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Sample records for african oceanic lithosphere

  1. The hydrothermal power of oceanic lithosphere

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

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

  2. 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.

  3. Global equivalent magnetization of the oceanic lithosphere

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

  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. Crustal Thickness and Oceanic Lithosphere Distribution in the Eastern Mediterranean from Satellite Gravity Anomaly Inversion

    NASA Astrophysics Data System (ADS)

    Cowie, L.; Kusznir, N. J.

    2010-12-01

    The distribution of oceanic and continental lithosphere in the eastern Mediterranean is not well understood. Gravity inversion, incorporating a lithosphere thermal gravity anomaly correction, has been used to map Moho depth, crustal thickness and continental lithosphere thinning factor for the eastern Mediterranean in order to determine the distribution of oceanic and continental lithosphere and the ocean-continent transition location. Data used in the gravity inversion are bathymetry, free-air gravity and sediment thickness data from Smith and Sandwell (1997), Sandwell and Smith (2009) and Laske and Masters (1997) respectively. Moho depths from the gravity inversion are dependent on the age of oceanic lithosphere and continental breakup because of the lithosphere thermal gravity correction; however, these ages are uncertain for the eastern Mediterranean. Gravity inversion sensitivities to break-up ages of 225Ma (late Triassic) and 100Ma (early Cretaceous) have been examined. Gravity inversion results show thin crust (5 - 10km thickness) for the Ionian Sea and the Herodotus Basin of the eastern Mediterranean consistent with these basins being underlain by oceanic or highly thinned continental crust. Predicted Moho depths from the gravity inversion are in agreement with published Ionian Sea ESP results (Voogd et al, 1992) and suggest a gravity inversion reference Moho depth increasing to the north, which we attribute to subduction dynamic subsidence. Calibration of gravity inversion Moho against ESP results show a trade-off between break-up age and reference Moho depth; a Cretaceous age ocean requires a larger Moho reference depth than a Triassic age ocean. Lithosphere thinning factor maps from gravity inversion for Africa do not show continuity between the Cretaceous African rift system (Benue Trough, Chad, CASZ and Sudan basins) and eastern Mediterranean basins. If the Ionian Sea is of Cretaceous age then it more probably links to Cretaceous rifting and sea

  7. 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

  8. 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.

  9. 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.

  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. Thermal stresses due to cooling of a viscoelastic oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Denlinger, Roger P.; Savage, William Z.

    1989-01-01

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

  12. Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere.

    PubMed

    Schlindwein, Vera; Schmid, Florian

    2016-07-14

    Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges. However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere-asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation. PMID:27362231

  13. 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.

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

    NASA Astrophysics Data System (ADS)

    Calmant, Stephane

    1987-09-01

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

  15. 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.

  16. 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.

  17. 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.

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

    NASA Astrophysics Data System (ADS)

    Karner, Garry D.; Weissel, Jeffrey K.

    1990-11-01

    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.

  19. 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.

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

    NASA Astrophysics Data System (ADS)

    Medvedev, Sergei

    2016-06-01

    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 toobserved 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

  1. Internal structure of oceanic lithosphere: A perspective from tectonic windows

    NASA Astrophysics Data System (ADS)

    Karson, Jeffrey A.

    Major faulted escarpments on the seafloor provide "tectonic windows" into oceanic crust and upper mantle. Direct observations in these settings reveal that the spatial arrangement, internal structure, and contacts between major rock units are significantly more complex than commonly anticipated on the basis of seismic studies and ophiolite analogs. From this perspective, a stratiform, ophiolite-like sequence of rock units, including basaltic volcanic rocks, sheeted diabase dike complex, isotropic and layered gabbroic and ultramafic rocks over upper mantle peridotites—all separated by generally horizontal contacts, may be much less common in the oceanic lithosphere than generally thought. Conversely, documented examples of large outcrop areas (tens of kilometers across) that lack the ophiolite-like sequence or that contain structures that do not conform to the ophiolite model call into question the basic assumptions made in the reconstruction and interpretation of ophiolite complexes. Historically, the stratiform ophiolite architecture has been the basis for inferences of the interaction between tectonism and magmatism at mid-ocean ridge spreading centers. A growing number of constraints on geological relations along seafloor escarpments hint at much broader range of interactions between tectonic deformation and magmatic construction. Along slow-spreading ridges, the magma budget (volume of magma per unit plate separation) is highly variable, giving rise to a wide range of morphologic and geologic features along wide rift valleys. The diversity of crustal architectures and internal structures seen in tectonic windows is correspondingly large. Although it is possible that a relatively simple, layered ophiolite-like crust develops in regions of relatively high magma budget (Reykjanes Ridge, Azores region, etc.), more complex structures that differ from those of stratiform ophiolites are present where lower magma budgets prevail. Significant deviations from a simple

  2. Osmium isotopic evidence for ancient subcontinental lithospheric mantle beneath the kerguelen islands, southern indian ocean

    PubMed

    Hassler; Shimizu

    1998-04-17

    Upper mantle xenoliths found in ocean island basalts are an important window through which the oceanic mantle lithosphere may be viewed directly. Osmium isotopic data on peridotite xenoliths from the Kerguelen Islands, an archipelago that is located on the northern Kerguelen Plateau in the southern Indian Ocean, demonstrate that pieces of mantle of diverse provenance are present beneath the Islands. In particular, peridotites with unradiogenic osmium and ancient rhenium-depletion ages (to 1.36 x 10(9) years old) may be pieces of the Gondwanaland subcontinental lithosphere that were incorporated into the Indian Ocean lithosphere as a result of the rifting process. PMID:9545216

  3. Seafloor morphology and the thermal evolution of oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Stein, Carol

    2010-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2007-12-01

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

  5. 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.

  6. 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).

  7. 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

  8. Mesozoic thermal evolution of the southern African mantle lithosphere

    NASA Astrophysics Data System (ADS)

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

    2003-12-01

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

  9. 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

  10. Lithospheric structure of southern Indian shield and adjoining oceans: integrated modelling of topography, gravity, geoid and heat flow data

    NASA Astrophysics Data System (ADS)

    Kumar, Niraj; Zeyen, H.; Singh, A. P.; Singh, B.

    2013-07-01

    For the present 2-D lithospheric density modelling, we selected three geotransects of more than 1000 km in length each crossing the southern Indian shield, south of 16°N, in N-S and E-W directions. The model is based on the assumption of local isostatic equilibrium and is constrained by the topography, gravity and geoid anomalies, by geothermal data, and where available by seismic data. Our integrated modelling approach reveals a crustal configuration with the Moho depth varying from ˜40 km beneath the Dharwar Craton, and ˜39 km beneath the Southern Granulite Terrane to about 15-20 km beneath the adjoining oceans. The lithospheric thickness varies significantly along the three profiles from ˜70-100 km under the adjoining oceans to ˜130-135 km under the southern block of Southern Granulite Terrane including Sri Lanka and increasing gradually to ˜165-180 km beneath the northern block of Southern Granulite Terrane and the Dharwar Craton. This step-like lithosphere-asthenosphere boundary (LAB) structure indicates a normal lithospheric thickness beneath the adjoining oceans, the northern block of Southern Granulite Terrane and the Dharwar Craton. The thin lithosphere below the southern block of Southern Granulite Terrane including Sri Lanka is, however, atypical considering its age. Our results suggest that the southern Indian shield as a whole cannot be supported isostatically only by thickened crust; a thin and hot lithosphere beneath the southern block of Southern Granulite Terrane including Sri Lanka is required to explain the high topography, gravity, geoid and crustal temperatures. The widespread thermal perturbation during Pan-African (550 Ma) metamorphism and the breakup of Gondwana during late Cretaceous are proposed as twin cause mechanism for the stretching and/or convective removal of the lower part of lithospheric mantle and its replacement by hotter and lighter asthenosphere in the southern block of Southern Granulite Terrane including Sri Lanka

  11. 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).

  12. Global Model of the Integrated Strength of Oceanic Lithosphere and Ridge-push Transmission

    NASA Astrophysics Data System (ADS)

    Mahatsente, R.

    2014-12-01

    Global models of the integrated lithospheric strength of oceanic plates and ridge-push force are presented. To assess the transmission of ridge-push related stress in oceanic plates, ridge-push force and lithospheric strength are compared. The integrated strength of oceanic plate has been estimated using plate cooling model and relevant rheology of the crust and upper mantle. The strength analysis includes Low-Temperature Plasticity (LTP) and assumes a range of possible tectonic conditions and rheology in the plates. The ridge-push force has been derived from thermal state of oceanic lithosphere, crustal age and seafloor depth data. The results of modelling show that both lithospheric strength and ridge-push force are age dependent. Old oceanic plates can be as strong as ~ 11 TN m-1. For plates of the same age, the magnitude of ridge-push force is in the order of ~ 3.5 TN m-1.The transmission of ridge-push is mainly controlled by the rheology and predominant tectonic condition in the plates. If the rheology of oceanic lithosphere is dry, the estimated lithospheric strength is larger than the ridge-push force at all ages for compressional tectonic regimes and at old ages for extension (> 75 Ma). Under such consideration, the ridge-push related stresses may not dissipate in a form of intraplate deformation. Instead, the stresses may be transmitted in the plate. If an oceanic lithosphere has wet rheology, the strength of young plates (< 75 Ma) is much less than the ridge-push force for extensional and compressional tectonic regimes. Consequently, the stress associated with ridge-push may result in intraplate deformation. This depends, however, on the balance of force resulting from far field forces, basal shear traction and Gravitational Potential Energy of elevated regions. In particular, strong basal shear traction can balance the ridge-push force, and as a result, the oceanic lithosphere may respond by deformation. The consideration of LTP in the upper mantle has

  13. 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.

  14. 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

  15. Lithospheric structure of the southern African subcontinent from surface wave tomography

    NASA Astrophysics Data System (ADS)

    Raveloson, A.; Nyblade, A.; Mulibo, G.; Mangongolo, A.; Tugume, F.

    2012-12-01

    In this study the lithospheric structure of the southern African subcontinent is examined using a new 3D shear wave velocity model. The lithospheric structure of Africa consists of several Archean cratons and Proterozoic mobile belts. Many intracratonic and rift basin are found within both the Archean and Protozoic terrains. We investigate the 3D shear wave velocity structure of the lithosphere by tomographically modeling Rayleigh wave dispersion curves. Rayleigh waves group velocities from 10 to 125 s period were determined by using multiple-filter and phase-matched techniques. We used seismic events with a magnitude greater than 4.5 and depth shallower than 100 km recorded from 1990 to 2011 on many stations belonging to temporary or permanent networks. We used events and stations within the African plate in an effort to minimize the contributions from outside structure. We constructed the 3D S wave model in two steps. In the first step we measured group velocities of fundamental mode Rayleigh waves. In the second stage we extracted a dispersion curve from the group velocity maps and inverted them to obtain 1D shear wave velocity models. The 1D models are then combined at a regular spatial interval to create a 3D shear wave velocity model. New features revealed in our model include a region of lower wave speeds beneath the cuvette central separating several Archean blocks of the Congo craton and a region of fast lithosphere in northern Mozambique associated with the Ruvuma microplate.

  16. 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

  17. 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.

  18. 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

  19. 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

  20. 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.

  1. 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

  2. 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.

  3. 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.

  4. Oceanic core complexes and ancient oceanic lithosphere: Insights from Iapetan and Tethyan ophiolites (Canada and Albania)

    NASA Astrophysics Data System (ADS)

    Tremblay, Alain; Meshi, Avni; Bédard, Jean H.

    2009-07-01

    Oceanic ridges are extensional environments characterized by brittle to ductile normal faulting and episodic magma supply. Studies in modern oceanic settings suggest that the interplay of low-angle detachments and high-angle normal faults locally lead to the exhumation of lower crust and upper mantle peridotites to the seafloor as oceanic core complexes. Along the Mid-Atlantic ridge, core complexes typically occur in inside corners of intersections between the ridge axis and major transform faults. The core complexes were exhumed by movement along long-lived detachment faults that extend for 10s of km, and that are crosscut by high-angle normal faults oriented parallel to the rift axis. Ridge segments with abundant detachments would be characterized by lithosphere having little or no upper crust. Similar features in ophiolites may also reflect the dominance of amagmatic extension. This contribution presents two examples of fossilized oceanic core complexes preserved in the Canadian Appalachian and Albanian Dinaride ophiolite belts. In the Canadian Appalachians, the Southern Quebec ophiolites experienced two Paleozoic orogenies after their obduction onto the Laurentian margin. Although locally obscured by tectonic fabrics and structures, the original relationships between the ophiolitic mantle, the overlying plutonic section, and onlapping Ordovician siliciclastic rocks are well-preserved in the Thetford-Mines ophiolite which also shows similarities with lithological and structural features of the Mirdita ophiolite, where conformable contacts between the sedimentary cover sequence (debris flows and flysch deposits) and the mantle and crustal sections are superbly exposed. The Jurassic Mirdita ophiolite in Albania underlies ca. 4000 km 2 and was only slightly affected by the Tertiary Alpine orogeny of Eastern Europe. Well-exposed sections of the ophiolite suggest that the oceanic mantle was exhumed prior to obduction and locally records the formation of an oceanic

  5. 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

  6. 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

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

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

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

  8. Lithospheric Thickness Modeled From Long Period Surface Wave Dispersion

    NASA Astrophysics Data System (ADS)

    Pasyanos, M. E.

    2007-12-01

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

  9. An analytical model of the free H2 produced by serpentinization within oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Worman, S. L.; Pratson, L.; Darrah, T. H.; Karson, J. A.; Klein, E. M.

    2014-12-01

    The free H2 produced by serpentinization within oceanic lithosphere figures prominently in theories regarding (1) the origin and early evolution of life of earth, (2) the basal fuel-source sustaining the sub-seafloor biosphere, (3) the formation of abiogenic hydrocarbons as well as (4) native metal alloys, (5) the geochemical cycles of various elements, and (5) the future use of H2 as a substitute for fossil fuels. Here we present an analytical model that ties H2 production to seafloor spreading rates and the cooling of oceanic lithosphere. If the assumptions underpinning our simplistic model are largely correct, we estimate that global H2 production by the serpentinization of peridoitite within oceanic-lithosphere may be as large as ~9.7x1012 moles H2/year, which is within one or two orders of magnitude of any previous estimates. The model, however, also predicts where globally H2 production should be greatest as well as how far from the axis of a mid-oceanic ridge H2 production should remain significant. We could not rigorously test our model due to sparse and varied measurements of H2 globally however the model suggests a number of systematic relationships and testable predications for future field-based studies. To demonstrate the potential validity and implications our results, we evaluate where all the free-H2 predicted by the model may go given the limited amount known about the H2 budget within the oceans.

  10. Heat Flow, Lower Crustal Thermochronology, and Transient Geotherms in the Mesozoic Southern African Continental Lithosphere

    NASA Astrophysics Data System (ADS)

    Schmitz, M. D.; Bell, D. R.; Bowring, S. A.

    2002-12-01

    There is a well-established debate regarding the role of higher mantle heat flux for accommodating the elevated average surface heat flow in the Proterozoic orogenic belts relative to the Archean cratonic regions of southern Africa1,2. Advocates of steeper off-craton lithospheric mantle thermal gradients commonly support their arguments with thermobarometric data indicating elevated temperatures at a given depth in off-craton versus cratonic mantle xenolith suites3,4, and together such inferences have cemented a paradigm of differential lithospheric thickness between Proterozoic orogenic belts (thin) and Archean cratons (thick). However, this inherently steady-state interpretation of lithospheric thermomechanical structure is at odds with emerging data pointing toward transient thermal perturbations and irreversible chemical modifications to the southern African lithosphere during the Late Mesozoic5,6. This contribution seeks to illustrate how an essentially "cratonic" thermal state in the Proterozoic lithospheric mantle of southern Africa remains compatible with nominally elevated surface heat flow in the same regions. Model geotherms have been constructed utilizing published surface heat flow, heat production and thermal conductivity measurements1, and seismically-derived crustal thickness estimates7. New heat-producing element concentration data for kimberlite-borne high-pressure granulite xenoliths from the Proterozoic domains constrain lower crustal heat production to be in the range of 0.15 to 0.2 μW/m3. A family of geologically realistic crustal heat production models accommodates surface heat flow >50 mW/m2, while maintaining moderate basal mantle heat flux <15 mW/m2, with associated lithospheric mantle geotherms and thickness indistinguishable from those of cratonic mantle. This family of models is further consistent with constraints on pre-Mesozoic lower crustal paleotemperatures of <450°C imposed by rutile U-Pb thermochronology in the craton

  11. 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.

  12. Eastern Termination of the Subducting African Lithosphere Beneath Anatolia Imaged by Teleseismic P-Wave Tomography

    NASA Astrophysics Data System (ADS)

    Biryol, C. B.; Zandt, G.; Beck, S. L.; Ozacar, A.; Schmandt, B.

    2009-12-01

    A variety of complex tectonic processes are active in Anatolia. Collision related plateau formation dominates the present lithospheric deformation toward the east and slab roll-back related back-arc extension takes place toward 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. 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 is less well constrained due to the lack of geophysical observations. In order to study how the deeper lithosphere and mantle structure evolves spatially from east to west, 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 - 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 15,000 P-wave travel time residuals, measured in multiple frequency bands, are inverted using approximate finite-frequency sensitivity kernels. Our tomographic model reveals 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 east of 36 degree longitude. This eastern edge of the slab also marks the western boundary of the EAP and Arabia-Eurasia collision zone. Beneath EAP our model reveals distributed slow anomalies down to 400 km and upper

  13. 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

  14. Lithosphere structure and subsidence evolution of the conjugate S-African and Argentine margins

    NASA Astrophysics Data System (ADS)

    Dressel, Ingo; Scheck-Wenderoth, Magdalena; Cacace, Mauro; Götze, Hans-Jürgen; Franke, Dieter

    2016-04-01

    The bathymetric evolution of the South Atlantic passive continental margins is a matter of debate. Though it is commonly accepted that passive margins experience thermal subsidence as a result of lithospheric cooling as well as load induced subsidence in response to sediment deposition it is disputed if the South Atlantic passive margins were affected by additional processes affecting the subsidence history after continental breakup. We present a subsidence analysis along the SW African margin and offshore Argentina and restore paleobathymetries to assess the subsidence evolution of the margin. These results are discussed with respect to mechanisms behind margin evolution. Therefore, we use available information about the lithosphere-scale present-day structural configuration of these margins as a starting point for the subsidence analysis. A multi 1D backward modelling method is applied to separate individual subsidence components such as the thermal- as well as the load induced subsidence and to restore paleobathymetries for the conjugate margins. The comparison of the restored paleobathymetries shows that the conjugate margins evolve differently: Continuous subsidence is obtained offshore Argentina whereas the subsidence history of the SW African margin is interrupted by phases of uplift. This differing results for both margins correlate also with different structural configurations of the subcrustal mantle. In the light of these results we discuss possible implications for uplift mechanisms.

  15. 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.

  16. 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.

  17. 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

  18. Characterising Antarctic and Southern Ocean Lithosphere with Magnetic and Gravity Imaging of East Antarctic Rift Systems

    NASA Astrophysics Data System (ADS)

    Vaughan, A. P.; Kusznir, N. J.; Ferraccioli, F.; Jordan, T. A.; Purucker, M. E.; Golynsky, A. V.; Rogozhina, I.

    2012-12-01

    Since the International Geophysical Year (1957), a view has prevailed that the lithospheric structure of East Antarctica is relatively homogeneous, forming a geological block of largely cratonic nature, consisting of a mosaic of Precambrian terranes, stable since the Pan-African orogeny ~500 million years ago. Recent recognition of a continental-scale rift system cutting the East Antarctic interior indicates that this is incorrect, and has crystallised an alternative view of much more recent geological activity with important implications for tectonic reconstructions and controls on ice sheet formation and stability. The newly defined East Antarctic Rift System 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 pioneered by Golynsky & Golynsky indicates that further rift zones may extend the East Antarctic Rift System into widely distributed extension zones within the continent. We have carried out a pilot study, using a newly developed gravity inversion technique with existing public domain satellite data, which shows that East Antarctica consists of distinct crustal thickness provinces with anomalously thick areas separated by thin, possibly rifted crust and overall high average thickness. Understanding the nature of crustal thickness in East Antarctica is critical because: 1) Better understanding of crustal thickness in Antarctica, especially along the ocean-continent transition (OCT), will make it possible to improve the plate reconstruction fit between Antarctica, Australia and India in Gondwana and also refine constraints on how and when these continents separated; 2) crustal thickness provinces can be used to aid supercontinent reconstructions and provide new assessments of the influence of basement architecture and mechanical properties on rifting processes; 3) tracking rift zones through

  19. 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. PMID:19390042

  20. Oceanic lithospheric magnetisation: Forward modelling and analysis using vector spherical harmonics (Invited)

    NASA Astrophysics Data System (ADS)

    Masterton, S. M.; Gubbins, D.; Müller, D.; Williams, S.

    2013-12-01

    The lithospheric contribution to the geomagnetic field arises from magnetised rocks that are cooler than the Curie temperature of their constituent minerals. Inversion of the magnetic field for this magnetisation is subject to inherent non-uniqueness, as many magnetisation distributions yield no potential field outside of the lithosphere. Such distributions are termed annihilators. We use a complete set of orthogonal vector spherical harmonics that separate the part of the magnetisation responsible for the magnetic field observed above the Earth's surface from the annihilators. A similar set of vector harmonics has been developed in Cartesian geometry suitable for small scale, industrial applications. In an attempt to quantify the significance of the annihilators, we first construct a global model of vertically integrated magnetisation (VIM) by combining a model of remanent magnetisation for the oceans with a previous model of induced magnetisation for the whole Earth. Remanence is computed by assigning magnetisations to the oceanic lithosphere acquired at the location and time of formation. The magnetising field is assumed to be an axial dipole that switches polarity with the reversal time scale. The magnetisation evolves with time by decay of thermal remanence and acquisition of chemical remanence. Remanence directions are calculated by implementing finite rotations of the original geomagnetic field direction with respect to an absolute reference frame. We then represent our estimated VIM in terms of vector spherical harmonics, to allow us to evaluate its relative contributions to a potential field that is observable outside of the lithosphere and to fields (both potential and non-potential) that are not observable. This analysis shows that our model of magnetisation is dominated by a part of the magnetisation that produces a potential field restricted to Earth's sub-lithospheric interior; it therefore contributes significantly to the huge null space in the

  1. 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.

  2. A radiogenic Os component in the oceanic lithosphere? Constraints from Hawaiian pyroxenite xenoliths

    NASA Astrophysics Data System (ADS)

    Sen, Indra Sekhar; Bizimis, Michael; Sen, Gautam; Huang, Shichun

    2011-09-01

    Platinum Group Element (PGE) concentrations in garnet pyroxenite xenoliths from Oahu, Hawaii, are significantly lower than those in mantle peridotites and show fractionated patterns (e.g. Pd N/Os N = 2-10, Pd N/Ir N = 4-24; N = chondrite normalized) and very high Re N/Os N ratios (˜9-248). Mass balance calculations show that the bulk rock pyroxenite PGE inventory is controlled by the presence of sulfide phases. The 187Os/ 188Os ratios of these pyroxenites vary from subchondritic to suprachondritic (0.123-0.164); and the 187Os/ 188Os ratios show good correlations with bulk rock and clinopyroxene major and trace element compositions, and bulk rock PGE and sulfur abundances. These observations suggest that the Os isotope compositions in these pyroxenites largely reflect primary processes in the oceanic mantle and Pacific lithosphere. In contrast, bulk rock 187Os/ 188Os ratios do not correlate with other lithophile isotopic tracers (e.g. Rb-Sr, Sm-Nd, Lu-Hf) which show limited isotopic variability ( Bizimis et al., 2005). This and the lack of 187Os/ 188Os vs. Re/Os correlations suggest that the range in Os isotope ratios is not likely the result of mixing between long-lived depleted and enriched components or aging of these pyroxenites within the Pacific lithosphere after its formation at a mid-oceanic ridge setting some 80-100 million years ago. We interpret the Os isotopes, PGE and lithophile element systematics as the result of melt-lithosphere interaction at the base of the Pacific lithosphere. The major and trace element systematics of the clinopyroxenes and bulk rock pyroxenites and the relatively constant lithophile element isotope systematics are best explained by fractional crystallization of a rather homogenous parental magma. We suggest that during melt crystallization and percolation within the lithosphere, the parental pyroxenite melt assimilated radiogenic Os from the grain boundaries of the peridotitic lithosphere. This radiogenic Os component may

  3. 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.

  4. 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

  5. Iron and magnesium isotope fractionation in oceanic lithosphere and sub-arc mantle: Perspectives from ophiolites

    NASA Astrophysics Data System (ADS)

    Su, Ben-Xun; Teng, Fang-Zhen; Hu, Yan; Shi, Ren-Deng; Zhou, Mei-Fu; Zhu, Bin; Liu, Fan; Gong, Xiao-Han; Huang, Qi-Shuai; Xiao, Yan; Chen, Chen; He, Yong-Sheng

    2015-11-01

    We present high-precision Fe and Mg isotopic data for the Purang ophiolite, southwestern Tibet, representing the first combined Fe and Mg isotopic study of the oceanic lithosphere hitherto. The δ56Fe and δ26Mg values of the ophiolitic peridotite, dunite and gabbro vary from -0.209 to 0.187‰ and from -0.28 to - 0.14 ‰, respectively. The average δ56Fe of the peridotites is - 0.030 ± 0.143 ‰ (2SD, n = 17), a value indistinguishable from abyssal peridotites and chondrites, and lower than oceanic basalts. The average δ26Mg value of the peridotites is - 0.20 ± 0.10 ‰, a value slightly higher than both chondrites and oceanic basalts. Correlations between δ56Fe and indices of partial melting indicate fractionation of 0.323‰ in δ56Fe between the oceanic lithospheric mantle and the overlying mafic crust during an early episode of partial melting, presumably beneath a spreading centre. Subsequent metasomatism in a supra-subduction zone caused elevated oxygen fugacity and heavy Fe isotopic compositions in the oceanic lithospheric mantle. The dunite with high Ba/La, a proxy for oxygen fugacity, and high δ56Fe values was likely formed during this process of sub-arc mantle-melt interaction. The negatively coupled Fe-Mg isotopic variations of the Purang ophiolite indicate that Mg isotope fractionation may also occur during high-temperature mantle processes. The observed isotopic variations among different lithologies in the ophiolite may satisfactorily account for the isotopic differences between arc lavas and mantle peridotites with respect to oceanic basalts, thus providing implications for crust-mantle differentiation.

  6. Intraplate earthquakes and the state of stress in oceanic lithosphere

    NASA Technical Reports Server (NTRS)

    Bergman, Eric A.

    1986-01-01

    The dominant sources of stress relieved in oceanic intraplate earthquakes are investigated to examine the usefulness of earthquakes as indicators of stress orientation. The primary data for this investigation are the detailed source studies of 58 of the largest of these events, performed with a body-waveform inversion technique of Nabelek (1984). The relationship between the earthquakes and the intraplate stress fields was investigated by studying, the rate of seismic moment release as a function of age, the source mechanisms and tectonic associations of larger events, and the depth-dependence of various source parameters. The results indicate that the earthquake focal mechanisms are empirically reliable indicators of stress, probably reflecting the fact that an earthquake will occur most readily on a fault plane oriented in such a way that the resolved shear stress is maximized while the normal stress across the fault, is minimized.

  7. A mechanism for decoupling within the oceanic lithosphere revealed in the Troodos ophiolite

    USGS Publications Warehouse

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

    1995-01-01

    Contrasting kinematic histories recorded in the sheeted dykes and underlying plutonic rocks of the Troodos ophiolite provide a new perspective on the mechanical evolution of oceanic spreading centres. The kinematic framework of the decoupling zone that partitions deformation between the sheeted dykes and plutonics contrasts with low-angle detachment models for slow-spreading ridges based on continental-rift analogues. A model for the generation of multiple, horizontal decoupling horizons, linked by planar normal faults, demonstrates new possibilities for the kinematic and rheological significance of seismic reflectors in oceanic lithosphere.

  8. Recovering the effective elastic thickness, Te, of oceanic lithosphere in the presence of long wavelength topography

    NASA Astrophysics Data System (ADS)

    Kalnins, L. M.; Watts, A. B.

    2010-05-01

    We have developed a moving window admittance technique to determine the relationship between free-air gravity anomaly and bathymetry as a function of wavelength over the world's ocean basins and their margins. Preliminary results from the western Pacific Ocean show that the technique resolves the effective elastic thickness of the oceanic lithosphere, Te, to better than ±5 km for Te < 30 km over horizontal distances of a few tens of km. In this paper, we investigate the robustness of our results using different tapering schemes (e.g. single versus multitaper) and synthetic tests that illustrate our ability to recover Te in the region of long wavelength features such as trench outer rises, mid-plate swells and mid-ocean ridges. By investigating observed admittances in the Pacific, Indian, and Atlantic Oceans, we have found that there is a 'critical wavelength' that separates the relatively short wavelength contributions of lithospheric flexure to the gravity field from longer wavelength effects such as those associated with mantle dynamics. We examine here this 'critical wavelength' and its implications for swell compensation depths, plate cooling models, and mantle convection.

  9. 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.

  10. Hf-Nd isotope decoupling in the oceanic lithosphere: constraints from spinel peridotites from Oahu, Hawaii

    NASA Astrophysics Data System (ADS)

    Bizimis, Michael; Sen, Gautam; Salters, Vincent J. M.

    2004-01-01

    We present a detailed geochemical investigation on the Hf, Nd and Sr isotope compositions and trace and major element contents of clinopyroxene mineral separates from spinel lherzolite xenoliths from the island of Oahu, Hawaii. These peridotites are believed to represent the depleted oceanic lithosphere beneath Oahu, which is a residue of a MORB-related melting event some 80-100 Ma ago at a mid-ocean ridge. Clinopyroxenes from peridotites from the Salt Lake Crater (SLC) show a large range of Hf isotopic compositions, from ɛHf=12.2 (similar to the Honolulu volcanics series) to extremely radiogenic, ɛHf=65, at nearly constant 143Nd/ 144Nd ratios ( ɛNd=7-8). None of these samples show any isotopic evidence for interaction with Koolau-type melts. A single xenolith from the Pali vent is the only sample with Hf and Nd isotopic compositions that falls within the MORB field. The Hf isotopes correlate positively with the degree of depletion in the clinopyroxene (e.g. increasing Mg#, Cr#, decreasing Ti and heavy REE contents), but also with increasing Zr and Hf depletions relative to the adjacent REE in a compatibility diagram. The Lu/Hf isotope systematics of the SLC clinopyroxenes define apparent ages of 500 Ma or older and these compositions cannot be explained by mixing between any type of Hawaiian melts and the depleted Pacific lithosphere. Metasomatism of an ancient (e.g. 1 Ga or older) depleted peridotite protolith can, in principle, explain these apparent ages and the Nd-Hf isotope decoupling, but requires that the most depleted samples were subject to the least amount of metasomatism. Alternatively, the combined isotope, trace and major element compositions of these clinopyroxenes are best described by metasomatism of the 80-100 Ma depleted oceanic lithosphere by melts products of extensive mantle-melt interaction between Honolulu Volcanics-type melts and the depleted lithosphere.

  11. On-, off-cratonic, orogenic and oceanic mantle roots: lithosphere unite

    NASA Astrophysics Data System (ADS)

    Wittig, Nadine; Pearson, D. Graham

    2010-05-01

    Associated with Earth's crust is the underlying lithospheric mantle, which is often categorized according its surface availability for sampling. For example, we frequently describe sub-continental lithospheric peridotite xenoliths as on- or off-cratonic to highlight the approximate lithosphere stabilization age as being Archean or post-Archean and contrast them against the lithospheric peridotites recovered from present-day oceanic basins. Orogenic peridotites and ophiolites are obducted in subduction zones and mark paleo-suture zones in amalgamated continents. Regardless of its present-day occurrence, lithospheric mantle is buoyant as a result of substantial extraction or mafic to ultramafic silicate melts. This depletion and buoyancy results in a thermal and mechanical boundary layer (lithosphere-asthenosphere boundary layer, LAB) and isolates Earth's lithosphere from the convecting mantle. The mineralogical and geochemical similarities of some of the oldest cratonic sub-continental lithosphere and highly-depleted young oceanic peridotites require common geochemical mechanisms that are capable of introducing well-correlated major element variability - until metasomatic enrichment masks the depletion signature. Efficient extraction of basaltic melts from the convecting mantle occurs at mid-ocean ridges and removes clinopyroxene and to some extent garnet leaving the olivine-rich buoyant residual lithospheric mantle. We will examine the oceanic heritage of garnet and spinel-facies sub-continental lithospheric mantle in general, but focus on xenoliths (n = 62) sampled across the North Atlantic Craton (~700km, NAC), West Greenland at c. 600 and 200 Ma. These Greenlandic samples are strongly serpentinized, harzburgitic to dunitic peridotites and generally comprise less than 15% orthopyroxene and very little clinopyroxene (<< 5%). If garnet is present it occurs in variable amounts (12 to 0.3%). Major element systematics of these peridotites are highly refractory with Al

  12. Origin of long-wavelength magnetic anomalies associated with subducting oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Williams, S.; Gubbins, D.; Müller, D.; Tetley, M. G.

    2015-12-01

    Many subduction zones correspond to strong long-wavelength magnetic signatures that are not predicted by forward models in which magnetisation is confined to the continental crust and unsubducted oceanic lithosphere. Previous studies have proposed this signature to be sourced within hydrated, serpentinised mantle wedge. Alternatively, thermal models of subducting oceanic lithosphere suggest slab material cool enough to hold magnetisation may persist >200 km below the Earth's surface. Here we investigate the origin of long wavelength magnetic anomalies by constructing alternative predictions of the lithospheric geomagnetic field, which we compare to the MF7 model based on satellite observations. We compare 2 models for subduction zone magnetisation, one in which the wedge is magnetised and one in which the subducted slab is magnetised. Magnetisation deeper than the Moho of the overriding plate helps to reconcile the observed anomalies for a number of subduction zones, including Sumatra, the Aleutians, and along the Pacific margin of North and Central America. North-south trending arcs, especially those near to the magnetic equator, have magnetisations within the null space that produces little or no external magnetic field, and are less distinct in the observations. A magnetic anomaly persists along the Baja and Mexican west coast, indicating that the fossil subduction zone continues from California through Central America despite limited seismic evidence of a slab there. Distinguishing between the two competing models is difficult because it depends critically on the trend in magnetic anomaly away from the trench.

  13. 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.

  14. 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

  15. 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.

  16. 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.

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

    PubMed

    Qin, Yanfang; Singh, Satish C

    2015-01-01

    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. PMID:26365624

  18. Geodynamic models of continental subduction and obduction of overriding plate forearc oceanic lithosphere on top of continental crust

    NASA Astrophysics Data System (ADS)

    Edwards, Sarah J.; Schellart, Wouter P.; Duarte, Joao C.

    2015-07-01

    Continental subduction takes place in the final stage of subduction when all oceanic lithosphere is consumed and continental passive margin is pulled into the mantle. When the overriding plate is oceanic, dense forearc oceanic lithosphere might be obducted onto light continental crust forming an ophiolite (Tethyan-style ophiolite obduction). Four-dimensional dynamic analog subduction models have been constructed to evaluate the mechanical feasibility of continental subduction and forearc oceanic lithosphere obduction on top of continental crust. The roles of continental crust thickness, passive margin length, subducting lithosphere thickness, and overriding plate thickness were investigated to determine the maximum continental subduction depth, maximum forearc obduction distance, and forearc deformation during continental subduction. Our buoyancy-driven experiments indicate that deep continental subduction occurs in most circumstances (down to ~560 km) and that obduction of dense oceanic forearc lithosphere on top of light continental crust is mechanically feasible. Maximum obduction distances are relatively small (~26-37 km) but are sufficient to explain obduction of short ophiolite sheets, such as observed in New Caledonia. When including the thin (5-10 km thick) accretionary wedge of off-scraped deep sea sediments, oceanic crust, and mantle, then maximum obduction distances are much larger, ~60-160 km, sufficient to account for the obducted Northland Allochthon in New Zealand. Results indicate that increasing continental crust thickness decreases continental subduction depth, whereas increasing passive margin length and subducting lithosphere thickness increases continental subduction depth. Notably, during continental subduction, backarc extension continues, while forearc deformation (shortening) increases moderately compared to the preceding phase of normal (oceanic) subduction.

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

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

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

  20. Inversion of Oceanic Lithosphere Density Structure in the South China Sea by Satellite Gravity

    NASA Astrophysics Data System (ADS)

    Zhang, S.

    2013-12-01

    In this paper, the satellite altimetry gravity anomaly is used to calculate the Bouguer gravity by Seafloor terrain correction in the South China Sea. The faults distribution and basin boundary is extracted to study the geological structure unit. The 3D density inversion method is applied to invert the oceanic lithosphere density structure. In the 3D density inversion method, the depth weighting function is an experience function derived by the relation between gravity anomaly and depth. It is difficult to artificially choose reasonable values of the weighting factors which have great effect on the inversion result. The power spectrum of the gravity anomaly is used to estimate the depth of the geological body, and the function of the weighting factors and the gravity anomaly is built. The weighting factors can be estimated automatically by the gravity anomaly power spectrum. Therefore, the artificial error can be avoided and the accuracy of inversion is improved. In the density inversion, the Bouguer gravity is firstly decomposed into different scales by wavelet transform method. The gravity anomaly in different scales is related to the density distribution in different depth. Then, the power spectrum is calculated by gravity anomaly in different scales to estimate the weighting factors. Finally, the ocean lithosphere density structure in the South China Sea is inverted and compared with its velocity structure.

  1. Sequential, Multi-stage Processes for Intraplate and Intraslab Seismogenesis in Oceanic Lithosphere

    NASA Astrophysics Data System (ADS)

    Kirby, S. H.

    2011-12-01

    Marine geoscience surveys and thermal modeling have shown that oceanic lithosphere has an extremely complex tenure under the ocean basins and during slab descent involving magmatic, hydrothermal, and tectonic processes, processes that undoubtedly leave a cumulative imprint on plate/slab mineralogy, structure, and fluid makeup. These processes clearly have implications for the physical states of plate materials from ridges to trenches to deep slabs and give insights into the geological and geophysical observables, especially intraplate and intraslab earthquakes. Likely stages of this inheritance include: (1) Shallow melting and stretching deformation at Mid Ocean Ridges (MOR's) produce brittle faulting and earthquakes as well as hydrothermal alteration, segregation into oceanic crust and mantle, and internal plate deformation as indicated by seismic anisotropy. Hydrothermal alteration of faults cutting oceanic crust at MOR's have been posited as possible sources of dehydration fluids or dehydration embrittlement near the original seafloor during subsequent slab descent as an explanation of some intermediate-depth earthquakes. (2) Ocean Island (Plume) Volcanism produce an additional component of magmatic CO2 of deeper origin as well as plate deformation due to mass loading by the island edifices. [Kirby presentation in Session T13, this meeting]. (3) Intraplate deformation especially near triple junctions or other settings where discontinuous lateral boundary stresses propagate into plate interiors. (4) Near-trench deformation associated with plate bending that is expressed by shallow normal-faulting earthquakes, fault scarps on the ocean floor off trenches, and seismic reflections of normal faults, all of which locally indicate that such faulting penetrates well into the lithospheric mantle. It seems likely that even though the thermal conditions in most oceanic plates near trenches are relatively cold, mineral alteration along normal-fault pathways from ingress of

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

    NASA Astrophysics Data System (ADS)

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

    2004-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2007-12-01

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

  4. Anomalous deep earthquakes beneath the East African Rift: evidence for rift induced delamination of the lithosphere?

    NASA Astrophysics Data System (ADS)

    Lindenfeld, Michael; Rümpker, Georg; Schmeling, Harro; Wallner, Herbert

    2010-05-01

    The over 5000 m high Rwenzori Mountains are situated within the western branch of the East African Rift System, at the border between Uganda and the Democratic Republic of Congo. They represent a basement block within the rift valley whose origin and relation to the evolution of the EARS are highly puzzling. During 2006/2007 a network of 27 seismological stations was operated in this area to investigate crustal and upper mantle structure in conjunction with local seismicity. The data analysis revealed unexpectedly high microseismic activity. On average more than 800 events per month could be located with magnitudes ranging from 0.5 to 5.1. Hypocentral depths go as deep as 30 km with a pronounced concentration of activity at a depth of about 15 km. This presentation focuses on a cluster of seven earthquakes that were located at anomalous depths between 53 and 60 km. According to our present knowledge these are the deepest events so far observed within the EARS and the African Plate. Their origin might be connected to magmatic intrusions. However, the existence of earthquakes at this depth is enigmatic, especially within a rifting regime were one expects hot and weak material close to the surface, which is not capable of seismogenic deformation. We think that these events are closely related to the evolution of the Rwenzoris. A recent hypothesis to explain the extreme uplift of the Rwenzori Mountains is rift induced delamination (RID) of mantle lithosphere that is captured between two approaching rift segments. By numerical modelling we show that the RID-process is also able to bring material that is cold and brittle enough to release seismic energy into greater depth. Therefore the RID-mechanism gives a consistent explanation for the detected deep events as well as for the uplift of a mountain block in a rift setting.

  5. Flexure of oceanic lithosphere along the north Algerian margin and tectonic implications

    NASA Astrophysics Data System (ADS)

    Hamai, Lamine; Petit, Carole; Abtout, Abdeslem; Yelles-chaouche, Abdekrim; Déverchère, Jacques; Klingelhoefer, Frauke

    2015-04-01

    The Algerian margin resulting from a back-arc basin Algerian opening associated with a withdrawal from the Tethyan break. Recent geophysical data acquired in the Algerian basin (MARADJA campaigns, 2003, 2005 (Active Margin of "el DJAzair" and SPIRAL, 2009 (Deep Seismic and Regional Investigations in Algeria) have identified signs of active or recent compressive deformation in the basin. New data from four wide-angle seismic profiles helped us image, for the first time, the deep structure of the Algerian margin and adjacent basins.We converted these velocity profiles into density models, then isostatic anomalies. This allowed us to image an isostatic imbalance (relative to a local isostasy model) scored at the margin toe. If we interpret this as the Moho depth variation over a depth of balance, then the Moho in oceanic part is too deep and Moho in continental part too shallow, on either side of a boundary located towards the margin toe. These abnormalities can be interpreted by a flexural mechanism in the presence of two lithospheres. We use a finite element formulation to model the lithospheric flexure at the plate boundary. The profiles are discretized into two distinct plates for the continental and oceanic parts, which allow us to apply opposite flexures on both domains. The two plates can be separated by a transition zone of variable width where no flexure is computed. We have focused our study on the oceanic part because the continental domain is much less constrained owing to the lack of wide-angle data. The deflection of the plates oppositely to their point of junction are set to zero, and the amount of deflection computed from the equivalent Moho deflection is applied at both ends of each plate. Preliminary interpretation of this modeling shows a flexural difference between central and lateral profiles, this is probably due to the difference in geometry margin segments and there evolution in the geodynamic and kinematic context.

  6. 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.

  7. 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

  8. Crustal Thickness Mapping of the Rifted Margin Ocean-Continent Transition using Satellite Gravity Inversion Incorporating a Lithosphere Thermal Correction

    NASA Astrophysics Data System (ADS)

    Hurst, N. W.; Kusznir, N. J.

    2005-05-01

    A new method of inverting satellite gravity at rifted continental margins to give crustal thickness, incorporating a lithosphere thermal correction, has been developed which does not use a priori information about the location of the ocean-continent transition (OCT) and provides an independent prediction of OCT location. Satellite derived gravity anomaly data (Sandwell and Smith 1997) and bathymetry data (Gebco 2003) are used to derive the mantle residual gravity anomaly which is inverted in 3D in the spectral domain to give Moho depth. Oceanic lithosphere and stretched continental margin lithosphere produce a large negative residual thermal gravity anomaly (up to -380 mgal), which must be corrected for in order to determine Moho depth. This thermal gravity correction may be determined for oceanic lithosphere using oceanic isochron data, and for the thinned continental margin lithosphere using margin rift age and beta stretching estimates iteratively derived from crustal basement thickness determined from the gravity inversion. The gravity inversion using the thermal gravity correction predicts oceanic crustal thicknesses consistent with seismic observations, while that without the thermal correction predicts much too great oceanic crustal thicknesses. Predicted Moho depth and crustal thinning across the Hatton and Faroes rifted margins, using the gravity inversion with embedded thermal correction, compare well with those produced by wide-angle seismology. A new gravity inversion method has been developed in which no isochrons are used to define the thermal gravity correction. The new method assumes all lithosphere to be initially continental and a uniform lithosphere stretching age is used corresponding to the time of continental breakup. The thinning factor produced by the gravity inversion is used to predict the thickness of oceanic crust. This new modified form of gravity inversion with embedded thermal correction provides an improved estimate of rifted

  9. 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

  10. “Imaging” the cross section of oceanic lithosphere: The development and future of electrical microresistivity logging through scientific ocean drilling

    NASA Astrophysics Data System (ADS)

    Tominaga, Masako

    2013-11-01

    A detailed understanding of the architecture of volcanic and magmatic lithologies present within the oceanic lithosphere is essential to advance our knowledge of the geodynamics of spreading ridges and subduction zones. Undertaking sub-meter scale observations of oceanic lithosphere is challenging, primarily because of the difficulty in direct continuous sampling (e.g., by scientific ocean drilling) and the limited resolution of the majority of geophysical remote sensing methods. Downhole logging data from drillholes through basement formations, when integrated with recovered core and geophysical remote sensing data, can provide new insights into crustal accretion processes, lithosphere hydrogeology and associated alteration processes, and variations in the physical properties of the oceanic lithosphere over time. Here, we introduce an alternative approach to determine the formation architecture and lithofacies of the oceanic sub-basement by using logging data, particularly utilizing downhole microresistivity imagery (e.g. Formation MicroScanner (FMS) imagery), which has the potential to become a key tool in deciphering the high-resolution internal architecture of the intact upper ocean crust. A novel ocean crust lithostratigraphy model based on meticulously deciphered lava morphology determined by in situ FMS electrofacies analysis of holes drilled during Ocean Drilling Program legs (1) advances our understanding of ocean crust formation and accretionary processes over both time and space; and (2) allows the linking of local igneous histories deciphered from the drillholes to the regional magmatic and tectonic histories. Furthermore, microresistivity imagery can potentially allow the investigation of (i) magmatic lithology and architecture in the lower ocean crust and upper mantle; and, (ii) void space abundances in crustal material and the determination of complex lithology-dependent void geometries.

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

    NASA Astrophysics Data System (ADS)

    Goodliffe, Andrew Mark

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

  12. 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

  13. A New Estimate for Global Hydrothermal Exchange Between the Oceans and Lithosphere

    NASA Astrophysics Data System (ADS)

    Hasterok, D. P.

    2013-12-01

    We revise the estimated global power deficit due to ventilated hydrothermal circulation (8.0 TW) using an updated global heat flow dataset with >14000 oceanic measurements and a new conductive cooling model of the oceanic lithosphere. This study differs from previous estimates by taking into account (1) non-Gaussian statistics, (2) an improved seafloor age model, (3) a new plate cooling model calibrated directly to heat flow, and (4) the effect of sediment cover on the heat flow deficit and ventilated cutoff age. We obtain the maximum heat flow deficit (difference between predicted and observed) when the data are separated by seafloor areas with <400 m and 400 m of sediment cover. The estimated power deficit (integrated heat flow deficit with respect to area) for areas of thin (<400 m) sediment cover is 7.8 TW and for areas of thick (400 m) is 0.2 TW. The total power deficit, 8.0 TW with 50% of estimates falling between 5.0 and 10.0 TW, represents a 30% reduction in magnitude compared with previous heat flow and fluid flow based estimates. Regions with thick, 400 m, sediment cover experience half the heat flow deficit for one-third of the duration (25 Ma) of regions with thin sediment cover (75 Ma). Based on this study, vigorous fluid exchange between the oceans and seafloor redistributes 30% of heat lost through young oceanic crust. Spatial variation in heat flow deficit relative to estimated conductive heat loss. Variations are calibrated to observed heat flow and constrained by sediment thickness variations.

  14. Proxies of oceanic Lithosphere/Asthenosphere Boundary from Global Seismic Anisotropy Tomography

    NASA Astrophysics Data System (ADS)

    Burgos, Gael; Montagner, Jean-Paul; Beucler, Eric; Trampert, Jeannot; Capdeville, Yann

    2013-04-01

    Surface waves provide essential information on the knowledge of the upper mantle global structure despite their low lateral resolution. This study, based on surface waves data, presents the development of a new anisotropic tomographic model of the upper mantle, a simplified isotropic model and the consequences of these results for the Lithosphere/Asthenosphere Boundary (LAB). As a first step, a large number of data is collected, these data are merged and regionalized in order to derive maps of phase and group velocity for the fundamental mode of Rayleigh and Love waves and their azimuthal dependence (maps of phase velocity are also obtained for the first six overtones). As a second step, a crustal a posteriori model is developped from the Monte-Carlo inversion of the shorter periods of the dataset, in order to take into account the effect of the shallow layers on the upper mantle. With the crustal model, a first Monte-Carlo inversion for the upper mantle structure is realized in a simplified isotropic parameterization to highlight the influence of the LAB properties on the surface waves data. Still using the crustal model, a first order perturbation theory inversion is performed in a fully anisotropic parameterization to build a 3-D tomographic model of the upper mantle (an extended model until the transition zone is also obtained by using the overtone data). Estimates of the LAB depth are derived from the upper mantle models and compared with the predictions of oceanic lithosphere cooling models. Seismic events are simulated using the Spectral Element Method in order to validate the ability of the anisotropic tomographic model of the upper mantle to re- produce observed seismograms.

  15. 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

  16. 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

  17. Intra-slab Fluid Flow and Eclogite-facies Metasomatism in Subducted Oceanic Lithosphere

    NASA Astrophysics Data System (ADS)

    Angiboust, Samuel; Pettke, Thomas; Agard, Philippe; De Hoog, Cees-Jan; Collins, Nathan; Oncken, Onno

    2013-04-01

    The Monviso ophiolite Lago Superiore Unit (LSU) constitutes a well-preserved, almost continuous fragment of upper oceanic lithosphere subducted down to ca. 80 km (between 50 and 40 Ma) and later exhumed along the subduction interface. The LSU is made of (i) a variably thick (50-500 m) section of eclogitized mafic crust (associated with minor calcschist lenses) overlying a 100-400 m thick metagabbroic body, and of (ii) a serpentinite sole (ca. 1000 m thick). This section is cut by two 10 to 100m thick eclogite-facies shear zones, found at the boundary between basalts and gabbros (Intermediate Shear Zone), and between gabbros and serpentinites (Lower Shear Zone: LSZ). Fragments of mylonitic basaltic eclogites and calcschists were dragged and dismembered within serpentinite schists along the LSZ during eclogite-facies deformation. Metasomatic rinds formed on these fragments at the contact with the surrounding antigorite schists during lawsonite-eclogite facies metamorphism, testifying to prominent fluid-rock interaction along with deformation. We present new petrological and geochemical data on four types of metasomatically altered eclogites (talc-, chlorite-, lawsonite- and phengite-bearing eclogites) and on a (serpentinite-derived) magnesite-bearing talc schist from the block rind. Bulk-rock compositions, in situ LA-ICP-MS analysis and X-ray maps of garnet demonstrate that (i) these samples underwent significant Cr, Mg, Ni enrichment and Fe, Al, V depletion during eclogitic metasomatism and (ii) garnet composition show strong variation from core to rim. These compositional patterns point to a massive, fluid-mediated element transfer along with deformation, originating from the surrounding serpentinite (locally, with possible contributions from metasediments-equilibrated fluids). Antigorite breakdown, occurring ca. 15 km deeper than the maximum depth reached by these eclogites, could have provided significant amounts of fluid promoting extensive fluid

  18. Localization of geoid anomalies and the evolution of oceanic lithosphere: A case study from the Mendocino Fracture Zone

    NASA Astrophysics Data System (ADS)

    Korenaga, J.; Cadio, C.

    2012-12-01

    The thermal evolution of oceanic lithosphere is investigated by focusing on geoid offsets occurring across the Mendocino fracture zone, where plates with different ages are juxtaposed. Various processing techniques have been devoted to separate the geoid signal of the age contrast from regional and shorter wavelength components unrelated to the thermal structure of lithosphere. Nevertheless, due to processing differences, estimates of geoid offsets vary, and no agreement on the thermal evolution of oceanic lithosphere has been found so far. In this study, we propose to use a continuous wavelet analysis to accurately characterize the components of the geoid at different spatial scales, and to estimate a new geoid slope-age relationship from localized signals. We also apply the same wavelet transform on a set of synthetic geoid calculated with different assumptions on plate cooling. The comparison of the observed geoid offsets with those predicted from cooling models indicates that our approach can successfully remove unwanted regional contributions and isolate the geoid signature due to lithospheric cooling. Our results suggest that, contrary to previous studies, geoid slopes measured at the Mendocino Fracture zone are compatible with both the half-space cooling model and the plate model.

  19. Localization of geoid anomalies and the evolution of oceanic lithosphere: A case study from the Mendocino Fracture Zone

    NASA Astrophysics Data System (ADS)

    Cadio, C.; Korenaga, J.

    2012-10-01

    The thermal evolution of oceanic lithosphere is investigated by focusing on geoid offsets occurring across the Mendocino Fracture Zone, where plates with different ages are juxtaposed. Various processing techniques have been devoted to separate the geoid signal of the age contrast from regional and shorter wavelength components unrelated to the thermal structure of lithosphere. Nevertheless, due to processing differences, estimates of geoid offsets vary, and no agreement on the thermal evolution of oceanic lithosphere has been found so far. In this study, we propose to use a continuous wavelet analysis to accurately characterize the components of the geoid at different spatial scales and to estimate a new geoid slope-age relationship from localized signals. We also apply the same wavelet transform on a set of synthetic geoid calculated with different assumptions on plate cooling. The comparison of the observed geoid offsets with those predicted from cooling models indicates that our approach can successfully remove unwanted regional contributions and isolate the geoid signature due to lithospheric cooling. Our results suggest that, contrary to previous studies, geoid slopes measured at the Mendocino Fracture Zone are compatible with both the half-space cooling model and the plate model.

  20. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  2. Lateral Temperature Variations in Upwelling Limbs of the Asthenosphere and its Implications for Thermal Models of the Oceanic Lithosphere

    NASA Astrophysics Data System (ADS)

    Hamza, V. M.; Cardoso, R. R.

    2008-05-01

    Thermal models of the lithosphere proposed to date (the Half-Space Cooling and Plate models) have failed to provide satisfactory accounts of some of the important features in large-scale variations of oceanic heat flow. The systematic difference between model values 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 model assumption that lateral temperature variations are absent in the sublithospheric mantle. We propose a new thermal model of the oceanic lithosphere that can overcome such inconsistencies. Designated CMI, the new model assumes existence of lateral temperature variations in up-welling limbs of the asthenosphere, similar in character to those commonly observed in tectonothermal processes in the upper crust and in laboratory experiments of thermal plumes. CMI model simulations indicate 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. As a result, the width of magma injection zone at mid-ocean ridges is relatively narrower in CMI model. Another noteworthy feature of the new model is its ability to provide vastly improved fits for observational heat flow data, in both young (ages less than 55 ma) and old (ages greater than 55 ma) oceanic lithosphere. More importantly, the improved fits to heat flow have been achieved without the need to invoke the ad-hoc hypothesis of large-scale hydrothermal circulation in stable ocean crust. Also, use of CMI model does not lead to artificial discontinuities in the temperature field of the lithosphere, as is the case with GDH reference models. The results of the CMI model provide a better understanding of the global heat flow variations and estimates of global heat loss. In particular, the model is capable of

  3. Thermal evolution of the oceanic lithosphere revisited with the help of an upper-mantle velocity model

    NASA Astrophysics Data System (ADS)

    Goutorbe, Bruno

    2010-05-01

    There are presently several candidate models for the thermal structure and evolution of the oceanic lithosphere, which essentially differ by the imposed basal condition. The latter condition represents the additional heat brought at the base of lithosphere (should it exist), and it can take the form of a fixed basal temperature or a fixed basal heat flow. I investigate this problem with the help of a shear velocity model of the upper mantle. Velocities of the lithospheric mantle are converted to temperatures using up-to-date constraints related to the velocity-temperature relationship (namely, constraints on thermoelastic parameters, mantle composition and attenuation factor), and an averaging by age interval is then applied. A particular care is dedicated to the estimation of final uncertainties on temperature, to allow for a quantitative interpretation. This work reconfirms that heat is brought by some mean at the base of the lithosphere, as a half-space cooling thermal evolution falls well below the seismically derived temperatures and their uncertainties. However, I do not observe within the age range (0-160 My) an asymptotic thickness for the lithosphere, which is at odds with the widely used plate model, whereby the additional heat supply is represented by a fixed temperature at some depth. Adding bathymetry and surface heat flow as joint constraints, I find that a model which prescribes a constant heat flow at some isotherm (the so-called Chablis model) provides a better fit to the data than the plate model. Only a strongly reduced thermal expansivity (reduction by at least 30 per cent with respect to the experimental value) allows the latter model achieving a joint-fitting comparable to the former model, and then with a fit to temperature that remains poor. The good fit of the plate model thus relies on a thermal expansivity reduced down to the lowest limit and on ocean depth, whose behaviour at old ages is considerably obscured by anomalous crust. The

  4. 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. PMID:11206543

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

    NASA Astrophysics Data System (ADS)

    Bourgeois, O.; Dauteuil, O.

    2010-12-01

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

  6. 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.

  7. 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

  8. Multi-scale Seismic Waveform Tomography and the Evolution of Oceanic Lithosphere

    NASA Astrophysics Data System (ADS)

    Auer, L.; Boschi, L.; Becker, T. W.; van Driel, M.; Stähler, S. C.; Nissen-Meyer, T.; Sigloch, K.

    2014-12-01

    The advent of high-resolution seismometer array deployments such as USArray, IberArray or the upcoming AlpArray pave the way for significantly enhanced tomographic resolution across their tectonically complex target areas. The optimal interpretation of these datasets requires a new generation of multiple-resolution tomographic imaging approaches. Our recent anisotropic S-wave tomography model SAVANI is adaptive and multi-scale in the sense that it relies on a data-driven adaptive parameterization scheme, which automatically adjusts grid size to local ray sampling density, where demanded by the data. Such automatic rescaling of the parameterization grid provides an efficient means to stepwise improve upon a global background model by updating the tomographic system whenever new observations become available. Since our method employs phase and dispersion measurements from the complete time- and frequency range of the seismic record, it is akin to other types of waveform inversion and sensitive across the entire depth extent of the mantle. We present our current work towards an update of the purely ray-theoretical first version of SAVANI, which involves the reinterpretation of the regional portion of our global dataset using more accurate full-waveform based sensitivity functions that should facilitate an adequate extraction of high-resolution regional structure in areas where data coverage permits it. Adaptive-resolution tomography models, as developed with our algorithm, honour the multi-scale nature of mantle convection, and have various advantages over purely global models when applied in the study of global and regional geodynamics. We focus here on upper mantle dynamics and the evolution of the oceanic lithosphere. Importantly, we find a distinct decorrelation between anisotropy patterns as observed in our tomography and conceptual half-space cooling models or dynamic predictions of anisotropic texture, respectively. This observation implies that sub-oceanic

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  10. Magma explains low estimates of lithospheric strength based on flexure of ocean island loads

    NASA Astrophysics Data System (ADS)

    Buck, W. Roger; Lavier, Luc L.; Choi, Eunseo

    2015-04-01

    One of the best ways to constrain the strength of the Earth's lithosphere is to measure the deformation caused by large, well-defined loads. The largest, simple vertical load is that of the Hawaiian volcanic island chain. An impressively detailed recent analysis of the 3D response to that load by Zhong and Watts (2013) considers the depth range of seismicity below Hawaii and the seismically determined geometry of lithospheric deflection. These authors find that the friction coefficient for the lithosphere must be in the normal range measured for rocks, but conclude that the ductile flow strength has to be far weaker than laboratory measurements suggest. Specifically, Zhong and Watts (2013) find that stress differences in the mantle lithosphere below the island chain are less than about 200 MPa. Standard rheologic models suggest that for the ~50 km thick lithosphere inferred to exist below Hawaii yielding will occur at stress differences of about 1 GPa. Here we suggest that magmatic accommodation of flexural extension may explain Hawaiian lithospheric deflection even with standard mantle flow laws. Flexural stresses are extensional in the deeper part of the lithosphere below a linear island load (i.e. horizontal stresses orthogonal to the line load are lower than vertical stresses). Magma can accommodate lithospheric extension at smaller stress differences than brittle and ductile rock yielding. Dikes opening parallel to an island chain would allow easier downflexing than a continuous plate, but wound not produce a freely broken plate. The extensional stress needed to open dikes at depth depends on the density contrast between magma and lithosphere, assuming magma has an open pathway to the surface. For a uniform lithospheric density ρL and magma density ρM the stress difference to allow dikes to accommodate extension is: Δσxx (z) = g z (ρM - gρL), where g is the acceleration of gravity and z is depth below the surface. For reasonable density values (i.e.

  11. Seismic pumping and cataclastic flow - important transport mechanisms during hydration and metasomatism of the oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Austrheim, H.; Jung, H.; Prestvik, T.

    2009-04-01

    Transform faults are the locus of earthquakes that extend down to 29 km below the seafloor (Choy and McGarr 2002). These faults are also associated with a mass deficiency (Gregg et al. 2007) possibly caused by extensive serpentinization. The mantle and crustal sections of the Leka ophiolite is characterized by deformation at all scales including faults and shear zones. Ongoing investigations of these structures reveal that olivine is strongly fractured and faulted. Tabular fragments of olivine are rotated relative to each other locally forming bookshelf-like fault pattern. Serpentine and diopside fill the space between the rotated fragments, and the texture suggests that these minerals grew in opening caused by the faults. The growth of secondary minerals indicated high fluid fluxes along these deformation zones. The space created by the faulting is estimated to ca 50 %.The main slip surfaces are decorated by banded serpentine. Each band represents a slip event and more than 10 slip events are recorded across a 1 cm broad fault zone. The faulting and fragmentation of the olivine, the opening of space between the fragments, combined with mineral growth and repeated faulting provides an efficient seismic (?) pumping system. Rodingites with textures indicating cataclastic flow are developed along some of the faults. The rodingites also show evidence of repeated deformations and growth of minerals such as vesuvianite, grossular and diposide. It is suggested that faulting including seismic pumping and cataclastic flow is an efficient mechanism to hydrate the oceanic lithosphere and cause serpentinization and metasomatism along transform/fracture zones. Choy and McGarr 2002. Geophys J. Int 150 506-523 Gregg et al. 2007. Nature 448, 183-187

  12. Seismic structure of the crust and lithospheric mantle of the southern African cratonic region

    NASA Astrophysics Data System (ADS)

    Youssof, M.; Thybo, H.; Artemieva, I. M.; Levander, A.

    2013-12-01

    We present a new seismic model for the structure of the crust and lithospheric mantle in southern Africa constrained by a joint study of seismic receiver functions and finite-frequency tomography, using the high-quality data from the South Africa Seismic Experiment (SASE). A) The crust has a highly heterogeneous structure with short wavelength variations in (i) thickness, (ii) composition (reflected in Vp/Vs-ratio calculated for all SASE stations), and (iii) Moho sharpness (which is quantified and mapped for the entire region) (Youssof et al., Tectonophysics, in review). By mapping these three parameters, we distinguish ~20 crustal blocks that do not everywhere coincide with surface tectonic features. Our RFs also demonstrate strong azimuthal anisotropy in the crust, with a typical crustal contribution to the total S-wave splitting of at least 30%. Spatial correlation of the S-wave polarization directions of crustal and mantle anisotropy indicates (i) the presence of three distinct Archean lithospheric terranes and (ii) coupling between the crust and lithospheric mantle in most of the study area, with a strong decoupling in western Kaapvaal where the crustal anisotropy is strongest. The similarity of anisotropy directions in the crust and mantle beneath much of the Kaapvaal craton indicates that (a) the seismic anisotropy originates at the time of cratonization and (b) the observed correspondence between the present direction of absolute plate motion (APM) and lithosphere anisotropy is coincidental. B) A new 3D high-resolution seismic model of the lithospheric mantle has been determined from finite frequency tomographic inversions of teleseismic P- and S- body wave data. The two velocity models are very similar in structure, but differ in the relative P- and S-wave velocity anomalies. We find that: 1) the fast lithospheric keels extends very deep, perhaps to depths of 300-350 km and 250 km beneath the Kaapvaal and Zimbabwe cratons, respectively, and 2) the Archean

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

    NASA Astrophysics Data System (ADS)

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

    2005-04-01

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

  14. 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

  15. 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.

  16. Lithospheric structures and Precambrian terrane boundaries in northeastern Botswana revealed through magnetotelluric profiling as part of the Southern African Magnetotelluric Experiment

    NASA Astrophysics Data System (ADS)

    Miensopust, M. P.; Jones, A. G.; Muller, M. R.; Garcia, X.; Evans, R. L.

    2011-02-01

    Within the framework of the Southern African Magnetotelluric Experiment a focused study was undertaken to gain improved knowledge of the lithospheric geometries and structures of the westerly extension of the Zimbabwe craton (ZIM) into Botswana, with the overarching aim of increasing our understanding of southern African tectonics. The area of interest is located in northeastern Botswana, where Kalahari sands cover most of the geological terranes and very little is known about lithospheric structures and thicknesses. Some of the regional-scale terrane boundary locations, defined based on potential field data, are not sufficiently accurate for local-scale studies. Investigation of the NNW-SSE orientated, 600 km long ZIM line profile crossing the Zimbabwe craton, Magondi mobile belt, and Ghanzi-Chobe belt showed that the Zimbabwe craton is characterized by thick (˜220 km) resistive lithosphere, consistent with geochemical and geothermal estimates from kimberlite samples of the nearby Orapa and Letlhakane pipes (˜175 km west of the profile). The lithospheric mantle of the Ghanzi-Chobe belt is resistive, but its lithosphere is only about 180 km thick. At crustal depths a northward dipping boundary between the Ghanzi-Chobe and the Magondi belts is identified, and two middle to lower crustal conductors are discovered in the Magondi belt. The crustal terrane boundary between the Magondi and Ghanzi-Chobe belts is found to be located further to the north, and the southwestern boundary of the Zimbabwe craton might be further to the west, than previously inferred from the regional potential field data.

  17. 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

  18. 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.

  19. 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

  20. 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. PMID:10504137

  1. Oceanization of the lithospheric mantle: the study case of the spinel peridotites from Monte Maggiore (Corsica, France).

    NASA Astrophysics Data System (ADS)

    Piccardo, G. B.

    2009-04-01

    (Rampone, 2004; Rampone et al., 2008; 2009) provide reliable geochronological informations (i.e. Sm-Nd cpx-plg-wr isochron ages and Sm-Nd model ages) and evidence that the whole mafic and ultramafic rocks show an overall Sm/Nd isotopic homogeneity. Cpx-plg-wr data from gabbroic dykes define internal isochrones yielding Jurassic ages (162+/-10 Ma and 159+/-15 Ma, respectively). The plg-cpx(-wr) isochrons for impregnated plagioclase peridotites yields age of 155+/-6 Ma. The initial ɛNd values (8.9-9.7) are indicative of a MORB affinity. Calculated DM model ages for both spinel and plagioclase peridotites point to a Late Jurassic age (150 Ma). Isotope ratios of cpx from spinel and plagioclase peridotites conform to the linear array defined by overall gabbroic rocks. The isotopic evidence from the melt-percolated, reactive and impregnated peridotites indicates that the pristine lithospheric mantle protoliths were isotopically homogenized by the melt-rock interaction during percolation/impregnation processes which erased any pre-existing isotopic signature. Moreover, the overall Sm/Nd isotopic homogeneity indicates that the asthenospheric mantle sources of the infiltrating melts were isotopically homogeneous. Accordingly, it is plausible that percolation and intrusion were operated by similar and coeval Late Jurassic MORB-type melts. In conclusion, petrologic and isotopic data allow to recognize that the extending sub-continental lithospheric mantle was infiltrated by Late Jurassic MORB melts, formed by asthenospheric decompression-induced partial melting during continental extension and rifting. Melt-peridotite interaction modified the compositional features of the lithospheric mantle and caused its isotopic resetting. Accordingly, the sub-continental lithospheric mantle underwent an "oceanization" process (i.e. isotope resetting to "oceanic" MORB signatures) during Late Jurassic times operated by asthenospheric MORB melts. Depending on the melt composition, the lithospheric

  2. Isotopic Equilibrium in Mature Oceanic Lithosphere: Insights From Sm-Nd Isotopes on the Corsica (France) Ophiolites

    NASA Astrophysics Data System (ADS)

    Rampone, E.; Hofmann, A. W.; Raczek, I.; Romairone, A.

    2003-12-01

    In mature oceanic lithosphere, formed at mid-ocean ridges, residual mantle peridotites and associated magmatic crust are, in principle, linked by a cogenetic relationship, because the times of asthenospheric mantle melting and magmatic crust production are assumed to be roughly coheval. This implies that oceanic peridotites and associated magmatic rocks should have similar isotopic compositions. Few isotope studies have been devoted to test this assumption. At mid-ocean ridges, similar Nd isotopic compositions in basalts and abyssal peridotites have been found by Snow et al. (1994), thus indicating that oceanic peridotites are indeed residues of MORB melting. By contrast, Salters and Dick (2002) have documented Nd isotope differences between abyssal peridotites and associated basalts, with peridotites showing higher 143Nd/144Nd values, and they concluded that an enriched pyroxenitic source component is required to explain the low end of the 143Nd/144Nd variation of the basalts. Here we present Sm/Nd isotope data on ophiolitic mantle peridotites and intruded gabbroic rocks from Mt.Maggiore (Corsica, France), interpreted as lithosphere remnants of the Jurassic Ligurian Tethys ocean. The peridotites are residual after low-degree (<10%) fractional melting. In places, spinel peridotites grade to plagioclase-rich impregnated peridotites. Clinopyroxene separates from both spinel- and plagioclase- peridotites display high 147Sm/144Nd (0.49-0.59) and 143Nd/144Nd (0.513367-0.513551) ratios, consistent with their depleted signature. The associated gabbros have Nd isotopic compositions typical of MORB (143Nd/144Nd = 0.51312-0.51314). Sm/Nd data on plag, whole rock and cpx from an olivine gabbro define an internal isochron with an age of 162 +/- 10 Ma, and an initial epsilon Nd value (9.0) indicating a MORB-type source. In the Sm-Nd isochron diagram, the peridotite data also conform to the above linear array, their initial (160 Ma) epsilon Nd values varying in the range 7

  3. 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). PMID:20471674

  4. 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.

  5. Large volume recycling of oceanic lithosphere over short time scales: geochemical constraints from the Caribbean Large Igneous Province

    NASA Astrophysics Data System (ADS)

    Hauff, F.; Hoernle, K.; Tilton, G.; Graham, D. W.; Kerr, A. C.

    2000-01-01

    Oceanic flood basalts are poorly understood, short-term expressions of highly increased heat flux and mass flow within the convecting mantle. The uniqueness of the Caribbean Large Igneous Province (CLIP, 92-74 Ma) with respect to other Cretaceous oceanic plateaus is its extensive sub-aerial exposures, providing an excellent basis to investigate the temporal and compositional relationships within a starting plume head. We present major element, trace element and initial Sr-Nd-Pb isotope composition of 40 extrusive rocks from the Caribbean Plateau, including onland sections in Costa Rica, Colombia and Curaçao as well as DSDP Sites in the Central Caribbean. Even though the lavas were erupted over an area of ˜3×10 6 km 2, the majority have strikingly uniform incompatible element patterns (La/Yb=0.96±0.16, n=64 out of 79 samples, 2σ) and initial Nd-Pb isotopic compositions (e.g. 143Nd/ 144Nd in=0.51291±3, ɛNdi=7.3±0.6, 206Pb/ 204Pb in=18.86±0.12, n=54 out of 66, 2σ). Lavas with endmember compositions have only been sampled at the DSDP Sites, Gorgona Island (Colombia) and the 65-60 Ma accreted Quepos and Osa igneous complexes (Costa Rica) of the subsequent hotspot track. Despite the relatively uniform composition of most lavas, linear correlations exist between isotope ratios and between isotope and highly incompatible trace element ratios. The Sr-Nd-Pb isotope and trace element signatures of the chemically enriched lavas are compatible with derivation from recycled oceanic crust, while the depleted lavas are derived from a highly residual source. This source could represent either oceanic lithospheric mantle left after ocean crust formation or gabbros with interlayered ultramafic cumulates of the lower oceanic crust. High 3He/ 4He in olivines of enriched picrites at Quepos are ˜12 times higher than the atmospheric ratio suggesting that the enriched component may have once resided in the lower mantle. Evaluation of the Sm-Nd and U-Pb isotope systematics on

  6. Resolving the fine-scale density structure of oceanic lithosphere and asthenosphere from small-scale geoid anomalies: a case study from the Mendocino Fracture zone

    NASA Astrophysics Data System (ADS)

    Cadio, C.; Korenaga, J.

    2013-12-01

    As oceanic lithosphere corresponds to the top boundary layer of mantle convection, its gross density structure reflects how the convecting mantle is actually cooled near the surface. A recent wavelet analysis of the geoid data around the Mendocino fracture zone has revealed 100- to 200-km scale anomalies, which notably deviate from any of reference evolution models and therefore call for prominent density anomalies at relatively shallow depths (Cadio and Korenaga, 2012). Given their spatial scales and weak correlation with seafloor topography, their sources must be within or right beneath the oceanic lithosphere. Such short-wavelength geoid anomalies carry important information regarding the fine-scale density structure of the oceanic lithosphere and asthenosphere. A new inversion scheme has been developed to investigate these local anomalies. To overcome the nonuniqueness of potential-field data, we reduce the model space by using spectral localization, reference models, and a priori bounds on the amplitude of geologically possible density perturbations that can be caused by thermal or chemical processes within the convecting mantle. Our approach is based on Bayesian statistics and is implemented by combining forward modeling with Markov chain Monte Carlo sampling. The depth and the vertical extent of density anomalies derived from our inversion indicate that they are intimately related to the structure of the lowermost lithosphere. They reflect very likely small-scale convection induced by lateral temperature gradient across the fracture zone. The amplitude of density perturbations varying between -25 to 30 kg m-3 and their spatial organization are also in agreement with predictions derived from numerical simulations of such instabilities (Huang et al., 2003; Dumoulin et al., 2008). A global analysis of these local anomalies will allow us to resolve the fine-scale density structure of shallow oceanic mantle not only along fracture zones, but also in other parts

  7. The chemical evolution of oceanic and continental lithosphere: Case studies in the US Cordillera

    NASA Astrophysics Data System (ADS)

    Jean, Marlon Mauricio

    Investigations into ophiolite from California demonstrated that these ultramafic rocks formed within the mantle wedge of a subduction zone. Fore-arc locales are dominated by highly refractory peridotite, formed by hydrous-fractional partial melting that began in the garnet stability field and ended in the spinel stability field. These ophiolites also displayed enriched fluid-mobile element concentrations. Based on melt models, these elements should have extremely low concentrations, yet all pyroxenes display enriched compositions. A new algorithm was derived to model this fluid enrichment process, which represents the total addition of material to the mantle wedge source region and can be applied to any refractory mantle peridotite that has been modified by melt extraction and/or metasomatism. Investigations into the interaction of a mantle plume with continental lithosphere demonstrated that Yellowstone-Snake River Plain olivine tholeiites are compatible with genesis from a deep-seated mantle plume and were modeled via mixing of three components. The variable age, thickness, and composition of North American lithosphere guide this process. Drill core near Twin Falls, ID was examined to assess (1) the chemical evolution of olivine tholeiite, (2) how basalt evolves in continental settings, and (3) the dominant fractionation process, e.g., fractional crystallization, Raleigh fractional crystallization, or assimilation fractional crystallization.

  8. 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

  9. 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.

  10. 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.

  11. Interactions between Oceanic Saharan Air Layer and African Easterly Jet- African Easterly Waves System

    NASA Astrophysics Data System (ADS)

    Hosseinpour, F.; Wilcox, E. M.

    2013-12-01

    Aerosols have robust influences on multi-scale climatic systems and variability. Non-linear aerosol-cloud-climate interactions depend on many parameters such as aerosol features, regional atmospheric dynamics and variability. Although there are remarkable modeling studies indicating that aerosols induce robust modifications in cloud properties, circulations and the hydrological cycle, many of the physical and dynamical processes involving in these complex interactions between aerosols and Earth's system are still poorly understood. Better understanding the contribution of aerosols with atmospheric phenomena and their transient changes are crucial for efforts to evaluate climate predictions by next generation climate models. This study provides strong evidence of mechanistic relationships between perturbations of the oceanic Saharan air layer (OSAL) and anomalies of atmospheric circulations over the eastern tropical Atlantic/Africa. These relationships are characterized using an ensemble of daily datasets including the Modern-Era Retrospective Analysis for Research and Applications (MERRA), the Moderate Resolution Imaging Spectro-radiometer (MODIS), and the Sea-viewing Wide Field-of-View Sensor (SeaWIFS) for the boreal summer season. The study is motivated by previous results suggesting that oceanic dust-induced large-scale to meso-scale climatic adjustments. Our hypothesis is that perturbations in OSAL significantly interact with regional climate variability through African Easterly Jet- African Easterly Waves (AEJ-AEW) system. Passive/ active phases of AEWs in the northern and southern-track wave packets are associated with dipole patterns of thermal/dynamical anomalies correlated with perturbations of aerosol optical depth (AOD) in OSAL. Enhanced (suppressed) dust AOD in OSAL are significantly correlated with convective re-circulation within subsidence region of Hadley cell as well as robust mid-level dipole vorticity disturbances downstream of the AEJ core

  12. Long-offset and multi-fold ocean bottom seismographic survey for imaging lithospheric scale structures in plate convergent margins

    NASA Astrophysics Data System (ADS)

    Kodaira, S.; Takahashi, N.; Nakanishi, A.; Fujie, G.; Ito, A.; Miura, S.; Sato, T.; Tsuru, T.; Park, J.; Kaneda, Y.

    2005-05-01

    Recent availability of a large number of ocean bottom seismographs (OBSs), a large volume of air-gun array and a long streamer cable for academics provide several new findings of lithospheric scale structures in plate convergent margins. Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has acquired long-offset seismic data using a super-densely deploy OBS (i.e. 1 - 5 km spacing OBSs along 100 - 500 km long profiles) since 1999. Long-offset multichannel seismic (MCS) data by a two-ship experiment, as well as conventional 2D MCS data, have been also acquired at a part of the profiles. Some of those profiles have been designed as combined onshore - offshore profiles for imaging a land-ocean transition zone. In a plate convergent margin, an oceanic plate subducts deep into a lithosphere and an island arc crust grows due to an accretion of melts welling up from subducted materials. Our obtained long-offset and multi-fold seismic data successfully provide fine images of subducting and overriding plates, which had not been imaged by conventional type of wide-angle seismic survey, in those complicated tectonic setting. One of the most striking findings is an image of several scales of subducted seamounts/ridges in the Nankai trough seismogenic zone, the SW Japan. We detected the subducted seamount/ridges, which are 50 - 100 km wide, distributing from near trough axis to ~ 40 km deep beneath the Japanese island. An important aspect, from a point of view of a geodynamic process, those structures are strongly correlated with slip zones of magnitude 8-class earthquakes, i.e.; subducted seamounts/ridge control the rupture propagations. We have also acquired very long offset seismic data (more than 500 km long) along and across the Izu-Bonin-Mariana (IBM) subduction zone in which ongoing crustal accretion process is proposed. Although the data is still processing, we expect, from our data, new and important information for the crustal accretion process at the IBM

  13. Minerals as mantle fingerprints: Sr-Nd-Pb-Hf in clinopyroxene and He in olivine distinguish an unusual ancient mantle lithosphere beneath the East African Rift System

    NASA Astrophysics Data System (ADS)

    Nelson, W. R.; Shirey, S. B.; Graham, D. W.

    2011-12-01

    The East African Rift System is a complex region that holds keys to understanding the fundamental geodynamics of continental break-up. In this region, the volcanic record preserves over 30 Myrs of geochemical variability associated with the interplay between shallow and deep asthenospheric sources, continental lithospheric mantle, and continental crust. One fundamental question that is still subject to debate concerns the relationship between the lithospheric mantle and the voluminous flood basalt province that erupted at ~30 Ma in Ethiopia and Yemen. Whole-rock Re-Os isotopic data demonstrate the high-Ti (HT2) flood basalts (187Os/188Ost = 0.1247-0.1329) and peridotite xenoliths (187Os/188Ost = 0.1235-0.1377) from NW Ethiopia have similar isotopic compositions. However, Sr-Nd-Pb-Hf isotopic signatures from peridotite clinopyroxene grains are different from those of the flood basalts. The peridotite clinopyroxene separates bear isotopic affinities to anciently depleted mantle (87Sr/86Sr = 0.7019-0.7029; ɛNd = 12.6-18.5; ɛHf = 13.8-27.6) - more depleted than the MORB source - rather than to the OIB-like 30 Ma flood basalts (87Sr/86Sr ~ 0.704; ɛNd = 4.7-6.7; ɛHf = 12.1-13.5). Peridotite clinopyroxenes display two groups of 206Pb/204Pb compositions: the higher 206Pb/204Pb group (18.7-19.3) is compositionally similar to the flood basalts (206Pb/204Pb = 18.97-19.02) whereas the lower 206Pb/204Pb group (17.1-17.9) overlaps with depleted mantle. This suggests that the Pb isotope systematics in some of the peridotites have been metasomatically perturbed. Helium isotopes were analyzed by crushing olivine separated from the peridotites and the flood basalts. Olivine in the peridotites has low He concentrations (0.78-4.7 ncc/g) and low 3He/4He (4.6-6.6 RA), demonstrating that they cannot be the petrogenetic precursor to the high 3He/4He (>12 RA) flood basalts. Notably, these peridotites have 3He/4He signatures consistent with a lithospheric mantle source. Therefore

  14. 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

  15. 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

  16. 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

  17. Channelized Fluid Flow and Metasomatism in Subducted Oceanic Lithosphere recorded in an Eclogite-facies Shear Zone (Monviso Ophiolite, Italy)

    NASA Astrophysics Data System (ADS)

    Angiboust, S.; Agard, P.; Pettke, T.

    2012-04-01

    The Monviso ophiolite Lago Superiore Unit (LSU) constitutes a well-preserved, almost continuous fragment of upper oceanic lithosphere subducted down to ca. 80 km (between 50 and 40 Ma) and later exhumed along the subduction interface. The LSU is made of (i) a variably thick (50-500 m) section of eclogitized mafic crust (associated with minor calcschist lenses) overlying a 100-400 m thick metagabbroic body, and of (ii) a serpentinite sole (ca. 1000 m thick). This section is cut by two 10 to 100m thick eclogite-facies shear zones, found at the boundary between basalts and gabbros (Intermediate Shear Zone: ISZ), and between gabbros and serpentinites (Lower Shear Zone: LSZ). Fragments of mylonitic basaltic eclogites and marbles were dragged and dismembered within serpentinite schists along the LSZ during eclogite-facies deformation [Angiboust et al., Lithos, 2011]. Metasomatic rinds formed on these fragments at the contact with the surrounding antigorite schists during lawsonite-eclogite facies metamorphism, testifying to prominent fluid-rock interaction along with deformation. We present new petrological and geochemical data on four types of metasomatically altered eclogites (talc-, chlorite-, lawsonite- and phengite-bearing eclogites) and on a (serpentinite-derived) talc schist from the block rind. Bulk-rock compositions, in situ LA-ICP-MS analysis and X-ray Cr/Mg maps of garnet demonstrate that (i) these samples underwent significant B, Cr, Mg, Ni and Co enrichment and Fe, V and As depletion during eclogite-facies metamorphism (while Li and Pb behaved inconsistently) and (ii) garnet composition and chemistry of inclusions show extreme variation from core to rim. These compositional patterns point to a massive, pulse-like, fluid-mediated element transfer along with deformation, originating from the surrounding serpentinite (locally, with contributions from metasediments-equilibrated fluids). Antigorite breakdown, occurring ca. 10 km deeper than the maximum depth

  18. 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

  19. Heterogeneity of the North Atlantic oceanic lithosphere based on integrated analysis of GOCE satellite gravity and geological data

    NASA Astrophysics Data System (ADS)

    Barantseva, Olga; Artemieva, Irina; Thybo, Hans; Herceg, Matija

    2015-04-01

    constrained by numerous published seismic profiles and potential-field models across the Norwegian off-shore crust (e.g. Breivik et al., 2005, 2007). The results demonstrate the presence of strong gravity and density heterogeneity of the upper mantle in the North Atlantic region. In particular, there is a sharp contrast at the continent-ocean transition, which also allows for recognising mantle gravity anomalies associated with continental fragments and with anomalous oceanic lithosphere.

  20. 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.

  1. The role of mantle plumes in the evolution of the African segment of Pangea and the formation of the Atlantic Ocean

    NASA Astrophysics Data System (ADS)

    Peyve, A. A.

    2015-09-01

    In this paper, we discuss a broad range of issues related to the formation of large igneous provinces in the African segment of Pangea on the basis of modern seismic tomography data. The formation of older igneous provinces (Central American and Karoo) is attributed to a prolonged phase of upwelling of hot mantle material or fluids in separate jets within a much larger area than the supposed plume head. Owing to its huge size and the thick, dense continental crust, Pangea acted as a shield promoting the accumulation and lateral channeling of heat energy beneath the lithosphere. The changes in global Earth dynamics and the generation of extensional stresses alone may have led to the breakup of Pangea, triggering the eruption of large volumes of magma over short period of time. The same factors led to the opening of the Atlantic Ocean. We provide arguments that the African superplume represents a Cenozoic structure not associated with the emplacement of the Karoo province. At the same time, the hot material brought under the lithosphere by this superplume synchronously with the start of magmatism in east Africa then spread out to the northwest to form local melting areas in Central and Northwestern Africa. We suggest that magmatic activity within the same region may have lasted, with interruptions, over tens of millions of years. Because of plate motion, these lowvelocity zones acting as heat sources appear to have lost their deep-seated roots, so that mantle reservoirs surviving at the base of the lithosphere may have fed magmatism and drifted together with the lithosphere.

  2. 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

  3. 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.

  4. Electric dipole fields over an anisotropic seafloor: theory and application to the structure of 40 MA Pacific Ocean lithosphere

    NASA Astrophysics Data System (ADS)

    Everett, Mark E.; Constable, Steven

    1999-01-01

    Seismic anisotropy has been detected in the oceanic crust and upper mantle, and likewise it is geologically reasonable to expect that a certain amount of lateral anisotropy exists in seafloor electrical properties. Anisotropy in Earth properties can often lead to surprising effects on geophysical responses that are not anticipated from simple isotropic theories. Here, we investigate the effects of lateral anisotropy on the frequency-domain, controlled-source electromagnetic (CSEM) response of a uniaxially conducting, non-magnetic seafloor excited by a horizontal electric dipole whose moment is oriented obliquely with respect to the electrical strike direction. A `paradox of anisotropy' is observed, in which the seafloor electric field strength is enhanced in the most conductive direction of the seafloor. This enhancement is opposite to what one would expect based on naive isotropic theory. We also show that it is possible in certain circumstances to extract the along-strike electrical conductivity from marine controlled-source electromagnetic data using only isotropic modelling. The extraction of across-strike conductivity, however, requires full anisotropic modelling. The physical insight into electromagnetic induction in uniaxial media that is presented here should greatly assist the geological interpretation of marine CSEM experimental data. Applying our algorithm to the PEGASUS data set (CSEM data collected over 40 Ma Pacific Ocean lithosphere) produces a model with conductivity in the fossil spreading direction that is seven x greater than the conductivity perpendicular to spreading. Strain-aligned mineralogical fabric, as predicted by tectonic modelling, would explain our result, with enhanced conductivities caused by hydrogen conduction along the olivine a-axis or connected accumulations of trace conductors such as graphite or magnetite.

  5. Obduction of old oceanic lithosphere due to reheating and plate reorganization: Insights from numerical modelling and the NE Anatolia - Lesser Caucasus case example

    NASA Astrophysics Data System (ADS)

    Hässig, Marc; Duretz, Thibault; Rolland, Yann; Sosson, Marc

    2016-05-01

    The ophiolites of NE Anatolia and of the Lesser Caucasus (NALC) evidence an obduction over ∼200 km of oceanic lithosphere of Middle Jurassic age (c. 175-165 Ma) along an entire tectonic boundary (>1000 km) at around 90 Ma. The obduction process is characterized by four first order geological constraints: Ophiolites represent remnants of a single ophiolite nappe currently of only a few kilometres thick and 200 km long. The oceanic crust was old (∼80 Ma) at the time of its obduction. The presence of OIB-type magmatism emplaced up to 10 Ma prior to obduction preserved on top of the ophiolites is indicative of mantle upwelling processes (hotspot). The leading edge of the Taurides-Anatolides, represented by the South Armenian Block, did not experience pressures exceeding 0.8 GPa nor temperatures greater than ∼300 °C during underthrusting below the obducting oceanic lithosphere. An oceanic domain of a maximum 1000 km (from north to south) remained between Taurides-Anatolides and Pontides-Southern Eurasian Margin after the obduction. We employ two-dimensional thermo-mechanical numerical modelling in order to investigate obduction dynamics of a re-heated oceanic lithosphere. Our results suggest that thermal rejuvenation (i.e. reheating) of the oceanic domain, tectonic compression, and the structure of the passive margin are essential ingredients for enabling obduction. Afterwards, extension induced by far-field plate kinematics (subduction below Southern Eurasian Margin), facilitates the thinning of the ophiolite, the transport of the ophiolite on the continental domain, and the exhumation of continental basement through the ophiolite. The combined action of thermal rejuvenation and compression are ascribed to a major change in tectonic motions occurring at 110-90 Ma, which led to simultaneous obductions in the Oman (Arabia) and NALC regions.

  6. Nitrogen recycling in subducted oceanic lithosphere: The record in high- and ultrahigh-pressure metabasaltic rocks

    NASA Astrophysics Data System (ADS)

    Halama, Ralf; Bebout, Gray E.; John, Timm; Schenk, Volker

    2010-03-01

    This paper provides the first measurements of the nitrogen (N) concentrations and isotopic compositions of high- and ultrahigh-pressure mafic eclogites, aimed at characterizing the subduction input flux of N in deeply subducting altered oceanic crust (AOC). The samples that were studied are from the Raspas Complex (Ecuador), Lago di Cignana (Italy), the Zambezi Belt (Zambia) and Cabo Ortegal (Spain), together representing subduction to 50-90 km depths. The eclogites contain 2-20 ppm N with δ 15N air values ranging from -1 to +8‰. These values overlap those of altered oceanic crust, but are distinct from values for fresh MORB (for the latter, ˜1.1 ppm N and δ 15N air ˜ -4‰). Based on N data in combination with other trace element data, the eclogite suites can be subdivided into those that are indistinguishable from their likely protolith, AOC, with or without superimposed effects of devolatilization (Lago di Cignana, Cabo Ortegal), and those that have experienced metasomatic additions during subduction-zone metamorphism (Zambezi Belt, Raspas). For the former group, the lack of a detectable loss of N in the eclogites, compared to various altered MORB compositions, suggests the retention of N in deeply subducted oceanic crust. The metasomatic effects affecting the latter group can be best explained by mixing with a (meta)sedimentary component, resulting in correlated enrichments of N and other trace elements (in particular, Ba and Pb) thought to be mobilized during HP/UHP metamorphism. Serpentinized and high-pressure metamorphosed peridotites, associated with the eclogites at Raspas and Cabo Ortegal, contain 3-15 ppm N with δ 15N air values ranging from +3 to +6‰, significantly higher than the generally accepted values for the MORB mantle (δ 15N air ˜ -5‰). Based on their relatively high N contents and their homogeneous and positive δ 15N values, admixing of sedimentary N is also indicated for the serpentinized peridotites. One possible pathway for the

  7. 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

  8. Oxygen - Osmium Isotopic Compositions of West Maui Lavas and the Link to Oceanic Lithosphere

    NASA Astrophysics Data System (ADS)

    Gaffney, A. M.; Nelson, B. K.; Reisberg, L.; Eiler, J. M.

    2003-12-01

    We use stratigraphically-controlled sequences of late shield-stage lavas from West Maui to investigate age-dependence and fine-scale complexity of chemical variation within this Kea-type Hawaiian volcano. With new O and Os isotope data we identify unique geochemical signals indicative of complex processes of mixing among source components. In W. Maui lavas, δ 18O and 187Os/188Os range from 4.5 to 5.2 and 0.1316 to 0.1394, respectively. One sample (187Os/188Os =0.158) has very low [Os] (20 ppt) and likely has been severely contaminated by shallow oceanic crust. These compositions, as well as 87Sr/86Sr (0.7034-0.7037) and 206Pb/204Pb (18.36-18.54), are typical of shield-stage lavas from other Kea-type volcanoes. Some W. Maui compositional variability correlates with stratigraphic height. All shallow samples have 187Os/188Os>=0.1333, and all deep samples have 187Os/188Os<=0.1330. The deep samples show a negative 187Os/188Os - 87Sr/86Sr correlation, but the shallow samples show no Os - Sr isotopic correlation. 187Os/188Os correlates with no other measured isotopic compositions except for δ 18O, for which the deep and shallow samples define two sub-parallel inversely correlated trends. δ 18O shows no other correlation with stratigraphy or other measured isotopic tracers. The inverse O-Os isotopic correlation of W. Maui is distinct from that observed for other Hawaiian volcanoes (e.g. Mauna Kea, Koolau) or within the Hawaiian suite as a whole. Age-dependent sampling of at least three components is required to describe the O-Os-Sr isotopic variation we observe at W. Maui. Similar intra-volcano correlation in 87Sr/86Sr - 206Pb/204Pb - trace element compositions is consistent with mixing of small-degree partial melts of gabbroic oceanic crust with plume-derived Kea-type magmas. We infer that this gabbroic endmemember is characterized by δ 18O < 4.5 and 187Os/188Os > 0.133. The remainder of the variability is the result of short-lived chemical heterogeneities in the Kea

  9. The relationship between Indian Ocean sea-surface temperature and East African rainfall.

    PubMed

    Black, Emily

    2005-01-15

    Knowledge of the processes that control East African rainfall is essential for the development of seasonal forecasting systems, which may mitigate the effects of flood and drought. This study uses observational data to unravel the relationship between the Indian Ocean Dipole (IOD), the El Nino Southern Oscillation (ENSO) and rainy autumns in East Africa. Analysis of sea-surface temperature data shows that strong East African rainfall is associated with warming in the Pacific and Western Indian Oceans and cooling in the Eastern Indian Ocean. The resemblance of this pattern to that which develops during IOD events implies a link between the IOD and strong East African rainfall. Further investigation suggests that the observed teleconnection between East African rainfall and ENSO is a manifestation of a link between ENSO and the IOD. PMID:15598619

  10. 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.

  11. Motions of Australia and surroundings since 43 Ma as recorded by subducted mantle lithosphere--evidence for a lost ocean between the Pacific and Indian Oceans

    NASA Astrophysics Data System (ADS)

    Renqi, L.; Wu, J. E.; Suppe, J.; Kanda, R. V.

    2013-12-01

    It is well known from seafloor spreading and hotspot data that the Australian plate has moved ~2500km northward in a mantle reference frame since 43Ma, during which time the Pacific plate moved approximately orthogonally ~3000km in a WNW direction. In addition the Australian plate has expanded up to 2000 km as a result of back arc spreading associated with evolving subduction systems on its northern and eastern margins. Here we attempt to account for this plate motion and subduction using new quantitative constraints of mapped slabs of subducted mantle lithosphere underlying the Australian plate and its surroundings. We have mapped a large swath of sub-horizontal slabs in the lower mantle under onshore and offshore NE Australia using global mantle seismic tomography. When restored together with other mapped slabs from the Asia Pacific region, these slabs reveal the existence of a major ocean between NE Australia, E. Asian, and the Pacific at 43 Ma, which we call the East Asian Sea. The southern half of this East Asian Sea was overrun and completely subducted by northward-moving Australia and the expanding Melanesian arcs, and the WNW-converging Pacific. This lost ocean fills a major gap in plate tectonic reconstructions and also constraints the possible motion of the Caroline Sea and New Guinea arcs. Slabs were mapped from MITP08 global P-wave seismic tomography data (Li and Hilst, 2008) and the TX2011 S-wave seismic tomography data (Grand and Simmons, 2011) using Gocad software. The mapped slabs were unfolded to the spherical Earth surface to assess their pre-subduction geometry. Gplates software was used to constrain plate tectonic reconstructions within a fully animated, globally consistent framework.

  12. 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.

  13. 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

  14. 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

  15. 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.

  16. Propagation characteristics of Po/So in the lithosphere of the Eastern Atlantic ocean revealed from automatic incoherent ocean bottom array processing

    NASA Astrophysics Data System (ADS)

    Dahm, T.; Krueger, F.; Hannemann, K.

    2013-12-01

    Contrary to continental lithosphere, the seismic shear wave anisotropy of the uppermost oceanic mantle is rarely sampled at local scales. Local anisotropy information from ocean bottom stations are often difficult to obtain because of the rare deployments and because of poor signal to noise (SNR) ratio at these stations. In a pilot study in the North Atlantic between Portugal mainland, Madeira and the Azores, we demonstrate that an ocean bottom mid-aperture array at 4-5 km depth allows for automatic retrieval of SHo, SVo and Po velocities from data filtered between 4 and 25 Hz from regional weak earthquakes with Ml < 3 in up to 500 km distance, even if the SNR is poor. We use incoherent array analysis applied to short-term average / long-term average (STA/LTA) characteristic functions. Contrary to conventional methods the array analysis reveals local, absolute velocities beneath the array that are not averages over long travelpaths. Additionally, earthquakes can be located using the backazimuth and So-Po difference times. For instance, we observe seismicity at an aseismic segment of the Gloria transform fault. For our pilot array at 38.4 N 18.38 W we detect and study more than 900 suited earthquakes over a period of 10 months, and retrieve a strong azimuthal anisotropy of SH and SV waves of about 8% with a fast direction striking 90E in accord with the direction of plate motion. Unexpectedly, the azimuthal anisotropy of P waves is small or even absent. We study furthermore the different propagation paths and find strong attenuation of Po and So for paths crossing the Azores hotspot region and attenuation of So only for the region directly west of Portugal. This indicates that Po and So phases are blocked or not generated in the hot upper mantle of active spreading zones The project is funded by the German Research Foundation (Da478/21-1, Kr1935/13-1). DEPAS (AWI, GFZ) and University of Hamburg supported the OBS deployment.

  17. The African Plate: A history of oceanic crust accretion and subduction since the Jurassic

    NASA Astrophysics Data System (ADS)

    Gaina, C.; Torsvik, T. H.; Labails, C.; van Hinsbergen, D.; Werner, S.; Medvedev, S.

    2012-04-01

    Initially part of Gondwana and Pangea, and now surrounded almost entirely by spreading centres, the African plate moved relatively slowly for the last 200 million years. Yet both Africa's cratons and passive margins were affected by tectonic stresses developed at distant plate boundaries. Moreover, the African plate was partly underlain by hot mantle (at least for the last 300 Ma) - either a series of hotspots or a superswell, or both - that contributed to episodic volcanism, basin-swell topography, and consequent sediment deposition, erosion, and structural deformation. A systematic study of the African plate boundaries since the opening of surrounding oceanic basins is presently lacking. This is mainly because geophysical data are sparse and there are still controversies regarding the ages of oceanic crust. The publication of individual geophysical datasets and more recently, global Digital Map of Magnetic Anomalies (WDMAM, EMAG2) prompted us to systematically reconstruct the ages and extent of oceanic crust around Africa for the last 200 Ma. Location of Continent Ocean Boundary/Continent Ocean Transition and older oceanic crust (Jurassic and Cretaceous) are updates in the light of gravity, magnetic and seismic data and models of passive margin formation. Reconstructed NeoTethys oceanic crust is based on a new model of microcontinent and intr-oceanic subduction zone evolution in this area.The new set of oceanic palaeo-age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and will be used as input for quantifying the paleo-ridge push and slab pull that contributed to the African plate palaeo-stresses and had the potential to influence the formation of sedimentary basins.

  18. 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.

  19. Arctic lithosphere - A review

    NASA Astrophysics Data System (ADS)

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

    2014-07-01

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

  20. 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.

  1. A high resolution seismic reflection image for the oceanic LAB (Lithosphere-Asthenosphere Boundary), beneath southern North Island, New Zealand

    NASA Astrophysics Data System (ADS)

    Stern, T. A.; Henrys, S. A.; Okaya, D. A.; Savage, M. K.; Sato, H.; Iwasaki, T.; Louie, J. N.; Lamb, S. H.

    2014-12-01

    We present the first high-resolution, multichannel, seismic-reflection image for the base of an oceanic plate. Our image is based on an 85 km-long, ~ 900 station deployment across the lower North Island of New Zealand. 12 x 500 kg dynamite shots were used as seismic sources. Strong reflections at a two way travel time of 9-12 s define the top of the plate that dips to the NW at ~ 12-15 degrees. Between 27-32 s we identify a pair of reflections on some shot gathers that are interpreted to come from a reflection 90-100 km deep, that dips to the NW at 15 degrees. We interpret the reflection pair as marking a Lithosphere-Asthenosphere Boundary (LAB) zone at the base of the Pacific plate. Using all 12 shots we made a CDP-stacked image (maximum fold = 15) that shows the LAB as a double event (2-3 s apart) dipping roughly parallel to the top of the plate and Benioff zone. Shot quality varies but the highest frequencies we record from the base of the plate are ~ 18 Hz, suggesting a boundary zone < 1 km thick. Seismic amplitude attributes, calibrated to the reflection from the top of the plate, indicate P-wave speed drops off at least 8% across the LAB boundary. The double reflection at the LAB is interpreted to be a 10 km-thick layer of low seismic wave speed. Because it is so sharp it cannot be a thermal boundary and must represent some form of mechanical change. Previous attempts to explain the abruptness of seismic wave speed changes at the LAB have appealed to layered zones of ponded melt, or anelastic relaxation due to water accumulating beneath the LAB. Both mechanisms may explain our observations and both would point to low viscosity below the LAB. However, the fact we see a ~ 10 km thick channel, with strong acoustic impedances each side of the channel, suggests a shear zone where plate motion ( ~ 9 cm/y in hotspot reference frame) is taken up and strain rates of ~3 x 10-13 s-1 are generated. This interpreted, low wave-speed, low-viscosity, shear zone appears to be

  2. 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. PMID:23325220

  3. Seismic Reflection Images of Deep Lithospheric Faults and Thin Crust at the Actively Deforming Indo-Australian Plate Boundary in the Indian Ocean

    NASA Astrophysics Data System (ADS)

    Singh, S. C.; Carton, H.; Chauhan, A.; Dyment, J.; Cannat, M.; Hananto, N.; Hartoyo, D.; Tapponnier, P.; Davaille, A.

    2007-12-01

    Recently, we acquired deep seismic reflection data using a state-of-the-art technology of Schlumberger having a powerful source (10,000 cubic inch) and a 12 km long streamer along a 250 km long trench parallel line offshore Sumatra in the Indian Ocean deformation zone that provides seismic reflection image down to 40 km depth over the old oceanic lithosphere formed at Wharton spreading centre about 55-57 Ma ago. We observe deep penetrating faults that go down to 37 km depth (~24 km in the oceanic mantle), providing the first direct evidence for full lithospheric-scale deformation in an intra-plate oceanic domain. These faults dip NE and have dips between 25 and 40 degrees. The majority of faults are present in the mantle and are spaced at about 5 km, and do not seem cut through the Moho. We have also imaged active strike-slip fault zones that seem to be associated with the re-activation of ancient fracture zones, which is consistent with previous seismological and seafloor observations. The geometries of the deep penetrating faults neither seem to correspond to faulting associated with the plate bending at the subduction front nor with the re-activation of fracture zone that initiated about 7.5 Ma ago, and therefore, we suggest that these deep mantle faults were formed due to compressive stress at the beginning of the hard collision between India and Eurasia, soon after the cessation of seafloor spreading in the Wharton basin. We also find that the crust generated at the fast Wharton spreading centre 55-57 Ma ago is only 3.5-4.5 km thick, the thinnest crust ever observed in a fast spreading environment. We suggest that this extremely thin crust is due to 40-50°C lower than normal mantle temperature in this part of the Indian Ocean during its formation.

  4. SEASAT observations of lithospheric flexure

    NASA Technical Reports Server (NTRS)

    Mcadoo, D. C.

    1984-01-01

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

  5. Seismic Anisotropy of the Lithosphere/Asthenosphere System Beneath the Rwenzori Region of the East-African Rift

    NASA Astrophysics Data System (ADS)

    Homuth, B.; Löbl, U.; Batte, A.; Link, K.; Kasereka, C.; Rumpker, G.

    2014-12-01

    We present results from a temporary seismic network of 32 broad-band stations located around the Rwenzori region of the Albertine rift at the border between Uganda and DR Congo. The study aims to constrain seismic anisotropy and mantle deformation processes in relation to the formation of the rift zone. Shear-wave splitting measurements from local and teleseismic earthquakes are used to investigate the seismic anisotropy in the crust and upper mantle beneath the Rwenzori region. At most stations, shear-wave splitting parameters obtained from individual earthquakes exhibit only minor variations with backazimuth. We therefore employ a joint inversion of SKS waveforms to derive hypothetical one-layer parameters. The corresponding fast polarizations are generally rift-parallel and the average delay time is about 1 s. On the other hand, shear phases from local events within the crust are characterized by an average delay time of 0.04 s. This observation suggests that the dominant source region for seismic anisotropy beneath the rift is located within the mantle. We use finite-frequency waveform modeling to test different models of anisotropy within the lithosphere/asthenosphere system of the rift. The results show that the rift-parallel fast polarizations are consistent with HTI anisotropy caused by magmatic intrusions or lenses located within the lithospheric mantle - as it would be expected during the early stages of continental rifting. Furthermore, the short-scale spatial variations in the fast polarizations observed in the southern part of the study area can be explained by effects due to sedimentary basins of low isotropic velocity in combination with a shift in the orientation of anisotropic fabrics in the upper mantle. A uniform anisotropic layer in relation to large-scale asthenospheric mantle flow is less consistent with the observed splitting parameters.

  6. Seismic anisotropy of the lithosphere/asthenosphere system beneath the Rwenzori region of the East-African Rift

    NASA Astrophysics Data System (ADS)

    Homuth, Benjamin; Löbl, Ulrike; Batte, Arthur; Link, Klemens; Kasereka, Celestine; Rümpker, Georg

    2014-05-01

    We present results from a temporary seismic network of 32 broad-band stations located around the Rwenzori region of the Albertine rift at the border between Uganda and DR Congo. The study aims to constrain seismic anisotropy and mantle deformation processes in relation to the formation of the rift zone. Shear-wave splitting measurements from local and teleseismic earthquakes are used to investigate the seismic anisotropy in the crust and upper mantle beneath the Rwenzori region. At most stations, shear-wave splitting parameters obtained from individual earthquakes exhibit only minor variations with backazimuth. We therefore employ a joint inversion of SKS waveforms to derive hypothetical one-layer parameters. The corresponding fast polarizations are generally rift-parallel and the average delay time is about 1 s. On the other hand, shear phases from local events within the crust are characterized by a bimodal pattern of fast polarizations and an average delay time of 0.04 s. This observation suggests that the dominant source region for seismic anisotropy beneath the rift is located within the mantle. We use finite-frequency waveform modeling to test different models of anisotropy within the lithosphere/asthenosphere system of the rift. The results show that the rift-parallel fast polarizations are consistent with HTI anisotropy caused by rift-parallel magmatic intrusions or lenses located within the lithospheric mantle - as it would be expected during the early stages of continental rifting. Furthermore, the short-scale spatial variations in the fast polarizations observed in the southern part of the study area can be explained by effects due to sedimentary basins of low isotropic velocity in combination with a shift in the orientation of anisotropic fabrics in the upper mantle. A uniform anisotropic layer in relation to large-scale asthenospheric mantle flow is less consistent with the observed splitting parameters.

  7. 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

  8. 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.

  9. Subtropical Indian Ocean SST dipole events and southern African rainfall

    NASA Astrophysics Data System (ADS)

    Reason, C. J. C.

    An atmospheric general circulation model (AGCM) is used to examine how the regional atmospheric circulation and rainfall over southern Africa respond to a recently observed dipole in subtropical sea surface temperature (SST) over the South Indian Ocean. Observations suggest that when SST is warm to the south of Madagascar and cool off Western Australia, increased summer rains occur over large areas of southeastern Africa. The model results suggest that this SST pattern leads to increased rainfall via enhanced convergence of moister than average air over the region. Increased evaporation occurs over the warm pole in the South West Indian Ocean and this moist air is advected towards Mozambique and eastern South Africa as a result of the low pressure anomaly generated over this pole which strengthens the onshore flow.

  10. Lattice-Preferred orientations of olivine in subducting oceanic lithosphere derived from the observed seismic anisotropies in double seismic zones

    NASA Astrophysics Data System (ADS)

    Han, Peng; Wei, Dongping; Zhang, Keliang; Sun, Zhentian; Zhou, Xiaoya

    2016-08-01

    Subduction zones can generally be classified into Mariana type and Chilean type depending on plate ages, plate thicknesses, subduction angles, back-arc deformation patterns, etc. The double seismic zones (DSZs) in subduction zones are mainly divided into type I and type II which, respectively, correspond to the Mariana type and Chilean type in most cases. Seismic anisotropy is an important parameter characterizing the geophysical features of the lithosphere, including the subduction zones, and can be described by the two parameters of delay time δt and fast wave polarization direction ϕ. We totally collected 524 seismic anisotropy data records from 24 DSZs and analyzed the statistical correlations between seismic anisotropy and the related physical parameters of DSZs. Our statistical analysis demonstrated that the fast wave polarization directions are parallel to the trench strike with no more than 30° for most type I DSZs, while being nearly perpendicular to the trench strike for type II DSZs. We also calculated roughly linear correlations that the delay time δt increases with dip angles but decreases with subduction rates. A linear equation was summarized to describe the strong correlation between DSZ's subduction angle α DSZ and seismic anisotropy in subduction zones. These results suggest that the anisotropic structure of the subducting lithosphere can be described as a possible equivalent crystal similar to the olivine crystal with three mutually orthogonal polarization axes, of which the longest and the second axes are nearly along the trench-perpendicular and trench-parallel directions, respectively.

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

    NASA Astrophysics Data System (ADS)

    Pasyanos, Michael E.; Nyblade, Andrew A.

    2007-11-01

    We study the lithospheric structure of Africa, Arabia and adjacent oceanic regions with fundamental-mode surface waves over a broad period range. Including group velocities with periods shorter than 35 s allows us to examine shallower features than previous studies of the whole continent. In the process, we have developed a crustal thickness map of Africa. Main features include crustal thickness increases under the West African, Congo, and Kalahari cratons. We find crustal thinning under Mesozoic and Cenozoic rifts, including the Benue Trough, Red Sea, and East, Central, and West African rift systems, along with less abrupt crustal thickness changes at passive continental margins. We also find crustal thickness differences in North Africa between the West African Craton and East Saharan Shield. Crustal shear wave velocities are generally faster in oceanic regions and cratons, and slower in more recent crust and in active and remnant orogenic regions. Deeper structure, related to the thickness of cratons and modern rifting, is generally consistent with previous work. Under cratons we find thick lithosphere and fast upper mantle velocities, while under rifts we find thinned lithosphere and slower upper mantle velocities. However, we also find the lack of a thick cratonic keel beneath the central portion of the Congo Craton. There are no consistent effects in areas classified as hotspots, indicating that there seem to be numerous origins for these features. Finally, it appears that the African Superswell, which is responsible for high elevation and uplift over large portions of Africa, has had a significantly different impact (as indicated by features such as temperature, time of influence, etc.) in the north and the south. This is consistent with episodic activity at shallow depths, which is well-expressed in northeastern Africa and Arabia today.

  12. Lithospheric instabilities. [associated with mantle geoid

    NASA Technical Reports Server (NTRS)

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

    1977-01-01

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

  13. 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

  14. 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.

  15. Reconstruction of the Flanks of the Mid-Atlantic Ridge, 28° to 29° N: Implications for Evolution of Young Oceanic Lithosphere at Slow-Spreading Centers

    NASA Astrophysics Data System (ADS)

    Sloan, H.; Patriat, P.

    2004-12-01

    We reconstruct the flanks of the Mid-Atlantic Ridge between 28° and 29° N from 1 to 10 Ma at intervals of 1 Myr for the purpose of investigating evolution of young oceanic lithosphere morphology and its variation through time using an innovative method that combines seafloor subsidence correction with interpolated isochrons and rotation poles. Reconstruction results are consistent with formation of abyssal hills every 1 to 2 Myr in 2-3 Myr old lithosphere at the outer edge of the ridge mountains as a result of transition from dynamic regime near the axis to isostatic regime of the flanks. The oblique passage of structures formed at the axial valley walls through this transition zone may play a role in the development of inside corner high bathymetry. Asymmetric juxtaposition of abyssal hill morphology in reconstruction is indicative of independent formation and evolution of the morphotectonic fabric on opposing flanks. The two major factors affecting asymmetric ridge flank morphology are found to be sense of axial offset and fluctuation of magmatic activity at the segment scale. Sense of axial offset determines the relative distribution of inside and outside corner bathymetry on the flanks. Enhanced magma supply and associated segment propagation may contribute to half-spreading rate asymmetry, accretion of thicker crust, and formation of larger abyssal hills on the faster-spreading flank. Consistent alignment in reconstruction of the base of the steep walls bounding inside corner bathymetry confirms that they mark the boundaries between segments on the flanks and that the deep sediment filled basins, typically identified as discordant zones, are attributable to outside corner bathymetry.

  16. 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.

  17. Lithospheric thickness jumps at the S-Atlantic continental margins from satellite gravity data and modelled isostatic anomalies

    NASA Astrophysics Data System (ADS)

    Shahraki, Meysam; Schmeling, Harro; Haas, Peter

    2016-04-01

    Isostatic equilibrium is a good approximation for passive continental margins. In these regions, geoid anomalies are proportional to the local dipole moment of density-depth distributions, which can be used to constrain the thickness of lithospheric jumps and corresponding tectonic stress. We analysed satellite derived geoid data and, after filtering, extracted typical averaged profiles across the Western and Eastern passive margins of the South Atlantic. They show geoid jumps of 8.1 m and 7.0 m for the Argentinian and African sides, respectively. Together with topography data and reasonable assumptions about densities these jumps are interpreted as isostatic geoid anomalies and yield best-fitting crustal and lithospheric thicknesses. They reveal a small asymmetry between the African and S-American crusts and lithospheres by a few kilometers. On both sides, the continental lithosphere is about 15 - 30km thicker than the oceanic lithosphere. To keep such geoid jumps stable over O(100Ma) fully dynamic models show that lithospheric viscosities must be of the order of 1e23 Pa s.

  18. The onshore-offshore ENCENS project: Imaging the stretching of the continental lithosphere and inception of oceanic spreading in the eastern Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Leroy, S.; Ebinger, C.; D'Acremont, E.; Stuart, G.; Al-Lazki, A.; Tiberi, C.; Autin, J.; Watremez, L.; Beslier, M.; Bellahsen, N.; Lucazeau, F.; Perrot, J.; Mouthereau, F.; Courrèges, E.; Huchon, P.; Rouzo, S.; Balahaf, S.; Sholan, J.; Unternehr, P.; Hello, Y.; Anglade, A.; Desprez, O.; Beguery, L.; Aouji, O.; Daniel, R.; Al Toubi, K.

    2006-12-01

    The eastern Gulf of Aden exemplifies several extensional processes that began 35 Ma ago from continental rifting to seafloor spreading at 2 cm/yr. Thin post-breakup sediment cover reveals the syn- and pre-rift basement fabric and the variable styles of conjugate margins along and across strike. A comprehensive multi- disciplinary study of the eastern part of the northern margin of the Gulf of Aden has been undertaken recently in the framework of the French margins program (GDR Marges) and the NERC with the long term objective to integrate in a consistent model of evolution, field observations where the margins crop out, results of marine geophysical survey where the margins are submerged and seismological observations of deep interior structure. A deep onshore-offshore seismic survey operated by IFREMER-GENAVIR was conducted between February, 3rd and March 14th 2006 off the Dhofar coast (southern Oman). We collect 67 multichannel seismic reflection profiles and 15 seismic refraction profiles acquired thanks to 60 stations. The shots have been recorded by 35 Ocean Bottom Seismometers (15 from IRD-Villefranche sur Mer and 20 from INSU Paris) and 25 seismological stations deployed onland in Southern Oman (18 stations from Encens-UK project funded by NERC and managed by RHUL ; 2 temporary and 4 permanent stations from Sultan Qaboos University ; 1 from ENS Paris). The network has been extended to the southern conjugate margin with 2 broadband stations deployed in Socotra island. The preliminary results show the evolution of the thickness of the crust from 35 km in the upper part of the margin to about 5 km in the ocean-continent transition. From our observations (low angle faults dipping toward the ocean or the continent imaged) we will propose rifting models responsible for the lithospheric stretching. The segmentation of the margins should be also precisely defined thanks to the multibeam bathymetry, the magnetism and the gravity acquired during the cruise.

  19. 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. PMID:11073447

  20. Role of the structural inheritance of the oceanic lithosphere in the magmato-tectonic evolution of Piton de la Fournaise volcano (La Réunion Island)

    NASA Astrophysics Data System (ADS)

    Michon, Laurent; Saint-Ange, Francky; Bachelery, Patrick; Villeneuve, Nicolas; Staudacher, Thomas

    2007-04-01

    La Réunion Island is located east of Madagascar, on the eastern rim of the tectonically inactive Mascarene Basin. This island is composed of three shield volcanoes of which only Piton de la Fournaise is currently active. Although the magmatic activity is restricted to Piton de la Fournaise, a scattered seismicity occurs on the whole 200 km wide volcanic edifice and in the underlying oceanic crust. We carried out a multiscale analysis to understand (1) the origin of the seismicity in the geodynamic context and (2) the role of the oceanic lithosphere in the deformation of Piton de la Fournaise and La Réunion Island. Analysis of the magmatic system suggests that the magma ascent is controlled by large N25-30 and N125-130 fracture zones located below the Enclos depression. We also show that the orientation difference between the eruptive fissures and the related dykes result from a rotation of the main principal stress σ1 from vertical to downslope through the surface. Combining a Digital Elevation Model (DEM) analysis, field observations and the geophysical data reveals that the volcano is affected by large fault zones. The fault distribution indicates the predominance of a main N70-80 trend. Magnetic data show the same N80 orientation characterizing the remnant part of the Alizés volcano. Such parallel alignment suggests a control exerted by the underlying Alizés volcano on Piton de la Fournaise. Furthermore, the alignment between the crustal orientations and the structures determined on the island suggests a control of the crustal structures in La Réunion's volcano-tectonic activity. Contrary to several volcanic islands such as Hawaii and Tenerife, La Réunion volcanoes lie on an upbending crust. Then, we interpret the reactivation of the crustal faults as resulting from a crustal uplift related to the thermal erosion of the base of the lithosphere and/or to strong underplating. The upward deformation may prevent the spreading of the volcanoes, as no evidence

  1. Metasomatism and Channelized Fluid Flow in Subducted Oceanic Lithosphere: the Record from an Eclogite-facies Shear Zone (Monviso Ophiolite, Italy)

    NASA Astrophysics Data System (ADS)

    Angiboust, S.; Pettke, T.; Agard, P.; Oncken, O.

    2012-12-01

    The Monviso ophiolite Lago Superiore Unit (LSU) constitutes a well-preserved, almost continuous fragment of upper oceanic lithosphere subducted down to ca. 80 km (between 50 and 40 Ma) and later exhumed along the subduction interface. The LSU is made of (i) a variably thick (50-500 m) section of eclogitized mafic crust (associated with minor calcschist lenses) overlying a 100-400 m thick metagabbroic body, and of (ii) a serpentinite sole (ca. 1000 m thick). This section is cut by two 10 to 100m thick eclogite-facies shear zones, found at the boundary between basalts and gabbros (Intermediate Shear Zone), and between gabbros and serpentinites (Lower Shear Zone: LSZ). Fragments of mylonitic basaltic eclogites and calcschists were dragged and dismembered within serpentinite schists along the LSZ during eclogite-facies deformation [Angiboust et al., Lithos, 2011]. Metasomatic rinds formed on these fragments at the contact with the surrounding antigorite schists during lawsonite-eclogite facies metamorphism, testifying to prominent fluid-rock interaction along with deformation. We present new petrological and geochemical data on four types of metasomatically altered eclogites (talc-, chlorite-, lawsonite- and phengite-bearing eclogites) and on a (serpentinite-derived) magnesite-bearing talc schist from the block rind. Bulk-rock compositions, in situ LA-ICP-MS analysis and X-ray Cr/Mg maps of garnet demonstrate that (i) these samples underwent significant Cr, Mg, Ni and Co enrichment and Fe, V (and to a lesser extent As) depletion during eclogitic metasomatism and (ii) garnet composition and chemistry of inclusions show extreme variation from core to rim. These compositional patterns point to a massive, pulse-like, fluid-mediated element transfer along with deformation, originating from the surrounding serpentinite (locally, with possible contributions from metasediments-equilibrated fluids). Antigorite breakdown, occurring ca. 15 km deeper than the maximum depth reached

  2. The impact of ENSO on Southern African rainfall in CMIP5 ocean atmosphere coupled climate models

    NASA Astrophysics Data System (ADS)

    Dieppois, Bastien; Rouault, Mathieu; New, Mark

    2015-11-01

    We study the ability of 24 ocean atmosphere global coupled models from the Coupled Model Intercomparison Project 5 (CMIP5) to reproduce the teleconnections between El Niño Southern Oscillation (ENSO) and Southern African rainfall in austral summer using historical forced simulations, with a focus on the atmospheric dynamic associated with El Niño. Overestimations of summer rainfall occur over Southern Africa in all CMIP5 models. Abnormal westward extensions of ENSO patterns are a common feature of all CMIP5 models, while the warming of the Indian Ocean that happens during El Niño is not correctly reproduced. This could impact the teleconnection between ENSO and Southern African rainfall which is represented with mixed success in CMIP5 models. Large-scale anomalies of suppressed deep-convection over the tropical maritime continent and enhanced convection from the central to eastern Pacific are correctly simulated. However, regional biases occur above Africa and the Indian Ocean, particularly in the position of the deep convection anomalies associated with El Niño, which can lead to the wrong sign in rainfall anomalies in the northwest part of South Africa. From the near-surface to mid-troposphere, CMIP5 models underestimate the observed anomalous pattern of pressure occurring over Southern Africa that leads to dry conditions during El Niño years.

  3. 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.

  4. Geochemistry of East African Rift basalts: An overview

    NASA Astrophysics Data System (ADS)

    Furman, Tanya

    2007-06-01

    Mafic lavas erupted along the East African Rift System from the Afar triangle in northern Ethiopia to the Rungwe province in southern Tanzania display a wide range of geochemical and isotopic compositions that reflect heterogeneity in both source and process. In areas with the lowest degree of crustal extension (the Western and Southern Kenya Rifts) primitive lavas record the greatest extent of lithospheric melting, manifest in elevated abundances of incompatible elements and highly radiogenic Sr-Nd-Pb isotopic compositions. Where prolonged extension has removed most or all of the mantle lithosphere (the Turkana and Northern Kenya Rifts), a larger role for sub-lithospheric processes is indicated. At intermediate degrees of extension (the Main Ethiopian Rift) both lithospheric and sub-lithospheric contributions are observed, and crustal assimilation occurs in some cases. Despite the wide compositional range of African Rift basalts, a restricted number of source domains contribute to magmatism throughout the area. These individual domains are: (1) the subcontinental mantle lithosphere; (2) a plume source with high-μ Sr-Nd-Pb-He isotopic affinities, present in all areas within and south of the Turkana Depression; and (3) a plume source with isotopic signatures analogous to those observed in some ocean islands, including high 3He/ 4He values, present throughout the Ethiopian Rift and the Afar region. The two plume sources may both be derived from the South African Superplume, which is likely to be a compositionally heterogeneous feature of the lower mantle.

  5. 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.

  6. 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

  7. 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. PMID:25455366

  8. 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, Chengyuan; Liu, Yongsheng; Min, Ning; Zong, Keqing; Hu, Zhaochu; Gao, Shan

    2016-09-01

    In-situ major and trace elements and Sr isotopic compositions of peridotite xenoliths of the Datong Quaternary alkaline basalt were analyzed to evaluate the influences of the southward subduction of the Paleo-Asian oceanic plate (PAOP) on the lithospheric mantle transformation of the North China Craton (NCC). These peridotite xenoliths including spinel harzburgites and lherzolites were classified into three groups. The type 1 peridotites have the lowest temperatures (961-1007 °C). Clinopyroxenes in these peridotites exhibit LREE-depleted REE patterns and have the lowest 87Sr/86Sr ratios of 0.70243-0.70411. The type 2 and 3 peridotites show higher temperatures (1017-1022 °C). Clinopyroxenes in the type 2 peridotite have V-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 enrichment in LILE and depletion in HFSE, and have the highest 87Sr/86Sr ratios of 0.71145-0.71285. The mineral chemistries and modal calculations suggest that the protolith of these peridotites experienced a variable degree of partial melting. The type 2 and 3 peridotites sampled from deeper depth experienced latter cryptic carbonatitic metasomatism. The carbonatitic veinlets have generally consistent trace element patterns and Sr isotopic ratios with the calculated melts equilibrated with clinopyroxenes in the type 3 peridotite, which may represent the percolated carbonatitic melt quickly solidified in the relatively cold and shallow mantle. The remarkable negative Eu anomalies (0.37-0.61) and highly radiogenic Sr isotopic compositions of the calculated metasomatic agents preclude indicate melt derived from carbonated peridotite or carbonated eclogite but point to a crustal sedimentary origin. Considering the tectonic setting and

  9. 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.

  10. 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.

  11. 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. PMID:17743571

  12. 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

  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 dynamics in eastern Mediterranean: Insights from seismic structure and anisotropy

    NASA Astrophysics Data System (ADS)

    Lebedev, S.; Neenan, E.; Knapmeyer-Endrun, B.; Meier, T.; Agius, M. R.; Schaeffer, A. J.; Tirel, C.; Friederich, W.

    2012-04-01

    The slow convergence of the African, Arabian and Eurasian Plates in eastern Mediterranean has been accompanied by widespread deformation of continental lithosphere within the region. The continuing southward retreat of the Hellenic Trench, where oceanic lithosphere is subducting northward, is accommodated by lateral movements and stretching of continental blocks of the overriding plate. Both the boundary conditions and lateral variations in the mechanical strength of the lithosphere determine the distribution and character of the localised and diffuse deformation that is taking place. Seismic velocities within the lithosphere depend on its temperature and can be used to infer its strength. Azimuthal seismic anisotropy indicates fabrics within the crust, mantle lithosphere and asthenosphere that are a record of their deformation and flow. Seismic observations can thus provide key information on the flow patterns and underlying mechanisms of the complex lithospheric deformation. Here we present the results of the analysis of surface waves recorded at seismic stations in eastern Mediterranean. Frequency-dependent surface-wave anisotropy reveals depth-dependent orientations of anisotropic fabrics in the crust and mantle. Isotropic-average shear-velocity profiles and surface-wave tomography indicate variability in temperature and, therefore, in mechanical strength of the lithosphere. Published shear-wave splitting measurements show remarkably uniform, NNE-SSW fast-propagation orientations across much of eastern Mediterranean, suggesting large-scale flow within the asthenosphere towards the retreating Hellenic Trench. The motion and deformation of lithospheric blocks are also driven by the trench retreat but are very different from those of the asthenosphere beneath them. The character of deformation within the lithosphere varies laterally and, also, vertically, from the brittle upper crust to the ductile lower crust and lithospheric mantle. The shear associated with the

  15. Chromatographic metasomatism of the Arabian—Nubian lithosphere

    NASA Astrophysics Data System (ADS)

    Stein, Mordechai; Navon, Oded; Kessel, Ronit

    1997-11-01

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

  16. Chromatographic metasomatism of the Arabian-Nubian lithosphere

    NASA Astrophysics Data System (ADS)

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

    1997-11-01

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

  17. The formation of volcanic centers at the Colorado Plateau as a result of the passage of aqueous fluid through the oceanic lithosphere and the subcontinental mantle: New implications for the planetary water cycle in the western United States

    NASA Astrophysics Data System (ADS)

    Sommer, Holger; Regenauer-Lieb, Klaus; Gasharova, Biliana; Jung, Haemyeong

    2012-10-01

    We provide new petrological evidence for the strong influence of water on the formation of the oceanic lithospheric mantle, the subcontinental mantle above, and the continental lithosphere. Our analysis throws new light on the hypothesis that new continental lithosphere was formed by the passage of silicate-rich aqueous fluid through the sub-continental mantle. In order to investigate this hypothesis, we analyzed a representative collection of lherzolite and harzburgite xenoliths from the sample volcano known as "The Thumb", located in the center of the Colorado Plateau, western United States. The studied sample collection exhibits multi-stage water enrichment processes along point, line and planar defect structures in nominally anhydrous minerals and the subsequent formation of the serpentine polymorph antigorite along grain boundaries and in totally embedded annealed cracks. Planar defect structures act like monomineralic and interphase grain boundaries in the oceanic lithosphere and the subcontinental mantle beneath the North American plate, which was hydrated by the ancient oceanic Farallon plate during the Cenozoic and Mesozoic eras. We used microspectroscopical, petrological, and seismological techniques to confirm multi-stage hydration from a depth of ˜150 km to just below the Moho depth. High-resolution mapping of the water distribution over homogeneous areas and fully embedded point, line and planar defects in olivine crystals of lherzolitic and harzburgitic origin by synchrotron infrared microspectroscopy enabled us to resolve local wet spots and thus reconstruct the hydration process occurring at a depth of ˜150 km (T ≈ 1225 °C). These lherzolites originated from the middle part of the Farallon mantle slab; they were released during the break up of the Farallon mantle slab, caused by the instability of the dipping slab. The background hydration levels in homogeneous olivines reached ˜138 ppm wt H2O, and the water concentration at the planar defects

  18. 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.

  19. 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

  20. Lithospheric thinning beneath rifted regions of Southern California.

    PubMed

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

    2011-11-11

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

  1. Lithospheric dynamics near plate boundaries

    NASA Astrophysics Data System (ADS)

    Solomon, Sean C.

    1992-09-01

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

  2. 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.

  3. Half-precessional climate forcing of Indian Ocean monsoon dynamics on the East African equator

    NASA Astrophysics Data System (ADS)

    Verschuren, D.; Sinninghe Damste, J. S.; Moernaut, J.; Kristen, I.; Fagot, M.; Blaauw, M.; Haug, G. H.; Project Members, C.

    2008-12-01

    The EuroCLIMATE project CHALLACEA produced a detailed multi-proxy reconstruction of the climate history of equatorial East Africa, based on the sediment record of Lake Challa, a 4.2 km2, 92-m deep crater lake on the lower East slope of Mt. Kilimanjaro (Kenya/Tanzania). Relatively stable sedimentation dynamics over the past 25,000 years resulted in a unique combination of high temporal resolution, excellent radiometric (210Pb, 14C) age control, and confidence that recording parameters of the climatic proxy signals extracted from the sediment have remained constant through time. The equatorial (3 deg. S) location of our study site in East Africa, where seasonal migration of convective activity spans the widest latitude range worldwide, produced unique information on how varying rainfall contributions from the northeasterly and southeasterly Indian Ocean monsoons shaped regional climate history. The Challa proxy records for temperature (TEX86) and moisture balance (reflection-seismic stratigraphy and the BIT index of soil bacterial input) uniquely weave together tropical climate variability at orbital and shorter time scales. The temporal pattern of reconstructed moisture balance bears the clear signature of half- precessional insolation forcing of Indian Ocean monsoon dynamics, modified by northern-latitude influence on moisture-balance variation at millennial and century time scales. During peak glacial time (but not immediately before) and the Younger Dryas, NH ice sheet influences overrode local insolation influence on monsoon intensity. After the NH ice sheets had melted and a relatively stable interglacial temperature regime developed, precession-driven summer insolation became the dominant determinant of regional moisture balance, with anti-phased patterns of Holocene hydrological change in the northern and southern (sub)tropics, and a uniquely hybrid pattern on the East African equator. In the last 2-3000 years a series of multi-century droughts with links to

  4. 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

  5. Attributes of mesoscale convective systems at the land-ocean transition in Senegal during NASA African Monsoon Multidisciplinary Analyses 2006

    NASA Astrophysics Data System (ADS)

    Delonge, Marcia S.; Fuentes, Jose D.; Chan, Stephen; Kucera, Paul A.; Joseph, Everette; Gaye, Amadou T.; Daouda, Badiane

    2010-05-01

    In this study we investigate the development of a mesoscale convective system (MCS) as it moved from West Africa to the Atlantic Ocean on 31 August 2006. We document surface and atmospheric conditions preceding and following the MCS, particularly near the coast. These analyses are used to evaluate how thermodynamic and microphysical gradients influence storms as they move from continental to maritime environments. To achieve these goals, we employ observations from NASA African Monsoon Multidisciplinary Analyses (NAMMA) from the NASA S band polarimetric Doppler radar, a meteorological flux tower, upper-air soundings, and rain gauges. We show that the MCS maintained a convective leading edge and trailing stratiform region as it propagated from land to ocean. The initial strength and organization of the MCS were associated with favorable antecedent conditions in the continental lower atmosphere, including high specific humidity (18 g kg-1), temperatures (300 K), and wind shear. While transitioning, the convective and stratiform regions became weaker and disorganized. Such storm changes were linked to less favorable thermodynamic, dynamic, and microphysical conditions over ocean. To address whether storms in different life-cycle phases exhibited similar features, a composite analysis of major NAMMA events was performed. This analysis revealed an even stronger shift to lower reflectivity values over ocean. These findings support the hypothesis that favorable thermodynamic conditions over the coast are a prerequisite to ensuring that MCSs do not dissipate at the continental-maritime transition, particularly due to strong gradients that can weaken West African storms moving from land to ocean.

  6. Differential opening of the Central and South Atlantic Oceans and the opening of the West African rift system

    NASA Astrophysics Data System (ADS)

    Fairhead, J. D.; Binks, R. M.

    1991-02-01

    Plate tectonic studies of the development of the Central and South Atlantic Oceans using Seasat and Geosat altimeter and magnetic anomaly isochron data now provide quantitative models of seafloor spreading through time. Such models enable an initial assessment of the differential opening between these two oceanic basins to be determined. The Equatorial Atlantic is an integral part of this oceanic rifting process, allowing stresses arising from the differential opening to be dissipated into both the Caribbean and Africa along its northern and southern boundaries respectively. The tectonic model for the West African rift system, based on geological and geophysical studies, shows a series of strike-slip fault zones diverging into Africa from the Gulf of Guinea and dissipating their shear movement into the development of extensional basins orientated perpendicular to these faults zones. The development of the West African rift system was contemporaneous with the early opening of the South Atlantic, continued to develop well after the final breakup of South America from Africa and did not cease until the late Cretaceous when there was a major phase of basin inversion and deformation. Santonian ( ~ 80 Ma) deformation across the Benue Trough (Nigeria) is broadly contemporaneous with dextral shear reactivation of the central African fracture system which, in turn resulted in renewed extension in the Sudan basins during the late Cretaceous and early Tertiary. This paper illustrates the close linkage in both time and space between the history of the African rift basins and the opening of the Atlantic. Both exhibit distinct phases of evolution with the rift basins developing in direct response to the differential opening between the Central and South Atlantic in order to dissipate stresses generated by this opening. The Mesozoic tectonic model proposed is therefore one of an intimate interaction between oceanic and continental tectonics.

  7. 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.

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

    NASA Astrophysics Data System (ADS)

    Pasyanos, Michael E.

    2010-01-01

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

  9. 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.

  10. 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.

  11. The East African rift system

    NASA Astrophysics Data System (ADS)

    Chorowicz, Jean

    2005-10-01

    ), its almost 1000 km diameter panache weakening the lithosphere and preparing the later first rifting episode along a preexisting weak zone, a Pan-African suture zone bordering the future Afar region. From the Afar, the rift propagated afterward from north to south on the whole, with steps of local lithospheric failure nucleations along preexisting weak zones. These predisposed lines are mainly suture zones, in which partial activation of low angle detachment faults reworked former thrust faults verging in opposite directions, belonging to double verging ancient belts. This is responsible for eventual reversal in rift asymmetry from one basin to the next. Supposing the plume migrated southward, or other plumes emplaced, the rift could propagate following former weaknesses, even outside areas influenced by plumes. This view of rift formation reconciles the classical models: active plume effect triggered the first ruptures; passive propagations of failure along lithospheric scale weak zones were responsible for the onset of the main rift segments. Various other aspects are shortly considered, such as tectonics and sedimentation, and relationships of the 'cradle of Mankind' with human evolution. By its size, structure and occurrence of oceanic lithosphere in the Afar, the EARS can be taken as a model of the prelude of oceanic opening inside a continent.

  12. Global and Regional-scale Sst Variability and West African Monsoon. The Role of The Indian Ocean : A Numerical Study

    NASA Astrophysics Data System (ADS)

    Trzaska, S.; Fontaine, B.; Janicot, S.

    Interannual to decadal variability of the West African Monsoon has been commonly linked to Tropical Atlantic and Pacific SST variabilities (so called "Atlantic Dipole" and ENSO). Tropical Atlantic is thought to affect West African Monsoon via modi- fication of low-level thermal gradients driving the monsoon thus the location of the rainbelt over the continent. Warm events in the easten Pacific may affect it via up- per level zonal circulation and eventual subsidence over West Africa. However the teleconnections seem to have modified through time : main association with tropi- cal Atlantic during 50's and 60's i.e. the wetter period vs stronger association with ENSO and relative disconnection with tropical Atlantic during recent, dry decades. The role of the Indian Ocean has not been much investigated so far. The variability of this basin is dominated by a slow warming trend which compares well with the global warming. This study is aimed at investigating the possible effects of the Indian Ocean warming on the West African Monsoon dynamics and its teleconnections to ENSO and Tropical Atlantic. It is shown that this warming can potentially modify circulation anomalies related to ENSO in the Atlantic-African region by limiting the zonal extent of the zonal circulation anomalies and shifting the main subsidence branch to Africa and central Atlantic. In non-ENSO case monsoon circulation seems also to have more zonal orientation. The results are documented in the divergent circulation frame since it allows to unify a regional view of the monsoon as a meridional overturning with the global effects of ENSO in the zonal circulation. Modifications in the low-level moisture flux are also presented.

  13. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  14. 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

  15. Lithosphere-asthenosphere boundary: Where and why?

    NASA Astrophysics Data System (ADS)

    Aryasova, Olga; Khazan, Yakov

    2015-04-01

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

  16. 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.

  17. 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.

  18. 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

  19. Reconstruction of the flanks of the Mid-Atlantic Ridge, 28° to 29°N: Implications for evolution of young oceanic lithosphere at slow-spreading centers

    NASA Astrophysics Data System (ADS)

    Sloan, Heather; Patriat, Philippe

    2004-09-01

    We reconstruct the flanks of the Mid-Atlantic Ridge (MAR) between 28° and 29°N from 1 to 10 Ma at intervals of 1 Myr for the purpose of investigating evolution of young oceanic lithosphere morphology and its variation through time using an innovative method that combines seafloor subsidence correction with interpolated isochrons and rotation poles. Reconstruction results are consistent with formation of abyssal hills every 1 to 2 Myr in 2-3 Myr old lithosphere at the outer edge of the ridge mountains as a result of transition from dynamic regime near the axis to isostatic regime of the flanks. The oblique passage of structures formed at the axial valley walls through this transition zone may play a role in the development of inside corner high bathymetry. Asymmetric juxtaposition of abyssal hill morphology in reconstruction is indicative of independent formation and evolution of the morphotectonic fabric on opposing flanks. The two major factors affecting asymmetric ridge flank morphology are found to be sense of axial offset and fluctuation of magmatic activity at the segment scale. Sense of axial offset determines the relative distribution of inside and outside corner bathymetry on the flanks. Enhanced magma supply and associated segment propagation may contribute to half-spreading rate asymmetry, accretion of thicker crust, and formation of larger abyssal hills on the faster spreading flank. Consistent alignment in reconstruction of the base of the steep walls bounding inside corner bathymetry confirms that they mark the boundaries between segments on the flanks and that the deep sediment-filled basins, typically identified as discordant zones, are attributable to outside corner bathymetry.

  20. 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

  1. 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

  2. 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.

  3. 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

  4. 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.

  5. 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.

  6. Tag team tectonics: mantle upwelling and lithospheric heterogeneity ally to rift continents (Invited)

    NASA Astrophysics Data System (ADS)

    Nelson, W. R.; Furman, T.

    2013-12-01

    The configuration of continents we know today is the result of several billion years of active Wilson Cycle tectonics. The rifting of continents and subsequent development of ocean basins is an integral part of long-term planetary-scale recycling processes. The products of this process can be seen globally, and the East African Rift System (EARS) provides a unique view of extensional processes that actively divide a continent. Taken together with the adjoining Red Sea and Gulf of Aden, the EARS has experienced over 40 Ma of volcanism and ~30 Ma of extension. While early (pre-rift) volcanism in the region is attributed to mantle plume activity, much of the subsequent volcanism occurs synchronously with continental rifting. Numerous studies indicate that extension and magmatism are correlated: extension leads to decompression melting while magmatism accommodates further extension (e.g. Stein et al., 1997; Buck 2004; Corti 2012). Evaluation of the entire EARS reveals significant geochemical patterns - both spatial and temporal - in the volcanic products. Compositional variations are tied directly to the melt source(s), which changes over time. These variations can be characterized broadly by region: the Ethiopian plateau and Turkana Depression, the Kenya Rift, and the Western Rift. In the Ethiopian plateau, early flood basalt volcanism is dominated by mantle plume contributions with variable input from lherzolitic mantle lithosphere. Subsequent alkaline shield volcanism flanking the juvenile Main Ethiopian Rift records the same plume component as well as contributions from a hydrous peridotitic lithosphere. The hydrous lithosphere does not contribute indefinitely. Instead, young (< 2 Ma) volcanism taps a combination of the mantle plume and anhydrous depleted lithospheric mantle. In contrast, volcanism in the Kenya Rift and the Western Rift are derived dominantly from metasomatized lithospheric mantle rather than mantle plume material. These rifts lie in the mobile

  7. A lithospheric 3D temperature study from the South Atlantic

    NASA Astrophysics Data System (ADS)

    Hirsch, K. K.; Scheck-Wenderoth, M.; Maystrenko, Y.; Sippel, J.

    2009-04-01

    The East African continental margin is a passive volcanic margin that experienced a long post-rifting history after break up in Early Cretaceous times. The break up resulted in the formation of a number of basins along the margin. The by far largest depocentre in the South Atlantic, the Orange Basin, was the location of previously performed studies. These studies of the Orange Basin have been performed to investigate the crustal structure and the temperature evolution of the basin. In this way, they gave way to new insights and to a number of questions. With 3D gravity modelling we found the crust to include high density bodies. Furthermore, a rifting model was developed which explained both the geometry and the thermal constraints of the basin. Now, this study has been extended spatially to cover a larger area and into depth to include the deep lithosphere. The main goal is to combine information on the geometry and properties of the sedimentary part of the system with data on the geometry and physical properties of the deep crust. It was also aimed to integrate both the continental and the oceanic parts of the margin into a consistent 3D structural model on a lithospheric scale. A 3D temperature model was evaluated for the passive continental margin of the South Atlantic including the lithospheric structure of the margin. We evaluate a case study for different scenarios to estimate the influence of sediments and crustal structures on the thermal field. The calculated conductive field is constrained by temperature measurements and 3D gravity modelling. At the Norwegian continental margin it has been found that a differentiation of the physical properties of the lower crust and the mantle is needed between the oceanic and continental domains to explain the observations. We aim to compare the younger setting of the Norwegian continental margin with the old passive margin in the South Atlantic. In particular, the South Atlantic is interesting since the southern half

  8. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  9. 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.

  10. Rejuvenation of the lithosphere by the Hawaiian plume.

    PubMed

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

    2004-02-26

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

  11. Tectonic provinces of the Atlantic Ocean

    NASA Astrophysics Data System (ADS)

    Pushcharovsky, Yu. M.

    2009-05-01

    The tectonic structure of the floor of the Atlantic Ocean beyond the continental margins is insufficiently studied. This is also true of its tectonic demarcation. The segmentation of the floor into regional-scale tectonic provinces of several orders proposed in this paper is primarily based on structural and historical geological features. It is shown that deep oceanic basins and fault tectonics are of particular importance in this respect. Tectonic provinces of two orders are distinguished by a set of attributes. The first-order provinces are the North, Central, South, and Antarctic domains of the Atlantic Ocean. They are separated by wide demarcation fracture zones into Transatlantic (transverse) second-order tectonic provinces. Ten such provinces are recognized (from the north southward): Greenland-Lofoten, Greenland-Scandinavia, Greenland-Ireland, Newfoundland-European, North American-African, Antilles-African, Angola-Brazil, Cape-Argentine, North Antarctic, and South Antarctic. This subdivision demonstrates significant differentiation in the geodynamic state of the oceanic lithosphere that determines nonuniform ocean formation and the tectonic features of the ocean floor. The latitudinal orientation of the second-order provinces inherits the past tectonic pattern, though newly formed structural units cannot be ruled out. The Earth rotation exerts a crucial effect on the crust and the mantle.

  12. 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

  13. The Lithospheric Structure of Southern Africa from Magnetotelluric Sounding

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  14. 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.

  15. 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

  16. Mantle support of the East African Rift System

    NASA Astrophysics Data System (ADS)

    Lin, S.; van Keken, P. E.; Brandenburg, J. P.; Furman, T.; Bryce, J.

    2007-12-01

    The African Superplume is a region of slow seismic wave velocities in the lower mantle under southern Africa. The uplift, volcanism and rifting that defines the much of eastern and southern Africa suggest a dynamic link between lower mantle dynamics and near-surface processes affecting the African plate. The dynamic link between the lower mantle and the surface, and the structure and dynamics of the upper mantle below the East African Rift System (EARS) remain unclear. As part of a comprehensive geochemical and numerical investigation of basaltic magmatism in the EARS we have modeled the interaction between putative upper mantle plumes and the rifting continental lithosphere. The modeling provides dynamically tested scenarios that explain the observed episodes of Cenozoic volcanism. Results from recent models that provided an explanation for the present day distribution of volcanism (Lin et al., EPSL, 237, 2005) suggest two plumes below Afar and Tanzania whose uplift is influenced by lithospheric topography. In new 3D modeling we provide improved quantification of the mantle involvement in generating EARS volcanism as constrained by the timing of uplift and regional volcanism. The time scales of episodicity of the volcanism observed at Turkana (related to the Tanzania-Kenya plume) since 45 Ma can be explained by deep- seated time-dependent plume activity. We suggest that this time-dependence is due to thermochemical interactions of dense recycled oceanic crust in the thermally hot regions in the African superplume region (Lin and Van Keken, Nature, 436, 2005).

  17. 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.

  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. Effects of African dust deposition on phytoplankton in the western tropical Atlantic Ocean off Barbados

    NASA Astrophysics Data System (ADS)

    Chien, Chia-Te; Mackey, Katherine R. M.; Dutkiewicz, Stephanie; Mahowald, Natalie M.; Prospero, Joseph M.; Paytan, Adina

    2016-05-01

    Bioassay incubation experiments conducted with nutrients and local atmospheric aerosol amendments indicate that phosphorus (P) availability limited phytoplankton growth in the low-nutrient low-chlorophyll (LNLC) ocean off Barbados. Atmospheric deposition provides a relatively large influx of new nutrients and trace metals to the surface ocean in this region in comparison to other nutrient sources. However, the impact on native phytoplankton is muted due to the high ratio of nitrogen (N) to P (NO3:SRP > 40) and the low P solubility of these aerosols. Atmospheric deposition induces P limitation in this LNLC region by adding more N and iron (Fe) relative to P. This favors the growth of Prochlorococcus, a genus characterized by low P requirements and highly efficient P acquisition mechanisms. A global three-dimensional marine ecosystem model that includes species-specific phytoplankton elemental quotas/stoichiometry and the atmospheric deposition of N, P, and Fe supports this conclusion. Future increases in aerosol N loading may therefore influence phytoplankton community structure in other LNLC areas, thereby affecting the biological pump and associated carbon sequestration.

  20. 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

  1. The international lithosphere program

    NASA Astrophysics Data System (ADS)

    Flinn, Edward A.

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

  2. 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.

  3. Adakites from collision-modified lithosphere

    NASA Astrophysics Data System (ADS)

    Haschke, M.; Ben-Avraham, Z.

    2005-08-01

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

  4. 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.

  5. 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.

  6. 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.

  7. The electrical Lithosphere of the Alboran Domain

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  8. Formation of the Cameroon Volcanic Line by lithospheric basal erosion: Insight from mantle seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Elsheikh, A. A.; Gao, S. S.; Liu, K. H.

    2014-12-01

    The formation mechanism of intraplate volcanism such as that along the Cameroon Volcanic Line (CVL) is one of the controversial problems in global tectonics. Models proposed by previous studies include re-activation of ancient suture zones, lithospheric thinning by mantle plumes, and edge-driven mantle convection. To provide additional constraints on the models for the formation of the CVL, we measured shear-wave splitting parameters at 36 stations in the vicinity of the CVL using a robust procedure involving automatic batch processing and manual screening to reliably assess and objectively rank shear-wave splitting parameters (fast polarization directions and splitting times). The resulting 432 pairs of splitting parameters show a systematic spatial variation. Most of the measurements with ray-piercing points (at 200 km depth) beneath the CVL show a fast direction that is parallel to the volcanic line, while the fast directions along the coastline are parallel to the continental margin. The observations can best be interpreted using a model that involves a channel flow at the bottom of the lithosphere originated from the NE-ward movement of the asthenosphere relative to the African plate. We hypothesize that progressive thinning of the lithosphere through basal erosion by the flow leads to decompression melting and is responsible for the formation of the CVL. The model is consistent with the lack of age progression of the volcanoes in the CVL, can explain the formation of both the continental and oceanic sections of the CVL, and is supported by previous geophysical observations and geodynamic modeling results.

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

    PubMed

    Rychert, Catherine A; Shearer, Peter M

    2009-04-24

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

  10. 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

  11. 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.

  12. Basement control in the development of the early cretaceous West and Central African rift system

    NASA Astrophysics Data System (ADS)

    Maurin, Jean-Christophe; Guiraud, René

    1993-12-01

    -Early Cretaceous the St. Helena mantle plume may have had an important effect in weakening the lithosphere across a broad zone from Brazil to Sudan. However, causes of extension and stretching are likely to be found in a plate tectonic framework. At that time, the opening of both the Indian and Atlantic oceans were the dominant forces which favoured the development of the WCARS and its propagation by strike-slip movements and extensional displacements along pre-existing Pan-African lithospheric zones of weakness.

  13. Actual data of the allocation of oxygen from the Earth's lithosphere

    NASA Astrophysics Data System (ADS)

    Kapochkin, Borys; Mikhaylov, Valery; Kucherenko, Nataliya; Kapochkina, Anastasiya

    2010-05-01

    We present observational data of the allocation of oxygen from the lithosphere to the oceans and seas. Observational data include marine hydrochemical observations in the Pacific Ocean, performed in 1991-1998. Studied dissolved gases in seawater. Measurements of dissolved oxygen were carried out by different methods. The observations were made from the scientific ship every 4 hours. Measurements were made from surface to bottom. We also present data of the allocation of oxygen from the lithosphere to the deep layers of the hydrogen sulphide zone of the Black Sea. Nature of the oxygen of the lithosphere - is the destruction of minerals, that containing oxygen. The destruction of minerals occurs in conditions of discontinuous deformation in the Earth's lithosphere. The more intense geodeformations, than more oxygen is released from the minerals of the lithosphere.

  14. Seismically Imaging the Destruction of Continental Lithosphere beneath Afar

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  15. 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

  16. Are Archean lithospheric keels inverted?

    NASA Astrophysics Data System (ADS)

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

    2007-02-01

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

  17. Anorogenic plateau formation induced by a heated lithosphere

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  18. 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

  19. 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

  20. 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.

  1. 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

  2. Integrative Analysis of Mantle Lithosphere Rheology

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  3. Shear-wave velocity structure of young Atlantic Lithosphere from dispersion analysis and waveform modelling of Rayleigh waves

    NASA Astrophysics Data System (ADS)

    Grevemeyer, Ingo; Lange, Dietrich; Schippkus, Sven

    2016-04-01

    The lithosphere is the outermost solid layer of the Earth and includes the brittle curst and brittle uppermost mantle. It is underlain by the asthenosphere, the weaker and hotter portion of the mantle. The boundary between the brittle lithosphere and the asthenosphere is call the lithosphere-asthenosphere boundary, or LAB. The oceanic lithosphere is created at spreading ridges and cools and thickens with age. Seismologists define the LAB by the presence of a low shear wave velocity zone beneath a high velocity lid. Surface waves from earthquakes occurring in young oceanic lithosphere should sample lithospheric structure when being recorded in the vicinity of a mid-ocean ridge. Here, we study group velocity and dispersion of Rayleigh waves caused by earthquakes occurring at transform faults in the Central Atlantic Ocean. Earthquakes were recorded either by a network of wide-band (up to 60 s) ocean-bottom seismometers (OBS) deployed at the Mid-Atlantic Ridge near 15°N or at the Global Seismic Network (GSN) Station ASCN on Ascension Island. Surface waves sampling young Atlantic lithosphere indicate systematic age-dependent changes of group velocities and dispersion of Rayleigh waves. With increasing plate age maximum group velocity increases (as a function of period), indicating cooling and thickening of the lithosphere. Shear wave velocity is derived inverting the observed dispersion of Rayleigh waves. Further, models derived from the OBS records were refined using waveform modelling of vertical component broadband data at periods of 15 to 40 seconds, constraining the velocity structure of the uppermost 100 km and hence in the depth interval of the mantle where lithospheric cooling is most evident. Waveform modelling supports that the thickness of lithosphere increases with age and that velocities in the lithosphere increase, too.

  4. Temperature, lithosphere-asthenosphere boundary, and heat flux beneath the Antarctic Plate inferred from seismic velocities

    NASA Astrophysics Data System (ADS)

    An, Meijian; Wiens, Douglas A.; Zhao, Yue; Feng, Mei; Nyblade, Andrew; Kanao, Masaki; Li, Yuansheng; Maggi, Alessia; Lévêque, Jean-Jacques

    2015-12-01

    We estimate the upper mantle temperature of the Antarctic Plate based on the thermoelastic properties of mantle minerals and S velocities using a new 3-D shear velocity model, AN1-S. Crustal temperatures and surface heat fluxes are then calculated from the upper mantle temperature assuming steady state thermal conduction. The temperature at the top of the asthenosphere beneath the oceanic region and West Antarctica is higher than the dry mantle solidus, indicating the presence of melt. From the temperature values, we generate depth maps of the lithosphere-asthenosphere boundary and the Curie temperature isotherm. The maps show that East Antarctica has a thick lithosphere similar to that of other stable cratons, with the thickest lithosphere (~250 km) between Domes A and C. The thin crust and lithosphere beneath West Antarctica are similar to those of modern subduction-related rift systems in East Asia. A cold region beneath the Antarctic Peninsula is similar in spatial extent to that of a flat-subducted slab beneath the southern Andes, indicating a possible remnant of the Phoenix Plate, which was subducted prior to 10 Ma. The oceanic lithosphere generally thickens with increasing age, and the age-thickness correlation depends on the spreading rate of the ridge that formed the lithosphere. Significant flattening of the age-thickness curves is not observed for the mature oceanic lithosphere of the Antarctic Plate.

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

    NASA Astrophysics Data System (ADS)

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

    2005-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2009-04-01

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

  7. Circum-Arctic lithospheric transects from onshore to offshore

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  8. 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

  9. 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

  10. An elastoviscoplastic finite element model of lithospheric deformation

    NASA Astrophysics Data System (ADS)

    Albert, Richard Alan

    1998-12-01

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

  11. 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

  12. 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

  13. Mechanical heterogeneities and lithospheric extension

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  14. Lithospheric structure and compensation mechanisms of the Galapagos Archipelago

    NASA Astrophysics Data System (ADS)

    Feighner, Mark A.; Richards, Mark A.

    1994-04-01

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

  15. 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

  16. Continuous deformation versus faulting through the continental lithosphere of new zealand

    PubMed

    Molnar; Anderson; Audoine; Eberhart-Phillips; Gledhill; Klosko; McEvilly; Okaya; Savage; Stern; Wu

    1999-10-15

    Seismic anisotropy and P-wave delays in New Zealand imply widespread deformation in the underlying mantle, not slip on a narrow fault zone, which is characteristic of plate boundaries in oceanic regions. Large magnitudes of shear-wave splitting and orientations of fast polarization parallel to the Alpine fault show that pervasive simple shear of the mantle lithosphere has accommodated the cumulative strike-slip plate motion. Variations in P-wave residuals across the Southern Alps rule out underthrusting of one slab of mantle lithosphere beneath another but permit continuous deformation of lithosphere shortened by about 100 kilometers since 6 to 7 million years ago. PMID:10521344

  17. 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

  18. Convective thinning of the lithosphere: a model constrained by geoid observations

    NASA Astrophysics Data System (ADS)

    Dalloubeix, C.; Fleitout, L.

    1989-11-01

    Geoid observations in the oceans suggest that lithospheric thinning is effected by small-scale convection within hot-spot material trapped in dimples a few tens of kilometers deep at the base of the lithosphere. Significant thinning can occur within 5 Ma if the viscosity of the convecting material is 1016 Pa s. Partial melting can enhance considerably the vigour of convection and the same rate of lithospheric thinning is obtained for a viscosity about five times higher. These results are derived from convection models with a realistic temperature-dependent rheology using the mean field approximation.

  19. 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.

  20. 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.

  1. 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.

  2. Re-Os isotopic constraints on the evolution of the Bangong-Nujiang Tethyan oceanic mantle, Central Tibet

    NASA Astrophysics Data System (ADS)

    Huang, Qi-Shuai; Shi, Ren-Deng; O'Reilly, Suzanne Y.; Griffin, William L.; Zhang, Ming; Liu, De-Liang; Zhang, Xiao-Ran

    2015-05-01

    Geochemical (including Re-Os isotopic) studies of the mantle rocks of ophiolites in the Bangong-Nujiang suture zone in central Tibet have provided a coherent picture of the evolution of the Bangong-Nujiang Tethyan oceanic mantle from mid-ocean ridge (MOR) to subduction-zone (SSZ) settings. Clinopyroxene (cpx)-harzburgites and lherzolites in the Bangong Lake ophiolite were formed in a MOR setting, as demonstrated by the Cr# of spinels (< 0.60) and whole-rock LREE-depleted patterns. Suprachondritic 187Re/188Os ratios (up to 1.833) of cpx-harzburgites and their spinels can be explained by interaction with melts derived from high Re/Os sources. Re-depletion (TRD) model ages (0.48-0.55 Ga) suggest these rocks may represent a Pan-African domain beneath the Gondwana continent. High TiO2 contents of spinels and whole-rock samples imply that the lherzolites were formed through a refertilization process. Similarly, Re-Os isotopic systematics of sulfides in the lherzolites (187Re/188Os: 0.173-1.717, 187Os/188Os: 0.12646-0.17340) demonstrate that they are mixtures of primary and secondary sulfides. 187Os/188Os ratios (0.1211-0.1226) of whole-rock lherzolites give TRD ages of 0.73-0.97 Ga, indicating the presence of Neoproterozoic lithospheric mantle under the spreading ridges. Mantle rocks in the SSZ-type ophiolites from Bangong Lake, Dongqiao and Nagqu reflect the complex evolution of the Bangong-Nujiang oceanic mantle during the SSZ stage. Most harzburgites from the Bangong Lake ophiolite give TRD ages of 1.0-1.5 Ga, possibly representing relics of a Mesoproterozoic lithospheric mantle. However, three samples have both high Os contents (1.32-4.45 ppb) and near-chondritic 187Os/188Os (0.1260-0.1297), and may represent Mesozoic oceanic lithospheric mantle. 187Os/188Os ratios of dunites and harzburgites from the Dongqiao and Nagqu ophiolites vary from 0.1174 to 0.1316 and give TRD ages up to 1.43 Ga, also suggesting the existence of a Mesoproterozoic lithospheric mantle which

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

    NASA Astrophysics Data System (ADS)

    Babuška, V.; Plomerová, J.

    1992-06-01

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

  4. Magmatic expressions of continental lithosphere removal

    NASA Astrophysics Data System (ADS)

    Wang, Huilin; Currie, Claire A.

    2015-10-01

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

  5. Marine electromagnetic constraints on lithosphere/asthenosphere structure

    NASA Astrophysics Data System (ADS)

    Constable, Steven; Key, Kerry; Naif, Samer

    2014-05-01

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

  6. 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

  7. 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.

  8. 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

  9. Gravity anomalies, crustal structure and flexure of the lithosphere at the Baltimore Canyon Trough

    NASA Astrophysics Data System (ADS)

    Watts, A. B.

    1988-07-01

    Seismic and gravity anomaly data have been used to examine the long-term mechanical properties of the lithosphere beneath the Baltimore Canyon Trough, a 15 km deep sedimentary basin in the rifted U.S. Atlantic margin. Seismic data constrain the crustal structure at the time of rifting as well as the amount of sedimentation and erosion that has occurred. The gravity effect of the initial crustal structure, sedimentary loading and erosional unloading have been computed and compared to the observed free-air gravity anomaly. The best fitting model is one in which the elastic thickness of the lithosphere ( Te) changes at the East Coast magnetic anomaly. Landward of the anomaly, Te is significantly smaller than previously reported values for continental or oceanic lithosphere of the same thermal age. Seaward of the anomaly, however, Te follows the depth to the 450°C oceanic isotherm which also describes the response of oceanic lithosphere to seamount and oceanic island loads. These results are interpreted as indicating that the shelf part of the trough is underlain by continental lithosphere which, following rifting, did not acquire any significant long-term strength.

  10. Magmatic expression of lithospheric thinning across continental rifts

    NASA Astrophysics Data System (ADS)

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

    1994-05-01

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

  11. 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

  12. Reconstruction of Benguela Current Ocean Productivity and West African Vegetation During Oxygen Isotope Stages 100 and 101.

    NASA Astrophysics Data System (ADS)

    Maslin, M.; Denison, S.; Ettwein, V.; Boot, C.; Pancost, R.; Evershed, R.; Platzman, E.; Murray, R.; Rosell-Mele, A.

    2002-12-01

    The Intensification of the Northern Hemisphere Glaciation (3.2 to 2.5 Ma) is a key climatic transition in Earth History. Deep-sea sediments recovered from ODP Leg 175 Site 1083 cover this important time period. Site 1083 has the advantage that it monitors both changes in the Benguela Current Upwelling system as well as the adjacent African continent. In this study we have focused on the interglacial (OIS 101) and glacial (OIS 100) periods that make up the final intensification step in this climatic transition. With a resolution of approximately 1 ka, we have reconstructed the following climatic parameters: Global ice volume (benthic foraminifera oxygen isotopes), wind strength and land aridity (HIRM, MS, Al/Ti ratios), upwelling intensity (UK37'-SSTs), surface water productivity (TOC, Chaetoceros resting spores, alkenone abundance, pigments, Ba), surface water nutrient availability (organic nitrogen isotopes), nutrient source (diatom species abundance), and land vegetation type (n-alkane abundance and carbon isotopes). Two conclusions have been drawn from this unique data set: 1. Surface water productivity peaks during glaciation, but is in fact lower during the full glacial than the previous interglacial. It is suggested that this is caused by the antagonistic effects of upwelling intensity and nutrient supply to this area. 2. The carbon isotopic record of n-alkanes, which monitors the relative abundance of C3 (tress and higher plants) vs C4 (tropical grasses) plants, clearly shows that the vegetation of South West Africa during this time period co-varies with precession and is independent of glacial-interglacial cycles. We suggest that this is due to the strong influence of precession on the penetration of the ITCZ-driven North-Central African monsoons (and resultant moisture availability) into the Congo Basin and the rest of SW Africa.

  13. 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

  14. 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

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

    NASA Astrophysics Data System (ADS)

    Gerya, Taras; Burov, Evgenii

    2016-04-01

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

  16. Lithospheric structures and Precambrian terrane boundaries in northeastern Botswana revealed through magnetotelluric profiling

    NASA Astrophysics Data System (ADS)

    Miensopust, M. P.; Jones, A. G.; Muller, M. R.; Garcia, X. A.; Evans, R. L.; Khoza, D. T.

    2010-12-01

    Within the framework of the Southern African MagnetoTelluric EXperiment (SAMTEX) a focused study was undertaken to gain better knowledge of the lithospheric geometries and structures of the westerly extension of the Zimbabwe Craton into Botswana, with the overarching aim to increase our understanding of southern African tectonics. The area of interest is located in northeastern Botswana, where Kalahari sands cover most of the geological terranes, and little is known about lithospheric structures and thickness. Some of the regional scale terrane boundary locations, defined based on potential field data, may be not sufficiently accurate for local scale studies. Investigation of the NNW-SSE orientated, 600 km long ZIM line profile crossing the Zimbabwe craton, Magondi mobile belt and Ghanzi-Chobe belt showed that the Zimbabwe craton is characterized by thick ( ˜ 220 km) resistive lithosphere, consistent with geochemical and geothermal estimates from kimberlite samples of the Orapa and Letlhakane pipes ( ˜ 175 km west of the profile). The lithospheric mantle of the Ghanzi-Chobe belt is resistive but the lithosphere is only about 180 km thick. At crustal depths a northwards-dipping boundary between the Ghanzi-Chobe and the Magondi belts is identified, and two mid- to lower-crustal conductors are discovered in the Magondi belt. The crustal terrane boundary between the Magondi and Ghanzi-Chobe belts is found to be located further to the north, and the southwestern boundary of the Zimbabwe craton might be further to the west, than previously inferred from potential field data.

  17. 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.

  18. 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

  19. 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.

  20. Buoyancy and localizing properties of continental mantle lithosphere: Insights from thermomechanical models of the eastern Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Watremez, L.; Burov, E.; d'Acremont, E.; Leroy, S.; Huet, B.; Pourhiet, L.; Bellahsen, N.

    2013-08-01

    Physical properties of the mantle lithosphere have a strong influence on the rifting processes and rifted structures. In particular, in context of rifting, two of these properties have been overlooked: (1) Mohr-Coulomb plasticity (localizing pressure dependent) may not be valid at mantle depths as opposed to nonlocalizing pressure-independent plasticity (hereafter, perfect plasticity), and (2) lithosphere buoyancy can vary, depending on the petrological composition of the mantle. Focussing on the Arabian plate, we show that the lithosphere may be negatively buoyant. We use thermomechanical modeling to investigate the importance of mantle rheology and composition on the formation of a passive margin, ocean-continent transition (OCT) and oceanic basin. We compare the results of this parametric study to observations in the eastern Gulf of Aden (heat flow, refraction seismics and topography) and show that (1) mantle lithosphere rheology controls the margin geometry and timing of the rifting; (2) lithosphere buoyancy has a large impact on the seafloor depth and the timing of partial melting; and (3) a perfectly plastic mantle lithosphere 20 kg m-3 denser than the asthenosphere best fits with observed elevation in the Gulf of Aden. Finally, thermomechanical models suggest that partial melting can occur in the mantle during the Arabian crustal breakup. We postulate that the produced melt could then infiltrate through the remnant continental mantle lithosphere, reach the surface and generate oceanic crust. This is in agreement with the observed narrow OCT composed of exhumed continental mantle intruded by volcanic rocks in the eastern Gulf of Aden.

  1. 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

  2. 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

  3. Columnar Aerosol Single-Scattering Albedo and Phase Function Retrieved from Sky Radiance Over the Ocean: Measurements of African Dust

    NASA Technical Reports Server (NTRS)

    Cattrall, Christopher; Carder, Kendall L.; Gordon, Howard R.

    2001-01-01

    The single-scattering albedo and phase function of African mineral dust are retrieved from ground-based measurements of sky radiance collected in the Florida Keys. The retrieval algorithm employs the radiative transfer equation to solve by iteration for these two properties which best reproduce the observed sky radiance using an assumed aerosol vertical structure and measured aerosol optical depth. Thus, no assumptions regarding particle size, shape, or composition are required. The single-scattering albedo, presented at fourteen wavelengths between 380 and 870 nm, displays a spectral shape expected of iron-bearing minerals but is much higher than current dust models allow. This indicates the absorption of light by mineral dust is significantly overestimated in climate studies. Uncertainty in the retrieved albedo is less than 0.02 due to the small uncertainty in the solar-reflectance-based calibration (12.2%) method employed. The phase function retrieved at 860 nm is very robust under simulations of expected experimental errors, indicating retrieved phase functions at this wavelength may be confidently used to describe aerosol scattering characteristics. The phase function retrieved at 443 nm is very sensitive to expected experimental errors and should not be used to describe aerosol scattering. Radiative forcing by aerosol is the greatest source of uncertainty in current climate models. These results will help reduce uncertainty in the absorption of light by mineral dust. Assessment of the radiative impact of aerosol species is a key component to NASA's Earth System Enterprise.

  4. 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

  5. Revealing the Fine Structures of the Lithosphere Asthenosphere Boundary

    NASA Astrophysics Data System (ADS)

    Olugboji, Tolulope Morayo

    Earth's near surface layer is made of relatively strong materials and often referred to as tectonic plates (or the lithosphere). Below this layer is a softer layer called the asthenosphere. The transition from strong lithosphere to weak asthenosphere is caused by temperature. However, recent high-resolution seismological observations suggest that the transition from the lithosphere to the asthenosphere cannot be attributed to the temperature alone: the change in seismic wave velocity at the boundary is too sharp and too large to be attributed solely to the gradual increase in temperature. There have been hot debates on what causes the observed sharp transition between two layers. In this dissertation, I synthesize insights from seismological observation, in particular receiver functions, with experiments and theory of anelasticity caused by grain-boundary sliding, to test and assess candidate models for the oceanic lithosphere asthenosphere boundary (LAB) as well as the Mid-lithospheric discontinues (MLD) observed in stable continental regions. I conduct new statistical analysis of the results of mineral physics experiments, providing a description of the uncertainties in the parameters of an elastically accommodated grain boundary sliding model (EAGBS). I extend the EAGBS model originally proposed by Karato (2012) to describe and explain the seismological signatures at the oceanic LAB, showing that this hypothesis suitably explains both the oceanic LAB and MLD in the continents within the limits of uncertainties in both mineral physics studies and seismological models. I then describe new supporting evidence for the specific predictions of the EAGBS model using novel seismological technique and data from stations in the oceanic regions. High-resolution receiver function (RF) stacking techniques can provide robust characterization of the age-dependence in the sharpness, depth, and anisotropic fabric within the normal oceanic LAB and underneath 'anomalous' Pacific

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

    PubMed

    Schmerr, Nicholas

    2012-03-23

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

  7. Formation of African Easterly Waves and Mesoscale Convective Systems over Eastern Africa and its Implication to Tropical Cyclogenesis over Eastern Atlantic Ocean

    NASA Astrophysics Data System (ADS)

    Lin, Yuh-Lang

    2009-07-01

    The formation of African easterly waves and mesoscale convective systems in eastern North Africa and its impacts on the tropical cyclogenesis over the eastern Atlantic Ocean is studied. Based on numerical simulations, AEWs can be produced by vortex shedding from the EH. The lee vortex is generated mainly by the blocking of the EH and helped by the horizontal shear associated with the northeasterly wind and the jet passing through the Turkana channel. The MCS was originated from the moist convection over the EH triggered by diurnal sensible heating. When the MCS moved to the lee of the mountain, it merged with the lee vortex of the AEW train and formed the coupled AEW-MCS system. Numerical simulations of a regional climate model indicate that the simulated fields do possess the AEW characteristics and the convection was generated over the EH, and the pre-Alberto AEW-MCS system was generated near the lee of the EH. Finer-resolution numerical simulations demonstrate that the vortex generated on the lee and MCS over the mountain eventually merge and become an AEW-MCS system which might serve as a precursor of tropical cyclone.

  8. Geoid, elevation and crustal thickness: Investigating the lithospheric structure of Africa

    NASA Astrophysics Data System (ADS)

    Globig, Jan; Fernandez, Manel; Torné, Montserrat; Faccenna, Claudio

    2014-05-01

    The African continent shows striking topographic features and has recently been target of numerous geophysical and seismic studies to determine its crustal structure, locally and regionally. Observation of surface tectonics coupled with knowledge of variations in crustal thickness provide a top to bottom frame to investigate sub-crustal processes, which affect the uppermost lithosphere and control today's African topography. As the significance of previous models of crustal thickness in Africa is debatable and favors premature conclusions here our motivation is to address the detailed structure of the African lithosphere revealing Moho and LAB geometry using 1D modeling of elevation, geoid and thermal data including the thermotectonic age of the crust as well as age and thickness of sediments to better account for lateral variations in crustal density. The four-layered model is composed of crust and lithospheric mantle plus sea water and asthenosphere, assuming Airy isostasy and is benchmarked against a detailed compilation of seismic Moho data from active and passive seismic experiments across the continent and its margins. Relating better surface topography with the depth of the Moho and the LAB contributes to improve knowledge on the lithospheric structure in Africa that mainly comes from global models, such as CRUST1.0, regional tomography models and gravity modeling, which unfortunately miss a proper relation between elevation, mean crustal density and crustal thickness. Our approach therefore is seen to support the discussion around the strongly debated processes responsible for the anomalous high elevation especially in the south eastern part of Africa and the observed undulations in Moho depth from about 20 km below the extended regions of the East African Rift System to 50 kilometers underneath the thickest Proterozoic belts. By linking differences in age, density and thermal state of the lithosphere with topography and geoid we want to provide new

  9. 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

  10. 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.

  11. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2008-12-01

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

  13. 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

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

    NASA Astrophysics Data System (ADS)

    Zhang, Kai-Jun

    2012-01-01

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

  15. 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.

  16. Strain rate dependency of oceanic intraplate earthquake b-values at extremely low strain rates

    NASA Astrophysics Data System (ADS)

    Sasajima, Ryohei; Ito, Takeo

    2016-06-01

    We discovered a clear positive dependence of oceanic intraplate earthquake (OCEQ) b-values on the age of the oceanic lithosphere. OCEQ b-values in the youngest (<10 Ma) oceanic lithosphere are around 1.0, while those in middle to old (>20 Ma) oceanic lithosphere 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 oceanic lithosphere has an anomalously higher strain rate compared with normal oceanic lithosphere, 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 oceanic lithosphere 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 oceanic lithosphere >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 oceanic lithosphere and the driving mechanism of plate tectonics.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Devlin, S.; Isacks, B. L.

    2007-12-01

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

  19. African dust phosphorus fertilizing the Amazon and the Atlantic Ocean is derived from marine sediments and igneous rocks - no indication for Bodélé diatomite contribution

    NASA Astrophysics Data System (ADS)

    Gross, Avner; Castido, Danilo; Pio, Casimero; Angert, Alon

    2013-04-01

    signatures. The diatomite in the Bodélé depression have lower P concentrations (550-900 µg P g/dust) and fall off the mixing line describe by our samples. These findings implies that the bio-available P delivered by dust from West Africa to the Central Atlantic Ocean and later to the Amazon basin is derived from a mixture of igneous origin and marine sedimentary origin, and that in contrast to previous claims, the Bodélé diatomite is not a major dust-P source. In addition, we found that African dust-P concentrations are between 2 folds to 10 folds higher then generally accounted for in modeling studies.

  20. Anatomy of lithosphere necking during orthogonal rifting

    NASA Astrophysics Data System (ADS)

    Nestola, Yago; Cavozzi, Cristian; Storti, Fabrizio

    2013-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2007-10-01

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

  2. Dependency of continental crustal rupture, decompression melt initiation and OCT architecture on lithosphere deformation modes during continental breakup: Numerical experiments

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    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 lithosphere thinning and stretching controls the rupture of continental crust and lithospheric mantle, the onset of decompression melting and their relative timing. We use a two dimensional finite element viscous flow model to describe lithosphere and asthenosphere deformation. This flow field is used to advect lithosphere 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 ocean ridges (Cannat, 1996), we assume that the topmost continental and oceanic lithosphere, corresponding to the cooler brittle seismogenic layer, deforms by extensional faulting (which we approximate to pure-shear deformation) and magmatic intrusion. Beneath this topmost lithosphere 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 lithosphere 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 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 ratio Vz/Vx and upper lithosphere pure-shear width W. Based on the numerical experiment results we explore a polyphase evolution of deformation modes leading to continental breakup, sea

  3. Using crustal thickness and subsidence history on the Iberia-Newfoundland margins to constrain lithosphere deformation modes during continental breakup

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    Observations at magma-poor rifted margins such as Iberia-Newfoundland show a complex lithosphere deformation history during continental breakup and seafloor spreading initiation leading to complex OCT architecture with hyper-extended continental crust and lithosphere, exhumed mantle and scattered embryonic oceanic crust and continental slivers. Initiation of seafloor spreading requires both the rupture of the continental crust and lithospheric 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 lithosphere deformation modes. A single lithosphere deformation mode leading to continental breakup and sea-floor spreading cannot explain observations. We have determined the sequence of lithosphere 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 lithosphere deformation modes, are generated by a 2D finite element viscous flow model (FeMargin), and used to advect lithosphere and asthenosphere temperature and material. FeMargin is kinematically driven by divergent deformation in the upper 15-20 km of the lithosphere inducing passive upwelling beneath that layer; extensional faulting and magmatic intrusions deform the topmost upper lithosphere, consistent with observations of deformation processes occurring at slow spreading ocean ridges (Cannat, 1996). Buoyancy enhanced upwelling, as predicted by Braun et al. (2000) is also kinematically included in the lithosphere deformation model. Melt generation by decompressional melting is predicted using the parameterization and methodology of Katz et al. (2003). The distribution of lithosphere deformation, the

  4. Deep India meets deep Asia: a seismological view of lithospheric slab interactions under Hindu Kush and Pamir

    NASA Astrophysics Data System (ADS)

    Schurr, Bernd; Kufner, Sofia; Sippl, Christian; Schneider, Felix; Yuan, Xiaohui; Ratschbacher, Lothar; Mechie, James

    2016-04-01

    It is part of the plate-tectonic paradigm that buoyant continental lithosphere subducts only in tow of a sinking oceanic plate after continent collision and that large deep (> 100 km) earthquakes occur exclusively in subducted oceanic lithosphere. Yet under the Pamir in Central Asia, far away from any (paleo-)ocean basins, continental lithosphere 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 lithosphere 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 lithosphere beneath the Pamir and a piece of Indian lithosphere 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 lithosphere involved: We suggest that the buoyant northwestern salient of (1) Cratonic India bulldozes into (2) Cratonic Asia forcing delamination and rollback of its lithosphere. 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.

  5. Lithospheric Structure and Seismotectonics of Central East Antarctica

    NASA Astrophysics Data System (ADS)

    Reading, A. M.

    2006-12-01

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

  6. Continents as lithological icebergs: The importance of buoyant lithospheric roots

    USGS Publications Warehouse

    Abbott, D.H.; Drury, R.; Mooney, W.D.

    1997-01-01

    An understanding of the formation of new continental crust provides an important guide to locating the oldest terrestrial rocks and minerals. We evaluated the crustal thicknesses of the thinnest stable continental crust and of an unsubductable oceanic plateau and used the resulting data to estimate the amount of mantle melting which produces permanent continental crust. The lithospheric mantle is sufficiently depleted to produce permanent buoyancy (i.e., the crust is unsubductable) at crustal thicknesses greater than 25-27 km. These unsubductable oceanic plateaus and hotspot island chains are important sources of new continental crust. The newest continental crust (e.g., the Ontong Java plateau) has a basaltic composition, not a granitic one. The observed structure and geochemistry of continents are the result of convergent margin magmatism and metamorphism which modify the nascent basaltic crust into a lowermost basaltic layer overlain by a more silicic upper crust. The definition of a continent should imply only that the lithosphere is unsubductable over ??? 0.25 Ga time periods. Therefore, the search for the oldest crustal rocks should include rocks from lower to mid-crustal levels.

  7. Multiple-frequency tomography of the upper mantle beneath the African/Iberian collision zone

    NASA Astrophysics Data System (ADS)

    Bonnin, Mickaël; Nolet, Guust; Villaseñor, Antonio; Gallart, Josep; Thomas, Christine

    2014-09-01

    During the Cenozoic, the geodynamics of the western Mediterranean domain has been characterized by a complex history of subduction of Mesozoic oceanic lithosphere. 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 African/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 lithospheric slab, thus favouring the hypothesis of a westward consumption of the Ligurian ocean 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 lithosphere 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.

  8. Tomography of the upper mantle beneath the African/Iberian collision zone

    NASA Astrophysics Data System (ADS)

    Mickael, B.; Nolet, G.; Villasenor, A.; Josep, G.; Thomas, C.

    2013-12-01

    During Cenozoic, geodynamics of the western Mediterranean domain has been characterized by a complex history of subduction of Mesozoic oceanic lithosphere. 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 broadband-station networks now available in Alborán Sea region, to develop a high-resolution 3D tomographic P velocity model of the upper mantle beneath the African/Iberian collision zone that will bring new constraints on 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 centered between 0.03 and 1.0 Hz, and interpreted using multiple frequency tomography. Our model shows, beneath Alborán Sea, a strong (~ 4%) 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 lithospheric slab, thus favoring the hypothesis of a westward consumption of the Ligurian ocean slab by roll-back during Cenozoic. In addition to this fast anomaly in the deep upper-mantle, several high intensity slow anomalies are widely observed in the lithosphere and asthenosphere beneath Morocco and southern Spain. These anomalies are correlated at surface with the position of the orogens (Rif and Atlas) and with Cenozoic volcanic fields. We thus confirm the presence, beneath Morocco, of an anomalous (hot) upper mantle, with piece of evidence for a lateral connection with the Canary volcanic islands, likely indicating a lateral spreading of the Canary plume to the east.

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

    NASA Astrophysics Data System (ADS)

    Sodoudi, Forough; Kind, Rainer

    2014-05-01

    depths of 260-280 km, which most likely represents the lithosphere-asthenosphere boundary. Based on our result, the Kalahari lithosphere 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 lithosphere, which most likely represents the Arabian lithosphere 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 lithosphere 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 lithosphere. 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 oceanic Neo-Tethyan slab beneath Central Iran, which results in an asthenospheric upwelling and thinning of the Iranian lithosphere beneath Central Iran and Alborz.

  10. An elastic-perfectly plastic analysis of the bending of the lithosphere at a trench

    NASA Technical Reports Server (NTRS)

    Turcotte, D. L.; Mcadoo, D. C.; Caldwell, J. G.

    1978-01-01

    A number of authors have modeled the flexure of the lithosphere at an oceanic 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 lithosphere 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 lithosphere 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.

  11. Rifting Thick Lithosphere - Canning Basin, Western Australia

    NASA Astrophysics Data System (ADS)

    Czarnota, Karol; White, Nicky

    2016-04-01

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

  12. Lithospheric Discontinuities Beneath North America (Invited)

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Artyushkov, Eugene

    2013-04-01

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

  14. Yellowstone hotspot-continental lithosphere interaction

    NASA Astrophysics Data System (ADS)

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

    2014-03-01

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

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

    USGS Publications Warehouse

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

    1996-01-01

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

  16. How does the lithosphere deformation mode during continental breakup affect mantle exhumation and subsidence history?

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

    Mantle exhumation at magma-poor rifted continental margin requires that continental crust ruptures prior to the onset of significant decompression melting. Both the relative timing of crustal rupture and melting, and therefore mantle exhumation, and rifted margin subsidence are dependent on the deformation mode of the continental lithosphere stretching and thinning leading to breakup. Fletcher et al. (2009) showed that for the Iberia-Newfoundland rifted margin, modelling of continental lithosphere stretching and thinning by pure-shear resulted in decompression melt initiation before continental crustal-rupture, while stretching and thinning by upwelling-divergent "corner flow" resulted in crustal-rupture before melt initiation. Observations at rifted continental margins (including Iberia-Newfoundland rifted margin) suggest a complex rifting evolution that cannot be explained by simplistic end-member pure-shear or "corner flow" deformation modes of lithosphere thinning and stretching (Péron-Pinvidic and Manatschal, 2009). By analogy with the deformation processes occurring at slow spreading ocean-ridges (Cannat, 1996), a more realistic lithosphere deformation mode for magma-poor continental breakup is extensional faulting for the colder brittle upper 12-15km above upwelling-divergent "corner flow" for the remaining lithosphere and asthenosphere. We use a kinematic numerical model of continental lithosphere thinning and stretching to examine decompression melt initiation, continental crustal rupture and subsidence for such a hybrid lithosphere deformation model represented by pure-shear deformation in the topmost brittle lithosphere above upwelling-divergent flow. We explore the relative contributions of pure-shear and upwelling-divergent "corner flow" deformation and its sensitivity to deformation rate, pure-shear half-width, the "corner flow" Vz/Vx ration and mantle potential temperature. The kinematic numerical model that we use represents lithosphere and

  17. Constraining lithosphere deformation mode evolution for the Iberia-Newfoundland rifted margins

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    The deformation of lithosphere 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 lithosphere, detachments faults, exhumed mantle, continental slivers and scattered embryonic oceanic crust. A coupled kinematic-dynamic model of lithosphere and asthenosphere deformation has been used to investigate the sequence of lithosphere 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 lithosphere and asthenosphere deformation models. A sequence of lithosphere 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 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 (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 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

  18. Wilson study cycles: Research relative to ocean geodynamic cycles

    NASA Technical Reports Server (NTRS)

    Kidd, W. S. F.

    1985-01-01

    The effects of conversion of Atlantic (rifted) margins to convergent plate boundaries; oceanic plateaus at subduction zones; continental collision and tectonic escape; southern Africa rifts; and global hot spot distribution on long term development of the continental lithosphere were studied.

  19. Preliminary models of normal fault development in subduction zones: lithospheric strength and outer rise deformation

    NASA Astrophysics Data System (ADS)

    Naliboff, J. B.; Billen, M. I.

    2010-12-01

    A characteristic feature of global subduction zones is normal faulting in the outer rise region, which reflects flexure of the downgoing plate in response to the slab pull force. Variations in the patterns of outer rise normal faulting between different subduction zones likely reflects both the magnitude of flexural induced topography and the strength of the downgoing plate. In particular, the rheology of the uppermost oceanic lithosphere is likely to strongly control the faulting patterns, which have been well documented recently in both the Middle and South American trenches. These recent observations of outer rise faulting provide a unique opportunity to test different rheological models of the oceanic lithosphere using geodynamic numerical experiments. Here, we develop a new approach for modeling deformation in the outer rise and trench regions of downgoing slabs, and discuss preliminary 2-D numerical models examining the relationship between faulting patterns and the rheology of the oceanic lithosphere. To model viscous and brittle deformation within the oceanic lithosphere we use the CIG (Computational Infrastructure for Geodynamics) finite element code Gale, which is designed to solve long-term tectonic problems. In order to resolve deformation features on geologically realistic scales (< 1 km), we model only the portion of the subduction system seaward of the trench. Horizontal and vertical stress boundary conditions on the side walls drive subduction and reflect, respectively, the ridge-push and slab-pull plate-driving forces. The initial viscosity structure of the oceanic lithosphere and underlying asthenosphere follow a composite viscosity law that takes into account both Newtonian and non-Newtonian deformation. The viscosity structure is consequently governed primarily by the strain rate and thermal structure, which follows a half-space cooling model. Modification of the viscosity structure and development of discrete shear zones occurs during yielding

  20. Evidence of a North-trending lithospheric detachment beneath the Betic Cordillera revealed by magnetotelluric data

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    Several and contradictory geodynamic models have been proposed to explain the opening of the Alboran Sea during the Oligocene - Miocene based on different geological and geophysical data, which do not allow to unequivocally distinguish between them. Using broad band and long period magnetotelluric data, we constructed a 3D model of the lithospheric electrical resistivity distribution beneath the Betic Cordillera. The model depicts the lithosphere - asthenosphere boundary, located deeper under the Iberian plate and the Betic Cordillera (160 km) than under the Alboran Basin (110 km). However, the main feature of the model is a N-S oriented conductive body in the lithospheric mantle located East of the 4W meridian. This conductive body coincides with a low velocity anomaly in an area without earthquake hypocenter locations. We interpret this N-S conductive body as asthenospheric material that has been intruded due to a lithospheric tearing process and the breaking-off of the East directed subducting slab. These new constraints, added to the available geodynamic models proposed, allow us to reconstruct a plausible geometry of the subduction and slab break-off of the Ligurian oceanic lithosphere under the Alboran Domain.

  1. Life in the lithosphere, kinetics and the prospects for life elsewhere.

    PubMed

    Cockell, Charles S

    2011-02-13

    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 lithosphere 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 oceans show the kinetic limitations of living directly off igneous rocks in the lithosphere. Using energy directly extracted from rocks the lithosphere 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 lithosphere 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 lithospheres 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. PMID:21220278

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

    PubMed Central

    Guex, Jean; Pilet, Sebastien; Müntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Sell, Bryan; Schaltegger, Urs

    2016-01-01

    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 lithosphere by plume activity or internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere. PMID:27009463

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

    NASA Astrophysics Data System (ADS)

    Guex, Jean; Pilet, Sebastien; Müntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Sell, Bryan; Schaltegger, Urs

    2016-03-01

    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 lithosphere by plume activity or internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere.

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

    PubMed

    Guex, Jean; Pilet, Sebastien; Müntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Sell, Bryan; Schaltegger, Urs

    2016-01-01

    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 lithosphere by plume activity or internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere. PMID:27009463

  5. Seismic anisotropy of the subcrustal lithosphere in Europe: Another clue to recognition of accreted terranes?

    NASA Astrophysics Data System (ADS)

    Babuška, Vladislav; Plomerová, J.

    P-wave residuals computed relative to a reference Earth model and normalized for effects originating in focal regions and along ray paths in the deep mantle provide information on deep lithospheric structure. The variations of the directionally independent representative average residuals, which are computed for waves arriving from different azimuths and with steep incidence angles, reflect compositional and thermal inhomogeneities. In our model they are attributed to variations of the lithosphere thickness. On the other hand, the variations of relative residuals that depend on the angles of azimuth and incidence form spatial patterns suggesting the existence of large-scale dipping anisotropic structures in the subcrustal lithosphere. The P-velocity anisotropy of these structures (9-11% on the average) agrees with the anisotropy of olivine ultramafites as measured in the laboratory. Orientations of the deep anisotropic structures change in the vicinity of important tectonic suture zones, for example, at the suture between the Saxothuringicum and Moldanubicum in central Europe, at the Insubric line in the Alps, and at the deep contact between the Rhodopean Massif and the Moesian Platform in the central Balkans. The structures probably retain preferred orientations of olivine crystals originating from an ancient oceanic lithosphere and may thus represent relict paleosubduction zones by which the continental lithosphere grew in the past.

  6. Multi-dimensional Crustal and Lithospheric Structure of the Atlas Mountains of Morocco by Magnetotelluric Imaging

    NASA Astrophysics Data System (ADS)

    Kiyan, D.; Jones, A. G.; Fullea, J.; Ledo, J.; Siniscalchi, A.; Romano, G.

    2014-12-01

    The PICASSO (Program to Investigate Convective Alboran Sea System Overturn) project and the concomitant TopoMed (Plate re-organization in the western Mediterranean: Lithospheric causes and topographic consequences - an ESF EUROSCORES TOPO-EUROPE project) project were designed to collect high resolution, multi-disciplinary lithospheric scale data in order to understand the tectonic evolution and lithospheric 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 lithospheric structure of the Atlas Mountains, and (ii) to test the hypotheses for explaining the purported lithospheric cavity beneath the Middle and High Atlas inferred from potential-field lithospheric 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 African 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

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

    NASA Astrophysics Data System (ADS)

    Miller, Meghan S.; Piana Agostinetti, Nicola

    2011-12-01

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

  8. Deformation and instability of underthrusting lithospheric plates

    NASA Technical Reports Server (NTRS)

    Liu, H.

    1972-01-01

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

  9. Coupled marine productivity and salinity and West African monsoon variability over the last 30,000 years in the eastern equatorial Atlantic Ocean

    NASA Astrophysics Data System (ADS)

    Marret, F.; Kim, S.-Y.; Scourse, J.; Kennedy, H.

    2009-04-01

    Marine cores collected off west equatorial Africa have highlighted transfer of terrigenous material in the close ocean that have had a deep influence on the marine productivity for the last 30,000 years. The strength of the West African 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

  10. Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution

    NASA Astrophysics Data System (ADS)

    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.

    2013-10-01

    The East African 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 oceanic 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 Ocean. The ocean 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 Ocean and experienced a ∼650-620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans 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 African 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 lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution of

  11. Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution☆

    PubMed Central

    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.

    2013-01-01

    The East African 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 oceanic 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 Ocean. The ocean 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 Ocean and experienced a ∼650–620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans 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 African 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 lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings

  12. The structure of the Ionian lithosphere

    NASA Technical Reports Server (NTRS)

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

    1984-01-01

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

  13. Enriched continental flood basalts from depleted mantle melts: modeling the lithospheric contamination of Karoo lavas from Antarctica

    NASA Astrophysics Data System (ADS)

    Heinonen, Jussi S.; Luttinen, Arto V.; Bohrson, Wendy A.

    2016-01-01

    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 lithosphere. Large-scale partial melting of the cold subcontinental lithospheric 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 lithosphere, 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 lithospheric and sublithospheric sources for two reasons: (1) recently discovered CFB dikes show isotopic characteristics akin to mid-ocean 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 lithospheric mantle contamination by using highly alkaline continental volcanic rocks (i.e., partial melts of mantle lithosphere) 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 lithospheric reservoirs. Our models imply that the role of continental lithosphere as a CFB source component or contaminant may have been overestimated in many cases. Thus, CFBs may represent major juvenile crustal

  14. Offshore Southern California lithospheric velocity structure from noise cross-correlation functions

    NASA Astrophysics Data System (ADS)

    Bowden, D. C.; Kohler, M. D.; Tsai, V. C.; Weeraratne, D. S.

    2016-05-01

    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 lithosphere at the westernmost edge of the North American continent. The Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment (ALBACORE) ocean 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 lithosphere to oceanic, 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 lithosphere 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.

  15. Melt-rich channel observed at the lithosphere-asthenosphere boundary.

    PubMed

    Naif, S; Key, K; Constable, S; Evans, R L

    2013-03-21

    The lithosphere-asthenosphere boundary (LAB) separates rigid oceanic 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 oceanic lithosphere, 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 lithosphere. 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 lithosphere 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. PMID:23518564

  16. Imaging the lithosphere of rifted passive margins using waveform tomography: North Atlantic, South Atlantic and beyond

    NASA Astrophysics Data System (ADS)

    Lebedev, Sergei; Schaeffer, Andrew; Celli, Nicolas Luca

    2016-04-01

    Lateral variations in seismic velocities in the upper mantle reflect variations in the temperature of the rocks at depth. Seismic tomography thus provides a proxy for lateral changes in the temperature and thickness of the lithosphere. It can map the deep boundaries between tectonic blocks with different properties and age of the lithosphere. Our 3D tomographic models of the upper mantle and the crust at the Atlantic and global scales are constrained by an unprecedentedly large global dataset of broadband waveform fits (over one million seismograms) and provide improved resolution of the lithosphere, compared to other available models. The most prominent high-velocity anomalies, seen down to 150-200 km depths, indicate the cold, thick, stable mantle lithosphere beneath Precambrian cratons, including those in North America, Greenland, northern and eastern Europe, Africa and South America. The dominant, large-scale, low-velocity feature is the global system of mid-ocean ridges, with broader low-velocity regions near hotspots, including Iceland. Currently active continental rifts show highly variable expression in the upper mantle, from pronounced low velocities to weak anomalies; this correlates with the amount of magmatism within the rift zone. Rifted passive margins have typically undergone cooling since the rifting and show more subtle variations in their seismic-velocity structure. Their thermal structure and evolution, however, are also shaped by 3D geodynamic processes since their formation, including cooling by the adjacent cratonic blocks inland and heating by warm oceanic asthenosphere.

  17. Lithospheric layering in the North American craton.

    PubMed

    Yuan, Huaiyu; Romanowicz, Barbara

    2010-08-26

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    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 lithosphere 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 oceanic islands basalts. However, available data suggest that the Karoo and CAMP areas were underlain by thick lithosphere (>200 km) prior to continental break up. The presence of thick lithosphere excludes significant melting of the asthenospheric mantle without initial stage of thermal erosion of the cratonic lithosphere. This initial step of thermal erosion / thermal heating of the cratonic lithosphere is critical to understand the volatile budget associated with LIPs while

  19. Microcrystalline diamonds in the oceanic lithosphere and their nature

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

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

  20. Creation and Deformation of Hydrous Lithosphere at the Southern Mariana Margin

    NASA Astrophysics Data System (ADS)

    Martinez, F.; Kelley, K. A.; Stern, R. J.

    2012-04-01

    Mantle lithosphere formed at mid-ocean 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 oceanic mantle lithosphere above solidus depths, perhaps helping to focus deformation and melt delivery to the narrow plate boundary zones observed at mid-ocean 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 lithosphere 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 lithosphere resulting in the present-day creation of new lithosphere 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-ocean ridges. This distributed

  1. Subduction of continental lithosphere in the Banda Sea region: Combining evidence from full waveform tomography and isotope ratios

    NASA Astrophysics Data System (ADS)

    Fichtner, Andreas; De Wit, Maarten; van Bergen, Manfred

    2010-09-01

    We provide new insight into the subduction of old continental lithosphere to depths of more than 100 km beneath the Banda arc, based on a spatial correlation of full waveform tomographic images of its lithosphere with He, Pb, Nd and Sr isotope signatures in its arc volcanics. The thickness of the subducted lithosphere of around 200 km coincides with the thickness of Precambrian lithosphere 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 ocean lithosphere north of the North Australian craton was capable of entraining large volumes of continental lithosphere. The Banda arc example demonstrates that continental lithosphere 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 oceanic lithosphere 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 lithosphere 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 lithosphere as inferred from the tomographic images. The subduction of continental lithosphere including crustal material beneath

  2. Complexity of the Fennoscandian lithosphere

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  3. Thermal structure of the lithosphere: a petrologic model.

    PubMed

    Macgregor, I D; Basu, A R

    1974-09-20

    A preliminary evaluation of the thermal history of the upper mantle as determined by petrologic techniques indicates a general correspondence with theoretically derived models. The petrologic data supply direct information which may be used as an independent calibration of calculated models, serve as a base for evaluating the assumptions of the theoretical approach, and allow more careful selection of the variables describing mantle thermal properties and processes. Like the theoretical counterpart, the petrological approach indicates that the lithosphere is dominated by two thermal regimes: first, there is a continental regime which cools at rates of the order of 10(9) years and represents the longterm cooling of the earth. Secondly, superimposed on the continental evolution is the thermal event associated with the formation of an oceanic basin, and which may be thought of as a 10(8) year convective perturbation on the continental cycle. Of special interest is petrologic evidence for a sudden steepening of the thermal gradients across the lithosphere-asthenosphere boundary not seen in the theoretical models. The unexpected change of slope points to the need for a critical reevaluation of the thermal processes and properties extant in the asthenosphere. The potential of the petrologic contribution has yet to be fully realized. For a start, this article points to an important body of independent evidence critical to our understanding of the earth's thermal history. PMID:17738235

  4. Renewal: Continential lithosphere evolution as a function of tectonic environment

    SciTech Connect

    McMillan, N.J.; Baldridge, W.S.

    1995-06-01

    The Cenozoic tectonic environment and stress regime of the southwestern United States have changed dramatically from compression during shallow-angle subduction during the Laramide orogeny in the early Cenozoic to the current mode of Basin and Range extension. Questions remain unresolved concerning the causes of this transition, including the timing of the initiation of extension (estimates range from 36 to 25 Ma), and is the Basin and Range simply an mega-example of back-arc extension, or is extension related to the subduction of an oceanic spreading center about 30 Ma? We have examined the patterns of magmagenesis and geochemical composition through Cenozoic time in southern New Mexico. We have defined four magma sources that have contributed to Cenozoic magmas. Immediately following the Laramide, magmas contain substantial contributions from the lower crust. Mid-Tertiary extension is related to the eruption of rhyolitic ash-flow tuffs and basalts. The basalts were generated by melting of the lithospheric mantle; intercalated rhyolites have a strong upper crustal signature. Eruption of basalts and andesites with sources in the lithospheric mantle and lower crust continued for several million years after rhyolitic volcanism ceased. The region was nearly void of volcanic activity for 16 million years despite continued extension, but at 10 Ma, basalts derived from the asthenosphere began to erupt.

  5. Renewal: Continental lithosphere evolution as a function of tectonic environment

    NASA Astrophysics Data System (ADS)

    McMillan, N. J.; Baldridge, W. S.

    1995-04-01

    The Cenozoic tectonic environment and stress regime of the southwestern United States have changed dramatically from compression during shallow-angle subduction during the Laramide orogeny in the early Cenozoic to the current mode of Basin and Range extension. Questions remain unresolved concerning the causes of this transition, including the timing of the initiation of extension (estimates range from 36 to 25 Ma), and is the Basin and Range simply an mega-example of back-arc extension, or is extension related to the subduction of an oceanic spreading center about 30 Ma? We have examined the patterns of magmagenesis and geochemical composition through Cenozoic time in southern New Mexico. We have defined four magma sources that have contributed to Cenozoic magmas. Immediately following the Laramide, magmas contain substantial contributions from the lower crust. Mid-Tertiary extension is related to the eruption of rhyolitic ash-flow tuffs and basalts. The basalts were generated by melting of the lithospheric mantle; intercalated rhyolites have a strong upper crustal signature. Eruption of basalts and andesites with sources in the lithospheric mantle and lower crust continued for several million years after rhyolitic volcanism ceased. The region was nearly void of volcanic activity for 16 million years despite continued extension, but at 10 Ma, basalts derived from the asthenosphere began to erupt.

  6. Ejecta from Ocean Impacts

    NASA Technical Reports Server (NTRS)

    Kyte, Frank T.

    2003-01-01

    Numerical simulations of deep-ocean impact provide some limits on the size of a projectile that will not mix with the ocean floor during a deep-ocean 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 ocean floor silicates. Given the typical water depths of 4 to 5 km in deep-ocean 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 ocean 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 oceanic impacts and more conventional continental impacts. Given that 60% of the Earth's surface is covered by oceanic lithosphere and 500 m projectiles impact the Earth on 10(exp 5) y timescales, there must be hundreds of oceanic impact deposits in the sediment record awaiting discovery.

  7. Imaging the continental lithosphere: Perspectives from global and regional anisotropic seismic tomography

    NASA Astrophysics Data System (ADS)

    Lebedev, Sergei; Schaeffer, Andrew

    2016-04-01

    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 lithosphere. It can map the deep boundaries between tectonic blocks with different properties and age of the lithosphere. 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 lithosphere 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 lithosphere 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 lithospheric deformation and mantle flow can be resolved, in particular in densely sampled regions. The depth of the boundary between the cold, rigid lithosphere (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 oceans, the lithosphere-asthenosphere boundary (LAB) is defined clearly by the layering of anisotropy, with a dependence on

  8. Stable isotopic constraints on formation of continental lithospheric mantle: a case study from the Colorado Plateau

    NASA Astrophysics Data System (ADS)

    Marshall, E. W., IV; Barnes, J.; Lassiter, J. C.

    2014-12-01

    Mantle melt depletion is fundamental to stabilization of thick lithospheric mantle and to craton formation and survival. Three potential mechanisms to grow lithospheric mantle are: partial melting of upwelling mantle plumes; imbrication and accretion of oceanic lithosphere at subduction zones; and flux melting of the mantle wedge at volcanic arcs. Stable isotopes can distinguish these models because stable isotopes fractionate substantially at low temperature, making the mantle sensitive to the incorporation of subducted material. Correlations between stable isotopes and trace elements can then distinguish seafloor weathering processes from flux melting processes, allowing subducted oceanic lithosphere to be discerned from flux-melted lithosphere. We find that indices of melt depletion (e.g. cpx Cr#, Yb concentrations and Mg#) in xenoliths from the Colorado Plateau correlate with δ18O values of olivine. Xenoliths from The Thumb volcanic neck have δ18Ool values ranging from +5.05 to +5.50‰ (n=12) that correlate positively with indices of melt depletion, and xenoliths from the Green Knobs diatreme have δ18Ool values that range from +4.85 to +5.36‰ (n=9) and correlate negatively with indices of melt depletion. These trends may reflect coupled fluid input and melt depletion, typical of flux melting. The different trends at each locality may be due to fluxing of different fluids- either high δ18O fluids derived from altered oceanic crust or low δ18O fluids derived from recycled serpentinite. Canil and Lee (2009) interpreted correlations between whole rock δ18O values and MgO/SiO2 in xenoliths from The Thumb to be due to seafloor weathering and Mg loss of abyssal peridotites followed by later accretion of the rocks to the Colorado Plateau lithosphere. However, chrome and alumina are immobile during seafloor weathering, so correlations between δ18O values and Cr# also present in The Thumb xenoliths are inconsistent with seafloor weathering. Evidence for flux

  9. PICASSO: Lithosphere Structure in the Western Mediterranean from Ps Receiver Functions and Rayleigh Wave Tomography

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

    The western Mediterranean is a diffuse plate boundary separating the African 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 lithospheric 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 lithospheric structure, and the lithosphere-asthenosphere boundary (LAB). From north to south along the PICASSO profile: The lithosphere is ~120 km thick beneath the Iberian Massif, where it has the highest shear velocity, 4.45 km/s. To the south the lithosphere 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 lithosphere, ~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

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

    NASA Technical Reports Server (NTRS)

    Morgan, P.

    1984-01-01

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

  11. Links between the mechanical, seismic and thermal thickness, rheological structure and mechanical stability of the continental lithosphere.

    NASA Astrophysics Data System (ADS)

    Burov, E.; Watts, A. B.; Francois, T.; Tesauro, M.

    2012-04-01

    To fulfill its plate-tectonics functions, the lithosphere has to remain mechanically strong over geological time spans and be capable to support important geological loads while transferring horizontal tectonic stresses at global scales. We use thermo-mechanically and thermo-dynamically coupled numerical models accounting for brittle-elastic-plastic rheology and petrologically and seismologically consistent pressure-temperature dependent density and elastic structure to obtain more robust insights on thickness of the mechanical lithosphere (Hm) and its links to the LAB depth and its seismic (Hs) and thermal thickness (Ht). Testing the mechanical stability of lithospheres with different thermo-rheological structures allows us to constrain rheological parameters needed for long-term survival of lithospheric plates and establish links between LAB,Hm,Hs and Ht. Mechanical lithosphere appears to be 1.5-2 times thinner than Hs and Ht and its mechanical thickness, Hm, is strongly dependent on thermal and rheological structure. The important contribution of inelastic components (brittle and ductile behavior) to the mechanical strength of the lithosphere suggests that Hm is also stress and strain dependent: within the same plate, it might drop by 30-50% in the areas of high strain or stress, and remain much higher in the areas where tectonic deformation is moderate. In some cases it is possible to establish direct links between the laterally variable mechanical, seismic and thermal lithosphere thickness. This is of special importance since tracking the mechanical thickness of the lithosphere allows us to put better constraints on its stress/strain dependent rheological properties. We explored relationships between Hs,Ht and Hm of the lithosphere in oceans and in more complex continental lithospheres. In oceanic plates, Hm corresponds to the observed equivalent elastic thickness (EET) multiplied by a factor of 1.2-1.5, and correlates well with Ht and Hs. In continents, the

  12. Insolation driven variations of Mercury's lithospheric strength

    NASA Astrophysics Data System (ADS)

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

    2011-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  14. Refertilization-driven destabilization of subcontinental mantle and the importance of initial lithospheric thickness for the fate of continents

    NASA Astrophysics Data System (ADS)

    Zheng, J. P.; Lee, C.-T. A.; Lu, J. G.; Zhao, J. H.; Wu, Y. B.; Xia, B.; Li, X. Y.; Zhang, J. F.; Liu, Y. S.

    2015-01-01

    Continents are underlain by thick, cold thermal boundary layers. Thermal contraction should render these boundary layers negatively buoyant and unstable; this is why old, cold oceanic lithospheres subduct. However, the ancient lithospheric roots of many continents appear to have existed for billions of years. In the common view, this preservation is due to the fact that the thermal boundary layers are compositionally distinct from the ambient mantle in that they are highly melt-depleted and dehydrated; the former provides positive buoyancy and the latter provides strength. Here, we show using mantle xenoliths that the Precambrian South China Block originally was underlain by highly depleted mantle, but has been refertilized via silicate melts generated from the asthenosphere. It is now more fertile than the ambient convecting mantle and is intrinsically denser by more than 1.5%. Achieving sufficient melt generation for refertilization is only possible if the lithosphere is thin enough to provide "headspace" for decompression melting. Thus, continental boundary layers thinner than the maximum depth of melting should experience refertilization, whereas thicker continents would altogether suppress melting and hence the potential for refertilization. We propose that refertilization, once initiated, will destabilize the base of the continent; this in turn will increase the amount of "headspace" and promote further refertilization, resulting in a positive feedback that could culminate in lithospheric destruction. By contrast, continents that are thick enough may not experience significant refertilization. This suggests that initial lithospheric thickness, as well as lithospheric composition, may be important for defining the fate of continents.

  15. Helium as a tracer for fluids released from Juan de Fuca lithosphere beneath the Cascadia forearc

    NASA Astrophysics Data System (ADS)

    McCrory, P. A.; Constantz, J. E.; Hunt, A. G.; Blair, J. L.

    2016-06-01

    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 lithosphere. 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 oceanic lithosphere and forearc mantle sources, whereas 3He detected seaward of the forearc mantle corner likely has only an oceanic source. The highest ratios in the Cascadia forearc coincide with slab depths (˜40-45 km) where metamorphic dehydration of young oceanic lithosphere 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.

  16. The magma ocean as an impediment to lunar plate tectonics

    NASA Technical Reports Server (NTRS)

    Warren, Paul H.

    1993-01-01

    The primary impediment to plate tectonics on the moon was probably the great thickness of its crust and particularly its high crust/lithosphere 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 ocean epoch, the moon's crust/lithosphere 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 ocean. However, these regions probably also tend to have uncommonly thin lithosphere, 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 lithosphere subduction was impeded by the magma ocean.

  17. The magma ocean as an impediment to lunar plate tectonics

    NASA Astrophysics Data System (ADS)

    Warren, P. H.

    1993-03-01

    The primary impediment to plate tectonics on the moon was probably the great thickness of its crust and particularly its high crust/lithosphere 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 ocean epoch, the moon's crust/lithosphere 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 ocean. However, these regions probably also tend to have uncommonly thin lithosphere, 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 lithosphere subduction was impeded by the magma ocean.

  18. Linking plate reconstructions with deforming lithosphere to geodynamic models

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

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

  20. Feedbacks between deformation and reactive melt transport in the mantle lithosphere during rifting

    NASA Astrophysics Data System (ADS)

    Tommasi, A.; Baptiste, V.; Vauchez, A. R.; Fort, A.

    2014-12-01

    The East-African rift associates lithospheric thinning with extensive volcanism. Melts, even at low fractions, reduce the mantle viscosity. They also carry and exchange heat, mainly via reactions (latent heat), modifying the temperature and the rheology, which in turn controls their transport through the lithospheric mantle. Analysis of microstructures and crystal preferred orientations of mantle xenoliths from different localities along the East-African rift system highlights strong feedbacks between deformation, melt transport, and thermal evolution in the lithospheric mantle. Microstructures change markedly from south (young) to north (mature rift). In Tanzania, mylonitic to porphyroclastic peridotites predominate in on-axis localities, while off-axis ones are coarse-granular to porphyroclastic, pointing to heterogeneous deformation and variable annealing due to local interaction with fluids or to different time lags between deformation and extraction. Mylonites point to strain localization but there is no evidence for dominant grain boundary sliding: ubiquituous intracrystalline deformation in olivine and orthopyroxene and strong CPO record dislocation creep with dominant [100] glide in olivine. Synkinematic replacement of opx by olivine in both mylonitic and porphyroclastic peridotites suggests that deformation continued in the presence of melt under near-solidus conditions. This heating was transient: exsolutions in opx record cooling before extraction. Mega peridotites, which sample the southern border of the Ethiopian plateau, are coarse-porphyroclastic and show widespread metasomatism by basalts or by evolved volatile-rich low melt fractions. The former predated or was coeval to deformation, since olivine and pyroxene CPO are coherent. Exsolutions in opx imply that the high primary equilibration temperatures, which are consistent with the coarse-grained microstructures, are linked to transient heating. Finally, the fine-grained polygonal microstructures

  1. Upper mantle flow and lithospheric dynamics beneath the Eurasian region

    NASA Astrophysics Data System (ADS)

    Zhang, G.; Jiang, G.; Jia, Z.; Gao, R.; Fu, R.

    2010-12-01

    Evidence from seismic tomography, geothermal and short wavelength geoid anomalies reveals the existence of small-scale convective systems in the upper mantle, with scales ranging from 500 km to 700 km. It is reasonable to suggest that these small-scale convective systems probably control the regional tectonic structure and the dynamical processes of the lithosphere. Here we have calculated the patterns of small-scale convection in the upper mantle for the Eurasian region (20°E~170°E,15°N~75°N), using the anomaly of isostatic gravity. The results show that the regional lithospheric tectonics is strongly correlated with the upper mantle flow in the Eurasian region. Two intensive convective belts against the weak background convection can be recognized from convection patterns in this region: Alpine-Himalayan collision belt and West Pacific island arc-underthrust belt. Alpine-Himalayan belt is caused by the collision between the northern plate (Eurasian plate) and the southern plates (African plate and Indian plate). West Pacific island arc-underthrust belt is caused by the subduction of the Pacific plate beneath the Eurasian plate. Both of them are also seismotectonic belts. The collision and the subduction are two important geological events occurred since Mesozoic era and Cenozoic era in the Eurasian region. Therefore, the mantle flows may be one of the main driving forces of two events. In addition, most plate boundaries in this region can be recognized and the characteristics of upper mantle convection are different completely between the Eurasian plate and the plates around it (African plate, Arabian plate, Indian plate, Philippine Sea plate and Pacific plate). Main structures and geodynamic characteristics of the Eurasian can also be explained by our model results. The Tibet plateau is located in the intensive convective belt. Around the belt, the upwelling materials push the lithosphere to lift unitarily and form the plateau. Towards the north of the Tibet

  2. Lithospheric stress patterns: A global view

    NASA Astrophysics Data System (ADS)

    Zoback, Mary Lou; Burke, Kevin

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

  3. Viscosity of the lithosphere of Enceladus

    NASA Technical Reports Server (NTRS)

    Passey, Q. R.

    1983-01-01

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

  4. Lithospheric structure in the Pacific geoid

    NASA Technical Reports Server (NTRS)

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

    1985-01-01

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

  5. Water and its influence on the lithosphere-asthenosphere boundary.

    PubMed

    Green, David H; Hibberson, William O; Kovács, István; Rosenthal, Anja

    2010-09-23

    The Earth has distinctive convective behaviour, described by the plate tectonics model, in which lateral motion of the oceanic lithosphere of basaltic crust and peridotitic uppermost mantle is decoupled from the underlying mechanically weaker upper mantle (asthenosphere). The reason for differentiation at the lithosphere-asthenosphere boundary is currently being debated with relevant observations from geophysics (including seismology) and geochemistry (including experimental petrology). Water is thought to have an important effect on mantle rheology, either by weakening the crystal structure of olivine and pyroxenes by dilute solid solution, or by causing low-temperature partial melting. Here we present a novel experimental approach to clarify the role of water in the uppermost mantle at pressures up to 6 GPa, equivalent to a depth of 190 km. We found that for lherzolite in which a water-rich vapour is present, the temperature at which a silicate melt first appears (the vapour-saturated solidus) increases from a minimum of 970 °C at 1.5 GPa to 1,350 °C at 6 GPa. We have measured the water content in lherzolite to be approximately 180 parts per million, retained in nominally anhydrous minerals at 2.5 and 4 GPa at temperatures above and below the vapour-saturated solidus. The hydrous mineral pargasite is the main water-storage site in the uppermost mantle, and the instability of pargasite at pressures greater than 3 GPa (equivalent to more than about 90 km depth) causes a sharp drop in both the water-storage capacity and the solidus temperature of fertile upper-mantle lherzolite. The presence of interstitial melt in mantle with more than 180 parts per million of water at pressures greater than 3 GPa alters mantle rheology and defines the lithosphere-asthenosphere boundary. Modern asthenospheric mantle acting as the source for mid-oceanic ridge basalts has a water content of 50-200 parts per million (refs 3-5). We show that this matches the

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

    NASA Astrophysics Data System (ADS)

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

    2014-02-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2012-08-01

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

  8. Water in the Cratonic Mantle Lithosphere

    NASA Technical Reports Server (NTRS)

    Peslier, A. H.

    2016-01-01

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

  9. Residual depth anomalies on the Iberian, Newfoundland, Labrador and Nova Scotian margins; implications for their lithosphere mass and density distribution

    NASA Astrophysics Data System (ADS)

    Cooper, C.; Kusznir, N.; Manatschal, G.

    2007-12-01

    The break-up of the Iberian and Newfoundland margins has resulted in large scale asymmetry between these conjugate margins. One documented asymmetry is that the oldest oceanic crust on the Newfoundland margin is elevated with respect to that of the Iberian margin. This work employs residual depth anomalies (RDA) to investigate this asymmetry. Observed bathymetry has been compared to the global oceanic bathymetry-age models of Parsons and Sclater (1977) and Stein and Stein (1992) in order to calculate RDA. The oceanic crust on the Nova Scotia and Labrador margins have also been included in this study in order to explore the lithosphere mass and density distribution across the wider region. Seismically derived cross-sections have been flexurally backstripped to correct the RDA for sediments. A correction has also been made to the RDA for seismically observed variation in oceanic crustal thickness, about the global mean thickness, using local isostasy. Key observations of the corrected RDA are: (1) oceanic crust on the Labrador and the Newfoundland margins are over a kilometre shallower than predicted by the global bathymetry-age models; (2) oceanic crust on the Newfoundland margin is elevated by 700 to 900m relative to its conjugate Iberian margin; (3) the RDA decrease oceanwards; (4) the RDA decrease southwards on both sides of the North Atlantic. Potential sources of the observed RDA have been investigated, including flexural coupling of oceanic and continental lithosphere, mantle plume uplift, serpentinization, magmatic intrusions and lithospheric mantle geochemical heterogeneity. Modelling shows flexural coupling of oceanic and continental lithosphere, during thermal subsidence of the oceanic lithosphere, may be the sole source of the ocean-wards decrease in RDA. However, the elevation of the Newfoundland margin relative to the Iberian margin can not be explained by flexural isostatic coupling. The Iceland plume might be the source of the southerly decrease in

  10. The lithosphere thermal structure of the Southeast Asia: constrained by Vs data

    NASA Astrophysics Data System (ADS)

    Chuanhai, Yu; Xiaobin, Shi; Qunshu, Tang; Xiaoqiu, Yang

    2016-04-01

    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 oceanic 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 lithosphere thermal structure and lithosphere 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 lithosphere 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 lithosphere. The results show that the temperature contours at 80km depth is about 200-300°C higher in the rifted basins and oceanic 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 lithosphere 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

  11. Using crustal thickness, subsidence and P-T-t history on the Iberia-Newfoundland & Alpine Tethys margins to constrain lithosphere deformation modes during continental breakup

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Observations at magma-poor rifted margins such as Iberia-Newfoundland show a complex lithosphere deformation history and OCT architecture, resulting in hyper-extended continental crust and lithosphere, exhumed mantle and scattered embryonic oceanic crust before continental breakup and seafloor spreading. Initiation of seafloor spreading requires both the rupture of the continental crust and lithospheric 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 lithosphere deformation modes. A single kinematic lithosphere deformation mode leading to continental breakup and sea-floor spreading cannot explain observations. We have determined the sequence of lithosphere 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. Lithosphere deformation modes, represented by flow fields, are generated by a 2D finite element viscous flow model (FeMargin), and used to advect lithosphere and asthenosphere temperature and material. FeMargin is kinematically driven by divergent deformation in the topmost upper lithosphere inducing passive upwelling beneath that layer; the upper lithosphere is assumed to deform by extensional faulting and magmatic intrusions, consistent with observations of deformation processes occurring at slow spreading ocean ridges (Cannat, 1996). 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 lithosphere

  12. Abnormal lithium isotope composition from the ancient lithospheric mantle beneath the North China Craton

    PubMed Central

    Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M.; Xiao, Yan

    2014-01-01

    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 lithospheric 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 lithospheric mantle. Our data demonstrate that abnormal δ7Li may be preserved in the ancient lithospheric mantle as observed in our study from the central North China Craton, which suggest that the subcontinental lithospheric mantle has experienced modification of fluid/melt derived from recycled oceanic crust. PMID:24589693

  13. Abnormal lithium isotope composition from the ancient lithospheric mantle beneath the North China Craton.

    PubMed

    Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M; Xiao, Yan

    2014-01-01

    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 lithospheric 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 lithospheric mantle. Our data demonstrate that abnormal δ(7)Li may be preserved in the ancient lithospheric mantle as observed in our study from the central North China Craton, which suggest that the subcontinental lithospheric mantle has experienced modification of fluid/melt derived from recycled oceanic crust. PMID:24589693

  14. Lithospheric strength and its relationship to the elastic and seismogenic layer thickness

    NASA Astrophysics Data System (ADS)

    Watts, A. B.; Burov, E. B.

    2003-08-01

    Plate flexure is a phenomenon that describes how the lithosphere responds to long-term (>105 yr) geological loads. By comparing the flexure in the vicinity of ice, volcano, and sediment loads to predictions based on simple plate models it has been possible to estimate the effective elastic thickness of the lithosphere, Te. In the oceans, Te is the range 2-50 km and is determined mainly by plate and load age. The continents, in contrast, are characterised by Te values of up to 80 km and greater. Rheological considerations based on data from experimental rock mechanics suggest that Te reflects the integrated brittle, elastic and ductile strength of the lithosphere. Te differs, therefore, from the seismogenic layer thickness, Ts, which is indicative of the depth to which anelastic deformation occurs as unstable frictional sliding. Despite differences in their time scales, Te and Ts are similar in the oceans where loading reduces the initial mechanical thickness to values that generally coincide with the thickness of the brittle layer. They differ, however, in continents, which, unlike oceans, are characterised by a multi-layer rheology. As a result, Te≫Ts in cratons, many convergent zones, and some rifts. Most rifts, however, are characterised by a low Te that has been variously attributed to a young thermal age of the rifted lithosphere, thinning and heating at the time of rifting, and yielding due to post-rift sediment loading. Irrespective of their origin, the Wilson cycle makes it possible for low values to be inherited by foreland basins which, in turn, helps explain why similarities between Te and Ts extend beyond rifts into other tectonic regions such as orogenic belts and, occasionally, the cratons themselves.

  15. Spatial variations of effective elastic thickness of the Lithosphere in the Southeast Asia regions

    NASA Astrophysics Data System (ADS)

    Shi, Xiaobin; Kirby, Jon; Yu, Chuanhai; Swain, Chris; Zhao, Junfeng

    2016-04-01

    The effective elastic thickness Te corresponds to the thickness of an idealized elastic beam that would bend similarly to the actual lithosphere 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 lithospheric 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 oceanic 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, lithospheric 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 lithosphere strength characterizes the oceanic basins, such as the South China Sea, the Banda sea area, the Celebes Sea, the Sulu Sea and the Andaman Sea. Unlike the oceanic basins, the continental fragments show a complex pattern of Te variations. The Khorat plateau and its adjacent area show strong lithosphere characteristics with a Te range of 20-50 km, suggesting that the Khorat plateau is the strong core of the Indochina Block. The West

  16. Imaging Lithospheric Structure beneath the Indian continent

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2009-09-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

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

  19. Magnetic mineralogy of the Mercurian lithosphere

    NASA Astrophysics Data System (ADS)

    Strauss, Becky; Feinberg, Joshua; Johnson, Catherine

    2016-04-01

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

  20. A comparison of Sr-Nd-Pb isotopes in young and old continental lithospheric mantle: Patagonia and eastern China

    USGS Publications Warehouse

    Zartman, R.E.; Futa, K.; Peng, Z.C.

    1991-01-01

    It is commonly accepted that beneath the continental crust lies a keel of lithospheric mantle, which extends 50-200 kilometres downward to a transition zone into the asthenosphere. The chemical and physical properties of this reservoir are best known through studies of the basalts and xenoliths that provide samples of the subcrustal mantle. Although sharing many characteristics with oceanic island basalts, some continental basalts become increasingly distinct isotopically as crustal age increases, strongly supporting a permanent association between crust and mantle. Five models are discussed that relate the isotopic composition of the continental lithospheric mantle to that of other parts of the terrestrial system, which may be involved in its origin and evolution. The potential locations of the contribution components and the mechanisms and timing of their assembly into lithosphere are considered. -from Authors

  1. The continental lithospheric mantle: characteristics and significance as a mantle reservoir.

    PubMed

    Pearson, D G; Nowell, G M

    2002-11-15

    The continental lithospheric 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 oceanic 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 lithospheric peridotites and metasomites. These data, combined with other data for spinel peridotites from non-cratonic regions, show that neither hydrous nor anhydrous lithospheric mantle xenoliths make suitable sources for continental or oceanic 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 lithospheric chemistry with depth, indicate that the elemental and isotopic composition of lithospheric 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 oceanic magmas and to promote consideration of a mantle that

  2. Understanding plate-motion changes over the past 100 Myr with quantitative models of the coupled lithosphere/mantle system

    NASA Astrophysics Data System (ADS)

    Stotz, Ingo; Iaffaldano, Giampiero; Rhodri Davies, D.

    2015-04-01

    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. Ocean-floor observations now allow one to resolve past plate motions (e.g. in the North Atlantic and Indian Ocean 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 lithospheric 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 lithosphere/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 lithosphere 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 lithosphere/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 lithosphere dynamics (e.g. Bird 1998). Our efforts are focused, in particular, on achieving the technical ability to: (i) simulate the

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

    NASA Astrophysics Data System (ADS)

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

    2003-04-01

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

  4. Eclogite xenoliths in west African kimberlites as residues from Archaean granitoid crust formation

    NASA Astrophysics Data System (ADS)

    Rollinson, Hugh

    1997-09-01

    Eclogites are a comparatively rare but petrologically important member of kimberlite xenolith suites. Their broadly basaltic chemistry has led many authors to propose that they represent ancient, subducted ocean crust. Recent studies, however, have suggested an alternative origin and propose that kimberlitic eclogites are residues from the process of Archaean granitoid crust formation. Geochemical arguments in support of this new model were previously based on the trace-element chemistry of eclogitic minerals. Here I report that the major-element chemistry of eclogite xenoliths also supports a crustal residue model. I examine eclogite xenoliths from kimberlite pipes at Koidu, Sierra Leone, which sample the lithospheric mantle underlying the Archaean (2.8Gyr) granitoid crust of the West African craton. Geochemical plots of major elements measured in unaltered, whole-rock samples of low-silica eclogite demonstrate that they are complementary to the granitoids of the West African craton and have compositions which indicate that both were derived from a common basaltic parent rock.

  5. Continental collision slowing due to viscous mantle lithosphere rather than topography.

    PubMed

    Clark, Marin Kristen

    2012-03-01

    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 oceanic lithosphere, 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 lithosphere 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 lithosphere beneath Tibet is intact, which supports the interpretation that the long-term strain history of Tibet reflects deformation of the mantle lithosphere. Under conditions of constant stress and strength, the deforming continental lithosphere creates a type of viscous resistance that affects plate motion irrespective of how topography evolved. PMID:22382982

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  7. Delamination of sub-crustal lithosphere beneath the Isthmus of Tehuantepec, Mexico

    NASA Astrophysics Data System (ADS)

    Manea, Marina; Constantin Manea, Vlad; Ferrari, Luca; Orozco-Esquivel, Maria Teresa

    2015-04-01

    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 oceanic lithosphere 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 lithospheric 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 lithosphere once it reached its base. The model geometry of the delaminated lithosphere 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 lithosphere is above 700

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  9. The Thinning of the lithosphere before Magmatic Spreading is Established at the Western End of the Cocos-Nazca Rift

    NASA Astrophysics Data System (ADS)

    Smith, D. K.; Schouten, H.

    2015-12-01

    The transition from rifting of oceanic lithosphere 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 lithosphere 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 lithosphere before magmatic spreading begins. The earliest faults show small outward rotation (1-5 degrees) for their offset suggesting that they cut thick lithosphere. Subsequent faults closer to the axis have larger outward rotations (up to 35-40 degrees) with larger offset indicating that the lithosphere 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 oceanic lithosphere, and place constraints on the supply of magma to a newly developing plate boundary.

  10. Constraining the rheology of the lithosphere and upper mantle with geodynamic inverse modelling

    NASA Astrophysics Data System (ADS)

    Kaus, Boris; Baumann, Tobias

    2016-04-01

    The rheology of the lithosphere is of key importance for the physics of the lithosphere. Yet, it is probably the most uncertain parameter in geodynamics as experimental rock rheologies have to be extrapolated to geological conditions and as existing geophysical methods such as EET estimations make simplifying assumptions about the structure of the lithosphere. In many geologically interesting regions, such as the Alps, Andes or Himalaya, we actually have a significant amount of data already and as a result the geometry of the lithosphere is quite well constrained. Yet, knowing the geometry is only one part of the story, as we also need to have an accurate knowledge on the rheology and temperature structure of the lithosphere. Here, we discuss a relatively new method that we developed over the last few years, which is called geodynamic inversion. The basic principle of the method is simple: we compile available geophysical data into a realistic geometric model of the lithosphere and incorporate that into a thermo-mechanical numerical model of lithospheric deformation. In order to do so, we have to know the temperature structure, the density and the (nonlinear) rheological parameters for various parts of the lithosphere (upper crust, upper mantle, etc.). Rather than fixing these parameters we assume that they are all 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, we first perform a geodynamic inversion of a synthetic forward model of intra-oceanic subduction with known parameters. This requires solving an inverse problem with 14-16 parameters, depending on whether temperature is assumed to be known or not. With the help of a massively parallel direct-search combined with a Markov Chain Monte Carlo method, solving the inverse problem

  11. Constraining the rheology of the lithosphere through joint geodynamic and gravity inversion

    NASA Astrophysics Data System (ADS)

    Kaus, Boris; Baumann, Tobias; Popov, Anton

    2013-04-01

    In order to understand the physics of the lithospheric defor