of the modelled M2 signals with those recovered by Comprehensive Inversion from one year of Swarm data is presented.

  1. Crustal Structure and Lithospheric Rupture Process of the Continent-Ocean Boundary of the South China Sea

    NASA Astrophysics Data System (ADS)

    Song, T. R.; Li, C. F.; Shi, H.; Ding, W.; Li, J.

    2014-12-01

    Seismic reflection profiles acquired in the continent-ocean transition zone (COT) of South China Sea provide a detailed view of Moho and deep crustal reflectors and continental lithosphere extension and breakup styles. At the north margin, rift basins are often bounded by listric normal faults, most of which are terminated at the base of the upper crust. The upper-lower crust interface corresponds to the brittle-ductile transition zone, where listric faults tend to converge into a low angle detachment fault. According to calculated stretch factors in different depth scales (upper crust, lower crust) along several profiles, hyper-extended continental crust is widespread, with the upper crust being often preferentially more thinned than the lower crust. The ductile lower crust is more resistant to faulting and therefore stretching. A high-velocity lower crustal layer represents either magmatic underplating or pervasive lower crustal intrusions in the northern margin. The possible lower crust ductile flow and the high-velocity lower crustal layer may have contributed to extension discrepancy, leading to direct exposure of lower crust material landward of the continent-ocean boundary. Hyper-extended continental crust and thick syn-rift sequence developed during a long period of rifting prior to the inception of seafloor spreading are also observed in the southern continental margin, further suggesting depth-dependent continental extension in the South China Sea. Basement highs and discontinuity in Moho reflector are common features around the continent-ocean boundary of South China Sea. The basement ridges are located at the landward edge of the continent-ocean boundary and possibly composed by lower crust material. The COT is ~50 km wide, where the gravity anomaly is approximately zero and the Moho reflector is discontinuous. The COT here is narrower than those found in other magma-poor margins (e.g., Iberia-Newfoundland type), indicating that normal oceanic crust

  2. Heat Flow and Magnetization in the Oceanic Lithosphere. Ph.D. ThesisSemiannual Report, Nov. 1987 - Apr. 1988

    NASA Technical Reports Server (NTRS)

    Hayling, Kjell Lennart

    1988-01-01

    Two aspects of the processing and interpretation of satellite measurements of the geomagnetic field are described. One deals with the extraction of the part of the geomagnetic field that originates from sources in the earth's atmosphere. The other investigates the possibility of using the thermal state of the oceanic lithosphere to further constrain modelling and interpretation of magnetic anomalies. It is shown that some of the magnetic signal in crustal anomaly maps can be an artifact of the mathematical algorithms that have been used to separate the crustal field from the observed data. Strong magnetic anomalies can be distorted but are probably real, but weak magnetic anomalies can arise from leakage of power from short wavelengths, and will also appear in anomaly maps as repetitions of the strong crustal anomaly. The distortion and the ghost anomalies follow the magnetic dip lines in a way that is similar to actual MAGSAT anomaly fields. This phenomenon will also affect the lower degree spherical harmonic terms in the power spectrum of the crustal field. A model of the magnetic properties of the oceanic crust that has been derived from direct measurements of the rock magnetic properties of oceanic rocks is presented. The average intensity of magnetization in the oceanic crust is not strong enough to explain magnetic anomalies observed over oceanic areas. This is the case for both near surface observations (ship and aeromagnetic data) and satellite altitude observations. It is shown that magnetic sources in the part of the upper mantle that is situated above the Curie isotherm, if sufficiently strong, can produce satellite magnetic anomalies that are comparable to MAGSAT data. The method developed for the study of depth to the Curie isotherm and magnetic anomalies can also be used in inverse modelling of satellite magnetic anomalies when the model is to be adjusted with an annihilator.

  3. Structure of the lithosphere of the northeastern part of the Indian Ocean according to results of two-dimensional structural-density modeling

    NASA Astrophysics Data System (ADS)

    Bulychev, A. A.; Gilod, D. A.; Dubinin, E. P.

    2016-05-01

    From a gravitational field analysis, the lithosphere was regionalized and a structural schematic map of the eastern part of the Indian Ocean was compiled. The area adjacent to the western margin of Australia was studied. The region is characterized by a complex lithospheric structure. It includes heterogeneous blocks of varying age, framed by structures with different morphological and geophysical expression and varying genesis. To clarify the peculiarities of tectonic structures of various genetic types, structural-density modeling was performed. This made it possible to establish certain gravimetric indicators characteristic of structures of various genesis.

  4. Roberts Victor Eclogites: The MacGregor Legacy of Archean Oceanic Lithosphere Subduction and its Role in Craton Formation

    NASA Astrophysics Data System (ADS)

    Shirey, S. B.; Schmitz, M. D.; Wiechert, U.; Carlson, R. W.

    2005-12-01

    Eclogite xenoliths from the 125 Ma old, Group II, Roberts Victor kimberlite have long been of interest (MacGregor et al, 1968) because of their diversity, abundance, large size, occurrence with peridotite and their high carbon/diamond content. Coesite, corundum, kyanite, Ca-, Mg-, and Fe- rich eclogites are available but those classified as Group I, Group II (as defined by MacGregor, 1970) or diamondiferous were selected with the goal to better understand eclogite petrogenesis, Kaapvaal cratonic keel evolution, diamond formation, and eclogite metasomatism. Recent laser fluorination oxygen isotope data (δ18O) on gt (GI = 5.8 to 6.9; GII = 2.1 to 5.1) match earlier data (Garlick et al, 1971; MacGregor and Manton, 1986), while ion-probe trace element contents of gt (e.g. chondrite normalized Ce G1 = 0.2 to 0.5; GII = 0.002 to 0.07) and cpx (G1 = 7 to 20; GII = 0.2 to 2) and whole-rock Re-Os (G1 Re = 0.19 to 3.41 ppb; GII Re = 0.006 to 0.38 ppb) highlight even more distinct differences between Groups I and II. These differences must be a pre-metamorphic signature of their original protoliths and not just due to pressure differences or partial melting during emplacement. Using ophiolites and composites of oceanic crust as a guide (e.g. MacGregor and Manton, 1986), Group I eclogites could represent the volcanic rocks of Layer 2 of Archean oceanic crust whereas Group II might represent the cumulate, intrusive rocks of Layer 3. Group II eclogites have positive Eu anomalies and lower REE and Re contents which support this idea. The Re-Os systematics of the oceanic lithosphere is poorly known, especially in the Archean, but Roberts Victor eclogite Re-Os and trace element abundances and major element compositions suggest a basaltic komatiitic protolith as might typify slightly hotter ocean ridges in the Archean. A U-Pb age of 3.061±0.006 Ga on zircon grains separated from a Group I Roberts Victor eclogite and a same-age but scattered whole-rock Re-Os isotope array

  5. A common Pan-African Lithospheric Mantle (PALM) source for HIMU-like Pb-isotope signatures in circum-Mediterranean magmas

    NASA Astrophysics Data System (ADS)

    Young, H. P.; Wang, Z.; Brandon, M. T.

    2013-12-01

    Isotopic compositions of widely distributed basaltic rocks of Europe and North Africa are clustered around a point that is displaced from modern MORB in 208Pb/204Pb vs. 206Pb/204Pb, pointing to the 'HIMU' component proposed by Zindler and Hart (1986). This observation was originally highlighted in an abstract by Cebria and Wilson (1995), who suggested that a reservoir of unknown origin exists in the convecting upper mantle of the Mediterranean and coin it the 'European asthenospheric reservoir' or EAR in order to distinguish it from the apparent influence of an additional 'lithospheric' component having a Sr-Nd isotope composition similar to continental crust that is observed in some, but not all, Cenozoic igneous rocks. While this study and most authors agree that the 'lithospheric' component in the model of Cebria and Wilson (1995) is crustal material associated with Cenozoic subduction, explanations for the origin of the HIMU-like EAR reservoir, however, are diverse, ranging from deep plumes to recently subducted slabs. These explanations are problematic. For example, neither plumes nor recent subduction are spatially broad enough to explain all of the EAR occurrences. Alternatively, we argue that both components (lithospheric and EAR) observed by Cebria and Wilson are lithospheric in origin. We propose that the origin of the HIMU-like Pb component is metasomatized sub-continental lithospheric mantle (SCLM). Comparison with synthetic evolution models of a veined mantle show the HIMU-like composition of European Cenozoic igneous rocks can be generated after ~500 Ma (Pilet et al., 2011). Major and trace element compositions of the European alkalic-basalts are similar to experimental melts of amphibole-pyroxenite veins in peridotite (a common feature of the SCLM) (Médard et al., 2006). A likely candidate for a veined 500 Ma SCLM in this region is the 'Pan-African' age terrane that is currently widely distributed from England to the Sahara as well as on the

  6. Inferences of Integrated Lithospheric Strength from Plate-Scale Analyses of Deformation Observed in the Aegean-Anatolian Region and the Indian Ocean

    NASA Astrophysics Data System (ADS)

    Houseman, Gregory

    2016-04-01

    In the context of a comprehensive review of the rheology and strength of the lithosphere (Marine and Petroleum Geology, 2011, doi:10.1016/j.marpetgeo.2011.05.008), Evgene Burov described the difficulty of extrapolating rock deformation laws derived from laboratory experiments to the time and length scales that apply when the Earth's lithosphere is deformed. Not only does the extrapolation introduce a large uncertainty, but even the relative importance of different possible mechanisms of deformation may be uncertain. Even though lithospheric deformation has a strong conceptual and theoretical basis, it is therefore essential, as Burov argued, that deformation laws for the lithosphere must be calibrated by using observations of deformation that occurs on a lithospheric length scale and at geological strain rates. The influence of regionally varying factors like crustal thickness, geothermal gradient and tectonic environment may induce large variations in how rapidly the lithosphere may deform in response to an applied load, not least in the contrast from continent to ocean. Plates may be deformed by different loading mechanisms but, when deformation is distributed over a broad region, the strain-rate field may be approximately constant with depth and we may integrate the in-plane stress components across the thickness of the lithosphere to derive a depth-averaged constitutive law for the deformation. This approximation is the basis for the thin viscous sheet formulation of lithospheric deformation and, in combination with appropriate observations, it allows us to calibrate the integrated resistance to processes like regional extension or convergence. In this talk I will summarise what we learn about effective lithospheric rheology from two recent studies of the distribution and rates of diffuse deformation of the lithosphere in, firstly the Anatolian-Aegean region, and secondly the Central Indian Ocean. In the first case the distribution of deformation is consistent

  7. Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere

    USGS Publications Warehouse

    Kirby, S.H.; Stein, S.; Okal, E.A.; Rubie, David C.

    1996-01-01

    Earth's deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km. This depth interval closely brackets the mantle transition zone, characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher-pressure phases. Deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal, thermodynamic, and mechanical ferment inside slabs. Numerical simulations of reaction rates show that the olivine ??? spinel transformation should be kinetically hindered in old, cold slabs descending into the transition zone. Thus wedge-shaped zones of metastable peridotite probably persist to depths of more than 600 km. Laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting. This instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser, finer-grained phase. Hence in cold slabs, such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite. It is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs. This consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features. It explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone, where minerals in downgoing slabs should transform to spinel and ilmenite structures. The onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab. Even if a slab penetrates into the lower mantle, earthquakes

  8. Younger and older zircons from rocks of the oceanic lithosphere in the Central Atlantic and their geotectonic implications

    NASA Astrophysics Data System (ADS)

    Skolotnev, S. G.; Bel'Tenev, V. E.; Lepekhina, E. N.; Ipat'eva, I. S.

    2010-11-01

    Local U-Pb dating of zircons separated from various rocks in the crest zone of the Mid-Atlantic Ridge (MAR) and Carter Seamount (Sierra Leone Rise) is performed. Younger zircons formed in situ in combination with older xenogenic zircons are revealed in enriched basalts, alkaline volcanic rocks, gabbroic rocks, and plagiogranites. Only older zircons are found in depleted basalts and peridotites. Older zircons are ubiquitous in the young oceanic lithosphere of the Central Atlantic. The age of the younger zircons from the crest zone of the MAR ranges from 0.38 to 11.26 Ma and progressively increases receding from the axial zone of the ridge. This fact provides additional evidence for spreading of the oceanic floor. The rate of half-spreading calculated from the age of the studied zircons is close to the rate of half-spreading estimated from magnetic anomalies. The age of the younger zircons from Carter Seamount (58 Ma) corresponds to the age of the volcanic edifice. Older zircons make up an age series from 53 to 3200 Ma. Clusters of zircons differing in age reveal quasiperiodicity of about 200 Ma, which approximately corresponds to the global tectonic epochs in the geological evolution of the Earth. Several age groups of older zircons combine grains close in morphology and geochemistry: (1) Neoproterozoic and Phanerozoic (53-700 Ma) prismatic grains with slightly resorbed faces, well-preserved or translucent oscillatory zoning, and geochemical features inherent to magmatic zircons; (2) prismatic grains dated at 1811 Ma with resorbed faces and edges, fragmentary or translucent zoning, and geochemical features inherent to magmatic zircons; (3) ovoid and highly resorbed prismatic grains with chaotic internal structure and metamorphic geochemical parameters; the peak of their ages is 1880 Ma. The performed study indicates that older xenogenic zircons from young rocks in the crest zone of the MAR were captured by melt or incorporated into refractory restite probably in the

  9. Magmatic lithospheric heating and weakening during continental rifting: A simple scaling law, a 2-D thermomechanical rifting model and the East African Rift System

    NASA Astrophysics Data System (ADS)

    Schmeling, Harro; Wallner, Herbert

    2012-08-01

    Continental rifting is accompanied by lithospheric thinning and decompressional melting. After extraction, melt is intruded at shallower depth thereby heating and weakening the lithosphere. In a feedback mechanism this weakening may assist rifting and melt production. A one-dimensional kinematic lithospheric thinning model is developed including decompressional melting and intrusional magma deposition. The intrusional heating effect is determined as a function of thinning rate and amount, melting parameters, potential temperature, and the depth range of emplacement. The temperature increases approximately proportionally to the square root of the thinning rate and to the square of the supersolidus potential temperature. Simple scaling laws are derived allowing predicting these effects and the surface heat flux for arbitrary scenarios. Two-dimensional thermomechanical extension models are carried out for a multicomponent (crust-mantle) two-phase (melt-matrix) system with a rheology based on laboratory data including magmatic weakening. In good agreement with the 1-D kinematic models it is found that the lithosphere may heat up by several 100 K. This heating enhances viscous weakening by one order of magnitude or more. In a feedback mechanism rifting is dynamically enforced, leading to a significant increase of rift induced melt generation. Including the effect of lateral focusing of magma toward the rift axis the laws are applied to different segments of the East African Rift System. The amount of intrusional heating increases with maturity of the rift from O(10 K) to up to 200 K or 400 K at the Afar Rift depending on the depth range of the magmatic emplacement.

  10. Trench Outer Rise Flexure Models with Laterally Variable Plate Rigidity Derived from Oceanic Lithosphere Strength Profiles

    NASA Astrophysics Data System (ADS)

    Garcia, E. S. M.; Sandwell, D. T.; Bassett, D.

    2015-12-01

    We generate model bathymetry and free-air gravity grids for the seafloor seaward of subduction zones that capture the broad trends of deformation due to lithospheric flexure. By using a thin elastic plate formulation with rigidity variations along both horizontal dimensions and which accounts for the effects of plastic yielding, our models are able to reproduce the observed rapid change in curvature at the outer trench wall. Forward models for flexural deflection and plate rigidity are fitted to satellite altimetry-derived marine gravity anomalies jointly with shipboard sonar soundings and swath bathymetry data sets. The estimated parameters are the applied vertical shear and bending moment at the trench axis, and the data misfits are minimized with respect to the L1-norm subject to Tikhonov regularization for smooth variation of the load parameters along the strike of the trench. We perform pre-processing of the input data to enhance the recovery of the flexural signal. Short-wavelength features such as seamounts are isolated using a directional median filter and then excluded from the parameter estimation process. The advantage of adopting anisotropic filtering over similar methods that separate local scale topography from regional swells is that it provides consistent performance in feature detection with minimal user supervision. Our preliminary results from several examples show that the plate rigidity progressively decreases with increasing proximity to the trench axis. These zones of plate weakening correspond to the occurrence of trench-parallel seafloor fractures at the outer trench wall as seen in high-resolution bathymetry data. We seek to determine whether a correlation exists between the distribution or morphology of these fractures and the amount or trend in the reduction of plate rigidity for outer rise regions across the Pacific Rim.

  11. Evolution of the lithosphere in the Indian Ocean from combined earthquake and ambient noise tomography

    NASA Astrophysics Data System (ADS)

    Ma, Zhitu; Dalton, Colleen A.

    2017-01-01

    Rayleigh wave dispersion extracted from ambient seismic noise has been widely used to image crustal and uppermost mantle structure. Applications of this approach in continental settings are abundant, but there have been relatively few studies within ocean basins. By cross-correlating ambient noise recorded at broadband seismic stations around the Indian Ocean, we demonstrate the feasibility of extracting high-quality, long-period (10-30 mHz) Rayleigh waves that traverse the entire ocean basin. High-quality Rayleigh wave cross-correlation functions can be obtained from stacking waveforms over less than 2 years at land stations and less than 4 years at island stations. We show that adding the dispersion information extracted from ambient noise to a global earthquake data set can improve the resolution of phase velocity maps by about 20% in the northern Indian Ocean, where the station distribution is the best. We find that a plate cooling model with a potential temperature of 1450°C and plate thickness of 125 km can fit both the seismic observations and seafloor topography. The Seychelles-Mascarene Plateau is characterized by anomalously slow velocity at 30 mHz. The inclusion of ambient noise data in the tomographic inversion shifts the slow velocity anomaly into better agreement with the topographic relief, allowing us to estimate its crustal thickness and confirm that the plateau's elevation is supported by thick crust. The 10 and 20 mHz phase velocity maps show a strong asymmetry across the Central Indian Ridge that is best explained by eastward asthenospheric flow emanating from nearby hot spots.

  12. Evolution of young oceanic lithosphere and the meaning of seafloor subsidence rate

    NASA Astrophysics Data System (ADS)

    Korenaga, Tomoko; Korenaga, Jun

    2016-09-01

    Plate tectonics, a special class of mantle convection so far observed only on the Earth, is responsible for a vast array of geological processes, from the generation of continental crust to the modulation of atmospheric composition. Whereas conditions for its operation are still debated, the minimum requirement is generally thought that surface plates become denser than the underlying asthenosphere so that they can subduct. Recent studies, however, have raised the possibility that even mature oceanic plates remain buoyant because chemical buoyancy is too high to be overcome by negative thermal buoyancy, challenging the basic tenet of plate tectonics. Here we show that on the basis of new integrated geophysical and petrological modeling, oceanic plates do become negatively buoyant after ˜30 Myr. Our modeling also indicates that the seafloor would subside at a rate of ˜500 m Myr-1/2, which is considerably faster than the observed rate of ˜320 m Myr-1/2. We suggest that this discrepancy in subsidence rate is best explained by the combined effect of incomplete viscous relaxation within oceanic plates, radiogenic heat production in the convecting mantle, and the secular cooling of the Earth.

  13. Magmatic activity at Islas Marias Archipelago, Gulf of California: Oceanic lithosphere with gabbroic sills versus Jurassic-Cretaceous arc components.

    NASA Astrophysics Data System (ADS)

    Schaaf, P. E. G.; Solis-Pichardo, G.; Hernandez-Trevino, T.; Villanueva, D.; Arrieta, G. F.; Rochin, H.; Rodriguez, L. F.; Bohnel, H.; Weber, B.

    2015-12-01

    Islas Marias Archipelago consists of four islands located in the mouth of the Gulf of California. Lithologically three of them (Maria Madre, San Juanito, and Maria Cleofas) are quite similar with a 165-170 Ma metamorphic basement, 75-85 Ma intrusive and extrusive rocks, and a sedimentary sandstone cover, which according to its foraminiferous content recorded multiple uplift and subsidence events related to the opening of the Gulf. However, these units are absent on Maria Magdalena island which is positioned between the other islands. Here, instead, oceanic lithosphere with pillow lavas and gabbroic sills, intercalated with sandstones form the dominant outcrops. Their geochemical and isotopic characteristics are similar to N-MORB with epsilon Nd values around +10 and 87Sr/86Sr of 0.70290. The gabbros are not older than 22 Ma. Magdalena island was obviously uplifted separately from the other islands of the archipelago, probably along a now hidden transform fault system along the East Pacific Rise. Metamorphic and igneous rocks of the other islands can be correlated to lithologically similar units in the Los Cabos Block, Baja California, or to the continental margin units in Sinaloa, Nayarit and Jalisco states when looking at their geochemical and geochronological signatures. Paleomagnetic studies on 35 sampling sites from all 4 islands give evidence for relatively small scale tectonic movements.

  14. Oceanic provenance of lithospheric mantle beneath Lower Silesia (SW Poland) and the two kinds of its "Fe-metasomatism"

    NASA Astrophysics Data System (ADS)

    Puziewicz, Jacek; Matusiak-Małek, Magdalena; Ntaflos, Theodoros; Kukuła, Anna; Ćwiek, Mateusz

    2016-04-01

    Lusatia is surrounded to the West and South West by Al-richer domains (the first Al-rich orthopyroxene occurrences in mantle xenoliths are located in the Rhön Mts. to the West and in Upper Palatinate to the South-West. The low Al content in orthopyroxene, corresponding to that typical for the Lower Silesian European mantle domain, is characteristic for (1) oceanic mantle formed in the mid ocean ridges (MOR) and (2) mantle wedge affected by extreme melting in the supra-subduction zones (SSZ). The SSZ harzburgites contain usually orthopyroxene which is more Al-poor (< 2.0 wt. % Al2O3) than that of the MOR ones (2.0 - 6. 0 wt. % Al2O3; Bonatti & Michael 1989, EPSL 91, 297-311). Thus, we infer that the Lower Silesian SCLM originated rather in the MOR setting. The Lower Silesian B harzburgites were formed by reactive basaltic melt percolation, which lowered the forsterite content in olivine and Mg# in orthopyroxene ("Fe metasomatism"). The B1 harzburgites contain orthopyroxene which is Al poor (see above) irrespectively of forsterite content of coexisting olivine. Thus, the medium which led to the "Fe-metasomatism" must have been also Al-poor. This criterion is met by tholeiitic basaltic melts which originate by multiple polybaric mantle melting in MOR environment. Their percolation in oceanic mantle leads to production of low-Al orthopyroxene (e. g in the peridotites from East Pacific Rise, Dick & Natland 1996 Proc ODP Sci Res, 147, 103-234). Therefore, we suggest that the B1 harzburgites originated by "Fe-metasomatism" also in the MOR setting. The coexistence of A and B1 harzburgites suggests that they represent lithospheric mantle generated in the magma-rich, thus rather fast-spreading, MOR. Textural relationships show that the Al-enriched B2 harzburgites were also affected by "Fe metasomatism", but by alkaline basaltic melt percolating in SCLM during Cenozoic rifting. The crust overlying western part of the Lower Silesian domain of European SCLM belongs to the easternmost

  15. Seismic attenuation and velocity constraints on the formation of oceanic lithosphere and the origin of the low-velocity zone

    NASA Astrophysics Data System (ADS)

    Yang, Y.; Forsyth, D. W.; Weeraratne, D. S.

    2005-12-01

    attenuation in the hot, young lithosphere is less pronounced. In addition to seismic reduction owing to attenuation, about 1% partial melting is required to explain the minimum value of shear wave velocities in the low velocity zone beneath young seafloor. The ~1% melt is produced in the depth range between the (°)wet solidus and the (°)dry solidus due to the presence of relatively small amount of water. Our seismic observations provide strong constraints on the argument that the structure of oceanic plates is controlled by compositional as well as thermal parameters.

  16. Widespread Occurrence of Zircon in Slow- and Ultraslow Spreading Ocean Crust: A Tool for Studying Ocean Lithospheric Processes

    NASA Astrophysics Data System (ADS)

    Grimes, C. B.; John, B. E.; Cheadle, M. J.; Schwartz, J. J.

    2005-12-01

    The presence of igneous zircon in oceanic gabbro and peridotite provides a new opportunity to constrain absolute ages, and the processes and rates of crustal accretion in oceanic environments. Our recent investigations show zircon to be common in slow and ultraslow spreading oceanic crust including several locations along the Mid-Atlantic Ridge (MAR) and Southwest Indian Ridge (SWIR), and in rock types ranging from trondjhemite dikes to peridotite. Zircon is typically found in felsic intrusions and oxide gabbro, and in many cases may be due to late stage saturation in small pockets of residual melt. We report the morphologic and chemical characteristics of zircon grains collected from >100 rock samples recovered both from the seafloor by manned submersible and ROV, and with depth by ODP/IODP drilling. Grains range from euhedral and faceted to anhedral and fractured, with internal zonation that may be homogeneous, concentric, or patchy, and rarely contain relict cores. Sizes range from <5 μm to >1 mm. Measurements of major, minor, and trace element concentrations and high-resolution Pb/U ages were collected with the SHRIMP-RG. Chondrite-normalized rare earth element (REE) patterns for more than 50 zircon grains are uniform in shape and closely resemble patterns for known terrestrial igneous zircon. This is in contrast to mantle affinity zircon (e.g. kimberlite), which typically show depleted and relatively unfractionated patterns. Observed total REE concentrations range from 330-3765 ppm. Patterns are convex upward and rise sharply towards the HREE, with normalized Sm/La ratios = 16-320 and Lu/Gd ratios = 20-51. Positive Ce and negative Eu anomalies are ubiquitous. Hf abundances range from 5988 to 14,266 ppm. Other elements occurring at minor abundance levels include Y (463-6949 ppm), P (253-2288 ppm), U (7-2827 ppm), and Th (3-7403 ppm). Preliminary Ti concentrations range from 13 to 270 ppm, indicating crystallization temperatures of 765 to 1147°C based on Ti in

  17. The role of mantle temperature and lithospheric thickness during initial oceanic crust production: numerical modelling constraints from the southern South Atlantic

    NASA Astrophysics Data System (ADS)

    Taposeea, C.; Armitage, J. J.; Collier, J.

    2015-12-01

    Evidence from seaward dipping reflector distributions has recently suggested that segmentation plays a major role in the pattern of volcanism during breakup, particularly in the South Atlantic. At a larger scale, variations in mantle temperature and lithosphere thickness can enhance or reduce volcanism. To understand what generates along strike variation of volcanism at conjugate margins, we measure the thickness of earliest oceanic crust in the South Atlantic, south of the Walvis and Rio Grande ridges. We use data from 29 published wide-angle and multichannel seismic profiles and at least 14 unpublished multichannel seismic profiles. A strong linear trend between initial oceanic crustal thickness and distance from hotspot centres, defined as the commencement of Walvis and Rio Grande ridges, with a regression coefficient of 0.7, is observed. At 450km south of the Walvis Ridge, earliest oceanic crustal thickness is found to be 11.7km. This reduces to 7.0km in the south at a distance of 1,420km. Such a linear trend suggests rift segmentation plays a secondary role on volcanism during breakup. To explore the cause of this trend, we use a 2D numerical model of extension capable of predicting the volume and composition of melt generated by decompressional melting during extension to steady state seafloor spreading. We explore the effect of both mantle temperature and lithosphere thickness on melt production with a thermal anomaly (hot layer) 100km thick located below the lithosphere with an excess temperature of 50-200°C, and lithospheric thickness ranging from 125-140km, covering the thickness range estimated from tomographic studies. By focusing on a set of key seismic profiles, we show a reduction in hot layer temperature is needed in order to match observed oceanic crustal thickness, even when the effect of north to south variations in lithosphere thickness are included. This model implies that the observed oceanic thickness requires the influence of a hot layer up

  18. The Generation of Oceanic Lithosphere in an Embryonic Oceanic Crust : the Example of the Chenaillet Ophiolite in the Western Alps

    NASA Astrophysics Data System (ADS)

    Masini, E.; Manatschal, G.; Muntener, O.

    2007-12-01

    The Chenaillet Ophiolite exposed in the Franco-Italian Alps represents a well-preserved ocean-floor sequence that was only weakly affected by later Alpine convergence. Based on the similarity between rock types and structures reported from ultraslow spreading ridges and those observed in the Chenaillet Ophiolite, it may represent a field analogue for slow to ultraslow spreading ridges such as the Gakkel Ridge or the Southwest Indian Ridge. Mapping of the Chenaillet Ophiolite enabled to identify an oceanic detachment fault that extends over a surface of about 16 km2 capping exhumed mantle and gabbros onto which clastic sediments have been deposited. The footwall of the detachment is formed by mafic and ultramafic rocks. The mantle rocks are strongly serpentinized lherzolites and subordinate harzburgites and dunites. Microstructures reminiscent of impregnation, and cpx major and trace element chemistry indicate that spinel peridotite is (locally) replaced by plagioclase-bearing assemblages. Pyroxene thermometry on primary minerals indicates high temperatures of equilibration ( max 1200°C) for the mantle rocks. Gabbros range from troctolite and olivine-gabbros to Fe-Ti gabbros and show clear evidence of syn-magmatic deformation, partially obliterated by retrograde amphibolite and low-grade metamorphic conditions. In sections perpendicular to the detachment within the footwall, syn-tectonic gabbros and serpentinized peridotites grade over some tens of meters into cataclasites that are capped by fault gouges. Petro-structural investigations of the fault rocks reveal a syn-tectonic retrograde metamorphic evolution. Clasts of dolerite within the fault zone suggest that detachment faulting was accompanied by magmatic activity. Hydrothermal alteration is indicated by strong mineralogical and chemical modifications. Gabbro and serpentinized peridotite, together with serpentinite cataclasites occur as clasts in tectono-sedimentary breccias overlying directly the detachment

  19. Lithospheric cooling and thickening as a basin forming mechanism

    NASA Astrophysics Data System (ADS)

    Holt, Peter J.; Allen, Mark B.; van Hunen, Jeroen; Bjørnseth, Hans Morten

    2010-12-01

    Widely accepted basin forming mechanisms are limited to flexure of the lithosphere, and lithospheric stretching followed by cooling and thermal subsidence. Neither of these mechanisms works for a group of large basins, sometimes known as "intracontinental sags". In this paper we investigate cooling and thickening of initially thin lithosphere as a basin forming mechanism, by a combination of forward modelling and a backstripping study of two Palaeozoic North African basins: Ghadames and Al Kufrah. These are two of a family of basins, once unified, which lie over the largely accretionary crust of North Africa and Arabia. Such accretionary crust tends to be juvenile, consisting of amalgamated island arcs, accretionary prisms and melanges, and typically has near-normal crustal thicknesses but initially thin mantle lithosphere. Post-accretion subsidence is modelled using a plate cooling model similar to cooling models for oceanic lithosphere. The crustal composition and thickness used in the models are varied around average values of accretionary crust to represent likely heterogeneity. The model allows the lithosphere to thicken as it cools and calculates the resulting isostatic subsidence. Water-loaded tectonic subsidence curves from these forward models are compared to tectonic subsidence curves produced from backstripped wells from Al Kufrah and Ghadames Basins. A good match between the subsidence curves for the forward model and backstripping is produced when the best estimates for the crustal structure, composition and the present day thickness of the lithosphere for North Africa are used as inputs for the forward model. The model produces sediment loaded basins of 2-7 km thickness for the various crustal assemblies over ~ 250 Myr. This shows that lithospheric cooling provides a viable method for producing large basins with prolonged subsidence, without the need for initial extension, provided the condition of initially thin mantle lithosphere is met.

  20. Thick lithosphere, deep crustal earthquakes and no melt: a triple challenge to understanding extension in the western branch of the East African Rift

    NASA Astrophysics Data System (ADS)

    O'Donnell, J. P.; Selway, K.; Nyblade, A. A.; Brazier, R. A.; Tahir, N. El; Durrheim, R. J.

    2016-02-01

    Geodynamic models predict that rifting of thick, ancient continental lithosphere should not occur unless it is weakened by heating and magmatic intrusion. Therefore, the processes occurring along sections of the western branch of the East African Rift, where ˜150 km thick, Palaeoproterozoic lithosphere is rifting with no surface expression of magmatism, are a significant challenge to understand. In an attempt to understand the apparently amagmatic extension we probed the regional uppermost mantle for signatures of thermal alteration using compressional (Vp) and shear (Vs) wave speeds derived from Pn and Sn tomography. Pervasive thermal alteration of the uppermost mantle and possibly the presence of melt can be inferred beneath the Rungwe volcanic centre, but no signatures on a similar scale were discerned beneath amagmatic portions of the western rift branch encompassing the southern half of the Lake Tanganyika rift and much of the Rukwa rift. In this region, Vp and Vs wave speeds indicate little, if any, heating of the uppermost mantle and no studies have reported dyking. Vp/Vs ratios are consistent with typical, melt-free, olivine-dominated upper mantle. Although our resolution limit precludes us from imaging potential localised magmatic intrusions with dimensions of tens of kilometres, the absence of surface volcanism, the amagmatic upper crustal rupture known to have occurred at disparate locations on the western branch, the presence of lower crustal seismicity and the low temperatures implied by the fast seismic wave speeds in the lower crust and uppermost mantle in this region suggests possible amagmatic extension. Most dynamic models predict that this should not happen. Indeed even with magmatic intrusion, rifting of continental lithosphere >100 km thick is considered improbable under conditions found on Earth. Yield strength envelopes confirm that currently modelled stresses are insufficient to produce the observed deformation along these portions of the

  1. The meteorology of the Western Indian Ocean, and the influence of the East African Highlands.

    PubMed

    Slingo, Julia; Spencer, Hilary; Hoskins, Brian; Berrisford, Paul; Black, Emily

    2005-01-15

    This paper reviews the meteorology of the Western Indian Ocean and uses a state-of-the-art atmospheric general circulation model to investigate the influence of the East African Highlands on the climate of the Indian Ocean and its surrounding regions. The new 44-year re-analysis produced by the European Centre for Medium range Weather Forecasts (ECMWF) has been used to construct a new climatology of the Western Indian Ocean. A brief overview of the seasonal cycle of the Western Indian Ocean is presented which emphasizes the importance of the geography of the Indian Ocean basin for controlling the meteorology of the Western Indian Ocean. The principal modes of inter-annual variability are described, associated with El Nino and the Indian Ocean Dipole or Zonal Mode, and the basic characteristics of the subseasonal weather over the Western Indian Ocean are presented, including new statistics on cyclone tracks derived from the ECMWF re-analyses. Sensitivity experiments, in which the orographic effects of East Africa are removed, have shown that the East African Highlands, although not very high, play a significant role in the climate of Africa, India and Southeast Asia, and in the heat, salinity and momentum forcing of the Western Indian Ocean. The hydrological cycle over Africa is systematically enhanced in all seasons by the presence of the East African Highlands, and during the Asian summer monsoon there is a major redistribution of the rainfall across India and Southeast Asia. The implied impact of the East African Highlands on the ocean is substantial. The East African Highlands systematically freshen the tropical Indian Ocean, and act to focus the monsoon winds along the coast, leading to greater upwelling and cooler sea-surface temperatures.

  2. Seismic azimuthal anisotropy in the oceanic lithosphere and asthenosphere from broadband surface wave analysis of OBS array records at 60 Ma seafloor

    NASA Astrophysics Data System (ADS)

    Takeo, A.; Kawakatsu, H.; Isse, T.; Nishida, K.; Sugioka, H.; Ito, A.; Shiobara, H.; Suetsugu, D.

    2016-03-01

    We analyzed seismic ambient noise and teleseismic waveforms of nine broadband ocean bottom seismometers deployed at a 60 Ma seafloor in the southeastward of Tahiti island, the South Pacific, by the Tomographic Investigation by seafloor ARray Experiment for the Society hotspot project. We first obtained one-dimensional shear wave velocity model beneath the array from average phase velocities of Rayleigh waves at a broadband period range of 5-200 s. The obtained model shows a large velocity reduction at depths between 40 and 80 km, where the lithosphere-asthenosphere boundary might exist. We then estimated shear wave azimuthal anisotropy at depths of 20-100 km by measuring azimuthal dependence of phase velocities of Rayleigh waves. The obtained model shows peak-to-peak intensity of the azimuthal anisotropy of 2%-4% with the fastest azimuth of NW-SE direction both in the lithosphere and asthenosphere. This result suggests that the ancient flow frozen in the lithosphere is not perpendicular to the strike of the ancient mid-ocean ridge but is roughly parallel to the ancient plate motion at depths of 20-60 km. The fastest azimuths in the current asthenosphere are subparallel to current plate motion at depths of 60-100 km. Additional shear wave splitting analysis revealed possible perturbations of flow in the mantle by the hot spot activities and implied the presence of azimuthal anisotropy in the asthenosphere down to a depth of 190-210 km.

  3. Metamorphosed oceanic lithosphere from the Chunky Gal Mountain complex, Blue Ridge province, North Carolina

    SciTech Connect

    Ranson, W.A.; Garihan, J.M. . Dept. of Geology)

    1993-03-01

    Closely associated dunite and layered troctolite, gabbro, and anorthosite from the Chunky Gal Mountain mafic-ultramafic complex suggest a related ocean floor origin for these lithologies, with regional emplacement by the pre-metamorphic Hayesville-Fries thrust of Taconic age. Anhydrous mineral assemblages of dunitic and troctolitic rocks were more resistant to granulite and upper amphibolite metamorphic episodes, retaining much of their original mineralogy. Dunite consists of fresh polycrystalline olivine with local development of anthophyllite, serpentine, and talc along fractures. Dunite adjacent to mafic amphibolites of the complex contains distinctive cm-scale bands or layers of recrystallized plagioclase( ) of uncertain affinity, possibly veins or rhythmic layers. Troctolitic rocks display reaction textures around fresh olivine and plagioclase. Orthopyroxene growing normal to olivine grain boundaries forms an inner corona, in turn surrounded by a complex symplectite of Cpx + Plag [+-] Grt [+-] Spl. Gabbroic rocks show nearly complete replacement of original mafic minerals. Orthopyroxene survives in a few gabbros but mostly has been replaced by emerald green alumino-magnesio-hornblende. Calcic plagioclase is abundant as subhedral crystals or as oval, polycrystalline clots and pink corundum constitutes an accessory phase. A possible reaction resulting in the observed aluminous assemblage is: Na-Plag + Opx + Di + Spl + fluid = Mg-Hbl + Crn + Ca-Plag. Anorthosites occur as layers 10--50 cm in width within layered troctolites and consist of beautifully recrystallized plagioclase with seriate texture and minor amounts of alumino-magnesio hornblende occurring as fine-grained clots. Contacts between anorthosite and troctolite display the same sort of symplectite formed as an outer corona around olivine in the troctolite.

  4. Semi-brittle flow during dehydration of lizardite chrysotile serpentinite deformed in torsion: Implications for the rheology of oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Hirose, Takehiro; Bystricky, Misha; Kunze, Karsten; Stünitz, Holger

    2006-09-01

    Serpentinite is a significant component of the oceanic lithosphere and plays an important role in subduction dynamics. However, the rheology of serpentine-bearing rocks is poorly understood, especially at large strains and during the dehydration of serpentine. We have investigated the mechanical behavior and microstructural evolution of serpentinite during dehydration reaction to olivine, talc and water at temperatures of 550 and 600 °C, pressures of 300 and 400 MPa and shear strain rates of 1 × 10 - 5 to 1 × 10 - 4 s - 1 under drained conditions. The non-coaxial deformation experiments were performed in a gas-medium apparatus equipped with a torsion system to shear strains of up to 3.3. Strong crystallographic preferred orientations (CPO) of partially dehydrated serpentine, shear localization through the development of S-C structures and widespread microfracturing in deformed specimens indicate that deformation took place in the semi-brittle field. Microstructural observations reveal that the CPO results from slip and rotation of (001) planes in partially dehydrated lizardite, possibly assisted by fluid released by the reaction. Despite the development of a strong CPO and shear localization, strain hardening accompanied by repeated transitory stress drops of a few MPa was observed and may be explained by the following combined effects: (1) progressive decrease of excess local pore pressures due to specimen dilatancy and the formation of pore networks during the reaction, leading to an increase in effective pressure, (2) subsequent collapse of this pore space by shear-enhanced compaction, resulting in work hardening, and (3) formation of a mechanically stronger assemblage of reaction products. The experiments imply that the bulk strength of a serpentinized subducting slab increases during dehydration as fluid escapes from the slab, while local embrittlement and shear localization take place.

  5. 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.

  6. Hyperextension of continental to oceanic-like lithosphere: The record of late gabbros in the shallow subcontinental lithospheric mantle of the westernmost Mediterranean

    NASA Astrophysics Data System (ADS)

    Hidas, Károly; Varas-Reus, Maria Isabel; Garrido, Carlos J.; Marchesi, Claudio; Acosta-Vigil, Antonio; Padrón-Navarta, José Alberto; Targuisti, Kamal; Konc, Zoltán

    2015-05-01

    lithospheric section. These data suggest that gabbro-forming melts in the Betic Peridotite record a mantle igneous event at very shallow depths and provide evidence for the hyperextension of the continental lithosphere compatible with extreme backarc basin extension induced by the slab rollback of the Cenozoic subduction system in the westernmost Mediterranean.

  7. 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.

  8. A comparison of surface wave tomography in the Atlantic Ocean with the plate model: mismatches and implications for the lithosphere-asthenosphere system

    NASA Astrophysics Data System (ADS)

    Fishwick, S.; Crosby, A. G.

    2009-12-01

    The oceans should be one of the most straightforward regions within which to investigate the lithosphere-asthenosphere boundary, because their thermal structure appears to be so simple. However, this assertion assumes our present understanding of the oceanic lithosphere is correct, which is derived principally from observations of heat flow and subsidence beneath the ocean floor. These observations show large positive deviations from half-space cooling models after 60-80 Ma, which indicate that conductive cooling does not continue forever as the plate moves away from the ridge. Instead, data from the oldest ocean floor are better fit by a thermal plate model with a thickness of 90-100 km, which is gently modulated by convection within the upper mantle. The physical interpretation of the fixed temperature basal boundary condition is that heat is supplied to the base of the plate by small-scale convection in the thermal boundary layer underlying the rigid lid. We compare the expected shear velocities given the plate cooling model with the velocities observed from surface wave tomography. Tomographic data sets for both Africa and South America have been combined to place constraints on the velocity structure beneath the Atlantic Ocean. The velocities observed in the tomography are faster than expected at lithospheric depths, and an age-velocity relationship continues to around 125km - a depth at which the plate model predicts uniform temperatures. Some aspect of these discrepancies could be due to the vertical parameterisation and smearing within the surface wave tomography. To test this possibility, we compare the dispersion characteristics for the theoretical model with the surface wave dispersion data, and also observe a similar mismatch. A more likely source of error is the conversion between velocity and temperature, particularly given our limited knowledge of the attenuation structure of the region. These results have important implications for our understanding

  9. Evaluating CMIP5 Models' Representation of Oceanic Drivers of North African Climate

    NASA Astrophysics Data System (ADS)

    Notaro, M.; Wang, F.; Yu, Y.; Mao, J.; Shi, X.; Wei, Y.

    2015-12-01

    North Africa is highly vulnerable to hydrologic variability and extremes, including impacts of climate change. Prior North African studies largely disagree regarding the dominant oceanic drivers of the region's hydrologic variability and the likely impacts of climate change. Here, we aim to apply a multivariate statistical method, the Generalized Equilibrium Feedback Assessment (GEFA), to evaluate the representation of the oceanic drivers of North African rainfall in the Coupled Model Intercomparison Project Phase Five (CMIP5) in comparison to a GEFA-based observational benchmark. The reliability of the statistical GEFA method is first evaluated against dynamical experiments within the Community Earth System Model (CESM), with initial focus on the atmospheric response to sea-surface temperature anomalies in the tropical Pacific and Indian Oceans. In order to estimate the minimum number of years of data needed to obtain stable GEFA response estimates to individual SST forcings, given large atmospheric internal variability, we apply GEFA to data records of varying durations from the CESM Large Ensemble historical simulations. After validating the GEFA method, we apply the statistical approach to both CESM and a range of CMIP5 models in order to elucidate the models' representation of oceanic drivers of North African rainfall. The observed oceanic forcings of North African rainfall, as investigated in a parallel study, serves as the benchmark for evaluating each model's performance and credibility in terms of future climate projections.

  10. Multidecadal variability in East African hydroclimate controlled by the Indian Ocean.

    PubMed

    Tierney, Jessica E; Smerdon, Jason E; Anchukaitis, Kevin J; Seager, Richard

    2013-01-17

    The recent decades-long decline in East African rainfall suggests that multidecadal variability is an important component of the climate of this vulnerable region. Prior work based on analysing the instrumental record implicates both Indian and Pacific ocean sea surface temperatures (SSTs) as possible drivers of East African multidecadal climate variability, but the short length of the instrumental record precludes a full elucidation of the underlying physical mechanisms. Here we show that on timescales beyond the decadal, the Indian Ocean drives East African rainfall variability by altering the local Walker circulation, whereas the influence of the Pacific Ocean is minimal. Our results, based on proxy indicators of relative moisture balance for the past millennium paired with long control simulations from coupled climate models, reveal that moist conditions in coastal East Africa are associated with cool SSTs (and related descending circulation) in the eastern Indian Ocean and ascending circulation over East Africa. The most prominent event identified in the proxy record--a coastal pluvial from 1680 to 1765--occurred when Indo-Pacific warm pool SSTs reached their minimum values of the past millennium. Taken together, the proxy and model evidence suggests that Indian Ocean SSTs are the primary influence on East African rainfall over multidecadal and perhaps longer timescales.

  11. An Investigation of Anatolian-African Subduction Zone in Southwestern Aegean:Lithospheric Structure Beneath Isparta Angle(IA)and the Surroundings from Surface Wave Tomography

    NASA Astrophysics Data System (ADS)

    Teoman, U. M.; Sandvol, E. A.; Turkelli, N.

    2009-12-01

    Our primary objective is to obtain the lithospheric structure of Anatolian-African Subduction zone and the surroundings including the Isparta Angle(IA) from Phase velocity inversion of Rayleigh waves. IA is seismically active and formed by the intersection of two very different subduction zones: The Hellenic arc to the west and the Cyprian arc to the east. The Hellenic arc is characterized by relatively steep, retreating subduction, whereas the Cyprus arc appears to involve a shallower subduction. The Geometric difference between Hellenic Arc and Cyprus Arc might indicate a tear in the subducting African Lithosphere beneath the Anatolian Plate responsible for the active deformation. A temporary seismic network consisting of 10 3-component BB stations were installed in August 2006 with the support from University of Missouri and 9 more stations in March 2007 in addition to the 21 existing permanent stations of Kandilli Observatory and Earthquake Research Institute(KOERI) and two from Süleyman Demirel University(SDU). 8 stations from Geofon Network were also included in the data to extend the station coverage. We used earthquakes in a distance range of 30-120 degrees with body wave magnitude larger than 5.5. Signal to noise ratio, azimuthal coverage of events, and coherence from station to station were considered during event selection.80 events provided high-quality data for our analysis. The distribution of events shows a good azimuthal coverage, which is important for resolving both lateral heterogeneity and azimuthal anisotropy. We adopted a two-plane-wave inversion technique of Forsyth and Li(2003) to simultaneously solve for the incoming wave field and phase velocity. The wave field is represented by the sum of two plane waves with initially unknown phase, amplitude and propagation direction.This relatively simpler representation of a more complex wavefield provided the pattern of amplitude variations effectively in many cases.This method will also help us

  12. Petrogenesis of Middle-Late Triassic volcanic rocks from the Gangdese belt, southern Lhasa terrane: Implications for early subduction of Neo-Tethyan oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Wang, Chao; Ding, Lin; Zhang, Li-Yun; Kapp, Paul; Pullen, Alex; Yue, Ya-Hui

    2016-10-01

    The Gangdese belt is dominantly composed of igneous rocks that formed during the northward subduction of Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane and has played a crucial role in understanding the pre-collisional evolution of southern Tibet. This paper presents new geochronological and geochemical (whole-rock major and trace element and Sr-Nd and zircon Hf isotope) data for recently identified volcanic rocks exposed in Changguo area, southernmost part of the Lhasa terrane. Zircon U-Pb dating from six samples yields consistent ages of 237.1 ± 1.1 Ma to 211.7 ± 1.5 Ma for magma emplacement through volcanic eruption, showing the Middle-Late Triassic magmatic activity in the southernmost Gangdese Belt. The Changguo volcanic rocks are mainly composed of basaltic and andesitic rocks and exhibit LILE enrichment and HFSE depletion. They also exhibit relatively uniform Nd-Hf isotopic compositions (εNd(t) = + 5.20 to + 7.74 and εHf(t)zircon = + 10.2 to + 15.9). The basaltic magmas were likely sourced from partial melting of sub-arc mantle wedge that was metasomatized by not only the aqueous fluid derived from subducting altered oceanic crust but also hydrous melt derived from subducting seafloor sediments, and subsequently experienced fractional crystallization and juvenile crustal contamination during ascent. The andesitic magmas were generated by partial melting of mafic-ultramafic metasomes through melt/fluid-peridotite reaction at slab-mantle interface. Taking into account the temporal and spatial distribution of the Early Mesozoic magmatic rocks and regional detrital zircon data, we further propose that the northward subduction of Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane commenced by Middle Triassic.

  13. Petrogenesis of fertile mantle peridotites from the Monte del Estado massif (southwest Puerto Rico): a preserved section of Proto-Caribbean oceanic lithospheric mantle?

    NASA Astrophysics Data System (ADS)

    Marchesi, Claudio; Jolly, Wayne T.; Lewis, John F.; Garrido, Carlos J.; Proenza, Joaquín. A.; Lidiak, Edward G.

    2010-05-01

    The Monte del Estado massif is the largest and northernmost serpentinized peridotite belt in southwest Puerto Rico. It is mainly composed of spinel lherzolite and minor harzburgite with variable clinopyroxene modal abundances. Mineral and whole rock major and trace element compositions of peridotites coincide with those of fertile abyssal peridotites from mid ocean ridges. Peridotites lost 2-14 wt% of relative MgO and variable amounts of CaO by serpentinization and seafloor weathering. HREE contents in whole rock indicate that the Monte del Estado peridotites are residues after low to moderate degrees (2-15%) of fractional partial melting in the spinel stability field. However, very low LREE/HREE and MREE/HREE in clinopyroxene cannot be explained by melting models of a spinel lherzolite source and support that the Monte del Estado peridotites experienced initial low fractional melting degrees (~ 4%) in the garnet stability field. The relative enrichment of LREE in whole rock is not due to secondary processes but probably reflects the capture of percolating melt fractions along grain boundaries or as microinclusions in minerals, or the presence of exotic micro-phases in the mineral assemblage. We propose that the Monte del Estado peridotite belt represents a section of ancient Proto-Caribbean (Atlantic) lithospheric mantle originated by seafloor spreading between North and South America in the Late Jurassic-Early Cretaceous. This portion of oceanic lithospheric mantle was subsequently trapped in the forearc region of the Greater Antilles paleo-island arc generated by the northward subduction of the Caribbean plate beneath the Proto-Caribbean ocean. Finally, the Monte del Estado peridotites belt was emplaced in the Early Cretaceous probably as result of the change in subduction polarity of the Greater Antilles paleo-island arc without having been significantly modified by subduction processes.

  14. Trace element behaviour during serpentinisation/deserpentinisation of an eclogitised oceanic lithosphere: a LA-ICPMS study of the Lanzo ultramafic massif (Western Alps)

    NASA Astrophysics Data System (ADS)

    Debret, B.; Andreani, M.; Godard, M.; Nicollet, C.; Schwartz, S.

    2012-04-01

    Serpentinites are a major component of the oceanic lithosphere. During oceanic mantle hydration and alteration, they trap fluid mobile elements and then play a major role in the fluid and elemental transfer occurring between the dehydrating slab and the mantle wedge in subduction context. The Lanzo massif is an eclogitised oceanic lithosphere that records different serpentinisation and de-serpentinisation steps, from oceanic mantle denudation, to subduction prograde metamorphism, up to serpentine dehydration, and finally retrograde metamorphism during exhumation. Thus, it constitutes a suitable place to study the chemical mass transfer associated with serpentinites during cycling of the oceanic lithosphere. Oceanic serpentinisation is preserved in slightly serpentinised peridotites where the lizardite crystallises as veins crossing orthopyroxene and olivine or, when the hydration is more important, as mesh and bastite textures in serpentinised channels of 1-2mm of width. The trace element composition of lizardite veins is similar to that of the host mineral. It suggests that their composition is inherited from the primary mantle phases. Mesh and bastite trace element compositions and patterns are homogenous at the scale of the thin section and are intermediate between orthopyroxene and olivine. Lizardites have a higher B content (2-75 ppm) and display a positive Eu anomaly (EuN/Eu*=1.74-7.54) relative to primary phases. In the slightly serpentinised peridotites, prograde antigorites crystallised during subduction as veins crossing all primary minerals and lizardites. Their trace element patterns are homogenous in one thin section. They are marked by flat HREE patterns (GdN/YbNPM=0.45-1.18) close to clinopyroxene (GdN/YbNPM=1.08-1.45). This suggests that clinopyroxene destabilisation is contemporaneous with antigorite crystallisation. The composition of antigorite veins is close to that of the host mineral. In the massive serpentinites, where previous phases are

  15. An Oceanic Perspective on the African Humid Period

    NASA Astrophysics Data System (ADS)

    Cole, J. M.; Demenocal, P. B.; Goldstein, S. L.; Hemming, S. R.; Grousset, F. E.; Eglinton, T. I.; Wagner, T.

    2008-12-01

    Data gathered from cores drilled off NW Africa have been instrumental in understanding recent climate change in northern Africa, including the most recent African Humid Period (AHP). Early work by deMenocal et al. (2000, Quat. Sci. Rev. 19: 347-361) showed that dust (as terrigenous percent) increased and then decreased abruptly at the onset and termination of the AHP, and linked that on/off switch to a threshold crossing in precessional index. However, this perspective differs from recent lacustrine and palynological evidence from Lake Yoa in the eastern Sahara suggesting a more gradual transition out of the AHP (Kroepelin et al., 2008, Science 320: 765-768). We will present major and trace element and radiogenic isotope data from the last ~25 kyr at ODP 658C off Mauritania. Large and abrupt shifts, notably in Sr isotope ratios, are contemporaneous with changes observed in dust flux and cannot be explained simply by changes in geologic source terrane or grain size sorting that might result from a shift in wind direction or wind speed. We envisage the addition of highly weathered, authigenic mineral phase(s) formed in lakes during the AHP. Additionally, we will present new data from the recent CHEETA (Changes in the Holocene Environment of the Eastern Tropical Atlantic) sediment coring cruise, a transect of 28 coring stations from Gibraltar to Senegal. Preliminary results suggest that the abrupt termination of the AHP near 5.5 ka BP can be traced from Senegal to the Canary Islands (roughly 18-28°N), suggesting that the end of the AHP was indeed abrupt along the western African margin whereas it was more gradual in the eastern Sahara.

  16. The lithosphere

    SciTech Connect

    Not Available

    1983-01-01

    This document is the report of a week-long workshop on problems relating to the interpretations of the composition and dynamics of the lithosphere. A wide range of topics was discussed, dealing not only with the lithosphere itself, but also with possible interactions between the lithosphere and underlying mantle, down to and including the core-mantle boundary zone. Emphasis, very broadly, was on the physical and chemical properties of the lower crust and the subcrustal lithosphere: the physical and chemical characteristics of the prominent seismic discontinuities down to the core-mantle boundary; the nature and patterns of possible convection within the mantle and its relation to the generation, subduction, and intermixing of lithospheric and mantle material; the location and nature and evolution of reservoirs supplying magmas to the crust; and the various models that have been proposed to account for the location, nature, and geological history of these magma reservoirs. The general applicability of the plate tectonics model was assumed, but virtually every widely accepted explanation for the dynamics of that model and of possible unrelated phenomena such as deep-mantle plumes and hot spots was brought into question. 83 refs., 19 figs.

  17. 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.

  18. African hot spot volcanism: small-scale convection in the upper mantle beneath cratons.

    PubMed

    King, S D; Ritsema, J

    2000-11-10

    Numerical models demonstrate that small-scale convection develops in the upper mantle beneath the transition of thick cratonic lithosphere and thin oceanic lithosphere. These models explain the location and geochemical characteristics of intraplate volcanos on the African and South American plates. They also explain the presence of relatively high seismic shear wave velocities (cold downwellings) in the mantle transition zone beneath the western margin of African cratons and the eastern margin of South American cratons. Small-scale, edge-driven convection is an alternative to plumes for explaining intraplate African and South American hot spot volcanism, and small-scale convection is consistent with mantle downwellings beneath the African and South American lithosphere.

  19. The birth and death of the oceanic lithosphere: Geochemical and tectonic investigations of the Juan de Fuca Ridge and Mariana Trench

    NASA Astrophysics Data System (ADS)

    Hulme, Samuel M.

    The research I conducted for my dissertation addresses specifically: (1) the role of fluid alteration of the ocean crust during its maturing stages near a spreading center using the Juan de Fuca Ridge as the model; and (2) the role of fluids in the demise of the crust as it is recycled within a subduction zone, using the non-accretioanry Mariana system as the model. The first 2 chapters detail the progressive alteration of a region of young oceanic crust (at the Juan de Fuca Ridge) by combining the results of multiple deep-sea drilling legs, long-term borehole observatory sampling, and hydrothermal vent chemistry. The third chapter examines fluid transfer during serpentinization of the suprasubduction lithosphere, based on the relationship between trace-element patterns and serpentine phases in serpentinite mud pore fluids and serpentinite mud/rocks collected at serpentinite mud volcanoes on the Mariana forearc. The final chapter presents detailed bathymetric and Hybrid Remote Operated Vehicle (HROV) Nereus surveys conducted in the deepest regions of the Mariana Trench (and therefore the world) that are interpreted to explain the regional tectonic processes and to guide future exploration efforts. These new efforts may lead to further discoveries of unique geologic features that will allow researchers to better understand the ongoing evolution of the Earth.

  20. Formation and evolution of a metasomatized lithospheric root at the motionless Antarctic plate: the case of East Island, Crozet Archipelago (Indian Ocean)

    NASA Astrophysics Data System (ADS)

    Meyzen, Christine; Marzoli, Andrea; Bellieni, Giuliano; Levresse, Gilles

    2016-04-01

    Sitting atop the nearly stagnant Antarctic plate (ca. 6.46 mm/yr), the Crozet archipelago midway between Madagascar and Antarctica constitutes a region of unusually shallow (1543-1756 m below sea level) and thickened oceanic crust (10-16.5 km), high geoid height, and deep low-velocity zone, which may reflect the surface expression of a mantle plume. Here, we present new major and trace element data for Quaternary sub-aerial alkali basalts from East Island, the easterly and oldest island (ca. 9 Ma) of the Crozet archipelago. Crystallization at uppermost mantle depth and phenocryst accumulation have strongly affected their parental magma compositions. Their trace element patterns show a large negative K anomaly relative to Ta-La, moderate depletions in Rb and Ba with respect to Th-U, and heavy rare earth element (HREE) depletions relative to light REE. These characteristics allow limits to be placed upon the composition and mineralogy of their mantle source. The average trace element spectrum of East Island basalts can be matched by melting of about 2 % of a garnet-phlogopite-bearing peridotite source. The stability field of phlogopite restricts melting depth to lithospheric levels. The modelled source composition requires a multistage evolution, where the mantle has been depleted by melt extraction before having been metasomatized by alkali-rich plume melts. The depleted mantle component may be sourced by residual mantle plume remnants stagnated at the melting locus due to a weak lateral flow velocity inside the melting regime, whose accumulation progressively edifies a depleted lithospheric root above the plume core. Low-degree alkali-rich melts are likely derived from the plume source. Such a mantle source evolution may be general to both terrestrial and extraterrestrial environments where the lateral component velocity of the mantle flow field is extremely slow.

  1. Integrated thermal and density modelling of the lithosphere across the Iberia-African plate boundary: the Gulf of Cadiz - Moroccan Atlas Geotransect

    NASA Astrophysics Data System (ADS)

    Zeyen, H.; Ayarza, P.; Fernàndez, M.; Rimi, A.

    2003-04-01

    The thermal and density structure of the lithosphere along a profile from the Gulf of Cadiz through the High Atlas to the Sahara is presented. The model is based on a numerical code that solves simultaneously the geopotential, lithostatic and heat transport equations integrating gravimetric, geoid, topographic and surface heat flow data, taking into account available geological and seismic constraints. Temperatures and densities are coupled by the thermal expansion, and the topography is considered to be in local isostatic equilibrium. The model presents an unusually thin lithosphere underneath the Atlas (60 to no more than 90 km) coinciding with areas of Neogene to Quaternary volcanism and relatively low P-wave velocities in the mantle. In contrast, the lithosphere underneath the Gulf of Cadiz thickens to 140-160 km depth. The relative amplitudes of geoid and gravity anomalies show that the high topography of the Atlas mountains cannot be produced only by crustal thickening. The lithospheric configuration, characterized by mantle thickening beneath the Gulf of Cadiz and thinning beneath the Atlas suggests a strong sub-lithospheric activity. Hence, westward drift of the lithospheric mantle in the Alboran region could be responsible for the observed mantle thickening and trigger sub-lithospheric erosion in the Atlas region.

  2. Plate Kinematic model of the NW Indian Ocean and derived regional stress history of the East African Margin

    NASA Astrophysics Data System (ADS)

    Tuck-Martin, Amy; Adam, Jürgen; Eagles, Graeme

    2015-04-01

    normal to the plate divergence vector. Away from the active ridges, compressional horizontal stresses caused by ridge-push forces were transmitted through the subsiding oceanic lithosphere, with an SH max orientation parallel to plate divergence vectors. These changes are documented by the lower Bajocian continental breakup unconformity, which can be traced throughout East African basins. At 133 Ma, the plate boundary moved from north to south of Madagascar, incorporating it into the African plate and initiating its separation from Antarctica. The orientation of the plate divergence vector however did not change markedly. The second phase (89 - 61 Ma) led to the separation of India from Madagascar, initiating a new and dramatic change in stress orientation from N-S to ENE-WSW. This led to renewed tectonic activity in the sedimentary basins of western Madagascar. In the third phase (61 Ma to present) asymmetric spreading of the Carlsberg Ridge separated India from the Seychelles and the Mascarene Plateau via the southward propagation of the Carlsberg Ridge to form the Central Indian Ridge. The anti-clockwise rotation of the independent Seychelles microplate between chrons 28n (64.13 Ma) and 26n (58.38 Ma) and the opening of the short-lived Laxmi Basin (67 Ma to abandonment within chron 28n (64.13 - 63.10 Ma)) have been further constrained by the new plate kinematic model. Along the East African margin, SH max remained in a NE - SW orientation and the sedimentary basins experienced continued thick, deep water sediment deposition. Contemporaneously, in the sedimentary basins along East African passive margin, ridge-push related maximum horizontal stresses became progressively outweighed by local gravity-driven NE-SW maximum horizontal stresses trending parallel to the margin. These stress regimes are caused by sediment loading and extensional collapse of thick sediment wedges, predominantly controlled by margin geometry. Our study successfully integrates an interpretation

  3. 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.

  4. Lithospheric structure under the western African-European plate boundary: A transect across the Atlas Mountains and the Gulf of Cadiz

    NASA Astrophysics Data System (ADS)

    Zeyen, Hermann; Ayarza, Puy; Fernã Ndez, Manel; Rimi, Abdelkrim

    2005-04-01

    We present a two-dimensional lithospheric thermal and density model along a transect running from the southwestern Iberian Peninsula to the northwestern Sahara. The main goal is to investigate the lithosphere structure underneath the Gulf of Cadiz and the Atlas Mountains. The model is based on the assumption of topography in local isostatic equilibrium and is constrained by surface heat flow, gravity anomalies, geoid, and topography data. The crustal structure has been constrained by seismic and geological data where available. Mantle density is supposed to vary linearly with temperature, providing the link between thermal and density-related data. The lithospheric thickness varies strongly along the profile, going from near 100 km under the Iberian Peninsula to at least 160-190 km under the Gulf of Cadiz and the Gharb foreland basin in Morocco and to 70 km underneath the Atlas Mountains, coinciding with a region of Neogene volcanism. The thickening of the lithosphere is interpreted as a SW trending lithospheric slab extending from the western Betics to the Gulf of Cadiz and the Gharb Basin, whereas the thin lithosphere underneath the Atlas may be interpreted as plume-like asthenospheric upwelling similar to those observed in the west European Alpine foreland or as a side effect of a slab penetrating the less viscous asthenosphere.

  5. African dust carries microbes across the ocean: are they affecting human and ecosystem health?

    USGS Publications Warehouse

    Kellogg, Christina A.; Griffin, Dale W.

    2003-01-01

    Atmospheric transport of dust from northwest Africa to the western Atlantic Ocean region may be responsible for a number of environmental hazards, including the demise of Caribbean corals; red tides; amphibian diseases; increased occurrence of asthma in humans; and oxygen depletion (eutrophication) in estuaries. Studies of satellite images suggest that hundreds of millions of tons of dust are trans-ported annually at relatively low altitudes across the Atlantic Ocean to the Caribbean Sea and southeastern United States. The dust emanates from the expanding Sahara/Sahel desert region in Africa and carries a wide variety of bacteria and fungi. The U.S. Geological Survey, in collaboration with the NASA/Goddard Spaceflight Center, is conducting a study to identify microbes--bacteria, fungi, viruses--transported across the Atlantic in African soil dust. Each year, millions of tons of desert dust blow off the west African coast and ride the trade winds across the ocean, affecting the entire Caribbean basin, as well as the southeastern United States. Of the dust reaching the U.S., Florida receives about 50 percent, while the rest may range as far north as Maine or as far west as Colorado. The dust storms can be tracked by satellite and take about one week to cross the Atlantic.

  6. Intermediate-depth Fracturing of Oceanic Lithosphere in Subduction Zones: Memories from Exhumed High-Pressure Ophiolites

    NASA Astrophysics Data System (ADS)

    Angiboust, Samuel; Oncken, Onno; Agard, Philippe

    2014-05-01

    Understanding processes acting along the subduction interface is crucial to assess lithospheric scale coupling between tectonic plates and mechanisms causing intermediate-depth seismicity. Despite a wealth of geophysical studies aimed at better characterizing/localizing this seismicity, we still critically lack constrains on processes triggering fracturing in regions (40-100km depths; T > 400°C) where deformation is expected to be achieved by plastic flow. We herein attempt to bridge this gap by providing a review of available evidence from brittle deformation patterns in exhumed High Pressure (HP) ophiolites, together with some new, critical observations. Field examples from various ophiolitic terranes (New-Caledonia, W. Alps, Tian Shan…) indicate that brittle deformation under HP conditions generally implies vein filling and precipitation of HP minerals, probably under very high pore fluid pressure conditions. Coalescence of such vein networks could explain some of the seismic events recorded along the fluid-rich subduction interface region. By contrast, HP pseudotachylites (though reported in only few localities so far) are apparently restricted to somehow deeper slab regions where fluid-deficient conditions are prevalent (Corsica, Zambia, Voltri?). The recent discovery of eclogite breccias, found as m-sized dismembered fragments within an eclogite-facies shear zone from the Monviso area (W. Alps), provides a new opportunity to study the genesis of intermediate-depth earthquakes. We herein argue that these eclogite breccias constitute unique remnants from an ancient fault zone associated with intraslab, intermediate-depth seismicity at ca. 80 km depth. The breccia is internally made of 1-10 cm-sized rotated fragments of eclogite mylonite cemented by an eclogite-facies matrix attesting of fracturing and fault sealing under lawsonite-eclogite facies conditions (550°C, 2.5 GPa) during subduction of the Tethyan seafloor. Textural observations and polyphased

  7. Diamonds and the african lithosphere.

    PubMed

    Boyd, F R; Gurney, J J

    1986-04-25

    Data and inferences drawn from studies of diamond inclusions, xenocrysts, and xenoliths in the kimberlites of southern Africa are combined to characterize the structure of that portion of the Kaapvaal craton that lies within the mantle. The craton has a root composed in large part of peridotites that are strongly depleted in basaltic components. The asthenosphere boundary shelves from depths of 170 to 190 kilometers beneath the craton to approximately 140 kilometers beneath the mobile belts bordering the craton on the south and west. The root formed earlier than 3 billion years ago, and at that time ambient temperatures in it were 900 degrees to 1200 degrees C; these temperatures are near those estimated from data for xenoliths erupted in the Late Cretaceous or from present-day heat-flow measurements. Many of the diamonds in southern Africa are believed to have crystallized in this root in Archean time and were xenocrysts in the kimberlites that brought them to the surface.

  8. Dynamic evolution of continental and oceanic lithosphere in global mantle convection model with plate-like tectonics and one sided subduction.

    NASA Astrophysics Data System (ADS)

    Ulvrova, Martina; Coltice, Nicolas; Tackley, Paul

    2015-04-01

    Drifting of continents, spreading of the seafloor and subduction at convergent boundaries shape the surface of the Earth. On the timescales of several hundreds of millions of years, divergent boundaries at mid-ocean ridges are created and destroyed in within the Wilson cycle. This controls the evolution of the Earth as it determines the heat loss out. Presence of floating continents facilitates the Earth-like mobile lid style of convection as convective stresses are concentrated on the rheological boundary between oceanic and continental lithosphere. Subducting slabs allow for the surface material to be buried down into the mantle and have an important effect on surface tectonics. The main feature of the subduction zones observed on Earth is that it is single-sided forming the deep trenches. Recently, different numerical models were successful in reproducing one-sided subduction by allowing for the vertical deformation of the Earth surface (Crameri and Tackley 2014). In the meantime, advances were made in modelling continental break-up and formation (Rolf et al. 2014). In this study we perform numerical simulations of global mantle convection in spherical annulus geometry with strongly depth and temperature dependent rheology using StagYY code (Tackley 2008). In these models plate tectonics is generated self-consistently and features one-sided subduction on ocean-ocean plate boundary as well as floating continents. We focus on determining (1) the influence of one-sided subduction on the dynamics of the system (2) formation and breakup of continents. Rerefences: Crameri, F. and P. J. Tackley, Spontaneous development of arcuate single-sided subduction in global 3-D mantle convection models with a free surface, J. Geophys. Res., 119(7), 5921-5942, 2014. Rolf, T., N. Coltice and P. J. Tackley (2014), Statistical cyclicity of the supercontinent cycle, Geophys. Res. Lett. 41, 2014. Tackley, P. J., Modellng compressible mantle convection with large viscosity contrasts in

  9. Heat flow in the western abyssal plain of the Gulf of Mexico: Implications for thermal evolution of the old oceanic lithosphere

    SciTech Connect

    Nagihara, S.; Sclater, J.G.; Phillips, J.D.; Behrens, E.W.

    1996-02-10

    This report discusses the heat flow and constraints on the heat flux from the lithosphere under the thick sedimentary layer of the western abyssal plain of the Gulf of Mexico. This paper developes the best estimates for the lithospheric heat loss that is free from near-surface disturbance.

  10. Oceanic transform earthquakes with unusual mechanisms or locations - Relation to fault geometry and state of stress in the adjacent lithosphere

    NASA Technical Reports Server (NTRS)

    Wolfe, Cecily J.; Bergman, Eric A.; Solomon, Sean C.

    1993-01-01

    Results are presented of a search for transform earthquakes departing from the pattern whereby they occur on the principal transform displacement zone (PTDZ) and have strike-slip mechanisms consistent with transform-parallel motion. The search was conducted on the basis of source mechanisms and locations taken from the Harvard centroid moment tensor catalog and the bulletin of the International Seismological Center. The source mechanisms and centroid depths of 10 such earthquakes on the St. Paul's, Marathon, Owen, Heezen, Tharp, Menard, and Rivera transforms are determined from inversions of long-period body waveforms. Much of the anomalous earthquake activity on oceanic transforms is associated with complexities in the geometry of the PTDZ or the presence of large structural features that may influence slip on the fault.

  11. Low densities of drifting litter in the African sector of the Southern Ocean.

    PubMed

    Ryan, Peter G; Musker, Seth; Rink, Ariella

    2014-12-15

    Only 52 litter items (>1cm diameter) were observed in 10,467 km of at-sea transects in the African sector of the Southern Ocean. Litter density north of the Subtropical Front (0.58 items km(-2)) was less than in the adjacent South Atlantic Ocean (1-6 items km(-2)), but has increased compared to the mid-1980s. Litter density south of the Subtropical Front was an order of magnitude less than in temperate waters (0.032 items km(-2)). There was no difference in litter density between sub-Antarctic and Antarctic waters either side of the Antarctic Polar Front. Most litter was made of plastic (96%). Fishery-related debris comprised a greater proportion of litter south of the Subtropical Front (33%) than in temperate waters (13%), where packaging dominated litter items (68%). The results confirm that the Southern Ocean is the least polluted ocean in terms of drifting debris and suggest that most debris comes from local sources.

  12. Predicting East African spring droughts using Pacific and Indian Ocean sea surface temperature indices

    NASA Astrophysics Data System (ADS)

    Funk, C.; Hoell, A.; Shukla, S.; Bladé, I.; Liebmann, B.; Roberts, J. B.; Robertson, F. R.; Husak, G.

    2014-12-01

    In eastern East Africa (the southern Ethiopia, eastern Kenya and southern Somalia region), poor boreal spring (long wet season) rains in 1999, 2000, 2004, 2007, 2008, 2009, and 2011 contributed to severe food insecurity and high levels of malnutrition. Predicting rainfall deficits in this region on seasonal and decadal time frames can help decision makers implement disaster risk reduction measures while guiding climate-smart adaptation and agricultural development. Building on recent research that links more frequent East African droughts to a stronger Walker circulation, resulting from warming in the Indo-Pacific warm pool and an increased east-to-west sea surface temperature (SST) gradient in the western Pacific, we show that the two dominant modes of East African boreal spring rainfall variability are tied to SST fluctuations in the western central Pacific and central Indian Ocean, respectively. Variations in these two rainfall modes can thus be predicted using two SST indices - the western Pacific gradient (WPG) and central Indian Ocean index (CIO), with our statistical forecasts exhibiting reasonable cross-validated skill (rcv ≈ 0.6). In contrast, the current generation of coupled forecast models show no skill during the long rains. Our SST indices also appear to capture most of the major recent drought events such as 2000, 2009 and 2011. Predictions based on these simple indices can be used to support regional forecasting efforts and land surface data assimilations to help inform early warning and guide climate outlooks.

  13. Diamond formation by carbon saturation in C-O-H fluids during cold subduction of oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Frezzotti, Maria-Luce; Huizenga, Jan-Marten; Compagnoni, Roberto; Selverstone, Jane

    2014-10-01

    Microdiamonds in garnet of graphite-free ultrahigh pressure metamorphic (UHPM) rocks from Lago di Cignana (western Alps, Italy) represent the first occurrence of diamond in a low-temperature subduction complex of oceanic origin (T = ∼600 °C; P ⩾ 3.2 GPa). The presence of diamonds in fluid inclusions provides evidence for carbon transport and precipitation in an oxidized H2O-rich C-O-H crustal fluid buffered by mineral equilibria at sub-arc mantle depths. The structural state of carbon in fluid-precipitated diamonds was analyzed with 514 nm excitation source confocal Raman microspectroscopy. The first order peak of sp3-bonded carbon in crystalline diamonds lies at 1331 (±2) cm-1, similar to diamonds in other UHPM terranes. The analysis of the spectra shows additional Raman features due to sp2 carbon phases indicating the presence of both hydrogenated carbon (assigned to trans-polyacetylene segments) in grain boundaries, and graphite-like amorphous carbon in the bulk, i.e. showing a structural disorder much greater than that found in graphite of other UHPM rocks. In one rock sample, disordered microdiamonds are recognized inside fluid inclusions by the presence of a weaker and broader Raman band, downshifted from 1332 to 1328 cm-1. The association of sp3- with sp2-bonded carbon indicates variable kinetics during diamond precipitation. We suggest that precipitation of disordered sp2 carbon acted as a precursor for diamond formation outside the thermodynamic stability field of crystalline graphite. Diamond formation started when the H2O-rich fluid reached the excess concentration of C required for the spontaneous nucleation of diamond. The interplay of rock buffered fO2 and the prograde P-T path at high pressures controlled carbon saturation. Thermodynamic modeling confirms that the C-O-H fluids from which diamond precipitated must have been water rich (0.992

  14. 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.

  15. Predicting East African spring droughts using Pacific and Indian Ocean sea surface temperature indices

    NASA Astrophysics Data System (ADS)

    Funk, C.; Hoell, A.; Shukla, S.; Bladé, I.; Liebmann, B.; Roberts, J. B.; Robertson, F. R.; Husak, G.

    2014-03-01

    In southern Ethiopia, Eastern Kenya, and southern Somalia, poor boreal spring rains in 1999, 2000, 2004, 2007, 2008, 2009, and 2011 contributed to severe food insecurity and high levels of malnutrition. Predicting rainfall deficits in this region on seasonal and decadal time frames can help decision makers implement disaster risk reduction measures while guiding climate-smart adaptation and agricultural development. Building on recent research that links more frequent droughts in that region to a stronger Walker Circulation, warming in the Indo-Pacific warm pool, and an increased western Pacific sea surface temperature (SST) gradient, we show that the two dominant modes of East African boreal spring rainfall variability are tied, respectively, to western-central Pacific and central Indian Ocean SST. Variations in these rainfall modes can be predicted using two previously defined SST indices - the West Pacific Gradient (WPG) and Central Indian Ocean index (CIO), with the WPG and CIO being used, respectively, to predict the first and second rainfall modes. These simple indices can be used in concert with more sophisticated coupled modeling systems and land surface data assimilations to help inform early warning and guide climate outlooks.

  16. Olivine-gabbros and olivine-rich troctolites genesis through melt-rock reactions in oceanic spreading lithosphere: an experimental study up to 0.7 GPa

    NASA Astrophysics Data System (ADS)

    Francomme, Justine E.; Fumagalli, Patrizia; Borghini, Giulio

    2016-04-01

    Extensive melt-rock reaction and melt impregnation significantly affect not only the physical and chemical properties at mantle-crust transition, but also control the evolution of migrating melts. We performed reactive dissolution and crystallization experiments at pressure ≤ 0.7 GPa in a piston-cylinder apparatus to provide experimental constraints on genesis of olivine-rich troctolites and olivine-gabbros at mantle-crust transition in oceanic spreading lithosphere by melt-rock reaction. Our experiments are carried out by using Salt-Pyrex-Graphite-Magnesium assemblies and graphite-lined platinum capsules. Experimental charges are prepared with three layers: (1) basalt powder, (2) fine powder (1-10μm) of San Carlos olivine (Fo90.1), and (3) carbon spheres used as a melt trap. Three synthetic MORB-type melts have been used, two tholeiitic basalts (Mg#: 0.62, SiO2: 47.70 wt%, Na2O: 2.28 wt% and Mg#: 0.58, SiO2: 49.25 wt%, Na2O: 2.49 wt%) and a primitive one (Mg#: 0.74, SiO2: 48.25 wt%, Na2O: 1.80 wt%), in order to investigate the effect of melt composition. A rock/melt ratio of 0.7 has been kept fixed. Experiments have been conducted at temperatures from 1200 to 1300°C, at both step cooling and isothermal conditions for different run durations (from 12 to 72 hrs). They resulted in layered samples in which all the initial San Carlos olivine powder, analog of a dunitic pluton infiltrated by basaltic melt, is replaced by different lithologies from olivine-rich troctolite to olivine gabbro. In isothermal experiments, reacted melts have been successfully trapped in the carbon spheres allowing their chemical analysis; as expected the reacted melt has a higher Mg# than the initial one (e.g. from Mg#=0.62 to 0.73). Across the different lithologies Mg# of olivine is decreasing from the olivine-rich troctolite to the gabbro. Replacive olivine-rich troctolite has a poikilitic texture with rounded euhedral olivine and interstitial poikilitic plagioclase and clinopyroxene

  17. 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.

  18. 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

  19. Late Jurassic adakitic granodiorite in the Dong Co area, northern Tibet: Implications for subduction of the Bangong-Nujiang oceanic lithosphere and related accretion of the Southern Qiangtang terrane

    NASA Astrophysics Data System (ADS)

    Fan, Jian-Jun; Li, Cai; Wu, Hao; Zhang, Tian-Yu; Wang, Ming; Chen, Jing-Wen; Xu, Jian-Xin

    2016-11-01

    In this paper we present new major and trace element chemical data, zircon U-Pb age data, and zircon Hf isotopic compositions for the Dong Co granodiorites (DGs) that were emplaced directly in the Dong Co ophiolites (DOs) in the middle segment of the Bangong-Nujiang suture zone (BNSZ), northern Tibet. We use these new data to discuss the genesis of the DGs and implications for the evolution of the region. The DGs have high contents of SiO2, Al2O3, Na2O, and Sr, high values of Mg#, low contents of Yb and Y, and markedly high Sr/Y ratios, indicating an adakitic affinity, and we show that they were derived from the partial melting of subducted oceanic crust. The DGs contain large numbers of inherited zircons that have similar shapes and peak ages to the detrital zircons in the accretionary wedge graywackes that surround the DGs, suggesting that many of these sediments were assimilated during the formation of the DGs. Five DG samples yield the youngest zircon U-Pb ages of 155-160 Ma, indicating that the DGs formed during the Late Jurassic. Based on the present results and regional geological data, we infer that the Bangong-Nujiang oceanic lithosphere was subducted northwards beneath the Southern Qiangtang terrane during the formation of a continental arc system during the Late Jurassic. The Bangong-Nujiang oceanic lithosphere retreated rapidly after its initial subduction, and the Southern Qiangtang terrane to the north of the Bangong-Nujiang Ocean acquired a massive amount of new oceanic material in the form of an accretionary wedge. This wedge, which included the DOs and the Mugagangri Group, became sufficiently large (both in size and width) for arc-type magmas (DGs) to develop within it during the Late Jurassic.

  20. 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.

  1. Eastern segment of the Azores-Gibraltar line (central-eastern Atlantic) : An oceanic plate boundary with diffuse compressional deformation

    NASA Astrophysics Data System (ADS)

    Sartori, R.; Torelli, L.; Zitellini, N.; Peis, D.; Lodolo, E.

    1994-06-01

    New seismic-reflection images across the eastern segment of the Azores-Gibraltar line west of the collisional area between the African and Iberian plates have revealed a complex pattern of compressional deformation involving the Mesozoic oceanic lithosphere. The compressional deformation developed in a region of slow plate convergence and is diffused, at different lithospheric levels, across an area spanning ˜200 km from the Gorringe Ridge to the Seine Plain. The convergence between the African and Iberian plates has been active since Tertiary time, and our results indicate that no subduction zone exists across this part of the plate boundary.

  2. Land - Ocean Climate Linkages and the Human Evolution - New ICDP and IODP Drilling Initiatives in the East African Rift Valley and SW Indian Ocean

    NASA Astrophysics Data System (ADS)

    Zahn, R.; Feibel, C.; Co-Pis, Icdp/Iodp

    2009-04-01

    The past 5 Ma were marked by systematic shifts towards colder climates and concomitant reorganizations in ocean circulation and marine heat transports. Some of the changes involved plate-tectonic shifts such as the closure of the Panamanian Isthmus and restructuring of the Indonesian archipelago that affected inter-ocean communications and altered the world ocean circulation. These changes induced ocean-atmosphere feedbacks with consequences for climates globally and locally. Two new ICDP and IODP drilling initiatives target these developments from the perspectives of marine and terrestrial palaeoclimatology and the human evolution. The ICDP drilling initiative HSPDP ("Hominid Sites and Paleolakes Drilling Project"; ICDP ref. no. 10/07) targets lacustrine depocentres in Ethiopia (Hadar) and Kenya (West Turkana, Olorgesailie, Magadi) to retrieve sedimentary sequences close to the places and times where various species of hominins lived over currently available outcrop records. The records will provide a spatially resolved record of the East African environmental history in conjunction with climate variability at orbital (Milankovitch) and sub-orbital (ENSO decadal) time scales. HSPDP specifically aims at (1) compiling master chronologies for outcrops around each of the depocentres; (2) assessing which aspects of the paleoenvironmental records are a function of local origin (hydrology, hydrogeology) and which are linked with regional or larger-scale signals; (3) correlating broad-scale patterns of hominin phylogeny with the global beat of climate variability and (4) correlating regional shifts in the hominin fossil and archaeological record with more local patterns of paleoenvironmental change. Ultimately the aim is to test hypotheses that link physical and cultural adaptations in the course of the hominin evolution to local environmental change and variability. The IODP initiative SAFARI ("Southern African Climates, Agulhas Warm Water Transports and Retroflection

  3. Coplanar polychlorinated biphenyl congeners in shark livers from the north-western African Atlantic ocean

    SciTech Connect

    Serrano, R.; Fernandez, M.A.; Hernandez, L.M.

    1997-01-01

    Polychlorinated biphenyls have been widely used by industry throughout the world since 1930. Although their use has been banned in many countries since the late 1970s, they still represent an important class of priority pollutants due to their persistence. Most open uses of these chemicals have been severely curtailed in industrialized nations, but a considerable fraction of past productions is probably still cycling in the ecosphere. In recent years, attention has been focused on the toxicity of PCBs, especially of those congeners showing similar toxicity as the polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDFs). It has been shown that PCB congeners` toxicity largely depends on the chlorine substitution pattern. The most toxic PCB cogeners are those with two para chlorines, at least two meta chlorines and 0-2 ortho chlorines. These so-called {open_quotes}coplanar{close_quotes} (non- mono- and di-ortho) PCB cogeners are able to obtain planar conformation. Recently, toxic equivalence factors have been assigned to coplanar PCBs. Thus determination of individual PCB cogeners is important for evaluating the toxic potentials of PCB residues in, for example, wildlife. This paper presents preliminary results of a study looking at levels of PCB congeners, including coplanar ones, in the liver of six shark species, collected in the North African Atlantic Ocean. 15 refs., 2 figs., 2 tabs.

  4. High-pressure whiteschists from the Ti-N-Eggoleh area (Central Hoggar, Algeria): A record of Pan-African oceanic subduction

    NASA Astrophysics Data System (ADS)

    Adjerid, Zouhir; Godard, Gaston; Ouzegane, Khadidja

    2015-06-01

    the West Gondwana orogenic belt during the Neoproterozoic Pan-African orogeny. The decompression associated with the early phase of exhumation was followed by an important increase in temperature towards granulite-facies conditions, possibly determined by the intrusion of abundant mafic rocks in this region due to delamination of the lithospheric mantle. The Ti-N-Eggoleh area and its high-pressure meta-ophiolitic series apparently belong to the Sérouènout Terrane, which stretches along the eastern margin of the Western Gondwana orogenic belt and consists mainly of oceanic metasediments; they are possibly markers of an ancient, yet unidentified, subduction and suture zone.

  5. Electromagnetic Studies Of The Lithosphere And Asthenosphere

    NASA Astrophysics Data System (ADS)

    Heinson, Graham

    In geodynamic models of the Earth's interior, the lithosphere and asthenosphere are defined in terms of their rheology. Lithosphere has high viscosity, and can be divided into an elastic region at temperatures below 350 °C and an anelastic region above 650 °C. Beneath the lithosphere lies the ductile asthenosphere, with one- to two-orders of magnitude lower viscosity. Asthenosphere represents the location in the mantle where the melting point (solidus) is most closely approached, and sometimes intersected. Seismic, gravity and isostatic observations provide constraints on lithosphere-asthenosphere structure in terms of shear-rigidity, density and viscosity, which are all rheological properties. In particular, seismic shear- and surface-wave analyses produce estimates of a low-velocity zone (LVZ) asthenosphere at depths comparable to the predicted rheological transitions. Heat flow measurements on the ocean floor also provide a measure of the thermal structure of the lithosphere.Electromagnetic (EM) observations provide complementary information on lithosphere-asthenosphere structure in terms of electrical conductivity. Laboratory studies of mantle minerals show that EM observations are very sensitive to the presence of melt or volatiles. A high conductivity zone (HCZ) in the upper mantle therefore represents an electrical asthenosphere (containing melt and/or volatile) that may be distinct from a rheological asthenosphere and the LVZ. Additionally, the vector propagation of EM fields in the Earth provides information on anisotropic conduction in the lithosphere and asthenosphere. In the last decade, numerous EM studies have focussed on the delineation of an HCZ in the upper mantle, and the determination of melt/volatile fractions and the dynamics of the lithosphere-asthenosphere. Such HCZs have been imaged under a variety of tectonic zones, including mid-ocean ridges and continental rifts, but Archaean shields show little evidence of an HCZ, implying that the

  6. 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.

  7. Investigating and Imaging the Lithospheric Structure of the Westernmost Mediterranean Using S Receiver Functions

    NASA Astrophysics Data System (ADS)

    Miller, Meghan S.; Butcher, Amber

    2013-04-01

    The Alboran System was created during the Neogene at the western edge of the Alpine-Himalayan orogenic belt, as the result of convergence between the European and African plates. This system includes the Gibraltar Arc, Rif-Betic chain, Atlas Mountains, and Alboran Sea. The evolution from ocean subduction to continental collision, particularly in complex three-dimensional settings such as this, is poorly understood. Advances in this subject are likely to come from multidisciplinary projects, such as PICASSO (Program to Investigate Convective Alboran Sea System Overturn): a study of the Alboran Sea, Atlas Mountains, and Gibraltar arc. Several models have been suggested to explain the tectonics of this system including: continental lithospheric delamination and drips, slab breakoff, and subducting slab rollback. Advances in defining the lithosphere - asthenosphere boundary (LAB) are crucial to understanding the geochemical and geodynamic evolution of the region. Seismic data from ~85 broadband instruments deployed in Morocco and Spain as part of the PICASSO project are being utilized to constrain lithospheric structure beneath this part of the Western Mediterranean via identification of S-to-p conversions from S receiver functions. A previous study indicates that the lithospheric thinning beneath the Atlas High may be the result of mantle upwelling induced thermal erosion, while a more recent imaging study suggests that the LAB could be at depths >200 km, tens of kilometers thicker than previous models. Our preliminary results indicate LAB depths down to ~100 - 110 km near the Straight of Gibraltar and as shallow as ~65 - 80 km under the Atlas High. The primary purpose of this project is to advance our understanding of the structure and evolution of the lithosphere - asthenosphere boundary (LAB) of the Atlas Mountains and surrounding areas.

  8. 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.

  9. 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

  10. DECOUPLED ACCOMMODATION OF CONVERGENCE BETWEEN AFRICA AND EURASIA. MODELLING THE LITHOSPHERIC STRUCTURE ACROSS THE GORRINGE BANK AND THE NW MOROCCAN MARGIN

    NASA Astrophysics Data System (ADS)

    Jimenez-Munt, I.; Fernandez, M.; Verges, J.; Garcia-Castellanos, D.; Perez-Gussinye, M.; Afonso, J.; Fullea, J.

    2009-12-01

    We have modelled the lithospheric structure across the NW Moroccan margin using an integrated methodology that combines elevation, heat flow, gravity, geoid and seismic data. The modelled profile is 1360 km long and extends NW-SE from the Iberian Abyssal Plane to the Sahara Platform, crossing the Gorringe Bank, the west Iberian-Africa plate boundary, the Moroccan continental margin, and the Atlas Mountains. Offshore, the profile coincides with the IAM-4 and SISMAR-04 deep seismic profiles whereas onshore, where no seismic data are available, it follows previous modelled lithospheric profiles. The present configuration of the Gorringe Bank is explained by a subcrustal NW-directed thrust carrying exhumed upper mantle rocks and transitional African crust on top of flexed-down Eurasian oceanic crust along the Tagus Abyssal Plain. This is the result of a long-lasting evolution related to the African and Eurasian plate boundary characterized by: (1) Late Jurassic-Early Cretaceous extension with mantle exhumation, intrusion of gabbros, and mantle serpentinization; (2) early Miocene compression, which produced ~20 km of NW-directed thrusting of serpentinized upper mantle rocks and African transitional crust on top of the Eurasian oceanic crust and sedimentary cover. In the Moroccan continental margin, the most outstanding result is a prominent lithospheric thickening with thickness values of around 210 km. This thickening is the result of applying the crustal and density structure proposed from SISMAR survey. Assuming that this thickening is gained by plate convergence, our calculations predict a minimum shortening of 140 km in the lithospheric mantle affecting a 400 km wide region. Meanwhile, at crustal levels, the Africa-Iberia convergence has been accommodated in a wider area (at least of 900 km) with an observed shortening of only ~ 60 km. This model shows a clear decoupled accommodation of the Africa-Eurasia convergence and put additional constraints on the regional

  11. Lithospheric expression of geological units in central and eastern North America from full waveform tomography

    NASA Astrophysics Data System (ADS)

    Yuan, Huaiyu; French, Scott; Cupillard, Paul; Romanowicz, Barbara

    2014-09-01

    The EarthScope TA deployment has provided dense array coverage throughout the continental US and with it, the opportunity for high resolution 3D seismic velocity imaging of both lithosphere and asthenosphere in the continent. Building upon our previous long-period waveform tomographic modeling in North America, we present a higher resolution 3D isotropic and radially anisotropic shear wave velocity model of the North American lithospheric mantle, constructed tomographically using the spectral element method for wavefield computations and waveform data down to 40 s period. The new model exhibits pronounced spatial correlation between lateral variations in seismic velocity and anisotropy and major tectonic units as defined from surface geology. In the center of the continent, the North American craton exhibits uniformly thick lithosphere down to 200-250 km, while major tectonic sutures of Proterozoic age visible in the surface geology extend down to 100-150 km as relatively narrow zones of distinct radial anisotropy, with Vsv>Vsh. Notably, the upper mantle low velocity zone is present everywhere under the craton between 200 and 300 km depth. East of the continental rift margin, the lithosphere is broken up into a series of large, somewhat thinner (150 km) high velocity blocks, which extend laterally 200-300 km offshore into the Atlantic Ocean. Between the craton and these deep-rooted blocks, we find a prominent narrow band of low velocities that roughly follows the southern and eastern Laurentia rift margin and extends into New England. We suggest that the lithosphere along this band of low velocities may be thinned due to the combined effects of repeated rifting processes and northward extension of the hotspot related Bermuda low-velocity channel across the New England region. We propose that the deep rooted high velocity blocks east of the Laurentia margin represent the Proterozoic Gondwanian terranes of pan-African affinity, which were captured during the Rodinia

  12. Properties of the lithosphere and asthenosphere deduced from geoid observations

    NASA Technical Reports Server (NTRS)

    Turcotte, D. L.

    1985-01-01

    Data from the GEOS-3 and SEASAT Satellites provided a very accurate geoid map over the oceans. Broad bathymetric features in the oceans such as oceanic swells and plateaus are fully compensated. It is shown that the geoid anomalies due to the density structures of the lithosphere are proportional to the first moment of the density distribution. The deepening of the ocean basins is attributed to thermal isostasy. The thickness of the oceanic lithosphere increases with age due to the loss of heat to the sea floor. Bathymetry and the geoid provide constraints on the extent of this heat loss. Offsets in the geoid across major fracture zones can also be used to constrain this problem. Geoid bathymetry correlations show that the Hawaiian and Bermuda swells and the Cape Verde Rise are probably due to lithospheric thinning.

  13. Seasat observations of lithospheric flexure seaward of trenches

    NASA Technical Reports Server (NTRS)

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

    1984-01-01

    Lithospheric flexure seaward of deep ocean trenches is evident 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 has been used for the oceanic lithosphere and, after least squares adjustments, estimates have been recovered of model parameters including outer rise (OR) amplitude, OR wavelength, and effective lithospheric thickness. Effective lithospheric thicknesses have been recovered for six regions: the Mariana, the Kuril, the Philippine, the Aleutian, the Izu-Bonin, and the Middle America OR's. These results support the proposition that effective thickness Te increases with age of lithosphere in approximate accord with the relation Te approximately C x square root of age where C approximately 4 km/square root of (m.y.). In fact, 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 Izu-Bonin trenches. This thickness-age relation implies that flexural strength of the oceanic lithosphere is temperature controlled.

  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 buoyancy and continental intraplate stresses

    USGS Publications Warehouse

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

    2003-01-01

    gravitational potential energy by taking a vertical integral over the computed lithosphere density. Our computed values suggest that the thick roots beneath cratons lead to strong negative potential energy differences relative to surrounding regions, and hence exert compressive stresses superimposed on the intraplate stresses derived from plate boundary forces. Forces related to this lithosphere structure thus may explain the dominance of reverse-faulting earthquakes in cratons. Areas of high elevation and a thin mantle lid (e.g., western U.S. Basin and Range, East African rift, and Baikal rift) are predicted to be in extension, consistent with the observed stress regime in these areas.

  16. 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

  17. 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.

  18. Undergraduate Research Experience in Ocean/Marine Science (URE-OMS) with African Student Component

    DTIC Science & Technology

    2011-01-01

    students by faculty and advisement of students on applying for future ocean and marine science internships . WORK COMPLETED The URE program... Geology , Chemistry, Math Education, Marine Biology, Computer Engineering, Mechanical Engineering, Geography, Geology , and Mathematics. Minority serving

  19. Anomalous seafloor mounds in the northern Natal Valley, southwest Indian Ocean: Implications for the East African Rift System

    NASA Astrophysics Data System (ADS)

    Wiles, Errol; Green, Andrew; Watkeys, Mike; Jokat, Wilfried; Krocker, Ralph

    2014-09-01

    The Natal Valley (southwest Indian Ocean) has a complicated and protracted opening history, as has the surrounding southwest Indian Ocean. Recently collected multibeam swath bathymetry and 3.5 kHz seismic data from the Natal Valley reveal anomalous seafloor mounds in the northern Natal Valley. The significance, of these domes, as recorders of the geological history of the Natal Valley and SE African Margin has been overlooked with little attempt made to identify their origin, evolution or tectonic significance. This paper aims to describe these features from a morphological perspective and to use their occurrence as a means to better understand the geological and oceanographic evolution of this basin. The seafloor mounds are distinct in both shallow seismic and morphological character from the surrounding seafloor of the Natal Valley. Between 25 km and 31 km long, and 16 km and 18 km wide, these features rise some 400 m above the sedimentary deposits that have filled in the Natal Valley. Such macro-scale features have not previously been described from the Natal Valley or from other passive margins globally. They are not the result of bottom water circulation, salt tectonics; rather, igneous activity is favoured as the origin for these anomalous seafloor features. We propose a hypothesis that the anomalous seafloor mounds observed in the Natal Valley are related to igneous activity associated with the EARS. The complicated opening history and antecedent geology, coupled with the southward propagation of the East African Rift System creates a unique setting where continental rift associated features have been developed in a marine setting.

  20. Paleo-Asian oceanic subduction-related modification of the lithospheric mantle under the North China Craton: evidence from peridotite xenoliths in the Datong basalts

    NASA Astrophysics Data System (ADS)

    Wang, C.; Liu, Y.; Min, N.; Zong, K.; Hu, Z.; Gao, S.

    2015-12-01

    In-situ major and trace elements and Sr isotopic compositions of peridotite xenoliths carried by the Datong Quaternary alkaline basalt were analyzed. These peridotite xenoliths were classified into three groups. The type 1 peridotites preserve depleted trace element and Sr isotopic signatures and record the lowest temperature (930 - 980 °C). Clinopyroxenes in these peridotites exhibit LREE-depleted REE patterns, and have the lowest 87Sr/86Sr ratios of 0.70243 - 0.70411. The types 2 and 3 peridotites are featured by enriched trace element and Sr isotopic signatures and record a higher temperature (1003 - 1032 °C). Clinopyroxenes in the type 2 peridotite have U-shaped REE patterns and relatively higher 87Sr/86Sr ratios of 0.70418 - 0.70465. Clinopyroxenes in the type 3 peridotite have concave-downward REE patterns and unusually high 87Sr/86Sr ratios of 0.70769 - 0.70929. Carbonatitic veinlets are found in the type 1 peridotites. They show steep LREE-enriched REE patterns with enrichments in LILE and depletions in HFSE, and have the highest 87Sr/86Sr ratios of 0.71147 - 0.71285. The types 2 and 3 peridotites suffered latter cryptic carbonatitic metasomatism, as indicated by the decreased Ti/Eu and increased Zr/Hf and CaO/Al2O3 ratios of clinopyroxenes. The carbonatitic veinlets have generally consistent trace element patterns and Sr isotopic ratios with the calculated melts being equilibrated with the clinopyroxenes in the type 3 peridotite, and may represent the metasoamtic agent solidified in the relatively cold and shallow mantle. The negative Eu anomalies (0.37 - 0.61) and high 87Sr/86Sr ratios of the calculated melts indicate a crustal sedimentary origin. It is speculated that the REE-rich and high-87Sr/86Sr metasoamtic agent should be carbonatitic melt derived from the carbonated pelite carried by the subducted PAOP, which could have contributed to the transformation of the lithospheric mantle beneath the NCC.

  1. 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.

  2. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    Subduction recycling of oceanic lithosphere, a central theme of plate tectonics, is relatively well understood. Recycling continental lithosphere is more difficult to recognize, can take a number of different forms, and appears to require an external trigger for initiation. Delamination and localized convective downwelling are two processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. We describe a related 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. Body wave tomograms from dense broadband seismograph arrays in northeastern South America (SA) and the western Mediterranean show larger than expected volumes of positive velocity anomalies which we identify as the subducted Atlantic slab under northeastern SA, and the Alboran slab beneath the Gibraltar arc (GA). The positive anomalies lie under and are aligned with the continental margins at sublithospheric depths. The continental margins along which the subduction zones have traversed, i.e. the northeastern SA plate boundary and east of GA, have significantly thinner lithosphere than expected. The thinner than expected lithosphere extends inland as far as the edges of nearby cratons as determined from receiver function images and surface wave tomography. 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 and include the lower crust, as around GA, inferred by results from active and passive seismic experiments. Viscous removal of continental margin lithosphere creates LAB topography leading

  3. The Arctic lithosphere: an overview

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

    The Arctic is comprised of three deepwater oceanic basins, the Norwegian-Greenland, Eurasia, and Amerasia basins, surrounded by continental masses of the Achaean to Early Proterozoic North American, Baltica and Siberian cratons and intervening Neoproterozoic and Phanerozoic fold belts. Though the tectonic history of the Arctic continental realm spans almost three billions of years, the formation of the Arctic began with the creation of Pangaea-II supercontinent at end of Permian epoch. Between 250 and 150 Ma the Proto-Arctic was represented by the Anyui Ocean, or Angayuchum Sea - a Paleo-Pacific embayment into Pangaea II. During the Mesozoic Pangaea II was destroyed and the Anyi Ocean was isolated from the Paleo-Pacific, finally leading to the separation of Arctic Alaska-Chukchi Microcontinent from the North American side of Laurasia; the collision of this microplate with the Siberian margin occurred at ca. 125 Ma in association with the opening of the Canada Basin. The final stage of the Arctic formation took place in the Cenozoic, and was related to the propagation of the divergent Atlantic lithospheric plate boundary between North America and Baltica with the separation of the Lomonosov continental sliver from the Eurasian margin and opening of the Eurasia oceanic basin between 56 and 0 Ma. The present-day Arctic, especially its shelves and oceanic basins, is one of the least studied places on the Earth. Though we know the geology of the surrounding continental masses, there are still many questions remaining about major lithospheric divides beneath the Arctic seas, such as: • Where are the plate boundaries associated with the Amerasia Basin? • How and when did the Canada Basin open? • What was the pre-drift setting of the Chukchi Borderland? • Which tectonic processes formed the East Siberian shelves? • How and when did the major ridges in the Amerasia Basin form? • Where are the Early Tertiary plate boundaries in the Arctic? • What is the

  4. 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.

  5. Imaging the Lithospheric - Asthenosphere Boundary Structure of the Westernmost Mediterranean Using S Receiver Functions

    NASA Astrophysics Data System (ADS)

    Butcher, A.; Miller, M. S.; Diaz Cusi, J.

    2013-12-01

    The Iberian microcontinent, in the westernmost portion of the Mediterranean is comprised of the Betic Cordillera Zone, the South Portuguese Zone, the Ossa-Morena Zone, the Central Iberian Zone, the Galicia-Tras Os Montes Zone, the West Asturian-Leonese Zone, and the Cantabrian Zone. These zones were created as a result of three primary stages of Iberian evolution, with the last being the collision of Iberia with in the Late Cretaceous. In northeastern Africa, Neogene convergence between the European and African plates created the Alboran System: comprised of the Gibraltar Arc, Rif-Betics, Atlas Mountains, and Alboran Sea. The primary purpose of this study is to advance our understanding of the structure and evolution of the lithosphere, as well as the lithosphere - asthenosphere boundary (LAB) of the Iberian microcontinent and surrounding areas. Of particular interest is improving our understanding of the evolution from ocean subduction to continental collision that has been taking place in the late stage convergence of this part of the Mediterranean., The region is a particularly complex three-dimensional settings and, several models have been suggested to explain the tectonics of this system including: continental lithospheric delamination and drips, slab breakoff, and subducting slab rollback. Here we use broadband seismic data from 272 broadband instruments deployed in Morocco and Spain as part of the PICASSO and IBERArray (Díaz, J., et al., 2009) projects to constrain lithospheric structure via identification of S-to-p conversions from S receiver functions (SRF). We use SRFs to image the characteristics and structure in terms of seismic velocity discontinuities, including the crust-mantle boundary (Moho) and the lithosphere-asthenosphere boundary (LAB) beneath the region. Our SRFs agree with previous work that suggests that the lithospheric thickness is shallow (~65 km) beneath the Atlas and thickest (~120 km) beneath the Rif. Additionally, LAB structures

  6. Temporal variations of the segmentation of slow to intermediate spreading mid-ocean ridges 2. A three-dimensional model in terms of lithosphere accretion and convection within the partially molten mantle beneath the ridge axis

    NASA Astrophysics Data System (ADS)

    Rabinowicz, Michel; Briais, Anne

    2002-06-01

    We present three-dimensional numerical models of convection within the partially molten mantle beneath the ridge axis. The modeling takes into account the cavity flow driven by plate spreading, the diffuse upwelling due to plate accretion, and the shearing movement generated by large-scale mantle flow. The ridge axis is free to move in the spreading direction to adjust to the maxima of tension at the lithosphere-mantle interface induced by the convective circulation. The melt distribution in the mantle and the crustal production at the ridge axis are estimated using the formalism of McKenzie and Bickle [1988]. During the experiments the record of the ridge axis positions and crustal production is used to compute synthetic maps of the isochrons and oceanic crustal thickness. Close to the ridge, the ascending convective flow consists of 80- to 100-km-long hot sheets oriented either roughly parallel or orthogonal to spreading. Most ridge segments fit with the top of hot upwelling sheets, while transient transform faults coincide with the top of cold downwelling flows. The crustal maps display lineations subparallel or slightily oblique to spreading, a few tens of million years long, and separated by ~60-50 km, resulting from the lithospheric record of the excess crust produced at the junction of hot sheets. When a junction of two hot sheets migrates outside the ridge axial plane, the crustal thickness maximum splits into two maxima along axis, and the induced lineation in the crustal map splits into two branches. The merging of lineations occurs when the ridge plane traps the junction of hot sheets. When the large-scale mantle circulation moves parallel to the ridge crest, it slowly pushs the spreading-parallel convective sheets. The resulting lineations form V shapes pointing in the same direction as the large-scale flow. When the large-scale flow parallels spreading, it slowly pushes the ridge-parallel hot sheets in the upflow direction. Thus the ridge segments

  7. Seismic Structure and Inferred Lithology of the Heterogeneous Upper Lithosphere at Atlantis Massif Oceanic Core Complex, 30°N MAR

    NASA Astrophysics Data System (ADS)

    Henig, A. S.; Blackman, D. K.; Harding, A. J.; Kent, G.

    2010-12-01

    MultiChannel Seismic refraction data at Atlantis Massif targets the lithospheric structure in the upper 1-1.5 km. Initial findings showed heterogeneity on vertical and lateral scales of as little as ~1km with three distinct velocity regimes: similar to average young Atlantic shallow (~1-1.5 kmbsf) crustal velocities that likely pertain to alteration and porosity rather than Penrose-type crust; intermediate shallow velocities and gradients that are higher than typical young Atlantic crustal values below ~200 mbsf; and high shallow velocities (~4.5 km/s at seafloor) and steep gradients (~4/s) in the upper km. Our SOBE (Synthetic On Bottom Experiment) analysis of the data supports these general findings and provides additional structural detail. We determine a systematic decrease in seismic velocities from east to west (younger to older crust) across both the Southern Ridge and the Central Dome. In the north part of the study area, velocities grade from the high dome values to typical young Atlantic crustal values. High domal velocities are interpreted as gabbroic pluton(s). The top of such body(ies) shoal to the seafloor on the eastern slope of the Southern Ridge and are suggested (by required gradients) to occur at depths just beneath the ray coverage in the central and western portions. These observations are consistent with an exhumation model for Atlantis Massif where greater unroofing has taken place for the Southern Ridge than for the Central Dome, with extents of unroofing in the east of the domes being less than in the west. Veneers of low velocity material (150-350 m thick, 2.5-3.5 km/s range) likely mark alteration patterns based on little evidence from dredges and submersible studies of extrusive lavas representative of crustal layer 2A. Lateral variation (~1.5 km/s over ~2 km) within the few-km wide higher-velocity bodies could indicate different intrusions that may feed late-stage diking observed in drill cores. An interesting transition zone between the

  8. Undergraduate Research Experience In Ocean/Marine Science (URE-OMS) with African Student Component

    DTIC Science & Technology

    2009-01-01

    mentoring of students by faculty and advisement of students on applying for future ocean and marine science internships . WORK COMPLETED The URE...programs representing 27 institutions and 12 majors. Among the majors included were Physics, Computer Science, Biology, Geology , Chemistry, Math...Education, Marine Biology, Computer Engineering, Mechanical Engineering, Geography, Geology , and Mathematics. Minority serving institutions comprised 78

  9. Lithospheric cooling as a basin forming mechanism within accretionary crust.

    NASA Astrophysics Data System (ADS)

    Holt, P. J.; Allen, M.; van Hunen, J.; Björnseth, H. M.

    2009-04-01

    Widely accepted basin forming mechanisms are limited to flexure of the lithosphere, lithospheric stretching, lithospheric cooling following rifting and, possibly, dynamic topography. In this work forward models have been used to investigate lithospheric growth due to cooling beneath accretionary crust, as a new basin forming mechanism. Accretionary crust is formed from collision of island arcs, accretionary complexes and fragments of reworked older crust at subduction zones, and therefore has thin lithosphere due to melting and increased convection. This is modeled using a 1D infinite half space cooling model similar to lithospheric cooling models for the oceans. The crustal composition and structure used in the models has been varied around average values of accretionary crust to represent the heterogeneity of accretionary crust. The initial mantle lithosphere thickness used in the model was 20 km. The model then allows the lithosphere to thicken as it cools and calculates the subsidence isostatically. The model produces sediment loaded basins of 2-7 km for the various crustal structures over 250 Myrs. Water-loaded tectonic subsidence curves from the forward models were compared to tectonic subsidence curves produced from backstripping wells from the Kufrah and Ghadames basins, located on the accretionary crust of North Africa. A good match between the subsidence curves for the forward model and backstripping is produced when the best estimates for the crustal structure, composition and the present day thickness of the lithosphere for North Africa are used as inputs for the forward model. This shows that lithospheric cooling provides a good method for producing large basins with prolonged subsidence in accretionary crust without the need for initial extension.

  10. The Natural History and Conservation of Indian Ocean Humpback Dolphins (Sousa plumbea) in South African Waters.

    PubMed

    Plön, Stephanie; Cockcroft, Victor G; Froneman, William P

    2015-01-01

    Although most knowledge on the biology of Sousa plumbea has primarily come from South African waters, a number of research gaps remain on the natural history and status of the species in the region. Research on two populations in South African waters for which some historical data exist may aid in highlighting long-term changes in the biology and natural history of this little known coastal delphinid. Recent studies on the age, growth and reproduction of animals incidentally caught in shark nets in Richards Bay, KwaZulu-Natal, yielded a lower maximum age estimate of 24 (previously 46) growth-layer-groups (GLGs), sexual maturity of 7.5 and 8 GLGs in males and females (previously 12-13 and 10 GLGs, respectively), an ovulation rate of 0.2 and a 5-year calving interval (previously 0.3 and 3-year calving interval) than previously reported. These differences may be due to a difference in the interpretation of GLGs between observers or a predominance of young males being caught in the shark nets. Stomach content analysis revealed a change in the relative proportions of the main prey items over the past 25 years, but no difference in species richness or diversity was found between the sexes. No change in trophic level was recorded between 1972 and 2009. Field studies in Algoa Bay, Eastern Cape, conducted 16 years apart indicated a decline in the mean group size (from 7 to 3 animals), a decline in the maximum group size (from 24 to 13 animals), an increase in solitary individuals (15.4-36%), and a change in behaviour from predominantly foraging (64-18%) to mainly travelling (24-49%). The observed changes are suggestive of a change in food availability, resulting in a range shift or a potential decline in numbers. These studies indicate the importance of long-term studies to monitor population changes and their possible causes. A number of threats, such as shark nets, pollution (noise and chemical), and coastal development and disturbance, to the humpback dolphin populations

  11. Ocean internal waves off the North American and African coasts from ERTS-1

    NASA Technical Reports Server (NTRS)

    Apel, J. R.; Charnell, R. L.

    1974-01-01

    Periodic features observed in the ocean portions of certain ERTS-1 images have been identified with reasonable certainty as surface manifestations of oceanic internal gravity waves. A series of images taken over the New York Bight, commencing with the 16 July 1972 overpass and continuing on into autumn of 1973, has shown the internal waves to be present when summer solar heating stratifies the water sufficiently well to support such oscillations. When fall and winter wind action mixes the shelf water down to the bottom, the waves no longer appear. In the Bight, the wavelengths range from approximately 400 to 1000 m, with the wave field being most sharply delineated near the edges of the continental shelf, at the mouth of the Hudson Canyon. They appear in packets consisting of several waves separated by 10-15 km, which propagate up on the shelf and disappear.

  12. Predicting East African spring droughts using Pacific and Indian Ocean sea surface temperature indices

    USGS Publications Warehouse

    Funk, Christopher C.; Hoell, Andrew; Shukla, Shraddhanand; Blade, Ileana; Liebmann, Brant; Roberts, Jason B.; Robertson, Franklin R.

    2014-01-01

    In southern Ethiopia, Eastern Kenya, and southern Somalia poor boreal spring rains in 1999, 2000, 2004, 2007, 2008, 2009 and 2011 contributed to severe food insecurity and high levels of malnutrition. Predicting rainfall deficits in this region on seasonal and decadal time frames can help decision makers support disaster risk reduction while guiding climate-smart adaptation and agricultural development. Building on recent research that links more frequent droughts to a stronger Walker Circulation, warming in the Indo-Pacific warm pool, and an increased western Pacific sea surface temperature (SST) gradient, we explore the dominant modes of East African rainfall variability, links between these modes and sea surface temperatures, and a simple index-based monitoring-prediction system suitable for drought early warning.

  13. Electrical lithosphere beneath the Kaapvaal craton, southern Africa

    NASA Astrophysics Data System (ADS)

    Evans, Rob L.; Jones, Alan G.; Garcia, Xavier; Muller, Mark; Hamilton, Mark; Evans, Shane; Fourie, C. J. S.; Spratt, Jessica; Webb, Susan; Jelsma, Hielke; Hutchins, Dave

    2011-04-01

    A regional-scale magnetotelluric (MT) experiment across the southern African Kaapvaal craton and surrounding terranes, called the Southern African Magnetotelluric Experiment (SAMTEX), has revealed complex structure in the lithospheric mantle. Large variations in maximum resistivity at depths to 200-250 km relate directly to age and tectonic provenance of surface structures. Within the central portions of the Kaapvaal craton are regions of resistive lithosphere about 230 km thick, in agreement with estimates from xenolith thermobarometry and seismic surface wave tomography, but thinner than inferred from seismic body wave tomography. The MT data are unable to discriminate between a completely dry or slightly "damp" (a few hundred parts per million of water) structure within the transitional region at the base of the lithosphere. However, the structure of the uppermost ˜150 km of lithosphere is consistent with enhanced, but still low, conductivities reported for hydrous olivine and orthopyroxene at levels of water reported for Kaapvaal xenoliths. The electrical lithosphere around the Kimberley and Premier diamond mines is thinner than the maximum craton thickness found between Kimberley and Johannesburg/Pretoria. The mantle beneath the Bushveld Complex is highly conducting at depths around 60 km. Possible explanations for these high conductivities include graphite or sulphide and/or iron metals associated with the Bushveld magmatic event. We suggest that one of these conductive phases (most likely melt-related sulphides) could electrically connect iron-rich garnets in a garnet-rich eclogitic composition associated with a relict subduction slab.

  14. Descending lithosphere slab beneath the Northwest Dinarides from teleseismic tomography

    NASA Astrophysics Data System (ADS)

    Šumanovac, Franjo; Dudjak, Darko

    2016-12-01

    The area of study covers the marginal zone between the Adriatic microplate (African plate) and the Pannonian segment (Eurasian plate). We present a tomography model for this area, with special emphasis on the northwest Dinarides. A dense distribution of temporary seismic stations in the area of the Northern Dinarides along with permanent seismic stations located in the area, allowed us to construct this P-wave tomographic model. We assembled our travel-time dataset based on 26 seismic stations were used to collect the dataset. Teleseismic events were recorded for a period of 18 months and a set of 76 distant earthquakes were used to calculate the P-wave travel-time residuals. We calculated relative rather than absolute arrival-time residuals in the inversion to obtain depths of 0-400 km. We imaged a pronounced fast velocity anomaly below the NW Dinarides which directly indicates a lithosphere slab downgoing beneath the Dinarides. This fast anomaly extends towards the NW direction to at least 250 km depth, and we interpreted it as a descending lithosphere slab. The thrusting of the Adriatic microplate may be brought about by sub-lithosphere rising movement beneath the Pannonian region, along with a push from African plate. In our interpretation, the Adriatic lower lithosphere has been detached from the crust, and steeply sinks beneath the Dinarides. A lithosphere model of the contact between the Adriatic microplate and Pannonian tectonic segment was constructed based on the tomographic velocity model and results of previous crustal studies.

  15. 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.

  16. Geochemistry of Archean Mafic Amphibolites from the Amsaga Area, West African Craton, Mauritania: Occurrence of Archean oceanic plateau

    NASA Astrophysics Data System (ADS)

    El Atrassi, Fatima; Debaille, Vinciane; Mattielli, Nadine; Berger, Julien

    2015-04-01

    While Archean terrains are mainly composed of a TTG (Tonalite-trondhjemite-granodiorite) suite, more mafic lithologies such as amphibolites are also a typical component of those ancient terrains. Although mafic rocks represent only ~10% of the Archean cratons, they may provide key evidence of the role and nature of basaltic magmatism in the formation of the Archean crust as well as the evolution of the Archean mantle. This study focuses on the Archean crust from the West African craton in Mauritania (Amsaga area). The Amsaga Archean crust mainly consists of TTG and thrust-imbricated slices of mafic volcanic rocks, which have been affected by polymetamorphic events from the amphibolite to granulite facies. We report the results of a combined petrologic, Sm-Nd isotopic, major element and rare earth element (REE) study of the Archean amphibolites in the West African craton. This study was conducted in order to characterize these rocks, to constrain the time of their formation and to evaluate their tectonic setting and their possible mantle source. Our petrological observations show that these amphibolites have fine to medium granoblastic and nematoblastic textures. They are dominated by amphibolite-facies mineral assemblages (mainly amphibole and plagioclase), but garnet and clinopyroxene occur in a few samples. These amphibolites have tholeiitic basalt composition. On a primitive mantle-normalized diagram, they display fairly flat patterns without negative anomalies for either Eu or Nb-Ta. We have shown using Sm-Nd whole rock isotopic data that these amphibolites formed at 3.3 ±0.075 Ga. They have positive ɛNdi values (+5.2 ± 1.6). These samples show isotopically juvenile features, which rule out the possibility of significant contamination of the protolith magmas by ancient continental crust. Based on these geochemical data we propose that the tholeiitic basalts were formed in an oceanic plateau tectonic setting from a mantle plume source and that they have a

  17. Constraints on Lithosphere Rheology from Observations of Volcano-induced Deformation

    NASA Astrophysics Data System (ADS)

    Zhong, S.; Watts, A. B.

    2011-12-01

    Mantle rheology at lithospheric conditions (i.e., temperature < 1200 oC) is important for understanding fundamental geodynamic problems including the dynamics of plate tectonics, subducted slabs, and lithosphere-mantle interaction. Laboratory studies suggest that the rheology at lithospheric conditions can be approximately divided into three different regimes: brittle or frictional sliding, semi-brittle, and plastic flow. In this study, we seek to constrain lithospheric rheology, using observations of deformation at seamounts and oceanic islands caused by volcanic loading. Volcano-induced surface deformation depends critically on lithospheric rheology at the time of seamount and oceanic island emplacement and while it changes rapidly on short time-scales it does not change significantly on long time-scales. In an earlier study [Watts and Zhong, 2000], we used the effective elastic thickness at seamounts and oceanic islands inferred from the observations of deformation and gravity to determine an effective activation energy of 120 KJ/mol for lithospheric mantle with Newtonian rheology. We have now expanded this study to incorporate non-Newtonian power-law and frictional sliding rheologies, and more importantly, to include realistic 3-D volcanic load geometries. We use the Hawaiian Islands as an example. We construct 3-D loads for the Hawaiian Islands by applying an appropriate median filter to remove Hawaiian swell topography and correcting for lithospheric age effect on the bathymetry. The loads are then used in 3-D finite element loading models with viscoelastic, non-Newtonian and frictional sliding rheologies to determine the lithospheric response including surface vertical motions and lithospheric stresses. Comparisons of our new model predictions to observations suggest that the activation energy of lithospheric mantle is significantly smaller than most experimentally determined values for olivine at high temperatures, but may be consistent with more recent

  18. High-pressure metamorphic age and significance of eclogite-facies continental fragments associated with oceanic lithosphere in the Western Alps (Etirol-Levaz Slice, Valtournenche, Italy)

    NASA Astrophysics Data System (ADS)

    Fassmer, Kathrin; Obermüller, Gerrit; Nagel, Thorsten J.; Kirst, Frederik; Froitzheim, Nikolaus; Sandmann, Sascha; Miladinova, Irena; Fonseca, Raúl O. C.; Münker, Carsten

    2016-05-01

    The Etirol-Levaz Slice in the Penninic Alps (Valtournenche, Italy) is a piece of eclogite-facies continental basement sandwiched between two oceanic units, the blueschist-facies Combin Zone in the hanging wall and the eclogite-facies Zermatt-Saas Zone in the footwall. It has been interpreted as an extensional allochthon from the continental margin of Adria, emplaced onto ultramafic and mafic basement of the future Zermatt-Saas Zone by Jurassic, rifting-related detachment faulting, and later subducted together with the future Zermatt-Saas Zone. Alternatively, the Etirol-Levaz Slice could be derived from a different paleogeographic domain and be separated from the Zermatt-Saas Zone by an Alpine shear zone. We present Lu-Hf whole rock-garnet ages of two eclogite samples, one from the center of the unit and one from the border to the Zermatt-Saas Zone below. These data are accompanied by a new geological map of the Etirol-Levaz Slice and the surrounding area, as well as detailed petrology of these two samples. Assemblages, mineral compositions and garnet zoning in both samples indicate a clockwise PT-path and peak-metamorphic conditions of about 550-600 °C/20-25 kbar, similar to conditions proposed for the underlying Zermatt-Saas Zone. Prograde garnet ages of the two samples are 61.8 ± 1.8 Ma and 52.4 ± 2.1 Ma and reflect different timing of subduction. One of these is significantly older than published ages of eclogite-facies metamorphism in the Zermatt-Saas Zone and thus contradicts the hypothesis of Mesozoic emplacement. The occurrence of serpentinite and metagabbro bodies possibly derived from the Zermatt-Saas Zone inside the Etirol-Levaz Slice suggests that the latter is a tectonic composite. The basement slivers forming the Etirol-Levaz Slice and other continental fragments were subducted earlier than the Zermatt-Saas Zone, but nonetheless experienced similar pressure-temperature histories. Our results support the hypothesis that the Zermatt-Saas Zone and the

  19. Response of the African monsoon to orbital forcing and ocean feedbacks in the middle holocene

    SciTech Connect

    Kutzbach, J.E.; Liu, Z.

    1997-10-17

    Simulations with a climate model that asynchronously couples the atmosphere and the ocean showed that the increased amplitude of the seasonal cycle of insolation in the Northern Hemisphere 6000 years ago could have increased tropical Atlantic sea surface temperatures in late summer. The simulated increase in sea surface temperature and associated changes in atmospheric circulation enhanced the summer monsoon precipitation of northern Africa by more than 25 percent, compared with the middle Holocene simulation with prescribed modern sea surface temperatures, and provided better agreement with paleorecords of enhanced monsoons. 28 refs., 4 figs., 1 tab.

  20. 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.

  1. Accretion and Subduction of Oceanic Lithosphere: 2D and 3D Seismic Studies of Off-Axis Magma Lenses at East Pacific Rise 9°37-40'N Area and Downgoing Juan de Fuca Plate at Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Han, Shuoshuo

    Two thirds of the Earth's lithosphere is covered by the ocean. The oceanic lithosphere is formed at mid-ocean ridges, evolves and interacts with the overlying ocean for millions of years, and is eventually consumed at subduction zones. In this thesis, I use 2D and 3D multichannel seismic (MCS) data to investigate the accretionary and hydrothermal process on the ridge flank of the fast-spreading East Pacific Rise (EPR) at 9°37-40'N and the structure of the downgoing Juan de Fuca plate at the Cascadia subduction zone offshore Oregon and Washington. Using 3D multichannel seismic (MCS) data, I image a series of off-axis magma lenses (OAML) in the middle or lower crust, 2-10 km from the ridge axis at EPR 9°37-40'N. The large OAMLs are associated with Moho travel time anomalies and local volcanic edifices above them, indicating off-axis magmatism contributes to crustal accretion though both intrusion and eruption (Chapter 1). To assess the effect of OAMLs on the upper crustal structure, I conduct 2-D travel time tomography on downward continued MCS data along two across-axis lines above a prominent OAML in our study area. I find higher upper crustal velocity in a region ~ 2 km wide above this OAML compared with the surrounding crust. I attribute these local anomalies to enhanced precipitation of alteration minerals in the pore space of upper crust associated with high-temperature off-axis hydrothermal circulation driven by the OAML (Chapter 2). At Cascadia, a young and hot end-member of the global subduction system, the state of hydration of the downgoing Juan de Fuca (JdF) plate is important to a number of subduction processes, yet is poorly known. As local zones of higher porosity and permeability, faults constitute primary conduits for seawater to enter the crust and potentially uppermost mantle. From pre-stack time migrated MCS images, I observe pervasive faulting in the sediment section up to 200 km from the deformation front. Yet faults with large throw and

  2. Melt inclusion evidence for CO2-rich melts beneath the western branch of the East African Rift: implications for long-term storage of volatiles in the deep lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Hudgins, T. R.; Mukasa, S. B.; Simon, A. C.; Moore, G.; Barifaijo, E.

    2015-05-01

    We present new major element, trace element, and volatile (H2O, CO2, S, F, and Cl) concentrations of olivine-hosted melt inclusions from five high-K, low-silica basanites from the western branch of the East African Rift System and use these data to investigate the generation of H2O- and CO2-rich melts at up to ~150 km depth. Measured H2O and CO2 concentrations reach ~2.5 and ~1 wt%, respectively, representing some of the highest CO2 concentrations measured in a melt inclusion to date. These measurements represent direct evidence of the high CO2 and H2O concentrations required to generate high-K alkaline lavas, and the CO2 that has been previously inferred to be necessary for the low mantle potential temperatures in the area. Ratios of CO2/Nb, CO2/Ba, and CO2/Cl are used to estimate an initial melt CO2 concentration of 5-12 wt%. The measured CO2 concentrations are consistent with CO2 solubilities determined by molecular dynamics calculations and high-pressure experiments for melt generation at 3-6 GPa; the depth of melting suggested by previous studies in the area. These melt inclusions measurements represent direct evidence for the presence of H2O- and CO2-rich melts in the deep upper mantle that have been proposed based on experimental and seismic evidence. Primitive-mantle normalized trace element patterns more closely resemble those found in subduction settings rather than ocean island basalt, and ratios of slab fluid tracers such as Li/Dy and B/Be indicate that the measured volatile abundances may be related to Neoproterozoic subduction during the assembly of Gondwana, implying the storage of volatiles in the mantle by subduction-related metasomatism.

  3. 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.

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

    PubMed

    Rychert, Catherine A; Shearer, Peter M

    2009-04-24

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  6. Heterogeneity of Water Concentrations in the Mantle Lithosphere Beneath Hawaii

    NASA Technical Reports Server (NTRS)

    Bizimis, M.; Peslier, A. H.; Clague, D.

    2017-01-01

    The amount and distribution of water in the oceanic mantle lithosphere has implications on its strength and of the role of volatiles during plume/lithosphere interaction. The latter plays a role in the Earth's deep water cycle as water-rich plume lavas could re-enrich an oceanic lithosphere depleted in water at the ridge, and when this heterogeneous lithosphere gets recycled back into the deep mantle. The main host of water in mantle lithologies are nominally anhydrous minerals like olivine, pyroxene and garnet, where hydrogen (H) is incorporated in mineral defects by bonding to structural oxygen. Here, we report water concentrations by Fourier transform infrared spectrometry (FTIR) on olivine, clino- and orthopyroxenes (Cpx & Opx) from spinel peridotites from the Pali vent and garnet pyroxenite xenoliths from Aliamanu vent, both part of the rejuvenated volcanism at Oahu (Hawaii). Pyroxenes from the Aliamanu pyroxenites have high water concentrations, similar to the adjacent Salt Lake Crater (SLC) pyroxenites (Cpx 400-500 ppm H2O, Opx 200 ppm H2O). This confirms that pyroxenite cumulates form water-rich lithologies within the oceanic lithosphere. In contrast, the Pali peridotites have much lower water concentrations than the SLC ones (<25 ppm vs. 50-96 ppm H2O respectively) despite being relatively fertile with >10% modal Cpx and low spinel Cr# (0.09-0.10). The contrast between the two peridotite suites is also evident in their trace elements and radiogenic isotopes. The Pali Cpx are depleted in light REE, consistent with minimal metasomatism. Those of SLC have enriched light REE patterns and Nd and Hf isotopes consistent with metasomatism by alkaline melts. These observations are consistent with heterogeneous water distribution in the oceanic lithosphere that may be related to metasomatism, as well as relatively dry peridotites cross-cut by narrow (?) water-rich melt reaction zones.

  7. The oceanic nature of the African slab subducted under Peloponnesus: thin-layer resolution from multiscale analysis of teleseismic P-to-S converted waves

    NASA Astrophysics Data System (ADS)

    Gesret, A.; Laigle, M.; Diaz, J.; Sachpazi, M.; Hirn, A.

    2010-11-01

    In the Hellenic subduction zone, the lithospheric slab may comprise continental and oceanic units juxtaposed downdip and along strike. For stations along eastern Peloponnesus, teleseismic P-wave receiver-function (RF) processing in the standard frequency band produces an image of a low-velocity layer (LVL) at the top of the slab apparently twice thicker than for an oceanic crust. To assess if this could come from a lack of resolution of the standard processing, we develop a multiscale approach with the RFs based on the wavelet-response of the medium, akin to the wavelet-transform of the velocity-depth function. The synthetic response in conversion is obtained for a multiscale singularity formed by two opposite velocity-steps at the boundaries of a crust embedded in mantle material. This indicates that only wavelet periods shorter than about 0.8 s will allow to identify clearly a 7 km thin oceanic crust. Going to longer periods leads to underestimate or overestimate the time-thickness of the LVL, due to interference phenomena. The analysis of the response in conversion from full waveform synthetic seismograms in a dipping slab model validates a multiresolution approach to real observations. With earthquakes of broad-enough spectrum towards high frequencies, yielding energy to provide wavelet periods significantly shorter than 1 s, the P-to-S conversions obtained allow us to resolve for the first time a standard oceanic crust at the slab top beneath the eastern coast of Peloponnesus. This documents the subduction of a purely oceanic slab of most reduced buoyancy since 4-5 Myr under the rapidly southwestward extending upper plate continental material.

  8. Metasomatized lithosphere and the origin of alkaline lavas.

    PubMed

    Pilet, Sébastien; Baker, Michael B; Stolper, Edward M

    2008-05-16

    Recycled oceanic crust, with or without sediment, is often invoked as a source component of continental and oceanic alkaline magmas to account for their trace-element and isotopic characteristics. Alternatively, these features have been attributed to sources containing veined, metasomatized lithosphere. In melting experiments on natural amphibole-rich veins at 1.5 gigapascals, we found that partial melts of metasomatic veins can reproduce key major- and trace-element features of oceanic and continental alkaline magmas. Moreover, experiments with hornblendite plus lherzolite showed that reaction of melts of amphibole-rich veins with surrounding lherzolite can explain observed compositional trends from nephelinites to alkali olivine basalts. We conclude that melting of metasomatized lithosphere is a viable alternative to models of alkaline basalt formation by melting of recycled oceanic crust with or without sediment.

  9. 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

  10. The relative role of ocean-atmosphere interaction and African easterly waves in the generation and development of Tropical cyclones in the North Atlantic

    NASA Astrophysics Data System (ADS)

    Cabos, William; Sein, Dmitry; Hodges, Kevin; Jacob, Daniela

    2016-04-01

    We use the regionally coupled ocean - atmosphere model ROM and its atmospheric component REMO in standalone configuration in order to assess the relative role of ocean feedbacks and the African easterly waves in the simulation of tropical cyclonic activity in the Atlantic ocean. To this end, a number of coupled and uncoupled simulations forced by ERA-Interim boundary conditions have been carried out. In one set of simulations, the atmospheric domain includes the Northern Africa land masses, where the easterly waves are formed. In a second set of simulations, the easterly waves are taken from the ERA Interim reanalysis, as atmospheric domain excludes explicitly the African land masses. We study the statistics of modeled tracks of the tropical cyclones in the simulations. We found that the coupling has a strong impact on the number of tropical cyclones generated in the Northern Tropical Atlantic. In the coupled run it was close to the observations, while in the uncoupled runs the number of tropical cyclones was strongly overestimated. The coupling also influences the simulated position of the ITCZ.

  11. Nature and evolution of Neoproterozoic ocean-continent transition: Evidence from the passive margin of the West African craton in NE Mali

    NASA Astrophysics Data System (ADS)

    Renaud, Caby

    2014-03-01

    The Timétrine massif exposed west of the Pan-African suture zone in northeastern Mali belongs to the passive margin of the West African craton facing to the east intra-oceanic arc assemblages and 730 Ma old pre-collisional calc-alkaline plutons. The Timétrine lithologic succession includes from the base to the top Mesoproterozoic cratonic to passive margin formations overlain by deep-sea Fe-Mg schists. Submarine metabasalts and two ultramafic massifs of serpentinized mantle peridotites are inserted as olistoliths towards the top whereas turbidites of continental origin represent the younger unit. Field and petrological data have revealed a distinct metasedimentary sequence attached to the serpentinized peridotites. It essentially consists of impure carbonates, Fe jaspers and polymictic breccias containing altered blocks of mantle peridotites, most rocks being enriched in detrital chromite. This association is interpreted as reworked chemical and detrital sediments derived from the alteration of mafic-ultramafic rocks. It is argued that mantle exhumation above sea floor took place during the Neoproterozoic rifting and crustal thinning period under possible tropical conditions, as suggested by the large volume of silicified serpentinites. In spite of greenschist facies metamorphic overprint characterized by widespread Fe-rich blue amphiboles that are not diagnostic of high-pressure conditions, it is possible to reconstruct a former ocean-continent transition similar to that evidenced for the Mesozoic period, followed by the deposition of syn-to post rift terrigeneous turbidites roughly coeval with ocean spreading some time before 800 Ma. It is concluded that the serpentinite massifs were tectonically emplaced first in an extensional setting, then incorporated within deep-sea sediments as olistoliths and finally transported westward during late Neoproterozoic collisional tectonics onto the West African craton.

  12. 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.

  13. Lithospheric Response of the Anatolian Plateau in the Realm of the Black Sea and the Eastern Mediterranean

    NASA Astrophysics Data System (ADS)

    Ergun, Mustafa

    2016-04-01

    The Eastern Mediterranean and the Middle East make up the southern boundary of the Tethys Ocean for the last 200 Ma by the disintegration of the Pangaea and closure of the Tethys Ocean. It covers the structures: Hellenic and Cyprus arcs; Eastern Anatolian Fault Zone; Bitlis Suture Zone and Zagros Mountains. The northern boundary of the Tethys Ocean is made up the Black Sea and the Caspian Sea, and it extends up to Po valley towards the west (Pontides, Caucasus). Between these two zones the Alp-Himalayan orogenic belt is situated where the Balkan, Anatolia and the Iran plateaus are placed as the remnants of the lost Ocean of the Tethys. The active tectonics of the eastern Mediterranean is the consequences of the convergence between the Africa, Arabian plates in the south and the Eurasian plate in the north. These plates act as converging jaws of vise forming a crustal mosaic in between. The active crustal deformation pattern reveals two N-S trending maximum compression or crustal shortening syntaxes': (i) the eastern Black Sea and the Arabian plate, (ii) the western Black Sea and the Isparta Angle. The transition in young mountain belts, from ocean crust through the agglomeration of arc systems with long histories of oceanic closures, to a continental hinterland is well exemplified by the plate margin in the eastern Mediterranean. The boundary between the African plate and the Aegean/Anatolian microplate is in the process of transition from subduction to collision along the Cyprus Arc. Since the Black Sea has oceanic lithosphere, it is actually a separate plate. However it can be considered as a block, because the Black Sea is a trapped oceanic basin that cannot move freely within the Eurasian Plate. Lying towards the northern margin of orogenic belts related to the closure of the Tethys Ocean, it is generally considered to be a result of back-arc extension associated with the northward subduction of the Tethyan plate to the south. Interface oceanic lithosphere at

  14. How thick is the lithosphere?

    PubMed

    Kanamori, H; Press, F

    1970-04-25

    A rapid decrease in shear velocity in the suboceanic mantle is used to infer the thickness of the lithosphere. It is proposed that new and highly precise group velocity data constrain the solutions and imply a thickness near 70 km.

  15. The influence of Pleistocene climatic changes and ocean currents on the phylogeography of the southern African barnacle, Tetraclita serrata (Thoracica; Cirripedia).

    PubMed

    Reynolds, Terry V; Matthee, Conrad A; von der Heyden, Sophie

    2014-01-01

    The evolutionary effects of glacial periods are poorly understood for Southern Hemisphere marine intertidal species, particularly obligatory sessile organisms. We examined this by assessing the phylogeographic patterns of the southern African volcano barnacle, Tetraclita serrata, a dominant species on rocky intertidal shores. Restricted gene flow in some geographical areas was hypothesized based on oceanic circulation patterns and known biogeographic regions. Barnacle population genetic structure was investigated using the mitochondrial cytochrome oxidase subunit 1 (COI) region for 410 individuals sampled from 20 localities spanning the South African coast. The mtDNA data were augmented by generating nuclear internal transcribed spacer 1 (ITS1) sequences from a subset of samples. Phylogenetic and population genetic analyses of mitochondrial DNA data reveal two distinct clades with mostly sympatric distributions, whereas nuclear analyses reveal only a single lineage. Shallow, but significant structure (0.0041-0.0065, P<0.01) was detected for the mtDNA data set, with the south-west African region identified as harbouring the highest levels of genetic diversity. Gene flow analyses on the mtDNA data show that individuals sampled in south-western localities experience gene flow primarily in the direction of the Benguela Current, while south and eastern localities experience bi-directional gene flow, suggesting an influence of both the inshore currents and the offshore Agulhas Current in the larval distribution of T. serrata. The mtDNA haplotype network, Bayesian Skyline Plots, mismatch distributions and time since expansion indicate that T. serrata population numbers were not severely affected by the Last Glacial Maximum (LGM), unlike other southern African marine species. The processes resulting in the two morphologically cryptic mtDNA lineages may be the result of a recent historical allopatric event followed by secondary contact or could reflect selective pressures

  16. The Influence of Pleistocene Climatic Changes and Ocean Currents on the Phylogeography of the Southern African Barnacle, Tetraclita serrata (Thoracica; Cirripedia)

    PubMed Central

    Reynolds, Terry V.; Matthee, Conrad A.; von der Heyden, Sophie

    2014-01-01

    The evolutionary effects of glacial periods are poorly understood for Southern Hemisphere marine intertidal species, particularly obligatory sessile organisms. We examined this by assessing the phylogeographic patterns of the southern African volcano barnacle, Tetraclita serrata, a dominant species on rocky intertidal shores. Restricted gene flow in some geographical areas was hypothesized based on oceanic circulation patterns and known biogeographic regions. Barnacle population genetic structure was investigated using the mitochondrial cytochrome oxidase subunit 1 (COI) region for 410 individuals sampled from 20 localities spanning the South African coast. The mtDNA data were augmented by generating nuclear internal transcribed spacer 1 (ITS1) sequences from a subset of samples. Phylogenetic and population genetic analyses of mitochondrial DNA data reveal two distinct clades with mostly sympatric distributions, whereas nuclear analyses reveal only a single lineage. Shallow, but significant structure (0.0041–0.0065, P<0.01) was detected for the mtDNA data set, with the south-west African region identified as harbouring the highest levels of genetic diversity. Gene flow analyses on the mtDNA data show that individuals sampled in south-western localities experience gene flow primarily in the direction of the Benguela Current, while south and eastern localities experience bi-directional gene flow, suggesting an influence of both the inshore currents and the offshore Agulhas Current in the larval distribution of T. serrata. The mtDNA haplotype network, Bayesian Skyline Plots, mismatch distributions and time since expansion indicate that T. serrata population numbers were not severely affected by the Last Glacial Maximum (LGM), unlike other southern African marine species. The processes resulting in the two morphologically cryptic mtDNA lineages may be the result of a recent historical allopatric event followed by secondary contact or could reflect selective pressures

  17. Continental collision with a sandwiched accreted terrane: Insights into Himalayan-Tibetan lithospheric mantle tectonics?

    NASA Astrophysics Data System (ADS)

    Kelly, Sean; Butler, Jared P.; Beaumont, Christopher

    2016-12-01

    Many collisional orogens contain exotic terranes that were accreted to either the subducting or overriding plate prior to terminal continent-continent collision. The ways in which the physical properties of these terranes influence collision remain poorly understood. We use 2D thermomechanical finite element models to examine the effects of prior 'soft' terrane accretion to a continental upper plate (retro-lithosphere) on the ensuing continent-continent collision. The experiments explore how the style of collision changes in response to variations in the density and viscosity of the accreted terrane lithospheric mantle, as well as the density of the pro-lithospheric mantle, which determines its propensity to subduct or compress the accreted terrane and retro-lithosphere. The models evolve self-consistently through several emergent phases: breakoff of subducted oceanic lithosphere; pro-continent subduction; shortening of the retro-lithosphere accreted terrane, sometimes accompanied by lithospheric delamination; and, terminal underthrusting of pro-lithospheric mantle beneath the accreted terrane crust or mantle. The modeled variations in the properties of the accreted terrane lithospheric mantle can be interpreted to reflect metasomatism during earlier oceanic subduction beneath the terrane. Strongly metasomatized (i.e., dense and weak) mantle is easily removed by delamination or entrainment by the subducting pro-lithosphere, and facilitates later flat-slab underthrusting. The models are a prototype representation of the Himalayan-Tibetan orogeny in which there is only one accreted terrane, representing the Lhasa terrane, but they nonetheless exhibit processes like those inferred for the more complex Himalayan-Tibetan system. Present-day underthrusting of the Tibetan Plateau crust by Indian mantle lithosphere requires that the Lhasa terrane lithospheric mantle has been removed. Some of the model results support previous conceptual interpretations that Tibetan

  18. Lithospheric mantle duplex beneath the central Mojave Desert revealed by xenoliths from Dish Hill, California

    NASA Astrophysics Data System (ADS)

    Luffi, Peter; Saleeby, Jason B.; Lee, Cin-Ty A.; Ducea, Mihai N.

    2009-03-01

    Low-angle subduction of oceanic lithosphere may be an important process in modifying continental lithosphere. A classic example is the underthrusting of the Farallon plate beneath North America during the Laramide orogeny. To assess the relevance of this process to the evolution and composition of continental lithosphere, the mantle stratigraphy beneath the Mojave Desert was constrained using ultramafic xenoliths hosted in Plio-Pleistocene cinder cones. Whole-rock chemistry, clinopyroxene trace element and Nd isotope data, in combination with geothermometry and surface heat flow, indicate kilometer-scale compositional layering. The shallow parts are depleted in radiogenic Nd (ɛNd = -13 to -6.4) and are interpreted to be ancient continental mantle that escaped tectonic erosion by low-angle subduction. The deeper samples are enriched in radiogenic Nd (ɛNd = +5.7 to +16.1) and reveal two superposed mantle slices of recent origin. Within each slice, compositions range from fertile lherzolites at the top to harzburgites at the bottom: the latter formed by 25-28% low-pressure melt depletion and the former formed by refertilization of harzburgites by mid-ocean-ridge-basalt-like liquids. The superposition and internal compositional zonation of the slices preclude recent fertilization by Cenozoic extension-related magmas. The above observations imply that the lower Mojavian lithosphere represents tectonically subcreted and imbricated lithosphere having an oceanic protolith. If so, the lherzolitic domains may be related to melting and refertilization beneath mid-ocean ridges. The present Mojavian lithosphere is thus a composite of a shallow section of the original North American lithosphere underlain by Farallon oceanic lithosphere accreted during low-angle subduction.

  19. Lithospheric Architecture Beneath Hudson Bay

    NASA Astrophysics Data System (ADS)

    Porritt, R. W.; Miller, M. S.; Darbyshire, F. A.

    2015-12-01

    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 lithosphere-asthenosphere boundary (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. Lithospheric structure beneath Eastern Africa from joint inversion of receiver functions and Rayleigh wave velocities

    NASA Astrophysics Data System (ADS)

    Dugda, Mulugeta Tuji

    Crust and upper mantle structure beneath eastern Africa has been investigated using receiver functions and surface wave dispersion measurements to understand the impact of the hotspot tectonism found there on the lithospheric structure of the region. In the first part of this thesis, I applied H-kappa stacking of receiver functions, and a joint inversion of receiver functions and Rayleigh wave group velocities to determine the crustal parameters under Djibouti. The two methods give consistent results. The crust beneath the GEOSCOPE station ATD has a thickness of 23+/-1.5 km and a Poisson's ratio of 0.31+/-0.02. Previous studies give crustal thickness beneath Djibouti to be between 8 and 10 km. I found it necessary to reinterprete refraction profiles for Djibouti from a previous study. The crustal structure obtained for ATD is similar to adjacent crustal structure in many other parts of central and eastern Afar. The high Poisson's ratio and Vp throughout most of the crust indicate a mafic composition, suggesting that the crust in Afar consists predominantly of new igneous rock emplaced during the late synrift stage where extension is accommodated within magmatic segments by diking. In the second part of this thesis, the seismic velocity structure of the crust and upper mantle beneath Ethiopia and Djibouti has been investigated by jointly inverting receiver functions and Rayleigh wave group velocities to obtain new constraints on the thermal structure of the lithosphere. Crustal structure from the joint inversion for Ethiopia and Djibouti is similar to previously published models. Beneath the Main Ethiopian Rift (MER) and Afar, the lithospheric mantle has a maximum shear wave velocity of 4.1-4.2 km/s and extends to a depth of at most 50 km. In comparison to the lithosphere away from the East African Rift System in Tanzania, where the lid extends to depths of ˜100-125 km and has a maximum shear velocity of 4.6 km/s, the mantle lithosphere under the Ethiopian Plateau

  1. The North West African Margin Magnetic Anomaly revisited : implications for the initial evolution of the Central Atlantic Ocean

    NASA Astrophysics Data System (ADS)

    Sahabi, M.; Olivet, J.-L.; Aslanian, D.; Patriat, M.; Géli, L.; Matias, L.; Réhault, J.-P.; Malod, J.; Bouabdelli, M.

    2003-04-01

    Due to the lack of data from the North West African margin, the Mesozïc evolution of the Central Atlantic is still controversial. Existing plate kinematics (Le Pichon et al, 1977), Wissmann and Roger (1982), Olivet et al, 1984, Klitgord and Schouten, 1986) reconstructions do not explain the characteristics of the S1 Magnetic Anomaly, nor the the presence and geometry of salt basins on the margins off NW Marocco and off Mauritania. We present a new magnetic compilation detailing the correspondance between the different conjugated magnetic anomalies that exist on each side of the Central Atlantic : the East Coast (ECMA), Brunswick (BMA) and Blake Spur (BSMA) Magnetic Anomalies on the American side, and the S1 and West African Coast (WACMA) magnetic anomalies on the African side. In addition, using all available, academic, seismic data, we mapped the ocenawards extension of the salt province of the 200 Ma old Seine Abyssal Plain basin, off Marocco, which is considered as autochtonous.

  2. The Lithospheric Geoid as a Constraint on Plate Dynamics

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

    100 years after Wegener's pioneering work there is still considerable debate about the dynamics of present-day plate motions. A better understanding of present-day dynamics is key to a better understanding of the supercontinent cycle. The Earth's gravity field is one of the primary data sets to help constrain horizontal density contrasts, and hence plate dynamic forces. Previous work has shown that the global average for the geoid step up from old oceanic lithosphere across passive continental margins to stable continental lithosphere is about 6-9m, and the global average for the geoid anomaly associated with cooling oceanic lithosphere (the so-called "ridge push") is 10-12m. The ridge geoid anomaly corresponds to a net force of ~3x1012N/m (averaged over the thickness of the lithosphere) due to 'ridge push.' However, for individual continental margins and mid-ocean ridge systems, there is considerable variation in the geoid step and geoid anomaly and consequently the associated forces contributing to the stress field. We explore the variation in geoid step across passive continental margins looking for correlations with age of continental breakup (and hence place within the supercontinent cycle), hot spot tracks, continental plate velocities, long-wavelength geoid energy (that may be masking signal), and small scale convection. For mid-ocean ridges, we explore variations in geoid anomaly looking for correlations with plate spreading rates, hot spot tracks, long-wavelength geoid energy (that may be masking signal), and small scale convection. We use a band-pass spherical harmonic filter on the full geoid (e.g., EGM2008-WGS84, complete to spherical harmonic degree and order 2159) between orders 6 and 80. The evaluation of the role of spatial variations in the geoid gradient for cooling oceanic lithosphere and across the continental margin in the dynamics of the intraplate stress field requires high spatial resolution modeling. We perform a high resolution finite

  3. 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

  4. 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

  5. Timing of maturation of a Neoproterozoic oceanic arc during Pan-African Orogeny: the Asmlil complex (Anti-Atlas, South Morocco)

    NASA Astrophysics Data System (ADS)

    Triantafyllou, Antoine; Berger, Julien; Baele, Jean-Marc; Bruguier, Olivier; Diot, Hervé; Ennih, Nasser; Plissart, Gaëlle; Monnier, Christophe; Watlet, Arnaud; Vandycke, Sara

    2016-04-01

    Many intra-oceanic paleo-arcs are exposed in the Pan-African belt surrounding the West African Craton. In the Moroccan Anti-Atlas, remnants of Intra-Oceanic Subduction Zone (IOSZ) are preserved in few erosional windows moulded along the Anti-Atlas Major fault. These complexes highlight a Neoproterozoic paleo-suture made of 760 My back-arc ophiolites thrusted to the south onto a dismembered band of oceanic arc relics. The Asmlil arc complex, located in the southern part of the Bou Azzer inlier, is made of (i) 755 to 745 My- intermediate banded gneiss interpreted as metavolcanic products of a juvenile oceanic arc. This latter has been intruded by (ii) medium-grained hornblende-gabbro and dioritic magmas, in turn intruded by (iii) medium- to coarse grained hornblenditic-granodioritic decametric intrusions under sub-magmatic HT conditions. Hornblende-gabbros are made of garnet + amphibole/cpx relics + epidote + rutile paragenesis. Calculated pseudosections yielded P ~ 11-12 kbar for T ranging between 600 and 720°C for garnet growth. Measured Zr-in-rutile thermometer gave slightly higher temperature ranging between 710-790°C. On the field, garnet-rich leucocratic veinlets suggest that moderate partial melting of the mafic rock or localized dehydration reactions took place under garnet-granulite conditions (>800°C for hydrated chemical system). New geochronological data on garnet-bearing leucogabbros constrain their emplacement at 700 ±7 My (U-Pb zircon with low Th/U < 0.3). Cooling age (< 700°C) of these HP-HT rocks yielded to a younger age of 654 ±7 My (U-Pb method on rutile). Geochemical data of each mafic and ultramafic facies (hornblende gabbro, garnet-bearing facies and hornblendite) show typical arc signature (marked by e.g. Nb-Ta anomaly, (La/Sm)N: 0.8-1.6 ; (Nb/La) < 0.46 ; high Nb/Ba ratio ; 0.4 < K2O < 2.1 wt%). Intrusive granodioritic magmas show depleted HREE trend similar to granitoids in the Kohistan paleo-arc. Melting modeling suggests they are

  6. 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.

  7. 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

  8. Episodic, Multi-staged Lithospheric Delamination Responsible for Destruction of the North China Craton

    NASA Astrophysics Data System (ADS)

    Huang, J.; Wang, Y.; Zhong, S.

    2013-12-01

    Archean cratons represent the oldest tectonic units on the Earth and most of them are tectonically stable for >3 Ga. The North China Craton (NCC), however, had undergone extensive destruction during the Mesozoic to Cenozoic as seen from surface volcanism, magmatism, and tectonic deformation and geochemical and seismic observations suggesting removal and replacement of thick, old, and fertile cratonic lithosphere with thin, young, and depleted oceanic-type lithosphere [Griffin et al., 1998; Xu, 2001; Menzies et al., 2007; Zhu et al., 2012; Zhang et al., 2012]. Lithospheric delamination has been proposed to explain different episodes of volcanism in the Jurassic [Gao et al., 2004; 2008] and Cretaceous [Yang et al., 2003; Wu et al., 2003] on NCC and hence as a mechanism for destruction of NCC. However, the relatively long period (~100 Myr) of volcanism associated with the destruction of NCC was considered as a challenge to the delamination process [Menzies et al., 2007] which typically lasts for several Myr [Conrad and Molnar, 1999]. Here we show that delamination for cratonic lithosphere with chemically buoyant root and non-Newtonian rheology, different from that for normal lithosphere that was considered in most previous geodynamic studies, is episodic and multi-staged and may last for tens to 100 Myrs. For cratonic lithosphere with non-Newtonian rheology with relatively large chemical buoyancy, the cold, shallow part of the lithosphere goes unstable first, causing significant stirring and mixing of asthenospheric mantle and cratonic lithosphere. This delamination process may explain the main geochemical signatures in the Jurassic and Cretaceous volcanic rocks found in the NCC including their eclogite component [Gao et al., 2004, 2008] and sourcing both cratonic lithosphere and asthenosphere [Zheng et al., 2000]. Subduction process, by increasing tectonic stress and water content, helps reduce the lithospheric viscosity sufficiently to delaminate the entire

  9. Constraining Lithosphere Deformation Modes during Continental Breakup for the Iberia-Newfoundland Conjugate Margins

    NASA Astrophysics Data System (ADS)

    Jeanniot, L.; Kusznir, N. J.; Mohn, G.; Manatschal, G.

    2014-12-01

    How the lithosphere and asthenosphere deforms during continental rifting leading to breakup and sea-floor spreading initiation is poorly understood. Observations at present-day and fossil analogue rifted margins show a complex OCT architecture which cannot be explained by a single simplistic lithosphere deformation modes. This OCT complexity includes hyper-extended continental crust and lithosphere, detachments faults, exhumed mantle, continental slivers and scattered embryonic oceanic crust. We use a coupled kinematic-dynamic model of lithosphere and asthenosphere deformation to determine the sequence of lithosphere deformation modes leading to continental breakup for Iberia-Newfoundland conjugate margin profiles. We quantitatively calibrate the models using observed present-day water loaded subsidence and crustal thickness, together with subsidence history and the age of melt generation. Flow fields, representing a sequence of lithosphere deformation modes, are generated by a 2D finite element viscous flow model (FE-Margin), and used to advect lithosphere and asthenosphere temperature and material. FE-Margin is kinematically driven by divergent deformation in the upper 15-20 km of the lithosphere inducing passive upwelling below. Buoyancy enhanced upwelling (Braun et al. 2000) is also kinematically included. Melt generation by decompressional melting is predicted using the methodology of Katz et al., 2003. The extension magnitudes used in the lithosphere deformation models are taken from Sutra et al (2013). The best fit calibrated models of lithosphere deformation evolution for the Iberia-Newfoundland conjugate margins require (i) an initial broad region of lithosphere deformation and passive upwelling, (ii) lateral migration of deformation, (iii) an increase in extension rate with time, (iv) focussing of deformation and (v) buoyancy induced upwelling. The preferred calibrated models predict faster extension rates and earlier continental crustal rupture and

  10. 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.

  11. 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

  12. 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.

  13. 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.

  14. 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.

  15. Microearthquake activity, lithospheric structure, and deformation modes at an amagmatic ultraslow spreading Southwest Indian Ridge segment

    NASA Astrophysics Data System (ADS)

    Schmid, Florian; Schlindwein, Vera

    2016-07-01

    While nascent oceanic lithosphere at slow to fast spreading mid-ocean ridges (MOR) is relatively well studied, much less is known about the lithospheric structure and properties at ultraslow MORs. Here we present microearthquake data from a 1 year ocean bottom seismometer deployment at the amagmatic, oblique supersegment of the ultraslow spreading Southwest Indian Ridge. A refraction seismic experiment was performed to constrain upper lithosphere P-velocities and results were used to construct a 1D velocity model for earthquake location. Earthquake foci were located individually and subsequently relocated relative to each other to sharpen the image of seismically active structures. Frequent earthquake activity extends to 31 km beneath the seafloor, indicating an exceptionally thick brittle lithosphere and an undulating brittle-ductile transition that implies significant variations in the along-axis thermal structure of the lithosphere. We observe a strong relation between petrology, microseismicity distribution, and topography along the ridge axis: Peridotite-dominated areas associate with deepest hypocenters, vast volumes of lithosphere that deforms aseismically as a consequence of alteration, and the deepest axial rift valley. Areas of basalt exposure correspond to shallower hypocenters, shallower and more rugged axial seafloor. Focal mechanisms deviate from pure extension and are spatially variable. Earthquakes form an undulating band of background seismicity and do not delineate discrete detachment faults as common on slow spreading ridges. Instead, the seismicity band sharply terminates to the south, immediately beneath the rift boundary. Considering the deep alteration, large steep boundary faults might be present but are entirely aseismic.

  16. Is the Venusian lithosphere subducting?

    NASA Technical Reports Server (NTRS)

    Sandwell, David T.; Schubert, Gerald

    1992-01-01

    Using data collected by the Magellan spacecraft, we are exploring the hypothesis that the cooler and more rigid outer layer of Venus (i.e., the lithosphere) is sinking (subducting) into the interior of Venus. If this process is occurring, it provides a mechanism for cooling the interior of Venus and also for recycling the lighter crustal rocks back into the interior. In addition, since subduction zones drive the plate tectonic motion on the Earth, evidence for lithospheric subduction on Venus raises the possibility of limited plate tectonic-like activity on Venus.

  17. Failure strength of icy lithospheres

    NASA Technical Reports Server (NTRS)

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

    1987-01-01

    Lithospheric strengths derived from friction on pre-existing fractures and ductile flow laws show that the tensile strength of intact ice under applicable conditions is actually an order of magnitude stronger than widely assumed. It is demonstrated that this strength is everywhere greater than that required to initiate frictional sliding on pre-existing fractures and faults. Because the tensile strength of intact ice increases markedly with confining pressure, it actually exceeds the frictional strength at all depths. Thus, icy lithospheres will fail by frictional slip along pre-existing fractures at yeild stresses greater than previously assumed rather than opening tensile cracks in intact ice.

  18. Seismic constraints on the lithosphere-asthenosphere boundary

    NASA Astrophysics Data System (ADS)

    Rychert, Catherine A.

    2014-05-01

    The basic tenet of plate tectonics is that a rigid plate, or lithosphere, moves over a weaker asthenospheric layer. However, the exact location and defining mechanism of the boundary at the base of the plate, the lithosphere-asthenosphere boundary (LAB) is debated. The oceans should represent a simple scenario since the lithosphere is predicted to thicken with seafloor age if it thermally defined, whereas a constant plate thickness might indicate a compositional definition. However, the oceans are remote and difficult to constrain, and studies with different sensitivities and resolutions have come to different conclusions. Hotspot regions lend additional insight, since they are relatively well instrumented with seismic stations, and also since the effect of a thermal plume on the LAB should depend on the defining mechanism of the plate. Here I present new results using S-to-P receiver functions to image upper mantle discontinuity structure beneath volcanically active regions including Hawaii, Iceland, Galapagos, and Afar. In particular I focus on the lithosphere-asthenosphere boundary and discontinuities related to the base of melting, which can be used to highlight plume locations. I image a lithosphere-asthenosphere boundary in the 50 - 95 km depth range beneath Hawaii, Galapagos, and Iceland. Although LAB depth variations exist within these regions, significant thinning is not observed in the locations of hypothesized plume impingement from receiver functions (see below). Since a purely thermally defined lithosphere is expected to thin significantly in the presence of a thermal plume anomaly, a compositional component in the definition of the LAB is implied. Beneath Afar, an LAB is imaged at 75 km depth on the flank of the rift, but no LAB is imaged beneath the rift itself. The transition from flank of rift is relatively abrupt, again suggesting something other than a purely thermally defined lithosphere. Melt may also exist in the asthenosphere in these regions

  19. East African cassava mosaic-like viruses from Africa to Indian ocean islands: molecular diversity, evolutionary history and geographical dissemination of a bipartite begomovirus

    PubMed Central

    2012-01-01

    Background Cassava (Manihot esculenta) is a major food source for over 200 million sub-Saharan Africans. Unfortunately, its cultivation is severely hampered by cassava mosaic disease (CMD). Caused by a complex of bipartite cassava mosaic geminiviruses (CMG) species (Family: Geminivirideae; Genus: Begomovirus) CMD has been widely described throughout Africa and it is apparent that CMG's are expanding their geographical distribution. Determining where and when CMG movements have occurred could help curtail its spread and reveal the ecological and anthropic factors associated with similar viral invasions. We applied Bayesian phylogeographic inference and recombination analyses to available and newly described CMG sequences to reconstruct a plausible history of CMG diversification and migration between Africa and South West Indian Ocean (SWIO) islands. Results The isolation and analysis of 114 DNA-A and 41 DNA-B sequences demonstrated the presence of three CMG species circulating in the Comoros and Seychelles archipelagos (East African cassava mosaic virus, EACMV; East African cassava mosaic Kenya virus, EACMKV; and East African cassava mosaic Cameroon virus, EACMCV). Phylogeographic analyses suggest that CMG’s presence on these SWIO islands is probably the result of at least four independent introduction events from mainland Africa occurring between 1988 and 2009. Amongst the islands of the Comoros archipelago, two major migration pathways were inferred: One from Grande Comore to Mohéli and the second from Mayotte to Anjouan. While only two recombination events characteristic of SWIO islands isolates were identified, numerous re-assortments events were detected between EACMV and EACMKV, which seem to almost freely interchange their genome components. Conclusions Rapid and extensive virus spread within the SWIO islands was demonstrated for three CMG complex species. Strong evolutionary or ecological interaction between CMG species may explain both their propensity

  20. Crustal Thickness and Lithospheric Structure in Northwestern Namibia from the WALPASS experiment

    NASA Astrophysics Data System (ADS)

    Heit, Benjamin; Yuan, Xiaohui; Geissler, Wolfram; Lushetile, Bufelo; Weber, Michael; Jokat, Wilfried

    2013-04-01

    An amphibian passive-source seismic network (WALPASS) have been deployed for a period of two years in the area where the Walvis Ridge intersects with the continental margin of northwestern Namibia. The deployment was intended to study and map the lithospheric and upper mantle structure in the ocean-continent transition beneath the passive continental margin. The main idea is to find seismic anomalies related to the postulated hotspot track from the continent to the ocean along the Walvis Ridge that links the Etendeka continental flood-basalt province to the Tristan da Cunha hotspot in the middle Atlantic ocean. This could provide clues that help us to better understand the role of plume-lithosphere interaction during the continental break-up. We present here first estimates of crustal and lithospheric thicknesses along with a map of distribution of local seismicity in this geophysically little studied region.

  1. The Middle Neoproterozoic Sidi Flah Group (Anti-Atlas, Morocco): synrift deposition in a Pan-African continent/ocean transition zone

    NASA Astrophysics Data System (ADS)

    Fekkak, A.; Pouclet, A.; Benharref, M.

    2003-08-01

    The Middle Neoproterozoic (Cryogenian) Sidi Flah Group rocks are located in the Saghro inlier of the Eastern Anti-Atlas and consists of siliciclastic detrital sediment, interbedded basaltic lavas and small ultramafic bodies. Sediment deposition occurred in three turbiditic formations of a deep-sea fan environment and was controlled by synsedimentary collapses. The composition of sandstones and typological study of zircons indicate that detrital material came from the gneisses and granites of a proximal craton. The lavas are synsedimentary subaqueous flows. They show chemical signatures of initial rift tholeiites and of plume-related alkaline intraplate basalts. The ultramafic rocks are serpentinized peridotites that were emplaced along N160° synsedimentary faults as numerous bodies 20-50 m in size. Their petrographical (Cr-spinel signature) and chemical features correspond to intracontinental ultramafic cumulates. The emplacement of the ultramafic rocks was associated with hydrothermal activity that generated calcareous and siliceous rocks such as ophicalcites and jaspers. All the features of the sediments, the lavas and the ultramafic bodies strongly suggest a continent-ocean transition geotectonic context, in an advanced stage of continental rifting that we attribute to the pre-Pan-African ocean passive margin extension.

  2. Investigating the Lithospheric Structure of Southern Madagascar

    NASA Astrophysics Data System (ADS)

    Tilmann, F. J.; Yuan, X.; Rumpker, G.; Heit, B.; Rambolamana, G.; Rindraharisaona, E.; Priestley, K. F.

    2013-12-01

    The island of Madagascar occupies a key region in both the assembly and the multi-stage breakup of Gondwanaland, itself part of the super-continent Pangaea. Madagascar consists of an amalgamation of continental material, with the oldest rocks being of Archaean age. Its ancient fabric is characterised by several shear zones, some of them running oblique to the N-S trend, in particular in the south of the island. More recently during the Neogene, moderate volcanism has occurred in the Central and Northern part of the island, and there are indications of uplift throughout Eastern Madagascar over the last 10 Ma. Although Madagascar is now located within the interior of the African plate and far away from major plate boundaries (> 1000 km from the East African rift system and even further from the Central and South-West Indian Ridges), its seismic activity indicates that some deformation is taking place, and present-day kinematic models based on geodetic data and earthquake moment tensors in the global catalogues identify a diffuse N-S-oriented minor boundary separating two microplates, which appears to pass through Madagascar. In spite of the presence of Archaean and Proterozoic rocks continent-wide scale studies indicate a thin lithosphere (<120 km) throughout Madagascar, but are based on sparse data and cannot resolve the difference between eastern and western Madagascar. We are operating a ENE-WSW oriented linear array of 25 broadband stations in southern Madagascar, extending from coast to coast and sampling the sedimentary basins in the west as well as the metamorphic rocks in the East, cutting geological boundaries seen at the surface at high angle. The array crosses the prominent Bongolava-Ranotsara shear zone which is thought to have been formed during Gondwanaland assembly. The array recorded the magnitude 5.3 earthquake of January 25, 2013 which occurred just off its western edge. In addition, in May 2013 we have deployed 25 short period sensors in the

  3. 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

  4. 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

  5. Seawater cycled throughout Earth's mantle in partially serpentinized lithosphere

    NASA Astrophysics Data System (ADS)

    Kendrick, M. A.; Hémond, C.; Kamenetsky, V. S.; Danyushevsky, L.; Devey, C. W.; Rodemann, T.; Jackson, M. G.; Perfit, M. R.

    2017-02-01

    The extent to which water and halogens in Earth's mantle have primordial origins, or are dominated by seawater-derived components introduced by subduction is debated. About 90% of non-radiogenic xenon in the Earth's mantle has a subducted atmospheric origin, but the degree to which atmospheric gases and other seawater components are coupled during subduction is unclear. Here we present the concentrations of water and halogens in samples of magmatic glasses collected from mid-ocean ridges and ocean islands globally. We show that water and halogen enrichment is unexpectedly associated with trace element signatures characteristic of dehydrated oceanic crust, and that the most incompatible halogens have relatively uniform abundance ratios that are different from primitive mantle values. Taken together, these results imply that Earth's mantle is highly processed and that most of its water and halogens were introduced by the subduction of serpentinized lithospheric mantle associated with dehydrated oceanic crust.

  6. Modes of Extension and Oceanization at Magma-Poor Margins: An Example from the Brazilian-African Margins

    NASA Astrophysics Data System (ADS)

    Perez-Gussinye, M.; Araujo, M. N.; Ros, E.; Andres-Martinez, M.; Morgan, J. P.

    2015-12-01

    It is well known that the amount of magmatism and occurrence of serpentinised mantle at rifted margins and oceanic ridges fundamentally depends on spreading rate and mantle potential temperature. Here we show that during continental extension the lower crustal strength exerts an additional fundamental control on the onset and amount of melting and serpentinisation and therefore the type of oceanization. Furthermore, using numerical modeling constrained by multi-channel seismic reflection and wide-angle data from the magma-poor margins of Brazil and Africa, we also show that it is possible to associate margin architectural styles to types of ocean-continent transitions. Observed margin architectural styles can be explained by a combination of extensional modes: core-complex, wide and narrow (Buck, 1991), with a fourth mode, sequential faulting, that accounts for conjugate margin asymmetry (Ranero & Perez-Gussinye, 2010, Brune et al., 2014). The prevalence of any of these modes during extension, which depends on lower crustal strength, controls mantle uplift velocity and hence the relative amounts and timing of melting and serpentinisation, and the character of the ocean-continent transition.

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

    NASA Astrophysics Data System (ADS)

    Wittig, Nadine

    2014-05-01

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

  8. Neoproterozoic oceanic arc remnants in the Moroccan Anti-Atlas: reconstructing deep to shallow arc crustal sequence and tracking Pan-African subduction-accretion processes

    NASA Astrophysics Data System (ADS)

    Triantafyllou, Antoine; Berger, Julien; Baele, Jean-Marc; Bruguier, Olivier; Diot, Hervé; Ennih, Nasser; Plissart, Gaëlle; Monnier, Christophe; Spagna, Paul; Watlet, Arnaud; Vandycke, Sara

    2015-04-01

    The Pan-African belt of West and North Africa exposes many intra-oceanic arc complexes while they are rather uncommon in Phanerozoic orogenic belts. Intra-Oceanic Subduction Zone (IOSZ) in the Moroccan Anti-Atlas crop out in two tectonic windows moulded along the Anti-Atlas Major fault: the Sirwa (western-) and the Bou Azzer (eastern- part) inliers, associated with 760 Ma back-arc ophiolites. These arc sequences are located at the south of the ophiolites and are named the Iriri-Tachakoucht (Sirwa window) and the Asmlil arc complexes (Bou Azzer inlier). (i) The Iriri-Tachakoucht unit is composed of coarse grained hornblendite lenticular plugs, medium-grained hornblende gabbro dykes intruding andesitic to dacitic porphyroclastic gneiss. The contact between both lithologies is gradual and marked by an increasing migmatitization of the gneisses towards hornblendite intrusions. Phase diagram calculation were performed on garnet-bearing gneisses. Garnet cores have grown during a prograde P-T path up to upper amphibolite facies conditions (660°C at ~9 kbar) and recorded the burial of the Tachakoucht metavolcanics, while rims composition indicates that the rock recrystallized under higher temperature conditions (800°C at 4-5 kbar). These HT conditions match those for hornblendites igneous emplacement (850°C and 4 kbar) and this event leaded to more pronounced but still limited partial melting (< 10% melting) of the porphyroclastic gneisses. New geochronological data on the migmatitic gneiss (zircon U-Pb dating) constrain the protolith age at 733 ±7 Ma (zircons core) and the HT tectono-metamorphic event at 654 ±7 Ma (zircons rim). (ii) The Asmlil arc complex is made of hornblende gabbros and garnet-bearing gabbros intruded under HT conditions by dykes of medium-grained hornblendites, hornblende-gabbros and leucodiorites. These metagabbroic intrusions have been dated at 697 ± 8 Ma (U-Pb zircons). P-T pseudosections were calculated for garnet-bearing gabbros and

  9. 3D Crustal and Lithospheric Structures in the Southeastern Mediterranean and Northeastern Egypt

    NASA Astrophysics Data System (ADS)

    Saleh, Salah

    2013-12-01

    Crustal and lithospheric thicknesses of the southeastern Mediterranean Basin region were determined using 3D Bouguer and elevation data analysis. The model is based on the assumption of local isostatic equilibrium. The calculated regional and residual Bouguer anomaly maps were employed for highlighting both deep and shallow structures. Generally, the regional field in the area under study is considered to be mainly influenced by the density contrast between the crust and upper mantle. Use of the gravity and topographic data with earthquake focal depths has improved both the geometry and the density distribution in the 3-D calculated profiles. The oceanic-continental boundary, the basement relief, Moho depth and lithosphere-asthenosphere boundary maps were estimated. The results point to the occurrence of thick continental crust areas with a thickness of approximately 32 km in northern Egypt. Below the coastal regions, the thickness of crust decreases abruptly (transition zone). An inverse correlation between sediment and crustal thicknesses shows up from the study. Furthermore, our density model reveals the existence of a continental crustal zone below the Eratosthenes Seamount block. Nevertheless, the crustal type beneath the Levantine basin is typically oceanic; this is covered by sedimentary sequences more than 14 km thick. The modeled Moho map shows a depth of 28-30 km below Cyprus and a depth of 26-28 km beneath the south Florence Rise in the northern west. However, the Moho lies at a constant shallow depth of 22-24 km below the Levantine Basin, which indicates thinning of the crust beneath this region. The Moho map reveals also a maximum depth of about 33-35 km beneath both the northern Egypt and northern Sinai, both of which are of the continental crust. The resulting mantle density anomalies suggest important variations of the lithosphere-asthenosphere boundary (LAB) topography, indicating prominent lithospheric mantle thinning beneath south Cyprus (LAB ~90

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

    NASA Astrophysics Data System (ADS)

    Houseman, Gregory A.

    2015-04-01

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

  11. 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

  12. Warming of the Indian Ocean threatens eastern and southern African food security but could be mitigated by agricultural development.

    PubMed

    Funk, Chris; Dettinger, Michael D; Michaelsen, Joel C; Verdin, James P; Brown, Molly E; Barlow, Mathew; Hoell, Andrew

    2008-08-12

    Since 1980, the number of undernourished people in eastern and southern Africa has more than doubled. Rural development stalled and rural poverty expanded during the 1990s. Population growth remains very high, and declining per-capita agricultural capacity retards progress toward Millennium Development goals. Analyses of in situ station data and satellite observations of precipitation have identified another problematic trend: main growing-season rainfall receipts have diminished by approximately 15% in food-insecure countries clustered along the western rim of the Indian Ocean. Occurring during the main growing seasons in poor countries dependent on rain-fed agriculture, these declines are societally dangerous. Will they persist or intensify? Tracing moisture deficits upstream to an anthropogenically warming Indian Ocean leads us to conclude that further rainfall declines are likely. We present analyses suggesting that warming in the central Indian Ocean disrupts onshore moisture transports, reducing continental rainfall. Thus, late 20th-century anthropogenic Indian Ocean warming has probably already produced societally dangerous climate change by creating drought and social disruption in some of the world's most fragile food economies. We quantify the potential impacts of the observed precipitation and agricultural capacity trends by modeling "millions of undernourished people" as a function of rainfall, population, cultivated area, seed, and fertilizer use. Persistence of current tendencies may result in a 50% increase in undernourished people by 2030. On the other hand, modest increases in per-capita agricultural productivity could more than offset the observed precipitation declines. Investing in agricultural development can help mitigate climate change while decreasing rural poverty and vulnerability.

  13. Warming of the Indian Ocean threatens eastern and southern African food security but could be mitigated by agricultural development

    USGS Publications Warehouse

    Funk, C.; Dettinger, M.D.; Michaelsen, J.C.; Verdin, J.P.; Brown, M.E.; Barlow, M.; Hoell, A.

    2008-01-01

    Since 1980, the number of undernourished people in eastern and southern Africa has more than doubled. Rural development stalled and rural poverty expanded during the 1990s. Population growth remains very high, and declining per-capita agricultural capacity retards progress toward Millennium Development goals. Analyses of in situ station data and satellite observations of precipitation have identified another problematic trend: main growing-season rainfall receipts have diminished by ???15% in food-insecure countries clustered along the western rim of the Indian Ocean. Occurring during the main growing seasons in poor countries dependent on rain-fed agriculture, these declines are societally dangerous. Will they persist or intensify? Tracing moisture deficits upstream to an anthropogenically warming Indian Ocean leads us to conclude that further rainfall declines are likely. We present analyses suggesting that warming in the central Indian Ocean disrupts onshore moisture transports, reducing continental rainfall. Thus, late 20th-century anthropogenic Indian Ocean warming has probably already produced societally dangerous climate change by creating drought and social disruption in some of the world's most fragile food economies. We quantify the potential impacts of the observed precipitation and agricultural capacity trends by modeling 'millions of undernourished people' as a function of rainfall, population, cultivated area, seed, and fertilizer use. Persistence of current tendencies may result in a 50% increase in undernourished people by 2030. On the other hand, modest increases in per-capita agricultural productivity could more than offset the observed precipitation declines. Investing in agricultural development can help mitigate climate change while decreasing rural poverty and vulnerability. ?? 2008 by The National Academy of Sciences of the USA.

  14. Warming of the Indian Ocean threatens eastern and southern African food security but could be mitigated by agricultural development

    PubMed Central

    Funk, Chris; Dettinger, Michael D.; Michaelsen, Joel C.; Verdin, James P.; Brown, Molly E.; Barlow, Mathew; Hoell, Andrew

    2008-01-01

    Since 1980, the number of undernourished people in eastern and southern Africa has more than doubled. Rural development stalled and rural poverty expanded during the 1990s. Population growth remains very high, and declining per-capita agricultural capacity retards progress toward Millennium Development goals. Analyses of in situ station data and satellite observations of precipitation have identified another problematic trend: main growing-season rainfall receipts have diminished by ≈15% in food-insecure countries clustered along the western rim of the Indian Ocean. Occurring during the main growing seasons in poor countries dependent on rain-fed agriculture, these declines are societally dangerous. Will they persist or intensify? Tracing moisture deficits upstream to an anthropogenically warming Indian Ocean leads us to conclude that further rainfall declines are likely. We present analyses suggesting that warming in the central Indian Ocean disrupts onshore moisture transports, reducing continental rainfall. Thus, late 20th-century anthropogenic Indian Ocean warming has probably already produced societally dangerous climate change by creating drought and social disruption in some of the world's most fragile food economies. We quantify the potential impacts of the observed precipitation and agricultural capacity trends by modeling “millions of undernourished people” as a function of rainfall, population, cultivated area, seed, and fertilizer use. Persistence of current tendencies may result in a 50% increase in undernourished people by 2030. On the other hand, modest increases in per-capita agricultural productivity could more than offset the observed precipitation declines. Investing in agricultural development can help mitigate climate change while decreasing rural poverty and vulnerability. PMID:18685101

  15. 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.

  16. Potential of space-borne GNSS reflectometry to constrain simulations of the ocean circulation. A case study for the South African current system

    NASA Astrophysics Data System (ADS)

    Saynisch, Jan; Semmling, Maximilian; Wickert, Jens; Thomas, Maik

    2015-11-01

    The Agulhas current system transports warm and salty water masses from the Indian Ocean into the Southern Ocean and into the Atlantic. The transports impact past, present, and future climate on local and global scales. The size and variability, however, of the respective transports are still much debated. In this study, an idealized model based twin experiment is used to study whether sea surface height (SSH) anomalies estimated from reflected signals of the Global Navigation Satellite System reflectometry (GNSS-R) can be used to determine the internal water mass properties and transports of the Agulhas region. A space-borne GNSS-R detector on the International Space Station (ISS) is assumed and simulated. The detector is able to observe daily SSH fields with a spatial resolution of 1-5∘. Depending on reflection geometry, the precision of a single SSH observation is estimated to reach 3 cm (20 cm) when the carrier phase (code delay) information of the reflected GNSS signal is used. The average precision over the Agulhas region is 7 cm (42 cm). The proposed GNSS-R measurements surpass the radar-based satellite altimetry missions in temporal and spatial resolution but are less precise. Using the estimated GNSS-R characteristics, measurements of SSH are generated by sampling a regional nested general circulation model of the South African oceans. The artificial observations are subsequently assimilated with a 4DVAR adjoint data assimilation method into the same ocean model but with a different initial state and forcing. The assimilated and the original, i.e., the sampled model state, are compared to systematically identify improvements and degradations in the model variables that arise due to the assimilation of GNSS-R based SSH observations. We show that SSH and the independent, i.e., not assimilated model variables velocity, temperature, and salinity improve by the assimilation of GNSS-R based SSH observations. After the assimilation of 90 days of SSH observations

  17. 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

  18. Lithospheric Decoupling and Rotations: Hints from Ethiopian Rift

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    Plates move relative to the mantle because some torques are acting on them. The shear in the low-velocity zone (LVZ) at the base of the lithosphere is the expression of these torques. The decoupling is allowed by the low viscosity in the LVZ, which is likely few orders of magnitudes lower than previously estimated. The viscosity value in the LVZ controls the degree of coupling/decoupling between the lithosphere and the underlying mantle. Lateral variations in viscosity within the LVZ may explain the velocity gradient among tectonic plates as the one determining the Ethiopian Rift (ER) separating Africa from Somalia. While it remains not fully understood the mechanisms of the torques acting on the lithosphere (thermally driven mantle convection or the combination of mantle convection with astronomical forces such as the Earth's rotation and tidal drag), the stresses are transmitted across the different mechanical layers (e.g., the brittle upper crust, down to the viscous-plastic ductile lower crust and upper mantle). Differential basal shear traction at the base of the lithosphere beneath the two sides of the East African Rift System (EARS) is assumed to drive and sustain rifting. In our analysis, the differential torques acting on the lithospheric/crustal blocks drive kinematics and block rotations. Since, ER involves the whole lithosphere, we do not expect large amount of rotation. Rotation can be the result of the whole plate motion on the sphere moving along the tectonic equator, or the second order sub-rotation of a single plate. Further rotation may occur along oblique plate boundaries (e.g., left lateral transtensional setting at the ER). Small amount of vertical axis rotation of blocks in northern ER could be related to the presence of local, shallower decollement layers. Shallow brittle-ductile transition (BDT) zone and differential tilting of crustal blocks in the northern ER could hint a possibility of detachment surface between the flow in the lower

  19. Seismic and Thermal Structure of the Arctic Lithosphere, From Waveform Tomography and Thermodynamic Modelling

    NASA Astrophysics Data System (ADS)

    Lebedev, S.; Schaeffer, A. J.; Fullea, J.; Pease, V.

    2015-12-01

    Thermal structure of the lithosphere is reflected in the values of seismic velocities within it. Our new tomographic models of the crust and upper mantle of the Arctic are constrained by an unprecedentedly large global waveform dataset and provide substantially improved resolution, compared to previous models. The new tomography reveals lateral variations in the temperature and thickness of the lithosphere and defines deep boundaries between tectonic blocks with different lithospheric properties and age. The shape and evolution of the geotherm beneath a tectonic unit depends on both crustal and mantle-lithosphere structure beneath it: the lithospheric thickness and its changes with time (these determine the supply of heat from the deep Earth), the crustal thickness and heat production (the supply of heat from within the crust), and the thickness and thermal conductivity of the sedimentary cover (the insulation). Detailed thermal structure of the basins can be modelled by combining seismic velocities from tomography with data on the crustal structure and heat production, in the framework of computational petrological modelling. The most prominent lateral contrasts across the Arctic are between the cold, thick lithospheres of the cratons (in North America, Greenland and Eurasia) and the warmer, non-cratonic blocks. The lithosphere of the Canada Basin is cold and thick, similar to old oceanic lithosphere elsewhere around the world; its thermal structure offers evidence on its lithospheric age and formation mechanism. At 150-250 km depth, the central Arctic region shows a moderate low-velocity anomaly, cooler than that beneath Iceland and N Atlantic. An extension of N Atlantic low-velocity anomaly into the Arctic through the Fram Strait may indicate an influx of N Atlantic asthenosphere under the currently opening Eurasia Basin.

  20. 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.

  1. Gravity and multichannel seismic reflection constraints on the lithospheric structure of the Canary Swell

    NASA Astrophysics Data System (ADS)

    Ranero, C. R.; Torne, M.; Banda, E.

    1995-12-01

    Deep penetrating multichannel seismic reflection and gravity data have been used to study the lithospheric structure of the Canary Swell. The seismic reflection data show the transition from undisturbed Jurassic oceanic crust, away from the Canary Islands, to an area of ocean crust strongly modified by the Canary volcanism (ACV). Outside the ACV the seismic records image a well layered sedimentary cover, underlined by a bright reflection from the top of the igneous basement and also relatively continuous reflections from the base of the crust. In the ACV the definition of the boundary between sedimentary cover and igneous basement and the crust-mantle boundary remains very loose. Two-dimensional gravity modelling in the area outside the influence of the Canary volcanism, where the reflection data constrain the structure of the ocean crust, suggests a thinning of the lithosphere. The base of the lithosphere rises from 100 km, about 400 km west of the ACV, to 80 km at the outer limit of the ACV. In addition, depth conversion of the seismic reflection data and unloading of the sediments indicate the presence of a regional depth anomaly of an extension similar to the lithospheric thinning inferred from gravity modelling. The depth anomaly associated with the swell, after correction for sediment weight, is about 500 m. We interpret the lithospheric thinning as an indication of reheating of old Mesozoic lithosphere beneath the Canary Basin and along with the depth anomaly as indicating a thermal rejuvenation of the lithosphere. We suggest that the most likely origin for the Canary Islands is a hot spot.

  2. Ocean Pollution as a Result of Onshore Offshore Petroleum Activities in the African Gulf of Guinea Region

    NASA Astrophysics Data System (ADS)

    Abubakar, B.

    2007-05-01

    increasing cases of pollution of farmlands, rivers, wells and the environment in general. Apart from all these, what is even becoming more worrisome is that none of all these oil firms operating in the region is able to account on how it disposes its industrial toxic waste generated as a result of its industrial activities within the region. Finally Geological strata are adversely destroyed by seismographic activities, Sea creatures are destroyed by oil pollution and Means of livelihood of revering dwellers are often threatened by pollution. RECOMMENDATIONS After identifying how the pollution in the Gulf of Guinea region is increasing in relation to the increasing petroleum activities, I have come up with the following suggestions/recommendations. 1. AFRICAN UNION RESOLUTION The Organization of the Petroleum Exporting Countries (OPEC) in conjunction with the International Atomic Energy Agency (IAEA) should use their capacity to be able to influence the African Union (AU) to pass a resolution banning the illegal dumping of radioactive waste, Gas flaring and Costal bunkering in this part of the world. 2. RESEARCH AND INVESTIGATION The Organization of the Petroleum Exporting Countries, in conjunction with the United Nations Environmental Agency, the International Atomic Energy Agency and with the corporation of the African Union should send team of researchers to come and investigate this trend on petroleum pollution in the Gulf of Guinea region and proffer possible solutions in checking the menace.

  3. Airborne microorganisms in the African desert dust corridor over the mid-Atlantic ridge, Ocean Drilling Program, Leg 209

    USGS Publications Warehouse

    Griffin, Dale W.; Westphal, Douglas L.; Gray, Michael A.

    2006-01-01

    The objective of this study was to enhance our understanding of the fate and trans-Atlantic transport of dustborne microorganisms from Northern Africa to the Caribbean and Americas, and more specifically to determine if culturable populations could be detected at a mid-ocean site, closer to the source of dust relative to land-based Caribbean sites, during the early summer months of May and June. Between the dates of 22 May and 30 June 2003, daily air samples were collected and evaluated for the presence of culturable bacterial and fungal colony-forming units (CFU). Here we report a statistically significant correlation between daily atmospheric CFU counts at a mid-ocean research site (???15??N, 45??W) and daily desert dust concentrations as determined by the U.S. Navy's Naval Aerosol Analysis and Prediction System (NAAPS) Global Aerosol Model (Honrath et al. (2004). Journal of Geophysical Research, 109; Johnson et al. (2003). Global Biogeochemical Cycles, 17, 1063; Reid et al. (2004). Geophysical Research Letters, 31; Schollaert, Yoder, Westphal, & O'Reilly (2003). Journal of Geophysical Research, 108, 3191). ?? Springer Science+Business Media B.V. 2006.

  4. Arctic and Antarctic Crustal Thickness and Continental Lithosphere Thinning from Gravity Inversion

    NASA Astrophysics Data System (ADS)

    Kusznir, Nick J.; Alvey, Andy; Vaughan, Alan P. M.; Ferraccioli, Fausto; Jordan, Tom A. R. M.; Roberts, Alan M.

    2013-04-01

    Mapping crustal thickness, continental lithosphere thinning and oceanic lithosphere distribution represents a substantial challenge for the Polar Regions. The Arctic region formed as a series of small distinct ocean basins leading to a complex distribution of oceanic crust, thinned continental crust and rifted continental margins. Antarctica, both peripherally and internally, experienced poly-phase rifting and continental breakup. We determine Moho depth, crustal basement thickness, continental lithosphere thinning and ocean-continent transition location for the Polar Regions using a gravity inversion method which incorporates a lithosphere thermal gravity anomaly correction. The method is carried out in the 3D spectral domain and predicts Moho depth and incorporates a lithosphere thermal gravity anomaly correction. Ice thickness is included in the gravity inversion, as is the contribution from sediments which assumes a compaction controlled sediment density increase with depth. A correction to the predicted continental lithospheric thinning derived from gravity inversion is made for volcanic material addition produced by decompression melting during continental rifting and seafloor spreading. For the Arctic, gravity data used is from the NGA (U) Arctic Gravity Project, bathymetry is from IBCAO and sediment thickness is from a new regional compilation. For Antarctica and the Southern Oceans, data used are elevation and bathymetry, free-air gravity anomaly, ice and sediment thickness from Smith and Sandwell (2008), Sandwell and Smith (2008) and Laske and Masters (1997) respectively, supplemented by Bedmap2 data south of 60 degrees south. Using gravity anomaly inversion, we have produced the first comprehensive maps of crustal thickness and oceanic lithosphere distribution for the Arctic, Antarctica and the Southern Ocean. Our gravity inversion predicts thin crust and high continental lithosphere thinning factors in the Makarov, Podvodnikov, Nautilus and Canada

  5. Dynamic lithosphere within the Great Basin

    NASA Astrophysics Data System (ADS)

    Porter, Ryan C.; Fouch, Matthew J.; Schmerr, Nicholas C.

    2014-04-01

    place new constraints on the short-term, broad-scale lithospheric evolution of plate interiors, we utilize broadband seismic data from the Great Basin region of the Western United States to produce high-resolution images of the crust and upper mantle. Our results suggest that parts of the Great Basin lithosphere has been removed, likely via inflow of hot asthenosphere as subduction of the Farallon spreading center occurred and the region extended. In our proposed model, fragments of thermal lithosphere removed by this process were gravitationally unstable and subsequently sank into the underlying mantle, leaving behind less dense, stronger, chemically depleted lithosphere. This destabilization process promotes volcanism, deformation, and the reworking of continental lithosphere inboard from plate margins. Our results provide evidence for a new mechanism of lithospheric evolution that is likely common and significant in postsubduction tectonic settings.

  6. Constraints on the depth and thermal history of cratonic lithosphere from peridotite xenoliths, xenocrysts and seismology

    NASA Astrophysics Data System (ADS)

    Mather, Kathy A.; Pearson, D. Graham; McKenzie, Dan; Kjarsgaard, Bruce A.; Priestley, Keith

    2011-07-01

    the southern African lithosphere. We find very similar estimates for the heat flow and thickness of the lithosphere between SW Namibia (off-craton) and Bultfontein (on-craton). This supports suggestions of a cratonic thermal regime and equivalent lithospheric thickness across that region of southern Africa at the time of kimberlite sampling, with concurrent local thermal disturbance evident in Namibia. Complimentary, novel, seismically-obtained geotherm estimates show that the lithosphere in Namibia is now significantly thinner than the estimate at 70 Ma obtained from xenolith thermobarometry.

  7. A numerical study of forced lithospheric thinning

    NASA Technical Reports Server (NTRS)

    Schubert, G.; Anderson, A.; Fishbein, E.

    1985-01-01

    Subsolidus lithospheric thinning by mantle plumes may be involved in the creation of swells, hotspots, and rifts. Among the major questions concerning this process are the timescale on which it occurs and the structure of the plumes. The lithosphere is known to have been substantially thinned in 10 Ma or less. Current studies are focused on the lithospheric thinning by time-dependent plumes hypothesized to have large temperature differences across them.

  8. Dynamic topography as constraints on stress and viscosity in the mantle and lithosphere

    NASA Astrophysics Data System (ADS)

    Zhong, S.

    2015-12-01

    Mantle convection generates stress in the mantle and lithosphere. The lithosphere stress is responsible for localized deformation including seismic deformation at plate boundaries, and localized stress highs in lithosphere are also suggested to cause dynamically self-consistent generation of plate tectonics and continental lithosphere instability, as the stress exceeds a threshold or yield stress. Modeling load-induced deformation at oceanic islands (e.g., Hawaii) constrains lithospheric stress at 100-200 MPa in the plate interiors, leading to a lower limit on lithospheric yield stress (Zhong and Watts, 2013). However, convection-induced lithospheric stress is poorly understood, ranging from 500 MPa to tens of MPa as reported in mantle convection studies. The magnitude and distribution of lithospheric and mantle stress depend critically on buoyancy and viscosity, particularly the latter. Unfortunately, lithospheric and mantle viscosity is also poorly constrained. For example, the inferred lower mantle viscosity from post-glacial rebound and geoid modeling studies ranges from 1023 Pas to 1022 Pas (e.g., Mitrovica and Forte, 2004; Simons and Hager, 1996; Paulson et al., 2007). In addition to the stress, the lower mantle viscosity may also affect the time evolution of mantle structure including sinking rate of slabs and formation of the degree-2 mantle seismic structure. Therefore, it is important to develop independent constraints on mantle viscosity and convection-induced stress. In this study, I demonstrate that dynamic topography can be used to place first-order constraints on both lithospheric stress and mantle viscosity. For a given superadiabatic temperature difference across the mantle (e.g., 2500 K), a larger mantle viscosity (or a smaller Rayleigh number) leads to a larger lithospheric stress and a larger dynamic topography. To be consistent with the inferred dynamic topography, the lower mantle viscosity is constrained to be significantly smaller than 1023

  9. Convective thinning of the lithosphere: A mechanism for rifting and mid-plate volcanism on Earth, Venus, and Mars

    NASA Technical Reports Server (NTRS)

    Spohn, T.; Schubert, G.

    1982-01-01

    Thinning of the Earth's lithosphere by heat advected to its base is a possible mechanism for continental rifting and continental and oceanic mid-plate volcanism. It might also account for continental rifting-like processes and volcanism on Venus and Mars. Earth's continental lithosphere can be thinned to the crust in a few tens of million years by heat advected at a rate of 5 to 10 times the normal basal heat flux. This much heat is easily carried to the lithosphere by mantle plumes. The continent is not required to rest over the mantle hot spot but may move at tens of millimeters per year. Because of the constant level of crustal radioactive heat production, the ratio of the final to the initial surface heat flow increases much less than the ratio of the final to initial basal heat flow. For large increases in asthenospheric heat flow, the lithosphere is almost thinned to the crust before any significant change in surface heat flow occurs. Uplift due to thermal expansion upon thinning is a few kilometers. The oceanic lithosphere can be thinned to the crust in less than 10 million years if the heat advection is at a rate around 5 or more times the basal heat flow into 100 Ma old lithosphere. Uplift upon thinning can compensate the subsidence of spreading and cooling lithosphere.

  10. Venus Chasmata: A Lithospheric Stretching Model

    NASA Technical Reports Server (NTRS)

    Solomon, S. C.; Head, J. W.

    1985-01-01

    An outstanding problem for Venus is the characterization of its style of global tectonics, an issue intimately related to the dominant mechanism of lithospheric heat loss. Among the most spectacular and extensive of the major tectonic features on Venus are the chasmata, deep linear valleys generally interpreted to be the products of lithospheric extension and rifting. Systems of chasmata and related features can be traced along several tectonic zones up to 20,000 km in linear extent. A lithospheric stretching model was developed to explain the topographic characteristics of Venus chasmata and to constrain the physical properties of the Venus crust and lithosphere.

  11. Can We Probe the Conductivity of the Lithosphere and Upper Mantle Using Satellite Tidal Magnetic Signals?

    NASA Technical Reports Server (NTRS)

    Schnepf, N. R.; Kuvshinov, A.; Sabaka, T.

    2015-01-01

    A few studies convincingly demonstrated that the magnetic fields induced by the lunar semidiurnal (M2) ocean flow can be identified in satellite observations. This result encourages using M2 satellite magnetic data to constrain subsurface electrical conductivity in oceanic regions. Traditional satellite-based induction studies using signals of magnetospheric origin are mostly sensitive to conducting structures because of the inductive coupling between primary and induced sources. In contrast, galvanic coupling from the oceanic tidal signal allows for studying less conductive, shallower structures. We perform global 3-D electromagnetic numerical simulations to investigate the sensitivity of M2 signals to conductivity distributions at different depths. The results of our sensitivity analysis suggest it will be promising to use M2 oceanic signals detected at satellite altitude for probing lithospheric and upper mantle conductivity. Our simulations also suggest that M2 seafloor electric and magnetic field data may provide complementary details to better constrain lithospheric conductivity.

  12. Local recovery of lithospheric stress tensor from GOCE gravitational tensor

    NASA Astrophysics Data System (ADS)

    Eshagh, Mehdi

    2017-01-01

    SUMMARYThe sub-<span class="hlt">lithospheric</span> stress due to mantle convection can be computed from gravity data and propagated through the <span class="hlt">lithosphere</span> by solving the boundary-value problem of elasticity for the Earth's <span class="hlt">lithosphere</span>. In this case, a full tensor of stress can be computed at any point inside this elastic layer. Here, we present mathematical foundations for recovering such a tensor from gravitational tensor measured at satellite altitudes. The mathematical relations will be much simpler in this way than the case of using gravity data as no derivative of spherical harmonics or Legendre polynomials is involved in the expressions. Here, new relations between the spherical harmonic coefficients of the stress and gravitational tensor elements are presented. Thereafter integral equations are established from them to recover the elements of stress tensor from those of the gravitational tensor. The integrals have no closed-form kernels, but they are easy to invert and their spatial truncation errors are reducible. The integral equations are used to invert the real data of the gravity field and steady-state <span class="hlt">ocean</span> circulation explorer (GOCE) mission, in November 2009, over the South American plate and its surroundings to recover the stress tensor at a depth of 35 km. The recovered stress fields are in good agreement with the tectonic and geological features of the area.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=5035893','PMC'); return false;" href="https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=5035893"><span>Olivine anisotropy suggests Gutenberg discontinuity is not the base of the <span class="hlt">lithosphere</span></span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pmc">PubMed Central</a></p> <p>Qi, Chao; Warren, Jessica M.</p> <p>2016-01-01</p> <p>Tectonic plates are a key feature of Earth’s structure, and their behavior and dynamics are fundamental drivers in a wide range of large-scale processes. The operation of plate tectonics, in general, depends intimately on the manner in which <span class="hlt">lithospheric</span> plates couple to the convecting interior. Current debate centers on whether the transition from rigid <span class="hlt">lithosphere</span> to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth. Thus, the manner in which the rigid <span class="hlt">lithosphere</span> couples to the flowing asthenosphere is currently unclear. Here we present results from laboratory-based torsion experiments on olivine aggregates with and without melt, yielding an improved database describing the crystallographic alignment of olivine grains. We combine this database with a flow model for <span class="hlt">oceanic</span> upper mantle to predict the structure of the seismic anisotropy beneath <span class="hlt">ocean</span> basins. Agreement between our model and seismological observations supports the view that the base of the <span class="hlt">lithosphere</span> is thermally controlled. This model additionally supports the idea that discontinuities in velocity and anisotropy, often assumed to be the base of the <span class="hlt">lithosphere</span>, are, instead, intralithospheric features reflecting a compositional boundary established at midocean ridges, not a rheological boundary. PMID:27606485</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016PNAS..11310503H','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016PNAS..11310503H"><span>Olivine anisotropy suggests Gutenberg discontinuity is not the base of the <span class="hlt">lithosphere</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Hansen, Lars N.; Qi, Chao; Warren, Jessica M.</p> <p>2016-09-01</p> <p>Tectonic plates are a key feature of Earth’s structure, and their behavior and dynamics are fundamental drivers in a wide range of large-scale processes. The operation of plate tectonics, in general, depends intimately on the manner in which <span class="hlt">lithospheric</span> plates couple to the convecting interior. Current debate centers on whether the transition from rigid <span class="hlt">lithosphere</span> to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth. Thus, the manner in which the rigid <span class="hlt">lithosphere</span> couples to the flowing asthenosphere is currently unclear. Here we present results from laboratory-based torsion experiments on olivine aggregates with and without melt, yielding an improved database describing the crystallographic alignment of olivine grains. We combine this database with a flow model for <span class="hlt">oceanic</span> upper mantle to predict the structure of the seismic anisotropy beneath <span class="hlt">ocean</span> basins. Agreement between our model and seismological observations supports the view that the base of the <span class="hlt">lithosphere</span> is thermally controlled. This model additionally supports the idea that discontinuities in velocity and anisotropy, often assumed to be the base of the <span class="hlt">lithosphere</span>, are, instead, intralithospheric features reflecting a compositional boundary established at midocean ridges, not a rheological boundary.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013Geote..47..465P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013Geote..47..465P"><span>Gravity models of two-level collision of <span class="hlt">lithospheric</span> plates in northeastern Asia</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Petrishchevsky, A. M.</p> <p>2013-11-01</p> <p>Structural forms of emplacement of crustal and mantle rigid sheets in collision zones of <span class="hlt">lithospheric</span> plates in northeastern Asia are analyzed using formalized gravity models reflecting the rheological properties of geological media. Splitting of the <span class="hlt">lithosphere</span> of moving plates into crustal and mantle constituents is the main feature of collision zones, which is repeated in the structural units irrespective of their location, rank, and age. Formal signs of crustal sheet thrusting over convergent plate boundaries and subduction of the <span class="hlt">lithospheric</span> mantle beneath these boundaries have been revealed. The deep boundaries and thickness of <span class="hlt">lithospheric</span> plates and asthenospheric lenses have been traced. A similarity in the deep structure of collision zones of second-order marginal-sea buffer plates differing in age is displayed at the boundaries with the Eurasian, North American, and Pacific plates of the first order. Collision of <span class="hlt">oceanic</span> crustal segments with the Mesozoic continental margin in the Sikhote-Alin is characterized, as well as collision of the <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> with the Kamchatka composite island arc. A spatiotemporal series of deep-seated Middle Mesozoic, Late Mesosoic, and Cenozoic collision tectonic units having similar structure is displayed in the transitional zone from the Asian continent to the Pacific plate.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/27606485','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/27606485"><span>Olivine anisotropy suggests Gutenberg discontinuity is not the base of the <span class="hlt">lithosphere</span>.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Hansen, Lars N; Qi, Chao; Warren, Jessica M</p> <p>2016-09-20</p> <p>Tectonic plates are a key feature of Earth's structure, and their behavior and dynamics are fundamental drivers in a wide range of large-scale processes. The operation of plate tectonics, in general, depends intimately on the manner in which <span class="hlt">lithospheric</span> plates couple to the convecting interior. Current debate centers on whether the transition from rigid <span class="hlt">lithosphere</span> to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth. Thus, the manner in which the rigid <span class="hlt">lithosphere</span> couples to the flowing asthenosphere is currently unclear. Here we present results from laboratory-based torsion experiments on olivine aggregates with and without melt, yielding an improved database describing the crystallographic alignment of olivine grains. We combine this database with a flow model for <span class="hlt">oceanic</span> upper mantle to predict the structure of the seismic anisotropy beneath <span class="hlt">ocean</span> basins. Agreement between our model and seismological observations supports the view that the base of the <span class="hlt">lithosphere</span> is thermally controlled. This model additionally supports the idea that discontinuities in velocity and anisotropy, often assumed to be the base of the <span class="hlt">lithosphere</span>, are, instead, intralithospheric features reflecting a compositional boundary established at midocean ridges, not a rheological boundary.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014EGUGA..1610293T','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014EGUGA..1610293T"><span>Investigating the <span class="hlt">Lithospheric</span> Structure of Southern Madagascar</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Tilmann, Frederik; Yuan, Xiaohui; Rümpker, Georg; Gerard, Rambolamana; Elisa, Rindraharisaona; Priestley, Keith</p> <p>2014-05-01</p> <p>The island of Madagascar occupies a key region in both the assembly and the multi-stage breakup of Gondwanaland, itself part of the super-continent Pangaea. Madagascar consists of an amalgamation of continental material, with the oldest rocks being of Archaean age. Its ancient fabric is characterised by several shear zones, some of them running oblique to the N-S trend, in particular in the south of the island. More recently during the Neogene, moderate volcanism has occurred in the Central and Northern part of the island, and there are indications of uplift throughout Eastern Madagascar over the last 10 Ma. Although Madagascar is now located within the interior of the <span class="hlt">African</span> plate and far away from major plate boundaries (> 1000 km from the East <span class="hlt">African</span> rift system and even further from the Central and South-West Indian Ridges), its seismic activity indicates that some deformation is taking place, and present-day kinematic models based on geodetic data and earthquake moment tensors in the global catalogues identify a diffuse N-S-oriented minor boundary separating two microplates, which appears to pass through Madagascar. In spite of the presence of Archaean and Proterozoic rocks continent-wide scale studies indicate a thin <span class="hlt">lithosphere</span> (<120 km) throughout Madagascar, but are based on sparse data. We are operating a ENE-WSW oriented linear array of 25 broadband stations in southern Madagascar, extending from coast to coast and sampling the sedimentary basins in the west as well as the metamorphic rocks in the East, cutting geological boundaries seen at the surface at high angle. The array crosses the prominent Bongolava-Ranotsara shear zone which is thought to have been formed during Gondwanaland assembly, although this interpretation has recently been questioned. The array recorded the magnitude 5.3 earthquake of January 25, 2013 which occurred just off its western edge. In addition, in May 2013 we have deployed 25 short period sensors in the eastern part of the</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013JGRB..118.6025Z','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013JGRB..118.6025Z"><span><span class="hlt">Lithospheric</span> deformation induced by loading of the Hawaiian Islands and its implications for mantle rheology</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Zhong, Shijie; Watts, A. B.</p> <p>2013-11-01</p> <p>long-term rheological properties of the <span class="hlt">lithosphere</span> are fundamental for understanding both surface tectonics and mantle dynamics on Earth. In this study, we have developed 3-D finite element models for computing the load-induced surface deformation and stress for <span class="hlt">lithosphere</span> and mantle with realistic nonlinear viscoelastic rheology including the frictional sliding, low-temperature plasticity, and high-temperature creep. We have determined the <span class="hlt">lithospheric</span> deformation and stress due to volcano loading in the Hawaiian Islands region for the last few million years. By comparing model predictions with seismic observations of the depth to the top of <span class="hlt">oceanic</span> crust and depth dependence of seismicity in the Hawaiian Islands region, we have sought to constrain <span class="hlt">lithospheric</span> rheology. Our calculations show that the load-induced surface deformation is controlled by low-temperature plasticity and frictional sliding but is insensitive to high-temperature creep. <span class="hlt">Lithospheric</span> strength predicted from laboratory-derived low-temperature plasticity needs to be reduced significantly, and a frictional coefficient μf ranging from 0.1 to 0.7 is required in order to account for the observations. However, μf = 0.1 weakens the shallow part of the <span class="hlt">lithosphere</span> so much that it causes the minima in strain rate and stress to occur at too large depths to be consistent with the observed depth distribution of seismicity. Our results therefore suggest a value for μf between 0.25 and 0.7. Finally, the maximum stress that accumulates in the deformed <span class="hlt">lithosphere</span> beneath the Hawaiian Islands is about 100-200 MPa for models that match the observations, and this stress may be viewed as the largest <span class="hlt">lithospheric</span> stress on Earth.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015GeoJI.203.1961A','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015GeoJI.203.1961A"><span>Australia's <span class="hlt">lithospheric</span> density field, and its isostatic equilibration</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Aitken, A. R. A.; Altinay, C.; Gross, L.</p> <p>2015-12-01</p> <p>Density is a key driver of tectonic processes, but it is a difficult property to define well in the <span class="hlt">lithosphere</span> 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 <span class="hlt">lithospheric</span> 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 <span class="hlt">lithospheric</span> 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 <span class="hlt">lithospheric</span> mantle structure is not extensively modified from the reference model, but the results reinforce the systematic difference between the density of the <span class="hlt">oceanic</span> and continental domains, and help identify</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013AGUFM.S31A2343R','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013AGUFM.S31A2343R"><span>Constraining <span class="hlt">Lithospheric</span> Structure across the California Borderland using Receiver Functions</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Reeves, Z. A.; Lekic, V.; Weeraratne, D. S.; Kohler, M. D.</p> <p>2013-12-01</p> <p>Due to its complex history of deformation that has included subduction, rifting, and translational motions, the California (Continental) Borderland provides a unique geological setting for studying how the structure of <span class="hlt">oceanic</span> and continental plates respond to deformation. Three component <span class="hlt">ocean</span> bottom data gathered as part of the ALBACORE experiment in 2010-2011 allows us to expand on previous receiver function studies that characterized the crustal (Zhu and Kanamori, 2000) and mantle <span class="hlt">lithosphere</span> structure (Lekic et al., 2011) from the inner borderland into the outer borderland; stations west of the Patten Escarpment allow us to extend the constraints into the abyssal plain of the Pacific Plate. However, compared to coastal and <span class="hlt">ocean</span> island stations used in previous studies, the <span class="hlt">ocean</span> bottom data is characterized by low signal-to-noise ratios making it difficult to obtain interpretable receiver functions; therefore, developing strategies for de-noising the seismograms and obtaining reliable receiver functions from <span class="hlt">ocean</span> bottom data is one of our main objectives. We explore multiple strategies in order to improve the signal-to-noise ratio and yield more easily interpretable receiver functions. In order to better isolate the sought-after structural signal, we carry out wavefield simulations through synthetic structures with varying thicknesses of sediment. By examining each waveform record, we find that highest signal-to-noise ratios are obtained by bandpass filtering the data in the 0.03-0.12 Hz frequency range. We show that the use of elastic wave-field decomposition method proposed by Bostock and Trehu (2012) improves the estimation of the P and S wavefields in comparison to the free-surface transform matrix (Kennett 1991) used for stations on the land surface. We then calculate and map variations in absolute shear wavespeed needed for optimal wavefield separation. Finally, we present both Ps and Sp receiver functions across the California Borderland and interpret</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_13");'>13</a></li> <li><a href="#" onclick='return showDiv("page_14");'>14</a></li> <li class="active"><span>15</span></li> <li><a href="#" onclick='return showDiv("page_16");'>16</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_15 --> <div id="page_16" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_14");'>14</a></li> <li><a href="#" onclick='return showDiv("page_15");'>15</a></li> <li class="active"><span>16</span></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="301"> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2005AGUFM.S23B0254H','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2005AGUFM.S23B0254H"><span>DANUBE 2004 <span class="hlt">Lithosphere</span> Research Program</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Hegedus, E.; Brueckl, E.; Csabafi, R.; Fancsik, T.; Grad, M.; Guterch, A.; Hajnal, Z.; Keller, R.; Kovacs, A. C.; Komminaho, K.; Kozlovskaya, E.; Tiira, T.; Torok, I.; Yliniemi, J.</p> <p>2005-12-01</p> <p>The DANUBE 2004 (Deep imAgiNg of hUngarian BasEment) <span class="hlt">lithosphere</span> research program following significant seismic <span class="hlt">lithospheric</span> experiments in Central Europe (e.g., CELEBRATION 2000), was coordinated by the ELGI, on the commission of the Public Agency for Radioactive Waste Management (PURAM), in international cooperation. The goal of the research program was to allocate and characterize the potential geological site for high-level radioactive waste disposal in SW Hungary (Central Europe) using seismic methods. The research program comprised of two main fields: 1) 2D, 3D active seismic measurements 2) passive seismotectonic monitoring 1) Detailed 2D seismic reflection measurements were carried out along four profiles in the study area using 20 m geophone spacing with >100 folds in order to image the deep geological structure of the potential waste disposal site. Tomographic imaging of the reflection data along the four profiles was also carried out, whereas a 40 km long wide angle tomographic profile and a 50 square kilometers 3D tomography were performed in the prospective location. 2) Passive seismotectonic monitoring of the waste disposal site is also part of the program. 30 SP stations with continuous data recording (100 sps) are used to gather the seismic signals emerging from local tectonic activity in the 2000 square kilometers area so as to locate tectonically active zones in the region. The passive monitoring focuses on low (M< or =1) magnitude seismic signals that are expected from the study area.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2010EGUGA..1210342G','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2010EGUGA..1210342G"><span>Thin-layer resolution from multiscale analysis of teleseismic P-to-S converted waves: the <span class="hlt">oceanic</span> nature of the <span class="hlt">African</span> slab subducted under Peloponnesus.</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Gesret, Alexandrine; Laigle, Mireille; Diaz, Jordi; Sachpazi, Maria; Hirn, Alfred</p> <p>2010-05-01</p> <p> the first results of the application of this analysis to examples of a real dataset recently collected in the frame of the European Union THALES WAS RIGHT project. A tight-array teleseismic receiver-function network was dedicated to resolve at depth the heterogeneity at the top of the <span class="hlt">African</span> slab under the Hellenic region and its variations along-dip and along-strike. With earthquakes of broad-enough spectrum towards the short periods yielding energy to provide wavelet periods significantly shorter than 1s, the P to S conversions obtained beneath the eastern coast of Peloponnesus allow resolving for the first time a standard <span class="hlt">oceanic</span> crust at the slab top. This result is consistent with the fast trench retreat model, attested by the southwestward fast motion of the edge of the Aegean upper plate domain documented from GPS. Indeed, if this occurred since 4-5 million years ago, the slab underlying now eastern Peloponnesus to about 180 km from the backstop tip should be the most reduced buoyancy <span class="hlt">oceanic</span> slab of the Ionian sea basin.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2005AGUFM.T34A..03F','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2005AGUFM.T34A..03F"><span>Depth of the <span class="hlt">Lithosphere</span>-Asthenosphere Transition and Related Viscosity Structure</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Faul, U.; Jackson, I.</p> <p>2005-12-01</p> <p>The <span class="hlt">lithosphere</span> - asthenosphere transition can be defined in a number of different ways, for example rheologically, thermally or chemically. A thermal definition of the <span class="hlt">lithosphere</span> - asthenosphere transition can be the depth at which the geotherm approaches the adiabat to within 10 K. With our recent experimental measurements of shear modulus and attenuation as a function of temperature and grain size (Faul and Jackson, EPSL, 05) seismological models of the upper mantle can be used to constrain temperatures. Shear velocities calculated for geotherms of conductively cooling <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> compare well with seismological models of the Pacific. In particular, the age-dependent deepening and lessening of the low velocity zone is reproduced, implying that the low velocity zone can be explained by temperature effects alone. However, an important aspect is that the velocity minimum occurs at sub-adiabatic temperatures, i.e. is part of the (thermally defined) <span class="hlt">lithosphere</span>. The low velocity zone is due to the transition from the relatively steep temperature gradients of the conductively cooling lid to the much smaller adiabatic temperature gradients in the asthenosphere where pressure effects dominate. The magnitude of the velocity minimum depends on the depth at which the geotherm joins the adiabat. The shallow depth for young <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> results in a pronounced velocity minimum, whereas for archean cratons the very minor or absent velocity minimum implies a thermal boundary layer that is > 300 km thick. Similarly, viscosities calculated for deformation of olivine (e.g. Hirth and Kohlstedt, Geophys. Monogr., 03) are affected by the trade-offs between temperature and pressure effects, such that the viscosity minimum is near the velocity minimum. Young <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> has a relatively pronounced, shallow viscosity minimum, whereas for archean cratons the viscosity minimum is significantly deeper and less pronounced.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2012AGUFM.T43C2700J','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2012AGUFM.T43C2700J"><span>Dependency of continental crustal rupture, decompression melt initiation and OCT architecture on <span class="hlt">lithosphere</span> deformation modes during continental breakup: Numerical experiments</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Jeanniot, L.; Kusznir, N. J.; Manatschal, G.</p> <p>2012-12-01</p> <p>During the continental breakup process, the initiation of sea-floor spreading requires both the rupture of the continental crust and the initiation of decompression melting. Using numerical experiments, we investigate how the deformation mode of continental <span class="hlt">lithosphere</span> thinning and stretching controls the rupture of continental crust and <span class="hlt">lithospheric</span> mantle, the onset of decompression melting and their relative timing. We use a two dimensional finite element viscous flow model to describe <span class="hlt">lithosphere</span> and asthenosphere deformation. This flow field is used to advect <span class="hlt">lithosphere</span> and asthenosphere material and temperature. Decompression melting is predicted using the parameterization scheme of Katz et al. (2003). Consistent with the observations of deformation processes occurring at slow spreading <span class="hlt">ocean</span> ridges (Cannat, 1996), we assume that the topmost continental and <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span>, corresponding to the cooler brittle seismogenic layer, deforms by extensional faulting (which we approximate to pure-shear deformation) and magmatic intrusion. Beneath this topmost <span class="hlt">lithosphere</span> layer approximately 15-20 km thick, we assume that deformation occurs in response to passive upwelling and thermal and melt buoyancy driven small-scale convection. The relative contribution of these deformation components is parameterised by the ratio Vz/Vx, where Vx is the half spreading rate applied to the topmost <span class="hlt">lithosphere</span> deformation and Vz is the upwelling velocity associated with the small scale convection. We use a series of numerical experiments to investigate the dependency of continental crust and <span class="hlt">lithosphere</span> rupture, decompression melt initiation, rifted margin <span class="hlt">ocean</span>-continent transition architecture and subsidence history on the half-spreading rate Vx, buoyancy driven upwelling rate Vz, the ratio Vz/Vx and upper <span class="hlt">lithosphere</span> pure-shear width W. Based on the numerical experiment results we explore a polyphase evolution of deformation modes leading to continental breakup, sea</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014EGUGA..1612949J','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014EGUGA..1612949J"><span>Using crustal thickness and subsidence history on the Iberia-Newfoundland margins to constrain <span class="hlt">lithosphere</span> deformation modes during continental breakup</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Jeanniot, Ludovic; Kusznir, Nick; Manatschal, Gianreto; Mohn, Geoffroy</p> <p>2014-05-01</p> <p>Observations at magma-poor rifted margins such as Iberia-Newfoundland show a complex <span class="hlt">lithosphere</span> deformation history during continental breakup and seafloor spreading initiation leading to complex OCT architecture with hyper-extended continental crust and <span class="hlt">lithosphere</span>, exhumed mantle and scattered embryonic <span class="hlt">oceanic</span> crust and continental slivers. Initiation of seafloor spreading requires both the rupture of the continental crust and <span class="hlt">lithospheric</span> mantle, and the onset of decompressional melting. Their relative timing controls when mantle exhumation may occur; the presence or absence of exhumed mantle provides useful information on the timing of these events and constraints on <span class="hlt">lithosphere</span> deformation modes. A single <span class="hlt">lithosphere</span> deformation mode leading to continental breakup and sea-floor spreading cannot explain observations. We have determined the sequence of <span class="hlt">lithosphere</span> deformation events for two profiles across the present-day conjugate Iberia-Newfoundland margins, using forward modelling of continental breakup and seafloor spreading initiation calibrated against observations of crustal basement thickness and subsidence. Flow fields, representing a sequence of <span class="hlt">lithosphere</span> deformation modes, are generated by a 2D finite element viscous flow model (FeMargin), and used to advect <span class="hlt">lithosphere</span> and asthenosphere temperature and material. FeMargin is kinematically driven by divergent deformation in the upper 15-20 km of the <span class="hlt">lithosphere</span> inducing passive upwelling beneath that layer; extensional faulting and magmatic intrusions deform the topmost upper <span class="hlt">lithosphere</span>, consistent with observations of deformation processes occurring at slow spreading <span class="hlt">ocean</span> ridges (Cannat, 1996). Buoyancy enhanced upwelling, as predicted by Braun et al. (2000) is also kinematically included in the <span class="hlt">lithosphere</span> deformation model. Melt generation by decompressional melting is predicted using the parameterization and methodology of Katz et al. (2003). The distribution of <span class="hlt">lithosphere</span> deformation, the</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016JGRB..121.4523S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016JGRB..121.4523S"><span>Strain rate dependency of <span class="hlt">oceanic</span> intraplate earthquake b-values at extremely low strain rates</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Sasajima, Ryohei; Ito, Takeo</p> <p>2016-06-01</p> <p>We discovered a clear positive dependence of <span class="hlt">oceanic</span> intraplate earthquake (OCEQ) b-values on the age of the <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span>. OCEQ b-values in the youngest (<10 Ma) <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> are around 1.0, while those in middle to old (>20 Ma) <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> exceed 1.5, which is significantly higher than the average worldwide earthquake b-value (around 1.0). On the other hand, the b-value of intraplate earthquakes in the Ninety East-Sumatra orogen, where <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> has an anomalously higher strain rate compared with normal <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span>, is 0.93, which is significantly lower than the OCEQ b-value (about 1.9) with the same age (50-110 Ma). Thus, the variation in b-values relates to the strain rate of the <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> and is not caused by a difference in thermal structure. We revealed a negative strain rate dependency of the b-value at extremely low strain rates (<2 × 10-10/year), which can clearly explain the above b-values. We propose that the OCEQ b-value depends strongly on strain rate (either directly or indirectly) at extremely low strain rates. The high OCEQ b-values (>1.5) in <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> >20 Ma old imply that future improvement in seismic observation will capture many smaller magnitude OCEQs, which will provide valuable information on the evolution of the <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> and the driving mechanism of plate tectonics.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..18.8843S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..18.8843S"><span>Deep India meets deep Asia: a seismological view of <span class="hlt">lithospheric</span> slab interactions under Hindu Kush and Pamir</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Schurr, Bernd; Kufner, Sofia; Sippl, Christian; Schneider, Felix; Yuan, Xiaohui; Ratschbacher, Lothar; Mechie, James</p> <p>2016-04-01</p> <p>It is part of the plate-tectonic paradigm that buoyant continental <span class="hlt">lithosphere</span> subducts only in tow of a sinking <span class="hlt">oceanic</span> plate after continent collision and that large deep (> 100 km) earthquakes occur exclusively in subducted <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span>. Yet under the Pamir in Central Asia, far away from any (paleo-)<span class="hlt">ocean</span> basins, continental <span class="hlt">lithosphere</span> appears to subduct by itself and the Pamir-Hindu Kush seismic zone is one of the most active intermediate (100-300 km) depth earthquake zones globally. We show that large-scale indentation of cratonic Asia by a promontory of the Indian plate is causing subduction of continental <span class="hlt">lithosphere</span> under the Pamir and that the Hindu Kush earthquakes are due to detachment of a narrow plate sliver. New precise earthquake hypocenters, a large number of source mechanisms and detailed receiver function sections and tomographic images allow us to distinguish an arcuate, stretched and partly torn slab of Asian <span class="hlt">lithosphere</span> beneath the Pamir and a piece of Indian <span class="hlt">lithosphere</span> beneath the deepest Hindu Kush earthquakes. This peculiar double subduction zone arises by contrasting modes of convergence under Pamir and Hindu Kush imposed by the different mechanical properties of the three types of <span class="hlt">lithosphere</span> involved: We suggest that the buoyant northwestern salient of (1) Cratonic India bulldozes into (2) Cratonic Asia forcing delamination and rollback of its <span class="hlt">lithosphere</span>. At the same time (3) India's thinned margin tears off from Cratonic India and subducts under Asia. The narrow swath of the subducted Indian continental margin forms a prominent high-velocity anomaly down to the mantle transition zone. Its uppermost section is thinned or already severed and intermediate depth earthquakes cluster at the final neck connecting it to the deeper slab. These images provide a rare glimpse of the ephemeral process of slab break-off.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/1996Natur.382..773S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/1996Natur.382..773S"><span>From plume head to continental <span class="hlt">lithosphere</span> in the Arabian-Nubian shield</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Stein, Mordechai; Goldstein, Steven L.</p> <p>1996-08-01</p> <p>The <span class="hlt">lithosphere</span> of the Arabian-Nubian shield was mainly formed during an interval of about 150 million years near the end of the Proterozoic aeon. The events recorded in the rocks of the shield indicate that an <span class="hlt">oceanic</span> plateau, formed by the head of an upwelling mantle plume, was later overprinted with continent-like characteristics by plate convergence and its associated magmatism. Similar sequences of events are seen in the geological record from Archaean to recent times, suggesting that the transformation from plume head to continental <span class="hlt">lithosphere</span> has been an important component of continent generation throughout Earth history.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2007JGRB..112.8305D','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2007JGRB..112.8305D"><span>Thin <span class="hlt">Lithosphere</span> Beneath the Ethiopian Plateau Revealed by a Joint Inversion of Rayleigh Wave Group Velocities and Receiver Functions</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Dugda, Mulugeta T.; Nyblade, Andrew A.; Julia, Jordi</p> <p>2007-08-01</p> <p>The seismic velocity structure of the crust and upper mantle beneath Ethiopia and Djibouti has been investigated by jointly inverting receiver functions and Rayleigh wave group velocities to obtain new constraints on the thermal structure of the <span class="hlt">lithosphere</span>. Most of the data for this study come from the Ethiopia broadband seismic experiment, conducted between 2000 and 2002. Shear velocity models obtained from the joint inversion show crustal structure that is similar to previously published models, with crustal thicknesses of 35 to 44 km beneath the Ethiopian Plateau, and 25 to 35 km beneath the Main Ethiopian Rift (MER) and the Afar. The <span class="hlt">lithospheric</span> mantle beneath the Ethiopian Plateau has a maximum shear wave velocity of about 4.3 km/s and extends to a depth of ˜70-80 km. Beneath the MER and Afar, the <span class="hlt">lithospheric</span> mantle has a maximum shear wave velocity of 4.1-4.2 km/s and extends to a depth of at most 50 km. In comparison to the <span class="hlt">lithosphere</span> away from the East <span class="hlt">African</span> Rift System in Tanzania, where the lid extends to depths of ˜100-125 km and has a maximum shear velocity of 4.6 km/s, the mantle <span class="hlt">lithosphere</span> under the Ethiopian Plateau appears to have been thinned by ˜30-50 km and the maximum shear wave velocity reduced by ˜0.3 km/s. Results from a 1D conductive thermal model suggest that the shear velocity structure of the Ethiopian Plateau <span class="hlt">lithosphere</span> can be explained by a plume model, if a plume rapidly thinned the <span class="hlt">lithosphere</span> by ˜30-50 km at the time of the flood basalt volcanism (c. 30 Ma), and if warm plume material has remained beneath the <span class="hlt">lithosphere</span> since then. About 45-65% of the 1-1.5 km of plateau uplift in Ethiopia can be attributed to the thermally perturbed <span class="hlt">lithospheric</span> structure.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013AGUFM.T42D..07Z','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013AGUFM.T42D..07Z"><span><span class="hlt">Lithospheric</span> Rheology Constrained by Loading of the Hawaiian Islands and its Implications for the Dynamics of Plate Tectonics</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Zhong, S.; Watts, A. B.</p> <p>2013-12-01</p> <p><span class="hlt">Lithospheric</span> rheology is important for understanding crustal and <span class="hlt">lithospheric</span> dynamics, and the conditions for plate tectonics. For example, numerical modeling studies suggest that plate tectonics emerge from the dynamics of mantle convection when a small coefficient of friction (<0.1) or small yield stress for <span class="hlt">lithosphere</span> is used. However, <span class="hlt">lithospheric</span> rheology is not well understood. In this study, we developed 3-D finite element models for computing load-induced surface deformation and stress for <span class="hlt">lithosphere</span> and mantle with realistic non-linear viscoelastic rheology including the frictional sliding, low-temperature plasticity, and high-temperature creep. We determined the deformation and stress due to volcano loading in the Hawaiian region in the last few million years. By comparing model predictions with seismic observations of the depth to the top of <span class="hlt">oceanic</span> crust and depth-dependence of seismicity in the Hawaiian region, we sought to constrain <span class="hlt">lithospheric</span> rheology. Our calculations show that the load-induced surface deformation in the Hawaiian region is controlled by low-temperature plasticity and frictional sliding but is insensitive to high-temperature creep. <span class="hlt">Lithospheric</span> strength predicted from laboratory derived low-temperature plasticity needs to be reduced significantly to match the observations, together with frictional coefficient in the range from 0.1 to 0.7. However, the small coefficient of friction weakens the shallow part of the <span class="hlt">lithosphere</span> so much that it causes the minima in strain rate and stress to occur at too large depths to be consistent with the depth distribution of seismicity at Hawaii. Our results therefore suggest that the coefficient of friction is between 0.25 and 0.7. Finally, maximum <span class="hlt">lithospheric</span> stress under Hawaiian loads is about 100-200 MPa for models that match the observations, and this stress may be viewed as the largest <span class="hlt">lithospheric</span> stress on the Earth.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70019809','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70019809"><span>Continents as lithological icebergs: The importance of buoyant <span class="hlt">lithospheric</span> roots</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Abbott, D.H.; Drury, R.; Mooney, W.D.</p> <p>1997-01-01</p> <p>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 <span class="hlt">oceanic</span> plateau and used the resulting data to estimate the amount of mantle melting which produces permanent continental crust. The <span class="hlt">lithospheric</span> mantle is sufficiently depleted to produce permanent buoyancy (i.e., the crust is unsubductable) at crustal thicknesses greater than 25-27 km. These unsubductable <span class="hlt">oceanic</span> 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 <span class="hlt">lithosphere</span> 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.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014EGUGA..16.2260S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014EGUGA..16.2260S"><span>Seismic evidence for the layered mantle <span class="hlt">lithosphere</span>: a comparsion between Zagros and South Africa</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Sodoudi, Forough; Kind, Rainer</p> <p>2014-05-01</p> <p> depths of 260-280 km, which most likely represents the <span class="hlt">lithosphere</span>-asthenosphere boundary. Based on our result, the Kalahari <span class="hlt">lithosphere</span> may have survived multiple episodes of intense magmatism and collisional rifting during the billions of years of its history, which left their imprint in its internal layering. Beneath the Zagros collision zone we find a 200 km thick <span class="hlt">lithosphere</span>, which most likely represents the Arabian <span class="hlt">lithosphere</span> that has been strongly deformed, thickened and depleted. Thus, similar processes such as those that occurred beneath shields may have taken place beneath the Zagros. In contrast, we observe a thin <span class="hlt">lithosphere</span> of about 80-90 km beneath Central Iran and Alborz. Our results also suggest the presence of remnants of the fossil Neo-Tethys subduction at depths ranging between 80-150 km within the Arabian <span class="hlt">lithosphere</span>. This dipping structure can be seen beneath the Zagros collision zone and disappear towards the northeast beneath Central Iran and Alborz. These findings may support the idea of a breakoff of the <span class="hlt">oceanic</span> Neo-Tethyan slab beneath Central Iran, which results in an asthenospheric upwelling and thinning of the Iranian <span class="hlt">lithosphere</span> beneath Central Iran and Alborz.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014GeoJI.198.1458B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014GeoJI.198.1458B"><span>Multiple-frequency tomography of the upper mantle beneath the <span class="hlt">African</span>/Iberian collision zone</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Bonnin, Mickaël; Nolet, Guust; Villaseñor, Antonio; Gallart, Josep; Thomas, Christine</p> <p>2014-09-01</p> <p>During the Cenozoic, the geodynamics of the western Mediterranean domain has been characterized by a complex history of subduction of Mesozoic <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span>. The final stage of these processes is proposed to have led to the development of the Calabria and Gibraltar arcs, whose formation is still under debate. In this study, we take advantage of the dense broad-band station networks now available in the Alborán Sea region, to develop a high-resolution 3-D tomographic P velocity model of the upper mantle beneath the <span class="hlt">African</span>/Iberian collision zone that will better constraint the past dynamics of this zone. The model is based on 13200 teleseismic arrival times recorded between 2008 and 2012 at 279 stations for which cross-correlation delays are measured with a new technique in different frequency bands centred between 0.03 and 1.0 Hz, and for the first time interpreted using multiple frequency tomography. Our model shows, beneath the Alborán Sea, a strong (4 per cent) fast vertically dipping anomaly observed to at least 650 km depth. The arched shape of this anomaly, and its extent at depth, are coherent with a <span class="hlt">lithospheric</span> slab, thus favouring the hypothesis of a westward consumption of the Ligurian <span class="hlt">ocean</span> slab by roll-back during Cenozoic. In addition to this fast anomaly in the deep upper mantle, high intensity slow anomalies are widespread in the <span class="hlt">lithosphere</span> and asthenosphere beneath Morocco and southern Spain. These anomalies are correlated at the surface with the position of the Rif and Atlas orogens and with Cenozoic volcanic fields. We thus confirm the presence, beneath Morocco, of an anomalous (hot?) upper mantle, but without clear indication for a lateral spreading of the Canary plume to the east.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/1995RvGeS..33..379B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/1995RvGeS..33..379B"><span><span class="hlt">Lithosphere</span> dynamics and continental deformation</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Bird, Peter</p> <p>1995-07-01</p> <p>The unifying theme in this section is the remarkable weakness of major faults. I will consider the diverse new evidence for weakness, and the evidence for high pore pressure localized in faults as a fundamental cause. With this background one can better understand why faults remain active even after large rotations with respect to stress: I will look at large Neogene (≤23.7 million year old) rotations about horizontal axes in the Basin and Range province, and about vertical axes along the Pacific margin. Recent developments will be summarized from studies of Neogene tectonics (neotectonics) in California, Alaska, and the Mississippi embayment, in the context of a weak North American stress field that results mainly from topographic forces. To close, I will present new geophysical studies relevant to the continuing controversy over whether the basic structure of the North American mantle <span class="hlt">lithosphere</span> was altered by an early Tertiary episode of flat subduction.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://ntrs.nasa.gov/search.jsp?R=19830031652&hterms=bathymetric&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D60%26Ntt%3Dbathymetric','NASA-TRS'); return false;" href="https://ntrs.nasa.gov/search.jsp?R=19830031652&hterms=bathymetric&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D60%26Ntt%3Dbathymetric"><span><span class="hlt">Lithospheric</span> flexure at fracture zones</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Sandwell, D.; Schubert, G.</p> <p>1982-01-01</p> <p>Studies attempting to demonstrate that <span class="hlt">lithospheric</span> flexure occurs across the Pioneer and Mendocino fracture zones, and that the flexural topography is a topographic expression at these fracture zones, are presented. The flexure is modelled and compared with predicted depths with five bathymetric profiles which cross the two fracture zones at different ages. The model uses a thin elastic plate overlying an incompressible fluid half-space, and incorporates a temperature-dependent effective elastic thickness. Several conclusions were derived from this study. First, it is found that no significant slip on the fossil fault planes of the Mendocino and Pioneer fracture zones exists. In addition, the flexural amplitude is determined to increase with age. Finally, it is concluded that there is elastic coupling between the Mendocino and Pioneer fracture zones since the separation is less than a flexural wavelength.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/1990EOSTr..71R1759.','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/1990EOSTr..71R1759."><span>Deformation in the continental <span class="hlt">lithosphere</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p></p> <p></p> <p>The Physical Properties of Earth Materials Committee, a technical committee of AGU's Tectonophysics Section, is organizing a dinner/colloquium as part of the Fall Meeting in San Francisco, Calif. This event will be held Monday, December 3rd, in the Gold Rush Room of the Holiday Inn Golden Gateway Hotel at 1500 Van Ness St. There will be a no-host bar from 6:30 to 7:30 P.M., followed by dinner from 7:30 to 8:30 P.M. Paul Tapponnier will deliver the after-dinner talk, “Large-Scale Deformation Mechanisms in the Continental <span class="hlt">Lithosphere</span>: Where Do We Stand?” It will start at 8:30 P.M. and a business meeting will follow at 9:30 P.M.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://ntrs.nasa.gov/search.jsp?R=19780070009&hterms=ocean+plastic&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D40%26Ntt%3Docean%2Bplastic','NASA-TRS'); return false;" href="https://ntrs.nasa.gov/search.jsp?R=19780070009&hterms=ocean+plastic&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D40%26Ntt%3Docean%2Bplastic"><span>An elastic-perfectly plastic analysis of the bending of the <span class="hlt">lithosphere</span> at a trench</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Turcotte, D. L.; Mcadoo, D. C.; Caldwell, J. G.</p> <p>1978-01-01</p> <p>A number of authors have modeled the flexure of the <span class="hlt">lithosphere</span> at an <span class="hlt">oceanic</span> trench using a thin elastic plate with a hydrostatic restoring force. In some cases good agreement with observed topography is obtained but in other cases the slope of the <span class="hlt">lithosphere</span> within the trench is greater than that predicted by the elastic theory. In this paper the bending of a thin plate is considered using an elastic-perfectly plastic rheology. It is found that the <span class="hlt">lithosphere</span> behaves elastically seaward of the trench, but that plasticity decreases the radius of curvature within the trench. The results are compared with a number of observed trench profiles. The elastic-perfectly plastic profiles are in excellent agreement with those profiles that deviate from elastic behavior.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..1811002C','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..1811002C"><span>Rifting Thick <span class="hlt">Lithosphere</span> - Canning Basin, Western Australia</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Czarnota, Karol; White, Nicky</p> <p>2016-04-01</p> <p>The subsidence histories and architecture of most, but not all, rift basins are elegantly explained by extension of ~120 km thick <span class="hlt">lithosphere</span> followed by thermal re-thickening of the <span class="hlt">lithospheric</span> 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 <span class="hlt">lithosphere</span> (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 <span class="hlt">lithosphere</span> ≥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 <span class="hlt">lithosphere</span> 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 <span class="hlt">lithosphere</span> 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 <span class="hlt">lithospheric</span> 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</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013EGUGA..15.8867A','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013EGUGA..15.8867A"><span>The Neotectonic crustal uplift and <span class="hlt">lithospheric</span> softening in plate interiors caused by infiltration of mantle fluids into the <span class="hlt">lithosphere</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Artyushkov, Eugene</p> <p>2013-04-01</p> <p>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 <span class="hlt">lithosphere</span> 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 <span class="hlt">African</span> continent, in Greenland and East Siberia, and in many other regions. Neotectonic crustal uplift widely occurred on the Phanerozoic <span class="hlt">lithosphere</span>. 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 <span class="hlt">lithosphere</span> 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 <span class="hlt">lithosphere</span> 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</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014E%26PSL.389..119J','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014E%26PSL.389..119J"><span>Yellowstone hotspot-continental <span class="hlt">lithosphere</span> interaction</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Jean, Marlon M.; Hanan, Barry B.; Shervais, John W.</p> <p>2014-03-01</p> <p>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 <span class="hlt">lithosphere</span> end-members, heterogeneous in age and composition. We use the SRP Pb, Sr, and Nd isotope data to model the Yellowstone Hotspot-continental <span class="hlt">lithosphere</span> interaction by three component mixing between two continental <span class="hlt">lithospheric</span> components, Archean <span class="hlt">lithosphere</span> (CL1) that represents older <span class="hlt">lithosphere</span> underlying the Yellowstone Plateau in the east, and Paleoproterozoic <span class="hlt">lithosphere</span> (CL2) representing the younger <span class="hlt">lithosphere</span> 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 <span class="hlt">lithosphere</span> migrated over the upwelling hotspot along a shoaling gradient in the sub-continental mantle <span class="hlt">lithosphere</span>. The model shows a decrease in Total <span class="hlt">Lithosphere</span> end-members (CL1 + CL2) and the <span class="hlt">Lithosphere</span> Ratio (CL1/CL2</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_14");'>14</a></li> <li><a href="#" onclick='return showDiv("page_15");'>15</a></li> <li class="active"><span>16</span></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_16 --> <div id="page_17" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_15");'>15</a></li> <li><a href="#" onclick='return showDiv("page_16");'>16</a></li> <li class="active"><span>17</span></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="321"> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70018621','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70018621"><span>Inelastic models of <span class="hlt">lithospheric</span> stress - I. Theory and application to outer-rise plate deformation</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Mueller, S.; Choy, G.L.; Spence, W.</p> <p>1996-01-01</p> <p>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 <span class="hlt">lithospheric</span> 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 <span class="hlt">lithospheric</span> yield strength will result in an accurate characterization of <span class="hlt">lithospheric</span> stress only under relatively restrictive circumstances. In contrast to elastic-plate models, which predict that <span class="hlt">lithospheric</span> 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 <span class="hlt">lithospheric</span> 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 <span class="hlt">lithospheric</span> 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 <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span>.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015EGUGA..1712087J','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015EGUGA..1712087J"><span>Constraining <span class="hlt">lithosphere</span> deformation mode evolution for the Iberia-Newfoundland rifted margins</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Jeanniot, Ludovic; Kusznir, Nick; Mohn, Geoffroy; Manatschal, Gianreto</p> <p>2015-04-01</p> <p>The deformation of <span class="hlt">lithosphere</span> and asthenosphere and its evolution during continental rifting leading to breakup and seafloor spreading initiation is poorly understood. The resulting margin architecture and OCT structure is complex and diverse, and observations at magma poor margins includes hyper-extended continental crust and <span class="hlt">lithosphere</span>, detachments faults, exhumed mantle, continental slivers and scattered embryonic <span class="hlt">oceanic</span> crust. A coupled kinematic-dynamic model of <span class="hlt">lithosphere</span> and asthenosphere deformation has been used to investigate the sequence of <span class="hlt">lithosphere</span> deformation modes for 2 conjugate margin profiles for the Iberia-Newfoundland rifted margins. We use the observed water-loaded subsidence and crustal thickness, together with subsidence history and the age of melt generation, to test and constrain <span class="hlt">lithosphere</span> and asthenosphere deformation models. A sequence of <span class="hlt">lithosphere</span> deformation modes is represented by a succession of flow-fields, which are generated by a 2D finite element viscous flow model (FE-Margin), and is used to advect <span class="hlt">lithosphere</span> and asthenosphere temperature and material. FE-Margin is kinematically driven by divergent deformation in the upper 15-20 km of the <span class="hlt">lithosphere</span> inducing passive upwelling below. Buoyancy enhanced upwelling (e.g. Braun et al. 2000) is also kinematically included. The methodology of Katz et al., 2003 is used to predict melt generation by decompressional melting. The magnitude of extension used in the modelling is consistent with that proposed by Sutra et al (2013). The best fit calibrated models of <span class="hlt">lithosphere</span> deformation evolution for the Iberia-Newfoundland conjugate margins require (i) an initial broad region of <span class="hlt">lithosphere</span> 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</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/21220278','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/21220278"><span>Life in the <span class="hlt">lithosphere</span>, kinetics and the prospects for life elsewhere.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Cockell, Charles S</p> <p>2011-02-13</p> <p>The global contiguity of life on the Earth today is a result of the high flux of carbon and oxygen from oxygenic photosynthesis over the planetary surface and its use in aerobic respiration. Life's ability to directly use redox couples from components of the planetary <span class="hlt">lithosphere</span> in a pre-oxygenic photosynthetic world can be investigated by studying the distribution of organisms that use energy sources normally bound within rocks, such as iron. Microbiological data from Iceland and the deep <span class="hlt">oceans</span> show the kinetic limitations of living directly off igneous rocks in the <span class="hlt">lithosphere</span>. Using energy directly extracted from rocks the <span class="hlt">lithosphere</span> will support about six orders of magnitude less productivity than the present-day Earth, and it would be highly localized. Paradoxically, the biologically extreme conditions of the interior of a planet and the inimical conditions of outer space, between which life is trapped, are the locations from which volcanism and impact events, respectively, originate. These processes facilitate the release of redox couples from the planetary <span class="hlt">lithosphere</span> and might enable it to achieve planetary-scale productivity approximately one to two orders of magnitude lower than that produced by oxygenic photosynthesis. The significance of the detection of extra-terrestrial life is that it will allow us to test these observations elsewhere and establish an understanding of universal relationships between <span class="hlt">lithospheres</span> and life. These data also show that the search for extra-terrestrial life must be accomplished by 'following the kinetics', which is different from following the water or energy.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2017GGG....18..676A','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2017GGG....18..676A"><span>Conductivity structure of the <span class="hlt">lithosphere</span>-asthenosphere boundary beneath the eastern North American margin</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Attias, Eric; Evans, Rob. L.; Naif, Samer; Elsenbeck, Jimmy; Key, Kerry</p> <p>2017-02-01</p> <p>Tectonic plate motion and mantle dynamics processes are heavily influenced by the characteristics of the <span class="hlt">lithosphere</span>-asthenosphere boundary (LAB), yet this boundary remains enigmatic regarding its properties and geometry. The processes involved in rifting at passive margins result in substantial alteration of the <span class="hlt">lithosphere</span> through the transition from continental to <span class="hlt">oceanic</span> lithologies. Here we employ marine magnetotelluric (MT) data acquired along a ˜135 km long profile, offshore Martha's Vineyard, New England, USA, to image the electrical conductivity structure beneath the New England continental margin for the first time. We invert the data using two different MT 2-D inversion algorithms and present a series of models that are obtained using three different parameterizations: fully unconstrained, unconstrained with an imposed LAB discontinuity and a priori constrained <span class="hlt">lithosphere</span> resistivity. This suite of models infers variability in the depth of the LAB, with an average depth of 115 km at the eastern North America passive margin. Models robustly detect a ˜350 Ωm <span class="hlt">lithospheric</span> anomalous conductivity zone (LACZ) that extends vertically through the entire <span class="hlt">lithosphere</span>. Our preferred conductivity model is consistent with regional P-to-S receiver function data, shear-wave velocity, gravity anomalies, and prominent geological features. We propose that the LACZ is indicative of paleolithospheric thinning, either resulting from kimberlite intrusions associated with rifting and the New England Great Meteor hot spot track, or from shear-driven localized deformation related to rifting.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=4806358','PMC'); return false;" href="https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=4806358"><span>Thermal erosion of cratonic <span class="hlt">lithosphere</span> as a potential trigger for mass-extinction</span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pmc">PubMed Central</a></p> <p>Guex, Jean; Pilet, Sebastien; Müntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Sell, Bryan; Schaltegger, Urs</p> <p>2016-01-01</p> <p>The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here we present a synthesis of ammonite biostratigraphy, isotopic data and high precision U-Pb zircon dates from the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. We explain these transitions as a result of changing gas species emitted during the progressive thermal erosion of cratonic <span class="hlt">lithosphere</span> by plume activity or internal heating of the <span class="hlt">lithosphere</span>. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic <span class="hlt">lithosphere</span> before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic <span class="hlt">lithosphere</span>. PMID:27009463</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016NatSR...623168G','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016NatSR...623168G"><span>Thermal erosion of cratonic <span class="hlt">lithosphere</span> as a potential trigger for mass-extinction</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Guex, Jean; Pilet, Sebastien; Müntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Sell, Bryan; Schaltegger, Urs</p> <p>2016-03-01</p> <p>The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here we present a synthesis of ammonite biostratigraphy, isotopic data and high precision U-Pb zircon dates from the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. We explain these transitions as a result of changing gas species emitted during the progressive thermal erosion of cratonic <span class="hlt">lithosphere</span> by plume activity or internal heating of the <span class="hlt">lithosphere</span>. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic <span class="hlt">lithosphere</span> before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic <span class="hlt">lithosphere</span>.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/27009463','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/27009463"><span>Thermal erosion of cratonic <span class="hlt">lithosphere</span> as a potential trigger for mass-extinction.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Guex, Jean; Pilet, Sebastien; Müntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Sell, Bryan; Schaltegger, Urs</p> <p>2016-03-24</p> <p>The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here we present a synthesis of ammonite biostratigraphy, isotopic data and high precision U-Pb zircon dates from the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. We explain these transitions as a result of changing gas species emitted during the progressive thermal erosion of cratonic <span class="hlt">lithosphere</span> by plume activity or internal heating of the <span class="hlt">lithosphere</span>. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic <span class="hlt">lithosphere</span> before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic <span class="hlt">lithosphere</span>.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014AGUFMGP31A3672K','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014AGUFMGP31A3672K"><span>Multi-dimensional Crustal and <span class="hlt">Lithospheric</span> Structure of the Atlas Mountains of Morocco by Magnetotelluric Imaging</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Kiyan, D.; Jones, A. G.; Fullea, J.; Ledo, J.; Siniscalchi, A.; Romano, G.</p> <p>2014-12-01</p> <p>The PICASSO (Program to Investigate Convective Alboran Sea System Overturn) project and the concomitant TopoMed (Plate re-organization in the western Mediterranean: <span class="hlt">Lithospheric</span> causes and topographic consequences - an ESF EUROSCORES TOPO-EUROPE project) project were designed to collect high resolution, multi-disciplinary <span class="hlt">lithospheric</span> scale data in order to understand the tectonic evolution and <span class="hlt">lithospheric</span> structure of the western Mediterranean. The over-arching objectives of the magnetotelluric (MT) component of the projects are (i) to provide new electrical conductivity constraints on the crustal and <span class="hlt">lithospheric</span> structure of the Atlas Mountains, and (ii) to test the hypotheses for explaining the purported <span class="hlt">lithospheric</span> cavity beneath the Middle and High Atlas inferred from potential-field <span class="hlt">lithospheric</span> modeling. We present the results of an MT experiment we carried out in Morocco along two profiles: an approximately N-S oriented profile crossing the Middle Atlas, the High Atlas and the eastern Anti-Atlas to the east (called the MEK profile, for Meknes) and NE-SW oriented profile through western High Atlas to the west (called the MAR profile, for Marrakech). Our results are derived from three-dimensional (3-D) MT inversion of the MT data set employing the parallel version of Modular system for Electromagnetic inversion (ModEM) code. The distinct conductivity differences between the Middle-High Atlas (conductive) and the Anti-Atlas (resistive) correlates with the South Atlas Front fault, the depth extent of which appears to be limited to the uppermost mantle (approx. 60 km). In all inverse solutions, the crust and the upper mantle show resistive signatures (approx. 1,000 Ωm) beneath the Anti-Atlas, which is the part of stable West <span class="hlt">African</span> Craton. Partial melt and/or exotic fluids enriched in volatiles produced by the melt can account for the high middle to lower crustal and uppermost mantle conductivity in the Folded Middle Atlas, the High Moulouya Plain and the</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2011GeoRL..3823301M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2011GeoRL..3823301M"><span>Erosion of the continental <span class="hlt">lithosphere</span> at the cusps of the Calabrian arc: Evidence from S receiver functions analysis</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Miller, Meghan S.; Piana Agostinetti, Nicola</p> <p>2011-12-01</p> <p>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 <span class="hlt">lithospheric</span> structure of the convergent, Apulian and Hyblean forelands at the cusps of the arc. Specifically we investigate the crustal and <span class="hlt">lithospheric</span> 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 <span class="hlt">lithospheric</span> thicknesses range between 70-120 km beneath Apulia and 70-90 km beneath Sicily. The <span class="hlt">lithosphere</span> of the forelands at each end of the Calabrian arc are continental in nature, buoyant compared to the subducting <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> and have previously been interpreted as mostly undeformed carbonate platforms. Our receiver function images also show evidence of <span class="hlt">lithospheric</span> 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 <span class="hlt">lithosphere</span> resists being subducted it is being thermo-mechanically modified by toroidal flow around the edges of the subducting <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> of the Calabrian arc.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016CoMP..171....9H','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016CoMP..171....9H"><span>Enriched continental flood basalts from depleted mantle melts: modeling the <span class="hlt">lithospheric</span> contamination of Karoo lavas from Antarctica</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Heinonen, Jussi S.; Luttinen, Arto V.; Bohrson, Wendy A.</p> <p>2016-01-01</p> <p>Continental flood basalts (CFBs) represent large-scale melting events in the Earth's upper mantle and show considerable geochemical heterogeneity that is typically linked to substantial contribution from underlying continental <span class="hlt">lithosphere</span>. Large-scale partial melting of the cold subcontinental <span class="hlt">lithospheric</span> mantle and the large amounts of crustal contamination suggested by traditional binary mixing or assimilation-fractional crystallization models are difficult to reconcile with the thermal and compositional characteristics of continental <span class="hlt">lithosphere</span>, however. The well-exposed CFBs of Vestfjella, western Dronning Maud Land, Antarctica, belong to the Jurassic Karoo large igneous province and provide a prime locality to quantify mass contributions of <span class="hlt">lithospheric</span> and sublithospheric sources for two reasons: (1) recently discovered CFB dikes show isotopic characteristics akin to mid-<span class="hlt">ocean</span> ridge basalts, and thus help to constrain asthenospheric parental melt compositions and (2) the well-exposed basaltic lavas have been divided into four different geochemical magma types that exhibit considerable trace element and radiogenic isotope heterogeneity (e.g., initial ɛ Nd from -16 to +2 at 180 Ma). We simulate the geochemical evolution of Vestfjella CFBs using (1) energy-constrained assimilation-fractional crystallization equations that account for heating and partial melting of crustal wall rock and (2) assimilation-fractional crystallization equations for <span class="hlt">lithospheric</span> mantle contamination by using highly alkaline continental volcanic rocks (i.e., partial melts of mantle <span class="hlt">lithosphere</span>) as contaminants. Calculations indicate that the different magma types can be produced by just minor (1-15 wt%) contamination of asthenospheric parental magmas by melts from variable <span class="hlt">lithospheric</span> reservoirs. Our models imply that the role of continental <span class="hlt">lithosphere</span> as a CFB source component or contaminant may have been overestimated in many cases. Thus, CFBs may represent major juvenile crustal</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..18.1772P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..18.1772P"><span>Thermal erosion of cratonic <span class="hlt">lithosphere</span> as a potential trigger for mass-extinction</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Pilet, Sebastien; Guex, Jean; Muntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Schaltegger, Urs</p> <p>2016-04-01</p> <p>The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here, we present a synthesis of stratigraphic constraints on the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries combined with geochronological data in order to establish the sequence of events that initiate two of the major mass extinctions recorded in Earth's history. This synthesis demonstrates that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. The initial regressive events recorded at T-J and Pl-To boundaries seem difficult to reconcile either with large initial CO2 degassing associated with plume activity or by volatile-release (CO2, CH4, Cl2) from deep sedimentary reservoirs during contact metamorphism associated to dykes and sills intrusion because massive CO2 degassing is expected to produce super greenhouse conditions. We evaluate, here, an alternative suggesting that the initial cooling could be due to gas release during the initial thermal erosion of the cratonic <span class="hlt">lithosphere</span> due to emplacement of the CAMP and Karoo-Ferrar volcanic provinces. Petrological constraints on primary magmas indicate that the mantle is hotter and melts more extensively to produce LIP lavas than for current <span class="hlt">oceanic</span> islands basalts. However, available data suggest that the Karoo and CAMP areas were underlain by thick <span class="hlt">lithosphere</span> (>200 km) prior to continental break up. The presence of thick <span class="hlt">lithosphere</span> excludes significant melting of the asthenospheric mantle without initial stage of thermal erosion of the cratonic <span class="hlt">lithosphere</span>. This initial step of thermal erosion / thermal heating of the cratonic <span class="hlt">lithosphere</span> is critical to understand the volatile budget associated with LIPs while</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/20740006','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/20740006"><span><span class="hlt">Lithospheric</span> layering in the North American craton.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Yuan, Huaiyu; Romanowicz, Barbara</p> <p>2010-08-26</p> <p>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 <span class="hlt">lithosphere</span>-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 <span class="hlt">lithospheric</span> 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 <span class="hlt">lithosphere</span>-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 <span class="hlt">lithosphere</span> in North America. They also suggest that the horizon detected in receiver function studies probably corresponds to the sharp mid-<span class="hlt">lithospheric</span> boundary rather than to the more gradual <span class="hlt">lithosphere</span>-asthenosphere boundary.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..1815366L','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..1815366L"><span>Imaging the <span class="hlt">lithosphere</span> of rifted passive margins using waveform tomography: North Atlantic, South Atlantic and beyond</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Lebedev, Sergei; Schaeffer, Andrew; Celli, Nicolas Luca</p> <p>2016-04-01</p> <p>Lateral variations in seismic velocities in the upper mantle reflect variations in the temperature of the rocks at depth. Seismic tomography thus provides a proxy for lateral changes in the temperature and thickness of the <span class="hlt">lithosphere</span>. It can map the deep boundaries between tectonic blocks with different properties and age of the <span class="hlt">lithosphere</span>. Our 3D tomographic models of the upper mantle and the crust at the Atlantic and global scales are constrained by an unprecedentedly large global dataset of broadband waveform fits (over one million seismograms) and provide improved resolution of the <span class="hlt">lithosphere</span>, compared to other available models. The most prominent high-velocity anomalies, seen down to 150-200 km depths, indicate the cold, thick, stable mantle <span class="hlt">lithosphere</span> beneath Precambrian cratons, including those in North America, Greenland, northern and eastern Europe, Africa and South America. The dominant, large-scale, low-velocity feature is the global system of mid-<span class="hlt">ocean</span> ridges, with broader low-velocity regions near hotspots, including Iceland. Currently active continental rifts show highly variable expression in the upper mantle, from pronounced low velocities to weak anomalies; this correlates with the amount of magmatism within the rift zone. Rifted passive margins have typically undergone cooling since the rifting and show more subtle variations in their seismic-velocity structure. Their thermal structure and evolution, however, are also shaped by 3D geodynamic processes since their formation, including cooling by the adjacent cratonic blocks inland and heating by warm <span class="hlt">oceanic</span> asthenosphere.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/23518564','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/23518564"><span>Melt-rich channel observed at the <span class="hlt">lithosphere</span>-asthenosphere boundary.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Naif, S; Key, K; Constable, S; Evans, R L</p> <p>2013-03-21</p> <p>The <span class="hlt">lithosphere</span>-asthenosphere boundary (LAB) separates rigid <span class="hlt">oceanic</span> plates from the underlying warm ductile asthenosphere. Although a viscosity decrease beneath this boundary is essential for plate tectonics, a consensus on its origin remains elusive. Seismic studies identify a prominent velocity discontinuity at depths thought to coincide with the LAB but disagree on its cause, generally invoking either partial melting or a mantle dehydration boundary as explanations. Here we use sea-floor magnetotelluric data to image the electrical conductivity of the LAB beneath the edge of the Cocos plate at the Middle America trench offshore of Nicaragua. Underneath the resistive <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span>, the magnetotelluric data reveal a high-conductivity layer confined to depths of 45 to 70 kilometres. Because partial melts are stable at these depths in a warm damp mantle, we interpret the conductor to be a partially molten layer capped by an impermeable frozen lid that is the base of the <span class="hlt">lithosphere</span>. A conductivity anisotropy parallel to plate motion indicates that this melt has been sheared into flow-aligned tube-like structures. We infer that the LAB beneath young plates consists of a thin, partially molten, channel of low viscosity that acts to decouple the overlying brittle <span class="hlt">lithosphere</span> from the deeper convecting mantle. Because this boundary layer has the potential to behave as a lubricant to plate motion, its proximity to the trench may have implications for subduction dynamics.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016JGRB..121.3415B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016JGRB..121.3415B"><span>Offshore Southern California <span class="hlt">lithospheric</span> velocity structure from noise cross-correlation functions</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Bowden, D. C.; Kohler, M. D.; Tsai, V. C.; Weeraratne, D. S.</p> <p>2016-05-01</p> <p>A new shear wave velocity model offshore Southern California is presented that images plate boundary deformation including both thickening and thinning of the crustal and mantle <span class="hlt">lithosphere</span> at the westernmost edge of the North American continent. The Asthenospheric and <span class="hlt">Lithospheric</span> Broadband Architecture from the California Offshore Region Experiment (ALBACORE) <span class="hlt">ocean</span> bottom seismometer array, together with 65 stations of the onshore Southern California Seismic Network, is used to measure ambient noise correlation functions and Rayleigh wave dispersion curves which are inverted for 3-D shear wave velocities. The resulting velocity model defines the transition from continental <span class="hlt">lithosphere</span> to <span class="hlt">oceanic</span>, illuminating the complex history and deformation in the region. A transition to the present-day strike-slip regime between the Pacific and North American Plates resulted in broad deformation and capture of the now >200 km wide continental shelf. Our velocity model suggests the persistence of the uppermost mantle volcanic processes associated with East Pacific Rise spreading adjacent to the Patton Escarpment, which marks the former subduction of Farallon Plate underneath North America. The most prominent of these seismic structures is a low-velocity anomaly underlying the San Juan Seamount, suggesting ponding of magma at the base of the crust, resulting in thickening and ongoing adjustment of the <span class="hlt">lithosphere</span> due to the localized loading. The velocity model also provides a robust framework for future earthquake location determinations and ground-shaking simulations for risk estimates.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2007Icar..192...92B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2007Icar..192...92B"><span>Unstable extension of Enceladus' <span class="hlt">lithosphere</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Bland, Michael T.; Beyer, Ross A.; Showman, Adam P.</p> <p>2007-12-01</p> <p>Regions near Enceladus' equator, Sarandib and Diyar Planitia, contain extensive sets of parallel ridges and troughs that may be diagnostic of the region's formation conditions. We present photoclinometry profiles across these ridges and troughs, which indicate that they are periodic, low-slope features with dominant wavelengths of 3 to 4 km and amplitudes between 100 and 400 m. The morphology of these terrains is consistent with formation via unstable extension of the <span class="hlt">lithosphere</span>. Our numerical modeling demonstrates that unstable extension can generate large-scale topography under Enceladus-like conditions. Comparison of our photoclinometry profiles with the dominant wavelengths produced by our numerical model permits estimation of the background heat flow at the time the Sarandib-Diyar province formed. We estimate heat flows of 110 to 220mWm, suggesting that resurfacing of the planitiae was accompanied by strong, localized heating. The extension necessary to produce the ridges and troughs may have been caused by now-inactive diapirs, internal phase changes, or other mechanisms. Our heat flux estimates imply elastic thickness at the time of resurfacing of 0.4 to 1.4 km, which are sufficient to have allowed satellite reorientation if the province was underlain by a low-density region. It is therefore plausible that Enceladus has experienced multiple heating events, each leading to localized resurfacing and global reorientation.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013JAfES..86...65F','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013JAfES..86...65F"><span>Orogen styles in the East <span class="hlt">African</span> Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Fritz, H.; Abdelsalam, M.; Ali, K. A.; Bingen, B.; Collins, A. S.; Fowler, A. R.; Ghebreab, W.; Hauzenberger, C. A.; Johnson, P. R.; Kusky, T. M.; Macey, P.; Muhongo, S.; Stern, R. J.; Viola, G.</p> <p>2013-10-01</p> <p>The East <span class="hlt">African</span> Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world´s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual <span class="hlt">oceanic</span> domains and continental fragments between the Archean Sahara-Congo-Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian-Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique <span class="hlt">Ocean</span>. The <span class="hlt">ocean</span> closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite-Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique <span class="hlt">Ocean</span> and experienced a ∼650-620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East <span class="hlt">African</span> Orogeny and was related to closure of the Mozambique <span class="hlt">Ocean</span>. <span class="hlt">Oceans</span> persisted after 620 Ma between East Antarctica, India, southern parts of the Congo-Tanzania-Bangweulu Cratons and the Zimbabwe-Kalahari Craton. They closed during the ∼600-500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East <span class="hlt">African</span> and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600-550 Ma extension is recorded in the Arabian-Nubian Shield and the Eastern Granulite-Cabo Delgado Nappe Complex. Later ∼550-480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of <span class="hlt">lithospheric</span> delamination. Along the strike of the East <span class="hlt">African</span> Orogen, different geodynamic settings resulted in the evolution of</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=4802517','PMC'); return false;" href="https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=4802517"><span>Orogen styles in the East <span class="hlt">African</span> Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution☆</span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pmc">PubMed Central</a></p> <p>Fritz, H.; Abdelsalam, M.; Ali, K.A.; Bingen, B.; Collins, A.S.; Fowler, A.R.; Ghebreab, W.; Hauzenberger, C.A.; Johnson, P.R.; Kusky, T.M.; Macey, P.; Muhongo, S.; Stern, R.J.; Viola, G.</p> <p>2013-01-01</p> <p>The East <span class="hlt">African</span> Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world́s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual <span class="hlt">oceanic</span> domains and continental fragments between the Archean Sahara–Congo–Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian–Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique <span class="hlt">Ocean</span>. The <span class="hlt">ocean</span> closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite–Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique <span class="hlt">Ocean</span> and experienced a ∼650–620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East <span class="hlt">African</span> Orogeny and was related to closure of the Mozambique <span class="hlt">Ocean</span>. <span class="hlt">Oceans</span> persisted after 620 Ma between East Antarctica, India, southern parts of the Congo–Tanzania–Bangweulu Cratons and the Zimbabwe–Kalahari Craton. They closed during the ∼600–500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East <span class="hlt">African</span> and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600–550 Ma extension is recorded in the Arabian–Nubian Shield and the Eastern Granulite–Cabo Delgado Nappe Complex. Later ∼550–480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of <span class="hlt">lithospheric</span> delamination. Along the strike of the East <span class="hlt">African</span> Orogen, different geodynamic settings</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/27065752','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/27065752"><span>Orogen styles in the East <span class="hlt">African</span> Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Fritz, H; Abdelsalam, M; Ali, K A; Bingen, B; Collins, A S; Fowler, A R; Ghebreab, W; Hauzenberger, C A; Johnson, P R; Kusky, T M; Macey, P; Muhongo, S; Stern, R J; Viola, G</p> <p>2013-10-01</p> <p>The East <span class="hlt">African</span> Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world́s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual <span class="hlt">oceanic</span> domains and continental fragments between the Archean Sahara-Congo-Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian-Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique <span class="hlt">Ocean</span>. The <span class="hlt">ocean</span> closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite-Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique <span class="hlt">Ocean</span> and experienced a ∼650-620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East <span class="hlt">African</span> Orogeny and was related to closure of the Mozambique <span class="hlt">Ocean</span>. <span class="hlt">Oceans</span> persisted after 620 Ma between East Antarctica, India, southern parts of the Congo-Tanzania-Bangweulu Cratons and the Zimbabwe-Kalahari Craton. They closed during the ∼600-500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East <span class="hlt">African</span> and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600-550 Ma extension is recorded in the Arabian-Nubian Shield and the Eastern Granulite-Cabo Delgado Nappe Complex. Later ∼550-480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of <span class="hlt">lithospheric</span> delamination. Along the strike of the East <span class="hlt">African</span> Orogen, different geodynamic settings resulted in the evolution</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/1993Litho..30..309D','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/1993Litho..30..309D"><span>Earth's earliest continental <span class="hlt">lithosphere</span>, hydrothermal flux and crustal recycling</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>de Wit, Maarten J.; Hart, Roger A.</p> <p>1993-09-01</p> <p>The Kaapvaal craton in southern Africa and the Pilbara craton of northwestern Australia are the largest regions on Earth to have retained relatively pristine mid-Archaean rocks (3.0-4.0 Ga). The Kaapvaal craton covers about 1.2×10 6 km 2, and varies in <span class="hlt">lithospheric</span> thickness between 170 and 350 km. At surface, the craton can be subdivided into a number of Archaean sub-domains; some of the subdomains are also well defined at depth, and local variations in tomography of the <span class="hlt">lithosphere</span> correspond closely with subdomain boundaries at surface. The Archaean history of the Kaapvaal craton spans about 1 Gyr and can be conveniently subdivided into two periods, each of about the same length as the Phanerozoic. The first period, from circa 3.7-3.1 Ga, records the initial separation of the cratonic <span class="hlt">lithosphere</span> from the asthenosphere, terminating with a major pulse of accretion tectonics between 3.2 and 3.1 Ga, which includes the formation of "paired metamorphic belts". This period of continental growth can be compared to plate tectonic processes occurring in modern-day <span class="hlt">oceanic</span> basins. However, the difference is that in the mid-Archaean, these <span class="hlt">oceanic</span> processes appear to have occurred in shallower water depths than the modern <span class="hlt">ocean</span> basins. The second period, from circa 3.1-2.6 Ga, records intra-continental and continental-edge processes: continental growth during this period occurred predominantly through a combination of tectonic accretion of crustal fragments and subduction-related igneous processes, in much the same way as has been documented along the margins of the Pacific and Tethys <span class="hlt">oceans</span> since the Mesozoic. The intra-<span class="hlt">oceanic</span> processes resulted in small, but deep-rooted continental nucleii; the first separation of this early continental <span class="hlt">lithosphere</span> could only have occurred when the mean elevation of mid-oceanicridges sank below sea-level. Substantial recycling of continental <span class="hlt">lithosphere</span> into the mantle must have occurred during this period of Earth history. During the</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_15");'>15</a></li> <li><a href="#" onclick='return showDiv("page_16");'>16</a></li> <li class="active"><span>17</span></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_17 --> <div id="page_18" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_16");'>16</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li class="active"><span>18</span></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="341"> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015PEPI..238....8B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015PEPI..238....8B"><span>Assimilating <span class="hlt">lithosphere</span> and slab history in 4-D Earth models</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Bower, Dan J.; Gurnis, Michael; Flament, Nicolas</p> <p>2015-01-01</p> <p>We develop methods to incorporate paleogeographical constraints into numerical models of mantle convection. Through the solution of the convection equations, the models honor geophysical and geological data near the surface while predicting mantle flow and structure at depth and associated surface deformation. The methods consist of four constraints determined a priori from a plate history model: (1) plate velocities, (2) thermal structure of the <span class="hlt">lithosphere</span>, (3) thermal structure of slabs in the upper mantle, and (4) velocity of slabs in the upper mantle. These constraints are implemented as temporally- and spatially-dependent conditions that are blended with the solution of the convection equations at each time step. We construct Earth-like regional models with <span class="hlt">oceanic</span> and continental <span class="hlt">lithosphere</span>, trench migration, oblique subduction, and asymmetric subduction to test the robustness of the methods by computing the temperature, velocity, and buoyancy flux of the <span class="hlt">lithosphere</span> and slab. Full sphere convection models demonstrate how the methods can determine the flow associated with specific tectonic environments (e.g., back-arc basins, intraoceanic subduction zones) to address geological questions and compare with independent data, both at present-day and in the geological past (e.g., seismology, residual topography, stratigraphy). Using global models with paleogeographical constraints we demonstrate (1) subduction initiation at the Izu-Bonin-Mariana convergent margin and flat slab subduction beneath North America, (2) enhanced correlation of model slabs and fast anomalies in seismic tomography beneath North and South America, and (3) comparable amplitude of dynamic and residual topography in addition to improved spatial correlation of dynamic and residual topography lows.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/5023538','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/5023538"><span>Strength and survival of subducted <span class="hlt">lithosphere</span> during the Laramide orogeny</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Spencer, J.E. )</p> <p>1993-04-01</p> <p>The strength of subducted <span class="hlt">ocean</span> <span class="hlt">lithosphere</span> is influenced primarily by two competing processes. During subduction brittle rock strength increases because of increasing compressive stress across fracture surfaces which increases frictional resistance to sliding. The strength of rocks hot enough to be in the plastic deformation regime decreases primarily because of heat conducted from the overriding plate and the asthenosphere. A one-dimensional finite-element heat-flow program was used to simulate subduction in two dimensions where conductive heat flow parallel to the slab and to the upper plate could be neglected. Temperatures determined with this method, and pressures based on depth, were then used to calculate the form of the brittle-plastic failure envelope for subducted <span class="hlt">lithosphere</span>. An olivine flow law and strain rate of 10[sup [minus]15] s[sup [minus]1] were used for the plastic part of the failure envelope. The failure envelope was then used to calculate slab-parallel compressive strength and maximum sustainable bending moment. Modeling of Maramide subduction beneath southwestern North America, using slab ages and subduction rates for the Farallon plate from Engebretson et al., suggests that the subducted slab will not retain much strength beyond 1,000 to 1,200 km inland unless the thickness of the North American <span class="hlt">lithosphere</span>, and depth to the top of the slab, are significantly less than 200 km. Slab survival for distances of 1000 km seems assured. Survival for much greater distances is possible. The slab is predicted to have been up to several times stronger beneath southwestern North America than at the trench because much rock remains in the brittle regime and is under high confining pressure.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013JGRB..118.3080F','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013JGRB..118.3080F"><span>Generation of continental rifts, basins, and swells by <span class="hlt">lithosphere</span> instabilities</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Fourel, LoïC.; Milelli, Laura; Jaupart, Claude; Limare, Angela</p> <p>2013-06-01</p> <p>Continents may be affected simultaneously by rifting, uplift, volcanic activity, and basin formation in several different locations, suggesting a common driving mechanism that is intrinsic to continents. We describe a new type of convective instability at the base of the <span class="hlt">lithosphere</span> that leads to a remarkable spatial pattern at the scale of an entire continent. We carried out fluid mechanics laboratory experiments on buoyant blocks of finite size that became unstable due to cooling from above. Dynamical behavior depends on three dimensionless numbers, a Rayleigh number for the unstable block, a buoyancy number that scales the intrinsic density contrast to the thermal one, and the aspect ratio of the block. Within the block, instability develops in two different ways in an outer annulus and in an interior region. In the outer annulus, upwellings and downwellings take the form of periodically spaced radial spokes. The interior region hosts the more familiar convective pattern of polygonal cells. In geological conditions, such instabilities should manifest themselves as linear rifts striking at a right angle to the continent-<span class="hlt">ocean</span> boundary and an array of domal uplifts, volcanic swells, and basins in the continental interior. Simple scaling laws for the dimensions and spacings of the convective structures are derived. For the subcontinental <span class="hlt">lithospheric</span> mantle, these dimensions take values in the 500-1000 km range, close to geological examples. The large intrinsic buoyancy of Archean <span class="hlt">lithospheric</span> roots prevents this type of instability, which explains why the widespread volcanic activity that currently affects Western Africa is confined to post-Archean domains.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://hdl.handle.net/2060/19860011621','NASA-TRS'); return false;" href="http://hdl.handle.net/2060/19860011621"><span>Wilson study cycles: Research relative to <span class="hlt">ocean</span> geodynamic cycles</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Kidd, W. S. F.</p> <p>1985-01-01</p> <p>The effects of conversion of Atlantic (rifted) margins to convergent plate boundaries; <span class="hlt">oceanic</span> plateaus at subduction zones; continental collision and tectonic escape; southern Africa rifts; and global hot spot distribution on long term development of the continental <span class="hlt">lithosphere</span> were studied.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2012Litho.149....4H','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2012Litho.149....4H"><span>Formation of cratonic <span class="hlt">lithosphere</span>: An integrated thermal and petrological model</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Herzberg, Claude; Rudnick, Roberta</p> <p>2012-09-01</p> <p>The formation of cratonic mantle peridotite of Archean age is examined within the time frame of Earth's thermal history, and how it was expressed by temporal variations in magma and residue petrology. Peridotite residues that occupy the <span class="hlt">lithospheric</span> mantle are rare owing to the effects of melt-rock reaction, metasomatism, and refertilization. Where they are identified, they are very similar to the predicted harzburgite residues of primary magmas of the dominant basalts in greenstone belts, which formed in a non-arc setting (referred to here as "non-arc basalts"). The compositions of these basalts indicate high temperatures of formation that are well-described by the thermal history model of Korenaga. In this model, peridotite residues of extensive ambient mantle melting had the highest Mg-numbers, lowest FeO contents, and lowest densities at ~ 2.5-3.5 Ga. These results are in good agreement with Re-Os ages of kimberlite-hosted cratonic mantle xenoliths and enclosed sulfides, and provide support for the hypothesis of Jordan that low densities of cratonic mantle are a measure of their high preservation potential. Cratonization of the Earth reached its zenith at ~ 2.5-3.5 Ga when ambient mantle was hot and extensive melting produced <span class="hlt">oceanic</span> crust 30-45 km thick. However, there is a mass imbalance exhibited by the craton-wide distribution of harzburgite residues and the paucity of their complementary magmas that had compositions like the non-arc basalts. We suggest that the problem of the missing basaltic <span class="hlt">oceanic</span> crust can be resolved by its hydration, cooling and partial transformation to eclogite, which caused foundering of the entire <span class="hlt">lithosphere</span>. Some of the <span class="hlt">oceanic</span> crust partially melted during foundering to produce continental crust composed of tonalite-trondhjemite-granodiorite (TTG). The remaining <span class="hlt">lithosphere</span> gravitationally separated into 1) residual eclogite that continued its descent, and 2) buoyant harzburgite diapirs that rose to underplate cratonic nuclei</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2010E%26PSL.297..405F','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2010E%26PSL.297..405F"><span>Subduction of continental <span class="hlt">lithosphere</span> in the Banda Sea region: Combining evidence from full waveform tomography and isotope ratios</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Fichtner, Andreas; De Wit, Maarten; van Bergen, Manfred</p> <p>2010-09-01</p> <p>We provide new insight into the subduction of old continental <span class="hlt">lithosphere</span> to depths of more than 100 km beneath the Banda arc, based on a spatial correlation of full waveform tomographic images of its <span class="hlt">lithosphere</span> with He, Pb, Nd and Sr isotope signatures in its arc volcanics. The thickness of the subducted <span class="hlt">lithosphere</span> of around 200 km coincides with the thickness of Precambrian <span class="hlt">lithosphere</span> as inferred from surface wave tomography. While the deep subduction of continental material in continent-continent collisions is widely recognised, the analogue process in the arc-continent collision of the Banda region is currently unique. The integrated data suggest that the late Jurassic <span class="hlt">ocean</span> <span class="hlt">lithosphere</span> north of the North Australian craton was capable of entraining large volumes of continental <span class="hlt">lithosphere</span>. The Banda arc example demonstrates that continental <span class="hlt">lithosphere</span> in arc-continent collisions is not generally preserved, thus increasing the complexity of tectonic reconstructions. In the particular case of Timor, the tomographic images indicate that this island is not located directly above the northern margin of the North Australian craton, and that decoupled <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> must be located at a considerable distance north of Timor, possibly as far north as the northern margin of the volcanically extinct arc sector. The tomographic images combined with isotope data suggest that subduction of the continental <span class="hlt">lithosphere</span> did not lead to the delamination of its complete crust. 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 <span class="hlt">lithosphere</span> as inferred from the tomographic images. The subduction of continental <span class="hlt">lithosphere</span> including crustal material beneath</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2012EGUGA..14.6643M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2012EGUGA..14.6643M"><span>Creation and Deformation of Hydrous <span class="hlt">Lithosphere</span> at the Southern Mariana Margin</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Martinez, F.; Kelley, K. A.; Stern, R. J.</p> <p>2012-04-01</p> <p>Mantle <span class="hlt">lithosphere</span> formed at mid-<span class="hlt">ocean</span> seafloor spreading centers is thought to be essentially anhydrous because water is strongly partitioned into melt and removed from the mantle during crustal formation. Since water weakens olivine this dehydration process is also thought to strengthen <span class="hlt">oceanic</span> mantle <span class="hlt">lithosphere</span> above solidus depths, perhaps helping to focus deformation and melt delivery to the narrow plate boundary zones observed at mid-<span class="hlt">ocean</span> ridges. In contrast, convergent margins are sites of high water flux from subducting slabs and thereby provide an opportunity to study the creation and deformation of <span class="hlt">lithosphere</span> in a hydrous environment. The southern Mariana margin presents a rare case in which the upper plate is undergoing active extension parallel to the trench and directly above the subducting slab. The extension has rifted preexisting Paleogene <span class="hlt">lithosphere</span> resulting in the present-day creation of new <span class="hlt">lithosphere</span> in this hydrous environment. Here we present preliminary results from R/V Thomas G. Thompson cruise TN273 in December 2011-January 2012 utilizing the Hawaii Mapping Research Group's IMI-30, a 30 kHz deep-towed side-scan sonar, and ship-based Simrad EM302 multibeam bathymetry. The sidescan sonar imagery and multibeam bathymetry map the tectonic and volcanic structure of a 32 x 80 km area referred to as the southeast Mariana forearc rifts (SEMFR), which extend from near the backarc spreading center toward the trench. The sonar imagery shows a complex volcanic and tectonic structure with no single spreading or rifting axis. Volcanism appears to be widely dispersed and separated by faulted areas. Bathymetry data show several rifts spanning this area but no single rift appears to be focusing tectonic activity as earthquake seismicity is broadly distributed across this region. The data depict a broad volcano-tectonic zone of complex deformation and distributed volcanism unlike the narrow plate boundary zones of mid-<span class="hlt">ocean</span> ridges. This distributed</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2007AGUSM.S44A..03H','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2007AGUSM.S44A..03H"><span>The crust and <span class="hlt">lithosphere</span> thicknesses in South America: trying to find the <span class="hlt">lithosphere</span>- asthenosphere boundary</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Heit, B.; Sodoudi, F.; Yuan, X.; Bianchi, M.; Kind, R.</p> <p>2007-05-01</p> <p>During the past years, a series of seismological investigations have been carried out to study the crustal and mantle structures all over the world. In South America, this investigation has not been an easy task as there are different regions where the geodynamics involves the subduction of an <span class="hlt">oceanic</span> plate, the building of a mountain range as the Andes, the interaction with older <span class="hlt">lithosphere</span> as the Brazilian Shield and the presence of active deformation fronts between the last two regions. In order to investigate the thickness of the <span class="hlt">lithosphere</span> in such a complex context we have performed S-wave receiver function analysis (Vinnik and Farra, 2000; Li et al., 2004). The S receiver function technique looks for the S-to-P converted waves at seismic discontinuities beneath a station in the same way as the conventional P receiver function method that deals with P-to-S conversions. The S receiver function technique have proved to be useful to map the Moho and the LAB in many regions where other methods (i.e. surface waves) failed to provide reliable information (e.g. Li et al., 2004; Kumar et al., 2004a, 2004b; Sodoudi et al., 2006). We present here the results of S receiver function technique that has been applied to all the available temporary seismic experiments (e.g. BANJO, SEDA, REFUCA, BLSP) and the permanent stations from the IRIS network. We have been able to investigate the upper mantle discontinuities at all the depths beneath the stations and obtained coherent Moho depths along the entire Andes and in other South American continental regions. The LAB has been clearly detected below some stations, particularly those that are located far away from the subduction zone. By comparing our results with those from the P receiver functions, we have been able to further constrain the thicknesses of the crust and LAB in different regions including shields, mobile belts, basins and mountain ranges. At many stations we have also been able to map the upper mantle</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://www.osti.gov/scitech/servlets/purl/81086','SCIGOV-STC'); return false;" href="http://www.osti.gov/scitech/servlets/purl/81086"><span>Renewal: Continential <span class="hlt">lithosphere</span> evolution as a function of tectonic environment</span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>McMillan, N.J.; Baldridge, W.S.</p> <p>1995-06-01</p> <p>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 <span class="hlt">oceanic</span> 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 <span class="hlt">lithospheric</span> mantle; intercalated rhyolites have a strong upper crustal signature. Eruption of basalts and andesites with sources in the <span class="hlt">lithospheric</span> 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.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/17738235','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/17738235"><span>Thermal structure of the <span class="hlt">lithosphere</span>: a petrologic model.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Macgregor, I D; Basu, A R</p> <p>1974-09-20</p> <p>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 <span class="hlt">lithosphere</span> 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 <span class="hlt">oceanic</span> 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 <span class="hlt">lithosphere</span>-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.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2009EGUGA..11.5793M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2009EGUGA..11.5793M"><span>Coupled marine productivity and salinity and West <span class="hlt">African</span> monsoon variability over the last 30,000 years in the eastern equatorial Atlantic <span class="hlt">Ocean</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Marret, F.; Kim, S.-Y.; Scourse, J.; Kennedy, H.</p> <p>2009-04-01</p> <p>Marine cores collected off west equatorial Africa have highlighted transfer of terrigenous material in the close <span class="hlt">ocean</span> that have had a deep influence on the marine productivity for the last 30,000 years. The strength of the West <span class="hlt">African</span> Monsoon has varied though time, from weak during glacial periods to strong during interglacials. In consequence, the amount of precipitation on the continent had drastic effect on the vegetation cover and soil erosion. Studies of marine cores have enabled the observation of changes in vegetation cover, from extended equatorial rainforest to expansion of savannahs. In association with open grassland association, soil is open to erosion, although precipitation is less; conversely, during periods of extended rainforest in a context of strong monsoon, soil erosion is minimised to the presence of trees. In both cases, terrigenous material is flushed out to the adjacent marine domain and has a profound influence on the marine biota. Three marine cores were studied from a north south transect, from Cameroon to Angola (off Sanaga, off Ogouée, and off Congo rivers), for their palynomorph contents. All cores contain a robust chronology based on radiocarbon dates and two have stable isotope data, allowing comparison. Dinoflagellate cysts were studied for retracing sea-surface conditions such as temperature, salinity and productivity whereas pollen were used to assess changes in the vegetation on the close continent for the last 30,000 years (1). A number of pollen records from terrestrial sequences from equatorial central Africa document the dynamics of the lowland rainforest and savannah in relation to climatic changes during the Holocene. Prior to the Holocene, continental records are scarce in this vast region and/or only allow reconstruction of the local vegetation. In our records, terrestrial proxies (pollen, spores, and charred grass cuticles) signal changes in the expansion/regression of the lowland rainforest which we relate to the</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014AGUFM.T43A4705S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014AGUFM.T43A4705S"><span>Regional 3D Numerical Modeling of the <span class="hlt">Lithosphere</span>-Mantle System: Implications for Continental Rift-Parallel Surface Velocities</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Stamps, S.; Bangerth, W.; Hager, B. H.</p> <p>2014-12-01</p> <p>The East <span class="hlt">African</span> Rift System (EARS) is an active divergent plate boundary with slow, approximately E-W extension rates ranging from <1-6 mm/yr. Previous work using thin-sheet modeling indicates <span class="hlt">lithospheric</span> buoyancy dominates the force balance driving large-scale Nubia-Somalia divergence, however GPS observations within the Western Branch of the EARS show along-rift motions that contradict this simple model. Here, we test the role of mantle flow at the rift-scale using our new, regional 3D numerical model based on the open-source code ASPECT. We define a thermal <span class="hlt">lithosphere</span> with thicknesses that are systematically changed for generic models or based on geophysical constraints in the Western branch (e.g. melting depths, xenoliths, seismic tomography). Preliminary results suggest existing variations in <span class="hlt">lithospheric</span> thicknesses along-rift in the Western Branch can drive upper mantle flow that is consistent with geodetic observations.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..1816657L','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..1816657L"><span>Imaging the continental <span class="hlt">lithosphere</span>: Perspectives from global and regional anisotropic seismic tomography</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Lebedev, Sergei; Schaeffer, Andrew</p> <p>2016-04-01</p> <p>Azimuthal seismic anisotropy, the dependence of seismic wave speeds on propagation azimuth, is largely due to fabrics within the Earth's crust and mantle, produced by deformation. It thus provides constraints on the distribution and evolution of deformation within the upper mantle. Lateral variations in isotropic-average seismic velocities reflect variations in the temperature of the rocks at depth. Seismic tomography thus also provides a proxy for lateral changes in the temperature and thickness of the <span class="hlt">lithosphere</span>. It can map the deep boundaries between tectonic blocks with different properties and age of the <span class="hlt">lithosphere</span>. Our new global, anisotropic, 3D tomographic models of the upper mantle and the crust are constrained by an unprecedentedly large global dataset of broadband waveform fits (over one million seismograms) and provide improved resolution of the <span class="hlt">lithosphere</span> at the global scale, compared to other available models. The most prominent high-velocity anomalies, seen down to around 200 km depths, indicate the cold, thick, stable mantle <span class="hlt">lithosphere</span> beneath Precambrian cratons. The tomography resolves the deep boundaries of the cratons even where they are not exposed and difficult to map at the surface. Our large waveform dataset, with complementary large global networks and high-density regional array data, also produces improved resolution of azimuthal anisotropy patterns, so that regional-scale variations related to <span class="hlt">lithospheric</span> deformation and mantle flow can be resolved, in particular in densely sampled regions. The depth of the boundary between the cold, rigid <span class="hlt">lithosphere</span> (preserving ancient, frozen anisotropic fabric) and the rheologically weak asthenosphere (characterized by fabric developed recently) can be inferred from the depth layering of seismic anisotropy and its comparison to the past and present plate motions. Beneath <span class="hlt">oceans</span>, the <span class="hlt">lithosphere</span>-asthenosphere boundary (LAB) is defined clearly by the layering of anisotropy, with a dependence on</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2012EGUGA..1411961P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2012EGUGA..1411961P"><span>PICASSO: <span class="hlt">Lithosphere</span> Structure in the Western Mediterranean from Ps Receiver Functions and Rayleigh Wave Tomography</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Palomeras, I.; Thurner, S.; Levander, A.; Humphreys, E.; Miller, M. S.; Carbonell, R.; Gallart, J.</p> <p>2012-04-01</p> <p>The western Mediterranean is a diffuse plate boundary separating the <span class="hlt">African</span> and Eurasian plates. Cenozoic deformation is centered on the Gibraltar arc and Alboran Sea, and occupies a wide area from the southern Iberian Massif in Spain to the Atlas Mountains in Morocco. We present a model of the <span class="hlt">lithospheric</span> structure of this region derived from Rayleigh wave tomography and Ps receiver functions, using data from the PICASSO (Program to Investigate Convective Alboran Sea System Overturn) linear broadband array of ~100 seismographs. This array is deployed from central Spain to the Morocco-Algerian border. We complement these data with some of that recorded by IberArray, an areal broadband array, operated by the Spanish seismological community, covering the same region with a uniform 50 km x 50 km grid of stations. Rayleigh phase velocities have been measured from 20-167s period using the two-plane-wave method to remove complications due to multi-pathing, and finite-frequency kernels to improve lateral resolution. The phase velocities were inverted for 1D structure on a 0.25 by 0.25 degree grid. Ps receiver functions at 1Hz and 2Hz were calculated for the same area using water-level and time-domain iterative deconvolution, and were then CCP stacked. The Rayleigh wave shear velocity model, jointly interpreted with the discontinuity structure from the CCP stack, shows the first-order <span class="hlt">lithospheric</span> structure, and the <span class="hlt">lithosphere</span>-asthenosphere boundary (LAB). From north to south along the PICASSO profile: The <span class="hlt">lithosphere</span> is ~120 km thick beneath the Iberian Massif, where it has the highest shear velocity, 4.45 km/s. To the south the <span class="hlt">lithosphere</span> thins dramatically beneath the Betic Mountains to ~85 km, and then varies in thickness and decreases in velocity beneath the Alboran Sea and Gibraltar Arc. The thinnest <span class="hlt">lithosphere</span>, ~60 km, is observed beneath the Rif mountains and Middle Atlas, with a low velocity feature (4.2 km/s) at ~60 km depth beneath a site of Late Cenozoic</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014AGUFMMR52A..01Z','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014AGUFMMR52A..01Z"><span>Observational Constraints on <span class="hlt">Lithospheric</span> Rheology and Their Implications for <span class="hlt">Lithospheric</span> Dynamics and Plate Tectonics</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Zhong, S.; Watts, A. B.</p> <p>2014-12-01</p> <p><span class="hlt">Lithospheric</span> rheology and strength are important for understanding crust and <span class="hlt">lithosphere</span> dynamics, and the conditions for plate tectonics. Laboratory studies suggest that <span class="hlt">lithospheric</span> rheology is controlled by frictional sliding, semi-brittle, low-temperature plasticity, and high-temperature creep deformation mechanisms as pressure and temperature increase from shallow to large depths. Although rheological equations for these deformation mechanisms have been determined in laboratory settings, it is necessary to validate them using field observations. Here we present an overview of <span class="hlt">lithospheric</span> rheology constrained by observations of seismic structure and load-induced flexure. Together with mantle dynamic modeling, rheological equations for high-temperature creep derived from laboratory studies (Hirth and Kohlstedt, 2003; Karato and Jung, 2003) satisfactorily explain the seismic structure of the Pacific upper mantle (Hunen et al., 2005) and Hawaiian swell topography (Asaadi et al., 2011). In a recent study that compared modeled surface flexure and stress induced by volcano loads in the Hawaiian Islands region with the observed flexure and seismicity, Zhong and Watts (2013) showed that the coefficient of friction is between 0.25 and 0.7, and is consistent with laboratory studies and also in-situ borehole measurements. However, this study indicated that the rheological equation for the low-temperature plasticity from laboratory studies (e.g., Mei et al., 2010) significantly over-predicts <span class="hlt">lithospheric</span> strength and viscosity. Zhong and Watts (2013) also showed that the maximum <span class="hlt">lithospheric</span> stress beneath Hawaiian volcano loads is about 100-200 MPa, which may be viewed as the largest <span class="hlt">lithospheric</span> stress in the Earth's <span class="hlt">lithosphere</span>. We show that the relatively weak <span class="hlt">lithospheric</span> strength in the low-temperature plasticity regime is consistent with seismic observation of reactivated mantle <span class="hlt">lithosphere</span> in the western US and the eastern North China. We discuss here the causes</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2011JGRE..116.1008W','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2011JGRE..116.1008W"><span>Insolation driven variations of Mercury's <span class="hlt">lithospheric</span> strength</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Williams, Jean-Pierre; Ruiz, Javier; Rosenburg, Margaret A.; Aharonson, Oded; Phillips, Roger J.</p> <p>2011-01-01</p> <p>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 <span class="hlt">lithosphere</span> and a long wavelength lateral variation in <span class="hlt">lithosphere</span> structure and strength that mirrors the insolation pattern. We employ a thermal evolution model for Mercury generating strength envelopes of the <span class="hlt">lithosphere</span> to demonstrate and quantify the possible effects the insolation pattern has on Mercury's <span class="hlt">lithosphere</span>. We find the heterogeneity in <span class="hlt">lithosphere</span> 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.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016JGRE..121.2225S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016JGRE..121.2225S"><span>Magnetic mineralogy of the Mercurian <span class="hlt">lithosphere</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Strauss, B. E.; Feinberg, J. M.; Johnson, C. L.</p> <p>2016-11-01</p> <p>Mercury and Earth are the only inner solar system planets with active, internally generated dynamo magnetic fields. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission recently detected magnetic fields on Mercury that are consistent with <span class="hlt">lithospheric</span> magnetization. We investigate the physical and chemical environment of Mercury's <span class="hlt">lithosphere</span>, past and present, to establish the conditions under which magnetization may have been acquired and modified. Three factors are particularly crucial to the determination of crustal composition and iron mineralogy: redox conditions in the planet's crust and mantle, the iron content of the <span class="hlt">lithosphere</span>, and, for any remanent magnetization, the temperature profile of the <span class="hlt">lithosphere</span> and its evolution over time. We explore potential mechanisms for remanence acquisition and alteration on Mercury, whose surface environment is both hot and highly reducing. The long-term thermal history of Mercury's crust plays an important role in the longevity of any remanent crustal magnetization, which may be subject to remagnetization through thermal, viscous, and shock mechanisms. This thermal and compositional framework is used both to constrain plausible candidate minerals that could carry magnetic remanence on Mercury and to evaluate their capacity to acquire and retain sufficient magnetization to be detectable from satellite orbit. We propose that iron metal and its alloys are likely to be the dominant contributors to induced and remanent magnetization in Mercury's <span class="hlt">lithosphere</span>, with additional contributions from iron silicides, sulfides, and carbides.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..18.8966G','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..18.8966G"><span>The evolution of fault geometry and <span class="hlt">lithosphere</span> mechanical response to faulting during <span class="hlt">lithosphere</span> hyper-extension at magma-poor rifted margins</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Gómez Romeu, Júlia; Kusznir, Nick; Manatschal, Gianreto; Roberts, Alan</p> <p>2016-04-01</p> <p>The geometry of upper <span class="hlt">lithosphere</span> extensional faulting and the mechanical response of the <span class="hlt">lithosphere</span> during continental breakup are controversial. The <span class="hlt">lithosphere</span> response to extensional faulting at magma-poor rifted margins controls the distribution of thinned continental crust, exhumed mantle, continental allochthons and syn-tectonic sediments leading to the complexity of heterogeneous structure of hyper-extended domain at these margins. In order to better understand the evolving fault geometry and <span class="hlt">lithosphere</span> mechanics during magma-poor rifted margin formation, we investigate extensional faulting for the tectonic end-members of continental rifting and slow sea-floor spreading. We presume that these end-members faulting styles both contribute to <span class="hlt">lithosphere</span> thinning during rifted margin evolution as continental rifting evolves into sea-floor spreading. For continental rifting, large extensional faults that rupture the seismogenic brittle upper <span class="hlt">lithosphere</span> have been shown to be planar and steeply dipping by earthquake seismology and geodesy (Stein and Barrientos 1985; Jackson 1987). These results are supported by seismic reflection imaging and structural modelling of rift basins (Kusznir et al., 1991, 1995). Individual fault heaves for continental rifting seldom exceeds approximately 10 km. The effective elastic thickness, used to parameterize <span class="hlt">lithosphere</span> flexural strength for syn-tectonic response to extensional faulting during continental rifting, are typically between 1.5 and 3 km. For slow-spreading <span class="hlt">ocean</span> ridges we examine extensional fault geometry and <span class="hlt">lithosphere</span> flexural response to cumulative faulting. We focus on the TAG area (deMartin et al., 2007) and the 15°N area (Schroeder et al., 2007) of the Mid-Atlantic Ridge using a flexural isostatic extensional faulting model (Buck 1988; Kusznir et al., 1991). Modelling of fault controlled bathymetry at slow-spreading <span class="hlt">ocean</span> ridges shows that active extensional faults at depth have a steep dip (50° - 70</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015GeoJI.202.1289B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015GeoJI.202.1289B"><span>Geodynamic inversion to constrain the non-linear rheology of the <span class="hlt">lithosphere</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Baumann, T. S.; Kaus, Boris J. P.</p> <p>2015-08-01</p> <p>One of the main methods to determine the strength of the <span class="hlt">lithosphere</span> is by estimating it's effective elastic thickness. This method assumes that the <span class="hlt">lithosphere</span> 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 <span class="hlt">oceanic</span> 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 <span class="hlt">lithosphere</span> and often show that it is rather complex. Yet, <span class="hlt">lithospheric</span> geometry by itself is insufficient to understand the dynamics of the <span class="hlt">lithosphere</span> as this also requires knowledge of the rheology of the <span class="hlt">lithosphere</span>. 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 <span class="hlt">lithosphere</span> 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 <span class="hlt">lithosphere</span> with an initial geometry that is constructed from geophysical data sets. We employ experimentally determined creep-laws for the various parts of the <span class="hlt">lithosphere</span>, but assume that the parameters of these creep-laws as well as the temperature structure of the <span class="hlt">lithosphere</span> 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</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2010AGUFMDI11A1823B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2010AGUFMDI11A1823B"><span>Global mapping of <span class="hlt">Lithosphere</span>/Asthenosphere Boundary from surface wave tomography</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Burgos, G.; Montagner, J.; Beucler, E.; Trampert, J.; Ritzwoller, M. H.; Capdeville, Y.; Shapiro, N. M.</p> <p>2010-12-01</p> <p>The coupling between rigid <span class="hlt">lithosphere</span> and the weaker underlying asthenosphere is a key point of Plate Tectonics and Mantle dynamics. The characterization of the properties of the <span class="hlt">Lithosphere</span>/Asthenosphere Boundary (LAB) is essential for understanding the Upper Mantle. Recent studies, using receiver functions for example, provide local constraints. In this study a global view by surface wave tomography is given. A large amount of data from different groups (Harvard, Boulder, Utrecht, Paris) has been collected. There are more than 100,000 phase and group velocicties measurements on the fundamental mode of Rayleigh and Love waves. This global scale dataset in the period range 15s-200s, enables us to investigate the LAB with an approximative lateral resolution of 500km. The regionalization of the path-averaged velocities is performed to extract isotropic and azimuthally anisotropic terms of local velocities. First we derive our own crustal model (taking account of topography-bathymetry, sediments and crustal thickness) by a MonteCarlo inversion with the shorter periods of the data. Second, to estimate the LAB properties and obtain a global map, we choose a very simple parameter space ajusted with the larger periods of the data. We report here a good correlation between <span class="hlt">oceanic</span> inverted LAB depth (~45km average) and <span class="hlt">ocean</span> floor age, in the sense of the classical thickening of <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span>. The determination of LAB beneath continents is more difficult, shallower depths (~90km average) are found in comparison with heat flux studies.</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_16");'>16</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li class="active"><span>18</span></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_18 --> <div id="page_19" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li class="active"><span>19</span></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="361"> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016GGG....17.2434M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016GGG....17.2434M"><span>Helium as a tracer for fluids released from Juan de Fuca <span class="hlt">lithosphere</span> beneath the Cascadia forearc</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>McCrory, P. A.; Constantz, J. E.; Hunt, A. G.; Blair, J. L.</p> <p>2016-06-01</p> <p>Helium isotopic ratios (3He/4He) observed in 25 mineral springs and wells above the Cascadia forearc provide a marker for fluids derived from Juan de Fuca <span class="hlt">lithosphere</span>. This exploratory study documents a significant component of mantle-derived helium within forearc springs and wells, and in turn, documents variability in helium enrichment across the Cascadia forearc. Sample sites arcward of the forearc mantle corner generally yield significantly higher ratios (˜1.2-4.0 RA) than those seaward of the corner (˜0.03-0.7 RA). 3He detected above the inner forearc mantle wedge may represent a mixture of both <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> and forearc mantle sources, whereas 3He detected seaward of the forearc mantle corner likely has only an <span class="hlt">oceanic</span> source. The highest ratios in the Cascadia forearc coincide with slab depths (˜40-45 km) where metamorphic dehydration of young <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> is expected to release significant fluid and where tectonic tremor occurs, whereas little fluid is expected to be released from the slab depths (˜25-30 km) beneath sites seaward of the corner. These observations provide independent evidence that tremor is associated with deep fluids, and further suggest that high pore pressures associated with tremor may serve to keep fractures open for 3He migration through the ductile upper mantle and lower crust.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2011AGUFM.T32A..05J','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2011AGUFM.T32A..05J"><span><span class="hlt">Lithospheric</span> geometries revealed through electromagnetic imaging: SAMTEX (Southern Africa MagnetoTelluric Experiment) observations and results</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Jones, A. G.; Muller, M. R.; Evans, R. L.; Miensopust, M. P.; Khoza, D. T.; Samtex Team</p> <p>2011-12-01</p> <p>The Southern <span class="hlt">African</span> Magnetotelluric Experiment (SAMTEX) is imaging the properties and geometries of the <span class="hlt">lithosphere</span> below southern Africa to depths of 200+ km. Electrical conductivity is highly sensitive to ambient temperature, and to the presence of an interconnected conducting phase, such as a solid phase like graphite or sulphides, a fluid phase like partial melt, or bound water through hydrogen diffusion. Thus, primary geometrical information can be readily obtained from <span class="hlt">lithospheric</span>-scale MT experiments about the three-dimensional variation in conductivity that can be related to formation and deformation processes. One important piece of information easily obtained from MT data is the depth to the <span class="hlt">lithosphere</span>-asthenosphere boundary (LAB), due to the sensitivity of conductivity to small fractions (<1%) of partial melt and/or of some hundreds of ppm of bound water. SAMTEX measurements have been made at a total of more than 750 MT sites over an area in excess of a million square kilometres, making it by far the largest-ever regional MT project undertaken. One of the most significant results from SAMTEX is the mapping of the LAB beneath the Archean cratons and bounding mobile belts of Southern Africa, particularly of the previously unknown regions of Namibia and Botswana. The LAB is shallow (150 km) beneath the mobile belts, deep (250 km) in the centres of the cratons, and transitional at the edges. Diamondiferous kimberlites are located primarily where <span class="hlt">lithosphere</span> is transitional in thickness, or where there is a change in its anisotropy properties, both of which are craton edge effects. The electrical properties of the continental mantle derived from SAMTEX data can be compared with seismic ones derived from data from the South <span class="hlt">African</span> Seismic Experiment (SASE) of the Kaapvaal Project. Generally there is very good predictive linear agreement between seismic velocity and log(conductivity), indicative of both being influenced by the same bulk property factors</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015AGUFMDI41A2590V','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015AGUFMDI41A2590V"><span><p>The consequences of hotspots on continental <span class="hlt">lithosphere</span> : a thermal case study on the Arabian Plate.</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Vicente De Gouveia, S.; Besse, J.; Greff-Lefftz, M.; Frizon de Lamotte, D.; Leparmentier, F.; Lescanne, M.</p> <p>2015-12-01</p> <p>Hotspots are thermal instabilities coming from various depths in the mantle. Their activity is often revealed by surface and sub-surface phenomena such as volcanic trapps or <span class="hlt">oceanic</span> plateaus, and volcanic island tracks on the seafloor. The two first are often linked to the eruption of a hotspot head, while the third is due to the volcanic material fed by the subsequent tail. Consequences of a hotspot tail on the <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> are well known, while its effect on the continental <span class="hlt">lithosphere</span> is most often masked by the thickness of the <span class="hlt">lithosphere</span>. The aim of our study is to try and link hotspot tracks with geological events in the continental <span class="hlt">lithosphere</span>. Hotspot tracks are first built using a modified version of the hybrid reference frame of Seton et al. (2012), and their effect on the continental <span class="hlt">lithosphere</span> is then evaluated using geological markers issued from petroleum wells, in particular the sedimentary record, backstripping, heat flux anomaly and temperature data. A case study is performed on the Arabian Plate, potentially crossed by two hotspots (Afar and Comores). Several W-E heat flux profiles display a large thermal anomaly close to the Red Sea, while a smaller N-S elongated heat flow anomaly more to the E suggests that a hotspot track could impact the thermal history of the Arabian plate.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016Tectp.693..143I','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016Tectp.693..143I"><span><span class="hlt">Lithospheric</span> architecture of the Levant Basin (Eastern Mediterranean region): A 2D modeling approach</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Inati, Lama; Zeyen, Hermann; Nader, Fadi Henri; Adelinet, Mathilde; Sursock, Alexandre; Rahhal, Muhsin Elie; Roure, François</p> <p>2016-12-01</p> <p>This paper discusses the deep structure of the <span class="hlt">lithosphere</span> underlying the easternmost Mediterranean region, in particular the Levant Basin and its margins, where the nature of the crust, continental versus <span class="hlt">oceanic</span>, remains debated. Crustal thickness and the depth of the <span class="hlt">lithosphere</span>-asthenosphere boundary (LAB) as well as the crustal density distribution were calculated by integrating surface heat flow data, free-air gravity anomaly, geoid and topography. Accordingly, two-dimensional, <span class="hlt">lithospheric</span> models of the study area are discussed, demonstrating the presence of a progressively attenuated crystalline crust from E to W (average thickness from 35 to 8 km). The crystalline crust is best interpreted as a strongly thinned continental crust under the Levant Basin, represented by two distinct components, an upper and a lower crust. Further to the west, the Herodotus Basin is believed to be underlain by an <span class="hlt">oceanic</span> crust, with a thickness between 6 and 10 km. The Moho under the Arabian Plate is 35-40 km deep and becomes shallower towards the Mediterranean coast. It appears to be situated at depths ranging between 20 and 23 km below the Levant Basin and 26 km beneath the Herodotus Basin, based on our proposed models. At the Levantine margin, the thinning of the crust in the transitional domain between the onshore and the offshore is gradual, indicating successive extensional regimes that did not reach the beak up stage. In addition, the depth to LAB is around 120 km under the Arabian and the Eurasian Plates, 150 km under the Levant Basin, and it plunges to 180 km under the Herodotus Basin. This study shows that detailed 2D <span class="hlt">lithosphere</span> modeling using integrated geophysical data can help understand the mechanisms responsible for the modelled <span class="hlt">lithospheric</span> architecture when constrained with geological findings.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://hdl.handle.net/2060/19860003395','NASA-TRS'); return false;" href="http://hdl.handle.net/2060/19860003395"><span><span class="hlt">Lithospheric</span> structure in the Pacific geoid</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Marsh, B. D.; Hinojosa, J. H.</p> <p>1985-01-01</p> <p>The high degree and order SEASAT geoid in the central Pacific correlates closely with the structure of the cooling <span class="hlt">lithosphere</span>. 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 <span class="hlt">lithosphere</span> while removing the subsidence of the <span class="hlt">lithosphere</span>.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://ntrs.nasa.gov/search.jsp?R=19920070460&hterms=Subduction+zones&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D20%26Ntt%3DSubduction%2Bzones','NASA-TRS'); return false;" href="https://ntrs.nasa.gov/search.jsp?R=19920070460&hterms=Subduction+zones&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D20%26Ntt%3DSubduction%2Bzones"><span>Evidence for retrograde <span class="hlt">lithospheric</span> subduction on Venus</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Sandwell, David T.; Schubert, Gerald</p> <p>1992-01-01</p> <p>Annular moats and outer rises around large Venus coronas such as Artemis, Latona, and Eithinoha are similar in arcuate planform and topography to the trenches and outer rises of terrestrial subduction zones. On earth, trenches and outer rises are modeled as the flexural response of a thin elastic <span class="hlt">lithosphere</span> to the bending moment of the subducted slab; this <span class="hlt">lithospheric</span> flexure model also accounts for the trenches and outer rises outboard of the major coronas on Venus. Accordingly, it is proposed that retrograde <span class="hlt">lithospheric</span> subduction may be occurring on the margins of the large Venus coronas while compensating back-arc extension is occurring in the expanding coronas interiors. Similar processes may be taking place at other deep arcuate trenches or chasmata on Venus such as those in the Dali-Diana chasmata area of aestern Aphrodite Terra.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/20865000','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/20865000"><span>Water and its influence on the <span class="hlt">lithosphere</span>-asthenosphere boundary.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Green, David H; Hibberson, William O; Kovács, István; Rosenthal, Anja</p> <p>2010-09-23</p> <p>The Earth has distinctive convective behaviour, described by the plate tectonics model, in which lateral motion of the <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> of basaltic crust and peridotitic uppermost mantle is decoupled from the underlying mechanically weaker upper mantle (asthenosphere). The reason for differentiation at the <span class="hlt">lithosphere</span>-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 <span class="hlt">lithosphere</span>-asthenosphere boundary. Modern asthenospheric mantle acting as the source for mid-<span class="hlt">oceanic</span> ridge basalts has a water content of 50-200 parts per million (refs 3-5). We show that this matches the</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://hdl.handle.net/2060/20160006653','NASA-TRS'); return false;" href="http://hdl.handle.net/2060/20160006653"><span>Water in the Cratonic Mantle <span class="hlt">Lithosphere</span></span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Peslier, A. H.</p> <p>2016-01-01</p> <p>The fact that Archean and Proterozoic cratons are underlain by the thickest (>200 km) <span class="hlt">lithosphere</span> on Earth has always puzzled scientists because the dynamic convection of the surrounding asthenosphere would be expected to delaminate and erode these mantle <span class="hlt">lithospheric</span> "keels" over time. Although density and temperature of the cratonic <span class="hlt">lithosphere</span> 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 <span class="hlt">lithospheres</span>, contributing to its strength. This presentation will review the distribution of water concentrations in four cratonic <span class="hlt">lithospheres</span>. 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 <span class="hlt">lithosphere</span> 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 <span class="hlt">lithosphere</span> with an over-­abundance of metasomatized peridotites with</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2006AGUSM.T32A..03M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2006AGUSM.T32A..03M"><span>Reconstructing vanished <span class="hlt">ocean</span> basins</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Müller, D.; Sdrolias, M.; Gaina, C.</p> <p>2006-05-01</p> <p>The large-scale patterns of mantle convection are mainly dependent on the history of subduction. Therefore some of the primary constraints for subduction models are given by of the location of subduction zones through time, and of the convergence vectors and age of subducted <span class="hlt">lithosphere</span>. This requires the complete reconstruction of <span class="hlt">ocean</span> floor through time, including the main <span class="hlt">ocean</span> basins, back-arc basins, and now subducted <span class="hlt">ocean</span> crust, and tying these kinematic models to geodynamic simulations. We reconstruct paleo- <span class="hlt">oceans</span> by creating "synthetic plates", the locations and geometry of which is established on the basis of preserved <span class="hlt">ocean</span> crust (magnetic lineations and fracture zones), geological data, paleogeography, and the rules of plate tectonics. We use a merged moving hotspot (Late Cretaceous-present) and palaeomagnetic/fixed hotspot (Early Cretaceous) reference frame, coupled with reconstructed spreading histories of the Pacific, Phoenix and Farallon plates and the plates involved in the Tethys <span class="hlt">oceanic</span> domain. Based on this approach we have created a set of global <span class="hlt">oceanic</span> paleo-isochrons and paleo-<span class="hlt">oceanic</span> age grids. The grids also provide the first complete global set of paleo-basement depth maps, including now subducted <span class="hlt">ocean</span> floor, for the last 130 million years based on a depth-age relationship. We show that the mid-Cretaceous sealevel highstand was primarily caused by two main factors: (1) the "supercontinent breakup effect", which resulted in the creation of the mid-Atlantic and Indian <span class="hlt">Ocean</span> ridges at the expense of subducting old <span class="hlt">ocean</span> floor in the Tethys and (2) by a changing age-area distribution of Pacific <span class="hlt">ocean</span> floor through time, resulting from the subduction of the Pacific-Izanagi, Pacific-Phoenix and Pacific-Farallon ridges. These grids provide model constraints for subduction dynamics through time and represent a framework for backtracking biogeographic and sediment data from <span class="hlt">ocean</span> drilling and for constraining the opening/closing of <span class="hlt">oceanic</span></p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://ntrs.nasa.gov/search.jsp?R=20030064112&hterms=ocean+floor&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D20%26Ntt%3Docean%2Bfloor','NASA-TRS'); return false;" href="https://ntrs.nasa.gov/search.jsp?R=20030064112&hterms=ocean+floor&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D20%26Ntt%3Docean%2Bfloor"><span>Ejecta from <span class="hlt">Ocean</span> Impacts</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Kyte, Frank T.</p> <p>2003-01-01</p> <p>Numerical simulations of deep-<span class="hlt">ocean</span> impact provide some limits on the size of a projectile that will not mix with the <span class="hlt">ocean</span> floor during a deep-<span class="hlt">ocean</span> impact. For a vertical impact at asteroidal velocities (approx. 20 km/s), mixing is only likely when the projectile diameter is greater than 112 of the water depth. For oblique impacts, even larger projectiles will not mix with <span class="hlt">ocean</span> floor silicates. Given the typical water depths of 4 to 5 km in deep-<span class="hlt">ocean</span> basins, asteroidal projectiles with diameters as large as 2 or 3 km may commonly produce silicate ejecta that is composed only of meteoritic materials and seawater salts. However, the compressed water column beneath the projectile can still disrupt and shock metamorphose the <span class="hlt">ocean</span> floor. Therefore, production of a separate, terrestrial ejecta component is not ruled out in the most extreme case. With increasing projectile size (or energy) relative to water depths, there must be a gradation between <span class="hlt">oceanic</span> impacts and more conventional continental impacts. Given that 60% of the Earth's surface is covered by <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> and 500 m projectiles impact the Earth on 10(exp 5) y timescales, there must be hundreds of <span class="hlt">oceanic</span> impact deposits in the sediment record awaiting discovery.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://ntrs.nasa.gov/search.jsp?R=19930049223&hterms=Plate+Tectonics&qs=N%3D0%26Ntk%3DAll%26Ntx%3Dmode%2Bmatchall%26Ntt%3DPlate%2BTectonics','NASA-TRS'); return false;" href="https://ntrs.nasa.gov/search.jsp?R=19930049223&hterms=Plate+Tectonics&qs=N%3D0%26Ntk%3DAll%26Ntx%3Dmode%2Bmatchall%26Ntt%3DPlate%2BTectonics"><span>The magma <span class="hlt">ocean</span> as an impediment to lunar plate tectonics</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Warren, Paul H.</p> <p>1993-01-01</p> <p>The primary impediment to plate tectonics on the moon was probably the great thickness of its crust and particularly its high crust/<span class="hlt">lithosphere</span> thickness ratio. This in turn can be attributed to the preponderance of low-density feldspar over all other Al-compatible phases in the lunar interior. During the magma <span class="hlt">ocean</span> epoch, the moon's crust/<span class="hlt">lithosphere</span> thickness ratio was at the maximum theoretical value, approximately 1, and it remained high for a long time afterwards. A few large regions of thin crust were produced by basin-scale cratering approximately contemporaneous with the demise of the magma <span class="hlt">ocean</span>. However, these regions probably also tend to have uncommonly thin <span class="hlt">lithosphere</span>, since they were directly heated and indirectly enriched in K, Th, and U by the same cratering process. Thus, plate tectonics on the moon in the form of systematic <span class="hlt">lithosphere</span> subduction was impeded by the magma <span class="hlt">ocean</span>.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..1810758C','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..1810758C"><span>The <span class="hlt">lithosphere</span> thermal structure of the Southeast Asia: constrained by Vs data</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Chuanhai, Yu; Xiaobin, Shi; Qunshu, Tang; Xiaoqiu, Yang</p> <p>2016-04-01</p> <p>The Southeast Asia, located in the southeastern part of the Eurasian Plate, comprises a complex collage of continental fragments, volcanic arcs, suture zones and marginal <span class="hlt">oceanic</span> basins, and is surrounded by tectonically active margins which exhibit intense seismicity and volcanism. As we all know, the tectonic evolution is closely related to the deep thermal structure state. Therefore, an accurate estimation of <span class="hlt">lithosphere</span> thermal structure and <span class="hlt">lithosphere</span> thickness is important in extracting information on tectonics and geodynamics. Though the thermal regime could be calculated with the observed surface heat flow, there are many uncertainties in the calculated deep thermal state. In this study, we calculated the deep <span class="hlt">lithosphere</span> thermal structure of Southeast Asia regions by employing an empirical relationship between Vs and temperature, from the calculated temperature-depth profiles, we can identify the base of the thermal <span class="hlt">lithosphere</span>. The results show that the temperature contours at 80km depth is about 200-300°C higher in the rifted basins and <span class="hlt">oceanic</span> basins such as Andaman Sea, Thailand Bay, Thailand Rift Basin, South China Sea than in the plateaus and subduction zones such as Khorat Plateau, Sumatra Island and Philippine Trench regions. Generally, the thermal state indicated by the temperature contours at 80 km depth is in agreement with those suggested by the observed surface heat flow. The temperature at 100 km and 200 km depth in Southeast Asia regions is 1450-1500°C and 1650-1780°C which suggest that the study regions might have a higher thermal state than other regions. Our results also show that the estimated thickness of the <span class="hlt">lithosphere</span> are 85-95 km in the regions of Subduction and collision regions surrounding the study area such as Java trench system, Sumatra trench system, Indo-Asian collision suture zone, Taiwan orogenic belt, Luzon Island, Celebes Island and Northeast of Borneo and becomes smaller toward the South China Sea. In the South China</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013AGUFM.T12B..05J','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013AGUFM.T12B..05J"><span>Using crustal thickness, subsidence and P-T-t history on the Iberia-Newfoundland & Alpine Tethys margins to constrain <span class="hlt">lithosphere</span> deformation modes during continental breakup</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Jeanniot, L.; Kusznir, N. J.; Manatschal, G.; Mohn, G.; Beltrando, M.</p> <p>2013-12-01</p> <p>Observations at magma-poor rifted margins such as Iberia-Newfoundland show a complex <span class="hlt">lithosphere</span> deformation history and OCT architecture, resulting in hyper-extended continental crust and <span class="hlt">lithosphere</span>, exhumed mantle and scattered embryonic <span class="hlt">oceanic</span> crust before continental breakup and seafloor spreading. Initiation of seafloor spreading requires both the rupture of the continental crust and <span class="hlt">lithospheric</span> mantle, and the onset of decompressional melting. Their relative timing controls when mantle exhumation may occur; the presence or absence of exhumed mantle provides useful information on the timing of these events and constraints on <span class="hlt">lithosphere</span> deformation modes. A single kinematic <span class="hlt">lithosphere</span> deformation mode leading to continental breakup and sea-floor spreading cannot explain observations. We have determined the sequence of <span class="hlt">lithosphere</span> deformation events, using forward modelling of crustal thickness, subsidence and P-T-t history calibrated against observations on the present-day Iberia-Newfoundland and the fossil analogue Alpine Tethys margins. <span class="hlt">Lithosphere</span> deformation modes, represented by flow fields, are generated by a 2D finite element viscous flow model (FeMargin), and used to advect <span class="hlt">lithosphere</span> and asthenosphere temperature and material. FeMargin is kinematically driven by divergent deformation in the topmost upper <span class="hlt">lithosphere</span> inducing passive upwelling beneath that layer; the upper <span class="hlt">lithosphere</span> is assumed to deform by extensional faulting and magmatic intrusions, consistent with observations of deformation processes occurring at slow spreading <span class="hlt">ocean</span> ridges (Cannat, 1996). Buoyancy enhanced upwelling is also included in the kinematic model as predicted by Braun et al (2000). We predict melt generation by decompressional melting using the parameterization and methodology of Katz et al., 2003. We use a series of numerical experiments, tested and calibrated against crustal thicknesses and subsidence observations, to determine the distribution of <span class="hlt">lithosphere</span></p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=3940973','PMC'); return false;" href="https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=3940973"><span>Abnormal lithium isotope composition from the ancient <span class="hlt">lithospheric</span> mantle beneath the North China Craton</span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pmc">PubMed Central</a></p> <p>Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M.; Xiao, Yan</p> <p>2014-01-01</p> <p>Lithium elemental and isotopic compositions of olivines in peridotite xenoliths from Hebi in the North China Craton provide direct evidence for the highly variable δ7Li in Archean <span class="hlt">lithospheric</span> mantle. The δ7Li in the cores of olivines from the Hebi high-Mg# peridotites (Fo > 91) show extreme variation from −27 to +21, in marked deviation from the δ7Li range of fresh MORB (+1.6 to +5.6) although the Li abundances of the olivines are within the range of normal mantle (1–2 ppm). The Li abundances and δ7Li characteristics of the Hebi olivines could not have been produced by recent diffusive-driven isotopic fractionation of Li and therefore the δ7Li in the cores of these olivines record the isotopic signature of the subcontinental <span class="hlt">lithospheric</span> mantle. Our data demonstrate that abnormal δ7Li may be preserved in the ancient <span class="hlt">lithospheric</span> mantle as observed in our study from the central North China Craton, which suggest that the subcontinental <span class="hlt">lithospheric</span> mantle has experienced modification of fluid/melt derived from recycled <span class="hlt">oceanic</span> crust. PMID:24589693</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/24589693','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/24589693"><span>Abnormal lithium isotope composition from the ancient <span class="hlt">lithospheric</span> mantle beneath the North China Craton.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M; Xiao, Yan</p> <p>2014-03-04</p> <p>Lithium elemental and isotopic compositions of olivines in peridotite xenoliths from Hebi in the North China Craton provide direct evidence for the highly variable δ(7)Li in Archean <span class="hlt">lithospheric</span> mantle. The δ(7)Li in the cores of olivines from the Hebi high-Mg# peridotites (Fo > 91) show extreme variation from -27 to +21, in marked deviation from the δ(7)Li range of fresh MORB (+1.6 to +5.6) although the Li abundances of the olivines are within the range of normal mantle (1-2 ppm). The Li abundances and δ(7)Li characteristics of the Hebi olivines could not have been produced by recent diffusive-driven isotopic fractionation of Li and therefore the δ(7)Li in the cores of these olivines record the isotopic signature of the subcontinental <span class="hlt">lithospheric</span> mantle. Our data demonstrate that abnormal δ(7)Li may be preserved in the ancient <span class="hlt">lithospheric</span> mantle as observed in our study from the central North China Craton, which suggest that the subcontinental <span class="hlt">lithospheric</span> mantle has experienced modification of fluid/melt derived from recycled <span class="hlt">oceanic</span> crust.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2001JVGR..110..299S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2001JVGR..110..299S"><span>Late Precambrian Balkan-Carpathian ophiolite — a slice of the Pan-<span class="hlt">African</span> <span class="hlt">ocean</span> crust?: geochemical and tectonic insights from the Tcherni Vrah and Deli Jovan massifs, Bulgaria and Serbia</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Savov, Ivan; Ryan, Jeff; Haydoutov, Ivan; Schijf, Johan</p> <p>2001-10-01</p> <p>The Balkan-Carpathian ophiolite (BCO), which outcrops in Bulgaria, Serbia and Romania, is a Late Precambrian (563 Ma) mafic/ultramafic complex unique in that it has not been strongly deformed or metamorphosed, as have most other basement sequences in Alpine Europe. Samples collected for study from the Tcherni Vrah and Deli Jovan segments of BCO include cumulate dunites, troctolites, wehrlites and plagioclase wehrlites; olivine and amphibole-bearing gabbros; anorthosites; diabases and microgabbros; and basalts representing massive flows, dikes, and pillow lavas, as well as hyaloclastites and umbers (preserved sedimentary cover). Relict Ol, Cpx and Hbl in cumulate peridotites indicate original orthocumulate textures. Plagioclase in troctolites and anorthosites range from An 60 to An 70. Cumulate gabbro textures range from ophitic to poikilitic, with an inferred crystallization order of Ol-(Plag+Cpx)-Hbl. The extrusive rocks exhibit poikilitic, ophitic and intersertal textures, with Cpx and/or Plag (Oligoclase-Andesine) phenocrysts. The major opaques are Ti-Magnetite and Ilmenite. The metamorphic paragenesis in the mafic samples is Chl-Trem-Ep, whereas the ultramafic rocks show variable degrees of serpentinization, with lizardite and antigorite as dominant phases. Our samples are compositionally and geochemically similar to modern <span class="hlt">oceanic</span> crust. Major element, trace element and rare earth element (REE) signatures in BCO basalts are comparable to those of MORB. In terms of basalt and dike composition, the BCO is a 'high-Ti' or '<span class="hlt">oceanic</span>' ophiolite, based on the classification scheme of Serri [Earth Planet. Sci. Lett. 52 (1981) 203]. Our petrologic and geochemical results, combined with the tectonic position of the BCO massifs (overlain by and in contact with Late Cambrian island arc and back-arc sequences), suggest that the BCO may have formed in a mid-<span class="hlt">ocean</span> ridge setting. If the BCO records the existence of a Precambrian <span class="hlt">ocean</span> basin, then there may be a relationship</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013AGUFM.T23G2678Y','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013AGUFM.T23G2678Y"><span>Structure of the Crust and Mantle <span class="hlt">Lithosphere</span> underneath NW Namibia Revealed by the WALPASS Seismic Experiment</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Yuan, X.; Jokat, W.; Weber, M. H.; Geissler, W.; Heit, B.; Eken, T.; Pandey, S.; Lushetile, B.; Hoffmann, K.</p> <p>2013-12-01</p> <p>The amphibian Walvis Ridge Passive-Source Seismic Experiment (WALPASS) have been operated for a period of two years from 2010 to 2012 in the area where the Walvis Ridge intersects the continental margin of northwestern Namibia. The deployment was intended to study the <span class="hlt">lithospheric</span> and upper mantle structure in the <span class="hlt">ocean</span>-continent transition area beneath the passive continental margin. The main idea is to find seismic anomalies related to the postulated hotspot track from the continent to the <span class="hlt">ocean</span> along the Walvis Ridge that links the Etendeka continental flood-basalt province to the Tristan da Cunha hotspot in the mid Atlantic <span class="hlt">ocean</span>. This could provide clues of the role of plume-<span class="hlt">lithosphere</span> interaction during the continental break-up. We present here seismic structures of the crustal and mantle <span class="hlt">lithosphere</span> in this geophysically little studied region using seismic methods including P and S receiver functions and shear wave splitting. The average crustal thickness in the continental Namibia is ~35 km with a relatively low Vp/Vs ratio of 1.7. Underneath the NE extension of the Walvis Ridge the crust is the thickest (45 km) with a high Vp/Vs ratio (>1.80). The thick crust and high Vp/Vs ratio beneath the Walvis Ridge are consistent with high Vp derived by controlled source seismics, implying a magmatic underplating. A low velocity zone in the mantle is observed at depths of 60-120 km, possibly representing the base of the <span class="hlt">lithosphere</span>. The P-to-S converted phases at the 410 and 660 km discontinuities arrive 2-3 s earlier, indicating higher upper mantle velocities (+5%). Seismic anisotropy in the mantle derived by the SKS splitting exhibits a pattern of the plume and plate interaction.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..18.3456S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..18.3456S"><span>Spatial variations of effective elastic thickness of the <span class="hlt">Lithosphere</span> in the Southeast Asia regions</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Shi, Xiaobin; Kirby, Jon; Yu, Chuanhai; Swain, Chris; Zhao, Junfeng</p> <p>2016-04-01</p> <p>The effective elastic thickness Te corresponds to the thickness of an idealized elastic beam that would bend similarly to the actual <span class="hlt">lithosphere</span> under the same applied loads, and could provide important insight into rheology and state of stress. Thus, it is helpful to improve our understanding of the relationship between tectonic styles, distribution of earthquakes and <span class="hlt">lithospheric</span> rheology in various tectonic settings. The Southeast Asia, located in the southeastern part of the Eurasian Plate, comprises a complex collage of continental fragments, volcanic arcs, and suture zones and marginal <span class="hlt">oceanic</span> basins, and is surrounded by tectonically active margins which exhibit intense seismicity and volcanism. The Cenozoic southeastward extrusion of the rigid Indochina Block due to the Indo-Asian collision resulted in the drastic surface deformation in the western area. Therefore, a high resolution spatial variation map of Te might be a useful tool for the complex Southeast Asia area to examine the relationships between surface deformation, earthquakes, <span class="hlt">lithospheric</span> structure and mantle dynamics. In this study, we present a high-resolution map of spatial variations of Te in the Southeast Asia area using the wavelet method, which convolves a range of scaled wavelets with the two data sets of Bouguer gravity anomaly and topography. The topography and bathymetry grid data was extracted from the GEBCO_08 Grid of GEBCO digital atlas. The pattern of Te variations agrees well with the tectonic provinces in the study area. On the whole, low <span class="hlt">lithosphere</span> strength characterizes the <span class="hlt">oceanic</span> basins, such as the South China Sea, the Banda sea area, the Celebes Sea, the Sulu Sea and the Andaman Sea. Unlike the <span class="hlt">oceanic</span> basins, the continental fragments show a complex pattern of Te variations. The Khorat plateau and its adjacent area show strong <span class="hlt">lithosphere</span> characteristics with a Te range of 20-50 km, suggesting that the Khorat plateau is the strong core of the Indochina Block. The West</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013AGUFMMR43A2370L','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013AGUFMMR43A2370L"><span><span class="hlt">Lithospheric</span> structure of the central Pacific: early returns from the NoMelt experiment</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Lin, P.; Gaherty, J. B.; Lizarralde, D.; Collins, J. A.; Evans, R. L.; Hirth, G.</p> <p>2013-12-01</p> <p>Recent advances in laboratory measurements and theoretical models of the seismic properties of mantle rocks predict seismic velocity profiles for mature <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> that are fundamentally inconsistent with the best observations of seismic velocities in two ways. Observations of strong positive velocity gradients with depth, and a very sharp and very shallow low-velocity asthenosphere boundary (LAB), both suggest that non-thermal factors such as bulk composition, mineral fabric, grain size, and dehydration play important roles in controlling the formation of the <span class="hlt">lithosphere</span>, and thus the underlying LAB. There is little consensus on which of these factors are dominant, in part because observations of detailed <span class="hlt">lithosphere</span> structure are limited. In 2011-2013, we conducted the NoMelt experiment on ~70 Ma Pacific <span class="hlt">lithosphere</span> between the Clarion and Clipperton fracture zones. The experiment consists of a 600x400 km array of broad-band (BB) <span class="hlt">ocean</span> bottom seismometers (OBS) and magnetotelluric (MT) instruments, and an active-source reflection/refraction experiment. The BB OBS array was recovered in January 2013, and we present preliminary observations derived from one year's recording of teleseismic earthquakes, which will constrain anisotropic velocity and anelasticity structure based on surface waves, shear-wave splitting, and direct and converted body waves. Of the 27 deployed instruments, 21 were recovered, all of which produced useful data on the seismometer and/or the differential pressure gauge in the 10-100 s period band. We have identified at least 38 energetic events that produced outstanding P and S body waves, which we will use for receiver-function analyses to look for the reflected and converted body waves. High signal-to-noise Rayleigh waves are observed from over 26 events with Mw 6.5 or larger, and useful Love waves are observed on several stations for 2 events greater than Mw 7.1. These observations will be used to produce a radially anisotropic</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://ntrs.nasa.gov/search.jsp?R=19950045244&hterms=lithosphere&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D10%26Ntt%3Dlithosphere','NASA-TRS'); return false;" href="https://ntrs.nasa.gov/search.jsp?R=19950045244&hterms=lithosphere&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D10%26Ntt%3Dlithosphere"><span><span class="hlt">Lithospheric</span> thickness and mantle/<span class="hlt">lithosphere</span> density contrast beneath Beta Rigio, Venus</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Moore, William B.; Schubert, Gerald</p> <p>1995-01-01</p> <p>The spatial variation of the geoid/topography ratio over the large Venusian volcanic highland Beta Regio is suggestive of thermal compensation, i.e., support of the highland's topography by <span class="hlt">lithospheric</span> thinning. Both the thickness of the <span class="hlt">lithosphere</span> and the density contrast at its base can be inferred from a quadratic regression of suitably filtered (600 km less than wavelength less than 4000 km) geoid vs. topography data. The regression yields a mean <span class="hlt">lithospheric</span> thickness of 270 km and a density contrast of magnitude 2.5% to 3.0%. Simple isostatic balance of the long-wavelength topography at Beta Regio requires thinning of the <span class="hlt">lithosphere</span> by 50-60% beneath the rise.</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li class="active"><span>19</span></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_19 --> <div id="page_20" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li class="active"><span>20</span></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li><a href="#" onclick='return showDiv("page_22");'>22</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="381"> <li> <p><a target="_blank" onclick="trackOutboundLink('http://hdl.handle.net/2060/19930005141','NASA-TRS'); return false;" href="http://hdl.handle.net/2060/19930005141"><span>Variations in <span class="hlt">lithospheric</span> thickness on Venus</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Johnson, C. L.; Sandwell, David T.</p> <p>1992-01-01</p> <p>Recent analyses of Magellan data have indicated many regions exhibiting topograhic flexure. On Venus, flexure is associated predominantly with coronae and the chasmata with Aphrodite Terra. Modeling of these flexural signatures allows the elastic and mechanical thickness of the <span class="hlt">lithosphere</span> to be estimated. In areas where the <span class="hlt">lithosphere</span> is flexed beyond its elastic limit the saturation moment provides information on the strength of the <span class="hlt">lithosphere</span>. Modeling of 12 flexural features on Venus has indicated <span class="hlt">lithospheric</span> thicknesses comparable with terrestrial values. This has important implications for the venusian heat budget. Flexure of a thin elastic plate due simultaneously to a line load on a continuous plate and a bending moment applied to the end of a broken plate is considered. The mean radius and regional topographic gradient are also included in the model. Features with a large radius of curvature were selected so that a two-dimensional approximation could be used. Comparisons with an axisymmetric model were made for some features to check the validity of the two-dimensional assumption. The best-fit elastic thickness was found for each profile crossing a given flexural feature. In addition, the surface stress and bending moment at the first zero crossing of each profile were also calculated. Flexural amplitudes and elastic thicknesses obtained for 12 features vary significantly. Three examples of the model fitting procedures are discussed.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015AGUFMDI11B2585M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015AGUFMDI11B2585M"><span>Imaging <span class="hlt">Lithospheric</span> Structure beneath the Indian continent</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Maurya, S.; Montagner, J. P.; Mangalampally, R. K.; Stutzmann, E.; Burgos, G.; Kumar, P.; Davuluri, S.</p> <p>2015-12-01</p> <p>The <span class="hlt">lithospheric</span> structure and thickness to the LAB are the most debated issues, especially beneath continents. In this context, the structure and thickness of the Indian <span class="hlt">lithosphere</span> 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 <span class="hlt">lithosphere</span> (≈100 km; Kumar et al., 2007) in contradiction with the thick <span class="hlt">lithosphere</span> 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 <span class="hlt">lithosphere</span>. Also, recent results of P and S wave travel time tomography of India suggest that the <span class="hlt">lithospheric</span> 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 <span class="hlt">lithosphere</span>. The LAB depths inferred from</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..1817956S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..1817956S"><span>Magnetic mineralogy of the Mercurian <span class="hlt">lithosphere</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Strauss, Becky; Feinberg, Joshua; Johnson, Catherine</p> <p>2016-04-01</p> <p>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 <span class="hlt">lithospheric</span> 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 <span class="hlt">lithosphere</span> 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 <span class="hlt">lithosphere</span> and its evolution over time, redox conditions in the planet's crust and mantle, and the iron content of the <span class="hlt">lithosphere</span>. 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 <span class="hlt">lithosphere</span> of Mercury, including iron alloys, silicides, and sulfides.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2009Tectp.474..322L','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2009Tectp.474..322L"><span><span class="hlt">Lithospheric</span> gravitational instability beneath the Southeast Carpathians</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Lorinczi, P.; Houseman, G. A.</p> <p>2009-09-01</p> <p>The Southeast corner of the Carpathians, known as the Vrancea region, is characterised by a cluster of strong seismicity to depths of about 200 km. The peculiar features of this seismicity make it a region of high geophysical interest. In this study we calculate the seismic strain-rate tensors for the period 1967-2007, and describe the variation of strain-rate with depth. The observed results are compared with strain-rates predicted by numerical experiments. We explore a new dynamical model for this region based on the idea of viscous flow of the <span class="hlt">lithospheric</span> mantle permitting the development of local continental mantle downwelling beneath Vrancea, due to a Rayleigh-Taylor instability that has developed since the cessation of subduction at 11 Ma. The model simulations use a Lagrangean frame 3D finite-element algorithm solving the equations of conservation of mass and momentum for a spatially varying viscous creeping flow. The finite deformation calculations of the gravitational instability of the continental <span class="hlt">lithosphere</span> demonstrate that the Rayleigh-Taylor mechanism can explain the present distribution of deformation within the downwelling <span class="hlt">lithosphere</span>, both in terms of stress localisation and amplitude of strain-rates. The spatial extent of the high stress zone that corresponds to the seismically active zone is realistically represented when we assume that viscosity decreases by at least an order of magnitude across the <span class="hlt">lithosphere</span>. The mantle downwelling is balanced by <span class="hlt">lithospheric</span> thinning in an adjacent area which would correspond to the Transylvanian Basin. Crustal thickening is predicted above the downwelling structure and thinning beneath the basin.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/9924022','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/9924022"><span>Electrical conductivity in the precambrian <span class="hlt">lithosphere</span> of western canada</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Boerner; Kurtz; Craven; Ross; Jones; Davis</p> <p>1999-01-29</p> <p>The subcrustal <span class="hlt">lithosphere</span> underlying the southern Archean Churchill Province (ACP) in western Canada is at least one order of magnitude more electrically conductive than the <span class="hlt">lithosphere</span> beneath adjacent Paleoproterozoic crust. The measured electrical properties of the <span class="hlt">lithosphere</span> underlying most of the Paleoproterozoic crust can be explained by the conductivity of olivine. Mantle xenolith and geological mapping evidence indicate that the <span class="hlt">lithosphere</span> beneath the southern ACP was substantially modified as a result of being trapped between two nearly synchronous Paleoproterozoic subduction zones. Tectonically induced metasomatism thus may have enhanced the subcrustal <span class="hlt">lithosphere</span> conductivity of the southern ACP.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015EGUGA..17.8108S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015EGUGA..17.8108S"><span>Understanding plate-motion changes over the past 100 Myr with quantitative models of the coupled <span class="hlt">lithosphere</span>/mantle system</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Stotz, Ingo; Iaffaldano, Giampiero; Rhodri Davies, D.</p> <p>2015-04-01</p> <p>The volume of geophysical datasets has grown substantially over recent decades. Our knowledge of continental evolution has increased due to advances in interpreting the records of orogeny and sedimentation. <span class="hlt">Ocean</span>-floor observations now allow one to resolve past plate motions (e.g. in the North Atlantic and Indian <span class="hlt">Ocean</span> over the past 20 Myr) at temporal resolutions of about 1 Myr. Altogether, these ever-growing datasets allow us to reconstruct the past evolution of Earth's <span class="hlt">lithospheric</span> plates in greater detail. This is key to unravelling the dynamics of geological processes, because plate motions and their temporal changes are powerful probe into the evolving force balance between shallow- and deep-rooted processes. However, such progress is not yet matched by the ability to quantitatively model past plate-motion changes and, therefore, to test hypotheses on the dominant controls. The main technical challenge is simulating the rheological behaviour of the <span class="hlt">lithosphere</span>/mantle system, which varies significantly from viscous to brittle. Traditionally computer models for viscous mantle flow on the one hand, and for the motions of the brittle <span class="hlt">lithosphere</span> on the other hand, have been developed separately. Coupling of these two independent classes of models has been accomplished only for neo-tectonic scenarios, without accounting for the impact of time-evolving mantle-flow (e.g. Iaffaldano and Bunge 2009). However, we have built a coupled model to simulate the <span class="hlt">lithosphere</span>/mantle system (using SHELLS and TERRA, respectively) through geological time, and to exploit the growing body of geophysical data as a primary constraint on these quantitative models. TERRA is a global spherical finite-element code for mantle convection (e.g. Baumgardner 1985, Bunge et al. 1996, Davies et al. 2013), whilst SHELLS is a thin-sheet finite-element code for <span class="hlt">lithosphere</span> dynamics (e.g. Bird 1998). Our efforts are focused, in particular, on achieving the technical ability to: (i) simulate the</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/6462242','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/6462242"><span>Shallow subduction, ridge subduction, and the evolution of continental <span class="hlt">lithosphere</span></span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Helmstaedt, H.; Dixon, J.M.; Farrar, E.; Carmichael, D.M.</p> <p>1985-01-01</p> <p>Subduction of <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> beneath continental crust at a shallow angle has occurred throughout the Phanerozoic Eon. Ridge subduction often follows shallow subduction and causes bimodal volcanism and crustal rifting, forming back-arc basins. Recent models for Archean plate tectonics propose very fast rates of spreading (400-800 km/Ma) and convergence, and sinking rates comparable to or slower (<10 km/Ma) than those of today. As faster convergence and slower sinking correspond to subduction at shallower angles, shallow subduction and ridge subduction must have been ubiquitous during the Archean permobile regime. This is compatible with a back-arc-basin origin for Archean greenstone belts. The common coexistence of tholeiitic and calc-alkaline igneous rocks in Archean greenstone belts, also implies ridge subduction. The authors envisage a transition, between 2.4 and 1.8 Ga., from a regime dominated by shallow subduction and repeated ridge subduction to one of normal plate tectonics with steeper subduction. Spreading rates decreased; continental plates became larger and stable shelves could develop at trailing margins. Shallow subduction became the exception, restricted to episodes of abnormally fast convergence; nevertheless, the long span of post-Archean time makes it unlikely that any part of the continental crust has escaped shallow subduction and ridge subduction. These processes recycle much volatile-rich <span class="hlt">oceanic</span> crust into the sub-continental upper mantle, thereby underplating the crust, effecting upper-mantle metasomatism and affecting intraplate magmatism.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/12460473','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/12460473"><span>The continental <span class="hlt">lithospheric</span> mantle: characteristics and significance as a mantle reservoir.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Pearson, D G; Nowell, G M</p> <p>2002-11-15</p> <p>The continental <span class="hlt">lithospheric</span> mantle (CLM) is a small-volumed (ca. 2.5% of the total mantle), chemically distinct mantle reservoir that has been suggested to play a role in the source of continental and <span class="hlt">oceanic</span> magmatism. It is our most easily identifiable reservoir for preserving chemical heterogeneity in the mantle. Petrological and geophysical constraints indicate that the maximum depth of the CLM is ca. 250 km. There is a clear secular variation of CLM composition, such that CLM formed in the last 2 Gyr is less depleted and therefore less dynamically stable than ancient CLM formed in the Archean. We present new trace-element data for kimberlite-hosted <span class="hlt">lithospheric</span> peridotites and metasomites. These data, combined with other data for spinel peridotites from non-cratonic regions, show that neither hydrous nor anhydrous <span class="hlt">lithospheric</span> mantle xenoliths make suitable sources for continental or <span class="hlt">oceanic</span> basalts. Addition of a hydrous phase, either amphibole or phlogopite, to depleted peridotite results in positive Nb and Ti anomalies that are the opposite of those predicted for some flood-basalt sources on the basis of their trace-element abundances. Overall, the Sr and Nd isotopic composition of cratonic and non-cratonic CLM is close to bulk Earth, with cratonic CLM showing small numbers of extreme compositions. Thus, while the CLM is certainly ancient in many locations, its average composition is not significantly 'enriched' over primitive upper mantle, in terms of either radiogenic isotopes or trace elements. These characteristics, plus a change in <span class="hlt">lithospheric</span> chemistry with depth, indicate that the elemental and isotopic composition of <span class="hlt">lithospheric</span> mantle likely to be re-incorporated into convecting mantle via delamination/thermal erosion processes is probably not very distinct from that of the convecting mantle. These observations lead us to question the requirement for CLM participation in the source of <span class="hlt">oceanic</span> magmas and to promote consideration of a mantle that</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2010AGUFM.T41E..05P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2010AGUFM.T41E..05P"><span>Imaging <span class="hlt">Lithospheric</span> Cascadia Structure with Ambient Noise Tomography</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Porritt, R. W.; Allen, R. M.; Brudzinski, M. R.; Boyarko, D. C.; O'Driscoll, L.; Zhai, Y.; Levander, A.; Humphreys, E.; Pollitz, F. F.</p> <p>2010-12-01</p> <p> the tremor recurrence interval is only on the order of ~10 months. A zone of low shear velocity in the subducting Juan de Fuca slab implies a potential zone of weakness or high pore fluid pressure in the relatively thin and young <span class="hlt">ocean</span> <span class="hlt">lithosphere</span>.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2007AGUFM.T51B0547P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2007AGUFM.T51B0547P"><span>Subduction Stability: <span class="hlt">Lithospheric</span> Strength and Roll-back</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Patel, P. I.; Lavier, L.; Grand, S.</p> <p>2007-12-01</p> <p>In exploring the issue of subduction zone stability, we ran a series of simulations representing subduction systems consisting of simple 2D representations of <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> subducting beneath continental <span class="hlt">lithosphere</span>. Our modelling software utilizes temperature dependent visco-elasto-plastic rheologies as well as a few proxies for significant chemical processes such as ecologitization and hydration. With externally imposed convergence rates, these models evolve from a contrived subduction initiation state to "normal-looking" subduction within approximately 10 million years. The simulations are then allowed to continue to evolve for up to 30 million more years. From our early results, we note that while most systems start with similar subduction geometries, they may deviate from each other over time. Notably, subduction initiated at "cooler" (and therefore stronger) junctures tend to form very stable subduction zones which maintain normal-looking geometries throughout the life of the simulation. However, subduction initiated at warmer margins tend to result in slab rollback relatively quickly. Systems with junctures of intermediate temperature also tend to subduct stably for a substantial amount of time, yet they too eventually result in rollback as the subducting slab entrains and removes some of the cooler <span class="hlt">lithosphere</span> near the juncture, allowing hotter asthenospheric material into the contact region between the plates. The hot, low-viscosity material sharply reduces the fluid-dynamically derived suction force that partially supports the stable subduction geometry, facilitating the retreat of the subducting slab as well as the rifting of the over-riding slab. These simulations incorporate a variety of approximations and assumptions which may not reflect the actual conditions within the Earth. However, they do offer a chance to observe how a system that at least appears geometrically similar to observed Earth systems may behave when subjected to varying</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2009EGUGA..11.8328B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2009EGUGA..11.8328B"><span>Surface topography changes in North Africa derived from combined <span class="hlt">lithosphere</span> and mantle modelling</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Buiter, S. J. H.; Steinberger, B.</p> <p>2009-04-01</p> <p>Northern Africa hosts several large intracratonic basins which record sedimentary processes since their formation in the (Pre)Cambrian. The absence of larger-scale normal faults indicates that they are not typical rift basins. Until now, no conclusive formation mechanism has been identified, though various processes (such as magmatism, phase changes, hydrothermal circulation and glacial loading) have been proposed. Here we focus on the contribution of deep-seated mantle processes to changes in surface topography. Traditionally, mantle flow models have relied on simplified translation of vertical stresses to changes in surface topography by using local isostasy or an elastic <span class="hlt">lithosphere</span>. We evaluate the role of the brittle-elastic-viscous <span class="hlt">lithosphere</span> rheology in controlling surface topography. We use models of mantle flow driven by density anomalies that are converted from seismic tomography, with prescribed surface plate motions and mantle viscosity structure inferred from mineral physics and surface observations. Density anomalies are advected backward in time. This approach provides reasonably accurate results back to 70 Myr ago and our models therefore span the entire Cenozoic. The mantle flow and pressure fields in the reference frame of the moving <span class="hlt">African</span> plate are applied to <span class="hlt">lithosphere</span> finite element models with an elastic-linear viscous-plastic rheology. Our first results focus on the Taoudenni, Kufrah and Chad basins. The Chad basin experienced Cretaceous extension and this allows us to evaluate the effects of rifting in combination with mantle processes.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://pubs.er.usgs.gov/publication/70021553','USGSPUBS'); return false;" href="http://pubs.er.usgs.gov/publication/70021553"><span>Crustal and <span class="hlt">lithospheric</span> structure of the west Antarctic Rift System from geophysical investigations: a review</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Behrendt, John C.</p> <p>1999-01-01</p> <p>-middle Cretaceous translation between East Antarctica and Pacific West Antarctica. Because a great amount of crustal extension in late Cenozoic time is unlikely, alternate mechanisms have been proposed for the late Cenozoic volcanism. Its vast volume and the <span class="hlt">ocean</span> island basalt chemistry of the exposed late Cenozoic alkaline volcanic rocks were interpreted as evidence for a mantle plume head. An alternative or supplemental explanation to the mantle plume hypothesis is significantly greater lower <span class="hlt">lithosphere</span> (mantle) stretching resulting in greater decompression melting than the limited Cenozoic crustal extension allows. Because of very slow rates of late Cenozoic extension in the West Antarctic Rift System, the amount of advected heat is small compared with the conductive heat. Therefore, phase transition probably would not explain the large subsidence with low extension observed in the West Antarctic Rift System. (C) 1999 Elsevier Science B.V.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.osti.gov/scitech/biblio/5545697','SCIGOV-STC'); return false;" href="https://www.osti.gov/scitech/biblio/5545697"><span>Analysis of geoid anomalies across the Mendocino fracture zone: implications for thermal models of the <span class="hlt">lithosphere</span></span></a></p> <p><a target="_blank" href="http://www.osti.gov/scitech">SciTech Connect</a></p> <p>Detrick, R.S. Jr.</p> <p>1981-12-10</p> <p>Geoid height increases in a step-like fashion across <span class="hlt">oceanic</span> fracture zones owing to the juxtaposition of <span class="hlt">lithosphere</span> with different ages and density structures. At a ridge/transform intersection, the initial geoid offset depends on the age difference across the fault and is approximately equal to 0.15 m/m.y. of age offset. The amplitude and shape of this anomaly will change along the length of a fracture zone depending on how the <span class="hlt">lithosphere</span> cools. If the <span class="hlt">lithosphere</span> thickens indefinitely (boundary layer model), then the geoid offset will remain constant although the width of the transition zone from one side of the fracture zone to the other will increase owing to both heat transfer across the fault and an increase in the average depth of compensation. If, however, the <span class="hlt">lithosphere</span> eventually approaches a constant thickness (plate model), the total geoid offset will decrease with time. An analysis of 18 SEASAT altimetry profiles across the Mendocino fracture zone (offset 25--30 m.y.) indicates geoid anomaly amplitudes systematically decrease from more than 4 m across the eastern (younger) part of the fracture zone to about 1 m across the western (older) part of the fault. Although the geoid offsets across the older portion of the Mendocino fracture zone may have been under-estimated by as much as 20--25%, this large decrease in geoid offset strongly suggests that the <span class="hlt">lithosphere</span> approaches a constant thickness along the older part of the fracture zone. A plate thickness of approx.100 km fits the observed change in geoid offset along the Mendocino, but with uncertainties in the offset determination, the age difference across the older part of the fault, and the presence of areas of anomalous depths on both sides of the fracture zone, this probably represents a minimum estimate of the total plate thickness.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015EGUGA..17.8157M','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015EGUGA..17.8157M"><span>Delamination of sub-crustal <span class="hlt">lithosphere</span> beneath the Isthmus of Tehuantepec, Mexico</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Manea, Marina; Constantin Manea, Vlad; Ferrari, Luca; Orozco-Esquivel, Maria Teresa</p> <p>2015-04-01</p> <p>Recent seismic data from a dense seismic array (VEOX), as well as from the permanent broadband network of the Mexican National Seismological Service (SSN), revealed several anomalous structures in the Isthmus of Tehuantepec. Seismic tomography imaged a high velocity body dipping ~30° from the Gulf of Mexico southward. Analysis of seismic noise detected a large well-defined low-velocity anomaly on top of this structure in the vicinity of the Late Miocene-Quaternary Los Tuxtlas volcanic field. The current interpretation of these observations propose the presence of a southward dipping slab resulting from the subduction of <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> prior to the collision of the Yucatán Block with Mexico ~12 Ma ago. However this interpretation contradicts many aspects of well-established models of Caribbean tectonics. Additionally such model does not explain how the southward dipping structure remained at a relatively low dipping angle (~30°) over the last 12 Ma, and why it is not seismically active. We propose an alternative model that reconciles the seismic observations with the tectonic evolution of the region. The south dipping seismic structure is the result of <span class="hlt">lithospheric</span> delamination produced by a thermal anomaly that migrated upwards through a slab gap in the Cocos slab located at ~200 km depth. Using high-resolution two-dimensional coupled petrological-thermomechanical numerical simulations of subduction, we show that hot and buoyant asthenospheric material flowing through a slab gap in the Cocos plate may have produced a rapid delamination of the <span class="hlt">lithosphere</span> once it reached its base. The model geometry of the delaminated <span class="hlt">lithosphere</span> is similar to the observed seismic anomaly, and the hot material from the plume impact is consistent with the low-velocity anomaly located at the north of the Isthmus of Tehuantepec, which feeds the Los Tuxtla volcanic field. Additionally our simulations show that the temperature of the delaminated <span class="hlt">lithosphere</span> is above 700</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014EGUGA..1612405C','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014EGUGA..1612405C"><span>Crossing the Iberian Plate from the Bay of Biscay to the Alboran Sea: a <span class="hlt">lithospheric</span> geotransect</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Carballo, Alberto; Fernandez, Manel; Torne, Montserrat; Jiménez-Munt, Ivone; Vergés, Jaume; Pedreira, David; Díaz, Jordi; Villaseñor, Antonio</p> <p>2014-05-01</p> <p>A ~1000-km-long <span class="hlt">lithospheric</span> transect running from the North-Iberian Margin to the Neogene Alboran Basin (W- Mediterranean) is investigated. The main goal is to image the lateral changes in the crustal and <span class="hlt">lithospheric</span> structure occurring in: i) the North-Iberian margin, whose deformation in Alpine times gave rise to the uplift of the Cantabrian Mountains related to incipient subduction; ii) the Spanish Meseta, characterized by the presence of Cenozoic basins on top of a Variscan basement with weak Alpine deformation in the Spanish Central System and localized Neogene-Quaternary deep volcanism; and iii) the Betic-Alboran system related to the roll-back of the Ligurian-Tethyan domain. The modeling approach based on the LitMod package combines potential fields, elevation, thermal, seismic and petrological data under a self-consistent scheme. The crustal structure is mainly constrained by active and passive seismic experiments whereas the upper mantle is constrained by tomography models. The results highlight the lateral variations in the topography of the <span class="hlt">lithosphere</span>-asthenosphere boundary (LAB), suggesting a strong <span class="hlt">lithospheric</span> mantle strain below the Cantabrian and Betic mountain belts. The LAB depth ranges from 160 km beneath the Cantabrian Mountains to 110-90 km beneath Iberia Meseta deepenly again to values of 190 km beneath Betic Mountain. The Spanish Central System, with elevations higher than 1400 m, has no noticeable signature on the LAB depth. We have considered three <span class="hlt">lithospheric</span> mantle compositions: a predominantly average Phanerozoic in the continental mainland, and two more fertile PUM (primitive upper mantle) compositions in the <span class="hlt">oceanic</span> domains of Cantabrian and Mediterranean seas, and in the Calatrava volcanic field. These compositional differences allowed us to reproduce the main trends of the geophysical observables as well as the inferred P-, Pn- and S-wave seismic velocities from tomography models and seismic experiments available in the study</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014AGUFM.T51D..05F','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014AGUFM.T51D..05F"><span>Imaging the Subduction of Continental <span class="hlt">Lithosphere</span> in the Banda Sea Region</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Fichtner, A.; De Wit, M.; van Bergen, M.</p> <p>2014-12-01</p> <p>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 <span class="hlt">lithosphere</span>. The integrated data suggest that the late Jurassic <span class="hlt">ocean</span> <span class="hlt">lithosphere</span> north of the North Australian craton was capable of entraining large volumes of continental <span class="hlt">lithosphere</span>. 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 <span class="hlt">lithosphere</span> as inferred from the tomographic images. The Banda arc example demonstrates that continental <span class="hlt">lithosphere</span> in arc-continent collisions may not generally be preserved, thus increasing the complexity of tectonic reconstructions and models of recycling continental crust.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/22382982','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/22382982"><span>Continental collision slowing due to viscous mantle <span class="hlt">lithosphere</span> rather than topography.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Clark, Marin Kristen</p> <p>2012-02-29</p> <p>Because the inertia of tectonic plates is negligible, plate velocities result from the balance of forces acting at plate margins and along their base. Observations of past plate motion derived from marine magnetic anomalies provide evidence of how continental deformation may contribute to plate driving forces. A decrease in convergence rate at the inception of continental collision is expected because of the greater buoyancy of continental than <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span>, but post-collisional rates are less well understood. Slowing of convergence has generally been attributed to the development of high topography that further resists convergent motion; however, the role of deforming continental mantle <span class="hlt">lithosphere</span> on plate motions has not previously been considered. Here I show that the rate of India's penetration into Eurasia has decreased exponentially since their collision. The exponential decrease in convergence rate suggests that contractional strain across Tibet has been constant throughout the collision at a rate of 7.03 × 10(-16) s(-1), which matches the current rate. A constant bulk strain rate of the orogen suggests that convergent motion is resisted by constant average stress (constant force) applied to a relatively uniform layer or interface at depth. This finding follows new evidence that the mantle <span class="hlt">lithosphere</span> beneath Tibet is intact, which supports the interpretation that the long-term strain history of Tibet reflects deformation of the mantle <span class="hlt">lithosphere</span>. Under conditions of constant stress and strength, the deforming continental <span class="hlt">lithosphere</span> creates a type of viscous resistance that affects plate motion irrespective of how topography evolved.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015AGUFM.T41F..08S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015AGUFM.T41F..08S"><span>The Thinning of the <span class="hlt">lithosphere</span> before Magmatic Spreading is Established at the Western End of the Cocos-Nazca Rift</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Smith, D. K.; Schouten, H.</p> <p>2015-12-01</p> <p>The transition from rifting of <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span> to full magmatic spreading is examined at the Galapagos triple junction (GTJ) where the tip of the Cocos-Nazca spreading center (called C-N Rift) is propagating westward and breaking apart 0.5 Ma <span class="hlt">lithosphere</span> formed at the East Pacific Rise near 2 15'N. Bathymetric mapping of the western section of the C-N Rift is limited, but sufficient to obtain a first-order understanding of how seafloor spreading is established. An initial rifting stage is followed by rifting with magma supply and lastly, full magmatic spreading is established. The flexural rotation of normal faults that border the rift basins is used to document thinning of the effective elastic thickness of the <span class="hlt">lithosphere</span> before magmatic spreading begins. The earliest faults show small outward rotation (1-5 degrees) for their offset suggesting that they cut thick <span class="hlt">lithosphere</span>. Subsequent faults closer to the axis have larger outward rotations (up to 35-40 degrees) with larger offset indicating that the <span class="hlt">lithosphere</span> was much thinner at the time of faulting and that low-angle detachment faults are forming. It is during late stage rifting and prior to full magmatic spreading that detachment faults such as the Intrarift ridge along Hess Deep rift are observed. Studies of low-angle detachment faulting during continental breakup at the Woodlark Basin suggest that their formation signals the input of magma beneath the rift. If this also is the case at the C-N Rift then magma is being supplied beneath Hess Deep rift. The axis of the segment immediately east of Hess Deep rift is characterized by a shallow graben with small seamounts scattered along it, typical of segments farther to the east, and we infer that full magmatic seafloor spreading has been established here. Our results provide new information on the formation of divergent boundaries in <span class="hlt">oceanic</span> <span class="hlt">lithosphere</span>, and place constraints on the supply of magma to a newly developing plate boundary.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2007JGRB..112.6412Z','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2007JGRB..112.6412Z"><span>Bathymetry of the Pacific plate and its implications for thermal evolution of <span class="hlt">lithosphere</span> and mantle dynamics</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Zhong, Shijie; Ritzwoller, Michael; Shapiro, Nikolai; Landuyt, William; Huang, Jinshui; Wessel, Paul</p> <p>2007-06-01</p> <p>A long-standing question in geodynamics is the cause of deviations of <span class="hlt">ocean</span> depth or seafloor topography from the prediction of a cooling half-space model (HSC). Are the deviations caused entirely by mantle plumes or <span class="hlt">lithospheric</span> reheating associated with sublithospheric small-scale convection or some other mechanisms? In this study we analyzed the age and geographical dependences of <span class="hlt">ocean</span> depth for the Pacific plate, and we removed the effects of sediments, seamounts, and large igneous provinces (LIPs), using recently available data sets of high-resolution bathymetry, sediments, seamounts, and LIPs. We found that the removal of seamounts and LIPs results in nearly uniform standard deviations in <span class="hlt">ocean</span> depth of ˜300 m for all ages. The <span class="hlt">ocean</span> depth for the Pacific plate with seamounts, LIPs, the Hawaiian swell, and South Pacific super-swell excluded can be fit well with a HSC model till ˜80-85 Ma and a plate model for older seafloor, particularly, with the HSC-Plate depth-age relation recently developed by Hillier and Watts (2005) with an entirely different approach for the North Pacific <span class="hlt">Ocean</span>. A similar <span class="hlt">ocean</span> depth-age relation is also observed for the northern region of our study area with no major known mantle plumes. Residual topography with respect to Hillier and Watts' HSC-Plate model shows two distinct topographic highs: the Hawaiian swell and South Pacific super-swell. However, in this residual topography map, the Darwin Rise does not display anomalously high topography except the area with seamounts and LIPs. We also found that the topography estimated from the seismic model of the Pacific <span class="hlt">lithosphere</span> of Ritzwoller et al. (2004) generally agrees with the observed topography, including the reduced topography at relatively old seafloor. Our analyses show that while mantle plumes may be important in producing the Hawaiian swell and South Pacific super-swell, they cannot be the only cause for the topographic deviations. Other mechanisms, particularly</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://hdl.handle.net/2060/19820004822','NASA-TRS'); return false;" href="http://hdl.handle.net/2060/19820004822"><span>Global isostatic geoid anomalies for plate and boundary layer models of the <span class="hlt">lithosphere</span></span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Hager, B. H.</p> <p>1981-01-01</p> <p>Commonly used one dimensional geoid models predict that the isostatic geoid anomaly over old <span class="hlt">ocean</span> basins for the boundary layer thermal model of the <span class="hlt">lithosphere</span> is a factor of two greater than that for the plate model. Calculations presented, using the spherical analogues of the plate and boundary layer thermal models, show that for the actual global distribution of plate ages, one dimensional models are not accurate and a spherical, fully three dimensional treatment is necessary. The maximum difference in geoid heights predicted for the two models is only about two meters. The thermal structure of old <span class="hlt">lithosphere</span> is unlikely to be resolvable using global geoid anomalies. Stripping the effects of plate aging and a hypothetical uniform, 35 km, isostatically-compensated continental crust from the observed geoid emphasizes that the largest-amplitude geoid anomaly is the geoid low of almost 120 m over West Antarctica, a factor of two greater than the low of 60 m over Ceylon.</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li class="active"><span>20</span></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li><a href="#" onclick='return showDiv("page_22");'>22</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_20 --> <div id="page_21" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li class="active"><span>21</span></li> <li><a href="#" onclick='return showDiv("page_22");'>22</a></li> <li><a href="#" onclick='return showDiv("page_23");'>23</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="401"> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016GeoJI.204.1756G','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016GeoJI.204.1756G"><span>Ambient noise tomography of the Cameroon Volcanic Line and Northern Congo craton: new constraints on the structure of the <span class="hlt">lithosphere</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Guidarelli, M.; Aoudia, A.</p> <p>2016-03-01</p> <p>We investigate the <span class="hlt">lithospheric</span> structure of Cameroon inverting Rayleigh waves obtained from the cross-correlation of ambient seismic noise. We correlate seismic records between 32 broad-band stations and we obtain good quality Rayleigh waves for 310 interstation paths. We measure group velocity dispersion curves from the reconstructed Rayleigh waves in the period range 10-35 s and we invert the group velocities for tomographic images. After the tomography the group velocities are then inverted, together with longer period group velocity measurements from existing literature, to compute a 3-D S-wave velocity model of the Cameroon <span class="hlt">lithosphere</span> down to 100 km depth. Our results provide an unprecedented mapping of the physical properties of the different crustal units and their correlations with surface geology, as well as with mantle <span class="hlt">lithospheric</span> variations. The Cameroon Volcanic Line (CVL) appears as a segmented feature exhibiting different physical properties along strike. The active Mt Cameroon volcano is underlain by very low velocities, unlike the other segments of the CVL. The along-strike variations in crustal structure suggest that lateral heterogeneities in <span class="hlt">lithospheric</span> thickness and physical properties have influenced the location and distribution of magmatism. The crust beneath the Central <span class="hlt">African</span> Shear Zone exhibits a sizeable low velocity anomaly. The <span class="hlt">lithosphere</span> beneath Cameroon is characterised by a heterogeneous crust with a relatively constant thickness and a low velocity uppermost mantle at the edge of the Congo Craton. Our results favour processes combining small-scale upwelling at the edge of a thick <span class="hlt">lithosphere</span> and reactivation of Precambrian basement structures to explain the distribution of Holocene-Recent magmatism and plateau uplift. Our results also indicate that Mt Cameroon and surroundings areas are the most at risk zones for magmatic activity during this stage of CVL development.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015AGUFMDI11B2583N','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015AGUFMDI11B2583N"><span>Mid-<span class="hlt">lithospheric</span> Discontinuity Beneath the Malawi Rift, Deduced from Gravity Studies and its Relation to the Rifting Process.</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Njinju, E. A.; Atekwana, E. A.; Mickus, K. L.; Abdelsalam, M. G.; Atekwana, E. A.; Laó-Dávila, D. A.</p> <p>2015-12-01</p> <p>The World Gravity Map satellite gravity data were used to investigate the <span class="hlt">lithospheric</span> structure beneath the Cenozoic-age Malawi Rift which forms the southern extension of the Western Branch of the East <span class="hlt">African</span> Rift System. An analysis of the data using two-dimensional (2D) power spectrum methods indicates the two distinctive discontinuities at depths of 31‒44 km and 64‒124 km as defined by the two steepest slopes of the power spectrum curves. The shallower discontinuity corresponds to the crust-mantle boundary (Moho) and compares well with Moho depth determined from passive seismic studies. To understand the source of the deeper discontinuity, we applied the 2D power spectrum analysis to other rift segments of the Western Branch as well as regions with stable continental <span class="hlt">lithospheres</span> where the <span class="hlt">lithospheric</span> structure is well constrained through passive seismic studies. We found that the deeper discontinuity corresponds to a mid-<span class="hlt">lithospheric</span> discontinuity (MLD), which is known to exist globally at depths between 60‒150 km and as determined by passive seismic studies. Our results show that beneath the Malawi Rift, there is no pattern of N-S elongated crustal thinning following the surface expression of the Malawi Rift. With the exception of a north-central region of crustal thinning (< 35 km), most of the southern part of the rift is underlain by thick crust (~40‒44 km). Different from the Moho, the MLD is shallower beneath the axis of the Malawi Rift forming a N-S trending zone with depths of 64‒80 km, showing a broad and gentle topography. We interpret the MLD as representing a sharp density contrast resulting from metasomatized <span class="hlt">lithosphere</span> due to lateral migration along mobile belts of hot mantle melt or fluids from a distant plume and not from an ascending asthenosphere. These fluids weaken the <span class="hlt">lithosphere</span> enhancing rift nucleation. The availability of satellite gravity worldwide makes gravity a promising technique for determining the MLD globally.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2002AGUFMGP51A0985B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2002AGUFMGP51A0985B"><span>Electrical Conductivity and Anisotropy in Pacific <span class="hlt">Lithosphere</span>: CSEM Results from APPLE</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Behrens, J.; Constable, S.; MacGregor, L.; Everett, M.</p> <p>2002-12-01</p> <p>Emplacement of the sheeted dyke complex and strain associated with plate formation at mid-<span class="hlt">ocean</span> ridge spreading centers may influence electrical conductivity at various depths in the <span class="hlt">lithosphere</span>, and may leave an anisotropic fabric frozen in place. By measuring <span class="hlt">lithospheric</span> electrical conductivity and anisotropy as a function of depth, insight may be gained regarding the formation and evolution of <span class="hlt">oceanic</span> crust and mantle. Controlled-source electromagnetic (CSEM) sounding of 35 Ma Pacific <span class="hlt">lithosphere</span> was undertaken as part of the Anisotropy and Physics of the Pacific <span class="hlt">Lithosphere</span> Experiment (APPLE), carried out in February/March 2001 approximately 1000 km west of San Diego, California. Twenty seafloor electric field sensors were deployed (and recovered with data) during this experiment. The transmitter (DASI), a 100 m horizontal electric dipole, was deep-towed in a 30 km radius circle around a central core and perimeter array of receivers. An additional radial tow to 70 km total range and a 15 km radius semi-circular tow around a perimeter receiver supplemented the geometry of the main tow. DASI transmitted a 4 Hz square wave throughout the CSEM phase of the experiment. Smooth (and layered) inversion of short-offset (2-20 km) data, using 1-D isotropic modeling, generates models with upper-crustal resistivities ~10 Ω m, varying by about an order of magnitude across the survey area. Lower crustal resistivities are on the order of 103 Ω m. Smooth inversion of the long radial tow data indicates upper mantle resistivities of ~104 Ω m, with an increase in conductivity below 20 km depth. This may be due to thermally-activated olivine conduction, indicating that the base of the <span class="hlt">lithosphere</span> has been detected. The integrated resistivity-thickness product for the top 100 km of our model is 1.1 x 109 Ω m2. A factor of two is observed in the electric fields measured during the circular tow, in a pattern that qualitatively resembles forward modeling results over an uniaxially</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2003AGUFMGP12B..02B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2003AGUFMGP12B..02B"><span>Electrical Conductivity and Anisotropy in Pacific <span class="hlt">Lithosphere</span>: CSEM Results from APPLE</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Behrens, J.; Constable, S.; Everett, M.; MacGregor, L.</p> <p>2003-12-01</p> <p>Strain associated with plate formation at mid-<span class="hlt">ocean</span> ridge spreading centers may influence electrical conductivity at various depths in the <span class="hlt">lithosphere</span>, and may leave an anisotropic fabric frozen in place. By measuring <span class="hlt">lithospheric</span> electrical conductivity and anisotropy, insight may be gained regarding the formation and evolution of <span class="hlt">oceanic</span> crust and mantle. Controlled-source electromagnetic (CSEM) sounding of 35 Ma Pacific <span class="hlt">lithosphere</span> was undertaken as part of the Anisotropy and Physics of the Pacific <span class="hlt">Lithosphere</span> Experiment (APPLE), carried out approximately 1000 km west of San Diego. The transmitter (DASI), with a 100 m horizontal electric dipole antenna, was deep-towed in a 30 km radius circle around an array of receivers. A radial tow to 70 km total range and a 15 km radius semi-circular tow supplemented the geometry of the main tow. DASI transmitted a 4 Hz square wave throughout the CSEM phase of the experiment. Smooth (and layered) inversions of short-offset (2-20 km) data, using 1-D isotropic modeling, generate models with upper-crustal resistivities ˜1 Ω m, varying by about an order of magnitude across the survey area. Lower crustal resistivities are on the order of 103 Ω m. Smooth inversion of the long radial tow data indicates upper mantle resistivities of ˜104 Ω m, with an increase in conductivity below 20 km depth. This may be due to thermally-activated olivine conduction, indicating that the base of the <span class="hlt">lithosphere</span> has been detected. The integrated resistivity-thickness product for the top 100 km of our model is 1.1 x 109 Ω m2. The electric field is characterized in terms of the polarization ellipse parameters. During the circular tow, the maximum axis varies by a factor of two, while the minimum axis varies by a factor of 5. Forward calculations were run on models with a uniaxially anisotropic layer sandwiched between two isotropic layers. Simulated data from models with increased conductivity perpendicular to the paleo-spreading direction in the</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://hdl.handle.net/2060/19850005056','NASA-TRS'); return false;" href="http://hdl.handle.net/2060/19850005056"><span><span class="hlt">Lithospheric</span> structure in the Pacific geoid</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Marsh, B. D.</p> <p>1984-01-01</p> <p>In order that sub-<span class="hlt">lithospheric</span> density variations be revealed with the geoid, the regional geoid anomalies associated with bathymetric variations must first be removed. Spectral techniques were used to generate a synthetic geoid by filtering the residual bathymetry assuming an Airy-type isostatic compensation model. An unbiased estimated of the admittances show that for region under study, no single compensation mechanism will explain all of the power in the geoid. Nevertheless, because topographic features are mainly coherent with the geoid, to first order an isostationally compensated <span class="hlt">lithosphere</span> cut by major E-W fracture zones accounts for most of the power in the high degree and other SEASAT geoid in the Pacific.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://hdl.handle.net/2060/19920005337','NASA-TRS'); return false;" href="http://hdl.handle.net/2060/19920005337"><span>Flexural deformation of the continental <span class="hlt">lithosphere</span></span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p></p> <p>1990-01-01</p> <p>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 <span class="hlt">lithosphere</span>.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2010AGUFM.T23C2281K','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2010AGUFM.T23C2281K"><span>Crustal and <span class="hlt">lithospheric</span> imaging of the Atlas Mountains of Morocco inferred from magnetotelluric data</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Kiyan, D.; Jones, A. G.; Fullea, J.; Hogg, C.; Ledo, J.; Sinischalchi, A.; Campanya, J.; Picasso Phase II Team</p> <p>2010-12-01</p> <p>The Atlas System of Morocco is an intra-continental mountain belt extending for more than 2,000 km along the NW <span class="hlt">African</span> plate with a predominant NE-SW trend. The System comprises three main branches: the High Atlas, the Middle Atlas, and the Anti Atlas. We present the results of a very recent multi-institutional magnetotelluric (MT) experiment across the Atlas Mountains region that started in September, 2009 and ended in February, 2010, comprising acquisition of broadband and long-period MT data. The experiment consisted of two profiles: (1) a N-S oriented profile crossing the Middle Atlas through the Central High Atlas to the east and (2) a NE-SW profile crossing the western High Atlas towards the Anti Atlas to the west. The MT measurements are part of the PICASSO (Program to Investigate Convective Alboran Sea System Overturn) and the concomitant TopoMed (Plate re-organization in the western Mediterranean: <span class="hlt">Lithospheric</span> causes and topographic consequences - an ESF EUROCORES TOPO-EUROPE project) projects, to develop a better understanding of the internal structure and evolution of the crust and <span class="hlt">lithosphere</span> of the Atlas Mountains. The MT data have been processed with robust remote reference methods and submitted to comprehensive strike and dimensionality analysis. Two clearly depth-differentiated strike directions are apparent for crustal (5-35 km) and <span class="hlt">lithospheric</span> (50-150 km) depth ranges. These two orientations are roughly consistent with the NW-SE Africa-Eurasia convergence acting since the late Cretaceous, and the NNE-SSW Middle Atlas, where Miocene to recent Alkaline volcanism is present. Two-dimensional (2-D) smooth electrical resistivity models were computed independently for both 50 degrees and 20 degrees E of N strike directions. At the crustal scale, our preliminary results reveal a middle to lower-crustal conductive layer stretching from the Middle Atlas southward towards the High Moulouya basin. The most resistive (and therefore potentially thickest</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016MsT..........2N','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016MsT..........2N"><span>Crustal and sub-continental <span class="hlt">lithospheric</span> mantle decoupling beneath the Malawi Rift</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Njinju, Emmanuel Atem</p> <p></p> <p>We analyzed satellite gravity and aeromagnetic data using the two-dimensional (2D) power-density spectrum technique to investigate the <span class="hlt">lithospheric</span> and thermal structure beneath the magma-starved Malawi Rift, which forms the southern extension of the Western Branch of the East <span class="hlt">African</span> Rift System. We observed: (1) lack of consistent pattern of crustal thinning and elevated heat flow along the surface expression of the rift. Beneath the Rungwe Volcanic Province (RVP) in the north, the crustal thickness ranges between 40 and 45 km and varies between 35 and 40 km along the entire length of the rift. (2) shallow <span class="hlt">lithosphere</span>-asthenosphere boundary (LAB) elevated to ˜64 km beneath the entire length of the rift and deeper than 100 km beneath the surrounding Precambrian terranes reaching in places ˜124 km. (3) localized zones of high heat flow (70-75 mWm-2) beneath the RVP, and the central and southern parts of the rift. The central and southern thermal anomalies are due to the presence of uranium deposits in the Karoo sedimentary rocks. We interpret the crustal thickness heterogeneity to have been inherited from pre-existing <span class="hlt">lithospheric</span> stretching, while strain during the extension of the Malawi Rift is preferentially localized in the sub-continental <span class="hlt">lithospheric</span> mantle (SCLM). Our interpretation is supported by 2D forward modeling of the gravity data showing uniform stretching of the SCLM by a factor of 1.5 to 1.8 beneath the entire length of the rift. Our results indicate decoupling of the crust from the SCLM during the early stages of the development of the Malawi Rift.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016JGRB..121.6966H','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016JGRB..121.6966H"><span>Identifying mantle <span class="hlt">lithosphere</span> inheritance in controlling intraplate orogenesis</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Heron, Philip J.; Pysklywec, Russell N.; Stephenson, Randell</p> <p>2016-09-01</p> <p>Crustal inheritance is often considered important in the tectonic evolution of the Wilson Cycle. However, the role of the mantle <span class="hlt">lithosphere</span> is usually overlooked due to its difficulty to image and uncertainty in rheological makeup. Recently, increased resolution in <span class="hlt">lithosphere</span> imaging has shown potential scarring in continental mantle <span class="hlt">lithosphere</span> to be ubiquitous. In our study, we analyze intraplate deformation driven by mantle <span class="hlt">lithosphere</span> heterogeneities from ancient Wilson Cycle processes and compare this to crustal inheritance deformation. We present 2-D numerical experiments of continental convergence to generate intraplate deformation, exploring the limits of continental rheology to understand the dominant <span class="hlt">lithosphere</span> layer across a broad range of geological settings. By implementing a "jelly sandwich" rheology, common in stable continental <span class="hlt">lithosphere</span>, we find that during compression the strength of the mantle <span class="hlt">lithosphere</span> is integral in generating deformation from a structural anomaly. We posit that if the continental mantle is the strongest layer within the <span class="hlt">lithosphere</span>, then such inheritance may have important implications for the Wilson Cycle. Furthermore, our models show that deformation driven by mantle <span class="hlt">lithosphere</span> scarring can produce tectonic patterns related to intraplate orogenesis originating from crustal sources, highlighting the need for a more formal discussion of the role of the mantle <span class="hlt">lithosphere</span> in plate tectonics.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016EGUGA..1810331F','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016EGUGA..1810331F"><span>What major faults look like, and why this matters for <span class="hlt">lithospheric</span> dynamics</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Fagereng, Ake</p> <p>2016-04-01</p> <p>Earthquakes involve seconds to minutes of frictional sliding on a discontinuity, likely of sub-cm thickness, within a damage zone. Earthquakes are separated by an interseismic period of hundreds to thousands of years, during which a number of healing and weakening processes occur within the fault zone. The next earthquake occurs as shear stress exceeds frictional resistance, on the same or a different discontinuity as the previous event, embedded within the fault damage zone. After incremental damage and healing in multiple earthquake cycles, the fault zone rock assemblage evolves to a structure and composition distinctly different from the host rock(s). This presentation presents field geology evidence from a range of settings, to discuss the interplay between the earthquake cycle, long-term deformation, and <span class="hlt">lithospheric</span> rheology. Classic fault zone models are based on continental transforms, which generally form discrete faults in the upper crust, and wide, anastomosing shear zones in the lower crust. In <span class="hlt">oceanic</span> crust, transforms are considered frictionally weak, and appear to exploit dyke margins and joint surfaces, but also locally cross-cut these structures in anastomosing networks. In the <span class="hlt">oceanic</span> lower crust and upper mantle, serpentinisation significantly alters fault structure. In old continental crust, previous deformation events leave a heterogeneous geology affecting active faulting. For example, the amagmatic, southern East <span class="hlt">African</span> Rift has long been thought to exploit weak Proterozoic 'mobile belts'. However, detailed look at the Bilila-Mtakataka border fault in Malawi indicates that this fault locally exploits weak foliation in existing deformed zones, but also locally forms a new set of anastomosing fault surfaces cross-cutting existing weak foliation. In exhumed lower crust, the Antarctic Maud Belt provides an example of multiple phases of plastic deformation, where the second event is only visible in localised shear zones, likely inherited from the</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2001AGUFM.T12A0896L','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2001AGUFM.T12A0896L"><span>Far-offset Airgun Imaging of the Mantle: <span class="hlt">Lithospheric</span> velocity structure of the North Atlantic</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Lizarralde, D.; Gaherty, J.; Collins, J. A.; Hirth, G.</p> <p>2001-12-01</p> <p>The extraction of <span class="hlt">oceanic</span> crust at mid-<span class="hlt">ocean</span> ridges leaves behind residual mantle depleted of basaltic constituents and volatiles, with an embedded structural fabric associated with melting and mantle flow beneath a spreading center. This basic layered structure should remain in the <span class="hlt">lithosphere</span> as it cools and translates from the ridge, providing a record of mid-<span class="hlt">ocean</span> ridge processes over time. Images of the seismic velocity structure within the uppermost <span class="hlt">o