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Sample records for continental mantle lithosphere

  1. The origin of thin lithosphere in continental backarcs: Effect of hydration on mantle lithosphere stability

    NASA Astrophysics Data System (ADS)

    Currie, C. A.; Huismans, R. S.; Beaumont, C.

    2006-12-01

    Nearly all continental backarcs have thin (~ 60 km) lithosphere for 100's of km behind the volcanic arc, even where there has been no extension. One mechanism to produce thin lithosphere is the erosion of normal thickness lithosphere by subduction-related mantle flow. The susceptibility of lithosphere to thinning largely depends on its rheology, which may be related to its state of hydration. Thin backarc lithosphere may reflect (1) a pre-existing weak rheology of lithosphere that has not been extensively dehydrated or (2) a response to rheological weakening by infiltration of slab-derived fluids. To study these processes, we use thermal- mechanical models of subduction of an old (90 Ma) oceanic plate beneath 120 km thick continental lithosphere. For (1), the backarc mantle lithosphere in the reference model has a wet olivine rheology. Subduction-induced mantle flow produces perturbations to the lowermost lithosphere, which are removed through gravitational instability and flow entrainment. The lithosphere then heats conductively, leading to subsequent thinning. Lithosphere that is stronger than wet olivine (less hydrated) does not thin, while weaker lithosphere undergoes more rapid thinning. Mantle lithosphere density influences stability, such that depleted (more buoyant) lithosphere is more stable. Thin backarc lithosphere may be limited to continental mantle that is both hydrated and fertile. This may explain why thin lithosphere of western North and South America backarcs coincides with Phanerozoic terranes that were accreted to older cratons, which may be drier, more refractory and resistant to thinning. For (2), the backarc mantle lithosphere is initially dehydrated with a scaled viscosity of wet olivine x 10 and the source of hydrating fluids is determined by tracking slab dehydration reactions (e.g., basalt-eclogite, serpentine breakdown). As most slab dehydration occurs at shallow depth, thinning of water-weakened lithosphere is restricted to the volcanic

  2. 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

  3. 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

  4. Fossilized Dipping Fabrics in Continental Mantle Lithosphere as Possible Remnants of Stacked Oceanic Paleosubductions

    NASA Astrophysics Data System (ADS)

    Babuska, V.; Plomerova, J.; Vecsey, L.; Munzarova, H.

    2015-12-01

    We have examined seismic anisotropy within the mantle lithosphere of Archean, Proterozoic and Phanerozoic provinces of Europe by means of shear-wave splitting and P-wave travel-time deviations of teleseismic waves observed at dense arrays of seismic stations (e.g., Vecsey et al., Tectonophys. 2007). Lateral variations of seismic-wave anisotropy delimit domains of the mantle lithosphere, each of them having a consistent fabric. The domains, modeled in 3D by olivine aggregates with dipping lineation a, or foliation (a,c), represent microplates or their fragments that preserved their pre-assembly fossil fabrics in the mantle lithosphere. Evaluating seismic anisotropy in 3D, as well as mapping boundaries of the domains helps to decipher processes of the lithosphere formation. Systematically dipping mantle fabrics and other seismological findings seem to support a model of continental lithosphere built from systems of paleosubductions of plates of ancient oceanic lithosphere (Babuska and Plomerova, AGU Geoph. Monograph 1989), or by stacking of the plates (Helmstaedt and Schulze, Geol. Soc. Spec. Publ. 1989). Seismic anisotropy in the oceanic mantle lithosphere, explained mainly by the olivine A- or D-type fabric (Karato et al., Annu. Rev. Earth Planet. Sci. 2008), was discovered a half century ago (Hess, Nature 1964). Field observations and laboratory experiments indicate the oceanic olivine fabric might be preserved in the subducting lithosphere to a depth of at least 200-300 km. We thus interpret the dipping anisotropic fabrics in domains of the European mantle lithosphere as systems of "frozen" paleosubductions (Babuska and Plomerova, PEPI 2006), and the lithosphere base as a boundary between a fossil anisotropy in the lithospheric mantle and an underlying seismic anisotropy related to present-day flow in the asthenosphere (Plomerova and Babuska, Lithos 2010).

  5. 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).

  6. Thermo-chemical heterogeneity of continental lithospheric mantle: examples from Europe, Siberia, and North America

    NASA Astrophysics Data System (ADS)

    Artemieva, I. M.

    2015-12-01

    I present models of lithosphere density and the non-thermal part of upper mantle Vs anomalies in different tectonic provinces of Eurasia and North America. The focus is on compositional heterogeneity of the lithospheric mantle, and therefore the effect of regional temperature variations on density and Vs is removed by applying regional temperature corrections, which are constrained by heat flow data. Significant parts of Precambrian cratons of Laurasia are characterized by extremely low surface heat flow values (<25-30 mW/m2), which imply the depth extent of the lithospheric keels down to 300-350 km, at least locally. These values are in apparent contradiction with a worldwide compilation of cratonic xenolith P-T arrays, which are usually consistent with surface heat flow of around 40 mW/m2 and the lithosphere thickness of 200-250 km depth. Models of lithosphere density and seismic velocity structure indicate that xenoliths do not sample mantle with the lowest density and the highest velocity. Density structure of continental lithosphere mantle correlates with crustal structure and surface tectonics. This observation is illustrated by examples from the East European and the Siberian cratons, where lateral variations in density structure of the lithospheric mantle are compared with petrological studies of mantle-derived xenoliths from the Fennoscandian and Siberian kimberlite provinces. The results indicate that in the Siberian craton isopycnicity is satisfied only in major kimberlite provinces. High lithosphere density in major sedimentary basins suggests the presence of eclogitic material. Since the depth distribution of density anomalies is unknown, the analysis is complemented by seismic data in order to understand better geodynamic causes of mantle density heterogeneity. Temperature-corrected seismic velocity structure based on published high-resolution tomography models indicates a pronounced stratification of lithospheric mantle in many Precambrian terranes

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

    PubMed

    Clark, Marin Kristen

    2012-02-29

    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.

  8. 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

  9. Relationship between the upper mantle high velocity seismic lid and the continental lithosphere

    NASA Astrophysics Data System (ADS)

    Priestley, Keith; Tilmann, Frederik

    2009-04-01

    useful in mapping the thickness of the high velocity upper mantle lid because this type of analysis often determines wave speed perturbations from an unknown horizontal average and not absolute velocities. Thus, any feature which extends laterally across the whole region beneath a seismic network becomes invisible in the teleseismic body-wave tomographic image. We compare surface-wave and body-wave tomographic results using southern Africa as an example. Surface-wave tomographic images for southern Africa show a strong, high velocity upper mantle lid confined to depths shallower than ~ 200 km, whereas body-wave tomographic images show weak high velocity in the upper mantle extending to depths of ~ 300 km or more. However, synthetic tests show that these results are not contradictory. The absolute seismic velocity structure of the upper mantle provided by surface wave analysis can be used to map the thermal lithosphere. Priestley and McKenzie (Priestley, K., McKenzie, D., 2006. The thermal structure of the lithosphere from shear wave velocities. Earth and Planetary Science Letters 244, 285-301.) derive an empirical relationship between shear wave velocity and temperature. This relationship is used to obtain temperature profiles from the surface-wave tomographic models of the continental mantle. The base of the lithosphere is shown by a change in the gradient of the temperature profiles indicative of the depth where the mode of heat transport changes from conduction to advection. Comparisons of the geotherms determined from the conversion of surface-wave wave speeds to temperatures with upper mantle nodule-derived geotherms demonstrate that estimates of lithospheric thickness from Vs and from the nodule mineralogy agree to within about 25 km. The lithospheric thickness map for Africa derived from the surface-wave tomographic results shows that thick lithosphere underlies most of the Archean crust in Africa. The distribution of diamondiferous kimberlites provides an

  10. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  12. 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

  13. 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.

  14. Isotopic characterisation of the sub-continental lithospheric mantle beneath Zealandia, a rifted fragment of Gondwana

    NASA Astrophysics Data System (ADS)

    Waight, Tod E.; Scott, James M.; van der Meer, Quinten H. A.

    2013-04-01

    The greater New Zealand region, known as Zealandia, represents an amalgamation of crustal fragments accreted to the paleo-Pacific Gondwana margin and which underwent significant thinning during the subsequent split from Australia and Antarctica in the mid-Cretaceous following opening of the Tasman Sea and the Southern Ocean. We present Sr, Nd and Pb isotopes and laser ablation trace element data for a comprehensive suite of clinopyroxene separates from spinel peridotite xenoliths (lherzolite to harzburgite) from the sub-continental lithospheric mantle across southern New Zealand. These xenoliths were transported to the surface in intra-plate alkaline volcanics that erupted across the region in the Eocene and Miocene (33-10 m.y.a.). Most of the volcanic suites have similar geochemical and isotopic properties that indicate melting of an OIB-like mantle source in the garnet stability zone and that contained a HIMU component. The volcanics have tapped two adjacent but chemically contrasting upper mantle domains: a fertile eastern domain and an extremely depleted western domain. Both domains underlie Mesozoic metasedimentary crust. Radiogenic isotope compositions of the clinopyroxene have 87Sr/86Sr between 0.7023 to 0.7035, 143Nd/144Nd between 0.5128 and 0.5132 (corresponding to ?Nd between +3 and +13) with a few samples extending to even more depleted compositions, 206Pb/204 Pb between ca. 19.5 to 21.5 and 208Pb/204 Pb between ca. 38.5 to 40.5. No correlations are observed between isotopic composition, age or geographical separation. These isotopic compositions indicate that the sub-continental lithospheric mantle under southern New Zealand has a regionally distinct and pervasive FOZO to HIMU - like signature. The isotopic signatures are also similar to those of the alkaline magmas that transported the xenoliths and suggest that most of the HIMU signature observed in the volcanics could be derived from a major source component in the sub-continental lithospheric mantle

  15. Impact of far-field stress distributions and thermo-rheological structure of continental lithosphere on mantle-lithosphere interactions.

    NASA Astrophysics Data System (ADS)

    Burov, E. B.; Koptev, A.; Gerya, T.; Calais, E.; Leroy, S. D.

    2015-12-01

    We implement fully-coupled high resolution 3D thermo-mechanical numerical models to investigate the impact of the laterally heterogeneous structure and rheological stratification of the continental lithosphere on the plume-activated rifting and continental break-up processes in presence of preexisting far-field tectonic stresses. In our experiments, "mantle plumes" represent short-lived diapiric upwellings that have no continuous feeding at depth. Such upwellings may be associated with "true" plumes but also with various instabilities in the convective mantle. Numerical models demonstrate strong dependence of crustal strain distributions and surface topography on the rheological composition of the lower crust and the initial thermal structure of the lithosphere. In contrast to the usual inferences from passive rifting models, distributed wide rifting takes place in case of cold (500 °C at Moho depth) initial isotherm and mafic composition of the lower crust, whereas hotter geotherms and weaker (wet quartzite) lower crustal rheology lead to strong localization of rifting. Moreover, it appears that the prerequisite of strongly anisotropic strain localization during plume-lithosphere interaction (linear rift structures instead of axisymmetric radial faulting) refers to simultaneous presence of a mantle upwelling and of (even extremely weak) directional stress field produced by far-field tectonic forces (i.e. ultra-slow far field extension at < 3 mm/y). Higher (than 1.5-3 mm/y) velocities of far-field extension lead to enlargement of the active fault zone for the same lapse of time. Yet, simultaneous rise of the lithospheric geotherm associated with active rifting has an opposite effect leading to the narrowing of the rift zone. Presence of heterogeneities (cratonic blocks) leads to splitting of the plume head onto initially nearly symmetrical parts, each of which flows towards beneath the craton borders. This craton-controlled distribution of plume material causes

  16. 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

  17. Processes accompanying of mantle plume emplacement into continental lithosphere: Evidence from NW Arabian plate, Western Syria

    NASA Astrophysics Data System (ADS)

    Sharkov, E. V.

    2015-12-01

    Lower crustal xenoliths occurred in the Middle Cretaceous lamprophyre diatremes in Jabel Ansaria (Western Syria) (Sharkov et al., 1992). They are represented mainly garnet granulites and eclogite-like rocks, which underwent by deformations and retrograde metamorphism, and younger fresh pegmatoid garnet-kaersutite-clinopyroxene (Al-Ti augite) rocks; mantle peridotites are absent in these populations. According to mineralogical geothermobarometers, forming of garnet-granulite suite rocks occurred under pressure 13.5-15.4 kbar (depths 45-54 kn) and temperature 965-1115oC. At the same time, among populations of mantle xenoliths in the Late Cenozoic platobasalts of the region, quite the contrary, lower crustal xenoliths are absent, however, predominated spinel lherzolites (fragments of upper cooled rim of a plume head), derived from the close depths (30-40 km: Sharkov, Bogatikov, 2015). From this follows that ancient continental crust was existed here even in the Middle Cretaceous, but in the Late Cenozoic was removed by extended mantle plume head; at that upper sialic crust was not involved in geomechanic processes, because Precambrian metamorphic rocks survived as a basement for Cambrian to Cenozoic sedimentary cover of Arabian platform. In other words, though cardinal rebuilding of deep-seated structure of the region occurred in the Late Cenozoic but it did not affect on the upper shell of the ancient lithosphere. Because composition of mantle xenolithis in basalts is practically similar worldwide, we suggest that deep-seated processes are analogous also. As emplacement of the mantle plume heads accompanied by powerful basaltic magmatism, very likely that range of lower (mafic) continental crust existence is very convenient for extension of plume heads and their adiabatic melting. If such level, because of whatever reasons, was not reached, melting was limited but appeared excess of volatile matters which led to forming of lamprophyre or even kimberlite.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  19. Origin of the Early Cretaceous continental intraplate volcanism, NW Syria: melting of a metasomatised lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Ma, G. S.; Malpas, J.; Xenophontos, C.; Suzuki, K.; Lo, C.

    2011-12-01

    The Mesozoic evolution of the Neotethys-Eastern Mediterranean between the African-Arabian and Eurasian continents was accompanied by intermittent eruption of alkaline-transitional basalts in Arabia. The causes of the prolonged volcanism remain controversial, whether related to the arrival(s) of mantle plume [1] or prolonged far-field extension of the passive continental margin [2]. In addition, the source(s) of the volcanism is not well constrained, as previous conclusions were drawn before recent understanding of the origin of intraplate magmas - (i) melting of hydrous metasomatic veins within the lithospheric mantle [3] or (ii) melting of an incompatible-element enriched peridotite source ± eclogites in the presence of CO2 [4, 5]. The Mesozoic basalts (ankaramites and transitional basalts) from the Coastal Ranges, NW Syria analysed in this study were dated at 106.3 ± 0.2 Ma and 103.4 ± 0.3 Ma (bulk-rock 40Ar/39Ar ages), representing the last instance of Mesozoic intraplate magmatism in the Levant region. Isotopic and geochemical analysis reveals distinct compositions between the two lava series (ankaramites: ɛNd(t) = 5.1-5.6, 87Sr/87Sr(t) = 0.70293-0.70302, 187Os/188Os(t) = 0.227-0.242; transitional basalts: ɛNd(t) = 4.0-4.6, 87Sr/87Sr(t) = 0.70320-0.70424, 187Os/188Os(t) = 0.392; and lower SiO2, higher TiO2, Nb/U, Nb/Th, Nb/La and Ce/Pb in the ankaramites). Fractional crystallisation and assimilation-fractional crystallisation modelling suggests minor roles for both processes during the evolution of the lavas, despite the generally high Os isotopic ratios. The modelling also precludes derivation of one lava series from the other, suggesting that the isotopic and geochemical distinctions must be inherited from the source. It is interpreted that the chemical characteristics represent a greater component derived from metasomatic amphibole-rich veins in the source region. Both the ankaramites and transitional basalts were generated from this metasomatised

  20. New constraints of subducted mantle lithosphere on plate-tectonic reconstructions of deformed continental blocks

    NASA Astrophysics Data System (ADS)

    Suppe, J.; Wu, J.; Kanda, R. V. S.; Lu, R.; Lin, C. D. J.

    2012-04-01

    Global seismic tomography and earthquake locations are now sufficiently good that many subducted slabs can be mapped in 3D, unfolded and restored to the surface of the Earth, thereby providing important new quantitative constraints on plate-tectonic reconstructions. The size, shape, present horizontal and vertical positions and seismic velocities of subducted slabs provide rich data constraints on plate-tectonic reconstructions of past plate networks into which the deformed continental regions such as Eurasia and SE Asia must fit. Commonly, we find that well-imaged and restored slabs of mantle lithosphere fit together along their edges in approximate "picture-puzzle" fashion, within seismic resolution. The slab edges correspond to plate transforms, slab tears, initial positions of trenches and edges of slab windows. This use of subducted slabs provides for more data-rich reconstructions of lost ocean basins such as those consumed between India and Eurasia and between Southeast Asia and Australia, and thereby constrains deformation of the adjacent continents. We describe our methodologies for mapping and unfolding slabs in Gocad, and using these restored slabs in GPlates. Examples are shown from Taiwan, the India-Asia collision, Southeast Asia, and Greater northeast Australia.

  1. 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.

  2. 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

  3. 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

  4. Rock Magnetic Mineral Assemblage in Mineral Separates from Xenoliths of Continental Lithospheric Mantle

    NASA Astrophysics Data System (ADS)

    Khakhalova, E.; Feinberg, J. M.; Ionov, D. A.; Ferre, E. C.; Friedman, S. A.; Hernandez, F. M.; Neal, C. R.; Conder, J. A.

    2014-12-01

    Studies of aeromagnetic anomalies suggest that the lithospheric mantle may contribute to long wavelength features. Examination of unaltered mantle xenoliths may reveal the mineralogical sources of these aeromagnetic anomalies. Prior work has reported microscopic inclusions of magnetic minerals in mantle silicates. Here we explore the magnetism of pure olivine, clinopyroxene, orthopyroxene, and spinel separated from peridotite xenoliths from the Dariganga and Tariat localities in Mongolia that sample the lithospheric mantle. All separates were leached with HF and HCl to remove secondary minerals adhering to the surface of the grains or in cracks. Separates were then mounted in cement to create monomineralic specimens for investigation using hysteresis loops, first order reversal curves (FORC), alternating field and thermal demagnetization of a 1T IRM, and low-temperature magnetometry. All specimens showed trace concentrations of ferromagnetic inclusions with Ms values of ~10-3 Am2kg-1. Thermal demagnetization showed a range of unblocking temperatures with median destructive temperatures of 300-400°C. Two specimens showed a dramatic demagnetization at 585°C, consistent with pure magnetite (Mt). The presence of Mt was confirmed by observations of the Verwey transition at 100-120K and by backfield remanence acquisition curves that plateau at ~300 mT. The median destructive alternating field was ~20 mT and 40-80 mT for specimens from Dariganga and Tariat, respectively. FORC diagrams show single-domain-like behavior with a median Hc of ~20 mT. The demagnetization experiments suggest that Mt inclusions in the lattice of olivine, opx, cpx and spinel carry magnetic remanence. Thus, the lithospheric mantle may exhibit in-situ ferromagnetism carried by Mt below 585°C. The magnetization of separates varies between xenolith localities but is consistent amongst minerals of the same locality. Future work will address whether the Mt formed before or during xenolith ascent.

  5. Mantle Water Fugacity is the Dominant Factor in Total Strength and Stability/Mobility of Continental Lithosphere

    NASA Astrophysics Data System (ADS)

    Lowry, A. R.; Schutt, D.; Perez-Gussinye, M.; Ma, X.; Berry, M. A.; Ravat, D.

    2014-12-01

    More than half a century after the plate tectonic revolution, the physical mechanism that distinguishes tectonically active plate boundaries from stable continental interiors remains nebulous. Rock flow strength and mass density variations both contribute to stress, so both are certain to be important, but these depend ambiguously on rock lithology, temperature, and concentrations of water. High seismic velocities observed to great depths often are interpreted as evidence that geothermal variations dominate patterns of lithospheric strength. However, mantle seismic velocities are sensitive to flow-induced anelastic attenuation as well as to temperature. A more ductile mantle will propagate waves more slowly regardless of whether low viscosity is a consequence of high temperature or of high water fugacity, complicating interpretations of seismic velocity in the absence of other constraints. Here we use EarthScope's USArray seismic data to independently constrain crustal thickness, bulk crustal lithology and Moho temperature of the lithosphere, and magnetic bottom measurements to refine the crustal geotherm. Strength models based on these quantities are then compared to integral measurements of western U.S. isostatic strength expressed as effective elastic thickness, Te. We show that mantle water is the primary factor that distinguishes stable lithosphere of North America's cratonic interior from actively deforming zones in the western U.S. Cordillera. Seismic and magnetic constraints on temperature and lithology variations can be reconciled with integral strength measurements only if water fugacity within the lithospheric column is permitted to vary from near-saturation in deforming, mobile lithosphere to nearly completely dry in the stable cratonic interior.

  6. Regional uplift associated with continental large igneous provinces: The roles of mantle plumes and the lithosphere

    USGS Publications Warehouse

    Saunders, A.D.; Jones, S.M.; Morgan, L.A.; Pierce, K.L.; Widdowson, M.; Xu, Y.G.

    2007-01-01

    The timing and duration of surface uplift associated with large igneous provinces provide important constraints on mantle convection processes. Here we review geological indicators of surface uplift associated with five continent-based magmatic provinces: Emeishan Traps (260??million years ago: Ma), Siberian Traps (251??Ma), Deccan Traps (65??Ma), North Atlantic (Phase 1, 61??Ma and Phase 2, 55??Ma), and Yellowstone (16??Ma to recent). All five magmatic provinces were associated with surface uplift. Surface uplift can be measured directly from sedimentary indicators of sea-level in the North Atlantic and from geomorpholocial indicators of relative uplift and tilting in Yellowstone. In the other provinces, surface uplift is inferred from the record of erosion. In the Deccan, North Atlantic and Emeishan provinces, transient uplift that results from variations in thermal structure of the lithosphere and underlying mantle can be distinguished from permanent uplift that results from the extraction and emplacement of magma. Transient surface uplift is more useful in constraining mantle convection since models of melt generation and emplacement are not required for its interpretation. Observations of the spatial and temporal relationships between surface uplift, rifting and magmatism are also important in constraining models of LIP formation. Onset of surface uplift preceded magmatism in all five of the provinces. Biostratigraphic constraints on timing of uplift and erosion are best for the North Atlantic and Emeishan Provinces, where the time interval between significant uplift and first magmatism is less than 1??million years and 2.5??million years respectively. Rifting post-dates the earliest magmatism in the case of the North Atlantic Phase 1 and possibly in the case of Siberia. The relative age of onset of offshore rifting is not well constrained for the Deccan and the importance of rifting in controlling magmatism is disputed in the Emeishan and Yellowstone

  7. Formation and Evolution of the Continental Lithospheric Mantle: Perspectives From Radiogenic Isotopes of Silicate and Sulfide Inclusions in Macrodiamonds

    NASA Astrophysics Data System (ADS)

    Shirey, S. B.; Richardson, S. H.

    2007-12-01

    Silicate and sulfide inclusions that occur in diamonds comprise the oldest (>3 Ga), deepest (>140 km) samples of mantle-derived minerals available for study. Their relevance to the evolution of the continental lithosphere is clear because terrestrial macrodiamonds are confined to regions of the Earth with continental lithospheric mantle keels. The goals of analytical work on inclusions in diamond are to obtain paragenesis constraints, radiogenic ages, and initial isotopic compositions. The purpose is to place diamond formation episodes into the broader framework of the geological processes that create and modify the continental lithosphere and to relate the source of the C and N in diamond-forming fluids to understanding the Earth's C and N cycles in the Archean. Although sulfide and silicate inclusions rarely occur in the same diamond, they both can be grouped according to their geochemical similarity with the chief rock types that comprise the mantle keel: peridotite and eclogite. Silicate inclusions are classified as harzburgitic (depleted; olivine > Fo91, garnet Cr2O3 > 3 wt% and CaO from 0 to 5 wt%), lherzolitic (fertile), or eclogitic (basaltic; garnet Cr2O3 < 2 wt% and CaO from 3 to 15 wt%, clinopyroxene with higher Na2O, Al2O3, and FeO); they are amenable for trace element study by SIMS and for Sm-Nd and Rb-Sr analysis by conventional P-TIMS after grouping by mineralogical similarity. Sulfide inclusions (chiefly FeS with lesser Ni, Cu, and Co) are classified as peridotitic (Ni > 14 wt%; Os > 2 ppm) versus eclogitic (Ni < 10 wt%; Os < 200 ppb); single sulfides are amenable for S isotopic study by SIMS or TIMS, and Re-Os analysis by N-TIMS. Work on inclusions in diamonds depends on the distribution of mined, diamond-bearing kimberlites, and the generosity of mining companies because of the extreme rarity of inclusions in suites of mostly gem-quality diamonds. Most isotopic work has been on the Kaapvaal-Zimbabwe craton with lesser work on the Slave, Siberian

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

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

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

  9. Sedimentary halogens and noble gases within Western Antarctic xenoliths: Implications of extensive volatile recycling to the sub continental lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Broadley, Michael W.; Ballentine, Chris J.; Chavrit, Déborah; Dallai, Luigi; Burgess, Ray

    2016-03-01

    Recycling of marine volatiles back into the mantle at subduction zones has a profound, yet poorly constrained impact on the geochemical evolution of the Earth's mantle. Here we present a combined noble gas and halogen study on mantle xenoliths from the Western Antarctic Rift System (WARS) to better understand the flux of subducted volatiles to the sub continental lithospheric mantle (SCLM) and assess the impact this has on mantle chemistry. The xenoliths are extremely enriched in the heavy halogens (Br and I), with I concentrations up to 1 ppm and maximum measured I/Cl ratios (85.2 × 10-3) being ∼2000 times greater than mid ocean ridge basalts (MORB). The Br/Cl and I/Cl ratios of the xenoliths span a range from MORB-like ratios to values similar to marine pore fluids and serpentinites, whilst the 84Kr/36Ar and 130Xe/36Ar ratios range from modern atmosphere to oceanic sediments. This indicates that marine derived volatiles have been incorporated into the SCLM during an episode of subduction related metasomatism. Helium isotopic analysis of the xenoliths show average 3He/4He ratios of 7.5 ± 0.5 RA (where RA is the 3He/4He ratio of air = 1.39 × 10-6), similar to that of MORB. The 3He/4He ratios within the xenoliths are higher than expected for the xenoliths originating from the SCLM which has been extensively modified by the addition of subducted volatiles, indicating that the SCLM beneath the WARS must have seen a secondary alteration from the infiltration and rise of asthenospheric fluids/melts as a consequence of rifting and lithospheric thinning. Noble gases and halogens within these xenoliths have recorded past episodes of volatile interaction within the SCLM and can be used to reconstruct a tectonic history of the WARS. Marine halogen and noble gas signatures within the SCLM xenoliths provide evidence for the introduction and retention of recycled volatiles within the SCLM by subduction related metasomatism, signifying that not all volatiles that survive

  10. Refertilization of deep continental arc lithosphere: constraints from major element and trace element systematics in mantle xenoliths from the Sierra Nevada, California

    NASA Astrophysics Data System (ADS)

    Chin, E. J.; Lee, C.; Luffi, P. I.

    2010-12-01

    Thickening of continental arc lithosphere, either by underplating, magmatic inflation, or other tectonic processes, is an important stage in the maturation of primitive island arcs into Phanerozoic continental crust. The Sierra Nevada continental arc is one example of a mature arc that has experienced significant thickening and recent orogenic collapse. Previous studies of mantle xenoliths from the Sierras point to delamination of a dense mafic root that represented the residua of the evolved crust distilled out of a >90 km thickened lithospheric column. What has not received much attention, however, is how this lithospheric column has been compositionally modified over the lifespan of the Sierra Nevada arc. As the interface between the crust and mantle, the base of the thickened lithospheric column (upper mantle and lowermost crust) is potentially a zone of refertilization, wherein previously depleted mantle is replenished with fertile components from ascending melts and/or fluids. Refertilization of deep lithosphere not only influences the composition of arc magmas in their early stages, but also has implications for the stability of highly melt-depleted, refractory mantle that may become “rejuvenated” by such refertilization. In this study, we evaluate the role of refertilization in continental arc evolution by investigating upper mantle xenoliths from the Sierra Nevada. Cr# in spinel cores is chosen as a baseline index of melt depletion, and is compared to whole-rock major- and trace-element concentrations (Yb, Ti, Mg, Al) to assess the degree of refertilization. Whole-rock trace element patterns indicate depletions in high field strength elements and complementary enrichments in fluid mobile elements, suggesting that the deep Sierran lithosphere was refertilized by a melt containing a large component of slab-derived fluid. Although high degrees of melting (>15%) are recorded by Cr# in spinel, many Sierran peridotites contain diffuse bands comprised of

  11. Geochemistry of basalts from small eruptive centers near Villarrica stratovolcano, Chile: Evidence for lithospheric mantle components in continental arc magmas

    NASA Astrophysics Data System (ADS)

    Hickey-Vargas, R.; Sun, M.; Holbik, S.

    2016-07-01

    and aged subduction-related pyroxenite in the mantle lithosphere complicates the identification of subducted, mantle and crustal inputs to magma erupted in continental arcs.

  12. Regional 3D Numerical Modeling of the Lithosphere-Mantle System: Implications for Continental Rift-Parallel Surface Velocities

    NASA Astrophysics Data System (ADS)

    Stamps, S.; Bangerth, W.; Hager, B. H.

    2014-12-01

    The East African Rift System (EARS) is an active divergent plate boundary with slow, approximately E-W extension rates ranging from <1-6 mm/yr. Previous work using thin-sheet modeling indicates lithospheric buoyancy dominates the force balance driving large-scale Nubia-Somalia divergence, however GPS observations within the Western Branch of the EARS show along-rift motions that contradict this simple model. Here, we test the role of mantle flow at the rift-scale using our new, regional 3D numerical model based on the open-source code ASPECT. We define a thermal lithosphere with thicknesses that are systematically changed for generic models or based on geophysical constraints in the Western branch (e.g. melting depths, xenoliths, seismic tomography). Preliminary results suggest existing variations in lithospheric thicknesses along-rift in the Western Branch can drive upper mantle flow that is consistent with geodetic observations.

  13. P/n/ velocity and cooling of the continental lithosphere. [upper mantle compression waves in North America

    NASA Technical Reports Server (NTRS)

    Black, P. R.; Braile, L. W.

    1982-01-01

    The average upper mantle compressional wave velocity and heat flow figures presently computed for continental physiographic provinces in North America exhibit an inverse relationship, and possess a statistically significant correlation coefficient. A correlation is also demonstrated between compressional wave velocity and material temperature by estimating crust-mantle boundary temperatures from heat flow values. The dependency of compressional wave velocity on temperature implies that the observed geographical distribution in upper mantle seismic velocity may be due to the temperature effect character of upper mantle compressional wave velocity variation.

  14. Characterization of the sub-continental lithospheric mantle beneath the Cameroon volcanic line inferred from alkaline basalt hosted peridotite xenoliths from Barombi Mbo and Nyos Lakes

    NASA Astrophysics Data System (ADS)

    Pintér, Zsanett; Patkó, Levente; Tene Djoukam, Joëlle Flore; Kovács, István; Tchouankoue, Jean Pierre; Falus, György; Konc, Zoltán; Tommasi, Andréa; Barou, Fabrice; Mihály, Judith; Németh, Csaba; Jeffries, Teresa

    2015-11-01

    We carried out detailed petrographic, major and trace element geochemical, microstructural and FTIR analyses on eight characteristic ultramafic xenoliths from Nyos and Barombi Mbo Lakes in the continental sector of the Cameroon Volcanic Line (CVL). The studied xenoliths are spinel lherzolites showing lithologies similar to the other xenoliths reported previously along the CVL. They have protogranular and porphyroclastic textures. One of the Barombi xenolith contains amphibole, which had not been previously reported in this locality. Amphibole is common in the Nyos xenoliths suite. Peridotite xenoliths from both localities show some chemical heterogeneity, but Barombi xenoliths generally are less depleted in basaltic elements with respect to Nyos xenoliths. Trace element compositions of Nyos spinel lherzolites show a moderately depleted initial (premetasomatic) composition and variable enrichment in REE. Evidence for both modal and cryptic metasomatism is present in Nyos xenoliths. Rare earth element patterns of clinopyroxene suggest that interaction between mafic melts and the upper mantle occurred beneath the Nyos locality. Barombi Mbo xenoliths, on the other hand, record a small degree of partial melting. The Barombi Mbo xenoliths have weak, dominantly orthorhombic olivine crystal preferred orientations, whereas Nyos ones have strong axial-[010] patterns, which may have formed in response to transpression. Nominally anhydrous mantle minerals (NAMs) of the Barombi Mbo xenoliths show generally higher bulk concentrations of 'water' (70-127 ppm) than Nyos xenoliths (32-81 ppm). The Barombi Mbo xenoliths could originate from a juvenile segment of the lithospheric mantle, which had been originally part of the asthenosphere. It became a part of the lithosphere in response to thermal relaxation following the extension, forming a weakly deformed lower lithospheric mantle region along the CVL. The Nyos xenoliths, however, represent a shallow lithospheric mantle bearing

  15. NSF Continental Lithosphere Program

    NASA Astrophysics Data System (ADS)

    Mayhew, Michael; MacGregor, Ian

    For several months the Continental Lithosphere Program (CL) of the National Science Foundation has been subject to a major review. The process was stimulated by a series of budget setbacks over the past few years. Although Presidential budget requests have been very favorable for the Division of Earth Sciences (EAR), and there has been strong support within the National Science Foundation and Congress, actual appropriations by Congress have been disappointing.In each year the final allocation to EAR has been affected by external factors beyond the control of the Foundation. In the four fiscal years from 1986 through 1989 the factors include reductions tied to the Gramm-Rudman deficit reduction measures, congressional reaction to the October 1987 stock market crash, and two years of protection for the Ocean Sciences part of the NSF budget that was paid for from the budgets of the Atmospheric and Earth Sciences divisions.

  16. Episodic Instabilities of Thick Continental Lithosphere (Invited)

    NASA Astrophysics Data System (ADS)

    Jaupart, C. P.; Fourel, L.; Farnetani, C. G.

    2009-12-01

    Although continental interiors are commonly described as stable, many have been subjected to major perturbations. The North American continent, for example, saw the formation or reactivation of four intracratonic basins (Williston, Hudson Bay, Illinois and Michigan) in the Paleozoic about 500 million years ago. These events occurred far from ocean basins and are not related to other tectonic events, and hence have usually been explained as late consequences of earlier orogenies or of mantle plumes impinging the base of the lithosphere. Why and how subsidence affected four neighbouring basins simultaneously in the Paleozoic has not been explained, however. Other important observations are that intracratonic basins and subsidence events tend to recur at the same locations, and that subsidence is rarely preceded by domal uplift. These observations can be explained by the behaviour of thick compositionally buoyant lithosphere that becomes unstable because it is being cooled from above. Laboratory analog experiments, stability theory and numerical simulations in 2-D and 3-D have been conducted to specify the necessary conditions for instability and to illustrate how flow develops and deforms the lithosphere. Numerical solutions accounting for temperature-dependent viscosity show that the compositional viscosity contrast between the lithospheric mantle and the underlying asthenosphere has only a weak effect on flow and deformation. Lithosphere behaviour depends on the Rayleigh number and the buoyancy ratio, which is equal to the ratio of compositional density contrast over the thermal density contrast through the unstable part of the lithosphere. Episodic instabilities are generated at small buoyancy numbers appropriate for geological conditions. Scaling laws for temperature-dependent viscosity fluids will be presented. Little uplift is generated by the instability because the hot upwelling asthenospheric mantle displaces compositionally buoyant colder lithospheric

  17. 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

  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. 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.

  20. Sub-continental lithospheric mantle structure beneath the Adamawa plateau inferred from the petrology of ultramafic xenoliths from Ngaoundéré (Adamawa plateau, Cameroon, Central Africa)

    NASA Astrophysics Data System (ADS)

    Nkouandou, Oumarou F.; Bardintzeff, Jacques-Marie; Fagny, Aminatou M.

    2015-11-01

    Ultramafic xenoliths (lherzolite, harzburgite and olivine websterite) have been discovered in basanites close to Ngaoundéré in Adamawa plateau. Xenoliths exhibit protogranular texture (lherzolite and olivine websterite) or porphyroclastic texture (harzburgite). They are composed of olivine Fo89-90, orthopyroxene, clinopyroxene and spinel. According to geothermometers, lherzolites have been equilibrated at 880-1060 °C; equilibrium temperatures of harzburgite are rather higher (880-1160 °C), while those of olivine websterite are bracketed between 820 and 1010 °C. The corresponding pressures are 1.8-1.9 GPa, 0.8-1.0 GPa and 1.9-2.5 GPa, respectively, which suggests that xenoliths have been sampled respectively at depths of 59-63 km, 26-33 km and 63-83 km. Texture and chemical compositional variations of xenoliths with temperature, pressure and depth on regional scale may be ascribed to the complex history undergone by the sub-continental mantle beneath the Adamawa plateau during its evolution. This may involve a limited asthenosphere uprise, concomitantly with plastic deformation and partial melting due to adiabatic decompression processes. Chemical compositional heterogeneities are also proposed in the sub-continental lithospheric mantle under the Adamawa plateau, as previously suggested for the whole Cameroon Volcanic Line.

  1. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  2. Continental crust subducted deeply into lithospheric mantle: the driving force of Early Carboniferous magmatism in the Variscan collisional orogen (Bohemian Massif)

    NASA Astrophysics Data System (ADS)

    Janoušek, Vojtěch; Schulmann, Karel; Lexa, Ondrej; Holub, František; Franěk, Jan; Vrána, Stanislav

    2014-05-01

    relamination mechanisms. The presence of refractory light material rich in radioactive elements under the denser upper plate would eventually result in gravity-driven overturns in the thickened crust. The contaminated lithospheric mantle domains yielded, soon thereafter, ultrapotassic magmas whose major- and compatible-trace element signatures point to equilibration with the mantle peridotite, while their LILE contents and radiogenic isotope signatures are reminiscent of the subducted continental crust. This research was financially supported by the GAČR Project P210-11-2358 (to VJ) and Ministry of Education of the Czech Republic program LK11202 (to KS). Becker, H. 1996. Journal of Petrology 37, 785-810. Kotková, J. et al. 2011. Geology 39, 667-670. Massonne, H.-J. 2001. European Journal of Mineralogy 13, 565-570. Naemura, K. et al. 2009. Journal of Petrolology 50, 1795-1827. Schulmann, K., et al., 2014. Geology, in print. Vrána, S. 2013. Journal of Geosciences 58, 347-378. Zheng, Y. F. 2012. Chemical Geology 328, 5-48.

  3. Water in the Lithospheric Mantle Beneath a Phanerozoic Continental Belt: FTIR Analyses of Alligator Lake Xenoliths (Yukon, Canada)

    NASA Technical Reports Server (NTRS)

    Gelber, McKensie; Peslier, Ann H.; Brandon, Alan D.

    2015-01-01

    Water in the mantle influences melting, metasomatism, viscosity and electrical conductivity. The Alligator Lake mantle xenolith suite is one of three bimodal peridotite suites from the northern Canadian Cordillera brought to the surface by alkali basalts, i.e., it consists of chemically distinct lherzolites and harzburgites. The lherzolites have equilibration temperatures about 50 C lower than the harzburgites and are thought to represent the fertile upper mantle of the region. The harzburgites might have come from slightly deeper in the mantle and/or be the result of a melting event above an asthenospheric upwelling detected as a seismic anomaly at 400-500 km depth. Major and trace element data are best interpreted as the lherzolite mantle having simultaneously experienced 20-25% partial melting and a metasomatic event to create the harzburgites. Well-characterized xenoliths are being analyzed for water by FTIR. Harzburgites contain 29-52 ppm H2O in orthopyroxene (opx) and (is) approximately140 ppm H2O in clinopyroxene (cpx). The lherzolites have H2O contents of 27-150 ppm in opx and 46-361 ppm in cpx. Despite correlating with enrichments in LREE, the water contents of the harzburgite pyroxenes are low relative to those of typical peridotite xenoliths, suggesting that the metasomatic agents were water-poor, contrarily to what has been suggested before. The water content of cpx is about double that of opx indicating equilibrium. Olivine water contents are low ((is) less than 5 ppm H2O) and out of equilibrium with those of opx and cpx, which may be due to H loss during xenolith ascent. This is consistent with olivines containing more water in their cores than their rims. Olivines exclusively exhibit water bands in the 3400-3000 cm-1 range, which may be indicative of a reduced environment.

  4. Water in the lithospheric mantle beneath a Phanerozoic continental belt: FTIR analyses of Alligator Lake Xenoliths (Yukon, Canada)

    NASA Astrophysics Data System (ADS)

    Gelber, M.; Peslier, A. H.; Brandon, A. D.

    2015-12-01

    Water in the mantle influences melting, metasomatism, viscosity and electrical conductivity. The Alligator Lake mantle xenolith suite is one of three bimodal peridotite suites from the northern Canadian Cordillera brought to the surface by alkali basalts, i.e., it consists of chemically distinct lherzolites and harzburgites [1-2]. The lherzolites have equilibration temperatures about 50 °C lower than the harzburgites and are thought to represent the fertile upper mantle of the region. The harzburgites might have come from slightly deeper in the mantle and/or be the result of a melting event above an asthenospheric upwelling detected as a seismic anomaly at 400-500 km depth [3]. Major and trace element data are best interpreted as the lherzolite mantle having simultaneously experienced 20-25% partial melting and a metasomatic event to create the harzburgites [3]. Well-characterized xenoliths are being analyzed for water by FTIR. Harzburgites contain 29-52 ppm H2O in orthopyroxene (opx) and ~140 ppm H2O in clinopyroxene (cpx). The lherzolites have H2O contents of 27-150 ppm in opx and 46-361 ppm in cpx. Despite correlating with enrichments in LREE, the water contents of the harzburgite pyroxenes are low relative to those of typical peridotite xenoliths [4], suggesting that the metasomatic agents were water-poor, contrarily to what has been suggested before [3]. The water content of cpx is about double that of opx indicating equilibrium. Olivine water contents are low (< 5 ppm H2O) and out of equilibrium with those of opx and cpx, which may be due to H loss during xenolith ascent. This is consistent with olivines containing more water in their cores than their rims. Olivines exclusively exhibit water bands in the 3400-3000 cm-1 range, which may be indicative of a reduced environment [5]. [1] Francis. 1987 JP 28, 569-97. [2] Eiche et al. 1987 CMP 95, 191-201. [3] Shi et al. 1997 CMP 131, 39-53. [4] Peslier et al. 2015 GGG 154, 98-117. [5] Bai et al. 1993 PCM 19, 460-71.

  5. Deep thermal structure and thickness of the continental lithosphere

    NASA Astrophysics Data System (ADS)

    Jaupart, C.; Mareschal, J.; Kaminski, E.

    2002-12-01

    There is no doubt that cratonic lithosphere is much thicker than oceanic lithosphere, but large uncertainties remain on its thickness, deep thermal structure, physical properties (density, rheology) and composition. Heat flow and crustal heat production data in North America demonstrate that, in steady-state conditions, heat flow variations are essentially due to changes of crustal heat production. Crustal models in several different geological provinces and considerations on the thermal stability of the crust through geological time constrain the mantle heat flow to be in the range 11-16 mWm-2. For such low values, heat flow data are not sensitive to small, but significant, lateral variations of the mantle heat flow (≈ 20%). Downward extrapolation of temperature profiles requires knowledge of thermal conductivity and heat production in the lithospheric mantle. For a thick lithosphere, surface heat flow is not in equilibrium with the instantaneous heat production and basal heat flow, and surface measurements record a time-average. With current estimates of heat production in the lithospheric mantle (≈ 0.02 mWm-3), thermal models rule out that lithospheric thickness is greater than 330 km. Using heat flow data and other constraints on lithospheric temperatures, such as xenolith (P,T) equilibration conditions, one can only construct geotherms in a stable conductive layer. Thus, comparison with seismic constraints cannot be made without considering the dynamical interactions with the convective mantle. A small-scale convection model shows that temperatures at the base of the continental lithosphere may not be equal to those of the well-mixed (isentropic) oceanic convecting mantle. Constraints on deep lithospheric structure can also be obtained using thermal transients, as recorded for example by sedimentary accumulations in intracratonic basins. The stability of thick roots requires the lithospheric mantle to be compositionally buoyant. Localized thinning results

  6. Two tales of the continental lithospheric mantle prior to the destruction of the North China Craton: Insights from Early Cretaceous mafic intrusions in western Shandong, East China

    NASA Astrophysics Data System (ADS)

    Huang, Xiao-Long; Zhong, Jun-Wei; Xu, Yi-Gang

    2012-11-01

    Weakened lithospheric zones such as the Dabie-Sulu orogenic belt and Tan-Lu fault zone played important roles in the destruction of the North China Craton (NCC) during the late Mesozoic. Early Cretaceous mafic intrusions in western Shandong, contemporary with extensive magmatism during the destruction of the NCC, delineate two spatially distinct mantle domains (EM1- and EM2-like) beneath the craton’s interior and weakened lithospheric zones, respectively. The Jinan and Zouping gabbros from the craton interior (∼128 Ma) show fractionated LREE and nearly flat HREE patterns ([La/Yb]N = 2.94-8.95; [Dy/Yb]N = 1.23-1.69) with notable negative Ta, Nb and Ti anomalies. They have strong negative εNd(t) (-15.7 to -7.1), low initial 87Sr/86Sr (0.7039-0.7060) and negative zircon εHf(t) of -20.0 to -6.2. These “crustal fingerprints” cannot be explained by crustal contamination, but were likely derived from a hybrid mantle source. Crustal delamination or detachment during the Early Paleoproterozoic might be responsible for the involvement of Early Precambrian crustal materials in the Mesozoic mantle source beneath the southeastern NCC. In comparison, the Early Cretaceous mafic igneous rocks from regions (e.g., Yinan, Mengyin and Fangcheng) adjacent to the Dabie-Sulu orogenic belt and Tan-Lu fault zone have higher 87Sr/86Sr ratios (0.7059-0.7119), suggesting modification of the lithospheric mantle by melts/fluids derived from the Yangtze crust. The Mesozoic crustal delamination may have triggered the destruction of the lithospheric root beneath the Dabie-Sulu orogenic belt, whereas the lithospheric thinning beneath the interior of the southeastern NCC is attributed to the thermo-mechanical erosion by lateral convective asthenosphere.

  7. Seismic images of the upper mantle velocities and structure of European mantle lithosphere

    NASA Astrophysics Data System (ADS)

    Plomerova, Jaroslava; Munzarova, Helena; Vecsey, Ludek; Babuska, Vladislav

    2014-05-01

    Tomography images of seismic velocities in the Earth mantle represent significant tool for recovering first order structural features. Regional studies, based on dense networks of temporary stations allow us to focus on structure of the continental upper mantle and to study variations of body-wave velocities in greater detail. However, the standard tomography exhibits only isotropic view of the Earth, whose structure is anisotropic in general, as shown by results of various studies exploiting a broad range of methods, types of waves and scales. We present results of our studies of seismic anisotropy in tectonically different provinces that clearly demonstrate the continental mantle lithosphere consists of domains with different fossil fabrics. We detect anisotropic signal both in teleseismic P-wave travel-time deviations and shear-wave splitting and show changes of the anisotropic parameters across seismic arrays, in which stations with similar characteristics form groups. The geographical variations of seismic-wave anisotropy delimit individual, often sharply bounded domains of the mantle lithosphere, each of them having a consistent fabric. The domains can be modelled in 3D by peridotite aggregates with dipping lineation a or foliation (a,c). These findings allow us to interpret the domains as micro-plate fragments retaining fossil fabrics in the mantle lithosphere, reflecting thus an olivine LPO created before the micro-plates assembled. Modelling anisotropic structure of individual domains of the continental mantle lithosphere helps to decipher boundaries of individual blocks building the continental lithosphere and hypothesize on processes of its formation (Plomerova and Babuska, Lithos 2010). Exploiting the long memory of the deep continental lithosphere fabric, we present the lithosphere-asthenosphere boundary (LAB) as a transition between a fossil anisotropy in the mantle lithosphere and an underlying seismic anisotropy related to the present-day flow in

  8. Deformation in the continental lithosphere

    NASA Astrophysics Data System (ADS)

    The Physical Properties of Earth Materials Committee, a technical committee of AGU's Tectonophysics Section, is organizing a dinner/colloquium as part of the Fall Meeting in San Francisco, Calif. This event will be held Monday, December 3rd, in the Gold Rush Room of the Holiday Inn Golden Gateway Hotel at 1500 Van Ness St. There will be a no-host bar from 6:30 to 7:30 P.M., followed by dinner from 7:30 to 8:30 P.M. Paul Tapponnier will deliver the after-dinner talk, “Large-Scale Deformation Mechanisms in the Continental Lithosphere: Where Do We Stand?” It will start at 8:30 P.M. and a business meeting will follow at 9:30 P.M.

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

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

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

  10. The Role of the Mantle Lithosphere in Continent Stability

    NASA Astrophysics Data System (ADS)

    Carlson, R. W.; Ancuta, L. D.; Fouch, M. J.; Idleman, B. D.; Ionov, D. A.; James, D. E.; Meltzer, A.; Pearson, G.; Shirey, S. B.; Zeitler, P. K.

    2012-12-01

    Most Archean cratons are underlain by up to 200 km thick sections of mantle characterized by high seismic velocities. Xenoliths from cratonic mantle lithosphere show them to consist of refractory peridotites that are the residues of very high degrees of partial melt removal leaving the majority with less than 2% Al2O3. The partial melt removal leaves the lithospheric mantle compositionally buoyant, strong, and with very little internal radioactive heat generating capacity so that even after cooling it contributes to the strength, longevity, and relative geologic inactivity of the overlying crust. Re-Os studies, particularly in the Kaapvaal Craton of southern Africa, show a strong correspondence between the ages of melt depletion of the cratonic mantle and significant crust building events. The main age peak in the Kaapvaal lithospheric mantle is 2.9 Ga, coincident with assembly of the western and eastern blocks of the craton. The only significant disruption to this age pattern is seen below the 2 Ga Bushveld intrusion where the mantle lithosphere is characterized by slower seismic velocities and xenolith ages closer to 2 than 3 Ga. The surrounding Proterozoic mobile belts have even slower seismic velocities and xenolith ages generally less than 1.5 Ga. An interesting contrast to this picture of cold, old, stable cratonic lithosphere is that displayed by central Mongolia. This area, more or less in the middle of the huge Asian continental plate, is far removed from plate boundary processes yet in the Hangay Mountains shows elevations approaching 4 km along with extensive late Cenozoic basaltic volcanism. In contrast to cratonic lithosphere, mantle xenoliths from the Hangay region are dominantly fertile peridotite. Fifty-six percent of a large collection of peridotites from 4 Mongolian localities have more than 3.5% Al2O3 and only 4% have Al2O3 contents of less than 2%. Cenozoic basalts from the region have subchondritic 143Nd/144Nd and MORB-like He isotopic

  11. Origin of Enriched Geochemical Signatures in Continental Arc Rocks by Antithetic Subduction of Continental Lithosphere

    NASA Astrophysics Data System (ADS)

    Glazner, A. F.; Brandon, M. T.

    2002-12-01

    The source materials for magmas in continental arcs are not well known. Magmas are typically assumed to be derived from the subducted slab, subducted sediment, and mantle wedge, but some continental arcs have isotopic signatures that are inconsistent with these sources. For example, a large volume of the Sierra Nevada batholith consists of rocks that have initial Sr ratios of about 0.706-0.708 and ɛNd values ranging from -3 to -8. Late Cenozoic volcanic rocks erupted through the batholith, and their entrained xenoliths, have similar isotopic compositions. These characteristics are inconsistent with derivation from only the sources above and are most easily explained by derivation from enriched continental lithospheric mantle. However, there was never enough lithospheric mantle under the arc to produce the observed volume of enriched arc rocks; mass balance fails by one or two orders of magnitude, and lithosphere cannot flow by viscous processes to provide material to the arc. A possible solution to this problem lies in kinematic mass balance of the arc system. In many continental arcs (e.g., Cenozoic Andes, Mesozoic northern Cordillera and Sierra Nevada), thrust faults have carried the arc at least 100-200 km toward the craton. This motion requires moving a comparable amount of sub-detachment lower crust and mantle toward the arc and produces a tremendous cross-sectional balance problem. Geologic field studies and geodynamic modeling indicate that this antithetic subduction feeds continental lithosphere into the convecting mantle wedge where it can contribute to magma genesis. This process provides a large volume of enriched and probably fertile continental lithospheric mantle to the arc system and may account for the unusual geochemical signatures of some arc rocks.

  12. The continental lithosphere: Reconciling thermal, seismic, and petrologic data

    NASA Astrophysics Data System (ADS)

    Artemieva, Irina M.

    2009-04-01

    The goal of the present study is to extract non-thermal signal from seismic tomography models in order to distinguish compositional variations in the continental lithosphere and to examine if geochemical and petrologic constraints on global-scale compositional variations in the mantle are consistent with modern geophysical data. In the lithospheric mantle of the continents, seismic velocity variations of a non-thermal origin (calculated from global Vs seismic tomography data [Grand S.P., 2002. Mantle shear-wave tomography and the fate of subducted slabs. Philosophical Transactions of the Royal Society of London. Series A, 360, 2475-2491.; Shapiro N.M., Ritzwoller M.H. 2002. Monte-Carlo inversion for a global shear velocity model of the crust and upper mantle. Geophysical Journal International 151, 1-18.] and lithospheric temperatures [Artemieva I.M., Mooney W.D., 2001. Thermal structure and evolution of Precambrian lithosphere: A global study. Journal of Geophysical Research 106, 16387-16414.] show strong correlation with tectono-thermal ages and with regional variations in lithospheric thickness constrained by surface heat flow data and seismic velocities. In agreement with xenolith data, strong positive velocity anomalies of non-thermal origin (attributed to mantle depletion) are clearly seen for all of the cratons; their amplitude, however, varies laterally and decreases with depth, reflecting either a peripheral growth of the cratons in Proterozoic or their peripheral reworking. These cratonic regions where kimberlite magmas erupted show only weakly positive compositional velocity anomalies, atypical for the "intact" cratonic mantle. A reduction in the amplitude of compositional velocity anomalies in kimberlite provinces is interpreted to result from metasomatic enrichment (prior or during kimberlite emplacement) of the cratonic mantle, implying that xenolith data maybe non-representative of the "intact" cratonic mantle.

  13. Lithospheric buoyancy and continental intraplate stresses

    USGS Publications Warehouse

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

    2003-01-01

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

  14. 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

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

    USGS Publications Warehouse

    Thatcher, W.; Pollitz, F.F.

    2008-01-01

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

  16. 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

  17. 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

  18. Asymmetric vs. symmetric deep lithospheric architecture of intra-plate continental orogens

    NASA Astrophysics Data System (ADS)

    Calignano, Elisa; Sokoutis, Dimitrios; Willingshofer, Ernst; Gueydan, Frédéric; Cloetingh, Sierd

    2015-08-01

    The initiation and subsequent evolution of intra-plate orogens, resulting from continental plate interior deformation due to transmission of stresses over large distances from the active plate boundaries, is controlled by lateral and vertical strength contrasts in the lithosphere. We present lithospheric-scale analogue models combining 1) lateral strength variations in the continental lithosphere, and 2) different vertical rheological stratifications. The experimental continental lithosphere has a four-layer brittle-ductile rheological stratification. Lateral heterogeneity is implemented in all models by increased crustal strength in a central narrow block. The main investigated parameters are strain rate and strength of the lithospheric mantle, both playing an important role in crust-mantle coupling. The experiments show that the presence of a strong crustal domain is effective in localizing deformation along its boundaries. After deformation is localized, the evolution of the orogenic system is governed by the mechanical properties of the lithosphere such that the final geometry of the intra-plate mountain depends on the interplay between crust-mantle coupling and folding versus fracturing of the lithospheric mantle. Underthrusting is the main deformation mode in case of high convergence velocity and/or thick brittle mantle with a final asymmetric architecture of the deep lithosphere. In contrast, lithospheric folding is dominant in case of low convergence velocity and low strength brittle mantle, leading to the development of a symmetric lithospheric root. The presented analogue modelling results provide novel insights for 1) strain localization and 2) the development of the asymmetric architecture of the Pyrenees.

  19. Numerical modeling of continental lithospheric weak zone over plume

    NASA Astrophysics Data System (ADS)

    Perepechko, Y. V.; Sorokin, K. E.

    2011-12-01

    The work is devoted to the development of magmatic systems in the continental lithosphere over diffluent mantle plumes. The areas of tension originating over them are accompanied by appearance of fault zones, and the formation of permeable channels, which are distributed magmatic melts. The numerical simulation of the dynamics of deformation fields in the lithosphere due to convection currents in the upper mantle, and the formation of weakened zones that extend up to the upper crust and create the necessary conditions for the formation of intermediate magma chambers has been carried out. Thermodynamically consistent non-isothermal model simulates the processes of heat and mass transfer of a wide class of magmatic systems, as well as the process of strain localization in the lithosphere and their influence on the formation of high permeability zones in the lower crust. The substance of the lithosphere is a rheologic heterophase medium, which is described by a two-velocity hydrodynamics. This makes it possible to take into account the process of penetration of the melt from the asthenosphere into the weakened zone. The energy dissipation occurs mainly due to interfacial friction and inelastic relaxation of shear stresses. The results of calculation reveal a nonlinear process of the formation of porous channels and demonstrate the diversity of emerging dissipative structures which are determined by properties of both heterogeneous lithosphere and overlying crust. Mutual effect of a permeable channel and the corresponding filtration process of the melt on the mantle convection and the dynamics of the asthenosphere have been studied. The formation of dissipative structures in heterogeneous lithosphere above mantle plumes occurs in accordance with the following scenario: initially, the elastic behavior of heterophase lithosphere leads to the formation of the narrow weakened zone, though sufficiently extensive, with higher porosity. Further, the increase in the width of

  20. Continental smokers couple mantle degassing and distinctive microbiology within continents

    NASA Astrophysics Data System (ADS)

    Crossey, Laura J.; Karlstrom, Karl E.; Schmandt, Brandon; Crow, Ryan R.; Colman, Daniel R.; Cron, Brandi; Takacs-Vesbach, Cristina D.; Dahm, Clifford N.; Northup, Diana E.; Hilton, David R.; Ricketts, Jason W.; Lowry, Anthony R.

    2016-02-01

    The discovery of oceanic black (and white) smokers revolutionized our understanding of mid-ocean ridges and led to the recognition of new organisms and ecosystems. Continental smokers, defined here to include a broad range of carbonic springs, hot springs, and fumaroles that vent mantle-derived fluids in continental settings, exhibit many of the same processes of heat and mass transfer and ecosystem niche differentiation. Helium isotope (3He/4He) analyses indicate that widespread mantle degassing is taking place in the western U.S.A., and that variations in mantle helium values correlate best with low seismic-velocity domains in the mantle and lateral contrasts in mantle velocity rather than crustal parameters such as GPS, proximity to volcanoes, crustal velocity, or composition. Microbial community analyses indicate that these springs can host novel microorganisms. A targeted analysis of four springs in New Mexico yield the first published occurrence of chemolithoautotrophic Zetaproteobacteria in a continental setting. These observations lead to two linked hypotheses: that mantle-derived volatiles transit through conduits in extending continental lithosphere preferentially above and at the edges of mantle low velocity domains. High CO2 and other constituents ultimately derived from mantle volatiles drive water-rock interactions and heterogeneous fluid mixing that help structure diverse and distinctive microbial communities.

  1. 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

  2. Continental flood basalts derived from the hydrous mantle transition zone

    NASA Astrophysics Data System (ADS)

    Wang, Xuan-Ce; Wilde, Simon A.; Li, Qiu-Li; Yang, Ya-Nan

    2015-07-01

    It has previously been postulated that the Earth's hydrous mantle transition zone may play a key role in intraplate magmatism, but no confirmatory evidence has been reported. Here we demonstrate that hydrothermally altered subducted oceanic crust was involved in generating the late Cenozoic Chifeng continental flood basalts of East Asia. This study combines oxygen isotopes with conventional geochemistry to provide evidence for an origin in the hydrous mantle transition zone. These observations lead us to propose an alternative thermochemical model, whereby slab-triggered wet upwelling produces large volumes of melt that may rise from the hydrous mantle transition zone. This model explains the lack of pre-magmatic lithospheric extension or a hotspot track and also the arc-like signatures observed in some large-scale intracontinental magmas. Deep-Earth water cycling, linked to cold subduction, slab stagnation, wet mantle upwelling and assembly/breakup of supercontinents, can potentially account for the chemical diversity of many continental flood basalts.

  3. Continental flood basalts derived from the hydrous mantle transition zone.

    PubMed

    Wang, Xuan-Ce; Wilde, Simon A; Li, Qiu-Li; Yang, Ya-Nan

    2015-07-14

    It has previously been postulated that the Earth's hydrous mantle transition zone may play a key role in intraplate magmatism, but no confirmatory evidence has been reported. Here we demonstrate that hydrothermally altered subducted oceanic crust was involved in generating the late Cenozoic Chifeng continental flood basalts of East Asia. This study combines oxygen isotopes with conventional geochemistry to provide evidence for an origin in the hydrous mantle transition zone. These observations lead us to propose an alternative thermochemical model, whereby slab-triggered wet upwelling produces large volumes of melt that may rise from the hydrous mantle transition zone. This model explains the lack of pre-magmatic lithospheric extension or a hotspot track and also the arc-like signatures observed in some large-scale intracontinental magmas. Deep-Earth water cycling, linked to cold subduction, slab stagnation, wet mantle upwelling and assembly/breakup of supercontinents, can potentially account for the chemical diversity of many continental flood basalts.

  4. Partial melting of an ancient sub-continental lithospheric mantle in the early Paleozoic intracontinental regime and its contribution to petrogenesis of the coeval peraluminous granites in South China

    NASA Astrophysics Data System (ADS)

    Zhong, Yufang; Wang, Lianxun; Zhao, Junhong; Liu, Lei; Ma, Changqian; Zheng, Jianping; Zhang, Zejun; Luo, Biji

    2016-11-01

    The appinite-granite association has been found in various tectonic regimes related to recent subduction, arc-continent or continent-continent collision and post-collision (orogen), and appinites generally originate from recently subduction-modified lithospheric mantle. We conducted a study on a rarely reported appinite-granite association formed in an intracontinental regime, the Zhangjiafang-Qinglongshan complex (ZQC), in which the appinites were derived from an ancient sub-continental lithospheric mantle (SCLM). The ZQC is located in the western Wugongshan domain, and consists of basaltic to intermediate appinites and granitoids. Ten dated samples (including massive and gneissoid granitoids, hornblende gabbro, and diorite) give zircon 206Pb/238U ages ranging from 444 ± 3 Ma to 452 ± 4 Ma, indicating that these various lithologies were emplaced synchronously. The basaltic appinites show radiogenic 87Sr/86Sri (0.71016-0.71431) and negative εNd(t) (- 6.1 to - 8.9) and zircon εHf(t) (- 4.2 to - 7.5) values. Combined with regional geological background, an origin from the Neoproterozoic metasomatised SCLM can be inferred for the appinites in the Wugongshan domain. The granitoids are peraluminous and almost high-K calc-alkaline to shoshonitic. They exhibit a wide range of isotopic compositions (87Sr/86Sri = 0.70828-0.71857, εNd(t) = - 6.2 to - 10.5, zircon εHf(t) = - 9.5 to - 26.6). Some of the granitoids display the most evolved Sr-Nd isotopic signatures among all the studied lithologies, which are consistent with those of the middle to upper crust, suggesting a pure crustal origin. Other granitoids show relatively mafic composition and less evolved isotopic signature. The intermediate appinites have intermediate chemical compositions between those of the basaltic appinites and granitoids, and similar Sr-Nd isotopic compositions to those of the basaltic appinites that have relatively evolved composition and isotopic signature. The petrographical and the

  5. Interaction between mantle and crustal detachments: a non-linear system controlling lithospheric extension

    NASA Astrophysics Data System (ADS)

    Rosenbaum, G.; Regenauer-Lieb, K.; Weinberg, R. F.

    2009-12-01

    We use numerical modelling to investigate the development of crustal and mantle detachment faults during lithospheric extension. Our models simulate a wide range of rift systems with varying values of crustal thickness and heat flow, showing how strain localization in the mantle interacts with localization in the upper crust and controls the evolution of extensional systems. Model results reveal a richness of structures and deformation styles, which grow in response to a self-organized mechanism that minimizes the internal stored energy of the system by localizing deformation at different levels of the lithosphere. Crustal detachment faults are well developed during extension of overthickened (60 km) continental crust, even when the initial heat flow is relatively low (50 mW/m2). In contrast, localized mantle deformation is most pronounced when the extended lithosphere has a normal crustal thickness (30-40 km) and an intermediate (60-70 mW/m2) heat flow. Results show a non-linear response to subtle changes in crustal thickness or heat flow, characterized by abrupt and sometime unexpected switches in extension modes (e.g. from diffuse rifting to effective lithospheric-scale rupturing) or from mantle- to crust-dominated strain localization. We interpret this non-linearity to result from the interference of doming wavelengths. Disharmony of crust and mantle doming wavelengths results in efficient communication between shear zones at different lithospheric levels, leading to rupturing of the whole lithosphere. In contrast, harmonious crust and mantle doming inhibits interaction of shear zones across the lithosphere and results in a prolonged rifting history prior to continental breakup.

  6. Thinning of Refertilized Sub-Continental Lithospheric Mantle (SLCM) beneath the Main Ethiopian Rift During Tertiary Rifting: Petrologic and Thermal Constraints from (Garnet)-Spinel Peridotite Xenoliths (Mega, Ethiopia).

    NASA Astrophysics Data System (ADS)

    Casagli, A.; Frezzotti, M. L.; Peccerillo, A.; Tiepolo, M.; De Astis, G.

    2014-12-01

    The East African Rift System (EARS) represents a key locality for the knowledge of the nature and evolution of SCLM during continental rifting processes, traditionally ascribed to ascending mantle plumes. We report petrological and geothermobarometric data from mantle xenoliths in Quaternary alkali-basalt lava flows and scoria cones at Mega (Sidamo Region; EARS) in the southern Main Ethiopian Rift (MER), that give evidence for refertilization of SCLM and for thinning during Tertiary rifting. Studied samples consist of seven lherzolites, five harzburgites and one olivine-websterite that contain spinel-pyroxene symplectites, interpreted as products of garnet breakdown reactions. These rocks were analyzed for major (whole rock and minerals) and trace elements (pyroxenes). Major element data have been used to reconstruct original garnet composition (pyrope). Equilibration temperatures range from 985 ± 40°C in the garnet facies (2.9-2.2 GPa) to 960 ± 55°C in the spinel facies (1.3 GPa). Xenoliths consist of depleted and fertile peridotites. Five lherzolites have up to 4 wt% of CaO, high CaO/Al2O3 (1.42-4.46), and the most fertile are more enriched than primitive mantle. Variations of major oxides in bulk rocks and minerals are consistent with variable degrees of melt extraction. Evidence for modal and cryptic metasomatism is given by addition of clinopyroxene ± phlogopite, and by LILE and LREE enrichment in clinopyroxene. Refertilization process appears to have been induced by sub-lithospheric volatile-rich melts at high melt/rock ratio, and were followed by cooling. To account for the geodynamic evolution of SCLM beneath the southern MER, which implies a temperature gradient from 50-60 to ˜ 90 mW/m2, we propose that thinning of the base of fertile SCLM from 90-95 to ˜45km depth and associated magmatism occurred along a normal-mantle adiabat above an upwelling asthenosphere (i.e., decompression melting) without the need for significant heat sources.

  7. Intra-cratonic melting as a result of delamination of mantle lithosphere - insight from numerical modelling

    NASA Astrophysics Data System (ADS)

    Gorczyk, W.; Vogt, K.; Gerya, T.; Hobbs, B. E.

    2012-12-01

    It is becoming increasingly apparent that intense deformation, metamorphism and metasomatism occur within continental cratonic blocks far removed form subducting margins Such changes may occur intra-cratonically arising from lithospheric thickening and the development of gravitational instabilities, but mostly occur at the boundary of cratonic blocks. The contact of two cratons is characterized by rheological lateral variations within mantle-lithosphere and overlying crust. Tectonic stresses acting on craton/craton boundaries may lead to thinning or thickening due to delamination of the mantle lithosphere. This is reflected in tectonic deformation, topography evolution, melting and crustal metamorphism. To understand the controls on these processes a number of 2D, coupled petrological thermo-mechanical numerical experiments has been performed to test the response of a laterally weakened zone to a compressional regime. The results indicate that the presence of water-bearing minerals in the lithosphere and lower crust is essential to initiate melting, which in the later stages may expand to dry melting of crust and mantle. In the case of anhydrous crust and lithosphere, no melting occurs. Thus a variety of instabilities, melting behaviour and topographic responses occurs at the base of the lithosphere as well as intensive faulting and buckling in the crust dependent on the strength and "water" content of the lithosphere.

  8. Interaction between mantle and crustal detachments: A nonlinear system controlling lithospheric extension

    NASA Astrophysics Data System (ADS)

    Rosenbaum, Gideon; Regenauer-Lieb, Klaus; Weinberg, Roberto F.

    2010-11-01

    We use numerical modeling to investigate the development of crustal and mantle detachments during lithospheric extension. Our models simulate a wide range of extensional systems with varying values of crustal thickness and heat flow, showing how strain localization in the mantle interacts with localization in the upper crust and controls the evolution of extensional systems. Model results reveal a richness of structures and deformation styles as a response to a self-organized mechanism that minimizes the internal stored energy of the system by localizing deformation. Crustal detachments, here referred as low-angle normal decoupling horizons, are well developed during extension of overthickened (60 km) continental crust, even when the initial heat flow is relatively low (50 mW m-2). In contrast, localized mantle deformation is most pronounced when the extended lithosphere has a normal crustal thickness (30-40 km) and an intermediate heat flow (60-70 mW m-2). Results show a nonlinear response to subtle changes in crustal thickness or heat flow, characterized by abrupt and sometimes unexpected switches in extension modes (e.g., from diffuse extensional deformation to effective lithospheric-scale rupturing) or from mantle- to crust-dominated strain localization. We interpret this nonlinearity to result from the interference of doming wavelengths in the presence of multiple necking instabilities. Disharmonic crust and mantle doming wavelengths results in efficient communication between shear zones at different lithospheric levels, leading to rupturing of the whole lithosphere. In contrast, harmonic crust and mantle doming inhibits interaction of shear zones across the lithosphere and results in a prolonged history of extension prior to continental breakup.

  9. Lithosphere-Mantle Interactions Associated with Flat-Slab Subduction

    NASA Astrophysics Data System (ADS)

    Gerault, M.; Becker, T. W.; Husson, L.; Humphreys, E.

    2014-12-01

    Episodes of flat-slab subduction along the western margin of the Americas may have lead to the formation of intra-continental basins and seas, as well as mountain belts and continental plateaux. Here, we explore some of the consequences of a flat slab morphology, linking dynamic topography and stress patterns in continents to slab and mantle dynamics. Using a 2-D cylindrical code, we develop general models and apply them to the North and South America plates. The results are primarily controlled by the coupling along the slab-continent interface (due to geometry and viscosity), the viscosity of the mantle wedge, and the buoyancy of the subducted lithosphere. All models predict broad subsidence, large deviatoric stresses, and horizontal compression above the tip of the flat slab and the deep slab hinge. In models where the slab lays horizontally for hundreds of kilometers, overriding plate compression focuses on both ends of the flat segment, where normal-dip subduction exerts a direct downward pull. In between, a broad low-stress region gets uplifted proportionally to the amount of coupling between the slab and the continent. Anomalously buoyant seafloor enhances this effect but is not required. The downward bending of the flat slab extremities causes its upper part to undergo extension and the lower part to compress. These results have potential for explaining the existence of relatively undeformed, uplifted regions surrounded by mountain belts, such as in the western U.S. and parts of the Andes. Adequately modeling topography and stress in the unusual setting of southwestern Mexico requires a low-viscosity subduction interface and mantle wedge. Our results are only partially controlled by the buoyancy of the subducting plate, suggesting that the viscosity and the morphology of the slab are important, and that the often-used low resolution and "Stokeslet" models may be missing substantial effects.

  10. Preservation of ancient and fertile lithospheric mantle beneath the southwestern United States.

    PubMed

    Lee, C T; Yin, Q; Rudnick, R L; Jacobsen, S B

    2001-05-01

    Stable continental regions, free from tectonic activity, are generally found only within ancient cratons-the centres of continents which formed in the Archaean era, 4.0-2.5 Gyr ago. But in the Cordilleran mountain belt of western North America some younger (middle Proterozoic) regions have remained stable, whereas some older (late Archaean) regions have been tectonically disturbed, suggesting that age alone does not determine lithospheric strength and crustal stability. Here we report rhenium-osmium isotope and mineral compositions of peridotite xenoliths from two regions of the Cordilleran mountain belt. We found that the younger, undeformed Colorado plateau is underlain by lithospheric mantle that is 'depleted' (deficient in minerals extracted by partial melting of the rock), whereas the older (Archaean), yet deformed, southern Basin and Range province is underlain by 'fertile' lithospheric mantle (not depleted by melt extraction). We suggest that the apparent relationship between composition and lithospheric strength, inferred from different degrees of crustal deformation, occurs because depleted mantle is intrinsically less dense than fertile mantle (due to iron having been lost when melt was extracted from the rock). This allows the depleted mantle to form a thicker thermal boundary layer between the deep convecting mantle and the crust, thus reducing tectonic activity at the surface. The inference that not all Archaean crust developed a strong and thick thermal boundary layer leads to the possibility that such ancient crust may have been overlooked because of its intensive reworking or lost from the geological record owing to preferential recycling. PMID:11333978

  11. Preservation of ancient and fertile lithospheric mantle beneath the southwestern United States.

    PubMed

    Lee, C T; Yin, Q; Rudnick, R L; Jacobsen, S B

    2001-05-01

    Stable continental regions, free from tectonic activity, are generally found only within ancient cratons-the centres of continents which formed in the Archaean era, 4.0-2.5 Gyr ago. But in the Cordilleran mountain belt of western North America some younger (middle Proterozoic) regions have remained stable, whereas some older (late Archaean) regions have been tectonically disturbed, suggesting that age alone does not determine lithospheric strength and crustal stability. Here we report rhenium-osmium isotope and mineral compositions of peridotite xenoliths from two regions of the Cordilleran mountain belt. We found that the younger, undeformed Colorado plateau is underlain by lithospheric mantle that is 'depleted' (deficient in minerals extracted by partial melting of the rock), whereas the older (Archaean), yet deformed, southern Basin and Range province is underlain by 'fertile' lithospheric mantle (not depleted by melt extraction). We suggest that the apparent relationship between composition and lithospheric strength, inferred from different degrees of crustal deformation, occurs because depleted mantle is intrinsically less dense than fertile mantle (due to iron having been lost when melt was extracted from the rock). This allows the depleted mantle to form a thicker thermal boundary layer between the deep convecting mantle and the crust, thus reducing tectonic activity at the surface. The inference that not all Archaean crust developed a strong and thick thermal boundary layer leads to the possibility that such ancient crust may have been overlooked because of its intensive reworking or lost from the geological record owing to preferential recycling.

  12. Mantle and crustal contributions to continental flood volcanism

    USGS Publications Warehouse

    Arndt, N.T.; Czamanske, G.K.; Wooden, J.L.; Fedorenko, V.A.

    1993-01-01

    Arndt, N.T., Czamanske, G.K., Wooden, J.L. and Fedorenko, V.A., 1993. Mantle and crustal contributions to continental flood volcanism. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near the Earth's Surface. Tectonophysics, 223: 39-52. Most continental flood basalts are enriched in incompatible elements and have high initial 87Sr/86Sr ratios and low ??{lunate}Nd values. Many are depleted in Nb and Ta. The commonly-held view that these characteristics are inherited directly from a source in metasomatized lithospheric mantle is inconsistent with the following arguments: (1) thermomechanical modelling demonstrates that flood basalt magmas come mainly from an asthenospheric or plume source, with minimal direct melting of the continental lithospheric mantle. The low water contents of most flood basalts argue against proposals that hydrous lithosphere was the source. (2) Lithospheric mantle normally has low concentrations of incompatible elements, and chondrite-normalized Nb and Ta contents similar to those of other incompatible elements. Such material cannot be the unmodified source of Nb-Ta-depleted basalts such as those from the Karoo, Ferrar, or Columbia River provinces. We suggest there are two main controls on the compositions of continental flood basalts. The first is lithospheric thickness, which strongly influences the depth and degree of mantle melting of a plume or asthenospheric source, and thus has an important influence on the composition of primary magmas. All liquids formed by partial melting of peridotite at sub-lithosphere depths are highly magnesian (20-25 wt.% MgO) but have variable trace-element contents. Where the lithosphere is thick, the source melts at high pressure, garnet is present, the degree of melting is low, and trace-element concentrations are high. This type of magma evolves to produce the high-Ti type of continental flood basalt. Where the lithosphere is

  13. Interrelationships between continental freeboard, tectonics and mantle temperature

    NASA Technical Reports Server (NTRS)

    Galer, S. J. G.

    1991-01-01

    Oceanic hypsometry and isentropic melting models are combined to address the question of freeboard from the middle Archean to the present. In addition to the fraction of continental crust, the factors governing the long-term balance of the continental freeboard include the mantle potential temperature (TP), the oceanic lithosphere thickness, and the plate creation rate (C0). It is shown that variation in TP far outweighs the other factors in importance, with freeboard decreasing by 1 km for every TP increase of about 80 C. The huge ten- to thirtyfold increases in C0 backward in geological time that have been invoked to explain near-constant freeboard are shown to be unnecessary. The low value found for TP is consistent with the preservation of ancient diamonds in the deep South African lithosphere. It is concluded from this that the present cooling rate of the earth of about 46 C/Ga has general applicability over much of geological time.

  14. Generation of Continental Rifts, Basins and Swells by Lithosphere Instabilities

    NASA Astrophysics Data System (ADS)

    Milelli, L.; Fourel, L.; Jaupart, C. P.

    2012-12-01

    Domal uplifts, volcanism, basin formation and rifting have often struck the same continent in different areas at the same time. Their characteristics and orientations are difficult to reconcile with mantle convection or tectonic forces and suggest a driving mechanism that is intrinsic to the continent. The rifts seem to develop preferentially at high angles to the edge of the continent whereas swells and basins seem confined to the interior. Another intriguing geometrical feature is that the rifts often branch out in complicated patterns at their landward end. In Western Africa, for example, magmatic activity currently occurs in a number of uplifted areas including the peculiar Cameroon Volcanic Line that stretches away from the continental margin over about 1000 km. Magmatic and volcanic activity has been sustained along this line for 70 My with no age progression. The mantle upwelling that feeds the volcanoes is not affected by absolute plate motions and hence is attached to the continent. The Cameroon Volcanic Line extends to the Biu swell to the North and the Jos plateau to the West defining a striking Y-shaped pattern. This structure segues into several volcanic domes including the Air, the Hoggar, the Darfur, the Tibesti and the Haruj domes towards the Mediterranean coast. Another example is provided by North America, where the late Proterozoic-early Ordovician saw the formation of four major basins, the Michigan, Illinois, Williston and Hudson Bay, as well as of major rifts in southern Oklahoma and the Mississipi Valley within a short time interval. At the same time, a series of uplifts developed, such as the Ozark and Nashville domes. Motivated by these observations, we have sought an explanation in the continental lithosphere itself. We describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent. We carried out fluid mechanics laboratory experiments on buoyant

  15. Constraining lithosphere deformation modes during continental breakup for the Iberia-Newfoundland conjugate rifted margins

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

    A kinematic model of lithosphere and asthenosphere deformation has been used to investigate lithosphere stretching and thinning modes during continental rifting leading to breakup and seafloor spreading. The model has been applied to two conjugate profiles across the Iberia-Newfoundland rifted margins and quantitatively calibrated using observed present-day water loaded subsidence and crustal thickness, together with observed mantle exhumation, subsidence and melting generation histories. The kinematic model uses an evolving prescribed flow-field to deform the lithosphere and asthenosphere leading to lithospheric breakup from which continental crustal thinning, lithosphere thermal evolution, decompression melt initiation and subsidence are predicted. We explore the sensitivity of model predictions to extension rate history, deformation migration and buoyancy induced upwelling. The best fit calibrated models of lithosphere deformation evolution for the Iberia-Newfoundland conjugate margins require; (1) an initial broad region of lithosphere deformation with passive upwelling, (2) lateral migration of deformation, (3) an increase in extension rate with time, (4) focussing of the deformation and (5) buoyancy induced upwelling. The model prediction of exhumed mantle at the Iberia-Newfoundland margins, as observed, requires a critical threshold of melting to be exceeded before melt extraction. The preferred calibrated models predict faster extension rates and earlier continental crustal separation and mantle exhumation for the Iberia Abyssal Plain-Flemish Pass conjugate margin profile than for the Galicia Bank-Flemish Cap profile to the north. The predicted N-S differences in the deformation evolution give insights into the 3D evolution of Iberia-Newfoundland margin crustal separation.

  16. Magmatism at passive margins: Effect of depth-dependent rifting and depleted continental lithospheric counterflow

    NASA Astrophysics Data System (ADS)

    Lu, Gang; Huismans, Ritske

    2016-04-01

    Rifted continental margins may have a variety of structural and magmatic styles, resulting in narrow or wide, magma-dominated or magma-poor conjugate margins. Some magma-poor margins differ from the classical uniform extension (McKenzie) model in that continental crust breaks up significantly earlier or later than continental mantle lithosphere and establishment of mature mid-ocean ridge is significantly delayed. The best-known examples are observed at: 1) the Iberia-Newfoundland conjugate margins (Type I) with a narrow transition between oceanic and continental crust; and 2) ultra-wide central South Atlantic margins (Type II) where the continental crust spans wide regions while the mantle lithosphere beneath has been removed. These margins are explained by depth-dependent extension. In this study, we perform 2D thermo-mechanical finite element numerical experiments to investigate magmatism at passive margins with depth-dependent extension. A melting prediction model is coupled with the thermo-mechanical model, in which temperature, density and viscosity feedbacks are considered. For the standard models, the crust is either strong and coupled (Type I-A models), or weak and decoupled (Type II-A models) with mantle lithosphere. In addition, models with a buoyant, depleted (cratonic) lower mantle lithosphere (referred as C models) are also investigated. We illustrate that Type I-A/C models develop Type I narrow margins, whereas Type II-A/C models develop Type II wide margins. In the C models, the buoyant lower mantle lithosphere flows laterally towards the ridge (i.e. the counterflow), resulting in the exhumation (in Type I-C models) or underplating (in Type II-C models) of the continental mantle lithosphere. Magmatic productivity is strongly prohibited when counterflow is developed. We argue that Type I-A and I-C models are comparable with the Aden Gulf rifted margins and the Iberia-Newfoundland conjugate margins, respectively. The Type II-A/C models are consistent

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

    NASA Astrophysics Data System (ADS)

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

    2009-04-01

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

  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. 87Sr/86Sr in spinel peridotites from Borée, Massif Central, France: melt depletion and metasomatism in the sub-continental lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Barnett, Caroline; Harvey, Jason

    2016-04-01

    Radiogenic isotopes and elemental concentrations in peridotite xenoliths may be used to model the timing and degree of partial melting in the upper mantle, but this primary melt depletion signature may be overwritten by subsequent episodes of melt or fluid infiltration. Spinel peridotites from the Maar de Borée, Massif Central, France have mainly poikilitic protogranular textures and clear petrographic evidence of a melt phase apparently unrelated to host basalt infiltration. Bulk rock major and compatible trace element concentrations are consistent with varying degrees of partial melting but incompatible trace element concentrations indicate cryptic metasomatism in some samples. Lithophile trace element mass balance cannot always be reconciled by the inclusion of the chemically characterized melt phase and suggest a contribution from a trace abundance grain boundary phase1. 87Sr/86Sr values for unleached bulk rocks and clinopyroxene mineral separates are higher than those for their leached equivalents, consistent with the removal of a radiogenic grain boundary phase. While unleached bulk rock 87Sr/86Sr is sometimes indistinguishable (within error) from its constituent unleached clinopyroxene, in two samples they show distinct patterns, as do the REE trends in these two xenoliths. BO01-01 bulk-rock is LREE-enriched (La/YbN = 3.6)2, and constituent clinopyroxene shows a similar relative enrichment trend. Bulk-rock 87Sr/86Sr is 0.70342±1 while that of clinopyroxene is lower at 0.70332±2. Clinopyroxene modal abundance is 11%. BO01-03 bulk-rock is only slightly LREE-enriched (La/YbN = 1.2) and both bulk-rock and clinopyroxene show a generally flatter profile. Bulk-rock 87Sr/86Sr is 0.70285±1 while that of clinopyroxene is in this case higher at 0.70296±2. Clinopyroxene modal abundance is also higher at 15%, consistent with a greater contribution by clinopyroxene to the bulk-rock Sr-isotope budget. The results appear to be inconsistent with a simple model of single

  20. Project Skippy explores lithosphere and mantle beneath Australia

    NASA Astrophysics Data System (ADS)

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

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

  1. Stability of Continental Lithosphere based on Analogue Experiments with Microwave Induced Internal Heating

    NASA Astrophysics Data System (ADS)

    Fourel, Loic; Limare, Angela; Surducan, Emanoil; Surducan, Vasile; Neamtu, Camelia; Vilella, Kenny; Farnetani, Cinzia; Kaminski, Edouard; Jaupart, Claude

    2015-04-01

    Continental lithosphere is usually depicted as the upper conductive layer of the Earth. Its formation is achieved through melt depletion that generates a residue that is less dense and more viscous than the underlying convecting mantle. As it is cooled from above, continental lithosphere can develop its own convective currents and may become unstable depending on its thickness and density contrast with the mantle. But chemical differentiation due to mantle magmatism also enriches continental lithosphere in heat producing elements. According to present estimates, the Earth's mantle may have lost as much as half of its radioactive elements in favour of continental crust and this stratified redistribution of heat sources has two main effects. First, mantle convection vigor decreases and becomes increasingly sensitive to heat supply from the core. Second, localized heat production at the top surface increases the continental insulating effects and competes against lithospheric instabilities. In the present study, we focus on the later and we determine which amount of internal heating is required to keep the lithosphere stable for a given rate of cooling from the top. The physics underlying instability triggering corresponds to the problem of a two differentially heated layered system cooled from above, where the top layer is less dense and more viscous than the bottom one, representative of the lithosphere-mantle system. Few studies have been devoted to the intrinsic characteristics of this layered type of convection. Here, we present a state of the art laboratory setup to generate internal heating in controlled conditions based on microwave (MW) absorption. The volumetric heat source can be localized in space and its intensity can be varied in time. Our tank prototype has horizontal dimensions of 30 cm x 30 cm and 5 cm height. A uniform and constant temperature is maintained at the upper boundary by an aluminium heat exchanger and adiabatic conditions are imposed at

  2. Lithospheric controls on magma composition along Earth's longest continental hotspot track.

    PubMed

    Davies, D R; Rawlinson, N; Iaffaldano, G; Campbell, I H

    2015-09-24

    Hotspots are anomalous regions of volcanism at Earth's surface that show no obvious association with tectonic plate boundaries. Classic examples include the Hawaiian-Emperor chain and the Yellowstone-Snake River Plain province. The majority are believed to form as Earth's tectonic plates move over long-lived mantle plumes: buoyant upwellings that bring hot material from Earth's deep mantle to its surface. It has long been recognized that lithospheric thickness limits the rise height of plumes and, thereby, their minimum melting pressure. It should, therefore, have a controlling influence on the geochemistry of plume-related magmas, although unambiguous evidence of this has, so far, been lacking. Here we integrate observational constraints from surface geology, geochronology, plate-motion reconstructions, geochemistry and seismology to ascertain plume melting depths beneath Earth's longest continental hotspot track, a 2,000-kilometre-long track in eastern Australia that displays a record of volcanic activity between 33 and 9 million years ago, which we call the Cosgrove track. Our analyses highlight a strong correlation between lithospheric thickness and magma composition along this track, with: (1) standard basaltic compositions in regions where lithospheric thickness is less than 110 kilometres; (2) volcanic gaps in regions where lithospheric thickness exceeds 150 kilometres; and (3) low-volume, leucitite-bearing volcanism in regions of intermediate lithospheric thickness. Trace-element concentrations from samples along this track support the notion that these compositional variations result from different degrees of partial melting, which is controlled by the thickness of overlying lithosphere. Our results place the first observational constraints on the sub-continental melting depth of mantle plumes and provide direct evidence that lithospheric thickness has a dominant influence on the volume and chemical composition of plume-derived magmas.

  3. Lithospheric controls on magma composition along Earth's longest continental hotspot track

    NASA Astrophysics Data System (ADS)

    Davies, D. R.; Rawlinson, N.; Iaffaldano, G.; Campbell, I. H.

    2015-09-01

    Hotspots are anomalous regions of volcanism at Earth's surface that show no obvious association with tectonic plate boundaries. Classic examples include the Hawaiian-Emperor chain and the Yellowstone-Snake River Plain province. The majority are believed to form as Earth's tectonic plates move over long-lived mantle plumes: buoyant upwellings that bring hot material from Earth's deep mantle to its surface. It has long been recognized that lithospheric thickness limits the rise height of plumes and, thereby, their minimum melting pressure. It should, therefore, have a controlling influence on the geochemistry of plume-related magmas, although unambiguous evidence of this has, so far, been lacking. Here we integrate observational constraints from surface geology, geochronology, plate-motion reconstructions, geochemistry and seismology to ascertain plume melting depths beneath Earth's longest continental hotspot track, a 2,000-kilometre-long track in eastern Australia that displays a record of volcanic activity between 33 and 9 million years ago, which we call the Cosgrove track. Our analyses highlight a strong correlation between lithospheric thickness and magma composition along this track, with: (1) standard basaltic compositions in regions where lithospheric thickness is less than 110 kilometres; (2) volcanic gaps in regions where lithospheric thickness exceeds 150 kilometres; and (3) low-volume, leucitite-bearing volcanism in regions of intermediate lithospheric thickness. Trace-element concentrations from samples along this track support the notion that these compositional variations result from different degrees of partial melting, which is controlled by the thickness of overlying lithosphere. Our results place the first observational constraints on the sub-continental melting depth of mantle plumes and provide direct evidence that lithospheric thickness has a dominant influence on the volume and chemical composition of plume-derived magmas.

  4. Lithospheric controls on magma composition along Earth's longest continental hotspot track.

    PubMed

    Davies, D R; Rawlinson, N; Iaffaldano, G; Campbell, I H

    2015-09-24

    Hotspots are anomalous regions of volcanism at Earth's surface that show no obvious association with tectonic plate boundaries. Classic examples include the Hawaiian-Emperor chain and the Yellowstone-Snake River Plain province. The majority are believed to form as Earth's tectonic plates move over long-lived mantle plumes: buoyant upwellings that bring hot material from Earth's deep mantle to its surface. It has long been recognized that lithospheric thickness limits the rise height of plumes and, thereby, their minimum melting pressure. It should, therefore, have a controlling influence on the geochemistry of plume-related magmas, although unambiguous evidence of this has, so far, been lacking. Here we integrate observational constraints from surface geology, geochronology, plate-motion reconstructions, geochemistry and seismology to ascertain plume melting depths beneath Earth's longest continental hotspot track, a 2,000-kilometre-long track in eastern Australia that displays a record of volcanic activity between 33 and 9 million years ago, which we call the Cosgrove track. Our analyses highlight a strong correlation between lithospheric thickness and magma composition along this track, with: (1) standard basaltic compositions in regions where lithospheric thickness is less than 110 kilometres; (2) volcanic gaps in regions where lithospheric thickness exceeds 150 kilometres; and (3) low-volume, leucitite-bearing volcanism in regions of intermediate lithospheric thickness. Trace-element concentrations from samples along this track support the notion that these compositional variations result from different degrees of partial melting, which is controlled by the thickness of overlying lithosphere. Our results place the first observational constraints on the sub-continental melting depth of mantle plumes and provide direct evidence that lithospheric thickness has a dominant influence on the volume and chemical composition of plume-derived magmas. PMID:26367795

  5. 187Os/188Os in Spinel Peridotites from Borée, Massif Central, France: Seeing through the Effects of Melt Infiltration in the Sub-continental Lithospheric Mantle

    NASA Astrophysics Data System (ADS)

    Barnett, C. J.; Harvey, J.

    2015-12-01

    The Re-Os isotope system can be used to model the timing of melt extraction in peridotites, although secondary metasomatic processes can obscure primary melt depletion signatures, implying that bulk-rock Os model ages should be treated with caution.1Spinel peridotites from the volcanic Maar de Borée (French Massif Central) have equigranular to protogranular and occasionally poikilitic textures. Their bulk-rock chemistry are consistent with moderate degrees of partial melting, but elevated incompatible trace element ratios (e.g. La/YbN) are indicative of subsequent secondary processes. Petrographic observation reveals no infiltration of host basalt, but melt infiltration unrelated to the host basalt has occurred, most likely within the sub-continental lithospheric mantle prior to entrainment as xenoliths. The peridotites have a mean [Os] concentration of 2.35 ng g-1 and 187Os/188Os values from 0.12081 ± 16 to 0.12639 ± 14 (cf. PUM = 0.1296 ± 00082), with rhenium depletion model ages (TRD) ranging from 0.48 to 1.30 Ga. Silicate melt contains up to 2 orders of magnitude less Os than peridotites3 but the 187Os/188Os of melt infiltrated peridotite can be skewed by the precipitation of immiscible sulfide when an infiltrating melt reaches S-saturation4. The Borée peridotites retain an unradiogenic Os-isotope signature despite silicate melt infiltration; this may be due to primary base metal sulfides enclosed in silicate minerals and therefore protected from interaction with infiltrating melts. TRD of enclosed sulphides should therefore be able to 'see through' any secondary metasomatic events and reveal melt depletion ages significantly older than those obtained from bulk-rock analyses (cf. 4). 1. Rudnick & Walker (2009) Lithos 112S, 1083-1095. 2. Meisel et al. (2001) Geochim Cosmochim Ac 65, 1311-1323. 3. Day, J.M.D. (2013) Chem Geol 341, 50-74. 4. Harvey et al. (2010) Geochim Cosmochim Acta 74, 293-320.

  6. Deformation of the Continental Lithosphere at the Margins of the North American Craton: Constraints from Seismic Anisotropy

    NASA Astrophysics Data System (ADS)

    Long, M. D.; Benoit, M. H.; Ford, H. A.; Wirth, E. A.; Aragon, J. C.; Abrahams, L.; McNamara, J.; Jackson, K.

    2015-12-01

    Earth's continents exhibit striking properties, including relatively thick and low-density crust and a strong, thick, long-lived mantle lithosphere. Major questions related to the formation, stability, evolution, and dynamics of cratonic lithosphere remain unanswered. One promising avenue for understanding the stability of cratonic lithosphere through geologic time is to understand how their margins are deformed via tectonic processes such as orogenesis and rifting. Here we present results of several recent and ongoing studies which aim to constrain past lithospheric deformation along the eastern margin of the North American craton. Each of these studies focuses on constraining seismic anisotropy, or the directional dependence of seismic wavespeeds, in the lithospheric upper mantle. Because there is a causative link between upper mantle deformation and the resulting seismic anisotropy, studies of anisotropic structure in the upper mantle beneath continental interiors can shed light on the deformation processes associated with past tectonic events. The recent explosion in the availability of seismic data in the eastern United States, largely due to the EarthScope initiative, has enabled detailed studies of lithospheric deformation using anisotropic receiver function (RF) analysis and SKS splitting analysis. A comparison of lithospheric structure inferred from RFs for stations located to the east of the Grenville deformation front with those located within the cratonic interior argues for extensive deformation of the lithosphere during the formation and/or breakup of Rodinia. The pattern of fast SKS splitting directions measured at USArray Transportable Array (TA) stations shows clear evidence for a specific lithospheric anisotropy signature at stations beneath the Appalachian Mountains, indicating strong, coherent lithospheric deformation associated with Appalachian orogenesis. The Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) experiment, a linear array

  7. 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

  8. Melting of the Earth's lithospheric mantle inferred from protactinium-thorium-uranium isotopic data

    PubMed

    Asmerom; Cheng; Thomas; Hirschmann; Edwards

    2000-07-20

    The processes responsible for the generation of partial melt in the Earth's lithospheric mantle and the movement of this melt to the Earth's surface remain enigmatic, owing to the perceived difficulties in generating large-degree partial melts at depth and in transporting small-degree melts through a static lithosphere. Here we present a method of placing constraints on melting in the lithospheric mantle using 231Pa-235U data obtained from continental basalts in the southwestern United States and Mexico. Combined with 230Th-238U data, the 231Pa-235U data allow us to constrain the source mineralogy and thus the depth of melting of these basalts. Our analysis indicates that it is possible to transport small melt fractions--of the order of 0.1%--through the lithosphere, as might result from the coalescence of melt by compaction owing to melting-induced deformation. The large observed 231Pa excesses require that the timescale of melt generation and transport within the lithosphere is small compared to the half-life of 231Pa (approximately 32.7 kyr). The 231Pa-230Th data also constrain the thorium and uranium distribution coefficients for clinopyroxene in the source regions of these basalts to be within 2% of one another, indicating that in this setting 230Th excesses are not expected during melting at depths shallower than 85 km.

  9. Melting of the Earth's lithospheric mantle inferred from protactinium-thorium-uranium isotopic data

    PubMed

    Asmerom; Cheng; Thomas; Hirschmann; Edwards

    2000-07-20

    The processes responsible for the generation of partial melt in the Earth's lithospheric mantle and the movement of this melt to the Earth's surface remain enigmatic, owing to the perceived difficulties in generating large-degree partial melts at depth and in transporting small-degree melts through a static lithosphere. Here we present a method of placing constraints on melting in the lithospheric mantle using 231Pa-235U data obtained from continental basalts in the southwestern United States and Mexico. Combined with 230Th-238U data, the 231Pa-235U data allow us to constrain the source mineralogy and thus the depth of melting of these basalts. Our analysis indicates that it is possible to transport small melt fractions--of the order of 0.1%--through the lithosphere, as might result from the coalescence of melt by compaction owing to melting-induced deformation. The large observed 231Pa excesses require that the timescale of melt generation and transport within the lithosphere is small compared to the half-life of 231Pa (approximately 32.7 kyr). The 231Pa-230Th data also constrain the thorium and uranium distribution coefficients for clinopyroxene in the source regions of these basalts to be within 2% of one another, indicating that in this setting 230Th excesses are not expected during melting at depths shallower than 85 km. PMID:10917528

  10. Inherited structure and coupled crust-mantle lithosphere evolution: Numerical models of Central Australia

    NASA Astrophysics Data System (ADS)

    Heron, Philip J.; Pysklywec, Russell N.

    2016-05-01

    Continents have a rich tectonic history that have left lasting crustal impressions. In analyzing Central Australian intraplate orogenesis, complex continental features make it difficult to identify the controls of inherited structure. Here the tectonics of two types of inherited structures (e.g., a thermally enhanced or a rheologically strengthened region) are compared in numerical simulations of continental compression with and without "glacial buzzsaw" erosion. We find that although both inherited structures produce deformation in the upper crust that is confined to areas where material contrasts, patterns of deformation in the deep lithosphere differ significantly. Furthermore, our models infer that glacial buzzsaw erosion has little impact at depth. This tectonic isolation of the mantle lithosphere from glacial processes may further assist in the identification of a controlling inherited structure in intraplate orogenesis. Our models are interpreted in the context of Central Australian tectonics (specifically the Petermann and Alice Springs orogenies).

  11. 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.

  12. 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

  13. Modes of continental extension in a lithospheric wedge

    NASA Astrophysics Data System (ADS)

    Wu, G.; Lavier, L. L.; Choi, E.

    2014-12-01

    We studied extension of a lithospheric wedge as an approximation to an orogenic belt or a continental margin. We ran a series of numerical models to quantify the effects of the strength of the lower crust and a mid-crustal shear zone (MCSZ) on the extension processes. When the MCSZ is present, we found that the regional lower crustal flow plays a critical role in controlling the modes of extension. The compensation is long-wavelength when the lower crust flows from the highest to the lowest elevation in order to compensate upper crustal thinning. In response to this motion, the mantle flows towards the highest elevation in order to balance for the lower crust leaving the area under the highest topography. For weak (wet quartz regime with partial melting) or intermediate (wet quartz regime), or strong (dry quartz regime) lower crust, we recognized three predominantly decoupled modes of extension characterized by 1) significant lower crustal exhumation exemplified as a large massif, 2) formation of core complexes and detachment faults, and 3) distributive domino faulting, respectively. Without the MCSZ, however, the lower crustal flow is essentially subdued with predominantly coupled extension. For weak or intermediate, or strong lower crust, we recognized three coupled modes characterized by 1) localized generally symmetric crustal exhumation, 2) distributed grabens and narrow rifts, and 3) wide continental margins, respectively. The MCSZ controls the degree of decoupling of the lower crustal flow such that a frictionally stronger MCSZ does not change the behaviors of the models but results in a more distributed extension. Due to the long-wavelength compensation, subhorizontal Moho is achieved where intensive extension occurred for all the decoupled models with a MCSZ. Natural counterparts for each mode may be easily identified, for instance, in the Basin and Range or the Aegean.

  14. Lithospheric Controls on Magma Composition along Earth's Longest Continental Hotspot-Track

    NASA Astrophysics Data System (ADS)

    Rawlinson, N.; Davies, R.; Iaffaldano, G.; Campbell, I. H.

    2014-12-01

    Hotspots are anomalous regions of volcanism at Earth's surface that show no obvious association with tectonic plate boundaries. Classic examples include the Hawaiian-Emperor chain and the Yellowstone-Snake River Plain province. The majority are believed to form as Earth's tectonic plates move over long-lived mantle plumes: buoyant upwellings that bring hot material from Earth's deep-mantle to its surface. It has long been recognised that lithospheric thickness limits the rise height of plumes and, thereby, their minimum melting pressure. It should, therefore, have a controlling influence on the geochemistry of plume-related magmas, although unambiguous evidence of this has, thus far, been lacking. Here we integrate observational constraints from surface geology, geochronology, plate-motion reconstructions, geochemistry and seismology to ascertain plume melting depths beneath Earth's longest continental hotspot-track, a ~2000 km long track in eastern Australia that displays a record of volcanic activity between ~33 and ~9 Ma, which we call the Cosgrove track. Our analyses highlight a strong correlation between lithospheric thickness and magma composition along this track, with: (i) standard basaltic compositions in regions where lithospheric thickness is less than ~110 km; (ii) volcanic gaps in regions where lithospheric thickness exceeds ~150 km; and (iii) low-volume, leucitite-bearing volcanism in regions of intermediate lithospheric thickness. Trace-element concentrations from samples along this track support the notion that these compositional variations result from different degrees of partial-melting, which is controlled by the thickness of overlying lithosphere. Our results place the first observational constraints on the subcontinental melting depth of mantle plumes and provide direct evidence that lithospheric thickness has a dominant influence on the volume and chemical composition of plume-derived magmas.

  15. Lithospheric Controls on Magma Composition along Earth's Longest Continental Hotspot-Track

    NASA Astrophysics Data System (ADS)

    Rawlinson, N.; Davies, R.; Iaffaldano, G.; Campbell, I. H.

    2015-12-01

    Hotspots are anomalous regions of volcanism at Earth's surface that show no obvious association with tectonic plate boundaries. Classic examples include the Hawaiian-Emperor chain and the Yellowstone-Snake River Plain province. The majority are believed to form as Earth's tectonic plates move over long-lived mantle plumes: buoyant upwellings that bring hot material from Earth's deep-mantle to its surface. It has long been recognised that lithospheric thickness limits the rise height of plumes and, thereby, their minimum melting pressure. It should, therefore, have a controlling influence on the geochemistry of plume-related magmas, although unambiguous evidence of this has, thus far, been lacking. Here we integrate observational constraints from surface geology, geochronology, plate-motion reconstructions, geochemistry and seismology to ascertain plume melting depths beneath Earth's longest continental hotspot-track, a ~2000 km long track in eastern Australia that displays a record of volcanic activity between ~33 and ~9 Ma, which we call the Cosgrove track. Our analyses highlight a strong correlation between lithospheric thickness and magma composition along this track, with: (i) standard basaltic compositions in regions where lithospheric thickness is less than ~110 km; (ii) volcanic gaps in regions where lithospheric thickness exceeds ~150 km; and (iii) low-volume, leucitite-bearing volcanism in regions of intermediate lithospheric thickness. Trace-element concentrations from samples along this track support the notion that these compositional variations result from different degrees of partial-melting, which is controlled by the thickness of overlying lithosphere. Our results place the first observational constraints on the subcontinental melting depth of mantle plumes and provide direct evidence that lithospheric thickness has a dominant influence on the volume and chemical composition of plume-derived magmas.

  16. Lithospheric mantle evolution in the Afro-Arabian domain: Insights from Bir Ali mantle xenoliths (Yemen)

    NASA Astrophysics Data System (ADS)

    Sgualdo, P.; Aviado, K.; Beccaluva, L.; Bianchini, G.; Blichert-Toft, J.; Bryce, J. G.; Graham, D. W.; Natali, C.; Siena, F.

    2015-05-01

    Detailed petrological and geochemical investigations of an extensive sampling of mantle xenoliths from the Neogene-Quaternary Bir Ali diatreme (southern Yemen) indicate that the underlying lithospheric mantle consists predominantly of medium- to fine-grained (often foliated) spinel-peridotites (85-90%) and spinel-pyroxenites (10-15%) showing thermobarometric estimates in the P-T range of 0.9-2.0 GPa and 900-1150 °C. Peridotites, including lherzolites, harzburgites and dunites delineate continuous chemical, modal and mineralogical variations compatible with large extractions of basic melts occurring since the late Proterozoic (~ 2 Ga, according to Lu-Hf model ages). Pyroxenites may represent intrusions of subalkaline basic melts interacting and equilibrated with the host peridotite. Subsequent metasomatism has led to modal changes, with evidence of reaction patches and clinopyroxene and spinel destabilization, as well as formation of new phases (glass, amphibole and feldspar). These changes are accompanied by enrichment of the most incompatible elements and isotopic compositions. 143Nd/144Nd ranges from 0.51419 to 0.51209 (εNd from + 30.3 to - 10.5), 176Hf/177Hf from 0.28459 to 0.28239 (εHf from + 64.4 to - 13.6), and 208Pb/204Pb from 36.85 to 41.56, thus extending from the depleted mantle (DM) towards the enriched OIB mantle (EM and HIMU) components. 3He/4He (R/RA) ratios vary from 7.2 to 7.9 with He concentrations co-varying with the most incompatible element enrichment, in parallel with metasomatic effects. These metasomatic events, particularly effective in harzburgites and dunites, are attributable to the variable interaction with alkaline basic melts related to the general extensional and rifting regime affecting the East Africa-Arabian domain during the Cenozoic. In this respect, Bir Ali mantle xenoliths resemble those occurring along the Arabian margins and the East Africa Rift system, similarly affected by alkaline metasomatism, whereas they are

  17. 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.

  18. LIMA U-Pb ages link lithospheric mantle metasomatism to Karoo magmatism beneath the Kimberley region, South Africa

    NASA Astrophysics Data System (ADS)

    Giuliani, Andrea; Phillips, David; Maas, Roland; Woodhead, Jon D.; Kendrick, Mark A.; Greig, Alan; Armstrong, Richard A.; Chew, David; Kamenetsky, Vadim S.; Fiorentini, Marco L.

    2014-09-01

    The Karoo igneous rocks (174-185 Ma) of southern Africa represent one of the largest continental flood basalt provinces on Earth. Available evidence indicates that Karoo magmas either originated in the asthenosphere and were extensively modified by interaction with the lithospheric mantle prior to emplacement in the upper crust; or were produced by partial melting of enriched mantle lithosphere. However, no direct evidence of interaction by Karoo melts (or their precursors) with lithospheric mantle rocks has yet been identified in the suites of mantle xenoliths sampled by post-Karoo kimberlites in southern Africa. Here we report U-Pb ages for lindsleyite-mathiasite (LIMA) titanate minerals (crichtonite series) from three metasomatised, phlogopite and clinopyroxene-rich peridotite xenoliths from the ∼84 Ma Bultfontein kimberlite (Kimberley, South Africa), located in the southern part of the Karoo magmatic province. The LIMA minerals appear to have formed during metasomatism of the lithospheric mantle by fluids enriched in HFSE (Ti, Zr, Hf, Nb), LILE (K, Ba, Ca, Sr) and LREE. LIMA U-Pb elemental and isotopic compositions were measured in situ by LA-ICP-MS methods, and potential matrix effects were evaluated by solution-mode analysis of mineral separates. LIMA minerals from the three samples yielded apparent U-Pb ages of 177±12 Ma, 178±29 Ma and 190±24 Ma (±2σ). A single zircon grain extracted from the ∼190 Ma LIMA-bearing sample produced a similar U-Pb age of 184±6 Ma, within uncertainty of the LIMA ages. These data provide the first robust evidence of fluid enrichment in the lithospheric mantle beneath the Kimberley region at ∼180-190 Ma, and suggest causation of mantle metasomatism by Karoo melts or their precursor(s). The results further indicate that U-Pb dating of LIMA minerals provides a new, accurate tool for dating metasomatic events in the lithospheric mantle.

  19. 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

  20. Sensitivity analysis of crustal correction for calculation of lithospheric mantle density from gravity data

    NASA Astrophysics Data System (ADS)

    Herceg, M.; Artemieva, I. M.; Thybo, H.

    2016-02-01

    compositional density anomalies expected for continental lithosphere mantle). Similar values of uncertainties may be caused by a 0.2 km s-1 error in average crustal VP velocities or by a 2 km uncertainty in the Moho depth. One of the largest uncertainties is caused by errors in thickness of the sedimentary layer, and a 2 km error leads to ca. 0.03 g cm-3 error in lithospheric mantle densities. Large deviations (locally ±10 km) of the Moho depth in global crustal models (CRUST 5.1, CRUST2.0 and CRUST1.0) from the high-resolution regional seismic model of the crust, SibCrust, may produce artefact residual mantle gravity anomalies of up to ±150 mGal locally, caused by large errors in crustal gravity corrections. These errors in gravity anomalies produce up to ca. 0.04 g cm-3 (ca. 1.2 per cent) errors in density of the lithospheric mantle, which may well correspond to the amplitude of real density anomalies in the mantle. Our results demonstrate that gravity modelling alone cannot reliably constrain the crustal structure, including the Moho depth and thickness of sediments.

  1. Helium isotope evidence for plume metasomatism of Siberian continental lithosphere

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    The Siberian craton contains more than 1000 kimberlite intrusions of various ages (Silurian to Jurassic), making it an ideal setting for understanding temporal and spatial variations in subcontinental lithospheric mantle (SCLM) composition and metasomatism. This region also experienced one of the largest flood basalt events in the geologic record. The Permo-Triassic Siberian Flood Basalts (SFB) are considered to have erupted in response to plume-head impingement under the Siberian SCLM. Here we present new He-isotope data for a suite of peridotitic xenoliths (n=19) from two temporally and petrologically-distinct kimberlite pipes (i.e., Late-Devonian Udachnaya and Jurassic Obnazhennaya) in Siberia that span the age of eruption of the SFB. All samples have previously been well-characterized, mineralogically, petrographically, and for major- and trace-element abundance geochemistry. He-isotope ratios (3He/4He) of garnet, pyroxene and olivine separates from 2.7-3.1 Ga Siberian peridotites range from 0.11 to 8.4 RA, displaying both strongly radiogenic (i.e., low 3He/4He) and mantle-like (i.e., SCLM = 6.1 × 0.9 RA; MORB = 8 × 1 RA) values. In contrast, SFB values extend up to ~13 RA [1]. Helium concentrations span ~ five orders of magnitude from 0.05 to 350 [4He]C (×10-6) cm3STP/g. These findings are consistent with previous studies [2], which suggested that the SCLM is heterogeneous with respect to He and that this heterogeneity is strongly dependent on lithospheric age. Notably, all but one Obnazhennaya sample displays 3He/4He values in the mantle range and are He depleted. In contrast, all but one Udachnaya samples are radiogenic and have higher He contents. Previous studies have suggested that partially-melted subducted ocean crust amalgamated to form the Siberian craton at ~3 Ga [3], followed by a complex history of metasomatism until eruption of xenolith samples within kimberlites [4]. For example, during the main stage of SFB emplacement (i.e., Siberian plume

  2. Numerical models of mantle lithosphere weakening, erosion and delamination induced by melt extraction and emplacement

    NASA Astrophysics Data System (ADS)

    Wallner, Herbert; Schmeling, Harro

    2016-09-01

    Continental rifting caused by extension and heating from below affects the lithosphere or cratons in various ways. Volcanism and melt intrusions often occur along with thinning, weakening and even breaking lithosphere. Although mechanical necking models of the lithosphere are often applied, the aspects of melting and the implications due to melt transport and emplacement at shallower depths are not well understood. A two-phase flow approach employing melt extraction and shallow emplacement associated with thermal weakening is developed and compared with observations. The results of this comparison indicate the importance of partial melts and an asthenospheric magma source for increasing the rising rate of the lithosphere-asthenosphere boundary during extension. Thermo-mechanical physics of visco-plastic flow is approximated using the Finite Difference method with Eulerian formulation in 2D. The conservation of mass, momentum and energy equations are solved for a multi-component (crust-mantle) and two-phase (melt-matrix) system. Rheology is temperature- and stress-dependent. In consideration of depletion and enrichment melting and solidification are controlled by a simplified linear binary solid solution model. Melt is extracted and emplaced in predefined depth regions (emplacement zones) in the lithospheric mantle and crust. The Compaction Boussinesq Approximation was applied; its validity was tested against the Full Compaction formulation and found fully satisfactory for the case of sublithospheric melting models. A simple model guided by the geodynamic situation of the Rwenzori region typically results in updoming asthenosphere with melt-assisted erosion of the lithosphere's base. Even with a conservative approach for a temperature anomaly melting alone doubles the lithospheric erosion rate in comparison with a model without melting. With melt extraction and intrusion lithospheric erosion and upwelling of the lithosphere-asthenosphere boundary speeds up by a

  3. Flexural deformation of the continental lithosphere

    NASA Technical Reports Server (NTRS)

    1990-01-01

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

  4. 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.

  5. Layered structure of the lithospheric mantle changes dynamics of craton extension

    NASA Astrophysics Data System (ADS)

    Liao, J.; Gerya, T.; Wang, Q.

    2013-11-01

    Although presence of weak layers due to hydration and/or metasomatism in the lithospheric mantle of cratons has been detected by both geophysical and geochemical studies, its influence on craton evolution remains elusive. Using a 2‒D thermomechanical viscoelastoplastic numerical model, we studied the craton extension of a heterogeneous lithospheric mantle with a rheologically weak layer. Our results demonstrate that the effect of the weak mantle layer is twofold: (1) enhances deformation of the overlying lithosphere and (2) inhibits deformation of the underlying lithospheric mantle. Depending on the weak‒layer depth, the Moho temperature and extension rate, three extension patterns are found (1) localized mantle necking with exposed weak layer, (2) widespread mantle necking with exposed weak layer, and (3) widespread mantle necking without exposed weak layer. The presence of the weak mantle layer reduces long‒term acting boundary forces required to sustain extensional deformation of the lithosphere.

  6. Lithospheric Mantle Contribution to High Topography in Central Mongolia

    NASA Astrophysics Data System (ADS)

    Carlson, R. W.; Ionov, D. A.

    2014-12-01

    Over 110 spinel peridotite xenoliths collected from four localities in the Tariat region, central Mongolia, show a predominance (over 90%) of fertile lherzolites with subordinant harzburgite and peridotites veined with pyroxenite. Equilibration temperatures are high (~900°C at 1.5 GPa [1]). Major element compositions of the fertile samples are consistent with them being the residues of 0-6% partial melt removal at shallow depths [2]. The clinopyroxenes in the lherzolites are moderately LREE depleted (average chondrite normalized La/Sm = 0.45) and most whole rocks show small, if any, depletions in Re and Pd compared to the other HSE. These data point to minimal metasomatic overprinting of these fertile lherzolites. 187Os/188Os for samples with more than 3.2% Al2O3 range only from 0.126 to 0.131, within the range of modern fertile asthenospheric mantle. In contrast to the indicators of fertility in most samples, Sr, Nd and Hf isotopic composition of acid-leached clinopyroxene separates from the lherzolites plot within the range of modern MORB with 87Sr/86Sr from 0.7021 to 0.7026, eNd from +7.7 to +9.8 and eHf from +13.3 to +18.5. The lherzolites thus appear to sample a section of mantle that has compositional and isotope characteristics consistent with modern fertile asthenosphere. The isotopic composition of the Tariat lherzolites are distinct from that of Cenozoic Mongolian basaltic volcanism pointing to limited involvement of the lithospheric mantle in magma generation in this area. The implied asthenospheric provenance of the mantle lithosphere suggests that it either could be the replacement for recently delaminated lithosphere or, more likely, a section of fertile mantle accreted to the base of the crust earlier, e.g. during construction of the Central Asian Orogenic Belt in the Mesozoic/Paleozoic. Although fertile, and hence compositionally dense, the high temperatures of the shallow lithospheric mantle under this section of Mongolia likely contribute to the

  7. Osmium Isotope Constraints on the Timing of Production and Destruction of Mantle Lithosphere in the Southwest United States

    NASA Astrophysics Data System (ADS)

    Brandon, A. D.

    2013-12-01

    When convecting mantle melts, the residual peridotite becomes less dense and may be become stabilized as lithosphere. The Re-Os isotope chronometer has been successfully applied to determining the timing of melt extraction in mantle peridotite. In continental regions where multiple mantle xenolith locales are present, the Re-Os chronometer can be applied to assessing the timing of mantle melting in relation to juvenile continental crust production, stabilization, and destruction of mantle lithosphere. This is evaluated here for the off-craton mantle lithosphere in the Southwest United States by examining 5 mantle xenolith suites from locales spanning a region hundreds of kilometers north to south and east to west - Dish Hill, California; Lunar Crater Nevada; Grand Canyon and San Carlos, Arizona; and Kilbourne Hole, New Mexico. Because Re is mobile in mantle peridotites at surface conditions, direct Re-Os isochrons representing mantle melting ages are typically absent. Instead melting proxies for Re such as Al2O3 can be used to obtain ';aluminachron' ages or to assess disturbances of the mantle lithosphere following partial melting. The Dish Hill, Grand Canyon, and Kilbourne Hole suites display lithophile element evidence for post-melting, multiple modal and cryptic metasomatic events in combination with positive and well correlated Os isotope versus Al2O3 trends. For example, each of these xenolith suites has samples with light rare earth element (LREE) depleted to LREE-enriched bulk rock and clinopyroxene compositions. However, no correlation exists between LREE differences and their Os isotope, bulk rock Al compositions, or other indices of melt-rock interaction. The Os-aluminachron age obtained for Dish Hill is 2.15 Ga, for Grand Canyon is 2.31 Ga, and for Kilbourne Hole is 1.96 Ga. These ages overlap TDM ages for the overlying crustal provinces confirming a link between melting that creates mantle lithosphere and production of juvenile continental crust. A

  8. 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

    -bounding orogenic belts8. We conclude that the elevated present-day surface heat flow in the Proterozoic belts can be wholly, if non-uniquely, attributed to variations in crustal radiogenic heat production. This modeling reconciles heat flow data with independent indications of a cratonic lithospheric thermal state from select off-craton kimberlite-borne mantle xenoliths suites3,6, while weakening inferences of differential Proterozoic versus Archean lithospheric thickness. These results also necessitate further consideration of advective heat transport mechanisms for the transient, and spatially heterogeneous elevation of Late Mesozoic thermal gradients throughout the southern African continent. Mesozoic thermal and chemical modification of the lithospheric mantle may be a consistent feature of the Gondwanide continental fragments. 1Ballard and Pollack (1987) EPSL 85:253-264; 2Jones (1987) JGR 92:6273-6289; 3Finnerty and Boyd (1987) in Nixon (ed) Mantle Xenoliths, pp. 381-402; 4Boyd and Gurney (1986) Science 232:472-477; 5Brown et al. (1998) Ext. Abs. 7IKC:105-107; 6Bell et al. (2002) Lithos in press; 7Nguuri et al. (2001) GRL 28:2501-2504; 8Schmitz and Bowring (2002) CMP in press.

  9. Global plate kinematic reconstructions and mantle flow models incorporating lithospheric deformation

    NASA Astrophysics Data System (ADS)

    Flament, N.; Williams, S.; Gurnis, M.; Seton, M.; Müller, R.; Heine, C.; Turner, M.

    2012-12-01

    The effect of mantle flow on surface topography has been the subject of considerable interest over the last few years. A common approach to the problem is to link plate tectonic reconstructions and global geodynamic models. An important limitation of this approach is that traditional plate tectonic reconstructions do not take the deformation of the lithosphere into account. Plates are represented as rigid blocks, resulting in continental overlap in full-fit reconstructions. Models that use topological polygons avoid continental overlaps, but plate velocities are still derived on the basis of Euler poles for rigid blocks. Our objective is to develop quantitative models of surface plate kinematics that include areas of deforming continental crust. We generated a series of global reconstructions including deforming plates in key areas, derived using tools developed within the open source reconstruction software GPlates. We used geological and geophysical data to define the areal and temporal extent of major crustal deformation phases. For convergent plate boundaries, we incorporated quantitative estimates of deformation to model the time-varying geometries of subduction zones such as the Andean margin of South America and east of the Lord Howe Rise. In reconstructions of continental breakup, for example between Australia and Antarctica or the opening of the South and Equatorial Atlantic, the timing and the intensity of continental extension is imposed by the progressive, diachronous breakup and initiation of seafloor spreading for each major margin system. We used these models as a boundary condition for geodynamic models. For each deforming domain, a topological mesh was generated such that surface velocity fields within the deforming regions are calculated by linear interpolation from velocities at the boundaries and from additional constraining points within the deforming regions. The velocity field derived from the plate reconstructions were used as a time

  10. 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

  11. 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

  12. Structure of the Lithosphere and Upper Mantle Across the Arabian Peninsula

    SciTech Connect

    Al-Amri, A; Rodgers, A

    2007-01-05

    -Nubian continental lithosphere. The step in the lithospheric thickness across the Shield-Platform boundary likely reveals a pre-existing difference in the lithospheric structure prior to accretion of the terranes composing the eastern Arabian Shield. Tomographic imaging of upper mantle velocities implies a single large-scale thermal anomaly underlies the Arabian Shield and is associated with Cenozoic uplift and volcanism.

  13. Lithosphere continental rifting and necking in 3D analogue experiments: role of plate divergence rate.

    NASA Astrophysics Data System (ADS)

    Nestola, Y.; Storti, F.; Cavozzi, C.

    2014-12-01

    The evolution of lithosphere necking is a fundamental parameter controlling the structural architecture and thermal state of rifted margins. Despite a large number of analogue and numerical modelling studies on lithosphere extension are available in the literature, a quantitative experimental description of lithosphere necking evolution is still lacking. Extensional strain rate and thermal layering of the lithosphere exert a fundamental control on necking shape and evolution. We focused our experimental work on the former parameter and simulated the progression of lithosphere thinning and necking during asymmetric orthogonal rifting at different plate divergence rates. Our models involve a 4-layer mechanical continental lithosphere, which rests on a glucose syrup asthenosphere. Both the topography and the base of the lithosphere were monitored by time-lapse laser scanning. This technical approach allowed us to quantify the evolution in space and time of the thinning factors for the crust, mantle, and lithosphere as a whole. Laser-scanning monitoring provided also a detailed picture of the evolving neck shape, which shows a strong dependency on the strain-rate. At low strain-rates, necking is "boxed" with steep flanks and a flat-lying roof, and few deep basins develop at surface. At high strain-rates, more distributed thinning occurs and isolates portions of less deformed mantle. More distributed deformation affects the model topography. Despite large differences in shape, the aspect ratio (amplitude/wavelength) of the cross-sectional neck shapes converges towards very similar values at the end of the experiments.The significant differences and evolutionary pathways produced by the plate divergence rate on the lithosphere necking profile, suggest that this parameter exert a fundamental control on localization vs. distribution of deformation in the crust as in the whole mechanical lithosphere. Furthermore, it can exert a fundamental control on the time and space

  14. The role of mechanical heterogeneities during continental breakup: a 3D lithospheric-scale modelling approach

    NASA Astrophysics Data System (ADS)

    Duclaux, Guillaume; Huismans, Ritske S.; May, Dave

    2015-04-01

    How and why do continents break? More than two decades of analogue and 2D plane-strain numerical experiments have shown that despite the origin of the forces driving extension, the geometry of continental rifts falls into three categories - or modes: narrow rift, wide rift, or core complex. The mode of extension itself is strongly influenced by the rheology (and rheological behaviour) of the modelled layered system. In every model, an initial thermal or mechanical heterogeneity, such as a weak seed or a notch, is imposed to help localise the deformation and avoid uniform stretching of the lithosphere by pure shear. While it is widely accepted that structural inheritance is a key parameter for controlling rift localisation - as implied by the Wilson Cycle - modelling the effect of lithospheric heterogeneities on the long-term tectonic evolution of an extending plate in full 3D remains challenging. Recent progress in finite-element methods applied to computational tectonics along with the improved accessibility to high performance computers, now enable to switch from plane strain thermo-mechanical experiments to full 3D high-resolution experiments. Here we investigate the role of mechanical heterogeneities on rift opening, linkage and propagation during extension of a layered lithospheric systems with pTatin3d, a geodynamics modeling package utilising the material-point-method for tracking material composition, combined with a multigrid finite-element method to solve heterogeneous, incompressible visco-plastic Stokes problems. The initial model setup consists in a box of 1200 km horizontally by 250 km deep. It includes a 35 km layer of continental crust, underlaid by 85 km of sub-continental lithospheric mantle, and an asthenospheric mantle. Crust and mantle have visco-plastic rheologies with a pressure dependent yielding, which includes strain weakening, and a temperature, stress, strain-rate-dependent viscosity based on wet quartzite rheology for the crust, and wet

  15. 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

  16. Tracing ancient events in the lithospheric mantle: A case study from ophiolitic chromitites of SW Turkey

    NASA Astrophysics Data System (ADS)

    Akbulut, Mehmet; González-Jiménez, José María; Griffin, William L.; Belousova, Elena; O'Reilly, Suzanne Y.; McGowan, Nicole; Pearson, Norman J.

    2016-04-01

    New major-, minor- and trace-element data on high-Cr chromites from several ophiolitic podiform chromitites from Lycian and Antalya peridotites in southwestern Turkey reveal a polygenetic origin from a range of arc-type melts within forearc and back-arc settings. These forearc and the back-arc related high-Cr chromitites are interpreted to reflect the tectonic juxtaposition of different lithospheric mantle segments during the obduction. The diversity of the γOs(t=0) values (-8.28 to +13.92) in the Antalya and Lycian chromitite PGMs and their good correlations with the sub- to supra-chondritic 187Os/188Os ratios (0.1175-0.1459) suggests a heterogeneous mantle source that incorporated up to 40% recycled crust, probably due to subduction processes of the orogenic events. The few model ages calculated define two significant peaks in TRD model ages at 1.5 and 0.25 Ga, suggesting that the chromitites are younger than 0.25 Ga and include relics of an at least Mesoproterozoic or older (>1.0 Ga) mantle protolith. Eight of the nine zircon grains separated from the chromitites, are interpreted as detrital and/or resorbed xenocrystic relics, whilst a significantly less reworked/resorbed one is considered to be of metasomatic origin. In-situ U-Pb dating of the xenocrystic zircon grains yielded a spread of ages within ca 0.6-2.1 Ga, suggesting recycling of crustal rocks younger than 0.6 Ga (Late Neoproterozoic). The notable coincidence between the lower age limit of the older zircons (ca 1.6 Ga) and the oldest Os model age peak (ca 1.5 Ga) from the PGM may suggest a Mesoproterozoic rifting stage. These findings imply a Paleoproterozoic sub-continental lithospheric mantle (SCLM) protolith for the SW Anatolian mantle which was later converted into an oceanic lithospheric mantle domain possibly following a rifting and continental break-up initiated during Mesoproterozoic (ca 1.5-1.0 Ga). The single metasomatic zircon of ca 0.09 Ga age coinciding with the initiation of the

  17. Mantle accommodation of lithospheric shortening as seen by combined surface wave and teleseismic imaging in the South Island, New Zealand

    NASA Astrophysics Data System (ADS)

    Fry, Bill; Eberhart-Phillips, Donna; Davey, Fred

    2014-10-01

    The Pacific and Australian plates in the South Island, New Zealand (NZ) converge at a rate of about 4 cm yr-1. Accommodation of the continental part of this convergence in the lithospheric mantle is both poorly understood and currently controversial yet it is a problem of fundamental importance for understanding lithospheric thickening. End-member possibilities range from the classical model of asymmetric subduction to symmetric viscous thickening. Seismic tomography has the potential to image this process. However, tomographic images to date are poorly constrained due to the lack of appropriate earthquakes. Improved teleseismic tomography of the region has been achieved by increasing data coverage and applying a novel scheme of correcting for crustal structure by ray tracing through a newly created model of shallow shear wave velocity derived from the inversion of noise-based dispersion measurements. Our resulting models suggest the lithospheric mantle high velocities at the continental plate boundary extend no deeper than approximately 125 km, evidence against both previous models of viscous drip and typical asymmetric subduction zones. This high velocity core extends from north to south along the axis of South Island suggesting that mantle convergence is accommodated along the older, mid-Cenozoic, plate boundary. West of South Island, a high velocity west dipping zone may define the remnant Cretaceous subduction zone that has been distorted by Cenozoic transcurrent deformation. We present our new 3-D seismic velocity models together with a compatible tectonic model and discuss their implications for the nature of lithospheric evolution at this convergent boundary.

  18. Mantle plumes, continental magmatism and asymmetry in the South Atlantic

    NASA Astrophysics Data System (ADS)

    Gallagher, Kerry; Hawkesworth, Chris

    1994-05-01

    Melt production rates are estimated along the ocean tracks (Rio Grande Rise, Walvis Ridge) and the onshore Paraná/Etendeka Flood Basalt Province for the mantle plume currently associated with Tristan da Cunha in the South Atlantic. Assuming that the volcanic edifice is locally compensated with a thickened crust, the maximum oceanic rate (˜ 0.3 km 3 a - 1) is similar to the value for the continental flood basalt, calculated using the surface volume of lava and a duration of 5 m.y. However, this latter estimate is conservative, and the appropriate rate is probably at least a factor of 5 higher. The maximum mantle potential temperature for the ocean plume tracks is estimated to have been 1450-1540°C, assuming a dry peridotite source. Using a similar model, temperatures in excess of 1600°C are required to explain the observed magma volumes onshore. This is because there is no significant extension in the region where the oldest lavas occur. It is argued that the observed volumes of melt can be generated with small amounts of extension and with plume temperatures of < 1500°C if melting in the lithosphere is strongly influenced by the relative enrichment of volatiles. Moreover, the model predicts the inferred change to an asthenosphere-dominated source when significant extension occurs. Recent geochonological data have shown that the continental volcanics are ˜ 138 Ma in the northwest of the Paraná province and become progressively younger to the southeast. Oceanic melt production rates decrease by a factor of 3-5 towards the mid-ocean ridge/Tristan da Cunha. Intriguingly, melt production rates on the Rio Grande Rise vary to more than twice that of the Walvis Ridge for oceanic crust of similar age. This asymmetry is also seen in the continental regions, most obviously in the preserved volume of lava in the Paraná and Etendeka, but also in the topography of the underplated continental margin and the orientation of dykes and oblique extension structures. These

  19. Subduction initiation, recycling of Alboran lower crust, and intracrustal emplacement of subcontinental lithospheric mantle in the Westernmost Mediterranean

    NASA Astrophysics Data System (ADS)

    Varas-Reus, María Isabel; Garrido, Carlos J.; Bosch, Delphine; Marchesi, Claudio; Hidas, Károly; Booth-Rea, Guillermo; Acosta-Vigil, Antonio

    2015-04-01

    Unraveling the tectonic settings and processes involved in the annihilation of subcontinental mantle lithosphere is of paramount importance for our understanding of the endurance of continents through Earth history. Unlike ophiolites -- their oceanic mantle lithosphere counterparts -- the mechanisms of emplacement of the subcontinental mantle lithosphere in orogens is still poorly known. The emplacement of subcontinental lithospheric mantle peridotites is often attributed to extension in rifted passive margins or continental backarc basins, accretionary processes in subduction zones, or some combination of these processes. One of the most prominent features of the westernmost Mediterranean Alpine orogenic arcs is the presence of the largest outcrops worldwide of diamond facies, subcontinental mantle peridotite massifs; unveiling the mechanisms of emplacement of these massifs may provide important clues on processes involved in the destruction of continents. The western Mediterranean underwent a complex Alpine evolution of subduction initiation, slab fragmentation, and rollback within a context of slow convergence of Africa and Europe In the westernmost Mediterranean, the alpine orogeny ends in the Gibraltar tight arc, which is bounded by the Betic, Rif and Tell belts that surround the Alboran and Algero-Balearic basins. The internal units of these belts are mostly constituted of an allochthonous lithospheric domain that collided and overthrusted Mesozoic and Tertiary sedimentary rocks of the Mesozoic-Paleogene, South Iberian and Maghrebian rifted continental paleomargins. Subcontinental lithospheric peridotite massifs are intercalated between polymetamorphic internal units of the Betic (Ronda, Ojen and Carratraca massifs), Rif (Beni Bousera), and Tell belts. In the Betic chain, the internal zones of the allochthonous Alboran domain include, from bottom to top, polymetamorphic rock of the Alpujarride and Malaguide complexes. The Ronda peridotite massif -- the

  20. Boron Isotopic Variation in the Subcontinental Lithospheric Mantle

    NASA Astrophysics Data System (ADS)

    Guild, M. R.; Bell, D. R.; Hervig, R. L.

    2013-12-01

    Boron contents and isotopic compositions (δ11B) of phlogopite mica, amphibole, and selected coexisting anhydrous phases were measured by secondary ion mass spectrometry in mantle xenolith samples from the Kaapvaal Craton of South Africa, in order to better understand processes of volatile element transfer in the mantle. We have documented a wide range of δ11B (>40‰) and B contents (<10ppb to 10's of ppm) in mica from three broad groups identified based on petrographic and compositional criteria, and B geochemistry. The first group, characterized by light δ11B values (-17‰ to -30‰) and low B contents (a few ppb to 100's ppb), consists of mica megacrysts in kimberlite and mica in garnet harzburgites (gt hz) and lherzolites (gt lz) containing variably abundant metasomatic mica, orthopyroxene (opx) and, in some cases, clinopyroxene (cpx). Boron contents and δ11B show a broad positive correlation with modal intensity of metasomatism from gt hz to mica-rich websteritic gt lz. Metasomatic fluids, parental to this group, are proposed to originate in partially-dehydrated subducting oceanic lithosphere, consistent with high LILE/HFSE mineral chemistry. The second group is characterized by relatively B-rich (~1ppm) micas and amphiboles from MARID xenoliths, cpx and gt in cpx-rich peridotite, and (B-poor) subcalcic cpx megacrysts, which all have δ11B of ~-10‰, indistinguishable from primitive mantle estimates. The fluids associated with the second group of samples may have originated in mantle plumes. The third group is heterogeneous showing δ11B values from ~-5 to +15‰ with B contents from 0.5-10 ppm. These samples (all micas) exhibit secondary textures that appear to result from fluid processes associated with kimberlite emplacement. Other analyzed samples (hydrous and anhydrous) may record contact with multiple fluids. The current dataset shows that boron is a useful tracer of fluids in the mantle and can contribute to the understanding of global geochemical

  1. Noble gas composition of subcontinental lithospheric mantle: An extensively degassed reservoir beneath Southern Patagonia

    NASA Astrophysics Data System (ADS)

    Jalowitzki, Tiago; Sumino, Hirochika; Conceição, Rommulo V.; Orihashi, Yuji; Nagao, Keisuke; Bertotto, Gustavo W.; Balbinot, Eduardo; Schilling, Manuel E.; Gervasoni, Fernanda

    2016-09-01

    Patagonia, in the Southern Andes, is one of the few locations where interactions between the oceanic and continental lithosphere can be studied due to subduction of an active spreading ridge beneath the continent. In order to characterize the noble gas composition of Patagonian subcontinental lithospheric mantle (SCLM), we present the first noble gas data alongside new lithophile (Sr-Nd-Pb) isotopic data for mantle xenoliths from Pali-Aike Volcanic Field and Gobernador Gregores, Southern Patagonia. Based on noble gas isotopic compositions, Pali-Aike mantle xenoliths represent intrinsic SCLM with higher (U + Th + K)/(3He, 22Ne, 36Ar) ratios than the mid-ocean ridge basalt (MORB) source. This reservoir shows slightly radiogenic helium (3He/4He = 6.84-6.90 RA), coupled with a strongly nucleogenic neon signature (mantle source 21Ne/22Ne = 0.085-0.094). The 40Ar/36Ar ratios vary from a near-atmospheric ratio of 510 up to 17700, with mantle source 40Ar/36Ar between 31100-6800+9400 and 54000-9600+14200. In addition, the 3He/22Ne ratios for the local SCLM endmember, at 12.03 ± 0.15 to 13.66 ± 0.37, are higher than depleted MORBs, at 3He/22Ne = 8.31-9.75. Although asthenospheric mantle upwelling through the Patagonian slab window would result in a MORB-like metasomatism after collision of the South Chile Ridge with the Chile trench ca. 14 Ma, this mantle reservoir could have remained unhomogenized after rapid passage and northward migration of the Chile Triple Junction. The mantle endmember xenon isotopic ratios of Pali-Aike mantle xenoliths, which is first defined for any SCLM-derived samples, show values indistinguishable from the MORB source (129Xe/132Xe =1.0833-0.0053+0.0216 and 136Xe/132Xe =0.3761-0.0034+0.0246). The noble gas component observed in Gobernador Gregores mantle xenoliths is characterized by isotopic compositions in the MORB range in terms of helium (3He/4He = 7.17-7.37 RA), but with slightly nucleogenic neon (mantle source 21Ne/22Ne = 0.065-0.079). We

  2. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-03-01

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

  4. Neodymium Isotope Variability at the Grain Scale in the Sub-Continental Lithospheric Mantle: NdO+ Analyses of Individual Clinopyroxene Grains (<5 ng Nd aliquots) from a Kilbourne Hole Harzburgitic Xenolith.

    NASA Astrophysics Data System (ADS)

    Harvey, J.; Honn, D.; Baxter, E. F.; Warren, J. M.; Hammond, S.; Walshaw, R.

    2014-12-01

    It is evident that at scales of 102 to 10-2 m there is significant isotopic heterogeneity in the mantle that is not always reflected in primitive melts. The "Os isotopic gap"[1] is one such manifestation of this phenomenon but a similar offset exists between the Nd isotope composition of abyssal peridotites and the mid-ocean ridge basalts that they are inferred to have produced[2]. This study takes advantage of recent advances in the analysis of Nd isotopes as NdO+[3,4] which permit the precise analysis of single clinopyroxene grains (<1 mg mass; <5 ng Nd) from a continental harzburgitic xenolith from Kilbourne Hole, NM. Analyses of aggregates of clinopyroxenes from 5 Kilbourne Hole xenoliths reveal a wide range of 143Nd/144Nd (0.513011 ± 28 to 0.513615 ±19)[5]. This study demonstrates significant grain-to-grain isotopic heterogeneity at a scale of 10-2 m (143Nd/144Nd = 0.513089 ± 78 to 0.513364 ± 74) which (i) is equivalent to the range of values for Pacific MORB[6] and (ii) is more primitive than local basalts with an asthenospheric signature[7]. This suggests that small-scale refractory domains exist within the mantle which are either not sampled during partial melting or whose presence is obscured by the melting of higher volumes of more fusible material. Ref:[1]Alard et al. (2005) Nature 436, 1005-1008 [2]Warren et al. (2009) JGR 114, B12203, doi:10.1029/2008JB006186 [3]Harvey and Baxter (2009) Chem. Geol. 258, 251-257 [4]Honn et al. (2013) AGU Fall abstr. V33-2722 [5]Harvey et al. (2012) J. Petrol. 53, 1709-1742 [6]Hofmann (1997) Nature 385, 219-229 [7]Thompson et al. (2005) J. Petrol. 46, 1603-1643

  5. Postcollisional mafic igneous rocks record crust-mantle interaction during continental deep subduction.

    PubMed

    Zhao, Zi-Fu; Dai, Li-Qun; Zheng, Yong-Fei

    2013-01-01

    Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins. PMID:24301173

  6. Postcollisional mafic igneous rocks record crust-mantle interaction during continental deep subduction

    PubMed Central

    Zhao, Zi-Fu; Dai, Li-Qun; Zheng, Yong-Fei

    2013-01-01

    Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins. PMID:24301173

  7. Postcollisional mafic igneous rocks record crust-mantle interaction during continental deep subduction.

    PubMed

    Zhao, Zi-Fu; Dai, Li-Qun; Zheng, Yong-Fei

    2013-12-04

    Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins.

  8. Delamination of the continental lithosphere and magmatogenesis: inferences from the Tyrrhenian area

    SciTech Connect

    Lavecchia, G.; Stoppa, F.

    1988-08-01

    On the basis of geologic and geophysical data, the Tyrrhenian rift zone is interpreted here as the result of an eastward-migrating process of delamination of the continental lithosphere. A fundamental consequence is the development of complex magmatogenetic processes within the crust, the lithosphere, and the asthenosphere. Starting from this point of view, the authors have reclassified the Neogene-Quaternary Italian magmas by means of multivariate analysis and defined new homogeneous magmatic series whose spatial and temporal distributions have been compared with the tectonic and geophysical setting of the Tyrrhenian Sea and its margins. In such a way a magmatogenetic model has been defined that helps them explain globally the different aspects of Per-Tyrrhenian magmatism and to insert them in the framework of a homogeneous geodynamic environment. The delamination of the Tyrrhenian lithosphere is accompanied by partial melting of the asthenosphere due to rapid unloading that enables partial melting of the more volatile fraction of the mantle rocks. In such a way alkaline and/or carbonate magmas are produced. In the meantime the footwall rocks across the extending lithosphere are subject to many kilobars of pressure drops, producing subalkaline magmas by partial melting of the lithosphere. Crustal simple shear deformations, on the other hand, enable anatexis of the lower continental crust, producing at its bottom granitic magmas. Thus, the Italian parent magmas are hypothesized to be produced by the above process, whereas the volcanic activity that takes place over the Peri-Tyrrhenian margins is supposed to be permitted by vertical deformations subsequent to the horizontal tension and caused by a sort of elastic rebound.

  9. Topography caused by mantle density variations: observation-based estimates and models derived from tomography and lithosphere thickness

    NASA Astrophysics Data System (ADS)

    Steinberger, Bernhard

    2016-04-01

    Large-scale topography may be due to several causes, including (1) variations in crustal thickness and density structure, (2) oceanic lithosphere age differences, (3) subcrustal density variations in the continental lithosphere and (4) convective flow in the mantle beneath the lithosphere. The last contribution in particular may change with time and be responsible for continental inundations; distinguishing between these contributions is therefore important for linking Earth's history to its observed geological record. As a step towards this goal, this paper aims at such distinction for the present-day topography: the approach taken is deriving a `model' topography due to contributions (3) and (4), along with a model geoid, using a geodynamic mantle flow model. Both lithosphere thickness and density anomalies beneath the lithosphere are inferred from seismic tomography. Density anomalies within the continental lithosphere are uncertain, because they are probably due to variations in composition and temperature, making a simple scaling from seismic to density anomalies inappropriate. Therefore, we test a number of different assumptions regarding these. As a reality check, model topography is compared, in terms of both correlation and amplitude ratio, to `residual' topography, which follows from observed topography after subtracting contributions (1) and (2). The model geoid is compared to observations as well. Comparatively good agreement is found if there is either an excess density of ≈0.2 per cent in the lithosphere above ≈150 km depth, with anomalies below as inferred from tomography, or if the excess density is ≈0.4 per cent in the entire lithosphere. Further, a good fit is found for viscosity ≈1020 Pa s in the asthenosphere, increasing to ≈1023 Pa s in the lower mantle above D'. Results are quite dependent on which tomography models they are based on; for some recent ones, topography correlation is ≈0.6, many smaller scale features are matched

  10. Subduction initiation, recycling of Alboran lower crust, and intracrustal emplacement of subcontinental lithospheric mantle in the Westernmost Mediterranean

    NASA Astrophysics Data System (ADS)

    Varas-Reus, María Isabel; Garrido, Carlos J.; Bosch, Delphine; Marchesi, Claudio; Hidas, Károly; Booth-Rea, Guillermo; Acosta-Vigil, Antonio

    2015-04-01

    Unraveling the tectonic settings and processes involved in the annihilation of subcontinental mantle lithosphere is of paramount importance for our understanding of the endurance of continents through Earth history. Unlike ophiolites -- their oceanic mantle lithosphere counterparts -- the mechanisms of emplacement of the subcontinental mantle lithosphere in orogens is still poorly known. The emplacement of subcontinental lithospheric mantle peridotites is often attributed to extension in rifted passive margins or continental backarc basins, accretionary processes in subduction zones, or some combination of these processes. One of the most prominent features of the westernmost Mediterranean Alpine orogenic arcs is the presence of the largest outcrops worldwide of diamond facies, subcontinental mantle peridotite massifs; unveiling the mechanisms of emplacement of these massifs may provide important clues on processes involved in the destruction of continents. The western Mediterranean underwent a complex Alpine evolution of subduction initiation, slab fragmentation, and rollback within a context of slow convergence of Africa and Europe In the westernmost Mediterranean, the alpine orogeny ends in the Gibraltar tight arc, which is bounded by the Betic, Rif and Tell belts that surround the Alboran and Algero-Balearic basins. The internal units of these belts are mostly constituted of an allochthonous lithospheric domain that collided and overthrusted Mesozoic and Tertiary sedimentary rocks of the Mesozoic-Paleogene, South Iberian and Maghrebian rifted continental paleomargins. Subcontinental lithospheric peridotite massifs are intercalated between polymetamorphic internal units of the Betic (Ronda, Ojen and Carratraca massifs), Rif (Beni Bousera), and Tell belts. In the Betic chain, the internal zones of the allochthonous Alboran domain include, from bottom to top, polymetamorphic rock of the Alpujarride and Malaguide complexes. The Ronda peridotite massif -- the

  11. Mantle xenoliths from Central Vietnam: evidence for at least Meso-Proterozoic formation of the lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Proßegger, Peter; Ntaflos, Theodoros; Ackerman, Lukáš; Hauzenberger, Christoph; Tran, Tuan Anh

    2016-04-01

    Intraplate Cenozoic basalts that are widely dispersed along the continental margin of East Asia belong to the Western Pacific "diffuse" igneous province. They consist mainly of alkali basalts, basanites,rarely nephelinites, which are mantle xenolith-bearing, potassic rocks and quartz tholeiites. The volcanism in this area has been attributed to the continental extension caused by the collision of India with Asia and by the subduction of the Pacific Ocean below Asia. We studied a suite of 24 mantle xenoliths from La Bang Lake, Dak Doa district and Bien Ho, Pleiku city in the Gia Province, Central Vietnam. They are predominantly spinel lherzolites (19) but spinel harburgites (3) and two garnet pyroxenites are present as well. The sizes of the xenoliths range from 5 to 40 cm in diameter with medium to coarse-grained protogranular textures. Whole rock major and trace element analyses display a wide range of compositions. The MgO concentration varies from 36.0 to 45.8 wt% whereas Al2O3 and CaO range from 0.63 to 4.36 wt% and from 0.52 to 4.21 wt% (with one sample having CaO of 6.63 wt%) respectively. Both CaO and Al2O3 positively correlate with MgO most likely indicating that the sampled rocks were derived from a common mantle source experienced variable degrees of partial melting. Mineral analyses show that the rock forming minerals are chemically homogeneous. The Fo contents of olivine vary between 89.2 and 91.2 and the Mg# of orthopyroxene and clinopyroxene range from 89 to 92 and 89 to 94 respectively. The range of Cr# for spinel is 0.06-0.26. Model calculations in both whole rock and clinopyroxenes show that lithospheric mantle underneath Central Vietnam experienced melt extractions that vary between 2-7, 12-15 and 20-30%. The majority of the primitive mantle-normalized whole rock and clinopyroxene REE patterns are parallel to each other indicating that clinopyroxene is the main repository of the trace elements. Clinopyroxenes are divided into two groups: group A

  12. Water contents in pyroxenes of intraplate lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Bonadiman, C.; Hao, Y.-T.; Coltorti, M.; Dallai, L.; Faccini, B.; Hu, H.; Qunke, X.

    2009-04-01

    Water contents of clinopyroxene and orthopyroxene in mantle peridotites from various xenolith occurrences in intraplate settings (both oceanic and continental) were determined by Fourier Transform Infrared Spectroscopy (FTIR). The localities are as follow: Sal Island (Cape Verde Archipelago); Baker Rocks and Greene Point (Northern Victoria Land, Antarctica); Panshishan and Lianshan (Subei Basin, Eastern China). They represent well-known localities where detailed petrographical and geochemical studies have already been carried out or areas which are currently under investigation. The water incorporated in these pyroxenes is low (cpx, 37-399ppm; opx: 9-166ppm)(or very low as in Greene Point, Antarctica; cpx, 5-16ppm; opx: 9-16ppm) and, among each population, no clear correlation with melting parameters (MgO contents) in single mineral is evident. Results are compared with the available literature data on water contents in mantle pyroxene which includes peridotites from on-craton (hosted by kimberlitic-type magmas) and off-craton (hosted by alkaline basic magmas), as well as subarc mantle settings. The "relatively dry" (cpx: 140-528 ppm; opx: 38-280 ppm) sub-arc mantle xenoliths (Peslier et al., 2002) are shown to be wetter than the intraplate (off-craton) xenoliths. Cratonic mantle pyroxenes are only represented by a few determinations on garnet peridotites and eclogite from Kaapvaal and Colorado Plateau. They record the highest water contents (cpx: 342-1012 ppm; opx: 180-491 ppm) so far measured in mantle pyroxenes from various tectonic settings. Despite the limited data set, the indication that the cratonic mantle is strongly hydrated is compelling. Rehydration for the Colorado Plateau craton may be due to the Farallon plate subduction (Li et al., 2008), while for Kaapvaal Craton it might be related to young (<100Ma) metasomatic enrichments (Griffin et al., 2003a; Kobussen et al., 2008). If this is the case then the Archean mantle water content needs to be

  13. Reflection Character of the Continental Lithosphere and Crustal Evolution

    NASA Astrophysics Data System (ADS)

    Brown, L. D.

    2015-12-01

    Reflection images represent the high-frequency end member of the suite of seismological tools commonly used to probe the lithosphere. The global inventory of deep reflection profiles has documented reflection characters as varied as the surface geology that provides the primary boundary condition for the interpretation of reflection images. Past reviews of reflection results have stressed similarities in reflection patterns between various geographic regions and attempted to associate these patterns with specific tectonic processes. Examples include: laminated sequences (shear fabrics formed during extension or collision), reflective Mohos (mafic underplating), bright spots (contemporary and fossil magma bodies), dipping mantle reflections rooted in the lower crust (fossil subduction zones) and subhorizontal mantle reflections (phase changes in the lower lithosphere). Here I focus on relating reflection character to the inversion and/or interpretation of results from broadband techniques such as receiver functions, body wave and surface wave tomography. Among the underappreciated aspects of reflectivity are its dependence upon density as well as velocity, and the limitations of 2D images in a 3D world. A core consideration is the need to meaningfully relate integrated physical properties (e.g. velocity inferred from refraction and surface wave measurements) with the differential physical properties (e.g. reflection coefficients) to which reflection images are primarily sensitive. Examples from Tibet and Eurasia are used to illustrate examples of successful integration of controlled (active) and natural (passive) source observations to constrain models of crustal evolution.

  14. Dating layered websterite formation in the lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Le Roux, V.; Nielsen, S. G.; Sun, C.; Yao, L.

    2016-11-01

    Pyroxenites are often documented among exhumed mantle rocks, and can be found in most tectonic environments, from supra-subduction to sub-continental and sub-oceanic mantle. In particular, websterites, i.e. orthopyroxene-clinopyroxene bearing pyroxenites, are found in parallel layers in most orogenic and ophiolitic peridotites. Their formation is often ascribed to melt infiltration and melt-rock reaction processes accompanied by variable amount of deformation. One outstanding question is whether the ubiquitous occurrence of layered websterites in exhumed rocks is generally linked to the exhumation process or truly represents large-scale melt infiltration processes at depth prior to exhumation. These two hypotheses can be distinguished by comparing the exhumation and formation ages of the websterites. However, determination of the layered websterite formation age is challenging. Here we present a novel approach to constrain the formation age of websterite layers using samples from the Lherz massif (France), where layered websterites and lherzolites have formed through melt-rock reaction. By combining high-resolution REE variations, isotope model ages, and diffusive re-equilibration timescales using REE closure temperatures across the websterite layers, we constrain a minimum age and a maximum age for the formation of layered websterites. We show that layered websterites in Lherz formed 1,500-1,800 Ma ago, and are thus clearly disconnected from the process of exhumation at 104 Ma. Multiple generations of layered websterites commonly found in ultramafic massifs, along with the evidence for ancient melt-rock reaction in Lherz, indicate that melt-rock reactions can happen episodically or continuously in the mantle and that layered websterites found in exhumed mantle rocks record ubiquitous melt infiltration processes in the mantle.

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

    NASA Astrophysics Data System (ADS)

    Chen, W.; Yang, Z.

    2009-12-01

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

  16. Lithosphere Structure and Mantle Characterization of the Alpine-Himalayan Belt: Atlas, Zagros and Tibet

    NASA Astrophysics Data System (ADS)

    Jiménez-Munt, I.; Tunini, L.; Fernandez, M.; Verges, J.; Garcia-Castellanos, D.

    2015-12-01

    By combining geophysical and petrological information, we investigate the crust and upper mantle of three orogens of the Alpine-Himalayan Belt (Atlas, Zagros and Tibet), characterizing the lithosphere from the thermal, compositional and seismological viewpoint. The modeling is based on an integrated geophysical-petrological methodology combining elevation, gravity, geoid, surface heat flow, seismic and geochemical data.The results show prominent lithospheric mantle thickening beneath the Moroccan margin followed by thinning beneath the Atlas Mountains. Different convergence accommodation between the crust and lithospheric mantle suggests a decoupled crustal-mantle mechanical response. In the northern Zagros the lithosphere-asthenosphere boundary rises sharply below the Sanandaj Sirjan Zone in a narrow region, whereas in the central Zagros the thinning is smoother and affects a wider region. The transition from the Arabian to the Eurasian lithospheric domain is located beneath the Zagros range, and it is marked by a change in the mantle velocity anomaly and in the lithospheric mantle composition. In the western Himalaya-Tibetan orogen, the lithosphere thickening is gradual reaching the maximum below the northern edge of the Plateau. The Indian lithospheric mantle underlies the whole Tibetan Plateau up to the boundary with the Tarim Basin. In the eastern sector, the thickening generates sharp steps beneath the Himalaya Range, and it thins abruptly beneath the Qiangtang and the Songpan Ganzi terrains. The Indian underthrusting is restricted to the southern Plateau. Different Eurasian domains have been also identified beneath the Tarim Basin, the Altaids region and NE Plateau by means of different lithospheric mantle compositions. The lithospheric models crossing Zagros and Tibetan Plateau show that the present-day lithosphere mantle structure of the Arabia-Eurasia and India-Eurasia collision zones are laterally-varying along the strike of both orogens, not just in

  17. 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

  18. 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.

  19. The age of the lithospheric mantle beneath the Northern Kerguelen Plateau

    NASA Astrophysics Data System (ADS)

    Debaille, V.; Mattielli, N. D.; Weis, D. A.

    2009-12-01

    The Kerguelen Plateau, in the Southern part of the Indian Ocean, provides a unique perspective on the building of Large Igneous Provinces. The Kerguelen Plateau by itself is divided in four parts, each presenting different geochemical characteristics that correspond to the various tectonic stages of the plateau evolution with time. The involvement of continental material has been evidenced in the Cretaceous lavas in the South of the plateau (>100 Myr). In contrast, there is no evidence for continental material in the young, Cenozoic lavas of the Kerguelen Archipelago, located in the Northern part of the plateau (NKP). On the other hand, the presence of subcontinental lithospheric material has been invoked in some basic and ultrabasic xenoliths from the archipelago [1]. These xenoliths, disseminated within alkaline lava series in the Southern and South-East part of the archipelago, have PT conditions generally comprised between 0.6 to 1.8 GPa (10-55 km) and 800 to 1000°C, corresponding to lithospheric conditions. Such petrogenetic conditions reflect underplated basaltic magmas and deep cumulates beneath the Kerguelen Plateau [2]. We have undertaken an Hf-Nd isotopic study on various xenoliths from the South East Province of the archipelago to decipher the fine structure of the mantle and the potential distribution of continental components under the NKP. Preliminary 143Nd/144Nd and 176Hf/177Hf results on websterite and spinel ± sapphirine bearing 2-pyroxenes metagabbro xenoliths show isotopic compositions overlapping those of the depleted ~29.5 Myr oldest lavas from the Archipelago (Mont Bureau). These lavas describe an isotopic alignment between the compositions of the South-East Indian Ridge and the flood basalts from Mont Crozier, which are representative of the enriched signature of the Kerguelen mantle plume. There is no evidence for contamination of the xenoliths from their host during their ascent. The xenoliths analyzed so far do not compare to the

  20. Lithosphere Structure in Southern Africa: Mantle Density, Dynamic Topography, Moho Sharpness, and Kimberlite Magmatism

    NASA Astrophysics Data System (ADS)

    Artemieva, I. M.; Vinnik, L. P.

    2015-12-01

    In southern Africa, both the Archean and Proterozoic blocks have the topography 500-700 m higher than in any other craton worldwide, except for the Tanzanian craton. An unusually high topography may be caused by a low density of the cratonic lithospheric mantle and/or by the dynamic support of the mantle with origin below the depth of isostatic compensation (assumed to be at the lithosphere base). We use free-board constraints to examine the relative contributions of the both factors to surface topography in the cratons of southern Africa and present regional model of density structure of the lithospheric mantle. The results indicate that 0.5-1.0 km of topography requires the dynamic contribution from the sublithospheric mantle because it cannot be explained by the lithosphere structure within the petrologically permitted range of mantle densities. The calculated lithospheric mantle density values are in an overall agreement with xenolith-based data and show an overall trend in mantle density increase from Archean to younger lithospheric terranes. Notable exceptions are the Limpopo belt and the Bushveld Intrusion Complex, which have an increased mantle density, probably as a result of melt-metasomatism. The Western Cape Fold Belt has a moderately depleted mantle with density within the range expected for Phanerozoic mantle, while mantle densities beneath the Eastern Cape Fold Belt require the presence of a significant amount of eclogite in the mantle. Mantle density structure correlates with distribution of kimberlites and with seismic velocity contrast across the Moho: kimberlite-rich regions have sharp Moho and low-density (3.32-3.33 g/cc) mantle, while kimberlite-poor regions have transient Moho and denser mantle (3.34-3.35 g/cc). We explain this pattern by melt-metasomatism which affects both mantle depletion and the Moho sharpness. We also find that regions with high mantle density host non-diamondiferous kimberlites, while diamondiferous kimberlites are

  1. Temporal distribution of mantle-derived potassic rocks and carbonatites linked to stabilization of mantle lithosphere and redox states during subduction

    NASA Astrophysics Data System (ADS)

    Foley, S. F.

    2014-12-01

    Mantle-derived potassic igneous rocks and carbonatites first appear in the geological record in the late Archean, coinciding with major crust-forming events on most continents. The compositions of potassic rocks require sources including discrete ultramafic rocks with phlogopite and pyroxenes, whereas carbonatites and ultramafic lamprophyres (carbonate-rich potassic rocks) require oxidizing conditions in which carbonate is stable. The presence of these source rocks from this time is probably related to the stabilization of mantle lithosphere. If mantle lithosphere had not been stable for considerable periods of time, then melting would be restricted to peridotite, which is not a viable option for strongly potassic rocks. The phlogopite-rich source-rock assemblages that are necessary precursors for potassic melts could be introduced into the lithosphere by either subduction processes or by multiple stages of low-degree melting. Many modern examples involve subducted sedimentary material, which concentrates potassium by the stabilization of micas in subduction metamorphism. Subduction involves a great variety of redox states, but the bulk effect is the return of oxidized material from the surface into the mantle. However, we cannot apply uniformitarianism unthinkingly, because subduction processes at and before 2.7 Ga may have had different redox states. Before the Great Oxidation Event the distribution and abundances of geological formations such as banded iron formations, red beds, and uraninites indicate that geological reservoirs became gradually oxidized, preventing an earlier increase in atmospheric oxygen. This means that the function of the subduction process to oxidize the upper mantle by the return of oxidized rocks from the surface was much weaker in the early Earth. Early continental mantle lithosphere was, therefore, likely to accumulate carbon in reduced form, which would be more easily remobilized in melts through low-temperature redox melting much

  2. Isotopic characteristics of mantle sources for Quaternary continental alkaline magmas in the northern Canadian Cordillera

    NASA Astrophysics Data System (ADS)

    Carignan, Jean; Ludden, John; Francis, Don

    1994-12-01

    Three mantle compositions are identified as potential source end members for Quaternary to recent alkaline volcanic rocks from Fort Selkirk, Llangorse-Hirschfeld, Alligator Lake and Mt. Edziza in the northern Canadian Cordillera. These are: (1) an amphibole-rich source, characterized by unradiogenic Sr, Nd and Pb, from which the olivine nephelinite lavas formed, (2) the continental lithospheric mantle which is characterised by high Pb-207/Pb-204 and appears to be involved in the formation of the alkali olivine basalts of Fort Selkirk, and (3) a mantle with radiogenic Pb and unradiogenic Sr (HIMU-type) represented by lavas from Mt. Edziza. The Mt. Edziza volcano is the largest of the volcanic centres in the region, and is considered to reflect melting of sublithospheric mantle of HIMU composition below central British Columbia. Incipient melting of amphibole-veined subcontinental mantle lithosphere resulted from plume upwelling and/or transtensional pressure release and produced the small nephelinite to olivine basalt centres of the northern Cordilleran Province. The source of the nephelinite magmas is slightly more radiogenic than present-day Pacific Mid-Ocean ridge basalts (MORB), and is best represented by the most depleted component of the Aleutian magmas. This suggests enrichment of the subcontinental lithosphere in the northern Cordillera by melts of this isotopic composition during Cretaceous subduction. The Alligator Lake complex is anomalous and charaterized by the most radiogenic lavas. Despite the presence of crustal xenoliths there is no clear geochemical signature for crustal contamination and, in contrast to the other volcanic centers which were erupted through the Intermontain Belt, the lavas of this center may have been derived from a highly radiogenic lithospheric mantle beneath the Coast Plutonic complex.

  3. Crustal and upper mantle structure of stable continental regions in North America and northern Europe

    USGS Publications Warehouse

    Masse, R.P.

    1987-01-01

    From an analysis of many seismic profiles across the stable continental regions of North America and northern Europe, the crustal and upper mantle velocity structure is determined. Analysis procedures include ray theory calculations and synthetic seismograms computed using reflectivity techniques. The P wave velocity structure beneath the Canadian Shield is virtually identical to that beneath the Baltic Shield to a depth of at least 800 km. Two major layers with a total thickness of about 42 km characterize the crust of these shield regions. Features of the upper mantle of these region include velocity discontinuities at depths of about 74 km, 330 km, 430 km and 700 km. A 13 km thick P wave low velocity channel beginning at a depth of about 94 km is also present. A number of problems associated with record section interpretation are identified and a generalized approach to seismic profile analysis using many record sections is described. The S wave velocity structure beneath the Canadian Shield is derived from constrained surface wave data. The thickness of the lithosphere beneath the Canadian and Baltic Shields is determined to be 95-100 km. The continental plate thickness may be the same as the lithospheric thickness, although available data do not exclude the possibility of the continental plate being thicker than the lithosphere. ?? 1987 Birkha??user Verlag.

  4. Water Content in the SW USA Mantle Lithosphere: FTIR Analysis of Dish Hill and Kilbourne Hole Pyroxenites

    NASA Technical Reports Server (NTRS)

    Gibler, Robert; Peslier, Anne H.; Schaffer, Lillian Aurora; Brandon, Alan D.

    2014-01-01

    Kilbourne Hole (NM, USA) and Dish Hill (CA, USA) mantle xenoliths sample continental mantle in two different tectonic settings. Kilbourne Hole (KH) is located in the Rio Grande rift. Dish Hill (DH) is located in the southern Mojave province, an area potentially affected by subduction of the Farallon plate beneath North America. FTIR analyses were obtained on well characterized pyroxenite, dunite and wehrlite xenoliths, thought to represent crystallized melts at mantle depths. PUM normalized REE patterns of the KH bulk-rocks are slightly LREE enriched and consistent with those of liquids generated by < 5% melting of a spinel peridotite source. Clinopyroxenes contain from 272 to 313 ppm weight H2O similar to the lower limit of KH peridotite clinopyroxenes (250-530 ppm H2O). This is unexpected as crystallized melts like pyroxenites should concentrate water more than residual mantle-like peridotites, given that H is incompatible. PUM normalized bulk REE of the DH pyroxenites are characterized by flat to LREE depleted REE profiles consistent with > 6% melting of a spinel peridotite source. Pyroxenite pyroxenes have no detectable water but one DH wehrlite, which bulk-rock is LREE enriched, has 4 ppm H2O in orthopyroxene and <1ppm in clinopyroxene. The DH pyroxenites may thus come from a dry mantle source, potentially unaffected by the subduction of the Farallon plate. These water-poor melts either originated from shallow oceanic lithosphere overlaying the Farallon slab or from continental mantle formed > 2 Ga. The Farallon subduction appears to have enriched in water the southwestern United States lithospheric mantle further east than DH, beneath the Colorado plateau.

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

    NASA Astrophysics Data System (ADS)

    Luth, Robert W.; Stachel, Thomas

    2014-11-01

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

  6. 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.

  7. Mantle xenoliths from Marosticano area (Northern Italy): a comparison with Veneto Volcanic Province lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Brombin, Valentina; Bonadiman, Costanza; Coltorti, Massimo

    2016-04-01

    redox conditions (Δlog fO2: +1.2 to -0.7, Ballhaus, 1991) to Lessinean and Val d'Adige xenoliths which may indicate a local oxidation of the mantle below this portion of VVP. References • Beccaluva L., Bianchini G., Bonadiman C., Coltorti M., Milani L., Salvini L., Siena F., Tassinari R. (2007). Intraplate lithospheric and sublithospheric components in the Adriatic domain: Nephelinite to tholeiite magma generation in the Paleogene Veneto Volcanic Province, Southern Alps. Geological Society of America, 131-152. • Beccaluva L., Bonadiman C., Coltorti M., Salvini L., Siena F. (2001). Depletion events, nature of metasomatizing agent and timing of enrichment processes in lithospheric mantle xenoliths from the Veneto Volcanic Province. Journal of Petrology, 42, 173-187. • Gasperini D., Bosch D., Braga R., Bondi M., Macera P., Morten L. (2006). Ultramafic xenoliths from the Veneto Volcanic Province (Italy): Petrological and geochemical evidence for multiple metasomatism of the SE Alps mantle lithospere. Geochemical Journal, 40, 377-404. • Siena F., Coltorti M. (1989). Lithospheric mantle evolution: evidences from ultramafic xenoliths in the Lessinean volcanics (Northern Itlay). Chemical Geology, 77, 347-364.

  8. The mantle lithosphere beneath the Hangay dome in central Mongolia : Microstructures and seismic properties from mantle xenoliths from the Tariat alkali basalts

    NASA Astrophysics Data System (ADS)

    Tommasi, A.; Ionov, D. A.; Higgie, K.; Carlson, R. W.

    2013-12-01

    The Hangay region in Mongolia displays a high-elevation, low relief topography, underlain by a low velocity zone in the upper mantle at depths between 100 to 200 km beneath central Mongolia. The relation between this active intra-continental uplift and the mantle dynamics are, however, still poorly understood. To constrain the deformation and thermal evolution and estimate the seismic properties of the mantle lithosphere beneath the Hangay, we analyzed the microstructures and crystal preferred orientations (CPO) of olivine, pyroxenes, and hydrous phases, when present, of 40 mantle xenoliths carried up by the Shavaryn-Tsaram, Zala, and Haer volcanics, which erupted between 8 and <1m.y. along the northeastern boundary of the dome. All xenoliths are medium to coarse-grained spinel-lherzolites. Some xenoliths contain rare amphibole, phlogopite and silicate glass. Coarse-granular, highly annealed microstructures predominate. They are characterized by equiaxed polygonal olivine crystals with rare, widely-spaced subgrains and either sinuous or straight grain boundaries and coarse irregularly-shaped pyroxenes. Tabular microstructures, marked by a weak shape preferred orientation of olivine crystals flattened normal to [010], but clear of internal deformation features (except for rare subgrains), are also common. Porphyroclastic microstructures recording a recent deformation related to the surface uplift are not present among the studied xenoliths. The annealed microstructures are associated with well-developed olivine and pyroxenes CPO, typical of deformation under high temperature, moderate pressure, dry conditions. This association indicates that ductile deformation was followed by static recrystallization, which annealed the deformation microstructure but preserved the CPO and, hence, the anisotropy of physical properties in the Hangay mantle lithosphere. This static recrystallization might be due to a long quiescence episode since the last deformation episode or to

  9. Hydration of the lithospheric mantle by the descending plate in a continent-continent collisional setting and its geodynamic consequences

    NASA Astrophysics Data System (ADS)

    Massonne, Hans-Joachim

    2016-05-01

    At the beginning of continent-continent collision the descending plate dehydrates. The influence of this dehydration on the adjacent lithospheric mantle was studied. For this reason, pressure (P), temperature (T) and T-H2O pseudosections were calculated for an average mantle composition using the computer software PERPLE_X. These pseudosections were contoured by isopleths, for instance, for volumes of amphibole, chlorite, and serpentine. In addition, P-T pseudosections were considered for four psammopelitic rocks, common in the upper portion of the continental crust, in order to quantify the release of H2O in these rocks during prograde metamorphism. At pressures around 1 GPa, a maximum of slightly more than 10 vol.% chlorite, almost 20 vol.% amphibole, and some talc but no serpentine forms when only 1.8 wt.% H2O is added to the dry ultrabasite at temperatures of 600 °C. For example, hydrous phases amount to about 35 vol.% serpentine and 10 vol.% each of chlorite and amphibole at 1 GPa, 550 °C, and 5 wt.% H2O. The modelled psammopelitic rocks can release 0.8-2.5 wt.% H2O between 450 and 650 °C at 0.8-1.4 GPa. On the basis of the above calculations, different collisional scenarios are discussed highlighting the role of hydrated lithospheric mantle. In this context a minimum hydration potential of the front region of the descending continental plate is considered, which amounts to 4.6 × 1016 kg releasable H2O for a 1000 km wide collisional zone, due to a thick sedimentary pile at the continental margin. Further suggestions are that (1) the lower crustal plate in a continent-continent collisional setting penetrates the lithospheric mantle, which is hydrated during the advancement of this plate, (2) the maximum depths of the subduction of upper continental crust is below 70 km and (3) hydrated mantle above the descending crustal plate is thrust onto this continental crust.

  10. Composition and thermal structure of the lithospheric mantle beneath kimberlite pipes from the Catoca cluster, Angola

    NASA Astrophysics Data System (ADS)

    Ashchepkov, I. V.; Rotman, A. Y.; Somov, S. V.; Afanasiev, V. P.; Downes, H.; Logvinova, A. M.; Nossyko, S.; Shimupi, J.; Palessky, S. V.; Khmelnikova, O. S.; Vladykin, N. V.

    2012-03-01

    Garnet, clinopyroxene and ilmenite xenocrysts from three Angolan kimberlite pipes belonging to the Catoca cluster (Angola Caquele, Camitongo I and II, and Catoca) from the SW part of the Congo-Kasai craton, reveal similar features which suggest a similarity of mantle structure. PT estimates for pyropes, Cr-diopsides and picroilmenites reveal similar geothermal conditions of ~ 37-40 mW/m2. This is slightly higher than the values determined for the Catoca pipe. Higher temperature conditions ~ 45 mW/m2 were determined for low-Cr pyroxenes and omphacites. The similar general mineralogy and suggested mantle lithology, as well as reconstructed layering of the sub-continental lithospheric mantle (SCLM), are similar for Camitongo I-II as well as for Caquele and Catoca pipes. Heating at depths of 7.5-4.5 GPa (240-140 km) is a general feature of the SCLM beneath the field. The high temperature trend for low-Cr and hybrid pyroxenes from the base of the SCLM up to 30 GPa (100 km) represents the PT path of the protokimberlite melts. PT conditions for ilmenites mainly correspond to colder conditions of crystallization in wall rocks and the outer parts of magmatic channels. Individual geochemical features of the minerals for each SCLM suggest pervasive metasomatism in lower part of the SCLM. Clinopyroxene trace element patterns from the Caquele pipe reveal a lherzolitic affinity; they are LILE-enriched with Ba peaks due to phlogopite melting, while those from Camitongo I-II show Ta-Nb enrichment and Pb troughs. The ilmenite trends trace the mantle column from deep to shallow mantle, evolving to Fe-ilmenites due to advanced AFC of protokimberlite magma that also produced abundant Fe-rich clinopyroxenes. The rise of calculated fO2 correlates with the position of protokimberlites. Comparison with the thermal gradient derived from peridotitic inclusions from Catoca cluster is lower than for Lesotho possibly related to the thicker lithospheric roots beneath the Congo-Kasai craton.

  11. 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.

  12. Crustal seismicity and the earthquake catalog maximum moment magnitudes (Mcmax) in stable continental regions (SCRs): correlation with the seismic velocity of the lithosphere

    USGS Publications Warehouse

    Mooney, Walter D.; Ritsema, Jeroen; Hwang, Yong Keun

    2012-01-01

    A joint analysis of global seismicity and seismic tomography indicates that the seismic potential of continental intraplate regions is correlated with the seismic properties of the lithosphere. Archean and Early Proterozoic cratons with cold, stable continental lithospheric roots have fewer crustal earthquakes and a lower maximum earthquake catalog moment magnitude (Mcmax). The geographic distribution of thick lithospheric roots is inferred from the global seismic model S40RTS that displays shear-velocity perturbations (δVS) relative to the Preliminary Reference Earth Model (PREM). We compare δVS at a depth of 175 km with the locations and moment magnitudes (Mw) of intraplate earthquakes in the crust (Schulte and Mooney, 2005). Many intraplate earthquakes concentrate around the pronounced lateral gradients in lithospheric thickness that surround the cratons and few earthquakes occur within cratonic interiors. Globally, 27% of stable continental lithosphere is underlain by δVS≥3.0%, yet only 6.5% of crustal earthquakes with Mw>4.5 occur above these regions with thick lithosphere. No earthquakes in our catalog with Mw>6 have occurred above mantle lithosphere with δVS>3.5%, although such lithosphere comprises 19% of stable continental regions. Thus, for cratonic interiors with seismically determined thick lithosphere (1) there is a significant decrease in the number of crustal earthquakes, and (2) the maximum moment magnitude found in the earthquake catalog is Mcmax=6.0. We attribute these observations to higher lithospheric strength beneath cratonic interiors due to lower temperatures and dehydration in both the lower crust and the highly depleted lithospheric root.

  13. Lithospheric roots beneath western Laurentia: The geochemical signal in mantle garnets

    USGS Publications Warehouse

    Canil, D.; Schulze, D.J.; Hall, D.; Hearn, B.C.; Milliken, S.M.

    2003-01-01

    This study presents major and trace element data for 243 mantle garnet xenocrysts from six kimberlites in parts of western North America. The geochemical data for the garnet xenocrysts are used to infer the composition, thickness, and tectonothermal affinity of the mantle lithosphere beneath western Laurentia at the time of kimberlite eruption. The garnets record temperatures between 800 and 1450??C using Ni-in-garnet thermometry and represent mainly lherzolitic mantle lithosphere sampled over an interval from about 110-260 km depth. Garnets with sinuous rare-earth element patterns, high Sr, and high Sc/V occur mainly at shallow depths and occur almost exclusively in kimberlites interpreted to have sampled Archean mantle lithosphere beneath the Wyoming Province in Laurentia, and are notably absent in garnets from kimberlites erupting through the Proterozoic Yavapai Mazatzal and Trans-Hudson provinces. The similarities in depths of equilibration, but differing geochemical patterns in garnets from the Cross kimberlite (southeastern British Columbia) compared to kimberlites in the Wyoming Province argue for post-Archean replacement and (or) modification of mantle beneath the Archean Hearne Province. Convective removal of mantle lithosphere beneath the Archean Hearne Province in a "tEctonic vise" during the Proterozoic terminal collisions that formed Laurentia either did not occur, or was followed by replacement of thick mantle lithosphere that was sampled by kimberlite in the Triassic, and is still observed there seismically today.

  14. Constrained potential field modeling of the crustal architecture of the Musgrave Province in central Australia: Evidence for lithospheric strengthening due to crust-mantle boundary uplift

    NASA Astrophysics Data System (ADS)

    Aitken, Alan R. A.; Betts, Peter G.; Weinberg, Roberto F.; Gray, Daniel

    2009-12-01

    We image the crustal architecture of the Musgrave Province with petrophysically constrained forward models of new potential field data. These models image divergent shallow-dipping crustal scale thrusts that, at depth, link with an axial zone defined by steeper, lithospheric scale transpressional shear zones. They also show that to permit a near-surface density distribution that is consistent with petrophysical and geological observations, approximately 15-20 km of crust-mantle boundary uplift is necessary beneath the axial zone. The long-term preservation of this crust-mantle boundary offset implies a change from relatively weak lithosphere to relatively strong lithosphere during the intraplate Petermann Orogeny. To explain this, we propose a model in which uplift of the axial zone of the orogen leads to local lithospheric strengthening as a result of the uplift of mantle rocks into the lower crust, coupled with long-term lithospheric cooling due to the erosion of a radioactive upper crust. Brace-Goetze lithospheric strength models suggest that these processes may have increased the integrated strength of the lithosphere by a factor of 1.4-2.8. Because of this strengthening, this system is self-limiting, and activity will cease when lithospheric strength is sufficient to resist external forces and support isostatic imbalances. A simple force-balance model demonstrates that the force required to uplift the axial zone is tectonically reasonable and that the system can subsequently withstand significant tensional forces. This example shows that crust-mantle boundary uplift coupled with reduced crustal heat production can profoundly affect the long-term strength of the continental lithosphere and may be a critical process in the tectonic stabilization of intraplate regions.

  15. Shear wave velocity, seismic attenuation, and thermal structure of the continental upper mantle

    USGS Publications Warehouse

    Artemieva, I.M.; Billien, M.; Leveque, J.-J.; Mooney, W.D.

    2004-01-01

    Seismic velocity and attenuation anomalies in the mantle are commonly interpreted in terms of temperature variations on the basis of laboratory studies of elastic and anelastic properties of rocks. In order to evaluate the relative contributions of thermal and non-thermal effects on anomalies of attenuation of seismic shear waves, QS-1, and seismic velocity, VS, we compare global maps of the thermal structure of the continental upper mantle with global QS-1 and Vs maps as determined from Rayleigh waves at periods between 40 and 150 S. We limit the comparison to three continental mantle depths (50, 100 and 150 km), where model resolution is relatively high. The available data set does not indicate that, at a global scale, seismic anomalies in the upper mantle are controlled solely by temperature variations. Continental maps have correlation coefficients of <0.56 between VS and T and of <0.47 between QS and T at any depth. Such low correlation coefficients can partially be attributed to modelling arrefacts; however, they also suggest that not all of the VS and QS anomalies in the continental upper mantle can be explained by T variations. Global maps show that, by the sign of the anomaly, VS and QS usually inversely correlate with lithospheric temperatures: most cratonic regions show high VS and QS and low T, while most active regions have seismic and thermal anomalies of the opposite sign. The strongest inverse correlation is found at a depth of 100 km, where the attenuation model is best resolved. Significantly, at this depth, the contours of near-zero QS anomalies approximately correspond to the 1000 ??C isotherm, in agreement with laboratory measurements that show a pronounced increase in seismic attenuation in upper mantle rocks at 1000-1100 ??C. East-west profiles of VS, QS and T where continental data coverage is best (50??N latitude for North America and 60??N latitude for Eurasia) further demonstrate that temperature plays a dominant, but non-unique, role in

  16. Implications for anomalous mantle pressure and dynamic topography from lithospheric stress patterns in the North Atlantic Realm

    NASA Astrophysics Data System (ADS)

    Schiffer, Christian; Nielsen, Søren Bom

    2016-08-01

    With convergent plate boundaries at some distance, the sources of the lithospheric stress field of the North Atlantic Realm are mainly mantle tractions at the base of the lithosphere, lithospheric density structure and topography. Given this, we estimate horizontal deviatoric stresses using a well-established thin sheet model in a global finite element representation. We adjust the lithospheric thickness and the sub-lithospheric pressure iteratively, comparing modelled in plane stress with the observations of the World Stress Map. We find that an anomalous mantle pressure associated with the Iceland and Azores melt anomalies, as well as topography are able to explain the general pattern of the principle horizontal stress directions. The Iceland melt anomaly overprints the classic ridge push perpendicular to the Mid Atlantic ridge and affects the conjugate passive margins in East Greenland more than in western Scandinavia. The dynamic support of topography shows a distinct maximum of c. 1000 m in Iceland and amounts <150 m along the coast of south-western Norway and 250-350 m along the coast of East Greenland. Considering that large areas of the North Atlantic Realm have been estimated to be sub-aerial during the time of break-up, two components of dynamic topography seem to have affected the area: a short-lived, which affected a wider area along the rift system and quickly dissipated after break-up, and a more durable in the close vicinity of Iceland. This is consistent with the appearance of a buoyancy anomaly at the base of the North Atlantic lithosphere at or slightly before continental breakup, relatively fast dissipation of the fringes of this, and continued melt generation below Iceland.

  17. Craton stability and continental lithosphere dynamics during plume-plate interaction

    NASA Astrophysics Data System (ADS)

    Wang, H.; Van Hunen, J.; Pearson, D.

    2013-12-01

    Survival of thick cratonic roots in a vigorously convecting mantle system for billions of years has long been studied by the geodynamical community. A high cratonic root strength is generally considered to be the most important factor. We first perform and discuss new numerical models to investigate craton stability in both Newtonian and non-Newtonian rheology in the stagnant lid regime. The results show that only a modest compositional rheological factor of Δη=10 with non-Newtonian rheology is required for the survival of cratonic roots in a stagnant lid regime. A larger rheological factor (100 or more) is needed to maintain similar craton longevity in a Newtonian rheology environment. Furthermore, chemical buoyancy plays an important role on craton stability and its evolution, but could only work with suitable compositional rheology. During their long lifespan, cratons experienced a suite of dynamic, tectonothermal events, such as nearby subduction and mantle plume activity. Cratonic nuclei are embedded in shorter-lived, more vulnerable continental areas of different thickness, composition and rheology, which would influence the lithosphere dynamic when tectonothermal events happen nearby. South Africa provides a very good example to investigate such dynamic processes as it hosts several cratons and there are many episodic thermal events since the Mesozoic as indicated by a spectrum of magmatic activity. We numerically investigate such an integrated system using the topographic evolution of cratons and surrounding lithosphere as a diagnostic observable. The post-70Ma thinning of pericratonic lithosphere by ~50km around Kaapvaal craton (Mather et al., 2011) is also investigated through our numerical models. The results show that the pericratonic lithosphere cools and grows faster than cratons do, but is also more likely to be effected by episodic thermal events. This leads to surface topography change that is significantly larger around the craton than within

  18. Metasomatism in the lithospheric mantle beneath southern Patagonia, Argentina

    NASA Astrophysics Data System (ADS)

    Kolosova-Satlberger, Olesya; Ntaflos, Theodoros; Bjerg, Ernesto

    2015-04-01

    Mantle xenoliths from Gobernador Gregores, southern Patagonia are spinel- lherzolites, harzburgites and wehrlites. Composite xenoliths consisting of websterites, olivine-websterites and spinel- lherzolites or harzburgites are present as well. The lithospheric mantle beneath Gobernador Gregores region was affected by multiple modal metasomatic events as can be inferred by the presence of amphibole, phlogopite and apatite. The existence of amphibole as inclusion in clinopyroxene suggests dehydration reaction of peridotites, which previously experienced modal metasomatism. This textural evidence records the earliest detectable metasomatic event. A second distinct modal metasomatic event consists of disseminated up to 6 mm in diameter coarse grained amphiboles (100*mg# =89.9) which show breakdown reactions and pseudomorphic replacement by glass and fine grained second generation of olivine, clinopyroxene and spinel. The intensity of the breakdown reaction is variable. In most cases amphibole occurs as a relict within these pseudomorphs. However, melt pockets of up to 10 mm in diameter are abundant, irregular in shape and having the same minerals such as in the pseudomorphs, indicate clearly amphibole breakdown because remnants of it were found enclosed by second generation clinopyroxene. Similar breakdown reactions experienced the phlogopite in the samples where is present. The Phlogopite (100*mg# =88.6) breakdown produces the same mineral phases as the amphibole. The second generation minerals formed after breakdown of amphibole and phlogopite show minor differences in their composition. However, the chemical composition of glass varies considerably. The glasses formed after breakdown of amphibole and phlogopite have trachyandesitic and tephriphonolitic composition, respectively. Some harzburgites and composite xenoliths reveal another metasomatic event: peridotite, enriched in orthopyroxene (mainly orthopyroxenite veinlets, rare websterite), suggests interaction with

  19. Counter-intuitive features of the dynamic topography unveiled by tectonically realistic 3D numerical models of mantle-lithosphere interactions

    NASA Astrophysics Data System (ADS)

    Burov, Evgueni; Gerya, Taras

    2013-04-01

    It has been long assumed that the dynamic topography associated with mantle-lithosphere interactions should be characterized by long-wavelength features (> 1000 km) correlating with morphology of mantle flow and expanding beyond the scale of tectonic processes. For example, debates on the existence of mantle plumes largely originate from interpretations of expected signatures of plume-induced topography that are compared to the predictions of analytical and numerical models of plume- or mantle-lithosphere interactions (MLI). Yet, most of the large-scale models treat the lithosphere as a homogeneous stagnant layer. We show that in continents, the dynamic topography is strongly affected by rheological properties and layered structure of the lithosphere. For that we reconcile mantle- and tectonic-scale models by introducing a tectonically realistic continental plate model in 3D large-scale plume-mantle-lithosphere interaction context. This model accounts for stratified structure of continental lithosphere, ductile and frictional (Mohr-Coulomb) plastic properties and thermodynamically consistent density variations. The experiments reveal a number of important differences from the predictions of the conventional models. In particular, plate bending, mechanical decoupling of crustal and mantle layers and intra-plate tension-compression instabilities result in transient topographic signatures such as alternating small-scale surface features that could be misinterpreted in terms of regional tectonics. Actually thick ductile lower crustal layer absorbs most of the "direct" dynamic topography and the features produced at surface are mostly controlled by the mechanical instabilities in the upper and intermediate crustal layers produced by MLI-induced shear and bending at Moho and LAB. Moreover, the 3D models predict anisotropic response of the lithosphere even in case of isotropic solicitations by axisymmetric mantle upwellings such as plumes. In particular, in presence of

  20. Basaltic volcanism in Ethiopia: Constraints on continental rifting and mantle interactions

    SciTech Connect

    Hart, W. K.; WoldeGabriel, G.; Walter, R. C.; Mertzman, S. A.

    1989-06-10

    Middle to late Cenozoic mafic lavas from the Ethiopian volcanic province exhibit considerable chemical and isotopic diversity that is linked to eruption age and eruption location. These variations provide a geochemical framework in which continental rifting can be examined. Trace element and Sr, Nd, and Pb isotopic data are interpreted to indicate involvement of up to two depleted and two enriched mantle reservoirs throughout Cenozoic rift development in Ethiopia. Superimposed on the characteristics imparted by varying degrees of melting of these distinct reservoirs are the effects of crystal fractionation and, in some instances, crustal contamnation. Initial stages of Oligocene rifting and volcanism, as manifested by the rift-bounding plateau flood basalts, are attributed to asthenospheric upwelling and melting of a heterogeneous, enriched subcontinental lithospheric mantle. Mildly alkaline lavas were produced from an enriched source with characteristics similar to those of the inferred source of other mantle/minus/derived lavas and xenoliths from east Africa (LoNd array, EMI to HIMU). Contemporaneous tholeiitic lavas were derived from a source similar to that producing oceanic basalts from Samoa and the Society Islands (EMII). As lithospheric thinning and rifting continued into the Miocene, upwelling depleted asthenosphere (depleted OIB reservoir, PREMA) interacted with the lithospheric sources producing lavas with hybrid elemental and isotopic characteristics (11-6 Ma plateau and rift margin basalts).

  1. Effective elastic thickness of the Venusian lithosphere with lateral viscosity variations in the mantle

    NASA Technical Reports Server (NTRS)

    Moresi, Louis

    1993-01-01

    Both the Earth and Venus have a convecting mantle at the top of which is a relatively strong, mechanical boundary layer. The surface topography and gravity signals which result from the convection within the viscous mantle are modified by the elastic properties of this lithospheric boundary layer. In particular the ability of the lithosphere to support loads and transmit stresses from below is a function of the wavelength of the load--the lithosphere is strong to loading at shorter wavelengths. As a consequence it is usual to expect that long wavelength topography cannot be supported by the mechanical strength of the lithosphere and must be compensated--isostatically or dynamically--within the uppermost mantle or the crust. The flexural rigidity of the lithosphere can therefore be determined by estimating the greatest wavelength at which uncompensated surface topography can be supported, usually by measuring the admittance as a function of wavelength. In fact this procedure for determining the elastic thickness relies upon being able to distinguish topography with underlying support from that supported by the brittle lithosphere on the basis of their each having a characteristic value of the admittance. However, in the presence of lateral viscosity variations in the mantle, it is possible for topography to be generated which is NOT compensated by density anomalies in the underlying mantle at the same wavelength. Although this effect is not likely to be important for the Earth, on Venus, where the high surface temperatures would be expected to give a weaker lithosphere, lateral viscosity variations in the mantle can give a misleadingly large apparent elastic thickness for the lithosphere.

  2. Metamorphism of peritotites in the mantle wedge above the subduction zone: Hydration of the lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Savelieva, G. N.; Raznitsin, Yu. N.; Merkulova, M. V.

    2016-05-01

    Two areas with different types of hydration (serpentinization), which occurred in two settings distinct in temperatures, pressures, and stresses, are spatially individualized in the ophiolitic ultramafic massifs of the Polar Urals. The high-temperature hydration of ultramafic rocks occurred in the lithosphere of the mantle wedge directly above the subducted slab. The initial conditions of hydration are limited to 1.2-2 GPa and 650-700°C; a stable assemblage of olivine + antigorite + magnetite → amphibole → talc → chlorite was formed at 0.9-1.2 GPa and 550-600°C. The low-temperature mesh lizardite-chrysotile serpentinization occurred in the crustal, near-surface conditions. Both types of hydration were accompanied by release of hydrogen, which participates in abiogenic CH4 synthesis in the presence of CO2 dissolved in water.

  3. Using Seismic Discontinuities to Image Melt and Dynamics in the Sub-Continental Upper Mantle

    NASA Astrophysics Data System (ADS)

    Schmerr, N. C.; Courtier, A. M.; Hier-Majumder, S.; Lekic, V.

    2014-12-01

    Continents are assembled from multiple Proterozoic and Archean terranes to form stable cratonic platforms with associated deformation typically localized to margins and/or rift zones. Successive episodes of subsequent extension, compression, magmatism, accretion, and rifting have left the sub-continental upper mantle with a complex signature of thermal and chemical heterogeneity. One key interest is the history of melt production, migration, and storage in sub-continental upper mantle as it provides a window into past and present dynamical processes, including the differentiation and formation of continental structure. Here we examine seismic discontinuities within the mantle that arise from a wide range of mechanisms, including changes in mineralogy, major element composition, melt content, volatile abundance, anisotropy, or a combination of the above. Using a dataset of broadband seismograms of underside reflected S-waves arriving as precursors to the seismic phase SS, we determine the depth and impedance contrast of discontinuities in the depth range of 80-410 km. Our observations are compared to predictions for the seismic moduli from a mineral physics database using the software MuMaP (Multiphase Material Properties). MuMaP modeling allows us to vary the average regional temperature, mantle composition and account for the effects of melt (if present). In our initial study of the western North American plate, we detect the presence of the 410 km discontinuity, a discontinuity at 300 km depth (X), and a G discontinuity at 60-80 km depth. The X is indicative of the coesite to stishovite phase transition in the upper mantle and suggests substantial mixing of subducted basalt with the mantle. The presence of the G may indicate partial melt in the asthenosphere, melt frozen into the lithosphere, and/or anisotropic fabrics preserved beneath the continent. These hypotheses are evaluated against MuMap predictions for melt content and anisotropic structure in the upper

  4. Seismic tomography of the Colorado Rocky Mountains upper mantle from CREST: Lithosphere-asthenosphere interactions and mantle support of topography

    NASA Astrophysics Data System (ADS)

    MacCarthy, J. K.; Aster, R. C.; Dueker, K.; Hansen, S.; Schmandt, B.; Karlstrom, K.

    2014-09-01

    The CREST experiment (Colorado Rocky Mountains Experiment and Seismic Transects) integrated the EarthScope USArray Transportable Array with portable and permanent stations to provide detailed seismic imaging of crust and mantle properties beneath the highest topography region of the Rocky Mountains. Inverting approximately 14,600 P- and 3600 S-wave arrival times recorded at 160 stations for upper mantle Vp and Vs structure, we find that large Vp perturbations relative to AK135 of 7% and Vs variations of 8% take place over very short (approaching tens of kilometers) lateral distances. Highest heterogeneity is observed in the upper 300 km of the mantle, but well resolved low velocity features extend to the top of the transition zone in portions of these images. The previously noted low velocity upper mantle Aspen Anomaly is resolved into multiple features. The lowest Vp and Vs velocities in the region are found beneath the San Juan Mountains, which is clearly distinguished from other low velocity features of the northern Rio Grande Rift, Taos/Latir region, Aspen region, and below the Never Summer Mountains. We suggest that the San Juan anomaly, and a similar feature below the Taos/Latir region of northern New Mexico, are related to delamination and remnant heat (and melt) beneath these sites of extraordinarily voluminous middle-Cenozoic volcanism. We interpret a northeast-southwest grain in velocity structure that parallels the Colorado Mineral belt to depths near 150 km as being reflective of control by uppermost mantle Proterozoic accretionary lithospheric architecture. Further to the north and west, the Wyoming province and northern Colorado Plateau show high velocity features indicative of thick (∼150 km) preserved Archean and Proterozoic lithosphere, respectively. Overall, we interpret the highly heterogeneous uppermost mantle velocity structure beneath the southern Rocky Mountains as reflecting interfingered chemical Proterozoic lithosphere that has been, is

  5. Model of the Arctic evolution since the Cretaceous to present, based on upper mantle convection linked with Pacific lithosphere subduction

    NASA Astrophysics Data System (ADS)

    Lobkovsky, Leopold

    2015-04-01

    The present paper comprises a model of Arctic basin evolution since early-mid Cretaceous to present. The model is based on the mechanism of upper mantle substance circulation beneath the Arctic lithosphere linked with Pacific lithosphere subduction. Seismic tomography data obtained for the Pacific-Eurasia-Arctic joint area indicate that Pacific lithosphere slab sinking to the mantle in subduction zone transforms into the horizontal layer upon reaching the upper mantle foot, this layer extending for two or more thousands km beneath the Eurasian continent. This pattern of seismic tomography indicates the presence of a horizontal convective cell where a flow of substance moving along the upper mantle foot from a subduction zone into the continent is compensated by a return flow moving along the lithosphere foot towards the subduction zone. The return mantle flow makes continental lithosphere extension, giving rise to processes of rifting, magmatism and spreading. The convective cell being continuously supplied with new substance which is transported through the subduction zone it is sure to expand horizontally. The above cell expansion occurs first, due to ocean ward movement of subduction zone (roll back) and secondly, due to the cell front propagation into the continent. The given model allows to understand main features for the Arctic evolution since early-mid Cretaceous to present. Numerous seismic profiling data obtained for shelf and deep water sedimentary basins in the Arctic Ocean as well as on land geological investigation reveal that since Aptian up to present the Arctic region has been characterized by sublatitudinal lithosphere extension. This extension is explained by the effect the return mantle flow related to the subduction of the Northern part of the Pacific plate acts on the Arctic lithosphere foot. The model shows the phenomenon of Arctic plume to be caused by the convective cell uprising flow. In fact lower horizontal flow of convective cell moving

  6. Structure and evolution of the lithospheric mantle beneath Siberian craton, thermobarometric study

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor V.; Pokhilenko, Nikolai P.; Vladykin, Nikolai V.; Logvinova, Alla M.; Afanasiev, Valentin P.; Pokhilenko, Lyudmila N.; Kuligin, Sergei S.; Malygina, Elena V.; Alymova, Natalia A.; Kostrovitsky, Sergey I.; Rotman, Anatolii Y.; Mityukhin, Sergey I.; Karpenko, Mikhail A.; Stegnitsky, Yuri B.; Khemelnikova, Olga S.

    2010-04-01

    70 kbar. Sub-continental lithospheric mantle (SCLM) beneath the Alakite field has been subjected to pervasive multistage metasomatism, as indicated by Fe-enriched Cr-diopsides and Ti-rich low-Ca garnets. Ilmenite PT trends were formed by rising protokimberlites that underwent AFC. In the Upper Muna field the mantle is similar in structure to that of the Alakite region. Fe-rich clinopyroxene-bearing rocks (60-55 kbar) are located between the ilmenite-forming systems (70-60 and 55-40 kbar), sub-Ca garnets start from 40 kbar and become more abundant downward. Beneath the Nakyn field, rhythmic layering is found for peridotites in the lower part ( P > 40 kbar), fertilization by Fe-Cpx (40-50 kbar) follow the Ilm-forming system ˜ 55-60 kbar correlating with the occurrence of depleted (low-Ca) peridotites. Beneath the Anabar fields highly depleted mantle at depth > 40 kbar has been subjected to Fe-metasomatism and pervasive metasomatism that accompanied protokimberlite feeders marked by low Cr-ilmenites accompanied by fertilization. In the upper section abundant garnet- and clinopyroxene-rich peridotites are typical. Comparison of mantle sections reconstructed from monomineral PT estimates from Paleozoic and Mesozoic kimberlites show differences in entrainment levels which were elevated after the Permian-Triassic superplume to > 55-40 kbar without delamination.

  7. Os-Nd-Sr isotopes in Miocene ultrapotassic rocks of southern Tibet: Partial melting of a pyroxenite-bearing lithospheric mantle?

    NASA Astrophysics Data System (ADS)

    Huang, Feng; Chen, Jian-Lin; Xu, Ji-Feng; Wang, Bao-Di; Li, Jie

    2015-08-01

    Miocene post-collisional ultrapotassic rocks in the southern and central parts of the Lhasa Terrane of southern Tibet provide an opportunity to explore the deep processes and lithospheric evolution of the Tibetan Plateau. The magmatic source of the ultrapotassic rocks is still debated. However, the source can be identified using the Re-Os isotopic system. In this paper, we provide comprehensive data on the Re-Os isotopic compositions of ultrapotassic rocks from Mibale and Maiga areas in southern Tibet, and we refine the age of the Mibale ultrapotassic rocks to 12.5 Ma. The Os isotopic data demonstrate that crustal assimilation affected the Os isotopic compositions of some ultrapotassic rocks with low Os contents, but samples with high Os contents have little or no evidence of crustal contamination. The initial 187Os/188Os ratios of the least-contaminated ultrapotassic rocks are higher than those of primitive upper mantle (PUM). The ultrapotassic rocks show a weak correlation between initial 187Os/188Os ratios and Mg# values, a negative correlation between εNd(t) and Mg# values, and high Ni contents and FeO/MnO ratios. These observations indicate that the ultrapotassic rocks were derived from a pyroxenite-bearing lithospheric mantle. Simple calculations indicate <20% pyroxenite in the lithospheric mantle, which is consistent with the pyroxenite xenoliths found in the ultrapotassic rocks of southern Tibet. The Os model ages for the ultrapotassic rocks in the south Lhasa Terrane range from 75 to 541 Ma, indicating that the lithospheric mantle beneath southern Tibet underwent multiple magmatic events. We conclude, therefore, that convective removal of a pyroxenite-bearing lithospheric mantle or break-off of the Indian continental lithospheric mantle could have resulted in the generation of the ultrapotassic rocks in southern Tibet.

  8. Upper mantle viscosity and lithospheric thickness under Iceland determined from a microphysical modelling approach of mantle rheology

    NASA Astrophysics Data System (ADS)

    Barnhoorn, A.; van der Wal, W.; Drury, M. R.

    2012-04-01

    The Vatnajökull glacier, located in the south-east of Iceland is the largest ice cap of Iceland having a mean radius of ~50 km covering an area of ˜8100 km2. The Vatnajökull glacier is situated directly on top of the spreading axis in the eastern volcanic zone (EVZ) of the Icelandic mid-ocean ridge and near the inferred center of the Icelandic hotspot. Due to the vicinity of the glacier to the active tectonic area, the response of the solid earth to melting of the ice cap is strongly controlled by the properties of the hot newly formed upper mantle underneath the mid-ocean ridge. The relatively high temperatures in the mantle during rifting result in relatively low upper mantle viscosities and fast relaxation times in comparison with tectonically inactive glaciated areas such as in. In this study, estimates for lithospheric thickness and upper mantle viscosity under Iceland are produced by a microphysical modelling approach using the theoretical temperature distribution under mid-ocean ridges combined with olivine diffusion and dislocation creep flow laws. Large lateral variations in upper mantle viscosity and especially lithospheric thickness are expected for Iceland perpendicular to the ridge axis due to the large changes in temperatures away from the ridge axis. The lithospheric thickness (27-40 km) and upper mantle viscosity (2 × 1018-1019 Pa s) outcomes for the recent glaciation are consistent with previous reports of viscosity and lithospheric thickness from glacial isostatic adjustment studies. A combination of a 40 km thick elastic lithosphere and an average upper mantle viscosity of 5 × 1018 Pa s would suggest that the upper mantle under Iceland is most likely dry. Also, the results indicate that the presence of a plume under Iceland cannot explain the recent low viscosity values reported for Iceland. Using a larger extent and larger thickness of the Icelandic icecap during the Weichselian glaciation event (˜10,000 BP) this study predicts that during

  9. Continental deformation and the mid-lithospheric discontinuity along the Grenville Front

    NASA Astrophysics Data System (ADS)

    Abrahams, L.; Long, M. D.; Ford, H. A.; Wirth, E. A.

    2015-12-01

    The existence of a mid-lithospheric discontinuity (MLD) within the stable continental mantle lithosphere of North America has been well established, but its interpretation remains difficult. Recent work with Ps receiver functions has found evidence for anisotropic structure at MLD depths within the western portion of the Granite-Rhyolite Province, suggesting that the MLD is the result of deformation associated with the formation of the continent. The last significant deformation to occur within the province was approximately 1.3-0.9 Ga and impacted the lithosphere east of the Grenville Front. In this study we analyzed six stations east of the front using Ps receiver functions in order to characterize anisotropy associated with the MLD in the region. Transverse and radial component Ps receiver functions were calculated for six stations (ACSO, BINY, ERPA, MCWV, SSPA, TZTN) using a multi-taper correlation technique and binned as a function of back azimuth and of epicentral distance. All six stations analyzed displayed significant positive phase energy on the radial component at ~6 seconds, which was interpreted as the Moho. At four of the six stations (ACSO, MCWV, SSPA, TZTN) the Moho showed moderate to significant complexity. At stations MCWV, SSPA, TZTN, all located along the Appalachian margin, there was significant transverse component energy at crustal depths with both two- and four-lobed anisotropy patterns observed. While ACSO, BINY, and ERPA displayed evidence of isotropic and/ or anisotropic crustal structure, a coherent pattern in back azimuth could not be established. The radial component receiver functions also exhibited negative phase energy, interpreted as the MLD, between 7.5 and 12.5 seconds (or ~80 to 120 km), at five of the six stations, with the exception of BINY, where no negative phase was observed. The transverse component receiver functions at stations ACSO, ERPA, SSPA and TZTN, also displayed a two-lobed pattern in back azimuth at MLD depths

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

    PubMed

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

    2002-11-21

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

  11. Thickening, refertilization, and the deep lithosphere filter in continental arcs: Constraints from major and trace elements and oxygen isotopes

    NASA Astrophysics Data System (ADS)

    Chin, Emily J.; Lee, Cin-Ty A.; Barnes, Jaime D.

    2014-07-01

    Arc magmatism is a complex process involving generation of primary melts in the mantle wedge and chemical refinement of these melts into differentiated products akin to continental crust. Interaction of magmas (cooling, crystallization and assimilation) with the overlying crust, particularly if it is thick, is one way by which primary basalts are refined into more evolved compositions. Here, we explore the role of the mantle lithosphere as a trap and/or reactive filter of magmas. We use mantle xenoliths from the Sierra Nevada continental arc in California as a probe into sub-Moho processes. Based on clinopyroxene modal abundance and major, minor and moderately incompatible trace element concentrations, the peridotites define a refertilization trend that increases with depth, grading from clinopyroxene-poor (<5%), undeformed spinel peridotites equilibrated at <3 GPa (<90 km) to clinopyroxene-rich (10-20%), porphyroclastic garnet peridotites equilibrated between 3 and 3.5 GPa (90-105 km), the latter presumably approaching the top of the subducting slab. The petrology and geochemistry of the xenoliths suggest that the fertile peridotites were originally depleted spinel peridotites, which were subsequently refertilized. Incompatible trace element geochemistry reveals a pervasive cryptic metasomatic overprint in all peridotites, suggesting involvement of small amounts of subduction-derived fluids from the long-lived Farallon plate beneath western North America. However, bulk reconstructed δOSMOW18 values of the peridotites, including the most refertilized, fall between 5.4 and 5.9‰, within the natural variability of unmetasomatized mantle (∼5.5±0.2‰). Together with Sm, Yb, and Ca compositional data, the oxygen isotope data suggest that the role of slab or sediment melts in refertilizing the peridotites was negligible (<5% in terms of added melt mass). Instead, binary mixing models suggest that many of the Sierran garnet peridotites, particularly those with high

  12. Osmium isotopic evidence for mesozoic removal of lithospheric mantle beneath the sierra nevada, california

    PubMed

    Lee; Yin; Rudnick; Chesley; Jacobsen

    2000-09-15

    Thermobarometric and Os isotopic data for peridotite xenoliths from late Miocene and younger lavas in the Sierra Nevada reveal that the lithospheric mantle is vertically stratified: the shallowest portions (<45 to 60 kilometers) are cold (670 degrees to 740 degrees C) and show evidence for heating and yield Proterozoic Os model ages, whereas the deeper portions (45 to 100 kilometers) yield Phanerozoic Os model ages and show evidence for extensive cooling from temperatures >1100 degrees C to 750 degrees C. Because a variety of isotopic evidence suggests that the Sierran batholith formed on preexisting Proterozoic lithosphere, most of the original lithospheric mantle appears to have been removed before the late Miocene, leaving only a sliver of ancient mantle beneath the crust. PMID:10988067

  13. Osmium isotopic evidence for mesozoic removal of lithospheric mantle beneath the sierra nevada, california

    PubMed

    Lee; Yin; Rudnick; Chesley; Jacobsen

    2000-09-15

    Thermobarometric and Os isotopic data for peridotite xenoliths from late Miocene and younger lavas in the Sierra Nevada reveal that the lithospheric mantle is vertically stratified: the shallowest portions (<45 to 60 kilometers) are cold (670 degrees to 740 degrees C) and show evidence for heating and yield Proterozoic Os model ages, whereas the deeper portions (45 to 100 kilometers) yield Phanerozoic Os model ages and show evidence for extensive cooling from temperatures >1100 degrees C to 750 degrees C. Because a variety of isotopic evidence suggests that the Sierran batholith formed on preexisting Proterozoic lithosphere, most of the original lithospheric mantle appears to have been removed before the late Miocene, leaving only a sliver of ancient mantle beneath the crust.

  14. Superplumes from the core-mantle boundary to the lithosphere: implications for heat flux.

    PubMed

    Romanowicz, Barbara; Gung, Yuancheng

    2002-04-19

    Three-dimensional modeling of upper-mantle anelastic structure reveals that thermal upwellings associated with the two superplumes, imaged by seismic elastic tomography at the base of the mantle, persist through the upper-mantle transition zone and are deflected horizontally beneath the lithosphere. This explains the unique transverse shear wave isotropy in the central Pacific. We infer that the two superplumes may play a major and stable role in supplying heat and horizontal flow to the low-viscosity asthenospheric channel, lubricating plate motions and feeding hot spots. We suggest that more heat may be carried through the core-mantle boundary than is accounted for by hot spot fluxes alone.

  15. Linking mantle upwelling with the lithosphere descent [corrected] and the Japan Sea evolution: a hypothesis.

    PubMed

    Ismail-Zadeh, Alik; Honda, Satoru; Tsepelev, Igor

    2013-01-01

    Recent seismic tomography studies image a low velocity zone (interpreted as a high temperature anomaly) in the mantle beneath the subducting Pacific plate near the Japanese islands at the depth of about 400 km. This thermal feature is rather peculiar in terms of the conventional view of mantle convection and subduction zones. Here we present a dynamic restoration of the thermal state of the mantle beneath this region assimilating geophysical, geodetic, and geological data up to 40 million years. We hypothesise that the hot mantle upwelling beneath the Pacific plate partly penetrated through the subducting plate into the mantle wedge and generated two smaller hot upwellings, which contributed to the rapid subsidence in the basins of the Japan Sea and to back-arc spreading. Another part of the hot mantle migrated upward beneath the Pacific lithosphere, and the presently observed hot anomaly is a remnant part of this mantle upwelling.

  16. Linking mantle upwelling with the lithosphere decent and the Japan Sea evolution: a hypothesis

    PubMed Central

    Ismail-Zadeh, Alik; Honda, Satoru; Tsepelev, Igor

    2013-01-01

    Recent seismic tomography studies image a low velocity zone (interpreted as a high temperature anomaly) in the mantle beneath the subducting Pacific plate near the Japanese islands at the depth of about 400 km. This thermal feature is rather peculiar in terms of the conventional view of mantle convection and subduction zones. Here we present a dynamic restoration of the thermal state of the mantle beneath this region assimilating geophysical, geodetic, and geological data up to 40 million years. We hypothesise that the hot mantle upwelling beneath the Pacific plate partly penetrated through the subducting plate into the mantle wedge and generated two smaller hot upwellings, which contributed to the rapid subsidence in the basins of the Japan Sea and to back-arc spreading. Another part of the hot mantle migrated upward beneath the Pacific lithosphere, and the presently observed hot anomaly is a remnant part of this mantle upwelling. PMID:23355951

  17. Water Distribution in the Continental and Oceanic Upper Mantle

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.

    2015-01-01

    Nominally anhydrous minerals such as olivine, pyroxene and garnet can accommodate tens to hundreds of ppm H2O in the form of hydrogen bonded to structural oxygen in lattice defects. Although in seemingly small amounts, this water can significantly alter chemical and physical properties of the minerals and rocks. Water in particular can modify their rheological properties and its distribution in the mantle derives from melting and metasomatic processes and lithology repartition (pyroxenite vs peridotite). These effects will be examined here using Fourier transform infrared spectrometry (FTIR) water analyses on minerals from mantle xenoliths from cratons, plume-influenced cratons and oceanic settings. In particular, our results on xenoliths from three different cratons will be compared. Each craton has a different water distribution and only the mantle root of Kaapvaal has evidence for dry olivine at its base. This challenges the link between olivine water content and survival of Archean cratonic mantle, and questions whether xenoliths are representative of the whole cratonic mantle. We will also present our latest data on Hawaii and Tanzanian craton xenoliths which both suggest the intriguing result that mantle lithosphere is not enriched in water when it interacts with melts from deep mantle upwellings (plumes).

  18. Re-Os systematics of the lithospheric mantle beneath the Western Ross Sea area, Antarctica: depletion ages and dynamic response during rifting

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    The West Antarctic Rift System (WARS) is situated between the East Antarctic craton and Marie Byrd Land. Seismic studies on the structure of the lithosphere beneath the WARS reveal thinned lithosphere [1] with crustal thickness ranging from 16 to 22 km in the Ross Sea basin [2,3] that is underlain by a low velocity zone at 80-200 km [4]. However, seismic studies alone provide little information on the age of the lithospheric mantle or its fate during rifting and the formation of the WARS. Geochemical studies on lithosphere surrounding Archean cratons have demonstrated the persistence of off-craton Proterozoic lithosphere and potentially Archean lithosphere (e.g. southeast Australia and southern Africa) [5,6], and suggest that it is possible to constrain the age and structure of the lithosphere in the WARS. Os isotope ratios can be used to date the melt depletion events in the asthenosphere that are considered to be equivalent to the stabilization age of the lithospheric mantle [7]. Here we present the first Re-Os isotope measurements on mantle xenoliths from 5.0 to <1.0 Ma-old volcanic rocks collected in a transection from the rift shoulder and into the rift basin in the Western Ross Sea area of the WARS, and suggest that these data can be used to examine the dynamic response of the lithosphere to rifting. For example, ancient Re-depletion ages across this margin could indicate thinning of the lithospheric mantle during continental extension and dynamic extension of the lithospheric mantle beneath the rift basin. In contrast, younger ages might suggest a more complex history or possibly the replacement by asthenosphere as a result of lithospheric delamination during rifting. Our 187Os/188Os isotope ratios show a large range throughout the rifted margin (0.1051 at Foster Crater to 0.1265 on Ross Island), yet define individual melt depletion trends at 7 locations across the rift. Alumachron model ages derived from 187Os/188Os vs. Al2O3 wt% depletion trends reveal

  19. Introducing tectonically and thermo-mechanically realistic lithosphere in the models of plume head -lithosphere interactions (PLI) including intra-continental plate boundaries.

    NASA Astrophysics Data System (ADS)

    Guillou-Frottier, L.; Burov, E.; Cloetingh, S.

    2007-12-01

    Plume-Lithosphere Interactions (PLI) in continets have complex topographic and magmatic signatures and are often identified near boundaries between younger plates (e.g., orogenic) and older stable plates (e.g., cratons), which represent important geometrical, thermal and rheological barriers that interact with the emplacement of the plume head (e.g., Archean West Africa, East Africa, Pannonian - Carpathian system). The observable PLI signatures are conditioned by plume dynamics but also by complex rheology and structure of continental lithosphere. We address this problem by considering a new free-surface thermo-mechanical numerical model of PLI with two stratified elasto-viscous-plastic (EVP) continental plates of contrasting age, thickness and structure. The results show that: (1) surface deformation is poly-harmonic and contains smaller wavelengths (50-500 km) than that associated with the plume head (>1000 km). (2) below intra-plate boundaries, plume head flattening is asymmetric, it is blocked from one side by the cold vertical boundary of the older plate, which leads to mechanical decoupling of crust from mantle lithosphere, and to localized faulting at the cratonic margin; (2) the return flow from the plume head results in sub-vertical down-thrusting (delamination) of the lithosphere at the margin, producing sharp vertical cold boundary down to the 400 km depth; (3) plume head flattening and migration towards the younger plate results in concurrent surface extension above the centre of the plume and in compression (pushing), down-thrusting and magmatic events at the cratonic margin (down-thrusting is also produced at the opposite border of the younger plate); these processes may result in continental growth at the "craton side"; (4) topographic signatures of PLI show basin-scale uplifts and subsidences preferentially located at cratonic margins. Negative Rayleigh-Taylor instabilities in the lithosphere above the plume head provide a mechanism for crustal

  20. Lithospheric mantle heterogeneities beneath the Zagros Mountains and the Iranian Plateau: a petrological-geophysical study

    NASA Astrophysics Data System (ADS)

    Tunini, Lavinia; Jiménez-Munt, Ivone; Fernandez, Manel; Vergés, Jaume; Villaseñor, Antonio

    2014-01-01

    We apply a combined geophysical-petrological methodology in order to study the thermal, compositional, density and seismological structure of the crust and upper mantle along two transects across the Arabia-Eurasia collision region. Results on the crustal thickness show minimum values beneath the Arabia Platform and Central Iran (42-43 km), and maximum values beneath the Sanandaj Sirjan zone (SSZ; 55-63 km), in agreement with seismic data. Major discrepancies in Moho depth from those derived from seismic data are locally found in the SSZ (central Zagros) and Alborz Mountains where more moderate crustal thicknesses are modelled. Results on the lithosphere thickness indicate that the Arabian lithosphere is ˜220 km thick along both profiles, whereas Eurasian lithosphere is up to ˜90 km thinner, especially below the Central Iran and Alborz Mountains. The lithosphere-asthenosphere boundary (LAB) shows different geometries between the two transects. In the northern profile (northern Zagros), the LAB rises sharply below the SSZ in a narrow region of ˜90 km, whereas in the southern profile (central Zagros), rising occurs in wider region, from the Zagros fold-and-thrust belt (ZFTB) to the SSZ. The best fit of seismic velocities (Vp, Vs) and densities requires lateral changes in the lithospheric mantle composition. Our results are compatible with Proterozoic peridotitic mantle compositions beneath the Arabian Platform, Mesopotamian Foreland Basin and the accreted terrains of Eurasia Plate, and with a more depleted Phanerozoic harzburgitic-type mantle composition below the ZFTB and imbricated zone.

  1. 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

  2. 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

  3. 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

    2014-03-04

    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.

  4. Peculiarities of mantle lithosphere beneath the large kimberlite pipes in different regions for Siberian craton

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Logvinova, Alla; Ntaflos, Theodoros; Vladykin, Nikolai; Spetsius, Zdislav; Kostrovitsky, Sergey; Stegnitsky, Yuri; Prokopyev, Sergey

    2016-04-01

    Comparison of the structure of the mantle columns and mineralogy of the large kimberlite pipes in Yakutia from the different regions, kimberlite fields and mantle terranes in Yakutia allowed several assumptions. 1. The large kimberlite pipes possibly trace the ancient magma feeders occurred in the time of the continent growth. Commonly kimberlites and large pipes are tracing the deep faults and lineaments tracing the ancient sutures, rift zones, trans -lithospheric faults and other permeable structures, which may be parallel to the ancient continental margins. Large pipes locate at the periodic distance like volcanoes in arc settings tracing the "volcanic fronts". 2. Large pipes commonly contain the higher amounts of the sub-calcic garnets representing the dunitic associations (Stachel et al., 2008). In ophiolites dunites veins are representing the channels for the melt transfer (Kelemen et al., 2002). It is likely that ancient large magmatic arc system could have also deep seated roots represented by the (sub calcic) garnet - bearing dunitic systems. 3. Many large pipes including Udachnaya (Pokhilenko et al., 1999) and Mir (Roden et al., 2006) contain in mantle roots high amount of various pyroxenites. The most ancient pyroxenites are supplementary to the dunitic associations. But mostly they represent the materials from the re-melted eclogites and partial and hybrid melts (plume and subduction -related). They are concentrating in the traps in the lithosphere base, in the middle part of mantle section and in the basaltic trap 2.0-3.0 GPa. Pyroxenites in the lithosphere base in some cases are vary abundant but mostly they are protokimberlitic cumulates from of the latest stages of plume activity. Products of the melts crystallization from the earlier stages represent easy melting material at the lithosphere base could be the traps for the later plume melts. 5. Large pipes as a rule reveal contrast layering which is favorite for the capturing of the material from

  5. Peculiarities of mantle lithosphere beneath the large kimberlite pipes in different regions for Siberian craton

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Logvinova, Alla; Ntaflos, Theodoros; Vladykin, Nikolai; Spetsius, Zdislav; Kostrovitsky, Sergey; Stegnitsky, Yuri; Prokopyev, Sergey

    2016-04-01

    Comparison of the structure of the mantle columns and mineralogy of the large kimberlite pipes in Yakutia from the different regions, kimberlite fields and mantle terranes in Yakutia allowed several assumptions. 1. The large kimberlite pipes possibly trace the ancient magma feeders occurred in the time of the continent growth. Commonly kimberlites and large pipes are tracing the deep faults and lineaments tracing the ancient sutures, rift zones, trans -lithospheric faults and other permeable structures, which may be parallel to the ancient continental margins. Large pipes locate at the periodic distance like volcanoes in arc settings tracing the "volcanic fronts". 2. Large pipes commonly contain the higher amounts of the sub-calcic garnets representing the dunitic associations (Stachel et al., 2008). In ophiolites dunites veins are representing the channels for the melt transfer (Kelemen et al., 2002). It is likely that ancient large magmatic arc system could have also deep seated roots represented by the (sub calcic) garnet - bearing dunitic systems. 3. Many large pipes including Udachnaya (Pokhilenko et al., 1999) and Mir (Roden et al., 2006) contain in mantle roots high amount of various pyroxenites. The most ancient pyroxenites are supplementary to the dunitic associations. But mostly they represent the materials from the re-melted eclogites and partial and hybrid melts (plume and subduction -related). They are concentrating in the traps in the lithosphere base, in the middle part of mantle section and in the basaltic trap 2.0-3.0 GPa. Pyroxenites in the lithosphere base in some cases are vary abundant but mostly they are protokimberlitic cumulates from of the latest stages of plume activity. Products of the melts crystallization from the earlier stages represent easy melting material at the lithosphere base could be the traps for the later plume melts. 5. Large pipes as a rule reveal contrast layering which is favorite for the capturing of the material from

  6. 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

  7. Recycling and transport of continental material through the mantle wedge above subduction zones: A Caribbean example

    NASA Astrophysics Data System (ADS)

    Rojas-Agramonte, Yamirka; Garcia-Casco, Antonio; Kemp, Anthony; Kröner, Alfred; Proenza, Joaquín A.; Lázaro, Concepción; Liu, Dunyi

    2016-02-01

    Estimates of global growth rates of continental crust critically depend upon knowledge of the rate at which crustal material is delivered back into the mantle at subduction zones and is then returned to the crust as a component of mantle-derived magma. Quantification of crustal recycling by subduction-related magmatism relies on indirect chemical and isotopic tracers and is hindered by the large range of potential melt sources (e.g., subducted oceanic crust and overlying chemical and clastic sediment, sub-arc lithospheric mantle, arc crust), whose composition may not be accurately known. There is also uncertainty about how crustal material is transferred from subducted lithosphere and mixed into the mantle source of arc magmas. We use the resilient mineral zircon to track crustal recycling in mantle-derived rocks of the Caribbean (Greater Antilles) intra-oceanic arc of Cuba, whose inception was triggered after the break-up of Pangea. Despite juvenile Sr and Nd isotope compositions, the supra-subduction zone ophiolitic and volcanic arc rocks of this Cretaceous (∼135-70 Ma) arc contain old zircons (∼200-2525 Ma) attesting to diverse crustal inputs. The Hf-O isotope systematics of these zircons suggest derivation from exposed crustal terranes in northern Central America (e.g. Mexico) and South America. Modeling of the sedimentary component in the most mafic lavas suggests a contribution of no more than 2% for the case of source contamination or less than 4% for sediment assimilation by the magma. We discuss several possibilities for the presence of inherited zircons and conclude that they were transported as detrital grains into the mantle beneath the Caribbean Plate via subduction of oceanic crust. The detrital zircons were subsequently entrained by mafic melts that were rapidly emplaced into the Caribbean volcanic arc crust and supra-subduction mantle. These findings suggest transport of continental detritus, through the mantle wedge above subduction zones, in

  8. Initiation and propagation of shear zones in a heterogeneous continental lithosphere

    SciTech Connect

    Tommasi, A.; Vauchez, A.

    1995-11-10

    Numerical methods were used to investigate the deformation of a continental plate in northeastern Brazil. Of particular interest are the perturbations induced by a stiff compressional deformation of a highly heterogeneous continental lithosphere on the development of a shear zone formed at the termination of a stiff block.

  9. Metasomatic Enrichment of the Lithosphere and its Potential Implications for the Formation of Oceanic and Continental Alkaline Magmas

    NASA Astrophysics Data System (ADS)

    Pilet, S.; Baker, M. B.; Stolper, E. M.; Muntener, O.

    2009-04-01

    The generation of oceanic and continental intra-plate magmas implied that the source of these magmas was enriched regarding the primitive mantle [1]. However, the nature and origin of the mantle components that melt below oceanic islands are still in debate. A hypothesis proposed that the enriched components which melt below oceanic islands correspond to recycled oceanic crust [2]. Alternatives suggest that these components could correspond to metasomatized continental or oceanic lithosphere [3-5]. However, if these two hypotheses are frequently presented as mutually exclusive, we suggest that theses hypotheses could be complementary. The fact that oceanic crust produces silica-saturated partial melts seems in agrement with the implication of this material in the generation of tholeiitic (i.e., hy- and qtz- normative) magmas from large oceanic islands and continental lava flows as proposed by [6]; however this fact makes it difficult to envision oceanic crust as a major component in the generation of alkaline (i.e., ne-normative) magmas. Experiments on metasomatic veins (hornblendites) and their dehydrated equivalents demonstrate that high-degree melting of these veins followed by variable amounts of interaction of the liquid with surrounding mantle can reproduce key features of the major- and trace-element compositions of alkaline magmas [7]. We suggest two scenarios for the production of alkaline magmas by melting metasomatized lithosphere: (i) the metasomatized lithosphere experiences a thermal perturbation or decompression and thereby melts in situ; or (ii) the metasomatized lithosphere is recycled into the convecting mantle by subduction or delamination and melts during later upwelling (e.g., in a plume). In continental alkaline magmas, the presence of amphibole xenocrysts compositionally similar to amphibole in metasomatic veins is consistent with the "in situ" hypothesis. While such veins may play a role in alkaline magmas for some oceanic islands and

  10. Mantle Discontinuities and the Origins of the U.S. Cratonic Lithosphere

    NASA Astrophysics Data System (ADS)

    Fischer, K. M.; Hopper, E.

    2014-12-01

    The goals of this work are to probe how mantle lithosphere discontinuity structure varies beneath the cratonic terranes of the northern U.S. and to relate this structure to the processes that created and modified the cratonic mantle. Our region samples the Archean Wyoming, Medicine Hat and Superior cratons, and the Proterozoic terranes that lie between them. We imaged the mantle using Sp phases recorded by permanent and temporary seismic networks, including EarthScope's Transportable Array. Sp receiver functions for individual waveforms were obtained by extended time multi-taper deconvolution, and migrated into a 3D volume using common conversion point stacking, a spline representation of phase Fresnel zones, and 3D models for crust and mantle structure. The stack was bootstrapped. In the cratonic mantle, we observe multiple mid-lithospheric discontinuities (MLDs) that are characterized by three types of structures: a relatively continuous negative discontinuity (velocity decrease with depth) that lies in the 65-100 km depth range; deeper negative MLDs (80-145 km) that are more discontinuous and intermittent; and occasional positive MLDs at the greatest depths (>125 km). In contrast to the tectonically active western U.S., beneath cratonic regions we typically do not observe a strong negative discontinuty at the base of the tomographically-defined lithosphere, indicating that the transition to asthenospheric properties is gradual. The MLDs indicate strong layering in the cratonic mantle lithosphere. In multiple cases, one negative discontinuity dips below another, consistent with a slab of lithosphere imbricated beneath pre-existing cratonic mantle. One of the clearest examples is a north-dipping phase at depths of 80-130 km beneath the Cheyenne Belt, the suture between the Wyoming Craton and the accreted Proterozoic terranes to its south. In Sept. 2013, an unusual earthquake occurred within the high velocity mantle of the Wyoming craton at ~76 km, a depth that

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

    NASA Astrophysics Data System (ADS)

    Sodoudi, Forough; Kind, Rainer

    2014-05-01

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

  12. Mantle discontinuities and temperature under the North American continental keel

    NASA Astrophysics Data System (ADS)

    Li, Aibing; Fischer, Karen M.; Wysession, Michael E.; Clarke, Timothy J.

    1998-09-01

    A ubiquitous feature of upper-mantle seismic velocity models has been the presence of high-velocity `keels' beneath stable continental interiors. Uncertainty remains, however, regarding the maximum depth to which continental keels extend, the degree to which they have cooled the mantle that surrounds them and their role in mantle flow. Here we investigate thermal anomalies across the eastern margin of the North American continental keel by imaging the seismic discontinuities at depths of 410 and 660km with compressional-to-shear converted waves recorded by a 1,500-km-long seismometer deployment in the eastern United States. The thickness of the transition zone (the region nominally between depths of 410 and 660km) and the depth to the `410-km' discontinuity indicate that cold keel material and sub-keel downwellings must be largely confined to the upper mantle and may impinge on the transition zone only in localized regions and with thermal anomalies of less than ~150K. A 20-km depression of the `660-km' discontinuity to the south of the westernmost stations coincides with a region of fast velocity in the deep transition zone and may be associated with the remnants of the subducted Farallon plate,,.

  13. Lithospheric mantle structure and the diamond potential of kimberlites in southern D.R. Congo

    NASA Astrophysics Data System (ADS)

    Batumike, J. M.; Griffin, W. L.; O'Reilly, S. Y.

    2009-11-01

    Mantle-derived peridotitic garnet xenocrysts from kimberlites in the Mbuji Mayi and Kundelungu areas and from heavy-mineral concentrates collected in the Luebo area, D.R. Congo, have been analysed for major- and trace-element compositions in order to understand the structure and composition of the subcontinental lithospheric mantle (SCLM) and the diamond potential of the kimberlites. The lithosphere beneath the Kundelungu Plateau is ca 175 km thick and has been affected by pronounced melt metasomatism. Garnets from the Kundelungu Plateau indicate an initially cool geotherm (~ 35 mW/m 2), which was disturbed by asthenospheric melts that penetrated the SCLM shortly before kimberlite intrusion ca 32 Ma ago. Harzburgitic garnets are very rare, but some lherzolitic garnets display compositions similar to garnets included in diamond. Garnets from the Mbuji Mayi region indicate a cool geotherm (35 mW/m 2); the SCLM is ~ 210 km thick and was affected by melt-related and phlogopite-related metasomatisms. Harzburgitic garnets form about 33% of the analysed population. The garnets from the Luebo region indicate a cool lithospheric geotherm (35 mW/m 2) typical of cratonic areas. The SCLM from which the garnets were derived was relatively thick (205 km), affected by melt-related and phlogopite-related metasomatisms and characterised by the presence of a ~ 80-km thick harzburgite-rich layer. In terms of peridotitic diamond potential, Mbuji Mayi and Luebo are more prospective than Kundelungu. The initially cool conductive geotherm, the presence of some garnets with compositions similar to garnets included in diamond and the presence of sporadic diamond in the Kundelungu Plateau suggest that diamond initially was present in the lithosphere and the observed paucity of diamond may be due to the melt-related metasomatism that affected the lithosphere in the region. We suggest that the lithospheric mantle beneath Kundelungu is a strongly modified Archean cratonic lithosphere that has

  14. Olivine water contents in the continental lithosphere and the longevity of cratons.

    PubMed

    Peslier, Anne H; Woodland, Alan B; Bell, David R; Lazarov, Marina

    2010-09-01

    Cratons, the ancient cores of continents, contain the oldest crust and mantle on the Earth (>2 Gyr old). They extend laterally for hundreds of kilometres, and are underlain to depths of 180-250 km by mantle roots that are chemically and physically distinct from the surrounding mantle. Forming the thickest lithosphere on our planet, they act as rigid keels isolated from the flowing asthenosphere; however, it has remained an open question how these large portions of the mantle can stay isolated for so long from mantle convection. Key physical properties thought to contribute to this longevity include chemical buoyancy due to high degrees of melt-depletion and the stiffness imparted by the low temperatures of a conductive thermal gradient. Geodynamic calculations, however, suggest that these characteristics are not sufficient to prevent the lithospheric mantle from being entrained during mantle convection over billions of years. Differences in water content are a potential source of additional viscosity contrast between cratonic roots and ambient mantle owing to the well-established hydrolytic weakening effect in olivine, the most abundant mineral of the upper mantle. However, the water contents of cratonic mantle roots have to date been poorly constrained. Here we show that olivine in peridotite xenoliths from the lithosphere-asthenosphere boundary region of the Kaapvaal craton mantle root are water-poor and provide sufficient viscosity contrast with underlying asthenosphere to satisfy the stability criteria required by geodynamic calculations. Our results provide a solution to a puzzling mystery of plate tectonics, namely why the oldest continents, in contrast to short-lived oceanic plates, have resisted recycling into the interior of our tectonically dynamic planet. PMID:20811455

  15. Olivine water contents in the continental lithosphere and the longevity of cratons.

    PubMed

    Peslier, Anne H; Woodland, Alan B; Bell, David R; Lazarov, Marina

    2010-09-01

    Cratons, the ancient cores of continents, contain the oldest crust and mantle on the Earth (>2 Gyr old). They extend laterally for hundreds of kilometres, and are underlain to depths of 180-250 km by mantle roots that are chemically and physically distinct from the surrounding mantle. Forming the thickest lithosphere on our planet, they act as rigid keels isolated from the flowing asthenosphere; however, it has remained an open question how these large portions of the mantle can stay isolated for so long from mantle convection. Key physical properties thought to contribute to this longevity include chemical buoyancy due to high degrees of melt-depletion and the stiffness imparted by the low temperatures of a conductive thermal gradient. Geodynamic calculations, however, suggest that these characteristics are not sufficient to prevent the lithospheric mantle from being entrained during mantle convection over billions of years. Differences in water content are a potential source of additional viscosity contrast between cratonic roots and ambient mantle owing to the well-established hydrolytic weakening effect in olivine, the most abundant mineral of the upper mantle. However, the water contents of cratonic mantle roots have to date been poorly constrained. Here we show that olivine in peridotite xenoliths from the lithosphere-asthenosphere boundary region of the Kaapvaal craton mantle root are water-poor and provide sufficient viscosity contrast with underlying asthenosphere to satisfy the stability criteria required by geodynamic calculations. Our results provide a solution to a puzzling mystery of plate tectonics, namely why the oldest continents, in contrast to short-lived oceanic plates, have resisted recycling into the interior of our tectonically dynamic planet.

  16. Rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones

    USGS Publications Warehouse

    Kirby, S.H.

    1985-01-01

    Emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures, conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones. Much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock-mechanics-based yield criteria for the inelastic deformations at the top and base. At mid-plate depths, stresses are likely to be supported elastically because bending strains and elastic stresses are low. The collisional tectonic regime, however, is far more complex because very large permanent strains are sustained at mid-plate depths and this requires us to include the broad transition between brittle and ductile flow. Moreover, important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid-plate depths. Two specific contributions of laboratory rock rheology research are considered in this paper. First, the high-temperature steady-state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks. Rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere. The thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm Tc above which (at geologic strain rates) the high-temperature ductile strength falls below some arbitrary strength isobar (e.g., 100 MPa). For olivine Tc is about 700??-800??C but for other crustal silicates, Tc may be as low as 400??-600??C, suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the Moho, as suggested by a number of workers on independent grounds. Put another way, the Moho is a rheological discontinuity. A second class of laboratory observations pertains to

  17. Does the "mantle" helium signature provide useful information about lithospheric architecture of Tibet/Himalaya?

    NASA Astrophysics Data System (ADS)

    Klemperer, S. L.; Liu, T.; Hilton, D. R.; Karlstrom, K. E.; Crossey, L. J.; Zhao, P.

    2015-12-01

    Measurements of 3He/4He > 0.1*Ra (where Ra = 3He/4He in Earth's atmosphere) in geothermal fluids are conventionally taken to represent derivation from a mantle source. 3He/4He values < 0.1*Ra are taken to represent only radiogenic helium with no modern mantle input (the canonical 3He/4He ratio for the crust is 0.02*Ra). Upward transport rates are hard to constrain, but transit times of 3He through the crust in a CO2-rich carrier fluid may be as short as a few years, so 3He/4He measurements offer a proxy for mantle temperature on geologically short time-scales. In Tibet, enhanced 3He/4He ratios could in principle represent (1) incipient partial melt of Indian lithospheric mantle; (2) of Asian lithospheric mantle; (3) upwelling asthenosphere north of underthrust India or along tears in the subducting Indian plate; and/or (4) high-T prograde metamorphism releasing previously trapped 3He from older, voluminous mafic/ultramafic rocks in the crust. We present data from our recent field campaigns and our compilations from the western and Chinese literature. Any individual observation of 3He/4He > 0.1*Ra may still be argued to result from mantle-derived 3He previously stored in the crust. However, our growing regional database of widely spaced observations of 3He/4He > 0.1*Ra, from the Karakoram Fault in the west to the Sangri-Cona rift and Yalaxiangbo Dome in the east, and from south of the Yarlung-Zangbo suture (YZS) to north of the Banggong-Nujiang suture, makes such special pleading increasingly implausible. The observation of 3He/4He > 0.1*Ra at the YZS and even within the Tethyan Himalaya south of the YZS cannot represent melting of Indian mantle close to the Moho unless existing thermal models are grossly in error. The source of 3He close to the YZS is likely either asthenosphere accessed by faults and shear zones that cut through subducting Indian lithospheric mantle; or incipient melt of Asian lithospheric mantle at the Moho north of the northern edge of

  18. Lithospheric deformation induced by loading of the Hawaiian Islands and its implications for mantle rheology

    NASA Astrophysics Data System (ADS)

    Zhong, Shijie; Watts, A. B.

    2013-11-01

    long-term rheological properties of the lithosphere are fundamental for understanding both surface tectonics and mantle dynamics on Earth. In this study, we have developed 3-D finite element models for computing the load-induced surface deformation and stress for lithosphere and mantle with realistic nonlinear viscoelastic rheology including the frictional sliding, low-temperature plasticity, and high-temperature creep. We have determined the lithospheric deformation and stress due to volcano loading in the Hawaiian Islands region for the last few million years. By comparing model predictions with seismic observations of the depth to the top of oceanic crust and depth dependence of seismicity in the Hawaiian Islands region, we have sought to constrain lithospheric rheology. Our calculations show that the load-induced surface deformation is controlled by low-temperature plasticity and frictional sliding but is insensitive to high-temperature creep. Lithospheric strength predicted from laboratory-derived low-temperature plasticity needs to be reduced significantly, and a frictional coefficient μf ranging from 0.1 to 0.7 is required in order to account for the observations. However, μf = 0.1 weakens the shallow part of the lithosphere so much that it causes the minima in strain rate and stress to occur at too large depths to be consistent with the observed depth distribution of seismicity. Our results therefore suggest a value for μf between 0.25 and 0.7. Finally, the maximum stress that accumulates in the deformed lithosphere beneath the Hawaiian Islands is about 100-200 MPa for models that match the observations, and this stress may be viewed as the largest lithospheric stress on Earth.

  19. Investigation of lithospheric deformation and mantle anisotropy beneath Central Anatolia from Shear Wave Splitting Analysis

    NASA Astrophysics Data System (ADS)

    Teoman, U.; Polat, G.; Sandvol, E. A.; Turkelli, N.; Kahraman, M.; Özacar, A.; Beck, S. L.; Delph, J. R.

    2015-12-01

    With the primary objective of investigating the upper mantle anisotropy beneath central Anatolia-Turkey, we have performed shear wave splitting analysis and calculated the fast polarization directions and time-delays benefiting from teleseismic earthquakes recorded by a dense temporary seismic network consisting of 65 broadband sensors that were deployed in early May 2013 and operated for two years as a part of CD-CAT project (Continental Dynamics Central Anatolian Tectonics, funded by NSF with instruments supplied by PASSCAL depository). To further enhance the station coverage in the region, we also included data from 45 permanent broadband stations of Kandilli Observatory and Earthquake Research Institute (KOERI). During the analysis, we have used the SplitLab software to determine splitting parameters of the records from only SKS and SKKS phases. Our initial results were derived from teleseismic earthquakes (with magnitudes greater than 5.8) that occurred within the time period from May-2013 to 2014. The average fast polarization directions obtained from stations located in the vicinity of the East Anatolia Fault Zone are well aligned with the fault trend indicating NE-SW orientations. Furthermore, we did not observe significant variations in the polarization directions and the delay times along the fault zone. Stations deployed in the vicinity of Central Anatolian fault zone exhibit N-S fast directions in good agreement with the fault trend. The average delay time for the whole study area is slightly higher than 1 second. Rapid spatial variations in splitting parameters are observed only in Adana region and the surrounding area. This probably suggests that the contribution of crustal anisotropy to mantle anisotropy is quite high. This observation is also consistent with the known tectonic structure of this region, which is presumably related to fabrics within deep crustal rocks preserving a record of deformation. This point should also need to be supported with

  20. 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

  1. The Diamondiferous Lithospheric Mantle Underlying the Eastern Superior Craton: Evidence From Mantle Xenoliths From the Renard Kimberlites, Quebec

    NASA Astrophysics Data System (ADS)

    Hunt, L.; Stachel, T.; Armstrong, J. P.; Simonetti, A.

    2009-05-01

    plot into the on-craton garnet peridotite field of Ramsay (1992), and follow the garnet peridotite trend of Grütter (2008). Using the single pyroxene geothermobarometer of Nimis and Taylor (2000), the clinopyroxene grains fall along a 38mW/m2 model geotherm. However, the majority fall on the low pressure side of the diamond graphite transition. Initial analysis on the garnet grains show that the majority plots in the on craton lherzolite field (G9A) of Grütter et al. (2006). A smaller eclogite population is also present, along with a minor harzburgitic (G10) population. Using the manganese in garnet thermometer of Creighton (2008) the majority of grains fall in the diamond window (T>950°C). This indicates a currently unexplained disconnect between clinopyroxene and garnet geothermobarometry. The newly developed technique of in situ Pb-Pb dating of clinopyroxene xenocrysts (Schmidberger et al. 2007) was applied to the microxenoliths. Initial results indicate an age of ˜2.7 Ga for the subcratonic lithospheric mantle beneath Renard. This date is significant, coinciding with the beginning of the break up of Vaalbara and a major phase of continental crust generation. Also at 2.7 Ga, Kenorland (including the Superior Province) was formed by accretion of granitoid-greenstone terranes at convergent margins (Barley et al., 2005).

  2. Deformation of "stable" continental interiors by mantle convection: Implications for intraplate stress in the New Madrid Seismic Zone

    NASA Astrophysics Data System (ADS)

    Forte, A. M.; Moucha, R.; Simmons, N. A.; Grand, S. P.; Mitrovica, J. X.

    2011-12-01

    The enigmatic origin of large-magnitude earthquakes far from active plate boundaries, especially those occurring in so-called "stable" continental interiors, is a source of continuing controversy that has eluded a satisfactory explanation using past geophysical models of intraplate deformation and faulting. One outstanding case of such major intraplate earthquakes is the 1811-1812 series of events in the New Madrid Seismic Zone (NMSZ). We contend that the origin of some of these enigmatic intraplate events is due to regional variations in the pattern of tectonic stress generated by mantle convective flow acting on the overlying lithosphere and crust. Mantle convection affects the entire surface of the planet, irrespective of the current configuration of surface plate boundaries. In addition, it must be appreciated that plate tectonics is not a 2-D process, because the convective flow that drives the observed horizontal motions of the tectonic plates also drives vertical displacements of the crust across distances as great as 2 to 3 km. This dynamic topography is directly correlated with convection-driven stress field variations in the crust and lithosphere and these stresses can be locally focussed if the mantle rheology below the lithosphere is characterised by sufficiently low viscosities. We have developed global models of convection-driven mantle flow [Forte et al. 2009,2010] that are based on recent high-resolution 3-D tomography models derived from joint inversions of seismic, geodynamic and mineral physics data [Simmons et al. 2007,2008,2010]. These tomography-based mantle convection models also include a full suite of surface geodynamic (postglacial rebound and convection) constraints on the depth-dependent average viscosity of the mantle [Mitrovica & Forte 2004]. Our latest tomography-based and geodynamically-constrained convection calculations reveal that mantle flow under the central US are driven by density anomalies within the lower mantle associated

  3. The evolution of lithosphere deformation due to infiltration of asthenosphere melt into a lithosphere with inherited weakness: insight from 2D numerical models of continental rifts

    NASA Astrophysics Data System (ADS)

    Havlin, C.; Parmentier, E.; Hirth, G.

    2013-12-01

    Melt formed in the asthenosphere affects lithosphere evolution through its accumulation and subsequent infiltration and heating of the lithosphere. Magmatic weakening of the lithosphere has received particular attention in the context of continental rifting because homogeneous continental lithosphere is too strong to rift under available tectonic forces without a weakening mechanism. But the observation that rifting generally initiates in heterogeneous continental mobile belts suggests that rift zones develop in lithosphere with some inherited compositional weakness, obscuring the importance of magmatic weakening. To test the relative roles of magmatic and compositional weakening, we construct a 2D numerical model that includes both effects. We treat the lithosphere and asthenosphere as separate, but coupled, domains. The lithosphere deforms via a composite brittle-ductile rheology with a strain rate that varies horizontally but is independent of depth. We apply an extensional force that is constant throughout the lithosphere, causing thinned or weakened regions of the lithosphere to extend at higher strain rate. We treat the asthenosphere as a viscous and partially molten and solve the 2D conservation equations for mass, momentum and energy for both the solid and melt phases. Melting and freezing are treated using a hydrated peridotite solidus. Solid velocities in the asthenosphere are calculated using the solid velocities from the lithosphere base as boundary conditions. The asthenosphere and lithosphere are coupled through magma infiltration and subsequent lithosphere heating. Melt fractions accumulating above an imposed critical melt fraction are extracted and emplaced within a few kms of the lithosphere-asthenosphere boundary, where it freezes, releasing latent heat. We test scenarios with a fixed critical melt fraction as well as a variable critical melt fraction determined by our previously published parametrization of dike propagation [Havlin et al., EPSL

  4. Upper mantle viscosity and dynamic subsidence of curved continental margins.

    PubMed

    Sacek, Victor; Ussami, Naomi

    2013-01-01

    Continental rifting does not always follow a straight line. Nevertheless, little attention has been given to the influence of rifting curvature in the evolution of extended margins. Here, using a three-dimensional model to simulate mantle dynamics, we demonstrate that the curvature of rifting along a margin also controls post-rift basin subsidence. Our results indicate that a concave-oceanward margin subsides faster than a convex margin does during the post-rift phase. This dynamic subsidence of curved margins is a result of lateral thermal conduction and mantle convection. Furthermore, the differential subsidence is strongly dependent on the viscosity structure. As a natural example, we analyse the post-rift stratigraphic evolution of the Santos Basin, southeastern Brazil. The differential dynamic subsidence of this margin is only possible if the viscosity of the upper mantle is >2-3 × 10(19) Pa s.

  5. Far-offset Airgun Imaging of the Mantle: Lithospheric Anisotropy of the North Atlantic

    NASA Astrophysics Data System (ADS)

    Gaherty, J. B.; Lizarralde, D.; Collins, J. A.; Hirth, G.

    2001-12-01

    Melt extraction and associated mantle flow beneath a spreading center produces coherent fabric in olivine-rich mantle rocks that is retained in the mantle lithosphere as it translates away from the ridge. Observations of seismic anisotropy provide a means to map this mantle fabric and thus flow associated with ridge processes. Refraction and surface-wave studies have successfully delineated the anisotropic structure of Pacific lithosphere, and to first order, these results agree with simple passive-spreading models of the mid-ocean ridge. Numerical models suggest that slow-spreading lithosphere may be characterized by anisotropy that is very different than that observed in the Pacific due to the possible influence of buoyancy-driven upwelling and along-axis flow. Unfortunately, observations of anisotropy in the Atlantic and other slow-spreading environments are limited to long-period surface waves, and the details of lithospheric fabric in such regions are largely unknown. We investigate the anisotropic structure of old Atlantic lithosphere using refraction data recorded during the FAIM (Far-offset Airgun Imaging of the Mantle) experiment. This experiment consisted of airgun shots to a 700-km-long linear array of 16 ocean-bottom seismometers (OBS), with an additional 3 OBS deployed perpendicular to the main array, located 150 km SW, 75-km SW, and 75-km NE from the line. The shots were also recorded by a broad-band seismometer deployed on Bermuda, perpendicular to the line approximately 350 km NE. Preliminary analysis of one of the off-line instruments indicates that refracted arrivals were successfully recorded to a source-receiver distance of over 250 km. These data will provide azimuthal record sections that span up to 160 degrees. The travel-time residuals from these data will be corrected for along-path heterogeneity using the results of the primary refraction profile, and the remaining azimuthal variation will be modeled for the direction, magnitude and depth

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

    NASA Astrophysics Data System (ADS)

    Peslier, Anne H.; Bizimis, Michael

    2015-04-01

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

  7. 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

  8. 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

  9. Layered anisotropy within the crust and lithospheric mantle beneath the Sea of Japan

    NASA Astrophysics Data System (ADS)

    Legendre, C. P.; Zhao, L.; Deschamps, F.; Chen, Q.-F.

    2016-10-01

    Continental rifting during the Oligocene to mid-Miocene caused the opening of the Sea of Japan and the separation between the Japanese Islands and the Eurasian Plate. The tectonic evolution in the Sea of Japan is important for understanding the evolution of back-arc regions in active convergent margins. Here, we use data from the seismic stations surrounding the Sea of Japan to map the Rayleigh-wave azimuthal anisotropy in the crust and lithospheric mantle beneath the Sea of Japan. We explore the variations of Rayleigh-wave phase-velocity beneath the Sea of Japan in a broad period range (30-80 s). Rayleigh-wave dispersion curves are measured by the two-station technique for a total of 231 interstation paths using vertical-component broad-band waveforms at 22 seismic stations around the Sea of Japan from 1411 global earthquakes. The resulting maps of Rayleigh-wave phase velocity and azimuthal anisotropy allow the examination of azimuthal anisotropy at specific periods. They exhibit several regions with different isotropic and anisotropic patterns: the Japan Basin displays fast velocities at shorter periods (30 and 40 s) with NNE-SSW anisotropy, whereas at 60 s and longer, the velocities become slow even if the anisotropy remains NE-SW; the East China Sea shows fast velocities at all periods (30-80 s) with constant NW-SE anisotropy. Trench-normal anisotropy beneath the Japanese Islands is found at short periods (30-40 s) and become trench-parallel at periods of 60 s and longer. Overall, our model resolves two layers of anisotropy, the shallowest and deepest layers being potentially related to frozen deformation due to recent geodynamic events, and asthenospheric flow, respectively.

  10. Late Cretaceous - recent lithosphere scale evolution of Turkey: linking the crustal surface evolution to the structure of the mantle

    NASA Astrophysics Data System (ADS)

    Bartol, J.; Govers, R. M. A.; Wortel, M. J. R.

    2015-12-01

    Central Anatolia (Central Turkey) possesses all the characteristics of a plateau. It experienced a period of rapid and substantial uplift (late Miocene, ˜8 Ma) while significant crustal shortening did not occur. Similar to other plateaus, the presence of volcanic ash and tuff within the sediments suggest that uplift was preceded by widespread volcanism (˜14-9Ma). The lithospheric context of these events is, however, unknown. For the Eastern Anatolian plateau, similar events have been attributed to southward retread followed by slab break-off of the northern Neotethys slab. Recent tomographic results indicate that this northern Neotethys slab extended beneath both the Eastern and Central Anatolian plateau prior to late Miocene delamination and possibly even beneath western Anatolia prior to the Eocene (?). We propose a new lithospheric scenario for the regional evolution for the Aegean-Anatolia-Near East region that combines a recent compilation of surface geology data with the structure of the upper mantle imaged with tomography. In our new scenario for the evolution of the Aegean-Anatolia-Near East region, a single continuous subduction zone south of the Pontides (Izmir - Ankara - Erzincan crustal suture zone) accommodated the Africa - Eurasia convergence until the end of the late Cretaceous. In the Late Cretaceous - Eocene the northern Neotethys Ocean closed followed by Anatolide - Taurides (south) and Pontides (north) continental collision along the Izmir - Ankara - Erzincan crustal suture zone. While the trench jumped to the south of Anatolide - Taurides terrane, subduction continued beneath the Izmir-Ankara-Erzincan suture where the northern Neotethys slab continued to sink into the deeper mantle. In the early Miocene (˜20-15Ma), the northern Neotethys slab started to retreat southward towards the trench, resulting in delamination of the lithospheric mantle. The last part of (early Miocene - recent) our scenario is testable. We use a coupled thermal

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  12. 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

  13. Issues of oxygen excess in the crust and upper mantle lithosphere

    NASA Astrophysics Data System (ADS)

    Balashov, Y. A.; Martynov, E. V.

    2012-04-01

    Application of a new geochemical buffer, 'CeB' - Ce+4/Ce+3 for zircons, is promising for oxygen fugacity (FO2) estimation in crust and mantle. Absence of Ce+4 and Eu+2-enriched zircons are typical of the lower lithosphere. Reducing setting dominate in mantle rocks. Subduction adds oxidized substance for lithosphere into deeper mantle (Balashov ea, 2011-2012). The zircons in upper lithosphere are oxidized. Peridotites minerals show increased H2O and OH- preserves to 150-160 km at ΔFMQ -1.4 - -0.1 (Babushkina et al, 2009) comparable with CeB 2.2 - 3.9. Increasing oceanic mass in the geological time controls water efflux and oxidation of upper the lithosphere. Oxygen source in crust and upper mantle is the most important, yet outstanding issues in geochemistry of Earth's upper shells. Oxygen excess in atmosphere correlating with long-term emergence and evolution of Earth's biosphere is an approach reflected in the schemes of cycle- and phase-wise biosphere evolution (Dobretsov et al, 2006; Sorokhtin et al, 2010). The both schemes demonstrate ideas for oxygen evolution of atmosphere, but are not confirmed by geochronology. Applying these outlines an actual picture FO2 evolution. Precambrian granitoids, detrital zircons and upper mantle lithosphere have similar CeB. The initial data include Australian Hadean and Archaean detrital zircons (Peck et al, 2001), CeB: 27.1 -1.96, and Eu+2/Eu+3: 0.015-0.12 (Balashov, Skublov, 2011). Greenland tonalities (3813 Ma) and granodiorite (3638 Ma) (Whitehouse, Kamber, 2002) CeB: 34 - 0.5. In oldest crust rocks dominated zircons with generation under high and heterogeneous FO2. Zircons in younger mantle-crustal rocks of S. American subduction zones (Ballard et al, 2002; Hoskin et al, 2000, etc.) show the same. Upper mantle lithosphere and crust represent continuously interacted with oxygen. If Progressively oxygen increase from Hadean to modern state (Dobretsov ea, 2006; Sorokhtin ea, 2010), contradicts with actual Archaean data. We

  14. 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.

  15. Seismic Structure and Geodynamic Evolution of the Lithosphere and Upper Mantle in the Pannonian - Carpathian Region

    NASA Astrophysics Data System (ADS)

    Houseman, G.; Brückl, E.; Hegedüs, E.; Radovanovic, S.; Brisbourne, A.; Lorinczi, P.; Dando, B.; Hausmann, H.; Kovács, A.; Török, I.

    2008-12-01

    The Pannonian Basin is the largest of a group of Miocene-age extensional basins within the arc of the Alpine-Carpathian Mountain Ranges. These basins are extensional in origin, but the surrounding Carpathians result from sustained convergence during and since the period of active extension. A significant part of the mantle lithosphere here has been replaced, as gravitational instability caused an overturn of the upper mantle. The Carpathian Basins Project (CBP) is a major international broadband seismology experiment, supported by geodynamical modelling and designed to improve our understanding of the structure and evolution of the lithosphere and upper mantle beneath the Pannonian and Vienna Basins. Between 2005 and 2007 we deployed 56 portable broadband seismic stations in Austria, Hungary and Serbia, spanning the Vienna Basin and the western part of the Pannonian Basin. Arrival time residuals from teleseismic earthquakes are delayed by about 0.8 sec in the Vienna Basin and early by a similar amount in southwest Hungary. Tomographic inversion of the travel time residuals shows relatively fast P-wave velocities in the upper mantle beneath the western Pannonian Basin and slow P-wave velocities beneath the West Carpathians. Seismic anisotropy (SKS) measurements reveal an intriguing pattern of lithospheric anisotropy: in the north-west the fast direction is generally elongated EW, perpendicular to the shortening direction across the Alps. Across the Vienna Basin the fast direction is NW-SE, perpendicular to the major bounding fault systems. Across the Pannonian Basin the dominant fast direction is EW, but in several locations the vectors are rotated toward NW-SE. The Mid-Hungarian Line, a major strike-slip structure already clearly identified in the gravity field, also is associated with abrupt changes in the azimuth of lithospheric anisotropy, and crustal receiver function signature. The object of these investigations is to use the seismic data to discriminate

  16. Seismic Structure and Geodynamic Evolution of the Lithosphere and Upper Mantle in the Pannonian - Carpathian Region

    NASA Astrophysics Data System (ADS)

    Houseman, G.; Stuart, G.; Dando, B.; Hetenyi, G.; Lorinczi, P.; Brueckl, E.; Hegedus, E.; Radovanovic, S.; Brisbourne, A.

    2009-04-01

    The Pannonian Basin is the largest of a group of Miocene-age extensional basins within the arc of the Alpine-Carpathian Mountain Ranges. These basins are extensional in origin, but the surrounding Carpathians result from sustained convergence during and since the period of active extension. A significant part of the mantle lithosphere here has been replaced, as gravitational instability caused an overturn of the upper mantle. The Carpathian Basins Project (CBP) is a major international broadband seismology experiment, supported by geodynamical modelling and designed to improve our understanding of the structure and evolution of the lithosphere and upper mantle beneath the Pannonian and Vienna Basins. Between 2005 and 2007 we deployed 56 portable broadband seismic stations in Austria, Hungary and Serbia, spanning the Vienna Basin and the western part of the Pannonian Basin. Arrival time residuals from teleseismic earthquakes are delayed by about 0.8 sec in the Vienna Basin and early by a similar amount in southwest Hungary. Tomographic inversion of the travel time residuals shows relatively fast P-wave velocities in the upper mantle beneath the western Pannonian Basin and slow P-wave velocities beneath the West Carpathians. Seismic anisotropy (SKS) measurements reveal an intriguing pattern of lithospheric anisotropy: in the north-west the fast direction is generally elongated EW, perpendicular to the shortening direction across the Alps. Across the Vienna Basin the fast direction is NW-SE, perpendicular to the major bounding fault systems. Across the Pannonian Basin the dominant fast direction is EW, but in several locations the vectors are rotated toward NW-SE. The Mid-Hungarian Line, a major strike-slip structure already clearly identified in the gravity field, also is associated with abrupt changes in the azimuth of lithospheric anisotropy. Receiver function analysis of the seismic discontinuity at 670 km shows significant structure on scales of order 100 km, and

  17. 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

  18. Geochemical evolution of lithospheric mantle underlying Intrasudetic Fault (SW Poland).

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    alkaline silicate melt. Xenolith where clinopyroxene shows constant enrichment in LREE may represent a part of mantle affected by pure carbonatitic metasomatism. Xenoliths with the lowest Fo content in olivine are probably cumulates of mafic silicate melt. Intergranular aggregates originated during reaction between primary phases and infiltrating, possibly mafic melt. The "glassy patch" might be a product of complete breakdown of amphibole in upper mantle conditions (Shaw, 2009, Lithos). However, presence of rhönite in host basanite suggests that some amphibole might have been broken also in lower pressures. Chemical composition of peridotites from Pilchowice resembles that recorded by group A mantle xenoliths from Krzeniów (Matusiak-Małek et al., 2014, JoP). In Pilchowice we have described the first xenolith affected by purely carbonatitic metasomatism. We also suggest that hydrous phases might have been present in upper mantle beneath this Pilchowice, which is believed to be nominally anhydrous (Puziewicz et al., 2015, IJES, DOI 10.1007/s00531-014-1134-2). This study was a part of MSc thesis of the first author and was possible thanks to the project NCN 2011/03/B/ST10/06248 of Polish National Centre for Science.

  19. Primitive off-rift basalts from Iceland and Jan Mayen: Os-isotopic evidence for a mantle source containing enriched subcontinental lithosphere

    NASA Astrophysics Data System (ADS)

    Debaille, Vinciane; Trønnes, Reidar G.; Brandon, Alan D.; Waight, Tod E.; Graham, David W.; Lee, Cin-Ty A.

    2009-06-01

    New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presented for basalts from the three volcanic flank zones in Iceland and from Jan Mayen Island. The 187Os/ 188Os ratios in lavas with <30 ppt Os ( n = 4) are elevated compared to ratios in coexisting olivine and appear to be contaminated at a shallow level. The 187Os/ 188Os ratios in the remaining lavas with >30 ppt Os ( n = 17) range between 0.12117 and 0.13324. These values are surprisingly low for oceanic island basalts and include some samples that are less than putative present-day primitive upper mantle (PUM with 187Os/ 188Os of 0.1296). These low 187Os/ 188Os preclude significant shallow-level contamination from oceanic crust. The 187Os/ 188Os ratios for Jan Mayen lavas are less than PUM, severely limiting the presence of any continental crust in their mantle source. A positive correlation between 143Nd/ 144Nd and 187Os/ 188Os ratios in Iceland and Jan Mayen lavas likely reflects the presence in their source of ancient subcontinental lithosphere that has undergone incompatible trace element enrichment that did not affect the Re-Os system. In addition, the Jan Mayen lava isotopic signature cannot be explained solely by the presence of subcontinental lithospheric mantle, and the influence of another geochemical component, such as a mantle plume appears required. Combined 87Sr/ 86Sr, 143Nd/ 144Nd, 3He/ 4He and 187Os/ 188Os data indicate a genetic relationship between Jan Mayen Island and the Iceland mantle plume. Material from the Iceland mantle plume likely migrates at depth until it reaches the tensional setting of the Jan Mayen Fracture Zone, where it undergoes low-degree partial melting. At a first-order, isotopic co-variations can be interpreted as broadly binary mixing curves between two primary end-members. One end-member, characterized in particular by its unradiogenic 187Os/ 188Os and 143Nd/ 144Nd, low 3He/ 4He and high 87Sr/ 86Sr, is represented by subcontinental

  20. Removal of Brown Colour From Diamonds During Storage in the Lithospheric Mantle

    NASA Astrophysics Data System (ADS)

    Smith, E. M.; Helmstaedt, H. H.

    2009-05-01

    Brown colour in natural diamonds is produced by plastic deformation during residence in the mantle. Dislocation movement generates vacancies, which aggregate into clusters of about 30-60 vacancies. The resulting electronic configuration at each cluster leads to the broad, featureless absorption pattern associated with common brown colour. Less commonly, other brownish colours can be attributed to hydrogen or isolated nitrogen atoms, H4 centres, or possibly oxygen. The common brown colour can be removed by high-pressure-high-temperature (HPHT) treatment. This process involves pressures and temperatures of 5-9 GPa and 1800-2700 ° C, respectively. Treatment may take several minutes or hours. It has been suggested that brown diamonds stored in the lithospheric mantle should lose their colour by analogy to HPHT treatment. If so, brown colour must be the product of late deformation (close to kimberlite eruption) or the brown diamonds must be stored above the diamond stability field (in a cooler part of the lithosphere). Is it reasonable to expect brown colour to be destroyed in the lithospheric mantle? An objective analysis of this question must consider temperature. Higher temperatures result in faster colour removal. HPHT treatment occurs at 1800-2700 ° C, whereas inclusion thermometry places most lithospheric diamonds in the range of 900-1400 ° C. Destruction of the brown colour centre involves breaking up vacancy clusters. The activation energy required to do this can be estimated as the energy of an isolated monovacancy, minus the energy per vacancy of the cluster, plus the vacancy migration energy. Data from recent literature produces a value of 7.7 eV. The Arrhenius equation can be modified to show how reaction time varies with temperature. The activation energy can be used in conjunction with experimental HPHT data to extrapolate reaction times from HPHT temperatures to lithospheric mantle temperatures. For a certain reduction in brown colour produced by HPHT

  1. 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

  2. Relative strength of lithospheric mantle compared to granulite lower crust in orogenic roots: insight from field laboratory.

    NASA Astrophysics Data System (ADS)

    Kusbach, V.; Ulrich, S.; Schulmann, K.

    2009-04-01

    The continental lithosphere is composed by strong lithospheric mantle and weak lower crust for average and hot geotherms. However, some experiments and seismic studies show that the strength contrast between mantle and crust can vary in order of several magnitudes. The internal zone of the European Variscan orogen (Bohemian Massif, Czech Republic) contains large complexes of Ky - K-feldspar granulites with incorporated spinel and garnet peridotites that can respond to question of mantle-lower crust strength contrast from the field perspective. The studied spinel-garnet harzburgite body (the Mohelno peridotite) represents probably a fragment of strongly depleted oceanic lithosphere showing peak conditions of 22,4-27,6 kbar and 1120-1275°C. The peridotite forms large folded sheet with steep hinge and vertical axial plane. It exhibits presence of spinel along the outer arc and the internal part of the fold and garnet along inner arc, both related to coarse-grained orthopyroxene - olivine microstructure. This coarse microstructure is dynamically recrystallized forming fine-grained matrix (~10 - 20 microns) and the EBSD measurements show presence of axial [100] LPO olivine pattern dominantly along the outer arc of the fold and in spinel harzburgite, while the inner arc of the fold and partly also garnet harzburgite reveals presence of axial [010] LPO pattern. Steep foliation and sub-horizontal to moderately plunging lineation determined from olivine EBSD data defines the shape of the megafold. Host rocks exhibit transposed mylonitic fabric S1-2 revealing peak conditions of 18 kbar, 800°C and heterogeneous D3 retrogression at about 10 - 7 kbar, 650°C. The foliation S2-3 is fully concordant with limbs of peridotite megafold, but close to the outer arc it is affected by asymmetrical folds with axial planar leucosomes coherent with the shape of the megafold hinge zone. In contrast, the S2 in the internal part of the megafold is affected by sinistral and dextral melt

  3. Survival of Brown Colour in Diamond During Storage in the Subcontinental Lithospheric Mantle

    NASA Astrophysics Data System (ADS)

    Smith, Evan; Helmstaedt, Herwart; Flemming, Roberta

    2010-05-01

    Brown is the most common colour of natural diamond. This colour is generally associated with plastic deformation of the crystal structure, which is imparted during residence in the mantle. Dislocation movement generates vacancies, which aggregate into clusters of perhaps 30-60 vacancies. The resulting electronic configuration leads to the broad, featureless absorption pattern associated with common brown colour. It is well-established that the common brown colour can be removed by high-pressure-high-temperature (HPHT) treatment. The process involves pressures and temperatures in the range of 5-9 GPa and 1800-2700 ° C, respectively. The treatment may take several minutes or hours. It has been suggested that the same colour removal process operates continuously in the subcontinental lithospheric mantle, causing any brown diamonds in the diamond window to quickly lose their colour. The present study examined the validity of this suggestion. Temperature is the important difference between HPHT conditions and diamond window conditions. Higher temperatures result in faster colour removal. HPHT treatment occurs at 1800-2700 ° C, whereas inclusion thermometry places most lithospheric diamonds in the range of 900-1400 ° C. Destruction of the brown colour centre involves breaking up vacancy clusters. How quickly this can be done depends on the concentration of clusters as well as the rate constant. The rate constant is changes with temperature, according to the Arrhenius equation. The key to this relationship is the activation energy required for the breakup of a vacancy cluster. This activation energy can be estimated as the energy of an isolated monovacancy, minus the energy per vacancy of the cluster, plus the vacancy migration energy. A value of 7.7±0.3 eV is obtained using data from recent literature. For any given brown diamond, the rate constant determines the time needed to remove a certain amount of colour. The Arrhenius equation can be rearranged to show how

  4. Crystal chemistry of amphiboles: implications for oxygen fugacity and water activity in lithospheric mantle beneath Victoria Land, Antarctica

    NASA Astrophysics Data System (ADS)

    Bonadiman, C.; Nazzareni, S.; Coltorti, M.; Comodi, P.; Giuli, G.; Faccini, B.

    2014-03-01

    Amphibole is the hydrous metasomatic phase in spinel-bearing mantle xenoliths from Baker Rocks, Northern Victoria Land, Antarctica. It occurs in veins or in disseminated form in spinel lherzolites. Both types derive from reaction between metasomatic melts and the pristine paragenesis of the continental lithospheric mantle beneath Northern Victoria Land. To determine the effective role of water circulation during the metasomatic process and amphibole formation, six amphibole samples were fully characterized. Accurate determination of the site population and the state of dehydrogenation in each of these amphiboles was carried out using single-crystal X-ray diffraction, electron microprobe and secondary ion mass spectroscopy on the same single crystal. The Fe3+/ΣFe ratio was determined by X-ray absorption near edge spectroscopy on amphibole powder. The degree of dehydrogenation determined by SIMS is 0.870-0.994 O3(O2-) a.p.f.u., primary and ascribed to the Ti-oxy component of the amphibole, as indicated by atom site populations; post-crystallization H loss is negligible. Estimates of aH2O (0.014-0.054) were determined from the dehydration equilibrium among end-member components assuming that amphiboles are in equilibrium with the anhydrous peridotitic phases. A difference up to 58 % in determination of aH2O can be introduced if the chemical formula of the amphiboles is calculated based on 23 O a.p.f.u. without knowing the effective amount of dehydrogenation. The oxygen fugacity of the Baker Rocks amphibole-bearing mantle xenoliths calculated based upon the dissociation constant of water (by oxy-amphibole equilibrium) is between -2.52 and -1.32 log units below the fayalite-magnetite-quartz (FMQ) buffer. These results are systematically lower and in a narrow range of values relative to those obtained from anhydrous olivine-orthopyroxene-spinel equilibria ( fO2 between -1.98 and -0.30 log units). A comparative evaluation of the two methods suggests that when amphibole

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

    NASA Astrophysics Data System (ADS)

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

    2013-07-01

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

  6. Joint modeling of lithosphere and mantle dynamics: Evaluation of constraints from global tomography models

    NASA Astrophysics Data System (ADS)

    Wang, Xinguo; Holt, William E.; Ghosh, Attreyee

    2015-12-01

    With the advances in technology, seismological theory, and data acquisition, a number of high-resolution seismic tomography models have been published. However, discrepancies between tomography models often arise from different theoretical treatments of seismic wave propagation, different inversion strategies, and different data sets. Using a fixed velocity-to-density scaling and a fixed radial viscosity profile, we compute global mantle flow models associated with the different tomography models and test the impact of these for explaining surface geophysical observations (geoid, dynamic topography, stress, and strain rates). We use the joint modeling of lithosphere and mantle dynamics approach of Ghosh and Holt (2012) to compute the full lithosphere stresses, except that we use HC for the mantle circulation model, which accounts for the primary flow-coupling features associated with density-driven mantle flow. Our results show that the seismic tomography models of S40RTS and SAW642AN provide a better match with surface observables on a global scale than other models tested. Both of these tomography models have important similarities, including upwellings located in Pacific, Eastern Africa, Iceland, and mid-ocean ridges in the Atlantic and Indian Ocean and downwelling flows mainly located beneath the Andes, the Middle East, and central and Southeast Asia.

  7. Mantle Dynamics and Lithosphere-Asthenosphere Interaction in the South Atlantic from Space Gravity Data

    NASA Astrophysics Data System (ADS)

    Maia, M.; Cadio, C.; Alodia, G.; Metivier, L.

    2015-12-01

    Geoid anomalies are essential tools for the understanding of mantle density distribution and of the patterns of convection. Previous wavelet analyses of the geoid revealed the existence of density anomalies in the deep mantle linked to the long-term volcanic activity of the Central Pacific (Cadio et al., 2011). The South Atlantic displays geodynamic characteristics similar to those of French Polynesia: long lasting volcanic activity, dating back to the opening of the Atlantic; wide area covered by volcanic edifices and seamount chains; irregularity of age patterns and a broad regional depth anomaly. Preliminary wavelet analyses of the geoid anomalies in this area revealed strong correlation with deep velocity anomalies from different tomographic models. However, here, the lithospheric signal linked to the ocean basin and continents must be estimated because it contributes to long wavelengths in geoid spectrum. We tested different lithospheric models in order to estimate this contribution. At intermediate and long wavelengths, the residual geoid confirms the existence of density anomalies in the mantle correlated with the surface expression of the volcanism. Cadio, C., I. Panet, A. Davaille, M. Diament, L. Métivier and O. de Viron, 2011. Pacific geoid anomalies revisited in light of thermochemical oscillating domes in the lower mantle, Earth Planetary Science Letters, 306, doi:10.1016/j.epsl.2011.03.040.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  9. Groundwater flow as a cooling agent of the continental lithosphere

    NASA Astrophysics Data System (ADS)

    Kooi, Henk

    2016-03-01

    Groundwater that flows through the outer shell of the Earth as part of the hydrologic cycle influences the distribution of heat and, thereby, the temperature field in the Earth’s crust. Downward groundwater flow in recharge areas lowers crustal temperatures, whereas upward flow in discharge areas tends to raise temperatures relative to a purely conductive geothermal regime. Here I present numerical simulations of generalized topography-driven groundwater flow. The simulations suggest that groundwater-driven convective cooling exceeds groundwater-driven warming of the Earth’s crust, and hence that groundwater flow systems cause net temperature reductions of groundwater basins. Moreover, the simulations demonstrate that this cooling extends into the underlying crust and lithosphere. I find that horizontal components of groundwater flow play a central role in this net subsurface cooling by conveying relatively cold water to zones of upward groundwater flow. The model calculations suggest that the crust and lithosphere beneath groundwater basins can cool by several tens of degrees Celsius where groundwater flows over large distances in basins that consist of crustal rock. In contrast, groundwater-induced cooling is small in unconsolidated sedimentary settings, such as deltas.

  10. Sulfide-scale insights into platinum-group element behavior during carbonate mantle metasomatism and evolution of Spitsbergen lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Kim, Nak Kyu; Choi, Sung Hi; Dale, Christopher W.

    2016-03-01

    We report combined Re-Os isotope and highly siderophile element data for whole-rock and whole-sulfide grains from Spitsbergen peridotites. The Os-Ir contents in whole-rocks are elevated compared to those of the primitive mantle, but the Pt-Pd-Re contents are depleted, reflecting refractory monosulfide solid solution (Mss) control during mantle melting. There are two general types of sulfide documented in global mantle samples: primary residual Mss with subchondritic Pd/Ir ratios and secondary metasomatic sulfides with suprachondritic Pd/Ir ratios. Most Spitsbergen sulfides have elevated Ir contents, and belong to the residual group. Most but not all Spitsbergen sulfides, however, are unusual in that they show a fractionation of Os (and Ru) from Ir which cannot be reconciled with a simple partial melting process. The Os(+ Ru) fractionation from Ir is most notable in a sample containing mantle-derived carbonate-bearing pockets. Infiltration of carbonate-rich S-undersaturated melt into the Spitsbergen lithospheric mantle may result in the formation of localized S-rich liquid by dissolving residual Mss. Such melt compositions may promote laurite crystallization before Mss, causing the combined depletion of Os + Ru relative to Ir in later-formed Mss. The Re-depletion model ages of residual sulfide grains from Spitsbergen peridotites coincide with crustal ages determined for Spitsbergen, indicating coupled mantle-crust evolution, and furthermore, they coincide with the previously proposed major peaks of pulsed crustal formation periods in Earth at ca. 2.7, 1.9 and 1.2 Ga.

  11. Petrofabric and seismic properties of lithospheric mantle xenoliths from the Calatrava volcanic field (Central Spain)

    NASA Astrophysics Data System (ADS)

    Puelles, P.; Ábalos, B.; Gil Ibarguchi, J. I.; Sarrionandia, F.; Carracedo, M.; Fernández-Armas, S.

    2016-06-01

    The microstructural and petrofabric study of peridotite xenoliths from the El Aprisco (Neogene Calatrava Volcanic Field) has provided new information on deformation mechanisms, ambient conditions and seismic properties of the central Iberian subcontinental mantle. Olivine, orthopyroxene, clinopyroxene, amphibole and spinel constitute the mineral assemblage in equilibrium. Their microstructure indicates that they accommodated crystal-plastic deformation under high water fugacity conditions. Crystallographic preferred orientation patterns of key minerals were determined with the EBSD technique. The xenoliths exhibit B, C and A olivine fabrics. B-type fabrics, involving the (010)[001] slip system, may develop in domains where deformation occurs under comparatively lower temperature, higher water-content and faster strain rates. They are interpreted here as the result of deformation in a suprasubduction mantle setting triggered by changing conditions imposed by a cooler subducting slab that incorporated fluids into the system. Xenoliths with olivine C-type fabrics involve activation of the dominant (100)[001] slip system, denote intracrystalline slip at higher temperatures and water-contents. They are here interpreted to sample lithospheric mantle domains where the impact of those new conditions was not so strong. Finally, the A-type fabrics, characteristic of the (010)[100] slip system, are frequent in the mantle under moderate to high temperature. These fabrics are considered here as characteristic of the mantle prior to subduction. The olivine fabrics constrain heterogeneous seismic properties. Propagation orientation of P waves (8.27-8.51 km/s) coincides with olivine [100] axis concentrations, whereas the fastest S1 waves (5.13-5.22 km/s) propagate parallel to [010] axis minima. The maximum shear wave birefringence (VS1-VS2 = 0.17-0.37 km/s) is close to the direction of the macroscopic lineation. Heterogeneity of calculated seismic properties would concur with

  12. Metasomatism in the lithospheric mantle beneath southern Patagonia, Argentina

    NASA Astrophysics Data System (ADS)

    Kolosova-Satlberger, Olesya; Ntaflos, Theodoros; Bjerg, Ernesto

    2014-05-01

    Mantle xenoliths from Gobernador Gregores, southern Patagonia are spinel- lherzolites, harzburgites and wherlites. A large number of the studied xenoliths have experienced cryptic and modal metasomatism. The xenoliths are mainly coarse-grained with prevalent protogranular texture but equigranular tabular and mosaic textures are present as well. Xenoliths that have undergone modal metasomatism bear hydrous phases such as amphibole, phlogopite ± apatite and melt pockets. The latter are of particular interest because of their unusually large size (up to 1 cm in diameter) and freshness. They consist of second generation olivine, clinopyroxene and spinel ± relict amphibole ± sulfides that are surrounded by a yellowish vesicular glass matrix. The melt pockets are found in amphibole- and/or phlogopite-bearing wehrlites and harzburgites as well as anhydrous lherzolites. Subhedral primary olivines enclosed by melt pockets show in the BSE images a dark grey margin up to 80 microns thick attributed to the reaction of the primatry olivine with melt. Fine grained spinel inclusions are always associated with the dark grey margin, indicating that they belong to the secong generation assemblage. There are considerable differences between first and second generation minerals found in melt pockets. While primary olivine has Fo-contents that range from 88.0 to 93.3, second generation olivines in melt pockets vary from Fo89.3 to Fo94.4. Both primary and second generation cpx are diopsides with the latter systematically enriched in TiO2. The glasses that occur in melt pockets or propagate intergranular have compositions varying from trachyandesite to phonolite. The variable composition of the glass could be attributed to host basalt infiltration and decompressional melting of amphiboles. Some of the studied xenoliths show melt propagation of two compositional different glasses crosscutting primary generation minerals and finally mixing with each other. Microprobe analyses suggest

  13. Deep Europe today: Geophysical synthesis of the upper mantle structure and lithospheric processes over 3.5 Ga

    USGS Publications Warehouse

    Artemieva, I.M.; Thybo, H.; Kaban, M.K.; ,

    2006-01-01

    We present a summary of geophysical models of the subcrustal lithosphere of Europe. This includes the results from seismic (reflection and refraction profiles, P- and S-wave tomography, mantle anisotropy), gravity, thermal, electromagnetic, elastic and petrological studies of the lithospheric mantle. We discuss major tectonic processes as reflected in the lithospheric structure of Europe, from Precambrian terrane accretion and subduction to Phanerozoic rifting, volcanism, subduction and continent-continent collision. The differences in the lithospheric structure of Precambrian and Phanerozoic Europe, as illustrated by a comparative analysis of different geophysical data, are shown to have both a compositional and a thermal origin. We propose an integrated model of physical properties of the European subcrustal lithosphere, with emphasis on the depth intervals around 150 and 250 km. At these depths, seismic velocity models, constrained by body-and surface-wave continent-scale tomography, are compared with mantle temperatures and mantle gravity anomalies. This comparison provides a framework for discussion of the physical or chemical origin of the major lithospheric anomalies and their relation to large-scale tectonic processes, which have formed the present lithosphere of Europe. ?? The Geological Society of London 2006.

  14. Understanding the interplays between Earth's shallow- and deep- rooted processes through global, quantitative model of the coupled brittle-lithosphere/viscous mantle system

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    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 permit reconstructing 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 a powerful probe into the evolving force balance between shallow- and deep-rooted processes. However, such a 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 and 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 and with some limitations as to accounting for the impact of time-evolving mantle-flow and lithospheric slabs. Here we present results in this direction that permit simulating the coupled plates/mantle system through geological time. We build on previous work aimed at coupling two sophisticated codes for mantle flow and lithosphere dynamics: TERRA and SHELLS. TERRA is a global spherical finite-element code for mantle convection. It has been developed by Baumgardner (1985) and Bunge et al. (1996), and further advanced by Yang (1997; 2000) and Davies et al. (2013), among others. SHELLS is a thin-sheet finite-element code for lithosphere dynamics, developed by

  15. Cobalt and precious metals in sulphides of peridotite xenoliths and inferences concerning their distribution according to geodynamic environment: A case study from the Scottish lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Hughes, Hannah S. R.; McDonald, Iain; Faithfull, John W.; Upton, Brian G. J.; Loocke, Matthew

    2016-01-01

    Abundances of precious metals and cobalt in the lithospheric mantle are typically obtained by bulk geochemical analyses of mantle xenoliths. These elements are strongly chalcophile and the mineralogy, texture and trace element composition of sulphide phases in such samples must be considered. In this study we assess the mineralogy, textures and trace element compositions of sulphides in spinel lherzolites from four Scottish lithospheric terranes, which provide an ideal testing ground to examine the variability of sulphides and their precious metal endowments according to terrane age and geodynamic environment. Specifically we test differences in sulphide composition from Archaean-Palaeoproterozoic cratonic sub-continental lithospheric mantle (SCLM) in northern terranes vs. Palaeozoic lithospheric mantle in southern terranes, as divided by the Great Glen Fault (GGF). Cobalt is consistently elevated in sulphides from Palaeozoic terranes (south of the GGF) with Co concentrations > 2.9 wt.% and Co/Ni ratios > 0.048 (chondrite). In contrast, sulphides from Archaean cratonic terranes (north of the GGF) have low abundances of Co (< 3600 ppm) and low Co/Ni ratios (< 0.030). The causes for Co enrichment remain unclear, but we highlight that globally significant Co mineralisation is associated with ophiolites (e.g., Bou Azzer, Morocco and Outokumpu, Finland) or in oceanic peridotite-floored settings at slow-spreading ridges. Thus we suggest an oceanic affinity for the Co enrichment in the southern terranes of Scotland, likely directly related to the subduction of Co-enriched oceanic crust during the Caledonian Orogeny. Further, we identify a distinction between Pt/Pd ratio across the GGF, such that sulphides in the cratonic SCLM have Pt/Pd ≥ chondrite whilst Palaeozoic sulphides have Pt/Pd < chondrite. We observe that Pt-rich sulphides with discrete Pt-minerals (e.g., PtS) are associated with carbonate and phosphates in two xenolith suites north of the GGF. This three

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  17. The Sandvik peridotite, Gurskøy, Norway: Three billion years of mantle evolution in the Baltica lithosphere

    NASA Astrophysics Data System (ADS)

    Lapen, Thomas J.; Medaris, L. Gordon, Jr.; Beard, Brian L.; Johnson, Clark M.

    2009-05-01

    Lu-Hf isotope analyses of the Sandvik ultramafic body, Norway. Contributions to Mineralogy and Petrology 150, 131-145.]. The isochron ages likely record lithospheric modification associated with the 1.25-1.00 Ga Sveconorwegian orogeny and represent the youngest age of the Stage 1 mineral assemblage equilibration. A 606 ± 39 Ma Sm-Nd isochron age of the Stage 2 kelyphite assemblage is consistent with partial re-equilibration of the porphyroclastic assemblage during continental rifting associated with opening of the Iapetus Ocean between Baltica and Laurentia at ~ 600 Ma, or extension between Baltica and Siberia that may have been associated with opening of the Ægir Sea. The age of kelyphite, therefore, places the Sandvik peridotite in the uppermost mantle prior to Silurian shortening between the Baltic and Laurentian continents.

  18. 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.

  19. Anomalous Subsidence at Rifted Continental Margins: Distinguishing Mantle Dynamic Topography from Anomalous Oceanic Crustal Thickness

    NASA Astrophysics Data System (ADS)

    Cowie, L.; Kusznir, N. J.

    2012-12-01

    It has been proposed that some continental rifted margins have anomalous subsidence histories and that at breakup they were elevated at shallower bathymetries than the isostatic response of classical rift models (McKenzie 1978) would predict. The existence of anomalous syn or post breakup subsidence of this form would have important implications for our understanding of the geodynamics of continental breakup and rifted continental margin formation, margin subsidence history and the evolution of syn and post breakup depositional systems. We have investigated three rifted continental margins; the Gulf of Aden, Galicia Bank and the Gulf of Lions, to determine whether the oceanic crust in the ocean-continent transition of these margins has present day anomalous subsidence and if so, whether it is caused by mantle dynamic topography or anomalous oceanic crustal thickness. Residual depth anomalies (RDA) corrected for sediment loading, using flexural backstripping and decompaction, have been calculated by comparing observed and age predicted oceanic bathymetries in order to identify anomalous oceanic bathymetry and subsidence at these margins. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions from Crosby & McKenzie (2009). Non-zero sediment corrected RDAs may result from anomalous oceanic crustal thickness with respect to the global average, or from mantle dynamic uplift. Positive RDAs may result from thicker than average oceanic crust or mantle dynamic uplift; negative RDAs may result from thinner than average oceanic crust or mantle dynamic subsidence. Gravity inversion incorporating a lithosphere thermal gravity anomaly correction and sediment thickness from 2D seismic data has been used to determine Moho depth and oceanic crustal basement thickness. The reference Moho depths used in the gravity inversion have been calibrated against seismic refraction Moho depths. The gravity inversion crustal basement thicknesses

  20. Controls on volatile content and distribution in the continental upper mantle of Southern Gondwana (Patagonia & W. Antarctica)

    NASA Astrophysics Data System (ADS)

    Rooks, Eve; Gibson, Sally; Petrone, Chiara; Leat, Phillip

    2015-04-01

    Water content is known to affect many physical and chemical properties of the upper mantle, including melting temperature and viscosity. Water is hosted by hydrous phases, such as amphibole and phlogopite, and also by more dominant, nominally-anhydrous mantle minerals (e.g. olivine and pyroxene). The latter have the potential to incorporate hundreds of ppm of water in point defects, and may explain geophysical observations such as seismic and conductivity anomalies in the upper mantle [1]. However, the significance of the reported concentrations of H2O in nominally anhydrous minerals in mantle xenoliths is still a subject of debate primarily due to the effects of post-entrainment loss [1,2]. Unlike H2O and Li, F is less susceptible to post entrainment loss and can potentially be used to constrain the source of volatiles. We present high-precision SIMS analyses of H2O, Li and F in mantle xenoliths hosted by recently-erupted (5-10 Ka) alkali basalts from south Patagonia and the Antarctic Peninsula. These two regions formed part of the southern margin of the Gondwana supercontinent, prior to break-up, and were located above long-lived subduction zones for at least 200 M.yr., making them highly-appropriate to investigating long term evolution of the sub-continental lithospheric mantle in this setting. The xenoliths are well characterised peridotites, sourced from the off-craton spinel- and spinel-garnet facies lithospheric mantle (40-80 km). Samples are relatively dry: H2O contents of olivine span 0-49 ppm, orthopyroxene 150-235 ppm and clinopyroxene 100-395 ppm. West Antarctic samples are more hydrated than Patagonian samples, on average. These H2O concentrations fall within the global measured range for off-craton mantle minerals [4]. We attribute low H2O concentrations in olivine to diffusive loss, either by exchange with the host magma, shallow level degassing or during cooling [2]. F shows less variability than H2O and is most highly concentrated in clinopyroxenes

  1. Dynamics of mantle rock metasomatic transformation in permeable lithospheric zones beneath Siberian craton

    NASA Astrophysics Data System (ADS)

    Sharapov, Victor; Sorokin, Konstantin; Perepechko, Yury

    2015-04-01

    The numerical descriptions of hydrodynamic model of two - velocity heat and mass transfer in permeable zones above the asthenospheric lenses was formulated and solved basing on the study the composition of inclusions in minerals of low crust ultra metamorphic rocks and lithospheric mantle metasomatites and estimation of thermodynamic conditions of the processes. Experimental study of influence of the simulated hot reduced gas flows on the minerals of low crust and mantle xenolith of the Siberian craton platform (SP) give the basic information for this processes. In detail: 1. Thermobarometric study of composition of inclusions in granulite and lithospheric mantle rocks beneath the diamondiferous cratons allowed to estimate the gas phase compositions during the metamorphism and metasomatism as well as products of their re equilibration during decompression. 2. Results of the pilot study of the influence of hot gas impact flows on minerals of mantle xenoliths are taken into account. This allowed to reproduce the elements and heterophase kinetics of interactions within a temperature range of about 300 to 1300o on relative to the interactions between the solid, liquid and gas phases. 3. Correct mathematical two-velocities model of fluid dynamics for compressible multiphase fluid -rock systems. 4. Numerical schemes are simulated and solved for the problems of quantitative description of 2D dynamics behavior of P and T within the permeable zone above the asthenospheric lens. 5. Quantitative description of heterophase non isothermal fluid-rock interaction within the framework of the approximation was obtained on the basis of the parallel solutions of the exchange between the ideal gas flow and solid phase according to the model of multi-reservoir reactors based on minimization of the Gibbs potential. Qualitatively the results of numerical simulation are as follows: 1) appearance in permeable zones of the any composition fluid flows from the upper mantle magma chambers

  2. 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

  3. Can we probe the conductivity of the lithosphere and upper mantle using satellite tidal magnetic signals?

    NASA Astrophysics Data System (ADS)

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

    2015-05-01

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

  4. Helium isotopes in lithospheric mantle: Evidence from Tertiary basalts of the western USA

    SciTech Connect

    Dodson, A.; DePaolo, D.J.; Kennedy, B.M. |

    1998-12-01

    The isotopic compositions of He, Sr, and Nd were measured in Tertiary-age basalts from the Basin and Range province of the western USA to evaluate models for the He isotopic character of subcontinental mantle lithosphere (SCML) and assess the role of recycled SCML in models of mantle evolution. Previous isotopic and trace element measurements suggested that most of these basalts were formed by melting of SCML. {sup 3}He/{sup 4}He ratios, measured by in-vacuo crushing of olivine phenocrysts, vary from 2.9 to 7.8 times the atmospheric value (2.9 to 7.8 Ra) consistently below the MORB value of 8.7 {+-} 0.5 Ra. The lowest R/Ra values, associated with low {epsilon}{sub Nd}, high {sup 87}Sr/{sup 86}Sr, and high La/Nb, are attributable to lithosphere mantle, and indicate that SCML is not dominated by MORB-type He, nor by high R/Ra, plume-type He. Consideration of geographic variability indicates there are two, and possibly three, distinct regions of SCML with differing He isotopic characteristics. SCML beneath the eastern Sierra Nevada is inferred to have {sup 3}He/{sup 4}He of {approximately}5.5 Ra and a He/Nd ratio slightly less than MORB-type mantle; SCML beneath the central Basin and Range has {sup 3}He/{sup 4}He of {approximately}4 Ra and a higher He/Nd ratio than MORB-type mantle. The SCML under southwestern Utah shows less systematic correlation of He isotopes with other geochemical parameters, but also has a lower bound R/Ra value of about 4 Ra. The inferred SCML helium ratios are consistent with retention of radiogenic {sup 4}He over 800 Ma for the eastern Sierra Nevada and 1700 Ma for the other two regions. The results are not consistent with models of He infiltration from the underlying asthenosphere and suggest the lithosphere of the Basin and Range region was not delaminated during the early Tertiary. The He, Sr, Nd, and Pb isotopic compositions inferred for the SCML of the southwestern USA are a reasonably good match to the characteristics of the EMII

  5. Structural, igneous and metamorphic footprints of Pangea break-up preserved in the subducted Austroalpine continental lithosphere of the European Alps.

    NASA Astrophysics Data System (ADS)

    Spalla, M. I.; Gosso, G.; Rebay, G.; Roda, M.; Zanoni, D.; Zucali, M.

    2015-12-01

    Although in the Alps a large amount of continental lithosphere from both European (lower plate) and Adriatic (upper plate) continental margins was absorbed in the sub-lithospheric mantle during Alpine subduction, relict metamorphic and igneous imprints of the Variscan convergence and the successive Pangea break-up are preserved in the continental crust of both margins. These pre-Alpine signatures are preserved either in small volumes within the exhumed continental crust slivers of the axial belt or in the Alpine hinterland and foreland crusts that were never reworked in deep-seated conditions. Because of the common occurrence of metamorphic and igneous markers of the Variscan convergence in the pre-Alpine continental crust, the Permian-Triassic high thermal regime, which left widespread metamorphic and igneous imprints, can be interpreted as the effect of lithospheric thinning leading to continental rifting and subsequent Tethyan ocean opening. In addition, even Permian igneous activity and related basin formation may be interpreted as linked to lithospheric extension leading to the Pangaea break-up and to subsequent oceanization, predating the marine transgression from the east, where the Neotethys Ocean was opening. Permian-Triassic HT-LP metamorphic imprints recorded in the Austroalpine continental units are correlatable to this scenario; these peculiar metamorphic patterns have been widely recognized in lower, intermediate and upper continental crust. HT assemblages mark newly differentiated foliations locally associated with discrete shear zones, mainly in metapelites. Ages inferred for Tmax conditions are comprised between 220 and 295 Ma and Jurassic mineral ages are obtained where LP-LT metamorphic imprints, recorded during uplift and associated with fluid circulation, are dominant. The uplift paths are usually characterized by a high T/P ratio and wide parts of the uplift paths occurred under high thermal regime and the exhumation of some of the deep

  6. Microscale effects of melt infiltration into the lithospheric mantle: Peridotite xenoliths from Xilong, South China

    NASA Astrophysics Data System (ADS)

    Lu, Jianggu; Zheng, Jianping; Griffin, William L.; O'Reilly, Suzanne Y.; Pearson, Norman J.

    2015-09-01

    Melting and reactions between minerals and melts are important processes in the evolution of the lithospheric mantle, and are usually inferred from their geochemical fingerprints in mantle samples. However, a suite of mantle-derived peridotite xenoliths from the Xilong area, South China, records the reaction of successive silicate melts of different compositions with mineral assemblages in the mantle, preserved by quenching during entrainment. These xenoliths form two groups and record a compositionally layered mantle. Group 1 has olivine Mg# ~ 91, (and is thus relatively refractory), is derived from depths of ~ 50-65 km, and shows the trace-element geochemical signature of "old" carbonatitic metasomatism. Group 2 is more fertile with olivine Mg# mainly ~ 89-90, is derived from ~ 40 to 55 km and has ubiquitous modal spinel. Xenoliths of both groups then show sequential infiltration by two compositionally distinct melts (Na-rich and K-rich) not long before eruption. The Na-rich melts are enclosed in spongy clinopyroxene and spinel rims and are inferred to have triggered the reactions that formed the spongy rims, which have lower Al2O3, Na2O and Mg#, but higher FeO, TiO2 and Cr# than the primary phases. The undersaturated Na-rich mafic melts were probably formed in the asthenosphere by low-degree melting. The K-rich melts occur mainly in reaction zones around orthopyroxene and in reaction patches containing fine-grained secondary olivine, clinopyroxene and minor spinel. The melts have high contents of SiO2, K2O (mean 14.3 wt.%), Rb, Ba, and LREE but very low Na2O/K2O (0.01-0.29), positive anomalies in Eu and Sr, and variable HFSE anomalies. These compositional characteristics are consistent with an origin as low-degree partial melts of pre-existing phlogopite-bearing rocks. The K-rich melts also react with primary olivine, and the spongy-textured secondary clinopyroxene and spinel inferred to have formed by reaction with the Na-rich melts, yielding secondary olivine

  7. Mesozoic-Cenozoic thermal evolution of lithospheric mantle beneath the North China Craton: evidence from REE-in-two-pyroxene temperatures of mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Wang, C.; Xu, W.; Liang, Y.

    2014-12-01

    Thermal state is an important property for us to understand the nature of the lithospheric mantle beneath the North China Craton (NCC). Traditionally, it was obtained by calculating equilibrium temperatures for the mantle xenoliths using thermometers based on major element compositions of coexisting minerals. A REE-in-two-pyroxene thermometer developed by Liang et al. (2013) is able to extract near-solidus temperatures,which can deduce the thermal histories of mantle rocks in combination with major-element-in-two-pyroxene temperatures (Tmaj). We calculated REE temperatures (TREE) for mantle samples from the NCC including ancient refractory peridotites entrained by Early Cretaceous high-Mg diorites from the central NCC (Fushan), Mantle pyroxenites entrained by Early Cretaceous basalts from the eastern NCC (Feixian and Fangcheng), and fertile/moderately depleted peridotites entrained by <100 Ma basalts from the central and eastern NCC. The Fushan peridotites have low Tmaj (<880°C) and mismatched high TREE (780-1150°C), indicating that the ancient mantle was subjected to melt-rock reactions. The Feixian and Fangcheng pyroxenites have both high Tmaj (>890°C) and high TREE - Tmaj values (80-220°C), suggestive of a hot and fast-cooled mantle in Early Cretaceous. The peridotites in <100 Ma basalts have similar TREE and Tmaj, characteristics of well-equilibrated mantle. Based on the thermometric data, we suggest that the transition of nature of the NCC lithospheric mantle is marked by the interaction between ancient lithospheric mantle and hot melt derived from recycling lower crust in Early Cretaceous. After that a fertile mantle was accreted from upwelling asthenosphere, which accomplished the replacement of the NCC lithsospheric mantle. Reference: Liang et al., 2013, A REE-in-two-pyroxene thermometer for mafic and ultramafic rocks. Geochimica et Cosmochimia Acta 102, 246-260.

  8. Rapid Cenozoic ingrowth of isotopic signatures simulating "HIMU" in ancient lithospheric mantle: Distinguishing source from process

    NASA Astrophysics Data System (ADS)

    McCoy-West, Alex J.; Bennett, Vickie C.; Amelin, Yuri

    2016-08-01

    Chemical and isotopic heterogeneities in the lithospheric mantle are increasingly being recognised on all scales of examination, although the mechanisms responsible for generating this variability are still poorly understood. To investigate the relative behaviour of different isotopic systems in off-cratonic mantle, and specifically the origin of the regional southwest Pacific "HIMU" (high time integrated 238U/204Pb) Pb isotopic signature, we present the first U-Th-Pb, Rb-Sr, Sm-Nd and Re-Os isotopic dataset for spinel peridotite xenoliths sampling the subcontinental lithospheric mantle (SCLM) beneath Zealandia. Strongly metasomatised xenoliths converge to a restricted range of Sr and Nd isotopic compositions (87Sr/86Sr = 0.7028-0.7033; εNd ≈ +3-+6) reflecting pervasive overprinting of their original melt depletion signatures by carbonatite-rich melts. In contrast, rare, weakly metasomatised samples possess radiogenic Nd isotopic compositions (εNd > +15) and unradiogenic Sr isotopic compositions (87Sr/86Sr < 0.7022). This is consistent with melt extraction at ca. 2.0 Ga and in accord with widespread Paleoproterozoic Re-Os model ages from both weakly metasomatised and the more numerous, strongly metasomatised xenoliths. The coupling of chalcophile (Os), and lithophile (Sr and Nd) melt depletion ages from peridotite xenoliths on a regional scale under Zealandia argues for preservation of a significant mantle keel (⩾2 million km3) associated with a large-scale Paleoproterozoic melting event. Lead isotopic compositions are highly variable with 206Pb/204Pb = 17.3-21.3 (n = 34) and two further samples with more extreme compositions of 22.4 and 25.4, but are not correlated with other isotopic data or U/Pb and Th/Pb ratios in either strongly or weakly metasomatised xenoliths; this signature is thus a recent addition to the lithospheric mantle. Lead model ages suggest that this metasomatism occurred in the last 200 m.y., with errorchrons from individual localities

  9. The redox conditions of anhydrous and hydrous xenoliths of suprasubduction and intraplate lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Bonadiman, C.; Coltorti, M.

    2012-12-01

    The oxidation state of the upper mantle, its relationship with C-H-O fluids speciation and tectonic settings has been debated for decades and the various modelling have considered the prevalent role of the hydrous minerals over nominally anhydrous minerals (and the opposite) as well as the dissolution of silicate minerals (as providers of Fe3+ to the system) as directly related to water activity and oxygen fugacity. Each of these modelling has different implications for mantle rheology, seismic structure, and the evolution of the lithosphere (i.e.: Karato and Jung, 1998, Hirshmann, 2006). Upper mantle is the only part of the Earth's mantle where the oxygen fugacity can be directly measured, its values/variation being dependent on various processes such as partial melting and metasomatism often operating in time and space without solution of continuity. Recent general reviews of oxygen thermobarometry measurements (Forst & McCammon, 2008; Foley, 2011) indicate that the oxygen fugacity at the top of the upper mantle falls within ±2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer. There is also a general consensus in considering H2O as the strongest oxidizing agent in mantle metasomatic fluids, its activity leading to the formation of amphibole and raising the mantle redox state. This contribution presents fO2 and water activity results from three spinel-bearing mantle xenolith localities and distinct geodynamic settings: Ichinomegata (Japan) amphibole-bearing peridotites entrained in calc-alkaline basalts and Cerro Fraile (South Patagonia, Argentina), mostly anhydrous lherzolites and pirossenites brought up to the surface by alkaline basalts representing fragments of sub-arc mantle and Baker Rocks, Victoria Land (Antarctica), amphibole-bearing lherzolites representing portion of intraplate subcontinental lithospheric mantle. The three mantle sectors records fO2 values in the range of -1.9 to +0.8 log units of the FQM buffer. and low to very low aH2O

  10. Flexural behavior of the continental lithosphere in Italy - Constraints imposed by gravity and deflection data

    NASA Technical Reports Server (NTRS)

    Royden, Leigh

    1988-01-01

    This paper presents a simple model that describes the geometry of subducted continental lithosphere as the result of flexural loading of a thin elastic sheet. The method involves a procedure for systematically matching the plate flexure and gravity data to the best-fitting two-dimensional flexural geometry obtained by a thin elastic sheet of specified elastic strength. It is shown that the model can explain both the deflection of the lithosphere and the gravity anomalies observed along four profiles across the Apennine convergent system in Italy. The results of calculations suggest that in subduction systems, like the Apennines, that are characterized by backarc extension, forces acting on subducted lithosphere at depth can be expressed at the surface in the geometry of the foredeep basin that forms adjacent to the subduction zone.

  11. Non-depleted sub-continental mantle beneath the Superior Province of the Canadian Shield: Nd-Sr isotopic and trace element evidence from Midcontinent Rift basalts

    SciTech Connect

    Paces, J.B. ); Bell, K. )

    1989-08-01

    Midcontinent Rift flood basalts represent a sample of the relatively shallow, sub-continental upper mantle beneath the Canadian Shield at 1.1 Ga. A thick sequence of olivine tholeiite lavas, including minor intermediate to rhyolitic lavas, from the Portage Lake Volcanics (PLV) in northern Michigan have initial Nd and Sr isotopic compositions which cluster near Bulk Earth values. The effects of assimilation of old LREE-enriched continental crust into mantle-derived fractionating liquids are isotopically discernible in evolved lavas as well as in olivine tholeiites from the lowest portion of the volcanic pile. However, the effects of crustal contamination decrease with stratigraphic height and are absent in more primitive lavas in the upper half of the section. The source for PLV tholeiites is substantially less depleted than previously reported mantle values from the Superior Province. An origin for the PLV source is compatible with either of several mantle evolution models. The PLV source may have been associated with upwelling of a LIL element-enriched, asthenospheric plume which emplaced non-depleted material from deeper sources into the shallow sub-continental mantle beneath the Midcontinent Rift during continental break-up. Alternatively, the PLV source may have originated by enrichment of refractory sub-continental lithospheric mantle which was previously depleted in incompatible trace elements during Archean-aged melt extraction and continental crust formation. Concurrent generation of carbonatite magmas in other areas beneath the Superior Province indicates the widespread presence of sub-continental mantle with substantially higher {epsilon}{sub Nd}(T) and lower {epsilon}{sub Sr}(T) than the PLV source.

  12. 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 mantle lithosphere due to late-stage melt impregnation.

  13. A Caledonian age for reflectors within the mantle lithosphere north and west of Scotland

    NASA Astrophysics Data System (ADS)

    Snyder, David B.; Flack, Catherine A.

    1990-08-01

    This paper synthesizes geophysical data from two areas north and west of the Scottish mainland, the sites of grids composed of over 1600 and 650 line km, respectively, of deep (≥15 s two-way travel time) seismic reflection profiles. These grids make possible migration of generally eastward-dipping reflections in order to locate correctly in three dimensions three reflector surfaces within the uppermost mantle lithosphere. North of Scotland part of the reflector surface strikes to the northwest and forms a plunging fold structure that arches several kilometers into the lower crust near its hingeline. At depths around 50 km the reflector splits into two surfaces, one subhorizontal and one dipping to the east. Gravity modeling suggests that this latter surface may be a normal fault or shear zone and therefore Permo-Triassic extensional features may be superposed on older, subhorizontal mantle reflectors. Similarities in reflection patterns observed both north and west of Scotland lead us to suggest that these reflections come from three parts of a once continuous structure over 700 km long. Late Caledonian age left-lateral offsets of over 200 km are inferred to have disrupted this structure. Restoration of this offset plus tens of kilometers of subsequent extension recreates an uppermost mantle feature parallel to the Caledonian erogenic front and dipping toward the Caledonian suture along the Laurentian plate margin. If our restoration is correct in assuming regional-scale coupling between crustal faulting and uppermost mantle deformation, some mantle structures have persisted for over 400 Ma. Eclogite or metasomatized mantle rock within zones several hundred meters thick appear likely sources of impedance contrast producing the mantle reflections.

  14. Petrology and geochemistry of mantle xenoliths from the Kapsiki Plateau (Cameroon Volcanic Line): Implications for lithospheric upwelling

    NASA Astrophysics Data System (ADS)

    Tamen, Jules; Nkoumbou, Charles; Reusser, Eric; Tchoua, Felix

    2015-01-01

    Mantle xenoliths hosted by Oligocene alkaline basalts of the Kapsiki Plateau, northern end of the Cameroon Volcanic Line consist of group I spinel and plagioclase peridotites, mainly protogranular and accessorily porphyroclastic. The sub-continental lithospheric mantle here is heterogeneous and encloses both depleted and fertile components. Minerals exhibit wide range major element compositions compared to Nyos and Kumba grabens equivalent rocks. Spinel occurs as homogeneous brown crystals or as composite (brown-core-dark-rim) crystals when in contact with diopside or swatted in melt pools. Clinopyroxene crystals are either spinel exsolution-bearing or exsolution-free, the latter being often skeletal or frameworked and riddled with intracrystalline melt pools. Intraxenolith melt pockets and veinlets are always associated to plagioclase-bearing samples. Feldspars depict two distinctive compositions (An37-66Ab57-32Or6-2 and An3-7Ab52-62Or31-48) partly attributed to host xenolith type and to the involvement in the spinel and/or diopside melting reaction of an infiltrating alkali and carbonate-rich liquid. Petrographic and geochemical data discriminate melt pockets from their host basalts, excluding thus infiltration of basaltic melt as prospective origin. Thermo-barometric estimates reveal that prior to their entrainment the Kapsiki mantle xenoliths experienced two P-T equilibrium stages resulting in subsolidus re-equilibration from spinel- to plagioclase-facies conditions. Furthermore mineral textural relations show that the occurrence of plagioclase and melts inclusions is linked to spinel and/or diopside breakdown, likely subsequent to decompression and/or metasomatic induced melting events predating Oligo-Miocene volcanism.

  15. Lithospheric flexure at the Hawaiian Islands and its implications for mantle rheology

    NASA Astrophysics Data System (ADS)

    Zhong, Shijie; Watts, Anthony

    2014-05-01

    The response of the lithosphere to long-term geological loads such as volcanoes, sediments and ice provide important insights to both plate mechanics and mantle dynamics. One of the largest loads on Earth's surface are the shield volcanoes that comprise the Hawaiian Islands in the Central Pacific Ocean. We have developed a 3-D finite element model for calculating the flexure and stress associated with the emplacement of an arbitrary-shaped volcano load on a crust and mantle with realistic non-linear viscoelastic rheology, including frictional sliding, low-temperature plasticity, and high-temperature creep. By comparing model predictions with seismic reflection and refraction observations of the depth to the top of the oceanic crust and the depth dependence of seismicity at the Hawaiian Islands, we have been able to constrain the long-term rheological properties of intraplate, plume influenced, Late Cretaceous (83-96 Ma) oceanic lithosphere. Our calculations show that while the load-induced surface flexure is insensitive to high-temperature creep, it is sensitive to both the frictional sliding and low-temperature plasticity laws. Results show that a frictional coefficient ranging from 0.25 to 0.70 and a low-temperature plasticity law that is significantly weaker than ones recently proposed from experimental rock mechanics data are required in order to account for the observations. For example, a frictional coefficient of 0.1 weakens the shallow part of the lithosphere so much that it causes the minima in strain rate and stress to occur at too large depths to be consistent with the observed depth distribution of seismicity while the low-temperature plasticity law of Mei et al (2010) strengthens the deep part of the lithosphere so much that it predicts too small an amplitude and long a wavelength flexure compared to the observed. Our best fit model suggest the maximum stress that accumulates in the flexed lithosphere beneath the Hawaiian Islands is 100-200 MPa, and

  16. Geochemistry of oceanic carbonatites compared with continental carbonatites: mantle recycling of oceanic crustal carbonate

    NASA Astrophysics Data System (ADS)

    Hoernle, Kaj; Tilton, George; Le Bas, Mike; Duggen, Svend; Garbe-Schönberg, Dieter

    2001-11-01

    Major and trace element and Sr-Nd-Pb-O-C isotopic compositions are presented for carbonatites from the Cape Verde (Brava, Fogo, Sáo Tiago, Maio and Sáo Vicente) and Canary (Fuerteventura) Islands. Carbonatites show pronounced enrichment in Ba, Th, REE, Sr and Pb in comparison to most silicate volcanic rocks and relative depletion in Ti, Zr, Hf, K and Rb. Calcio (calcitic)-carbonatites have primary (mantle-like) stable isotopic compositions and radiogenic isotopic compositions similar to HIMU-type ocean island basalts. Cape Verde carbonatites, however, have more radiogenic Pb isotope ratios (e.g. 206Pb/204Pb=19.3-20.4) than reported for silicate volcanic rocks from these islands (18.7-19.9 Gerlach et al. 1988; Kokfelt 1998). We interpret calcio-carbonatites to be derived from the melting of recycled carbonated oceanic crust (eclogite) with a recycling age of 1.6 Ga. Because of the degree of recrystallization, replacement of calcite by secondary dolomite and elevated ∂13C and ∂18O, the major and trace element compositions of the magnesio (dolomitic)-carbonatites are likely to reflect secondary processes. Compared with Cape Verde calcio-carbonatites, the less radiogenic Nd and Pb isotopic ratios and the negative Δ7/4 of the magnesio-carbonatites (also observed in silicate volcanic rocks from the Canary and Cape Verde Islands) cannot be explained through secondary processes or through the assimilation of Cape Verde crust. These isotopic characteristics require the involvement of a mantle component that has thus far only been found in the Smoky Butte lamproites from Montana, which are believed to be derived from subcontinental lithospheric sources. Continental carbonatites show much greater variation in radiogenic isotopic composition than oceanic carbonatites, requiring a HIMU-like component similar to that observed in the oceanic carbonatites and enriched components. We interpret the enriched components to be Phanerozoic through Proterozoic marine carbonate (e

  17. Geochemistry of oceanic carbonatites compared with continental carbonatites: mantle recycling of oceanic crustal carbonate

    NASA Astrophysics Data System (ADS)

    Hoernle, Kaj; Tilton, George; Le Bas, Mike; Duggen, Svend; Garbe-Schönberg, Dieter

    Major and trace element and Sr-Nd-Pb-O-C isotopic compositions are presented for carbonatites from the Cape Verde (Brava, Fogo, Sáo Tiago, Maio and Sáo Vicente) and Canary (Fuerteventura) Islands. Carbonatites show pronounced enrichment in Ba, Th, REE, Sr and Pb in comparison to most silicate volcanic rocks and relative depletion in Ti, Zr, Hf, K and Rb. Calcio (calcitic)-carbonatites have primary (mantle-like) stable isotopic compositions and radiogenic isotopic compositions similar to HIMU-type ocean island basalts. Cape Verde carbonatites, however, have more radiogenic Pb isotope ratios (e.g. 206Pb/204Pb=19.3-20.4) than reported for silicate volcanic rocks from these islands (18.7-19.9 Gerlach et al. 1988; Kokfelt 1998). We interpret calcio-carbonatites to be derived from the melting of recycled carbonated oceanic crust (eclogite) with a recycling age of 1.6 Ga. Because of the degree of recrystallization, replacement of calcite by secondary dolomite and elevated ∂13C and ∂18O, the major and trace element compositions of the magnesio (dolomitic)-carbonatites are likely to reflect secondary processes. Compared with Cape Verde calcio-carbonatites, the less radiogenic Nd and Pb isotopic ratios and the negative Δ7/4 of the magnesio-carbonatites (also observed in silicate volcanic rocks from the Canary and Cape Verde Islands) cannot be explained through secondary processes or through the assimilation of Cape Verde crust. These isotopic characteristics require the involvement of a mantle component that has thus far only been found in the Smoky Butte lamproites from Montana, which are believed to be derived from subcontinental lithospheric sources. Continental carbonatites show much greater variation in radiogenic isotopic composition than oceanic carbonatites, requiring a HIMU-like component similar to that observed in the oceanic carbonatites and enriched components. We interpret the enriched components to be Phanerozoic through Proterozoic marine carbonate (e

  18. The contribution of the Precambrian continental lithosphere to global H2 production

    NASA Astrophysics Data System (ADS)

    Lollar, Barbara Sherwood; Onstott, T. C.; Lacrampe-Couloume, G.; Ballentine, C. J.

    2014-12-01

    Microbial ecosystems can be sustained by hydrogen gas (H2)-producing water-rock interactions in the Earth's subsurface and at deep ocean vents. Current estimates of global H2 production from the marine lithosphere by water-rock reactions (hydration) are in the range of 1011 moles per year. Recent explorations of saline fracture waters in the Precambrian continental subsurface have identified environments as rich in H2 as hydrothermal vents and seafloor-spreading centres and have suggested a link between dissolved H2 and the radiolytic dissociation of water. However, extrapolation of a regional H2 flux based on the deep gold mines of the Witwatersrand basin in South Africa yields a contribution of the Precambrian lithosphere to global H2 production that was thought to be negligible (0.009 × 1011 moles per year). Here we present a global compilation of published and new H2 concentration data obtained from Precambrian rocks and find that the H2 production potential of the Precambrian continental lithosphere has been underestimated. We suggest that this can be explained by a lack of consideration of additional H2-producing reactions, such as serpentinization, and the absence of appropriate scaling of H2 measurements from these environments to account for the fact that Precambrian crust represents over 70 per cent of global continental crust surface area. If H2 production via both radiolysis and hydration reactions is taken into account, our estimate of H2 production rates from the Precambrian continental lithosphere of 0.36-2.27 × 1011 moles per year is comparable to estimates from marine systems.

  19. The contribution of the Precambrian continental lithosphere to global H2 production.

    PubMed

    Lollar, Barbara Sherwood; Onstott, T C; Lacrampe-Couloume, G; Ballentine, C J

    2014-12-18

    Microbial ecosystems can be sustained by hydrogen gas (H2)-producing water-rock interactions in the Earth's subsurface and at deep ocean vents. Current estimates of global H2 production from the marine lithosphere by water-rock reactions (hydration) are in the range of 10(11) moles per year. Recent explorations of saline fracture waters in the Precambrian continental subsurface have identified environments as rich in H2 as hydrothermal vents and seafloor-spreading centres and have suggested a link between dissolved H2 and the radiolytic dissociation of water. However, extrapolation of a regional H2 flux based on the deep gold mines of the Witwatersrand basin in South Africa yields a contribution of the Precambrian lithosphere to global H2 production that was thought to be negligible (0.009 × 10(11) moles per year). Here we present a global compilation of published and new H2 concentration data obtained from Precambrian rocks and find that the H2 production potential of the Precambrian continental lithosphere has been underestimated. We suggest that this can be explained by a lack of consideration of additional H2-producing reactions, such as serpentinization, and the absence of appropriate scaling of H2 measurements from these environments to account for the fact that Precambrian crust represents over 70 per cent of global continental crust surface area. If H2 production via both radiolysis and hydration reactions is taken into account, our estimate of H2 production rates from the Precambrian continental lithosphere of 0.36-2.27 × 10(11) moles per year is comparable to estimates from marine systems.

  20. The contribution of the Precambrian continental lithosphere to global H2 production.

    PubMed

    Lollar, Barbara Sherwood; Onstott, T C; Lacrampe-Couloume, G; Ballentine, C J

    2014-12-18

    Microbial ecosystems can be sustained by hydrogen gas (H2)-producing water-rock interactions in the Earth's subsurface and at deep ocean vents. Current estimates of global H2 production from the marine lithosphere by water-rock reactions (hydration) are in the range of 10(11) moles per year. Recent explorations of saline fracture waters in the Precambrian continental subsurface have identified environments as rich in H2 as hydrothermal vents and seafloor-spreading centres and have suggested a link between dissolved H2 and the radiolytic dissociation of water. However, extrapolation of a regional H2 flux based on the deep gold mines of the Witwatersrand basin in South Africa yields a contribution of the Precambrian lithosphere to global H2 production that was thought to be negligible (0.009 × 10(11) moles per year). Here we present a global compilation of published and new H2 concentration data obtained from Precambrian rocks and find that the H2 production potential of the Precambrian continental lithosphere has been underestimated. We suggest that this can be explained by a lack of consideration of additional H2-producing reactions, such as serpentinization, and the absence of appropriate scaling of H2 measurements from these environments to account for the fact that Precambrian crust represents over 70 per cent of global continental crust surface area. If H2 production via both radiolysis and hydration reactions is taken into account, our estimate of H2 production rates from the Precambrian continental lithosphere of 0.36-2.27 × 10(11) moles per year is comparable to estimates from marine systems. PMID:25519136

  1. Mantle dynamics in Mars and Venus: Influence of an immobile lithosphere on three-dimensional mantle convection

    SciTech Connect

    Schubert, G.; Bercovici ); Glatzmaier, G.A. )

    1990-08-30

    Numerical calculations of fully three-dimensional convection in constant viscosity, compressible spherical shells are interpreted in terms of possible convective motions in the mantles of Venus and Mars. The shells are heated both internally and from below to account for radiogenic heating, secular cooling, and heat flow from the core. The lower boundary of each of the shells is isothermal and shear stress free, as appropriate to the interface between a mantle and a liquid outer core. The upper boundary of each of the shells is rigid and isothermal, as appropriate to the base of a thick immobile lithosphere. Calculations with shear stress-free upper boundaries are also carried out to assess the role of the rigid surface condition. The ratio of the inner radius of each shell to its outer radius is in accordance with possible core sizes in both Venus and Mars. A calculation is also carried out for a Mars model with a small core to simulate mantle convection during early core formation. Different relative proportions of internal and bottom heating are investigated, ranging from nearly complete heating from within to almost all heating from below. The Rayleigh numbers of all the cases are approximately 100 times the critical Rayleigh numbers for the onset of convection. Cylindrical plumes are the prominent form of upwelling in the models independent of the surface boundary condition so long as sufficient heat derives from the core. Thus major volcanic centers on Mars, such as Tharsis and Elysium, and the coronae and some equatorial highlands on Venus may be the surface expressions of cylindrical mantle plumes.

  2. Mesoproterozoic orangeites of Karelia (Kostomuksha-Lentiira): evidence for composition of mantle lithosphere

    NASA Astrophysics Data System (ADS)

    Kargin, Alexey; Nosova, Anna; Larionova, Yulia; Kononova, Voctoria; Borisovskiy, Sergey; Kovalchuk, Elena; Griboedova, Irina

    2014-05-01

    The 1.23-1.20 Ga old diamondiferous lamproites and orangeites (kimberlites of II group) of the Kostomuksha-Taloveys and the Lentiira-Kuhmo dyke fields intrude the Archaean crust of the Karelian craton, NE of the East European Platform. Mineral (a trend of compositional evolution of mica, presence of carbonate minerals in basis, composition of olivine) and geochemical (major elements, ratio of trace elements, primitive mantle normalized trace elements patterns) characteristics of these rocks suggest an orangeitic rather than lamproitic or lamprophyric nature. The composition of Phl-Ol orangeites suggests intensive processes of fractional crystallization for their melts. Cpx-Phl-Ol orangeites indicate higher intensity of lithospheric mantle assimilation then other orangeitic types. Phl-Carb orangeites of the Taloveys area and Cpx-Phl-Ol one of the Lentiira area are closest to primary melts. The Ol-Phl-Cpx orangeites of the Lentiira area contain three generations of unaltered olivine that vary in composition and origin: a) xenocryst derived from depleted mantle peridotite; b) orangeitic olivine phenocryst and c) and olivine like early stage crystallization of megacryst assemblage or a product of metasomatic interaction between mantle peridotite and protokimberlitic melt. Orangeites of Kostomuksha-Lentiira have low- and medium-radiogenic value of (87Sr/86Sr)1200 that range from 0.7038 to 0.7067. Phl-Carb orangeites of Taloveys have less radiogenic isotopic composition of Nd (eNd -11 ... -12) then Cpx-Phl-Ol and Phl-Ol orangeites of Kostomuksha (eNd -6.9 ... -9.4). The study of Sm-Nd and Rb-Sr isotopic systems suggests that an ancient metasomatic mantle source took part in origin of orangeites. We propose a two-steps model of origin of their source (Kargin et al., 2014): 1) The metasomatic component of mantle source (like as MARID-type veins) formed during Lapland-Kola and/or Svecofennian orogeny events (2.1-1.8 Ga ago). 2) The intrusion of orangeites is comparable by

  3. Thermo-mechanical modeling of continental rift evolution over mantle upwelling in presence of far-field stresses

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    We conducted fully-coupled high resolution rheologically consistent 3D thermo-mechanical numerical models to investigate the processes of mantle-lithosphere interaction (MLI) in presence of preexisting far-field tectonic stresses. MLI-induced topography exhibits strongly asymmetric small-scale 3D features, such as rifts, flexural flank uplifts and complex faults structures. This suggests a dominant role of continental rheological structure and intra-plate stresses in controlling continental rifting and break-up processes above mantle upwelling while reconciling the passive (far-field tectonic stresses) versus active (plume-activated) rift concepts as our experiments show both processes in action. We tested different experiments by varying two principal controlling parameters: 1) horizontal extension velocity and 2) Moho temperature used as simplified indicator of the thermal and rheological lithosphere layering. An increase in the applied extension expectedly gives less localized deformation at lithospheric scale: the growth of external velocity from 1.5 mm/years to 6 mm/years leads to enlargement of the rift zones from 75-175 km to 150-425 km width. On the contrary, increasing of the lithospheric geotherm has an opposite effect leading to narrowing of the rift zone: the change of the Moho isotherm from 600°C to 800°C causes diminution of the rift width from 175-425 km to 75-150 km. Some of these finding are contra-intuitive in terms of usual assumptions. The models refer to strongly non-linear impact of far-field extension rates on timing of break-up processes. Experiments with relatively fast far-field extension (6 mm/years) show intensive normal fault localization in crust and uppermost mantle above the plume head at 15-20 Myrs after the onset of the experiment. When plume head material reaches the bottom of the continental crust (at 25 Myrs), the latter is rapidly ruptured (<1 Myrs) and several steady oceanic floor spreading centers develop. Slower (3 mm

  4. Formation of Secondary Lherzolite and Refertilization of the Subcontinental Lithospheric Mantle: The Record of Orogenic Peridotites

    NASA Astrophysics Data System (ADS)

    Garrido, Carlos J.; Varas-Reus, María Isabel; Bodinier, Jean-Louis; Marchesi, Claudio; Bosch, Delphine; Hidas, Károly

    2016-04-01

    Correlations observed between major and minor transition elements in tectonically-emplaced orogenic peridotites have classically been ascribed to variable degrees of melt extraction. There is a growing body of evidence indicating that these chemical variations mostly reflect melt redistribution and near solidus reactions superimposed onto previous melting depletion events. Here we will assess this hypothesis using a large database of peridotites from orogenic peridotites in the westernmost Mediterranean (Ronda and Beni Bousera peridotites). We show that lherzolite samples show some trends in major elements and modal variations that are inconsistent with their interpretation as depleted MORB mantle (DMM). These trends are more consistent with the secondary formation of lherzolites by refertilization processes involving a least two different near-solidus, melt-processes: refertilization by pyroxenite-derived melts and by hydrous melts leading, respectively, to secondary lherzolites with Ol/Opx and Cpx/Opx ratios greater than those expected from residues from a primitive upper mantle source. Together with their N-MORB, LREE-depleted pattern, their fertile lherzolitic composition may have been acquired as a result of melt-rock interaction processes associated with the thermomechanical erosion of lithospheric mantle by asthenosphere. Major refertilization of depleted subcontinental mantle is an alternative to the small degrees of melt extraction to account for LREE depletion in otherwise fertile orogenic lherzolites.

  5. The complex 3-D transition from continental crust to backarc magmatism and exhumed mantle in the Central Tyrrhenian basin

    NASA Astrophysics Data System (ADS)

    Prada, M.; Sallares, V.; Ranero, C. R.; Vendrell, M. G.; Grevemeyer, I.; Zitellini, N.; de Franco, R.

    2015-10-01

    Geophysical data from the MEDOC experiment across the Northern Tyrrhenian backarc basin has mapped a failed rift during backarc extension of cratonic Variscan lithosphere. In contrast, data across the Central Tyrrhenian have revealed the presence of magmatic accretion followed by mantle exhumation after continental breakup. Here we analyse the MEDOC transect E-F, which extends from Sardinia to the Campania margin at 40.5°N, to define the distribution of geological domains in the transition from the complex Central Tyrrhenian to the extended continental crust of the Northern Tyrrhenian. The crust and uppermost mantle structure along this ˜400-km-long transect have been investigated based on wide-angle seismic data, gravity modelling and multichannel seismic reflection imaging. The P-wave tomographic model together with a P-wave-velocity-derived density model and the multichannel seismic images reveal seven different domains along this transect, in contrast to the simpler structure to the south and north. The stretched continental crust under Sardinia margin abuts the magmatic crust of Cornaglia Terrace, where accretion likely occurred during backarc extension. Eastwards, around Secchi seamount, a second segment of thinned continental crust (7-8 km) is observed. Two short segments of magmatically modified continental crust are separated by the ˜5-km-wide segment of the Vavilov basin possibly made of exhumed mantle rocks. The eastern segment of the 40.5°N transect E-F is characterized by continental crust extending from mainland Italy towards the Campania margin. Ground truthing and prior geophysical information obtained north and south of transect E-F was integrated in this study to map the spatial distribution of basement domains in the Central Tyrrhenian basin. The northward transition of crustal domains depicts a complex 3-D structure represented by abrupt spatial changes of magmatic and non-magmatic crustal domains. These observations imply rapid variations

  6. Low crustal velocities and mantle lithospheric variations in southern Tibet from regional Pnl waveforms

    NASA Astrophysics Data System (ADS)

    Rodgers, Arthur J.; Schwartz, Susan Y.

    We report low average crustal P-wave velocities (5.9-6.1 km/s, Poisson's ratio 0.23-0.27, thickness 68-76 km) in southern Tibet from modelling regional Pnl waveforms recorded by the 1991-1992 Tibetan Plateau Experiment. We also find that the mantle lithosphere beneath the Indus-Tsangpo Suture and the Lhasa Terrane is shield-like (Pn velocity 8.20-8.25 km/s, lid thickness 80-140 km, positive velocity gradient 0.0015-0.0025 s-1). Analysis of relative Pn travel time residuals requires a decrease in the mantle velocities beneath the northern Lhasa Terrane, the Banggong-Nujiang Suture and the southern Qiangtang Terrane. Tectonic and petrologic considerations suggest that low bulk crustal velocities could result from a thick (50-60 km) felsic upper crust with vertically limited and laterally pervasive partial melt. These results are consistent with underthrusting of Indian Shield lithosphere beneath the Tibetan Plateau to at least the central Lhasa Terrane.

  7. Determination of sub-lithospheric stress due to mantle convection using GOCE gradiometric data over Iran

    NASA Astrophysics Data System (ADS)

    Eshagh, Mehdi; Romeshkani, Mohsen

    2015-11-01

    Sub-lithospheric stress due to mantle convection can be determined from gravimetric data based on Runcorn's theory. In this paper, the satellite gradiometric data of the recent European satellite mission, the Gravity field and steady-state Ocean Circulation Explorer (GOCE) is used to determine the sub-lithospheric stress locally in Iran. The method of S function (SF) with numerical differentiation is developed further and an integral equation connecting satellite gradiometric data to SF is presented. The integral equation will be used to invert the real gradiometric data of GOCE to recover the SF. Later on, the sub-lithospheric shear stresses, which are the northward and eastward derivatives of the SF, are computed numerically. Our numerical results show that the mean squares error of the recovered SF is smaller than the values of the SF meaning that the recovery process is successful. Also, the recovered stress has a good agreement with the tectonic boundaries and active seismic points of the world stress map (WSM) database. This stress reaches amplitude of 100 MPa in the territory.

  8. Structure of the mantle lithosphere beneath the Siberian kimberlite pipes reconstructed by monomineral thermobarometry

    NASA Astrophysics Data System (ADS)

    Ashchepkov, I. V.

    2009-04-01

    The original methods of the monomineral thermobarometry for clinopyroxene, garnet, ilmenite, chromite (Ashchepkov,2008) and orthopyroxene (Brey, Kohler, 1990- McGregor, 1974) thermobarometer allow to reconstruct the mantle columns. TP diagram for Udachnaya pipe suggests creation at least in tree stages of the melt percolation through the mantle column differing in Fe# and other parameters. The most high temperature (HT) (45 mvm-2) and Fe# rich refer to the last HT reactions with the protokimberlite melts formed the megacrystalline associations. Relict low temperature (LT) geotherm (35 45 mvm-2 and lower) is close to the conductive geotherm (Boyd et al., 1997). Most of ТР parameters for the minerals refer to the middle part of the geotherm (40- 45 mvm-2). Monomineral thermobarometry reconstructing the PTX values (Fe#; CrCpx, Cr-Ilm CaGar, TiChr) showing the high overlapping formed by the the melt percolation. The clinopyroxene growth in the mantle lithosphere in Daldyn, Akakite, Nakyn and Upper Muna are produced by the refertilization events under the influence of the protokimberlite melts. Their spreading in the lower part of mantle section of Garnet trend to subcalsic and pyroxenitic types is likely the result of submelting and heating of the mantle peridotites. Similar process for eclogites is responsible for the appearance of LT eclogites tracing subduction gradients and HT branches with the Ti- bearing associations corresponding to advective gradients. . For the larger pipes the scale of the perturbation is much higher then for smaller. The levels of the melt intrusions are reconstructed by the clotting of TP values inflections of TP paths and TiChr, CrIlm and Fe#. Ilmenite trends reveal the polybaric character of the fractionation and high degree interaction with the wall rock peridotites visible by CrIlm increase. The metasomatic associations differ in PTX diagrams by higher Cr and LT conditions the HT megacrystalls. The evident layered nature of the mantle

  9. Lithosphere structure and upper mantle characteristics below the Bay of Bengal

    NASA Astrophysics Data System (ADS)

    Rao, G. Srinivasa; Radhakrishna, M.; Sreejith, K. M.; Krishna, K. S.; Bull, J. M.

    2016-07-01

    The oceanic lithosphere in the Bay of Bengal (BOB) formed 80-120 Ma following the breakup of eastern Gondwanaland. Since its formation, it has been affected by the emplacement of two long N-S trending linear aseismic ridges (85°E and Ninetyeast) and by the loading of ca. 20-km of sediments of the Bengal Fan. Here, we present the results of a combined spatial and spectral domain analysis of residual geoid, bathymetry and gravity data constrained by seismic reflection and refraction data. Self-consistent geoid and gravity modelling defined by temperature-dependent mantle densities along a N-S transect in the BOB region revealed that the depth to the lithosphere-asthenosphere boundary (LAB) deepens steeply from 77 km in the south to 127 km in north, with the greater thickness being anomalously thick compared to the lithosphere of similar-age beneath the Pacific Ocean. The Geoid-Topography Ratio (GTR) analysis of the 85°E and Ninetyeast ridges indicate that they are compensated at shallow depths. Effective elastic thickness (Te) estimates obtained through admittance/ coherence analysis as well as the flexural modelling along these ridges led to the conclusions: (i) 85°E Ridge was emplaced in off-ridge environment (Te = 10-15 km); (ii) the higher Te values of ˜25 km over the Afanasy Nikitin Seamount (ANS) reflect the secondary emplacement of the seamount peaks in off-ridge environment, (iii) that the emplacement of the Ninetyeast Ridge north of 2°N occurred in an off-ridge environment as indicated by higher Te values (25-30 km). Furthermore, the admittance analysis of geoid and bathymetry revealed that the admittance signatures at wavelengths >800 km are compensated by processes related to upper mantle convection.

  10. Simulating the Thermochemical Magmatic and Tectonic Evolution of Venus's Mantle and Lithosphere: Intrusive vs. Extrusive Magmatism

    NASA Astrophysics Data System (ADS)

    Tackley, Paul; Armann, Marina

    2013-04-01

    Here we extend the models of [1]. Numerical convection models of the thermochemical evolution of Venus are compared to present-day topography and geoid, recent resurfacing history and surface deformation. The models include melting, magmatism, decaying heat-producing elements, core cooling, realistic temperature-dependent viscosity and either stagnant lid or episodic lithospheric overturn. In [1] it was found that in stagnant lid convection the dominant mode of heat loss is magmatic heat pipe, which requires massive magmatism and produces very thick crust, inconsistent with observations. Partitioning of heat-producing elements into the crust helps but does not help enough. Episodic lid overturn interspersed by periods of quiescence effectively loses Venus's heat while giving lower rates of volcanism and a thinner crust. Calculations predict 5-8 overturn events over Venus's history, each lasting ~150 Myr, initiating in one place and then spreading globally. During quiescent periods convection keeps the lithosphere thin. Magmatism keeps the mantle temperature constant over Venus's history. Crustal recycling occurs by entrainment in stagnant lid convection, and by lid overturn in episodic mode. Venus-like amplitudes of topography and geoid can be produced in either stagnant or episodic modes, with a viscosity profile that is Earth-like but shifted to higher values. The basalt density inversion below the olivine-perovskite transition causes compositional stratification around 730 km; breakdown of this layering increases episodicity but far less than episodic lid overturn. The classical stagnant lid mode with interior temperature rheological temperature scale lower than TCMB is not reached because mantle temperature is controlled by magmatism while the core cools slowly from a superheated start. Core heat flow decreases with time, possibly shutting off the dynamo, particularly in episodic cases. Here we extend [1] by considering intrusive magmatism as an alternative to

  11. Variable Water Concentrations in the Asthenospheric and Lithospheric Mantle Underneath the Eastern United States

    NASA Astrophysics Data System (ADS)

    Soles, B.; Brennan, G. W.; Johnson, E. A.; Mazza, S. E.; Gazel, E.

    2014-12-01

    An Eocene (47-48 Ma) volcanic swarm in NW Virginia represents the youngest episode of volcanism in the Eastern US, possibly initiated by delamination of lithospheric mantle (Mazza 2014). The Eocene swarm is located along the MAGIC seismic array (Crampton 2013). The phenocrysts and mantle xenocrysts within these volcanic rocks are the most direct constraints on the water content of the mantle in this region and will aid interpretation of geophysical data. In this study, we measured structural hydroxyl concentrations, [OH], in clinopyroxene (cpx) and olivine (ol) xenocrysts and cpx phenocrysts from three basaltic intrusions: Mole Hill, a volcanic neck, Trimble Knob, a diatreme, and Rt.631, a dike. Polarized FTIR spectra were obtained at JMU and the Smithsonian Institution. Mineral compositions were obtained on the electron microprobe at the USGS, Reston. The cpx xenocrysts show hydration profiles, whereas cpx phenocrysts have flat or dehydration profiles. Cpx xenocryst cores contain [OH]=25-300 ppm H2O and ol xenocrysts have [OH]<2 ppm. Cpx xenocryst rims contain [OH]=160-1300 ppm, and cpx phenocrysts have [OH]=100-570 ppm, and a cpx from Trimble Knob conservatively contains 1500-3500 ppm. Magmatic water contents calculated using O'Leary (2010) range from 0.3-4.9 wt% for xenocryst rims and phenocrysts, and >6 wt% at Trimble Knob. P and T were calculated using equilibrium exchange reactions from Putirka (2008). Xenocryst rims from Mole Hill have P=13.7±1.7 kbar and T=1287±24°C, and cpx phenocrysts from the Rt.631 dike record similar conditions of P=16.1±2.8 kbar and T=1339±37°C. A cpx phenocryst from Trimble Knob has P=23.8±4.0 kbar and T=1143±124°C. We interpret our data to indicate a dry lithospheric mantle as represented by the cpx and ol xenocrysts, underplated by a wet layer at the lithosphere-asthenosphere boundary produced by fractional crystallization of magma generated deeper in the asthenosphere, as represented by the cpx phenocrysts.

  12. Oxidation state of the lithospheric mantle beneath the Massif Central,France

    NASA Astrophysics Data System (ADS)

    Uenver-Thiele, L.; Woodland, A. B.; Downes, H.; Altherr, R.

    2012-04-01

    The Tertiary and Quaternary volcanism of the French Massif Central sampled the underlying subcontinental lithospheric mantle (SCLM) in the form of xenoliths over a wide geographic area of ~20.000km2. Such an extensive distribution of xenoliths provides an unique opportunity to investigate regional variations in mantle structure and composition. On the basis of textural and geochemical differences, Lenoir et al. (2000) and later Downes et al. (2003) identified two distinct domains in the SCLM lying north and south of latitude 45° 30' N, respectively. The northern domain is relatively refractory, but has experienced pervasive enrichment of LREE. The southern domain is generally more fertile, exhibiting depletion in LREE. A metasomatic overprint has developed to variable extents in many xenolith suites. The different histories of these two juxtaposed blocks of SCLM should also be reflected in their oxidation state, with local variations also to be expected due to metasomatic interactions. For example, if carbonate-melt metasomatism played a role in the LREE enrichment of the northern domain (Lenoir et al. 2000; Downes et al. 2003), then such mantle should be relatively oxidised. Since surprisingly little redox data are currently available, we are undertaking a study to determine the oxidation state of the SCLM beneath the Massif Central over the largest geographical area possible. All xenoliths investigated are spinel peridotites, mostly with protogranular textures (although some samples are porphyroclastic or equigranular). Most samples are nominally anhydrous although minor amphibole is present in some xenolith suites. Major element compositions of the individual minerals were determined by microprobe. Two-pyroxene temperatures (BKN) range from 750° to ~1200° C. Ferric iron contents of spinel were determined by Mössbauer spectroscopy and gave a range of Fe3+/ Fetot from 0.191 to 0.418, with a conservative uncertainty of ±0.02. These data were used to calculate

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

    NASA Astrophysics Data System (ADS)

    Kirby, Stephen H.; Stein, Seth; Okal, Emile A.; Rubie, David C.

    1996-05-01

    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

  14. Role of magmatism in continental lithosphere extension: an introduction to tectnophysics special issue

    SciTech Connect

    Van Wijk, Jolante W

    2008-01-01

    The dynamics and evolution of rifts and continental rifted margins have been the subject of intense study and debate for many years and still remain the focus of active investigation. The 2006 AGU Fall Meeting session 'Extensional Processes Leading to the Formation of Basins and Rifted Margins, From Volcanic to Magma-Limited' included several contributions that illustrated recent advances in our understanding of rifting processes, from the early stages of extension to breakup and incipient seafloor spreading. Following this session, we aimed to assemble a multi-disciplinary collection of papers focussing on the architecture, formation and evolution of continental rift zones and rifted margins. This Tectonophysics Special Issue 'Role of magmatism in continental lithosphere extension' comprises 14 papers that present some of the recent insights on rift and rifted margins dynamics, emphasising the role of magmatism in extensional processes. The purpose of this contribution is to introduce these papers.

  15. Joint geophysical and petrological models for the lithosphere structure of the Antarctic Peninsula continental margin

    NASA Astrophysics Data System (ADS)

    Yegorova, Tamara; Bakhmutov, Vladimir; Janik, Tomasz; Grad, Marek

    2011-01-01

    The Antarctic Peninsula (AP) is a composite magmatic arc terrane formed at the Pacific margin of Gondwana. Through the late Mesozoic and Cenozoic subduction has stopped progressively from southwest to northeast as a result of a series of ridge trench collisions. Subduction may be active today in the northern part of the AP adjacent to the South Shetland Islands. The subduction system is confined by the Shackleton and Hero fracture zones. The magmatic arc of the AP continental margin is marked by high-amplitude gravity and magnetic anomaly belts reaching highest amplitudes in the region of the South Shetland Islands and trench. The sources for these anomalies are highly magnetic and dense batholiths of mafic bulk composition, which were intruded in the Cretaceous, due to partial melting of upper-mantle and lower-crustal rocks. 2-D gravity and magnetic models provide new insights into crustal and upper-mantle structure of the active and passive margin segments of the northern AP. Our models incorporate seismic refraction constraints and physical property data. This enables us to better constrain both Moho geometry and petrological interpretations in the crust and upper mantle. Model along the DSS-12 profile crosses the AP margin near the Anvers Island and shows typical features of a passive continental margin. The second model along the DSS-17 profile extends from the Drake Passage through the South Shetland Trench/Islands system and Bransfield Strait to the AP and indicates an active continental margin linked to slow subduction and on-going continental rifting in the backarc region. Continental rifting beneath the Bransfield Strait is associated with an upward of hot upper mantle rocks and with extensive magmatic underplating.

  16. Lithospheric mantle evolution monitored by overlapping large igneous provinces: Case study in southern Africa

    NASA Astrophysics Data System (ADS)

    Jourdan, F.; Bertrand, H.; Féraud, G.; Le Gall, B.; Watkeys, M. K.

    2009-02-01

    Most of the studies on the large igneous provinces (LIPs) focus on Phanerozoic times, and in particular, those related to the disruption of Pangea (e.g. CAMP, Karoo, Parana-Etendeka) while Precambrian LIPs (e.g. Ventersdorpf, Fortescue) remain less studied. Although the investigation of Precambrian LIPs is difficult because they are relatively poorly preserved, assessment of their geochemical characteristics in parallel with younger overlapping LIP is fundamental for monitoring the evolution of the mantle composition through time. Recent 40Ar/ 39Ar dating of the Okavango giant dyke swarm (and related sills) in southern Africa showed that ~ 90% of the dykes were emplaced at 179 ± 1 Ma and belong to the Karoo large igneous province whereas ~ 10% of dykes yielded Proterozoic ages (~ 1-1.1 Ga). Here, we provide new major, trace and rare earth elements analyses of the low-Ti Proterozoic Okavango dyke swarm (PODS) that suggest, combined with age data, a cognate origin with the 1.1 Ga Umkondo large igneous province (UIP), southern Africa. The geochemical characteristics of the PODS and UIP basalts are comparable to those of overlapping low-Ti Karoo basalts, and suggest that both LIPs were derived from similar enriched mantle sources. A mantle plume origin for these LIPs is not easily reconciled with the geochemical dataset and the coincidence of two compositionally similar mantle plumes acting 900 Myr apart is unlikely. Instead, we propose that the Umkondo and Karoo large igneous provinces monitored the slight evolution of a shallow enriched lithospheric mantle from Proterozoic to Jurassic.

  17. Isotopic and trace-element constraints on mantle and crustal contributions to Siberian continental flood basalts, Noril'sk area, Siberia

    USGS Publications Warehouse

    Wooden, J.L.; Czamanske, G.K.; Fedorenko, V.A.; Arndt, N.T.; Chauvel, C.; Bouse, R.M.; King, B.-S.W.; Knight, R.J.; Siems, D.F.

    1993-01-01

    We present a tightly controlled and comprehensive set of analytical data for the 250-Ma Siberian flood-basalt province. Consideration of major- and trace-element compositions, along with strontium, lead and neodymium isotopic compositions, strongly supports earlier Russian subdivision of this magmatism into three magmatic cycles, giving rise to three assemblages of eleven basalt suites in the ascending order Ivakinsky-Gudchikhinsky, Khakanchansky-Nadezhdinsky and Morongovsky-Samoedsky. Geochemical and isotopic discontinuities of varying magnitude characterize most of the boundaries between the eleven recognized basalt suites in the Noril'sk area. Although we conclude that the dominant volume of erupted magma originated from an asthenospheric mantle plume, none of the lavas is interpreted to directly represent asthenospheric melts, which would have been far more magnesian. On the basis of thermal considerations, we consider it unlikely that vast volumes of basaltic melt were produced directly from the continental lithospheric mantle beneath the Siberian craton. Moreover, there is little evidence from mantle xenoliths that the geochemical signatures of such melts would correspond to those of the Siberian flood basalts. Studies of melt migration lead us to conclude that transport of asthenospheric melt through the lithospheric mantle would be rapid, by fracture propagation. Lavas from the Gudchikhinsky suite have negligible Ta-Nb anomalies and positive ??{lunate}Nd values and their parental magmas presumably interacted little with the continental lithospheric mantle or crust. All other lavas have negative Ta-Nb anomalies and lower ??{lunate}Nd values that we attribute to interaction with continental crust. The model that we have developed requires discrete contributions from the plume and complex processing of all erupted magmas in the continental crust. The earliest magmas represent small percentages of melt formed in equilibrium with garnet. Over time, the

  18. 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

  19. Seismic detection of folded, subducted lithosphere at the core-mantle boundary.

    PubMed

    Hutko, Alexander R; Lay, Thorne; Garnero, Edward J; Revenaugh, Justin

    2006-05-18

    Seismic tomography has been used to infer that some descending slabs of oceanic lithosphere plunge deep into the Earth's lower mantle. The fate of these slabs has remained unresolved, but it has been postulated that their ultimate destination is the lowermost few hundred kilometres of the mantle, known as the D'' region. Relatively cold slab material may account for high seismic velocities imaged in D'' beneath areas of long-lived plate subduction, and for reflections from a seismic velocity discontinuity just above the anomalously high wave speed regions. The D'' discontinuity itself is probably the result of a phase change in relatively low-temperature magnesium silicate perovskite. Here, we present images of the D'' region beneath the Cocos plate using Kirchhoff migration of horizontally polarized shear waves, and find a 100-km vertical step occurring over less than 100 km laterally in an otherwise flat D'' shear velocity discontinuity. Folding and piling of a cold slab that has reached the core-mantle boundary, as observed in numerical and experimental models, can account for the step by a 100-km elevation of the post-perovskite phase boundary due to a 700 degrees C lateral temperature reduction in the folded slab. We detect localized low velocities at the edge of the slab material, which may result from upwellings caused by the slab laterally displacing a thin hot thermal boundary layer.

  20. Pervasive, tholeiitic refertilisation and heterogeneous metasomatism in Northern Victoria Land lithospheric mantle (Antarctica)

    NASA Astrophysics Data System (ADS)

    Pelorosso, Beatrice; Bonadiman, Costanza; Coltorti, Massimo; Faccini, Barbara; Melchiorre, Massimiliano; Ntaflos, Theodoros; Gregoire, Michel

    2016-04-01

    The petrology of peridotite xenoliths in the Cenozoic volcanics from Greene Point (Northern Victoria Land, Antarctica) provides new constraints on the characterisation of the lithospheric mantle beneath the West Antarctic Rift. Based on mineral major and trace element models, this mantle domain is proposed to represent a residuum after 10% and 20% partial melting. Moreover, melting models and isotopic results for Sr and Nd systematics highlight the substantial contribution of tholeiitic melts percolating through peridotites. Close correlation with trace element contents in clinopyroxene phenocrysts from Ferrar and Karoo tholeiites allows us to ascribe this refertilisation event to the Jurassic. This asthenospheric melt was also able to transfer a garnet signature to the Northern Victoria Land mantle segment. The rare presence of glass and secondary phases indicate that Greene Point xenoliths were heterogeneously affected by alkaline metasomatism, probably related to the West Antarctic Rift System opening; this has also been widely observed in other Northern Victoria Land localities (i.e., Baker Rocks). Temperature and fO2 were calculated (950 °C; Δlog fO2 (QFM), - 1.70 to - 0.39) at a fixed pressure of 15 kbar, confirming the tendency of the anhydrous Greene Point xenolith population to have higher equilibration temperatures and comparable redox conditions, compared to the nearby amphibole-bearing peridotites from Baker Rocks.

  1. Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle?

    PubMed

    Giuliani, Andrea; Phillips, David; Woodhead, Jon D; Kamenetsky, Vadim S; Fiorentini, Marco L; Maas, Roland; Soltys, Ashton; Armstrong, Richard A

    2015-04-17

    Kimberlites and orangeites (previously named Group-II kimberlites) are small-volume igneous rocks occurring in diatremes, sills and dykes. They are the main hosts for diamonds and are of scientific importance because they contain fragments of entrained mantle and crustal rocks, thus providing key information about the subcontinental lithosphere. Orangeites are ultrapotassic, H2O and CO2-rich rocks hosting minerals such as phlogopite, olivine, calcite and apatite. The major, trace element and isotopic compositions of orangeites resemble those of intensely metasomatized mantle of the type represented by MARID (mica-amphibole-rutile-ilmenite-diopside) xenoliths. Here we report new data for two MARID xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) and we show that MARID-veined mantle has mineralogical (carbonate-apatite) and geochemical (Sr-Nd-Hf-O isotopes) characteristics compatible with orangeite melt generation from a MARID-rich source. This interpretation is supported by U-Pb zircon ages in MARID xenoliths from the Kimberley kimberlites, which confirm MARID rock formation before orangeite magmatism in the area.

  2. Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle?

    PubMed

    Giuliani, Andrea; Phillips, David; Woodhead, Jon D; Kamenetsky, Vadim S; Fiorentini, Marco L; Maas, Roland; Soltys, Ashton; Armstrong, Richard A

    2015-01-01

    Kimberlites and orangeites (previously named Group-II kimberlites) are small-volume igneous rocks occurring in diatremes, sills and dykes. They are the main hosts for diamonds and are of scientific importance because they contain fragments of entrained mantle and crustal rocks, thus providing key information about the subcontinental lithosphere. Orangeites are ultrapotassic, H2O and CO2-rich rocks hosting minerals such as phlogopite, olivine, calcite and apatite. The major, trace element and isotopic compositions of orangeites resemble those of intensely metasomatized mantle of the type represented by MARID (mica-amphibole-rutile-ilmenite-diopside) xenoliths. Here we report new data for two MARID xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) and we show that MARID-veined mantle has mineralogical (carbonate-apatite) and geochemical (Sr-Nd-Hf-O isotopes) characteristics compatible with orangeite melt generation from a MARID-rich source. This interpretation is supported by U-Pb zircon ages in MARID xenoliths from the Kimberley kimberlites, which confirm MARID rock formation before orangeite magmatism in the area. PMID:25882074

  3. Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle?

    NASA Astrophysics Data System (ADS)

    Giuliani, Andrea; Phillips, David; Woodhead, Jon D.; Kamenetsky, Vadim S.; Fiorentini, Marco L.; Maas, Roland; Soltys, Ashton; Armstrong, Richard A.

    2015-04-01

    Kimberlites and orangeites (previously named Group-II kimberlites) are small-volume igneous rocks occurring in diatremes, sills and dykes. They are the main hosts for diamonds and are of scientific importance because they contain fragments of entrained mantle and crustal rocks, thus providing key information about the subcontinental lithosphere. Orangeites are ultrapotassic, H2O and CO2-rich rocks hosting minerals such as phlogopite, olivine, calcite and apatite. The major, trace element and isotopic compositions of orangeites resemble those of intensely metasomatized mantle of the type represented by MARID (mica-amphibole-rutile-ilmenite-diopside) xenoliths. Here we report new data for two MARID xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) and we show that MARID-veined mantle has mineralogical (carbonate-apatite) and geochemical (Sr-Nd-Hf-O isotopes) characteristics compatible with orangeite melt generation from a MARID-rich source. This interpretation is supported by U-Pb zircon ages in MARID xenoliths from the Kimberley kimberlites, which confirm MARID rock formation before orangeite magmatism in the area.

  4. Sulfur and selenium systematics of the subcontinental lithospheric mantle: Inferences from the Massif Central xenolith suite (France)

    NASA Astrophysics Data System (ADS)

    Lorand, Jean-Pierre; Alard, Olivier; Luguet, Ambre; Keays, Reid R.

    2003-11-01

    Selenium has been analyzed in addition to S in 58 spinel peridotite xenoliths collected in Cenozoic alkali basalts from the Massif Central (France). The S concentration range now available for this suite, calculated from 123 samples, is the largest ever reported for alkali basalt-hosted xenoliths (<3-592 ppm). Likewise, the Se concentrations range between 0.2 and 67 ppb. No partial melting signature can be identified from the S and Se systematic. Half of the analyzed xenoliths have lost S during supergene weathering. By contrast, neither surficial alteration, nor loss of chalcophile elements during eruption can explain the regional-scale variations of S and Se concentrations. A first group of lherzolite xenoliths sampled in Southern Massif Central, from volcanic centers older and spatially unrelated to the Massif Central plume that triggered the Cenozoic volcanism, contains between 20 and 250 ppm S (with occasional S concentrations up to 592 ppm) and 12-67 ppb Se. It is clear that the highest S values, originally interpreted as representing S abundances in the primitive mantle, were in fact enriched by metasomatism. Highly variable S and Se contents (<5-360 ppm; 9-52 ppb) have also been observed in peridotite xenoliths collected in the Northern Massif Central, from volcanic centers mostly older than the plume. Like Group I xenoliths, these Group II xenoliths were strongly metasomatized by volatile-rich carbonated/silicated melts which precipitated Cu-rich sulfides. A third group of xenoliths from Plio-Quaternary basalts spatially related to the Massif Central Plume are uniformly poor in S (10-60 ppm) and Se (9-29 ppb). In this Group III, poikiloblastic textured xenoliths have lost most of their S and Se budget by peridotite-melt interactions at high melt/rock ratios. Taken as a whole, the Massif Central xenolith suite provides further evidence for strong heterogeneities in the S and Se budget of the sub-continental lithospheric mantle. However, the few LREE

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

    PubMed

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

    2003-05-29

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

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

    NASA Astrophysics Data System (ADS)

    Ziberna, L.; Klemme, S.

    2016-02-01

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

  7. Magmatic recycling of lithospheric mantle and production of Oligocene-Pliocene alkaline mantle magmas at the end of Laramide subduction, Four Corners region, USA

    NASA Astrophysics Data System (ADS)

    Lake, E. T.; Gonzales, D. A.

    2014-12-01

    Middle Cenozoic magmatism in the Four Corners region of the United States produced alkalic mantle magmas that were emplaced in diatreme-dike complexes of the 25-24 Ma Navajo volcanic field (NVF) and small-volume 20-5 Ma dike swarms on the northeastern San Juan Basin and western flanks of the San Juan Mountains. Magmatism involved partial melting of Proterozoic lithospheric mantle that underwent regional metasomatism during Laramide subduction. Emplacement of magmas was focused on zones that were broadly aligned with lithospheric-scale anisotropies (e.g., Colorado lineament). The majority of intrusive rocks produced are minette, but included small amounts of katungite formed by melting of carbonate rich mantle. A northward decrease in intrusion age is mimicked by subtle variations in major and trace element geochemistry. Compared to the 20-5 Ma dike rocks, NVF intrusions have higher overall Mg# (> 55), K2O (5-6 wt. %), La/Lu (300-1000), and elevated concentrations of incompatible trace elements (e.g. Rb, Ba, Zr). The katungite samples are marked by extreme silica undersaturation (SiO2 ~34 wt. %), and higher MgO (~15 wt. %) and FeO (~13 wt. %) than the minettes. Isotopic signatures of the minettes are relatively homogenous, falling within a lithospheric mantle field spanning 87Sr/86Sr 0.705-0.707 and ɛNd +2 to -2; except the Dulce dikes which have anomalously lower ɛNd. Isotopic signatures of the katungites plot between the minettes and average OIB. The spatial variations in geochemical signatures over time are attributed to either compositional variation in mantle sources ± different degrees of melting of lithospheric mantle; crustal contamination was insignificant. The lower concentrations of incompatible elements in the 20-5 Ma minettes might be explained by depletion of the lithospheric mantle caused by earlier voluminous volcanism in the San Juan Mountains. Alternatively, greater lithospheric fertility in Colorado may have promoted higher percentage partial

  8. Effective elastic thickness of the northern Australian continental lithosphere subducting beneath the Banda orogen (Indonesia): inelastic failure at the start of continental subduction

    NASA Astrophysics Data System (ADS)

    Tandon, K.; Lorenzo, J. M.; O'Brien, G. W.

    2000-12-01

    Pliocene-Recent continent-island arc collision of the northern Australian continental lithosphere across the Banda orogen from Roti to the Kai Plateau (˜121-137°E longitude) has formed an underfilled foreland basin within the Timor-Tanimbar-Aru Trough. Continental collision on northern Australian lithosphere is most advanced near central Timor Island in terms of shortening and absorbing the forearc basin (Savu Basin) within the accretionary prism. Australian continental lithosphere north of area around central Timor Island is believed to be detached from the oceanic lithosphere. Effective Elastic Thickness (EET) of the northern Australian continental lithosphere from Roti to the Kai Plateau are derived using an elastic half-beam model. Modeled deflection is matched to the seafloor bathymetry and the marine complete 3D Bouguer gravity anomalies. The EET varies from 27 to 75 km across the northern Australian continental lithosphere from Roti to Kai Plateau when the thickness of the elastic half-beam is kept constant. The highest EET values lies near central Timor. From the shelf to beneath the Banda orogen, the EET of the northern Australian continental lithosphere is reduced from ˜90 to ˜30 km when the thickness of the elastic half-beam is allowed to vary down dip. Elastic half-beam modeling approximates the Banda orogen as a triangular load and hidden subsurface loads as end-point loads. Wider triangular loads modeling the load contribution from Banda orogen need higher values of EET. Such an observation highlights the role of high EET in thin-skinned collisional tectonics by promoting the support of wider accretionary prisms by parts of foreland basins with higher EET. Variations in EET may result from inelastic yielding in the northern Australian continental lithosphere. Oroclinal bending of the Australian continental lithosphere in the east, from Tanimbar to the Kai Plateau, may create additional yielding and further decrease the EET. Change in EET occurred

  9. Generation of Northern Parana High Ti/Y Basalts By Progressive Lithospheric Thinning Above a "Gough"-like Mantle Source.

    NASA Astrophysics Data System (ADS)

    Peate, D. W.

    2015-12-01

    Stratigraphic and geochronologic data show that the high Ti/Y magma types (Pitanga & Paranapanema) of NW Paraná are the youngest magmatic phase in the Paraná-Etendeka flood basalt province and comprise ~50% of the total erupted volume. They are more homogeneous than low Ti/Y basalts in SE Paraná and the Etendeka, with a restricted range in Sr-Nd-Pb isotope composition (87Sr/86Sr 0.7055-0.7063; ɛNd -5 to -3; 206Pb/204Pb 17.7-18.2). Subtle differences between Pitanga and Paranapanema (Th/Ta, 206Pb/204Pb) are consistent with minor crustal assimilation. Pitanga show greater incompatible element enrichment compared to Paranapanema (Ti/Y 440-590 vs. 325-510; La/Yb 9.3-12.2 vs. 5.9-9.0), and have greater MREE/HREE enrichment (Dy/Yb 2.1-2.5 vs. 1.8-2.1). Boreholes and surface profiles reveal a consistent temporal transition from Pitanga to Paranapanema lavas, and the decrease in Dy/Yb requires a shallowing of the mean depth of melting, consistent with lithospheric thinning. Pitanga and Paranapanema lavas show Dupal characteristics (elevated Δ7/4Pb 8-13), distinct from Tristan hot-spot and S Atlantic MORB compositions, but similar to the EM-I endmember composition from Walvis Ridge DSDP Site 525A. Previous workers suggested a common origin for Parana high Ti/Y magmas and DSDP Site 525A in continental lithospheric mantle. However, recent comprehensive sampling of the Tristan - Gough - Walvis Ridge - Rio Grande Rise hotspot track has revealed spatial geochemical zonation with a northern "Tristan"-track and a southern "Gough"-track, and the "Tristan" component (Δ7/4Pb 3-6) is only found in samples < 70 Ma (Hoernle et al. 2015). The early hotspot track history is dominated by the "Gough" component (Δ7/4Pb 6-13), inferred to be derived from the African LLSVP, and this material has the compositional features (Sr-Nd-Pb-Hf isotopes, elevated La/Nb and Th/Nb) required for the mantle source for the Pitanga and Paranapanema magma types of the 135 Ma Paraná flood basalt

  10. Density and P-wave velocity structure beneath the Paraná Magmatic Province: Refertilization of an ancient lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Chaves, Carlos; Ussami, Naomi; Ritsema, Jeroen

    2016-08-01

    We estimate density and P-wave velocity perturbations in the mantle beneath the southeastern South America plate from geoid anomalies and P-wave traveltime residuals to constrain the structure of the lithosphere underneath the Paraná Magmatic Province (PMP) and conterminous geological provinces. Our analysis shows a consistent correlation between density and velocity anomalies. The P-wave speed and density are 1% and 15 kg/m3 lower, respectively, in the upper mantle under the Late Cretaceous to Cenozoic alkaline provinces, except beneath the Goiás Alkaline Province (GAP), where density (+20 kg/m3) and velocity (+0.5%) are relatively high. Underneath the PMP, the density is higher by about 50 kg/m3 in the north and 25 kg/m3 in the south, to a depth of 250 - 300 km. These values correlate with high-velocity perturbations of +0.5% and +0.3%, respectively. Profiles of density perturbation versus depth in the upper mantle are different for the PMP and the adjacent Archean São Francisco (SFC) and Amazonian (AC) cratons. The Paleoproterozoic PMP basement has a high-density root. The density is relatively low in the SFC and AC lithospheres. A reduction of density is a typical characteristic of chemically depleted Archean cratons. A more fertile Proterozoic and Phanerozoic subcontinental lithospheric mantle has a higher density, as deduced from density estimates of mantle xenoliths of different ages and composition. In conjunction with Re-Os isotopic studies of the PMP basalts, chemical and isotopic analyses of peridodite xenoliths from the GAP in the northern PMP, and electromagnetic induction experiments of the PMP lithosphere, our density and P-wave speed models suggest that the densification of the PMP lithosphere and flood basalt generation are related to mantle refertilization. Metasomatic refertilization resulted from the introduction of asthenospheric components from the mantle wedge above Proterozoic subduction zones, which surrounded the Paraná lithosphere

  11. Mantle phase changes and deep-earthquake faulting in subducting lithosphere.

    PubMed

    Kirby, S H; Durham, W B; Stern, L A

    1991-04-12

    Inclined zones of earthquakes are the primary expression of lithosphere subduction. A distinct deep population of subduction-zone earthquakes occurs at depths of 350 to 690 kilometers. At those depths ordinary brittle fracture and frictional sliding, the faulting processes of shallow earthquakes, are not expected. A fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics, including the discovery and characterization of transformational faulting, a shear instability connected with localized phase transformations under nonhydrostatic stress. These developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine-rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km. Predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes, the maximum depths of earthquakes in individual subductions zones, and key source characteristics of deep events. PMID:17769266

  12. Mantle phase changes and deep-earthquake faulting in subducting lithosphere

    USGS Publications Warehouse

    Kirby, S.H.; Durham, W.B.; Stern, L.A.

    1991-01-01

    Inclined zones of earthquakes are the primary expression of lithosphere subduction. A distinct deep population of subduction-zone earthquakes occurs at depths of 350 to 690 kilometers. At those depths ordinary brittle fracture and frictional sliding, the faulting processes of shallow earthquakes, are not expected. A fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics, including the discovery and characterization of transformational faulting, a shear instability connected with localized phase transformations under nonhydrostatic stress. These developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine-rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km. Predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes, the maximum depths of earthquakes in individual subductions zones, and key source characteristics of deep events.

  13. Mantle phase changes and deep-earthquake faulting in subducting lithosphere.

    PubMed

    Kirby, S H; Durham, W B; Stern, L A

    1991-04-12

    Inclined zones of earthquakes are the primary expression of lithosphere subduction. A distinct deep population of subduction-zone earthquakes occurs at depths of 350 to 690 kilometers. At those depths ordinary brittle fracture and frictional sliding, the faulting processes of shallow earthquakes, are not expected. A fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics, including the discovery and characterization of transformational faulting, a shear instability connected with localized phase transformations under nonhydrostatic stress. These developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine-rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km. Predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes, the maximum depths of earthquakes in individual subductions zones, and key source characteristics of deep events.

  14. Major crustal lineaments and their influence on the geological history of the continental lithosphere

    SciTech Connect

    Reading, H.G.; Watterson, J.; White, S.H.

    1987-01-01

    Thirty-eight contributors provide a synthesis and evaluation of our current understanding of the seismic signatures of major lineaments, their structure and reactivation, their subsequent control of geological processes, and their influence on the location of mineral and hydrocarbon deposits. Topics covered include architecture of the continental lithosphere, earthquakes and lineament infrastructure, crustal shear zones and thrust belts: their geometry and continuity in central Africa, fluid transport in lineaments, and the evolution of the Malvern Lineament. Originally published in Philosphical Transactions of the Royal Society.

  15. Simulating the thermochemical magmatic and tectonic evolution of Venus's mantle and lithosphere: Two-dimensional models

    NASA Astrophysics Data System (ADS)

    Armann, Marina; Tackley, Paul J.

    2012-12-01

    Numerical convection models of the thermochemical evolution of Venus are compared to present-day topography and geoid and recent resurfacing history. The models include melting, magmatism, decaying heat-producing elements, core cooling, realistic temperature-dependent viscosity and either stagnant lid or episodic lithospheric overturn. In stagnant lid convection the dominant mode of heat loss is magmatic heat pipe, which requires massive magmatism and produces very thick crust, inconsistent with observations. Partitioning of heat-producing elements into the crust helps but does not help enough. Episodic lid overturn interspersed by periods of quiescence effectively loses Venus's heat while giving lower rates of volcanism and a thinner crust. Calculations predict 5-8 overturn events over Venus's history, each lasting ˜150 Myr, initiating in one place and then spreading globally. During quiescent periods convection keeps the lithosphere thin. Magmatism keeps the mantle temperature ˜constant over Venus's history. Crustal recycling occurs by entrainment in stagnant lid convection, and by lid overturn in episodic mode. Venus-like amplitudes of topography and geoid can be produced in either stagnant or episodic modes, with a viscosity profile that is Earth-like but shifted to higher values. The basalt density inversion below the olivine-perovskite transition causes compositional stratification around 730 km breakdown of this layering increases episodicity but far less than episodic lid overturn. The classical stagnant lid mode with interior temperature ˜rheological temperature scale lower than TCMB is not reached because mantle temperature is controlled by magmatism while the core cools slowly from a superheated start. Core heat flow decreases with time, possibly shutting off the dynamo, particularly in episodic cases.

  16. Finite Frequency Traveltime Tomography of Lithospheric and Upper Mantle Structures beneath the Cordillera-Craton Transition in Southwestern Canada

    NASA Astrophysics Data System (ADS)

    Chen, Y.; Gu, Y. J.; Hung, S. H.

    2014-12-01

    Based on finite-frequency theory and cross-correlation teleseismic relative traveltime data from the USArray, Canadian National Seismograph Network (CNSN) and Canadian Rockies and Alberta Network (CRANE), we present a new tomographic model of P-wave velocity perturbations for the lithosphere and upper mantle beneath the Cordillera-cration transition region in southwestern Canada. The inversion procedure properly accounts for the finite-volume sensitivities of measured travel time residuals, and the resulting model shows a greater resolution of upper mantle velocity heterogeneity beneath the study area than earlier approaches based on the classical ray-theoretical approach. Our model reveals a lateral change of P velocities from -0.5% to 0.5% down to ~200-km depth in a 50-km wide zone between the Alberta Basin and the foothills of the Rocky Mountains, which suggests a sharp structural gradient along the Cordillera deformation front. The stable cratonic lithosphere, delineated by positive P-velocity perturbations of 0.5% and greater, extends down to a maximum depth of ~180 km beneath the Archean Loverna Block (LB). In comparison, the mantle beneath the controversial Medicine Hat Block (MHB) exhibits significantly higher velocities in the uppermost mantle and a shallower (130-150 km depth) root, generally consistent with the average depth of the lithosphere-asthenosphere boundary beneath Southwest Western Canada Sedimentary Basin (WCSB). The complex shape of the lithospheric velocities under the MHB may be evidence of extensive erosion or a partial detachment of the Precambrian lithospheric root. Furthermore, distinct high velocity anomalies in LB and MHB, which are separated by 'normal' mantle block beneath the Vulcan structure (VS), suggest different Archean assembly and collision histories between these two tectonic blocks.

  17. Geochemical and isotopic characteristics of lithospheric mantle beneath West Kettle River, British Columbia: Evidence from ultramafic xenoliths

    SciTech Connect

    Xue, Xianyu; Baadsgaard, H.; Scarfe, C.M. ); Irving, A.J. )

    1990-09-10

    A group of spinel peridotite xenoliths from West Kettle River, British Columbia, represents essentially undepleted to moderately depleted lithospheric mantle rocks in terms of major and compatible trace elements. Whole rock Sr isotopic composition for most of these xenoliths, and whole rock Sm-Nd isotopic composition and LREE contents for some of them, seem to have been perturbed by near-surface processes. Sr and Nd isotopic results for acid-cleaned clinopyroxenes separated from these spinel peridotites reveal an isotopically mid-ocean ridge basalt (MORB)-like mantle. Seven spinel lherzolites gave Nd model ages of 1.5-3.6 Ga, similar to MORB, and on a Sm-Nd isotope diagram plot close to a reference Nd isochron with an age of 0.7 Ga and an initial {var epsilon}{sub Nd} of +7. These features likely resulted from multiple mantle depletion. The isotopic similarities of these xenoliths with MORB suggest that this area is underlain by oceanic lithospheric mantle, possibly accreted to North America during the mid-Jurassic. The Nd isochron age could record the time when the oceanic lithosphere was isolated from the asthenosphere. Recent enrichment event may have acted on such a depleted mantle, as indicated by the low Sm/Nd ratios of two spinel harzburgites.

  18. 26 million years of mantle upwelling below a segment of the Mid Atlantic Ridge: The Vema Lithospheric Section revisited

    NASA Astrophysics Data System (ADS)

    Cipriani, Anna; Bonatti, Enrico; Brunelli, Daniele; Ligi, Marco

    2009-07-01

    Temporal variations of temperature and composition of the mantle upwelling below a 80-km long segment of the Mid Atlantic Ridge were reconstructed from 20 to 4 Ma ago from peridotites sampled along a > 300-km long section of oceanic lithosphere (Vema Lithospheric Section or VLS) exposed south of the Vema transform at 11° N [Bonatti, E., Ligi, M., Brunelli, D., Cipriani, A., Fabretti, P., Ferrante, V., Gasperini, L., Ottolini, L., 2003. Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variation in the formation of oceanic lithosphere, Nature, 423, 499-505]. We extended this time interval from 26 to 2 Ma by sampling mantle ultramafics at 18 new sites along the VLS. Peridotite orthopyroxene, clinopyroxene and spinel chemistry suggest a weak trend of decreasing extent of melting of the mantle from 26 to 18.5 Ma ago with superimposed short-wavelength (~ 4 Ma) oscillations followed by a steady increase of degree of melting from 18.5 to 2 Ma ago, with superimposed 3-4 Ma oscillations. Temporal variations of crustal thickness inferred from the Residual Mantle Bouguer Anomaly calculated from gravity data reveal similar trends. The older (26 to 18.5 Ma) and the younger (18.5 to 2 Ma) mantle suites differ in cpx Na 2O content and CaO/Al 2O 3 ratio, suggesting that not only the thermal regime, but also the composition of the mantle source might have been different in the two suites. The two trends are separated by a ~ 1.4 Ma-long stretch (from 18.2 to 16.8 Ma) where deformed ultramafic mylonites prevail, indicating probably an interval of nearly a-magmatic lithospheric emplacement at ridge axis, corresponding to a thermal minimum. Spatially offset correlation along the VLS of crustal thickness (i.e., quantity of basaltic melt released by the mantle) and mantle peridotite degree of melting led to an estimate of ~ 16.1 mm/a for the solid mantle average velocity of upwelling, a value close to the average half spreading rate for the 26 Ma interval covered by the

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

  20. 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

  1. Geochemical Evolution of Cratonic Lithospheric Mantle: A 3.6 Ga Story of Persistence and Transformation (Invited)

    NASA Astrophysics Data System (ADS)

    O'Reilly, S. Y.; Griffin, W. L.; Pearson, N. J.

    2013-12-01

    Over the last decade, our view of the Earth has changed dramatically with the rapid development of planetary- and lithosphere-scale seismic tomography at increasingly higher resolution, and other global datasets such as gravity, magnetic and magnetotellurics. We now have access to geochemical (including isotopic) data for a large proportion of the elements in the periodic table, at very low levels in bulk rocks and in tiny volumes of tiny volumes. These advances, along with imaging of microstructures, enormous databanks for tectonic syntheses, and robust geodynamic modelling, have generated new concepts in understanding lithosphere evolution, structure and processes. The 4-D Lithosphere Mapping methodology introduced the integration of geophysical and geochemical datasets to construct geologically realistic sections of rock types and major boundaries in the deep lithosphere, in different timeslices. Correlation of petrological and geophysical data allows broader extrapolation of Lithosphere Mapping beyond xenolith sampling sites. The lithosphere-asthenosphere boundary (LAB) is a first-order Earth discontinuity: it may shallow due to extension and metasomatism, or deepen due to vertical accretion. Its (paleo-)location can be traced geochemically (a proxy for paleogeophysics), and cratonic LAB is commonly marked by significant concentrations of eclogites. Convergent geochronology datasets of Hf isotopic model ages for zircons and Re-Os model ages for mantle sulfides and PGMs, reinforced by other geochemical and tectonic criteria, indicate that over 70% of the SCLM and its overlying crust (now mostly lower crust) formed at about 3.5 Ga, probably in a series of global overturn events that marked a change in Earth's fundamental geodynamic behaviour. This primitive SCLM, the roots of the Archean cratons up to 300km deep, was geochemically highly depleted, and subsequently played a major role in crustal metallogeny for many ore types. Firstly, the high degree of buoyancy

  2. Biotic vs. abiotic Earth: A model for mantle hydration and continental coverage

    NASA Astrophysics Data System (ADS)

    Höning, Dennis; Hansen-Goos, Hendrik; Airo, Alessandro; Spohn, Tilman

    2014-08-01

    The origin and evolution of life has undoubtedly had a major impact on the evolution of Earth's oceans and atmosphere. Recent studies have suggested that bioactivity may have had an even deeper impact and may have caused a change in the redox-state of the mantle and provided a path for the formation of continents. We here present a numerical model that assumes that bioactivity increases the continental weathering rate and that relates the sedimentation rate to the growth of continents and to the hydration of the mantle using elements of plate tectonics and mantle convection. The link between these factors is provided by assuming that an increase of the thickness of the sedimentary layer of low permeability on top of a subducting oceanic slab will reduce its dewatering upon subduction. This in turn leads to a greater availability of water in the source region of andesitic partial melt, resulting in an enhanced rate of continental crust production, and to an increased regassing rate of the mantle. The mantle in turn responds by reducing the mantle viscosity while increasing the convective circulation rate, degassing rate and plate speed. We use parameters that are observed for the present Earth and gauge uncertain parameters such that the present day continental surface area and mantle water concentration can be obtained. Our steady state results show two stable fixed points in a phase plane defined by the fractional continental surface area and the water concentration in the mantle, one of them pertaining to a wet mantle and the continental surface area of the present day Earth, and the other to a dry mantle and a small continental surface area. When the sedimentation rate is reduced, both fixed points move and the area of attraction of the latter fixed point increases systematically. We conclude that if the presence of life has increased the continental weathering rate, as is widely believed, and led to the observables of a wet mantle and a continental surface

  3. Continental growth and mantle hydration as intertwined feedback cycles in the thermal evolution of Earth

    NASA Astrophysics Data System (ADS)

    Höning, Dennis; Spohn, Tilman

    2016-06-01

    A model of Earth's continental coverage and mantle water budget is discussed along with its thermal evolution. The model links a thermal evolution model based on parameterized mantle convection with a model of a generic subduction zone that includes the oceanic crust and a sedimentary layer as carriers of water. Part of the subducted water is used to produce continental crust while the remainder is subducted into the mantle. The total length of the subduction zones is calculated from the total surface area of continental crust assuming randomly distributed continents. The mantle viscosity is dependent of temperature and the water concentration. Sediments are generated by continental crust erosion, and water outgassing at mid-oceanic ridges closes the water cycle. We discuss the strongly coupled, non-linear model using a phase plane defined by the continental coverage and mantle water concentration. Fixed points are found in the phase plane at which the rates of change of both variables are zero. These fixed points evolve with time, but in many cases, three fixed points emerge of which two are stable and an intermediate point is unstable with respect to continental coverage. With initial conditions from a Monte-Carlo scheme we calculate evolution paths in the phase plane and find a large spread of final states that all have a mostly balanced water budget. The present day observed 40% continental surface coverage is found near the unstable fixed point. Our evolution model suggests that Earth's continental coverage formed early and has been stable for at least 1.5 Gyr. The effect of mantle water regassing (and mantle viscosity depending on water concentration) is found to lower the present day mantle temperature by about 120 K, but the present day mantle viscosity is affected little. The water cycle thus complements the well-known thermostat effect of viscosity and mantle temperature. Our results further suggest that the biosphere could impact the feedback cycles by

  4. Upper-mantle seismic structure in a region of incipient continental breakup: northern Ethiopian rift

    NASA Astrophysics Data System (ADS)

    Bastow, Ian D.; Stuart, Graham W.; Kendall, J.-Michael; Ebinger, Cynthia J.

    2005-08-01

    The northern Ethiopian rift forms the third arm of the Red Sea, Gulf of Aden triple junction, and marks the transition from continental rifting in the East African rift to incipient oceanic spreading in Afar. We determine the P- and S-wave velocity structure beneath the northern Ethiopian rift using independent tomographic inversion of P- and S-wave relative arrival-time residuals from teleseismic earthquakes recorded by the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) passive experiment using the regularised non-linear least-squares inversion method of VanDecar. Our 79 broad-band instruments covered an area 250 × 350 km centred on the Boset magmatic segment ~70 km SE of Addis Ababa in the centre of the northern Ethiopian rift. The study area encompasses several rift segments showing increasing degrees of extension and magmatic intrusion moving from south to north into the Afar depression. Analysis of relative arrival-time residuals shows that the rift flanks are asymmetric with arrivals associated with the southeastern Somalian Plate faster (~0.65 s for the P waves; ~2 s for the S waves) than the northwestern Nubian Plate. Our tomographic inversions image a 75 km wide tabular low-velocity zone (δVP~-1.5 per cent, δVS~-4 per cent) beneath the less-evolved southern part of the rift in the uppermost 200-250 km of the mantle. At depths of >100 km, north of 8.5°N, this low-velocity anomaly broadens laterally and appears to be connected to deeper low-velocity structures under the Afar depression. An off-rift low-velocity structure extending perpendicular to the rift axis correlates with the eastern limit of the E-W trending reactivated Precambrian Ambo-Guder fault zone that is delineated by Quaternary eruptive centres. Along axis, the low-velocity upwelling beneath the rift is segmented, with low-velocity material in the uppermost 100 km often offset to the side of the rift with the highest rift flank topography. Our observations from this magmatic

  5. Refertilization process in the Patagonian subcontinental lithospheric mantle of Estancia Sol de Mayo (Argentina)

    NASA Astrophysics Data System (ADS)

    Melchiorre, Massimiliano; Coltorti, Massimo; Gregoire, Michel; Benoit, Mathieu

    2015-05-01

    Anhydrous mantle xenoliths equilibrated at 1003-1040 °C from Estancia Sol de Mayo (ESM, Central Patagonia, Argentina) and entrained in post-plateau alkaline lavas belonging to Meseta Lago Buenos Aires have been investigated aiming at reconstructing the depletion and enrichment processes that affected this portion of the Patagonia lithospheric mantle. Xenoliths are characterized by a coarse-grained protogranular texture and are devoid of evident modal metasomatism. They show two texturally different clinopyroxenes: protogranular (cpx1) and texturally related to spinel (cpx2). Three different types of orthopyroxenes are also recognized: large protogranular crystals with exsolution lamellae (opx1); small clean and undeformed grains without exsolution lamellae (opx2) and small grains arranged in a vein (opx3). Major element composition of clinopyroxenes and orthopyroxenes highlights two different trends characterized by i) a high Al2O3 content at almost constant mg# and ii) a slight increase in Al2O3 content with decreasing mg#. Clinopyroxenes are enriched in LREE and are characterized by prominent to slightly negative Nb, Zr and Ti anomalies. No geochemical differences are observed between cpx1 and cpx2, while a discrimination can be observed between opx1 and opx2 (LREE-depleted; prominent to slightly negative Ti and Zr anomalies) and opx3 (prominent positive Zr anomaly). Partial melting modeling using both major and trace elements indicates a melting degree between ~ 5% and ~ 13% (up to ~ 23% according to major element modeling) for lherzolites and between ~ 20% and ~ 30% for harzburgites (down to ~ 5% according to trace element modeling). La/Yb and Al2O3, as well as Sr and Al2O3 negative correlations in clinopyroxenes point to a refertilization event affecting this lithospheric mantle. The agent was most probably a transitional alkaline/subalkaline melt, as indicated by the presence of orthopyroxene in the vein and the similar geochemical features of ESM

  6. Osmium isotope evidence for Early to Middle Proterozoic mantle lithosphere stabilization and concomitant production of juvenile crust in Dish Hill, CA peridotite xenoliths

    NASA Astrophysics Data System (ADS)

    Armytage, Rosalind M. G.; Brandon, Alan D.; Peslier, Anne H.; Lapen, Thomas J.

    2014-07-01

    The 187Os/188Os compositions in peridotite samples from the sub-continental lithospheric mantle (SCLM) can be used to constrain the timing of melt extraction and potentially test the link between large-scale mantle melting and juvenile crust production. The SCLM has often experienced a complex history such that some lithophile elements such as REEs (rare earth elements) in these rocks typically record overprinting during metasomatism. New 187Os/188Os, major and trace element compositional data were obtained on sixteen Dish Hill peridotite xenoliths (California, USA) and are used to examine these issues. The samples show strong correlations between 187Os/188Os and indicators of melt depletion such as Lu abundance in clinopyroxene, modal abundance of clinopyroxene, bulk rock Al2O3 and the Cr# (Cr/(Cr + Al) in spinel. These relationships indicate that metasomatism did not compromise the 187Os/188Os systematics. The data appear to form two melt depletion trends consistent with Re depletion model ages (TRD) obtained from the two Al2O3 versus 187Os/188Os trends are 2.1 ± 0.5 Ga and 1.3 ± 0.3 Ga (±95% conf.). It has been suggested that the SCLM under Dish Hill may be fragments of oceanic lithosphere emplaced as the result of Farallon plate subduction during the Late Cretaceous (Luffi et al., 2009). However, the strong melt depletion trends, major element compositions and Re-depletion ages are not consistent with the interpretation of this suite of xenoliths having an oceanic lithospheric origin. Rather, the 2.1 Ga age coincides with Nd model ages of 2-2.3 Ga (Bennett and DePaolo, 1987; Rämö and Calzia, 1998) for the overlying Mojavia crustal province. The 1.3 Ga age is consistent with large-scale A-type magmatism in the nearby region at this time that is purported to be the result of mantle plume melting processes. Therefore, data from this study point to the SCLM under Dish Hill being formed by two ancient mantle-melting events, which could be the result of

  7. Multi-Observable Thermochemical Tomography of the lithosphere and upper mantle

    NASA Astrophysics Data System (ADS)

    Afonso, J. C.; Yang, Y.; Rawlinson, N.; Jones, A. G.; Fullea, J.; Qashqai, M.

    2015-12-01

    Current knowledge of the present-day physical state and structure of the lithosphere and upper mantle essentially derives from four independent sources: i) gravity field and thermal modelling, ii) modelling/inversion of different seismic datasets, iii) magnetotelluric studies, and iv) thermobarometric and geochemical data from exhumed mantle samples. Unfortunately, the integration of these different sources of information in modern geophysical studies is still uncommon and significant discrepancies and/or inconsistencies in predictions between these sources are still the rule rather than the exception.In this contribution we will present a thermodynamically-constrained multi-observable probabilistic inversion method capable of jointly inverting i) surface and body wave datasets, gravity anomalies, geoid height, gravity gradients, receiver functions, surface heat flow, magnetotelluric data, and elevation (static and dynamic) in 3D spherical coordinates. Key aspects of the method are: (a) it combines multiple geophysical observables with different sensitivities to deep/shallow, thermal/compositional anomalies into a single thermodynamic-geophysical framework; (b) it works with thermophysical models of the Earth rather than with parameterized structures of physical parameters (e.g. Vs, Vp, density, etc), (c) it uses a general probabilistic (Bayesian) formulation to appraise the data; (d) no initial model is needed; (e) a priori compositional information relies on robust statistical analyses of a large database of natural mantle samples; (f) it provides a natural platform to estimate realistic uncertainties; (g) it handles multiscale parameterizations and complex physical models, and (h) it includes dynamic (convection) effects on surface observables by solving the complete Stokes flow using multi-dimensional decomposition methods. We will present results for both synthetic and real case studies, which serve to highlight the advantages and limitations of this new

  8. Coupled mantle dripping and lateral dragging controlling the lithosphere structure of the NW-Moroccan margin and the Atlas Mountains: A numerical experiment

    NASA Astrophysics Data System (ADS)

    Zlotnik, Sergio; Jimenez-Munt, Ivone; Fernandez, Manel

    2015-04-01

    Recent studies integrating gravity, geoid, surface heat flow, elevation and seismic data indicate a prominent lithospheric mantle thickening beneath the NW-Moroccan margin (LAB > 200 km-depth) followed by thinning beneath the Atlas Domain (LAB about 80 km-depth). Such unusual configuration has been explained by the combination of mantle underthrusting due to oblique Africa-Eurasia convergence together with viscous dripping fed by asymmetric lateral mantle dragging, requiring a strong crust-mantle decoupling. In the present work we examine the physical conditions under which the proposed asymmetric mantle drip and drag mechanism can reproduce this lithospheric configuration. We also analyse the influence of kinematic boundary conditions as well as the mantle viscosity and the initial lithosphere geometry. Results indicate that the proposed drip- drag mechanism is dynamically feasible and only requires a lateral variation of the lithospheric strength. Further evolution of the gravitational instability can become either in convective removal of the lithospheric mantle, mantle delamination, or subduction initiation. The model reproduces the main trends of the present-day lithospheric geometry across the NW-Moroccan margin and the Atlas Mountains, the characteristic time of the observed vertical movements, the amplitude and rates of uplift in the Atlas Mountains and offers an explanation to the Miocene to Pliocene volcanism. An abnormal constant tectonic subsidence rate in the margin is predicted.

  9. Thermo-mechanical laboratory modeling of lithospheric-scale processes: the example of deep continental subduction and exhumation of UHP/LT rocks

    NASA Astrophysics Data System (ADS)

    Boutelier, David

    2010-05-01

    Purely mechanical (isothermal) laboratory experiments of continental subduction suggested existence of two principal regimes of this process, defined by the interplate pressure and inversely proportional to the pull-force. The pull-force itself depends on the difference between the average density of the subducting lithosphere and the surrounding mantle. In both high and low compression regimes, the continental crust reaches a critical depth proportional to its strength. Although these modeling results correspond quite well to geological data, it was purely mechanical and did not consider change in the mechanical properties during subduction. In nature, however, both pressure and temperature increase causing the strength of the subducting crust and mantle to be reduced by about one order of magnitude when reaching ~100 km-depth. Thermo-mechanical laboratory experiments revealed that such strong change deeply affect the subduction and exhumation processes. In the low compression regime, the crust can only subducts to ~120 km-depth in the asthenosphere. By then, it has become too hot and weak and undergoes large deformation, including upward ductile flow of the deeply subducted portions and a localized failure of the upper crust at depth of a few tens of kilometers. This deformation is accompanied by the delamination of the crustal and mantle layers. In the high compression regime, the deeply subducted continental crust reaches greater depth (~150 to 200 km) and maintains lower temperature when continental subduction triggers the subduction of the fore-arc block or the arc plate. 2-D thermo-mechanical laboratory modeling of continental subduction thus show that exhumation of deeply subducted continental crust is possible in the low compression regime (i.e. when the effective interplate pressure Pn is lower than the lithostatic pressure), while for the deeply subducted continental crust to be preserved at low temperature at great depth, the continental subduction

  10. MORB mantle hosts the missing Eu (Sr, Nb, Ta and Ti) in the continental crust: New perspectives on crustal growth, crust-mantle differentiation and chemical structure of oceanic upper mantle

    NASA Astrophysics Data System (ADS)

    Niu, Yaoling; O'Hara, Michael J.

    2009-09-01

    protolith consists of ~ 66.4% amphibole, ~ 29.2% plagioclase and 4.4% ilmenite. In terms of simple modal melting models, the bulk distribution coefficient ratios D2Eu/(Sm + Gd) = 1.21, D2Sr/(Pr + Nd) = 1.04, DNb/Th = 44, DTa/U = 57, DTi/Sm = 3.39 and DNb/Ta = 1.30 readily explains the small but significant negative Eu and Sr anomalies, moderate negative Ti anomaly and huge negative Nb and Ta anomalies as well as the more sub-chondritic Nb/Ta ratio in the syncollisional andesitic melt that is characteristic of and contributes to the continental crust mass. These results support the hypothesis that continental collision zones are primary sites of net continental crust growth, whereas the standard "island arc" model has many more difficulties than certainties. That is, it is the continental collision (vs. "island arc magmatism" or "episodic super mantle avalanche events") that produces and preserves the juvenile crust, and hence maintains net continental growth. The data also allow us to establish the robust composition of depleted and most primitive (or "primary") MORB melt with 13% MgO. This, together with the estimated positive Eu and Sr anomalies in the DMM, further permits estimation that the DMM may occupy the uppermost ~ 680 km of the convective mantle following the tradition that the DMM lies in the shallowest mantle. However, the tradition may be in error. The seismic low velocity zone (LVZ) may be compositionally stratified with small melt fractions concentrated towards the interface with the growing lithosphere because of buoyancy. Such small melt fractions, enriched in volatiles and incompatible elements, continue to metasomatize the growing lithosphere before it reaches the full thickness after ~ 70 Myrs. Hence, the oceanic mantle lithosphere is a huge enriched geochemical reservoir. On the other hand, deep portions of the LVZ, which are thus relatively depleted, become the primary source feeding the ridge because of ridge-suction-driven lateral material supply to

  11. A taxonomy of three species of negative velocity arrivals in the lithospheric mantle beneath the United States using Sp receiver functions

    NASA Astrophysics Data System (ADS)

    Foster, K.; Dueker, K.; McClenahan, J.; Hansen, S. M.; Schmandt, B.

    2012-12-01

    The Transportable Array, with significant supplement from past PASSCAL experiments, provides an unprecedented opportunity for a holistic view over the geologically and tectonically diverse continent. New images from 34,000 Sp Receiver Functions image lithospheric and upper mantle structure that has not previously been well constrained, significant to our understanding of upper mantle processes and continental evolution. The negative velocity gradient (NVG) found beneath the Moho has been elusive and is often loosely termed the "Lithosphere-Asthenosphere Boundary" (LAB).This label is used by some researchers to indicate a rheological boundary, a thermal gradient, an anisotropic velocity contrast, or a compositional boundary, and much confusion has arisen around what observed NVG arrivals manifest. Deconvolution across up to 400 stations simultaneously has enhanced the source wavelet estimation and allowed for more accurate receiver functions. In addition, Sdp converted phases are precursory to the direct S phase arrival, eliminating the issue of contamination from reverberated phases that add noise to Ps receiver functions in this lower-lithospheric and upper mantle depth range. We present taxonomy of the NVG arrivals beneath the Moho across the span of the Transportable Array (125° - 85° W). The NVG is classified into three different categories, primarily distinguished by the estimated temperature at the depth of the arrival. The first species of Sp NVG arrivals is found to be in the region west of the Precambrian rift hinge line, at a depth range of 70 - 90 km, corresponding to a temperature of >1150° C. This temperature and depth is predicted to be supersolidus for a 0.02% weight H2O Peridotite (Katz et al., 2004), supporting the theory that these arrivals are due to a melt-staging area (MSA), which could be correlated with the base of the thermal lithosphere. The current depth estimate of the cratonic US thermal LAB ranges from 150-220 km (Yuan and Romanowitz

  12. Petrogenesis of postcollisional magmatism at Scheelite Dome, Yukon, Canada: Evidence for a lithospheric mantle source for magmas associated with intrusion-related gold systems

    USGS Publications Warehouse

    Mair, John L.; Farmer, G. Lang; Groves, David I.; Hart, Craig J.R.; Goldfarb, Richard J.

    2011-01-01

    The type examples for the class of deposits termed intrusion-related gold systems occur in the Tombstone-Tungsten belt of Alaska and Yukon, on the eastern side of the Tintina gold province. In this part of the northern Cordillera, extensive mid-Cretaceous postcollisional plutonism took place following the accretion of exotic terranes to the continental margin. The most cratonward of the resulting plutonic belts comprises small isolated intrusive centers, with compositionally diverse, dominantly potassic rocks, as exemplified at Scheelite Dome, located in central Yukon. Similar to other spatially and temporally related intrusive centers, the Scheelite Dome intrusions are genetically associated with intrusion-related gold deposits. Intrusions have exceptional variability, ranging from volumetrically dominant clinopyroxene-bearing monzogranites, to calc-alkaline minettes and spessartites, with an intervening range of intermediate to felsic stocks and dikes, including leucominettes, quartz monzonites, quartz monzodiorites, and granodiorites. All rock types are potassic, are strongly enriched in LILEs and LREEs, and feature high LILE/HFSE ratios. Clinopyroxene is common to all rock types and ranges from salite in felsic rocks to high Mg augite and Cr-rich diopside in lamprophyres. Less common, calcic amphibole ranges from actinolitic hornblende to pargasite. The rocks have strongly radiogenic Sr (initial 87Sr/86Sr from 0.711-0.714) and Pb isotope ratios (206Pb/204Pb from 19.2-19.7), and negative initial εNd values (-8.06 to -11.26). Whole-rock major and trace element, radiogenic isotope, and mineralogical data suggest that the felsic to intermediate rocks were derived from mafic potassic magmas sourced from the lithospheric mantle via fractional crystallization and minor assimilation of metasedimentary crust. Mainly unmodified minettes and spessartites represent the most primitive and final phases emplaced. Metasomatic enrichments in the underlying lithospheric mantle

  13. Modeling Continental Growth and Mantle Hydration in Earth's Evolution and the Impact of Life

    NASA Astrophysics Data System (ADS)

    Höning, Dennis; Spohn, Tilman

    2016-04-01

    The evolution of planets with plate tectonics is significantly affected by several intertwined feedback cycles. On Earth, interactions between atmosphere, hydrosphere, biosphere, crust, and interior determine its present day state. We here focus on the feedback cycles including the evolutions of mantle water budget and continental crust, and investigate possible effects of the Earth's biosphere. The first feedback loop includes cycling of water into the mantle at subduction zones and outgassing at volcanic chains and mid-ocean ridges. Water is known to reduce the viscosity of mantle rock, and therefore the speed of mantle convection and plate subduction will increase with the water concentration, eventually enhancing the rates of mantle water regassing and outgassing. A second feedback loop includes the production and erosion of continental crust. Continents are formed above subduction zones, whose total length is determined by the total size of the continents. Furthermore, the total surface area of continental crust determines the amount of eroded sediments per unit time. Subducted sediments affect processes in subduction zones, eventually enhancing the production rate of new continental crust. Both feedback loops affect each other: As a wet mantle increases the speed of subduction, continental production also speeds up. On the other hand, the total length of subduction zones and the rate at which sediments are subducted (both being functions of continental coverage) affect the rate of mantle water regassing. We here present a model that includes both cycles and show how the system develops stable and unstable fixed points in a plane defined by mantle water concentration and surface of continents. We couple these feedback cycles to a parameterized thermal evolution model that reproduces present day observations. We show how Earth has been affected by these feedback cycles during its evolution, and argue that Earth's present day state regarding its mantle water

  14. Mantle lithosphere transition from the East European Craton to the Variscan Bohemian Massif imaged by shear-wave splitting

    NASA Astrophysics Data System (ADS)

    Vecsey, L.; Plomerová, J.; Babuška, V.

    2014-08-01

    We analyse splitting of teleseismic shear waves recorded during the PASSEQ passive experiment (2006-2008) focused on the upper mantle structure across and around the Trans-European Suture Zone (TESZ). Altogether 1009 pairs of the delay times of the slow split shear waves and orientations of the polarized fast shear waves exhibit lateral variations across the array, as well as back-azimuth dependences of measurements at individual stations. Variable components of the splitting parameters can be associated with fabrics of the mantle lithosphere of tectonic units. In comparison with a distinct regionalization of the splitting parameters in the Phanerozoic part of Europe that particularly in the Bohemian Massif (BM) correlate with the large-scale tectonics, variations of anisotropic parameters around the TESZ and in the East European Craton (EEC) are smooth and of a transitional character. No general and abrupt change in the splitting parameters (anisotropic structure) can be related to the Teisseyre-Tornquist Zone (TTZ), marking the edge of the Precambrian province on the surface. Instead, regional variations of anisotropic structure were found along the TESZ/TTZ. The coherence of anisotropic signals evaluated beneath the northern part of the Brunovistulian in the eastern rim of the BM and the pattern continuation to the NE towards the TTZ, support the idea of a common origin of the lithosphere micro-plates, most probably related to Baltica. Smooth changes in polarizations of the core-mantle boundary refracted shear waves (SKS), polarizations, or even a large number of null splits northward of the BM and further across the TESZ towards the EEC indicate less coherent fabrics and a transitional character of structural changes in the mantle beneath the surface trace of the TESZ/TTZ. The narrow and near-vertical TTZ in the crust does not seem to have a steep continuation in the mantle lithosphere. The mantle part of the TESZ, whose crust was formed by an assemblage of

  15. Rifting of the Tyrrhenian Basin: A Natural Laboratory to Study Extension of Continental Lithosphere and Conjugate Rifted Margin Formation

    NASA Astrophysics Data System (ADS)

    Ranero, C. R.; Sallares, V.; Grevemeyer, I.; Zitellini, N.; Vendrell, M. G.; Prada, M.; Moeller, S.; Party, M. C.; Medoc Cruise Party

    2011-12-01

    The Tyrrhenian basin has been created by extension of continental lithosphere above a retreating slab during the Neogene. The basin is not currently extending, but its structure preserves information of the time evolution of the rifting process. The basin opened from north to south with different amounts of extension. The northern region stopped opening after relatively low extension. The amount of extension increases southwards to a region where full crustal separation produced mantle exhumation. The final structure displays two conjugate margins with an asymmetric structure. We present results from a two-vessel seismic experiment that took place in spring 2010. The cruise was carried out with the Spanish R/V Sarmiento de Gamboa (SdG) and the Italian R/V Urania in a first leg. The ships collected 5 E-W trending wide-angle seismic (WAS) profiles across the entire basin using 17 Ocean Bottom Seismometers and 25 Ocean Bottom Hydrophones and a 4800 c.i. G-II gun array. During the second leg the R/V SdG collected 16 Multichannel Seismic Reflection (MCS) profiles using a 3.75 km-long streamer and a 3000 c.i. G-II gun array. MCS profiles were acquired coincident with the WAS profiles, and a number of additional lines concentrated in the central region of the basin where mantle exhumation took place. The seismic data covers the region of the basin that experienced different amount of extension from north to south. In this presentation we compare observations from different transects to study the evolution of the processes of continental margin formation by trading space (different areas with different extension factors) for time (evolution of extension). Each transect provides the tectonic structure, the geometry of sedimentary deposits, and seismic velocity distribution. This information allows to interpret the mechanisms of deformation and to study the symmetry-asymmetry structure of the conjugated margins, and thus of the processes involved in their formation. The

  16. 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.

  17. Constraints on the mantle and lithosphere dynamics from the observed geoid with the effect of visco-elasto-plastic rheology in the upper 300 km

    NASA Astrophysics Data System (ADS)

    Osei Tutu, Anthony; Steinberger, Bernhard; Rogozhina, Irina; Sobolev, Stephan

    2015-04-01

    Over the past decades rheological properties of the Earth's mantle and lithosphere have been extensively studied using numerical models calibrated versus a range of surface observations (e.g., free-air-gravity anomaly/geoid, dynamic topography, plate velocity, etc.).The quality of model predictions however strongly depends on the simplifying assumptions, spatial resolution and parameterizations adopted by numerical models. The geoid is largely (Hager & Richards, 1989) determined by both the density anomalies driving the mantle flow and the dynamic topography at the Earth surface and the core-mantle boundary. This is the effect of the convective processes within the Earth's mantle. The remainder is mostly due to strong heterogeneities in the lithospheric mantle and the crust, which also need to be taken into account. The surface topography caused by density anomalies both in the sub-lithospheric mantle and within the lithosphere also depends on the lithosphere rheology. Here we investigate the effects of complex lithosphere rheology on the modelled dynamic topography, geoid and plate motion using a spectral mantle flow code (Hager & O'Connell, 1981) considering radial viscosity distribution and a fully coupled code of the lithosphere and mantle accounting for strong heterogeneities in the upper mantle rheology in the 300 km depths (Popov & Sobolev, 2008). This study is the first step towards linking global mantle dynamics with lithosphere dynamics using the observed geoid as a major constraint. Here we present the results from both codes and compare them with the observed geoid, dynamic topography and plate velocities from GPS measurements. This method allows us to evaluate the effects of plate rheology (e.g., strong plate interiors and weak plate margins) and stiff subducted lithosphere on these observables (i.e. geoid, topography, plate boundary stresses) as well as on plate motion. This effort will also serve as a benchmark of the two existing numerical methods

  18. 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.

  19. Secondary Hotspots in the South Pacific as a Result of Mantle Plumelets and Lithospheric Extension?

    NASA Astrophysics Data System (ADS)

    Koppers, A.; Staudigel, H.; Wijbrans, J.; Pringle, M.

    2003-12-01

    has lead to new models that retain the concept of mantle plumes, but these lack both simplicity and predictive power. New models that call on "extension" are indeed simple and they may explain most characteristics of Earth's intra-plate volcanism, but they also have limited predictive power, making it more difficult to test for their validity. We argue that we require a combination of processes: one that forces regional magmatism from a large-scale source of buoyancy from below (like the rise of plumelets shooting off the top of a superplume) and one process that acts from above, as intra-plate extension opens up pathways that allow the lithosphere to be penetrated by magma.

  20. The Two Subduction Zones of the Southern Caribbean: Lithosphere Tearing and Continental Margin Recycling in the East, Flat Slab Subduction and Laramide-Style Uplifts in the West

    NASA Astrophysics Data System (ADS)

    Levander, A.; Bezada, M. J.; Niu, F.; Schmitz, M.

    2015-12-01

    The southern Caribbean plate boundary is a complex strike-slip fault system bounded by oppositely vergent subduction zones, the Antilles subduction zone in the east, and a currently locked Caribbean-South American subduction zone in the west (Bilham and Mencin, 2013). Finite-frequency teleseismic P-wave tomography images both the Atlanic (ATL) and the Caribbean (CAR) plates subducting steeply in opposite directions to transition zone depths under northern South America. Ps receiver functions show a depressed 660 discontinuity and thickened transition zone associated with each subducting plate. In the east the oceanic (ATL) part of the South American (SA) plate subducts westward beneath the CAR, initiating the El Pilar-San Sebastian strike slip system, a subduction-transform edge propagator (STEP) fault (Govers and Wortel, 2005). The point at which the ATL tears away from SA as it descends into the mantle is evidenced by the Paria cluster seismicity at depths of 60-110 km (Russo et al, 1993). Body wave tomography and lithosphere-asthenosphere boundary (LAB) thickness determined from Sp and Ps receiver functions and Rayleigh waves suggest that the descending ATL also viscously removes the bottom third to half of the SA continental margin lithospheric mantle as it descends. This has left thinned continental lithosphere under northern SA in the wake of the eastward migrating Antilles subduction zone. The thinned lithosphere occupies ~70% of the length of the El Pilar-San Sebastian fault system, from ~64oW to ~69oW, and extends inland several hundred kilometers. In northwestern SA the CAR subducts east-southeast at low angle under northern Colombia and western Venezuela. The subducting CAR is at least 200 km wide, extending from northernmost Colombia as far south as the Bucaramanga nest seismicity. The CAR descends steeply under Lake Maracaibo and the Merida Andes. This flat slab is associated with three Neogene basement cored, Laramide-style uplifts: the Santa Marta

  1. Deformation, annealing, reactive melt percolation, and seismic anisotropy in the lithospheric mantle beneath the southeastern Ethiopian rift: Constraints from mantle xenoliths from Mega

    NASA Astrophysics Data System (ADS)

    Tommasi, Andréa; Baptiste, Virginie; Vauchez, Alain; Holtzman, Benjamin

    2016-07-01

    We explore the relations between deformation, annealing, and melt percolation during rifting and the effect of these processes on seismic anisotropy by analyzing the microstructures and crystal preferred orientations (CPO) in a suite of mantle xenoliths from Mega, in the southern end of the Ethiopian rift. Previous geochemical studies on these xenoliths showed evidence for interactions with variable melt types and volumes during the rifting process. The peridotites have dominantly coarse-porphyroclastic microstructures, but coarse granular or partially recrystallized microstructures also occur. The olivine CPO, characterized by orthorhombic to fiber-[100] patterns and moderate intensities, the common occurrence of (100) tilt walls, and the predominance of <0vw> rotation axes accommodating low angle misorientations in olivine support deformation by dislocation creep with dominant activation of the [100](010) system. Annealing (static recrystallization) of variable intensity followed this deformation. Modal enrichment in pyroxenes in > 60% of the studied peridotites corroborates extensive, but spatially heterogeneous reactive melt percolation leading to refertilization of the lithospheric mantle beneath the southern Ethiopian rift. The common interstitial shapes of the pyroxenes and lack of correlation between the pyroxenes and the olivine CPOs in many samples suggest that part of the refertilization is post-kinematic. However, there is no simple relation between reactive melt percolation and annealing of the olivine deformation microstructure. Comparison with data from other xenolith localities points to changes in the metasomatic imprint in the lithospheric mantle along the East African rift system correlated with the evolution in the rift maturity. Seismic properties averaged over all samples show typical lithospheric mantle patterns with fast propagation of P- and polarization of the fast S-waves parallel to the lineation. The anisotropy is moderate (< 6% for P

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

    NASA Astrophysics Data System (ADS)

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

    2002-05-01

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

  3. Pb and Sr isotopic constraints on lithospheric magma sources during Mesozoic continental margin arc initiation, southern California

    SciTech Connect

    Barth, A.P. . Dept. of Geology); Tosdal, R.M.; Wooden, J.L. )

    1993-04-01

    Initiation of the Mesozoic Cordilleran arc in the southwestern US is marked by plutonism from about 241--213 Ma, exposed in the Granite Mountains in the southwestern Mojave Desert, through the Transverse Ranges to the Colorado River trough in southeastern California and southwestern Arizona. Plutons range in composition from diorite to granite, but quartz monzonite and monzodiorite predominate. Plutons intruded 1,700 to 1,100 Ma rocks of the Mojave crustal province [as defined isotopically by Wooden and Miller (1990) and Bennett and DePaolo (1987)], and episomal plutons locally intruded its deformed cratonal/miogeoclinal cover. Plutons emplaced during arc initiation overlap isotopically with local Proterozoic basement rocks, but typically have less radiogenic [sup 207]Pb/[sup 204]Pb, [sup 208]Pb/[sup 204]Pb and [sup 87]Sr/[sup 86]Sr than adjacent, more voluminous Middle-Late Jurassic and Late Cretaceous arc plutons. Among early arc plutons, an inter-suite trend toward more radiogenic Pb and [sup 87]Sr/[sup 86]Sr at similar silica contents reflects regional basement isotopic variability. Generally high Sr contents, low Rb/Sr and limited REE data suggest this inter-suite variations records heterogeneity in eclogite/garnet amphibolite facies mafic lithospheric magma sources, corresponding to mafic lower crust and/or upper mantle formed during 1,700 Ma orogenesis or rift-related magmatism at 1,100 Ma. Intra-suite trends toward less radiogenic Pb at constant or more radiogenic Sr reflect involvement of a U and Th depleted, silicic intracrustal contaminant. Distinctive magma sources and limited crustal interaction during emplacement probably reflects the relatively cool thermal structure of the nascent Cordilleran continental margin arc.

  4. Depleted subcontinental lithospheric mantle and its tholeiitic melt metasomatism beneath NE termination of the Eger Rift (Europe): the case study of the Steinberg (Upper Lusatia, SE Germany) xenoliths

    NASA Astrophysics Data System (ADS)

    Kukuła, Anna; Puziewicz, Jacek; Matusiak-Małek, Magdalena; Ntaflos, Theodoros; Büchner, Jörg; Tietz, Olaf

    2015-12-01

    The ca. 30 Ma Steinberg basanite occurs at the NE termination of the Eger (Ohře) Rift in the NW Bohemian Massif, Central Europe, and belongs to the Cenozoic alkaline Central European Volcanic Province. The basanite hosts a suite of mantle xenoliths, most of which are harzburgites containing relatively magnesian olivine (Fo 90.5-91.6) and Al-poor (0.04-0.13 a pfu) orthopyroxene (mg# 0.90-0.92). Some of these harzburgites also contain volumetrically minor clinopyroxene (mg# 0.92-0.95, Al 0.03-0.13 a pfu) and have U-shaped LREE-enriched REE patterns. The Steinberg harzburgites are typical for the Lower Silesian - Upper Lusatian domain of the European subcontinental lithospheric mantle. They represent residual mantle that has undergone extensive partial melting and was subsequently affected by mantle metasomatism by mixed carbonatite-silicate melts. The Steinberg xenolith suite comprises also dunitic xenoliths affected by metasomatism by melt similar to the host basanite, which lowered the Fo content in olivine to 87.6 %. This metasomatism happened shortly before xenolith entrainment in the erupting lava. One of the xenoliths is a wehrlite (olivine Fo 73 %, clinopyroxene mg# 0.83-0.85, subordinate orthopyroxene mg# 0.76-0.77). Its clinopyroxene REE pattern is flat and slightly LREE-depleted. This wehrlite is considered to be a tholeiitic cumulate. One of the studied harzburgites contains clinopyroxene with similar trace element contents to those in wehrlite. This type of clinopyroxene records percolation of tholeiitic melt through harzburgite. The tholeiitic melt might be similar to Cenozoic continental tholeiites occurring in the Central European Volcanic Province (e.g., Vogelsberg, Germany).

  5. Combined underthrusting and mantle dripping - lateral dragging controlling the lithosphere structure of the NW-Moroccan margin and the Atlas Mountains

    NASA Astrophysics Data System (ADS)

    Jiménez-Munt, I.; Fernandez, M.; Zlotnik, S.

    2012-04-01

    Recent studies carried out in NW-Africa indicate prominent variations of the lithosphere-asthenosphere boundary (LAB) depth. The studies integrate gravity, geoid, surface heat flow, elevation and seismic data along a profile running from the Tagus Abyssal Plain to the Sahara Platform and crossing the Gorringe Bank, the NW Moroccan Margin and the Atlas Mountains. The resulting mantle density anomalies show a prominent lithospheric mantle thickening beneath the margin (LAB >200 km-depth) followed by thinning beneath the Atlas Mountains (LAB ~90 km-depth). A combination of mantle underthrusting due to oblique convergence together with a viscous dripping fed by lateral mantle dragging can explain the imaged lithospheric structure. The model is consistent with a strong decoupled crustal-mantle mechanical response to the Africa-Eurasia convergence and results in positive/negative dynamic topography in regions with thickened/thinned crust. In the present work we go a step further analysing the role of the lithospheric mantle structure on the resulting dynamic topography and the dynamic conditions suitable to produce the inferred mantle density anomalies. Therefore, we calculate the dynamic topography rising from mantle thickness variations along the profile and those related to possible lateral variations of mantle composition. In addition, we study the key factors controlling the deformation of the lithospheric mantle when submitted to convergence by means the fully dynamic software UNDERWORLD. Chief among these factors are the mantle viscosity and its temperature dependence, the characteristic time of the process, and the resulting topography variation of the free upper surface. These results allow us to speculate on the past and future evolution of the NW-Moroccan margin which could show the appropriated conditions for subduction initiation.

  6. Teleseismic wave front anomalies at a Continental Rift: no mantle anomaly below the central Upper Rhine Graben

    NASA Astrophysics Data System (ADS)

    Kirschner, Stephanie; Ritter, Joachim; Wawerzinek, Britta

    2011-08-01

    The deep structure of the Upper Rhine Graben (URG), a continental rift in SW Germany and E France, is still poorly known. This deficit impedes a full understanding of the geodynamic evolution of this prominent rift. We study the lithosphere-asthenosphere structure using teleseismic waveforms obtained from the passive broad-band TIMO project across the central URG. The recovered, crust-corrected traveltime residuals relative to the iasp91 earth model are tiny (mostly less than 0.2-0.3 s). The average measured slowness (<1 s deg-1) and backazimuth (<5°) deviations are also very small and do not show any systematic wave front anomalies. These observed perturbation values are smaller than expected ones from synthetic 3-D ray tracing modelling with anomalies exceeding 2-3 per cent seismic velocity in the mantle. Thus there is no significant hint for any deep-seated anomaly such as a mantle cushion, etc. This result means that the rifting process did not leave behind a lower lithospheric signature, which could be clearly verified with high-resolution teleseismic experiments. The only significant traveltime perturbation at the central URG is located at its western side in the upper crust around a known geothermal anomaly. The upper crustal seismic anomaly with traveltime delays of 0.2-0.3 s cannot be explained with increased temperature alone. It is possibly related to a zone of highly altered granite. In the west of our network a traveltime anomaly (0.6-0.7 s delay) related with the Eifel plume is confirmed by the TIMO data set.

  7. New model of the mantle lithosphere beneath Kuoyka kimberlite field Yakutia.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Kostrovitsky, Sergey; Ovchinnikov, Yury; Tychkov, Nikolai; Khmelnikova, Olga; Palessky, Stanislav

    2013-04-01

    New data for the 11 pipes from Kuoyka field show that high Cr2O3 garnets to 10- 12% as well as high Cr chromites (to 64%Cr2O3) are found in several more pipes Zaozernaya, Seraya, Slyudyanka, Vodorasdelnaya, Titan, Lusya in addition to Djanga pipe. All garnets belong o lherzolite field and not less than 1/3 are TiO rich. The TiO2 rich chromites are dominating in the Cr- rich population. Metasomatic Cr2O3- rich (to 6%) ilmenites pre in the MgO and TiO2- part of the variation diagrams. The Cr- diopside variations show high variations of Fe and Na content to 4 % suggesting the hybridic origin similar to the Cr- pyroxeneis from Obnazhennaya pyroxenites (Taylor et al ., 2003). Omphicites (to 7 % Na2O) are rare. Cr-amphiboles (pargasites and hornblendes) are common in the upper part of the SCLM as well as in the Anabar and Kharamai region. Reconstructions of the mantle sections show the deep lithospheric roots beneath the Zosernaya pipe (7.5 GPa) traced by the PT conditions for Opx, Cpx, Gar, Cr and Ilm. SCLM is divided in to 4 sections and Ilm trace tow intervals in lower and upper part form 4 GPa. Th HT branch is sporadically found from 7 GPa to the Moho. In other pipes ilmenite and garnet PT estimates are more common in the lower part o mantle section while the Cpx trace mainly middle part of SCLM similar to the Obnazhennaya pip. It seems that kimberlites captured mainly the walls of feeders traced by Cr- low garnets and ilmenites in the lower part of SCLM while peridotitic mantle column was captured starting from the middle part of SCLM. The NS transsect of the Kuoyka field show more fertile mantle sections in the NNW part of the field. The TRE determined for the minerals from Kuoyka field show rather rounded patterns for REE of garnets with high variations in HREE part and small elevation in LREE . The depleted compositions reval the inflection in Eu TRE spidergrams well as relatively small Sr minima. Many of them show Ta peak, relatively small Pb elevation and Th

  8. Variations on the Lower Silesian (SW Poland) lithospheric mantle - the Grodziec xenoliths case study

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    The lithospheric mantle underlying the northern margin of Bohemian Massif (Lower Silesia, SW Poland) is in general characterized by presence of two ultramafic lithologies, both of mostly harzburgitic composition. The group A harzburgites are strongly depleted and record variable metasomatic events, which are however always related to reactions with mixed alkaline-carbonatite melts. The group B harzburgites also record reaction with mafic melts, but in this case the reaction resulted in enrichment of the peridotites in Fe ("Fe-metasomatism"). The xenoliths suites from Grodziec (this study), Krzeniów (Matusiak-Małek et al., 2014, JoP) and Wilcza Góra (Matusiak-Małek et al., submitted), all in the Złotoryja volcanic complex, follow the "A" and "B" lithological characteristics. The Grodziec suite contains, however, numerous lherzolitic xenoliths. The group A xenoliths from Grodziec are anhydrous lherzolites, scarcely harzburgites. The Fo content in olivine varies from 90.7 to 91.8%, Mg# in ortho-and clinopyroxene is 0.91-0.92. Al content in orthopyroxene is 0.05-0.14 a pfu (0.70 to 3.44 wt.%), which makes them one of the highest in region. Few lherzolites are characterized by slightly lower Fo content in olivine (89.16-90.10%) and are therefore classified as A- group. The Mg# of pyroxenes in this group varies from 0.89 to 0.90, but orthopyroxene is generally characterized by low Al content (< 0.08 a pfu, corresponding to <2 wt.% in majority of xenoliths). Group B xenoliths are orthopyroxene - free dunites, and wehrlite. Olivine contains from 85.14 to 86.14 % of Fo, the Mg# of clinopyroxene varies from 0.84 to 0.88. Clinopyroxene in all the groups is LREE enriched and has negative Sr, Zf-Hf and Ti anomalies, but the enrichment decreases from group A to B and so are the depths of negative anomalies. Temperatures of major element equilibration of group A and A- pyroxenes are from approximately 1010 to 1100°C with no specific differences between the groups. So high

  9. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere

    USGS Publications Warehouse

    Geist, E.L.; Andrews, D.J.

    2000-01-01

    Long-term slip rates on major faults in the San Francisco Bay area are predicted by modeling the anelastic deformation of the continental lithosphere in response to regional relative plate motion. The model developed by Bird and Kong [1994] is used to simulate lithospheric deformation according to a Coulomb frictional rheology of the upper crust and a dislocation creep rheology at depth. The focus of this study is the long-term motion of faults in a region extending from the creeping section of the San Andreas fault to the south up to the latitude of Cape Mendocino to the north. Boundary conditions are specified by the relative motion between the Pacific plate and the Sierra Nevada - Great Valley microplate [Argus and Gordon, 2000]. Rheologic-frictional parameters are specified as independent variables, and prediction errors are calculated with respect to geologic estimates of slip rates and maximum compressive stress directions. The model that best explains the region-wide observations is one in which the coefficient of friction on all of the major faults is less than 0.15, with the coefficient of friction for the San Andreas fault being approximately 0.09, consistent with previous inferences of San Andreas fault friction. Prediction error increases with lower fault friction on the San Andreas, indicating a lower bound of ??SAF > 0.08. Discrepancies with respect to previous slip rate estimates include a higher than expected slip rate along the peninsula segment of the San Andreas fault and a slightly lower than expected slip rate along the San Gregorio fault.

  10. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere

    NASA Astrophysics Data System (ADS)

    Geist, Eric L.; Andrews, D. J.

    2000-11-01

    Long-term slip rates on major faults in the San Francisco Bay area are predicted by modeling the anelastic deformation of the continental lithosphere in response to regional relative plate motion. The model developed by Bird and Kong [1994] is used to simulate lithospheric deformation according to a Coulomb frictional rheology of the upper crust and a dislocation creep rheology at depth. The focus of this study is the long-term motion of faults in a region extending from the creeping section of the San Andreas fault to the south up to the latitude of Cape Mendocino to the north. Boundary conditions are specified by the relative motion between the Pacific plate and the Sierra Nevada-Great Valley microplate [Argus and Gordon, 2000]. Rheologic-frictional parameters are specified as independent variables, and prediction errors are calculated with respect to geologic estimates of slip rates and maximum compressive stress directions. The model that best explains the region-wide observations is one in which the coefficient of friction on all of the major faults is less than 0.15, with the coefficient of friction for the San Andreas fault being approximately 0.09, consistent with previous inferences of San Andreas fault friction. Prediction error increases with lower fault friction on the San Andreas, indicating a lower bound of μSAF > 0.08. Discrepancies with respect to previous slip rate estimates include a higher than expected slip rate along the peninsula segment of the San Andreas fault and a slightly lower than expected slip rate along the San Gregorio fault.

  11. A mechanism to thin the continental lithosphere at magma-poor margins.

    PubMed

    Lavier, Luc L; Manatschal, Gianreto

    2006-03-16

    Where continental plates break apart, slip along multiple normal faults provides the required space for the Earth's crust to thin and subside. After initial rifting, however, the displacement on normal faults observed at the sea floor seems not to match the inferred extension. Here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults. Our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust (< 10 km) by exhumation of mid-crustal and mantle material. This transitional crust is capped by sub-horizontal detachment surfaces over distances exceeding 100 km with little visible deformation. Our rift model is based on numerical experiments constrained by geological and geophysical observations from the Alpine Tethys and Iberia/Newfoundland margins. Furthermore, we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust. The presence of such lower crustal gabbros is well constrained for the Alpine Tethys system. Initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution. This is achieved in our models by the formation of weak ductile shear zones.

  12. No thermal anomalies in the mantle transition zone beneath an incipient continental rift: evidence from the first receiver function study across the Okavango Rift Zone, Botswana

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

    Mechanisms leading to the initiation and early-stage development of continental rifts remain enigmatic, in spite of numerous studies. Among the various rifting models, which were developed mostly based on studies of mature rifts, far-field stresses originating from plate interactions (passive rifting) and nearby active mantle upwelling (active rifting) are commonly used to explain rift dynamics. Situated atop of the hypothesized African Superplume, the incipient Okavango Rift Zone (ORZ) of northern Botswana is ideal to investigate the role of mantle plumes in rift initiation and development, as well as the interaction between the upper and lower mantle. The ORZ developed within the Neoproterozoic Damara belt between the Congo Craton to the northwest and the Kalahari Craton to the southeast. Mantle structure and thermal status beneath the ORZ are poorly known, mostly due to a complete paucity of broad-band seismic stations in the area. As a component of an interdisciplinary project funded by the United States National Science Foundation, a broad-band seismic array was deployed over a 2-yr period between mid-2012 and mid-2014 along a profile 756 km in length. Using P-to-S receiver functions (RFs) recorded by the stations, the 410 and 660 km discontinuities bordering the mantle transition zone (MTZ) are imaged for the first time. When a standard Earth model is used for the stacking of RFs, the apparent depths of both discontinuities beneath the Kalahari Craton are about 15 km shallower than those beneath the Congo Craton. Using teleseismic P- and S-wave traveltime residuals obtained by this study and lithospheric thickness estimated by previous studies, we conclude that the apparent shallowing is the result of a 100-150 km difference in the thickness of the lithosphere between the two cratons. Relative to the adjacent tectonically stable areas, no significant anomalies in the depth of the MTZ discontinuities or in teleseismic P- and S-wave traveltime residuals are

  13. Thermal and mechanical structure of the upper mantle: A comparison between continental and oceanic models

    NASA Technical Reports Server (NTRS)

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

    1976-01-01

    Temperature, velocity, and viscosity profiles for coupled thermal and mechanical models of the upper mantle beneath continental shields and old ocean basins show that under the continents, both tectonic plates and the asthenosphere, are thicker than they are beneath the oceans. The minimum value of viscosity in the continental asthenosphere is about an order of magnitude larger than in the shear zone beneath oceans. The shear stress or drag underneath continental plates is also approximately an order of magnitude larger than the drag on oceanic plates. Effects of shear heating may account for flattening of ocean floor topography and heat flux in old ocean basins.

  14. Constraints on upper mantle rheology from modeling of plate motions with fully 3D visco-elasto-plastic lithosphere

    NASA Astrophysics Data System (ADS)

    Sobolev, S. V.; Popov, A.; Steinberger, B.

    2009-04-01

    The convection in deep Earth is linked to the surface through the heterogeneous and rheologically complex lithosphere and asthenosphere, which are usually strongly simplified in global geodynamic models. We use a newly developed 3D thermomechanical finite element numerical technique (Popov and Sobolev, PEPI 2008) to model a 300 km thick upper layer of the Earth in full 3D, coupled with the convecting mantle. The present day temperature distribution and crustal structure within the layer are taken from existing models. We also assume that the upper layer is composed from non-linear temperature- and stress-dependent visco-elastic rheology, corresponding to the dry or wet olivine (mantle) or naturally wet plagioclase (crust), combined with Mohr-Coulomb frictional plasticity. Plate boundaries are represented by the narrow zones of elasto-visco-plastic rheology with much lower frictional strength than within the plates. The mantle below the 300 km depth is modeled using Hager and O'Connell's mantle flow spectral modeling technique with present day density and viscosity distribution based on either interpretation of global seismic tomography or history of subduction. The upper layer and mantle modeling domains are coupled by iteratively achieved precise continuity of tractions and velocities at 300 km depth. Here we will show modeling results for the present day Earth structure focusing on the effect on the plate velocities of the frictional strength at plate boundaries, of mantle potential temperature and of rheology of the asthenosphere (dry versus wet). Modeling shows that deep convection generates plate tectonic-like velocity pattern only when effective friction at subduction plate boundaries becomes less than 0.1. Both magnitudes and directions of plate velocities are reproduced very well at friction in subduction zones around 0.005-0.05 and friction at other plate boundaries of 0.05-0.1. The best fit of the observed velocities is obtained assuming that

  15. Slab detachment during continental collision: Influence of crustal rheology and interaction with lithospheric delamination

    NASA Astrophysics Data System (ADS)

    Duretz, T.; Gerya, T. V.

    2013-08-01

    Collision between continents can lead to the subduction of continental material. If the crust remains coupled to the downgoing slab, a large buoyancy force is generated. This force slows down convergence and promotes slab detachment. If the crust resists to subduction, it may decouple from the downgoing slab and be subjected to buoyant extrusion. We employ two-dimensional thermo-mechanical modelling to study the importance of crustal rheology on the evolution of subduction-collision systems. We propose simple quantifications of the mechanical decoupling between lithospheric levels (σ*) and the potential for buoyant extrusion of the crust (ξ*). The modelling results indicate that a variable crustal rheological structure results in slab detachment, delamination, or the combination of both mechanisms. A strong crust provides coupling at the Moho (low σ*) and remains coherent during subduction (low ξ). It promotes deep subduction of the crust (180 km) and slab detachment. Exhumation occurs in coherent manners via eduction and thrusting. Slab detachment triggers the development of topography (> 4.5 km) close to the suture. A contrasting style of collision occurs using a weak crustal rheology. Mechanical decoupling at the Moho (high σ*) promotes the extrusion of the crust (high ξ), disabling slab detachment. Ongoing shortening leads to buckling of the crust and development of topography on the lower plate. Collisions involving rheologically layered crust allow decoupling at mid-crustal depths. This structure favours both the extrusion of upper crust and the subduction of the lower crust. Such collisions are successively affected by delamination and slab detachment. Topography develops together with the buoyant extrusion of crust onto the foreland and is further amplified by slab detachment. Our results suggest that the occurrence of both delamination (Apennines) and slab detachment (Himalayas) in orogens may indicate differences in the initial crustal structure of

  16. Crustal magmatism and lithospheric geothermal state of western North America and their implications for a magnetic mantle

    NASA Astrophysics Data System (ADS)

    Wang, Jian; Li, Chun-Feng

    2015-01-01

    The western North American lithosphere experienced extensive magmatism and large-scale crustal deformation due to the interactions between the Farallon and North American plates. To further understand such subduction-related dynamic processes, we characterize crustal structure, magmatism and lithospheric thermal state of western North America based on various data processing and interpretation of gravimetric, magnetic and surface heat flow data. A fractal exponent of 2.5 for the 3D magnetization model is used in the Curie-point depth inversion. Curie depths are mostly small to the north of the Yellowstone-Snake River Plain hotspot track, including the Steens Mountain and McDermitt caldera that are the incipient eruption locations of the Columbia River Basalts and Yellowstone hotspot track. To the south of the Yellowstone hotspot track, larger Curie depths are found in the Great Basin. The distinct Curie depths across the Yellowstone-Snake River Plain hotspot track can be attributed to subduction-related magmatism induced by edge flow around fractured slabs. Curie depths confirm that the Great Valley ophiolite is underlain by the Sierra Nevada batholith, which can extend further west to the California Coast Range. The Curie depths, thermal lithospheric thickness and surface heat flow together define the western edge of the North American craton near the Roberts Mountains Thrust (RMT). To the east of the RMT, large Curie depths, large thermal lithospheric thickness, and low thermal gradient are found. From the differences between Curie-point and Moho depth, we argue that the uppermost mantle in the oceanic region is serpentinized. The low temperature gradients beneath the eastern Great Basin, Montana and Wyoming permit magnetic uppermost mantle, either by serpentinization/metasomatism or in-situ magnetization, which can contribute to long-wavelength and low-amplitude magnetic anomalies and thereby large Curie-point depths.

  17. Influence of cratonic lithosphere on slab geometry and mantle flow: insights from 3D time-dependent modelling.

    NASA Astrophysics Data System (ADS)

    Taramón, Jorge M.; Rodríguez-González, Juan; Negredo, Ana M.

    2014-05-01

    Recent studies show a clear correlation between the occurrence of flat subduction and the proximity of areas of high elastic/thermal thickness in the overriding plate. A plausible explanation is that cold overriding plates lead to colder mantle wedge, increasing the hydrodynamic suction and decreasing the slab dip. In particular, recent numerical modeling has shown that the presence of cratonic lithosphere in the overriding plate has a significant effect on subducting slabs. In this study we quantify the influence of cratonic areas in the overriding plate on subduction dynamics. We present 3D thermo-mechanical and time-dependent numerical models of buoyancy-driven subduction processes. A non-Newtonian pseudo-plastic rheology is assumed. Different simulations have been performed to quantify the effect of different factors, such as the craton width, thermal thickness and distante to the trench. Modelling results indicate that presence of cratonic lithosphere in the overriding plate produces strong along-trench variations of the slab geometry. These variations are maintained and propagated at great depths as the slab sinks deeper into the mantle. Significant trench-parallel flow in the mantle wedge is generated by time-dependent changes in slab dip. For cases of reduced slab pull, the slab and the base of the craton become coupled, which causes a dramatic reduction of subduction velocity and the formation of a slab gap. The presence of cratons may have an important role on subduction episodicity and provide a new mechanism to explain slab gaps in areas where cratons have been located close to trenches, as is the case of South America and the Cenozoic subduction of North America. We further emphasize that the lithospheric structure of the overriding plate should be taken into account in analysis and modelling studies of subduction zones.

  18. Three-dimensional modeling of the lateral flow of the plume material under the continental lithosphere of Africa

    NASA Astrophysics Data System (ADS)

    Lin, S.; Kuo, B.

    2002-12-01

    Spreading of a giant plume head beneath the continental lithosphere, has been proposed to explain the extensive Cenozoic magmatism in east Africa (e.g., Ebinger and Sleep, 1998). Previous models using kinematic flow with simple geometry and constant rheology show that the progression of the lateral flow of the impinging plume material guided by the anti-valley along the lithosphere-asthenosphere interface agrees with the age of the magmatic activities. Here we revisit this problem with a 3-D, variable-viscosity numerical modeling approach to explore the combined effects of the temperature- and water-content rheology and the topography of the lithosphere's base. We examine the single-plume hypothesis by investigating how the buoyant plume material migrates from east Africa to the north and west and how far it could reach, with a more detailed modeling on rheology and geometry against geological constraints. The problem is characterized as a multi-component viscous flow model involving advection of the interfaces between continent, asthenosphere and plume head material. We design models for the continental lithosphere with different slopes and depths on the base and monitor the propagation of the plume head material along the channel.

  19. Temporal and Spatial Scales of Sub-Continental Mantle Convection: Comparison of Modern and Geological Observations of Dynamic Support

    NASA Astrophysics Data System (ADS)

    Jones, S. M.; Lovell, B.; Crosby, A. G.

    2011-12-01

    The topographies of Africa and Antarctica form patterns of interlocking swells. The admittance between swell topography and free-air gravity indicates that these swells are dynamically supported by mantle convection, with swell diameters of 1850±450 km and full heights between 800 and 1800 m. The implication is that mantle convection not only supports swells surrounding hotspots but also influences topography across the entire surface areas of Africa and Antarctica. Topographic swells and associated gravity anomalies with diameters over 1000 km are observed on other continents and throughout the oceans. Numerical models support the idea that dynamically supported swell topography is a worldwide phenomenon. We investigate whether dynamically supported swells are also observed throughout the geological record, focussing on intensively studied Mesozoic- Cenozoic sedimentary rocks around Britain and Ireland. Since 200 Ma, this region was affected by three dynamically supported swells that peaked during the Middle Jurassic, Early Cretaceous and Eocene (c. 175, 146 and 56 Ma), each several thousand kilometres in diameter, and the region now lies on the edge of the modern swell centred on Iceland. The diameters and maximum heights of the Mesozoic British swells and the modern African and Antarctic swells are similar. The ancient British swells grew in 5--10 Myr and decayed over 20--30 Myr, suggesting vertical motion rates comparable to those estimated from geomorphological studies of Africa. Igneous production rate and swell height are not correlated in the modern and the geological records. Vertical motions of Britain and Ireland, a typical piece of continental lithosphere far from a destructive plate boundary, have been demonstrably affected by convective support for over half of the past 200 Ma period. Mantle convection should be considered as a common control on regional sea-level at time periods from 10s down to 1 Myr or less, and with vertical motion rates in the

  20. Dynamics of upper mantle rocks decompression melting above hot spots under continental plates

    NASA Astrophysics Data System (ADS)

    Perepechko, Yury; Sorokin, Konstantin; Sharapov, Victor

    2014-05-01

    Numeric 2D simulation of the decompression melting above the hot spots (HS) was accomplished under the following conditions: initial temperature within crust mantle section was postulated; thickness of the metasomatized lithospheric mantle is determined by the mantle rheology and position of upper asthenosphere boundary; upper and lower boundaries were postulated to be not permeable and the condition for adhesion and the distribution of temperature (1400-2050°C); lateral boundaries imitated infinity of layer. Sizes and distribution of lateral points, their symmetry, and maximum temperature varied between the thermodynamic condition for existences of perovskite - majorite transition and its excess above transition temperature. Problem was solved numerically a cell-vertex finite volume method for thermo hydrodynamic problems. For increasing convergence of iterative process the method of lower relaxation with different value of relaxation parameter for each equation was used. The method of through calculation was used for the increase in the computing rate for the two-layered upper mantle - lithosphere system. Calculated region was selected as 700 x (2100-4900) km. The time step for the study of the asthenosphere dynamics composed 0.15-0.65 Ma. The following factors controlling the sizes and melting degree of the convective upper mantle, are shown: a) the initial temperature distribution along the section of upper mantleb) sizes and the symmetry of HS, c) temperature excess within the HS above the temperature on the upper and lower mantle border TB=1500-2000oC with 5-15% deviation but not exceed 2350oC. It is found, that appearance of decompression melting with HS presence initiate primitive mantle melting at TB > of 1600oC. Initial upper mantle heating influence on asthenolens dimensions with a constant HS size is controlled mainly by decompression melting degree. Thus, with lateral sizes of HS = 400 km the decompression melting appears at TB > 1600oC and HS

  1. Growth of the continental crust: A planetary-mantle perspective

    NASA Technical Reports Server (NTRS)

    Warren, Paul H.

    1988-01-01

    The lack of earth rocks older than about 3.8 Ga is frequently interpreted as evidence that the earth formed little or no subduction-resistant continental crust during the first 700 My of its history. Such models obviously imply that the pre-3.8 Ga earth was covered entirely or almost entirely by smoothly subducting oceanic crust. On the other hand, the thermal regime of the early earth probably tended to cause the oceanic crust at this time to be comparatively thin and comparatively mafic. The present earth is covered by about 50 percent oceanic crust, averaging about 7 km in thickness, and 41 percent continental crust, averaging roughly 40 km in thickness. Thus continentless-early-earth models would seem to imply a total mass of crust less than 1/3 that of the present day earth. Possible explanations are examined.

  2. A Thermal Evolution Model of the Earth Including the Biosphere, Continental Growth and Mantle Hydration

    NASA Astrophysics Data System (ADS)

    Höning, D.; Spohn, T.

    2014-12-01

    By harvesting solar energy and converting it to chemical energy, photosynthetic life plays an important role in the energy budget of Earth [2]. This leads to alterations of chemical reservoirs eventually affecting the Earth's interior [4]. It further has been speculated [3] that the formation of continents may be a consequence of the evolution life. A steady state model [1] suggests that the Earth without its biosphere would evolve to a steady state with a smaller continent coverage and a dryer mantle than is observed today. We present a model including (i) parameterized thermal evolution, (ii) continental growth and destruction, and (iii) mantle water regassing and outgassing. The biosphere enhances the production rate of sediments which eventually are subducted. These sediments are assumed to (i) carry water to depth bound in stable mineral phases and (ii) have the potential to suppress shallow dewatering of the underlying sediments and crust due to their low permeability. We run a Monte Carlo simulation for various initial conditions and treat all those parameter combinations as success which result in the fraction of continental crust coverage observed for present day Earth. Finally, we simulate the evolution of an abiotic Earth using the same set of parameters but a reduced rate of continental weathering and erosion. Our results suggest that the origin and evolution of life could have stabilized the large continental surface area of the Earth and its wet mantle, leading to the relatively low mantle viscosity we observe at present. Without photosynthetic life on our planet, the Earth would be geodynamical less active due to a dryer mantle, and would have a smaller fraction of continental coverage than observed today. References[1] Höning, D., Hansen-Goos, H., Airo, A., Spohn, T., 2014. Biotic vs. abiotic Earth: A model for mantle hydration and continental coverage. Planetary and Space Science 98, 5-13. [2] Kleidon, A., 2010. Life, hierarchy, and the

  3. Variations in timing of lithospheric failure on terrestrial planets due to chaotic nature of mantle convection

    NASA Astrophysics Data System (ADS)

    Wong, Teresa; Solomatov, Viatcheslav S.

    2016-05-01

    We perform numerical simulations of lithospheric failure in the stagnant lid regime of temperature-dependent viscosity convection, using the yield stress approach. We find that the time of failure can vary significantly for the same values of the controlling parameters due to the chaotic nature of the convective system. The general trend of the dependence of the time of lithospheric failure on the yield stress can be explained by treating lithospheric failure as a type of Rayleigh-Taylor instability. This study suggests that it is important to address not only the question of whether plate tectonics can occur on a planet but also when it would occur if conditions are favorable.

  4. Combining kimberlite (U-Th)/He dating with the mantle xenolith record to decipher elevation change in continental interiors: an example from the southern African Plateau

    NASA Astrophysics Data System (ADS)

    Stanley, J. R.; Flowers, R. M.; Bell, D. R.

    2013-12-01

    Deciphering the patterns and causes of erosion and elevation change histories in continental interiors is commonly not straightforward. Many continental shield regions are repeatedly intruded by small volume kimberlite magmas, which often contain rich xenolith records of the state of the lithosphere and the sedimentary cover at the time of eruption. Here we show that dating kimberlites with apatite (U-Th)/He thermochronometry (AHe), a tool used to constrain thermal histories within the upper 1-3 km of the crust, can tightly bracket the timing of erosion through comparison of cooling dates and eruption ages. Mantle xenoliths from kimberlites erupted at different times also record perturbations to the lithospheric mantle, indicators of changes to the mantle below. The coeval deep and shallow records of kimberlite pipes thus allow the potential to link deep earth processes with the surface response. The southern African Plateau was elevated from sea level to >1000 m elevation in post-Paleozoic time while distal from convergent plate boundaries and with little surface deformation. The timing and mechanisms of surface uplift are debated. AHe data for four kimberlites off the southwestern corner of the Kaapvaal Craton indicate a substantial Mesozoic unroofing episode that was largely completed by 90 Ma. This erosion phase is contemporaneous with significant warming, metasomatism, and thinning of the lithospheric mantle revealed in the peridotite xenoliths and garnet xenocrysts in these same pipes. We suggest that this surface signal is the erosional response to regional, mantle-induced surface uplift. These data also detect a lesser Cenozoic erosion signature in some pipes, focused around a proposed Tertiary paleo-tributary to the Orange River, suggesting that the Cenozoic signal is associated with drainage network evolution rather than long-wavelength surface uplift. Preliminary data from an E-W transect of kimberlites across the Kaapvaal Craton from Kimberley to the

  5. Upper mantle structures beneath the Carpathian-Pannonian region: Implications for the geodynamics of continental collision

    NASA Astrophysics Data System (ADS)

    Ren, Y.; Stuart, G. W.; Houseman, G. A.; Dando, B.; Ionescu, C.; Hegedüs, E.; Radovanović, S.; Shen, Y.; South Carpathian Project Working Group

    2012-10-01

    The Carpathian-Pannonian system of Eastern and Central Europe represents a unique opportunity to study the interaction between surface tectonic processes involving convergence, extension and convective overturn in the upper mantle. Here, we present high-resolution images of upper mantle structure beneath the region from P-wave finite-frequency teleseismic tomography to help constrain such geodynamical interactions. We have selected earthquakes with magnitude greater than 5.5 in the distance range 30°-95°, which occurred between 2006 and 2011. The data were recorded on 54 temporary stations deployed by the South Carpathian Project (2009-2011), 56 temporary stations deployed by the Carpathian Basins Project (2005-2007), and 131 national network broadband stations. The P-wave relative arrival times are measured in two frequency bands (0.5-2.0 Hz and 0.1-0.5 Hz), and are inverted for Vp perturbation maps in the upper mantle. Our images show a sub-vertical slab of fast material beneath the eastern Alps which extends eastward across the Pannonian basin at depths below ˜300km. The fast material extends down into the mantle transition zone (MTZ), where it spreads out beneath the entire basin. Above ˜300km, the upper mantle below the Pannonian basin is dominated by relatively slow velocities, the largest of which extends down to ˜200km. We suggest that cold mantle lithospheric downwelling occurred below the Pannonian Basin before detaching in the mid-Miocene. In the Vrancea Zone of SE Romania, intermediate-depth (75-180 km) seismicity occurs at the NE end of an upper mantle high velocity structure that extends SW under the Moesian Platform, oblique to the southern edge of the South Carpathians. At greater depths (180-400 km), a sub-circular high velocity anomaly is found directly beneath the seismicity. This sub-vertical high-velocity body is bounded by slow anomalies to the NW and SE, which extend down to the top of the MTZ. No clear evidence of a residual slab is

  6. Interrelation between rifting, faulting, sedimentation, and mantle serpentinization during continental margin formation

    NASA Astrophysics Data System (ADS)

    Rupke, L.; Schmid, D. W.; Perez-Gussinye, M.; Hartz, E. H.

    2013-12-01

    We explore the conditions under which mantle serpentinization may take place during continental rifting with 2D thermotectonostratigraphic basin models. The basic concept follows the idea that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur. The new model tracks the rheological evolution of the continental crust and allows for kinetically controlled mantle serpentinization processes. The isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions. A systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs. Sedimentation shifts this critical stretching factor to higher values as both deeper burial and the low thermal conductivity of sediments lead to higher crustal temperatures. Serpentinization reactions are therefore only likely in settings with low sedimentation rates and high stretching factors. In addition, we find that the rate of sediment supply has first order controls on the rheology of the lower crust, which may control the overall margin geometry. We further test these concepts in ideas in a case study for the Norwegian margin. In particular, we evaluate whether the inner lower crustal bodies (LCB) imaged beneath the More and Voring margin could be serpentinized mantle. For this purpose we reconstruct multiple 2D transects through a 3D data set. This reconstruction of the Norwegian margin shows that serpentinization reactions are indeed possible and likely during the Jurassic rift phase. Predicted present-day thicknesses and locations of partially serpentinized mantle rocks fit well to information on LCBs from seismic and gravity data. We conclude that some of the inner LCBs beneath the Norwegian margin may, in fact, be partially serpentinized mantle.

  7. Evolution of LILE-enriched small melt fractions in the lithospheric mantle: a case study from the East African Rift

    NASA Astrophysics Data System (ADS)

    Bedini, R. M.; Bodinier, J.-L.; Dautria, J.-M.; Morten, L.

    1997-12-01

    Spinel-peridotite xenoliths from Mega (East African Rift, Sidamo region, SE Ethiopia) show variable degrees of recrystallization coupled with trace-element variations. The less recrystallized samples (deformed xenoliths) consist of apatite-bearing porphyroclastic peridotites. They are strongly enriched in LILE (Ba, Th, U, Sr and LREE), with negative anomalies of the HFSE (Nb, Ta, Zr, Hf and Ti). The most recrystallized samples (granular xenoliths) consist of apatite-free peridotites with coarse-grained, granular textures. These samples are depleted or only slightly enriched in LILE and display no significant HFSE anomaly. We suggest that the inverse relationship between recrystallization and trace-element enrichment results from km-scale variation in volume and composition of melts pervasively infiltrated in the lithosphere. The deformed xenoliths record interaction with LILE-enriched small melt fractions, at low melt/rock ratio, while the granular xenoliths were extensively re-equilibrated with a higher fraction of basaltic melt, at higher melt/rock ratio. With a numerical simulation of reactive porous flow at the transition between adiabatic and conductive geotherms in the mantle, it is shown that these two processes were possibly coeval and associated with thermo-mechanical erosion of the lower lithosphere above a mantle plume.

  8. Distinct Thermal and Metasomatic Characteristics of Mantle Lithosphere Beneath Two Proterozoic Terranes Bordering the Kaapvaal Craton of Southern Africa

    NASA Astrophysics Data System (ADS)

    Janney, P. E.; Shiimi, E. T.

    2015-12-01

    There is a first order contrast in compositional and thermal properties between cold and infertile Archean cratonic mantle and younger, warmer and more fertile Proterozoic lithosphere, but it has also become apparent that coherent thermal and compositional differences exist between adjacent Proterozoic terranes, even in regions that have been stable for over 1 Ga. We report new thermobarometry and in-situ trace element data for garnet peridotite xenoliths from several late Cretaceous (100-70 Ma) kimberlite localities in the western Namaqua-Natal Belt (NNB) and Rehoboth Province (RP), which bound the Archean Kaapvaal craton to the west and south, respectively. The localities include some for which no data have been reported previously. Re-depletion model ages from Os isotopes indicate that the lithosphere beneath the NNB and RP is mainly Early Proterozoic (Pearson et al., Chem. Geol., 2004; Janney et al. J. Petrol., 2010) and there is no evidence from xenolith modal proportions for significant differences in average fertility between lithospheric terranes. Equilibration pressures for garnet peridotites from both terranes fall in a similar range (2 to 5 GPa). However, peridotites from the RP typically have P and T values that fall on or very close to the Kaapvaal cratonic geotherm (apart from a group of peridotites from the Gibeon kimberlites with pressures > 4 GPa that follow an adiabatic gradient; e.g. Franz et al., J. Geol., 1996) whereas peridotites from the western NNB have temperatures roughly 100°C warmer than the cratonic geotherm over the whole depth range. Peridotites from the 140 Ma Melton Wold kimberlite, also in the western NNB, lack these warmer temperatures and suggest that warming was contemporaneous with Late Cretaceous kimberlite magmatism. Metasomatic enrichment in incompatible elements (consistent with interaction with kimberlitic melts) is more pronounced in NNB as compared to RP peridotites. The association of higher temperatures with a greater

  9. Evolution of continental crust and mantle heterogeneity: Evidence from Hf isotopes

    USGS Publications Warehouse

    Jonathan, Patchett P.; Kouvo, O.; Hedge, C.E.; Tatsumoto, M.

    1982-01-01

    We present initial 176Hf/177 Hf ratios for many samples of continental crust 3.7-0.3 Gy old. Results are based chiefly on zircons (1% Hf) and whole rocks: zircons are shown to be reliable carriers of essentially the initial Hf itself when properly chosen on the basis of U-Pb studies. Pre-3.0 Gy gneisses were apparently derived from an unfractionated mantle, but both depleted and undepleted mantle are evident as magma sources from 2.9 Gy to present. This mantle was sampled mainly from major crustal growth episodes 2.8, 1.8 and 0.7 Gy ago, all of which show gross heterogeneity of 176Hf/177Hf in magma sources from ??Hf=0 to +14, or about 60% of the variability of the present mantle. The approximate ??Hf=2??Nd relationship in ancient and modern igneous rocks shows that 176Lu/177Hf fractionates in general twice as much as 147Sm/144Nd in mantle melting processes. This allows an estimation of the relative value of the unknown bulk solid/liquid distribution coefficient for Hf. DLu/DHf=??? 2.3 holds for most mantle source regions. For garnet to be an important residual mantle phase, it must hold Hf strongly in order to preserve Hf-Nd isotopic relationships. The ancient Hf initials are consistent with only a small proportion of recycled older cratons in new continental crust, and with quasi-continuous, episodic growth of the continental crust with time. However, recycling of crust less than 150 My old cannot realistically be detected using Hf initials. The mantle shows clearly the general positive ??Hf resulting from a residual geochemical state at least back to 2.9 Gy ago, and seems to have repeatedly possessed a similar degree of heterogeneity, rather than a continuously-developing depletion. This is consistent with a complex dynamic disequilibrium model for the creation, maintenance and destruction of heterogeneity in the mantle. ?? 1981 Springer-Verlag.

  10. Numerical Modelling of the Transition from Continental Rifting to Mantle Exhumation at the West Iberia Margin.

    NASA Astrophysics Data System (ADS)

    Reston, T. J.; Perez-Gussinye, M.; Phipps Morgan, J.

    2003-12-01

    The continental margin of West Iberia lacks significant synrift magmatism and exhibits a zone up to 100 km wide thought to consist mainly of serpentinized and exhumed mantle between the thinned continental and the oceanic crust. However, the existence of linear magnetic anomalies pose an ambiguity regarding the exact amount of synrift magmatism produced during mantle exhumation at the surface. We investigate how the thinned continental crust gives way to a broad zone of exhumed and serpentinized mantle with little synrift magmatism. For this we use a finite element code that includes brittle and ductile deformation in both crust and mantle, production of serpentine and melt. Serpentinisation is only allowed to occur when the entire crust has become brittle so that large amounts of water can reach the mantle through brittle faults. The increase in temperature due to the exothermic nature of serpentinisation and the decrease in the coefficient of friction where serpentinisation occurs is also taken into account. Melt production includes the effect of increased depletion in mantle temperatures. In a first approximation, melt is assumed to migrate instantaneously upwards and accumulate at crustal levels. We present tests with a range of extension velocities and asthenospheric temperatures. Preliminary model runs shows how the entire crust becomes brittle after it is has reached a thickness of less than 10 km. For slow rifting velocities (< 5 mm/yr), serpentinisation occurs prior to melting, whereas for faster rifting velocities the opposite is true. In all models, crustal separation and the exposure of mantle at the continent-ocean transition (COT) occurs after the entire crust has become brittle. The relative amount of serpentinite and melt in the COT depends on the rifting velocity, with slower velocities promoting the production of more serpentinite than melt. However, for a normal mantle temperature (1300 C), even for the slow extension rate of 5 mm/yr, 3-4 km of

  11. Cretaceous potassic intrusives with affinities to aillikites from Jharia area: Magmatic expression of metasomatically veined and thinned lithospheric mantle beneath Singhbhum Craton, Eastern India

    NASA Astrophysics Data System (ADS)

    Srivastava, Rajesh K.; Chalapathi Rao, N. V.; Sinha, Anup K.

    2009-11-01

    Cretaceous potassic dykes and sills at the Jharia area intrude the Permo-carboniferous coal-bearing Gondwana sediments of the Eastern Damodar Valley, Singhbhum craton. These intrusives are widely regarded as a part of the Mesozoic alkaline and Rajmahal flood basalt magmatism in the Eastern Indian shield. Jharia intrusives display a wide petrographic diversity; olivine, phlogopite and carbonate are the predominant phases whereas apatite and rutile constitute important accessories. Impoverishment in sodium, silica and alumina and enrichment in potassium, titanium and phosphorous are the hallmark of these rocks and in this aspect they are strikingly similar to the rift-related aillikites (ultramafic lamprophyres) of Aillik Bay, Labrador. Crustal contamination of the Jharia magmas is minimal and the incompatible trace element ratios demonstrate (i) their generation by greater degrees of partial melting of a sub-continental lithospheric mantle (SCLM) source similar to that of the kimberlites of Dharwar craton, southern India, and (ii) retention of long-term memories of ancient (Archaean) subduction experienced by their source regions. We infer that a metasomatically veined and thinned lithosphere located at the margin of the Singhbhum craton and the inheritance of an ancient (Archaean) subducted component has played a significant role in deciding the diverging petrological and geochemical characters displayed by the Jharia potassic intrusives: those of kimberlites (orangeites) and lamproites (cratonic signature) and those of aillikites (rift-related signature). A substantial melt component of Jharia potassic intrusives was derived from the SCLM and the melt contribution of the Kerguelen plume is inferred to be minimal.

  12. A Model of Continental Growth and Mantle Degassing Comparing Biotic and Abiotic Worlds

    NASA Astrophysics Data System (ADS)

    Höning, D.; Hansen-Goos, H.; Spohn, T.

    2012-12-01

    While examples for interaction of the biosphere with the atmosphere can be easily cited (e.g., production and consumption of O2), interaction between the biosphere and the solid planet and its interior is much less established. It has been argued (e.g., Rosing et al. 2006; Sleep et al, 2012) that the formation of continents could be a consequence of bioactivity harvesting solar energy through photosynthesis to help build the continents and that the mantle should carry a chemical biosignature. We present an interaction model that includes mantle convection, mantle water vapor degassing at mid-oceanic ridges and regassing through subduction zones, continental crust formation and erosion and water storage and transport in a porous oceanic crust that includes hydrous mineral phases. The mantle viscosity in this model depends on the water concentration in the mantle. We use boundary layer theory of mantle convection to parameterize the mantle convection flow rate and assume that the plate speed equals the mantle flow rate. The biosphere enters the calculation through the assumption that the continental erosion rate is enhanced by a factor of several through bioactivity and through an assumed reduction of the kinetic barrier to diagenetic and metamorphic reactions (e.g., Kim et al. 2004) in the sedimentary basins in subduction zones that would lead to increased water storage capacities. We further include a stochastic model of continent-to-continent interactions that limits the effective total length of subduction zones. We use present day parameters of the Earth and explore a phase plane spanned by the percentage of surface coverage of the Earth by continents and the total water content of the mantle. We vary the ratio of the erosion rate in a postulated abiotic Earth to the present Earth, as well as the activation barrier to diagenetic and metamorphic reactions that affect the water storage capacity of the subducting crust. We find stable and unstable fixed points in

  13. Fault-controlled hydration of the upper mantle during continental rifting

    NASA Astrophysics Data System (ADS)

    Bayrakci, G.; Minshull, T. A.; Sawyer, D. S.; Reston, T. J.; Klaeschen, D.; Papenberg, C.; Ranero, C.; Bull, J. M.; Davy, R. G.; Shillington, D. J.; Perez-Gussinye, M.; Morgan, J. K.

    2016-05-01

    Water and carbon are transferred from the ocean to the mantle in a process that alters mantle peridotite to create serpentinite and supports diverse ecosystems. Serpentinized mantle rocks are found beneath the sea floor at slow- to ultraslow-spreading mid-ocean ridges and are thought to be present at about half the world’s rifted margins. Serpentinite is also inferred to exist in the downgoing plate at subduction zones, where it may trigger arc magmatism or hydrate the deep Earth. Water is thought to reach the mantle via active faults. Here we show that serpentinization at the rifted continental margin offshore from western Spain was probably initiated when the whole crust cooled to become brittle and deformation was focused along large normal faults. We use seismic tomography to image the three-dimensional distribution of serpentinization in the mantle and find that the local volume of serpentinite beneath thinned, brittle crust is related to the amount of displacement along each fault. This implies that sea water reaches the mantle only when the faults are active. We estimate the fluid flux along the faults and find it is comparable to that inferred for mid-ocean ridge hydrothermal systems. We conclude that brittle processes in the crust may ultimately control the global flux of sea water into the Earth.

  14. Helium Isotopic Compositions of Antarctic High-Mg Rocks Related to the Karoo Continental Flood Basalts: Evidence for a Depleted Upper Mantle Source?

    NASA Astrophysics Data System (ADS)

    Heinonen, J. S.; Kurz, M. D.

    2014-12-01

    The isotopic composition of helium is often considered to be one of the key elements in resolving deep mantle plume vs. upper mantle origin of hotspot-related volcanic rocks. High 3He/4He values, greater than 10 times atmospheric (Ra), are generally thought to indicate plume-related sources in the lower mantle. The use of helium isotopes in continental flood basalt (CFB) provinces has been limited by the lack of fresh rock material, poor exposures, time-integrated ingrowth of radiogenic 4He, and strong lithospheric overprinting. Vestfjella mountain range at western Dronning Maud Land, Antarctica, is comprised of lava flows and intrusive rocks that belong to the Jurassic (~180 Ma) Karoo continental flood basalt province, the bulk of which is exposed in southern Africa. The Karoo CFBs and related rocks show strong lithospheric influence in their geochemistry in general, but some high-Mg dikes from Vestfjella show geochemical evidence of derivation from sublithospheric sources. In an attempt to determine the first estimate for the helium isotopic composition of the Karoo mantle sources, we performed He isotopic measurements on six primitive Vestfjella dike samples collected from variably exposed nunataks. Helium was extracted by in-vacuo stepwise crushing and melting of separated and carefully hand-picked olivine phenocrysts (Ø = 0.25-1 mm; ~10 000 grains in total; abraded and unabraded fractions). The results of coupled crushing and melting measurements show evidence of both cosmogenic and radiogenic helium contributions within the olivines (i.e. by having high He contents and anomalously low or high 3He/4He released by melting), which complicates interpretation of the data. As a best estimate for the mantle isotopic composition, we use the sample with the highest amount of He released (> 50%) during the first crushing step of an abraded coarse fraction, which gave 3He/4He of 7.03 ± 0.23 (2σ) Ra. This value is indistinguishable from those measured from Southwest

  15. A melt-focusing zone in the lithospheric mantle preserved in the Santa Elena Ophiolite, Costa Rica

    NASA Astrophysics Data System (ADS)

    Madrigal, Pilar; Gazel, Esteban; Denyer, Percy; Smith, Ian; Jicha, Brian; Flores, Kennet E.; Coleman, Drew; Snow, Jonathan

    2015-08-01

    The Santa Elena Ophiolite in Costa Rica is composed of a well-preserved fragment of the lithospheric mantle that formed along a paleo-spreading center. Within its exposed architecture, this ophiolite records a deep section of the melt transport system of a slow/ultra-slow spreading environment, featuring a well-developed melt-focusing system of coalescent diabase dikes that intrude the peridotite in a sub-vertical and sub-parallel arrangement. Here we present an integrated analysis of new structural data, 40Ar/39Ar geochronology, major and trace element geochemistry and radiogenic isotope data from the diabase dikes in order to elucidate the tectonic setting of the Santa Elena Ophiolite. The dikes are basaltic and tholeiitic in composition. Petrological models of fractional crystallization suggest deep pressures of crystallization of > 0.4 GPa for most of the samples, which is in good agreement with similar calculations from slow/ultra-slow spreading ridges and require a relatively hydrated (~ 0.5 wt.% H2O) MORB-like source composition. The diabase dikes share geochemical and isotope signatures with both slow/ultra-slow spreading ridges and back-arc basins and indicate mixing of a DMM source and an enriched mantle end-member like EMII. The 40Ar/39Ar geochronology yielded an age of ~ 131 Ma for a previous pegmatitic gabbroic magmatic event that intruded the peridotite when it was hot and plastic and an age of ~ 121 Ma for the diabase intrusions, constraining the cooling from near asthenospheric conditions to lithospheric mantle conditions to ~ 10 Ma. Our findings suggest a complex interplay between oceanic basin and back-arc extension environments during the Santa Elena Ophiolite formation. We propose an alternative hypothesis for the origin of Santa Elena as an obducted fragment of an oceanic core complex (OCC).

  16. Recycling of oceanic crust from a stagnant slab in the mantle transition zone: Evidence from Cenozoic continental basalts in Zhejiang Province, SE China

    NASA Astrophysics Data System (ADS)

    Li, Yan-Qing; Ma, Chang-Qian; Robinson, Paul T.; Zhou, Qin; Liu, Ming-Liang

    2015-08-01

    Cenozoic continental basalts from Zhejiang Province, southeast China are tholeiitic to weakly alkalic in composition, with moderate MgO contents (6-11 wt.%) and an average Mg# of 62. They display typical OIB-like trace element features, including enrichment in most incompatible elements, both LILE and LREE, and negative K, Pb, Zr, Hf anomalies. In particular, they are characterized by high Fe/Mn (73 ± 5), La/Yb (19 ± 6) and Nb/Ta (18.8 ± 0.4) ratios, which can be attributed to the presence of residual clinopyroxene, garnet and rutile in the mantle source. Based on these minerals, the following hybrid source rocks are hypothesized: garnet pyroxenite/eclogite and peridotite. Clinopyroxene-liquid thermobarometry indicates clinopyroxene crystallization temperatures of > 1257 °C. This is higher than the assumed temperature at the base of the sub-continental lithospheric mantle (SCLM) (~ 1220 °C) beneath Zhejiang, thus the magmas were presumably derived from the asthenosphere. Some typical geochemical features such as negative K, Pb anomalies, positive Ba, Sr, Nb, Ta anomalies and the extremely high Os isotopic signatures, suggest participation of EM-like mantle sources, indicative of ancient subducted oceanic crust. (87Sr/86Sr)i (0.7037-0.7046) and 143Nd/144Nd (0.512832-0.512990) isotope ratios point to the presence of mixed components in the source region, i.e., DMM, EM1 and EM2. Recent seismic tomographic images of the mantle beneath Zhejiang suggest the presence of a subducted slab of oceanic lithosphere in the transition zone. Based on the combined geophysical and geochemical evidence, we propose that the major source of the Zhejiang basaltic magmas was the ancient subducted oceanic slab in the transition zone with an EM-like signature. The other magma sources include depleted asthenospheric peridotite possessing a DMM-like signature. The dynamics of this upwelling hybrid magma was apparently related to westward subduction of the Pacific plate underneath the

  17. Stable isotopic (O, H) evidence for hydration of the central Colorado Plateau lithospheric mantle by slab-derived fluids

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    The Colorado Plateau is a tectonically stable, relatively undeformed Proterozoic lithospheric province in the North America Cordillera. Although the stability of the Colorado Plateau suggests that it is rheologically strong, evidence from xenoliths show that the lithospheric mantle is extensively hydrated (e.g., presence of hydrous minerals, 'high' water contents in nominally anhydrous minerals), and therefore weakened. In addition, LREE enrichments in clinopyroxene (cpx) imply that the lithospheric mantle has been metasomatized ([1],[2]). Here we analyze mineral separates from spinel and garnet peridotite xenoliths from the Navajo Volcanic Field (NVF), located in the center of the Plateau, for their oxygen and hydrogen isotope compositions. These compositions are compared to those of xenoliths at the margins of the Plateau: spinel peridotites from the Grand Canyon Volcanic Field (GCVF) in the west and Zuni-Bandera Volcanic Field (ZBVF) in the east. NVF xenoliths are significantly more hydrous than the xenoliths on the margins of the Colorado Plateau based on modal abundances of hydrous minerals and structural water in olivine (e.g. [3]). All hydrous phases have high δD values (antigorite = -71 to -46‰ (n = 6 xenoliths); chlorite = -49 to -31‰ (n=3); amphibole = -47‰ (n=1)) compared to normal mantle (~-80‰), suggesting the addition of a fluid that is enriched in D compared to typical mantle. δ18O values for the same hydrous minerals range from 6.0 to 6.6‰ (n=6). δ18O values of olivine from NVF spinel peridotites have a narrow range, 5.0 to 5.4‰ (n = 4), near mantle olivine values (~5.2‰). Olivines from spinel peridotites from the GCVF and ZBVF also have mantle-like δ18O values (5.1 to 5.2‰ (n=3) and 5.1 to 5.4‰ (n=7), respectively). However, olivines and orthopyroxenes (opx) from NVF garnet peridotites have a slightly larger range and some record 18O enrichment (olivine = 5.1 to 5.6‰ (n = 3); opx = 5.9‰ (n=1)). The high δ18O values of

  18. 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.

  19. 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

    Our recent studies (Puziewicz et al. 2015, IJES 104:1913-1924, and references therein) show that the subcontinental lithospheric mantle (SCLM) beneath Lower Silesia (SW Poland) and neighbouring part of Upper Lusatia (SE Germany) is dominated by harzburgites. Part of them contain small amounts of clinopyroxene which, despite its primary textural appearance, is a late addition to the protoliths which are residues after extensive (up to 30 %) partial melting. This clinopyroxene was added to the harzburgites in Cenozoic times by alkaline basaltic melts migrating upwards from their asthenospheric sources during rifting in the Variscan foreland of the Alpine-Carpathian chain. The pre-rifting history of the SCLM beneath the region is thus recorded in the olivine and orthopyroxene. The forsterite content in olivine divides the Lower Silesian harzburgites into two groups: A (olivine Fo 90.5 - 92.0), and B (olivine Fo 84.0 - 90.0; for data see Puziewicz et al. 2015, op. cit.). The Al content in orthopyroxene is low and similar in both A and part of B harzburgites, called B1 in the following. The orthopyroxene occurring in the B1 harzburgites contains typically 0.05 - 0.10 atoms of Al per formula unit (corresponding to 0.5 - 2.5 wt. % Al2O3), although slightly lower (down to 0.02 a pfu) and slightly higher (up to 0.13 a pfu) Al contents occur in subordinate number of samples. The Al content in the B1 orthopyroxene is not correlated with forsterite content in coexisting olivine. The B2 harzburgites occur only in one site (Księginki). They contain orthopyroxene which Al content exhibits negative correlation with forsterite content in coexisting olivine. The most Al -rich orthopyroxene (0.24 atoms of Al pfu, corresponding to ca. 5.7 wt % Al2O3) coexists with olivine Fo 86.5 in Księginki. The low contents of Al in orthopyroxene is specific for the Lower Silesian/Upper Lusatian domain of European lithospheric mantle. The Al-poor mantle domain below Lower Silesia and upper

  20. A detailed view of the crust and lithospheric mantle beneath eastern Australia from transportable seismic array tomography

    NASA Astrophysics Data System (ADS)

    Rawlinson, Nicholas; Pilia, Simone

    2014-05-01

    The WOMBAT transportable seismic array project has been ongoing in eastern Australia since 1998, when a 40 station temporary array of recorders was first installed in western Victoria. To date, 16 consecutive array deployments have taken place with a cumulative total of over 700 stations installed in an area spanning Tasmania, New South Wales, southern Queensland and much of South Australia. Station separation varies between 15 km in Tasmania and 50 km on the mainland, with the majority of stations 3-component 1 Hz instruments, although a number of broadband instruments are interspersed. Although best suited to P-wave tomography, the recorded seismic wavefield has also proven to be useful for ambient noise tomography and crustal receiver functions, thus allowing detailed information on both the crust and lithospheric mantle structure to be retrieved. In order to apply teleseismic tomography using a transportable array of instruments, a robust background model is required which contains the long wavelength features suppressed by the use of relative arrival time residual datasets which are array specific. Here, we use the recently released AuSREM mantle model which is based on regional surface and body wave datasets. Crustal and Moho structure, which is poorly resolved by teleseismic data, is also included (from the AuSREM crustal model) as prior information to minimise smearing of crustal information into the mantle. The final model exhibits a variety of well resolved features, including a low velocity zone associated with Quaternary intraplate volcanism; a pronounced velocity gradient transition zone between the Precambrian shield region of Australia in the west and the Palaeozoic orogens in the east; and the presence of a high velocity salient which extends almost to the east coast in northern New South Wales, which is interpreted to be Precambrian lithosphere. The ambient noise tomography results, which are now continuous between Tasmania and mainland Australia

  1. Interaction of extended mantle plume head with ancient lithosphere: evidence from deep-seated xenoliths in basalts and lamprophyre diatremes in Western Syria

    NASA Astrophysics Data System (ADS)

    Sharkov, Evgenii

    2016-04-01

    . Formation of clinopyroxene-hornblende rocks (analogs of the "black series" of mantle xenoliths in basalt) occurred at close P-T parameters: 12.6 kbar, 1100°C. Judging from the absence of deformations in the rocks, their parental melts were intruded into the stabilized lower crust. Hence, it follows that the ancient continental lower crust existed there in the mid-Cretaceous, but in the Late Cenozoic it was replaced by the spreading mantle plume head. In other words, the deep structure of the region was reconstructed radically in the Late Cenozoic, and only the uppermost horizon of the ancient lithosphere (sialic crust) was not changed. According to the geological and petrological data, the heads of mantle plumes reached the base of the upper sialic crust, and the level of the lower crust of the continents (30-40 km) is optimal for abundant adiabatic melting of the mantle plume head. If this level was not reached, melting was limited, and an excess of volatile components appeared, which resulted in the formation of lamprophyric and even kimberlitic diatremes. The work was supported by grant RFBR # 14-05-00468 and Project of ONZ RAS # 8.

  2. 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.

  3. Spatial distributions of mineral compositions in the southernmost part of Salahi mantle section, the Oman ophiolite: modification from oceanic lithosphere to subarc mantle

    NASA Astrophysics Data System (ADS)

    Fujii, S.; Takazawa, E.

    2013-12-01

    The northern Oman ophiolite is a former oceanic lithosphere that had been modified by arc-type magmatism during subduction initiation. To investigate how oceanic lithospheric mantle was transformed to subarc mantle we studied the southernmost part of Salahi mantle section in the Oman ophiolite. Our results indicate that mantle peridotites were variably modified from mid-ocean ridge-like signature to subarc-like signature as a result of fluid infiltration from the base of the ophiolite that caused flux melting of residual peridotite and formation of boninitic magma together with highly refractory peridotites. Harzburgites in the southern most Salahi mantle section contain spinels with Cr# (=Cr/[Cr+Al] atomic ratio) in a relatively narrow range (0.46-0.67) while dunites in the same area contain spinels with Cr# in a wider range of 0.43-0.80. Moreover, dunites with relatively high Cr# spinel (greater than 0.7) frequently occur in the eastern part of the study area where a structurally lower level of the mantle section is exposed (high Cr# domain). On the other hand, the dunites with relatively low Cr# spinel (0.47-0.57) occur in the central and basal parts. The southernmost part of Salahi block has foliation plane nearly horizontal. Because of gently wavy structure the central part of the study area exposes slightly higher stratigraphic level in the mantle section relative to the eastern part and to the basal part. Clinopyroxene (cpx) in harzburgites from the high Cr# domain and low Cr# domain show highly LREE-depleted chondrite-normalized REE pattern with [Yb]CH=2-3 and [Ce]CH =0.01-0.02. On the other hand, REE patterns for dunite cpxs in the low spinel Cr# domain are similar to those of harzburgites in the same outcrop. However, cpxs in the dunite from the high spinel Cr# domain are enriched in LREE relative to those of harzburgites in the same outcrop. This suggests a possibility that dunites were reacted with LREE-enriched fluid infiltrated from the base of the

  4. Proterozoic mantle lithosphere beneath the Tariat Depression and Dariganga Plateau, Mongolia: in situ Re-Os evidence

    NASA Astrophysics Data System (ADS)

    Wang, K.; O'Reilly, S.; Griffin, W.; Pearson, N.; Zhang, M.

    2007-12-01

    The Os isotope compositions of sulfides in mantle peridotites from the Tariat Depression (TD) in central Mongolia and the Dariganga Plateau (DP) in southeast Mongolia reveal the presence of Proterozoic lithospheric mantle beneath these regions. The least-disturbed sulfides, with 187Re/188Os<0.07, yield TMA model ages of 1.5±0.3, 1.1±0.1, 0.7±0.1 and 0.48±0.08 Ga (2σ) for TD and 2.0±0.2 and 1.4±0.4 Ga (2σ) for DP. Beside these low Re/Os sulfides, TRD model ages of other sulfides without later introduction/loss of Os, can still provide minimum estimates for the age of lithospheric mantle and record later metasomatic events. Ten TD Sulfides have TRD model ages ranging from 2.6 to 0.5 Ga, with peaks around 1.6 and 0.8-0.6 Ga; two DP sulfides have TRD model ages of 1.8 and 1.2 Ga. These old depletion/melting events are consistent with those from Nd model ages of peridotites from the same region (~2 Ga for TD [1]; >1 Ga [2] and 1.6 Ga [3] for DP). In the Tariat region, these mantle events are consistent with those known in the overlying crust as recorded by Mesoproterozoic Nd model ages (TDM = 1.4-1.1 Ga) of Paleozoic and Mesozoic granitoids from the Khangai microcontinental block, where the Tariat Depression is located, and similar Nd model ages of 1.5-1.0 Ga for crustal granulite xenoliths from the Shavaryn-Tsaram volcano in the Tariat Depression [4]. Younger sulfide Os ages (1.1, 0.7 and ~0.5 Ga) may date mantle thermal events that also affected the overlying crust, marking the beginning of the Central Asia Orogeny in Neoproterozoic time. Although the South Mongolia domain, where the Dariganga Plateau is located, is believed to consist of late Paleozoic accretionary complexes and arc terrains [5], the presence of Precambrian zircon xenocrysts in magmatic rocks and ancient detrital zircons in arc-derived sediments [6], and Proterozoic Nd model ages of basement rocks in the Xilinhot region (B. Chen, pers. comm.) suggest substantial reworking of old crust. The

  5. The lithosphere

    SciTech Connect

    Not Available

    1983-01-01

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

  6. An ancient depleted mantle sample from a 42-Ma dike in Montana: Constraints on persistence of the lithosphere during Eocene Magmatism

    USGS Publications Warehouse

    Dudas, F.O.; Harlan, S.S.

    1999-01-01

    Recent models for the Cenozoic tectonic evolution of the western margin of North America propose that delamination of ancient lithosphere accompanied asthenospheric upwelling, magmatism, and uplift subsequent to Laramide deformation. On the basis of the age of an alkaline dike in south-central Montana, thermometry of mantle xenoliths from the dike, and Sr, Nd, and Pb isotopic compositions of the dike and a xenocryst, we show that refractory lithosphere, derived from ancient, depleted mantle, remained in place under the Wyoming Craton as late as 42 Ma. The Haymond School Dike, a camptonite, yields a 40Ar/39Ar plateau date of 41.97 ?? 0.19 Ma (2??). Paleomagnetic data are consistent with this date and indicate intrusion during chron C19r. The dike has Sr, Nd, and Pb isotopic compositions similar to those of other Eocene alkaline rocks from central Montana. A clinopyroxene megacryst from the dike has ??42 = 17, and 87Sr/86Sr = 0.70288, indicating that it derives from ancient, depleted mantle isotopically distinct from the source of the host camptonite. Thermometry of xenoliths from the dike shows pyroxene populations that formed at 880?? and 1200??C. Combining thermometry with previous estimates of the regional Eocene geotherm inferred from xenoliths in kimberlites, and with the Al-in-orthopyroxene barometer, we infer that lithospheric mantle remained intact to depths of 110-150 km as late as 42 Ma. Eocene magmatism was not accompanied by complete removal of ancient lithosphere.

  7. Extension velocity partitioning, rheological crust-mantle and intra-crustal decoupling and tectonically inherited structures: consequences for continental rifting dynamics.

    NASA Astrophysics Data System (ADS)

    Wang, Kun; Mezri, Leila; Burov, Evgueni; Le Pourhiet, Laetitia

    2015-04-01

    We implemented series of systematic thermo-mechanical numerical models testing the importance of the rheological structure and extension rate partitioning for continental rift evolution. It is generally assumed that styles of continental rifting are mainly conditioned by the initial integrated strength of the lithosphere. For example, strong plates are expected to undergo extension in narrow rifting mode, while weak lithospheres would stretch in wide rifting mode. However, we show that this classification is largely insufficient because the notion of the integrated strength ignores the internal rheological structure of the lithosphere that may include several zones of crust-mantle or upper-crust-intermediate (etc) crust decoupling. As well, orogenic crusts characterizing most common sites of continental extension may exhibit inverted lithological sequences, with stronger and denser formerly lower crustal units on top of weaker and lighter upper crustal units. This all may result in the appearance of sharp rheological strength gradients and presence of decoupling zones, which may lead to substantially different evolution of the rift system. Indeed, strong jump-like contrasts in the mechanical properties result in mechanical instabilities while mechanical decoupling between the competent layers results in overall drop of the flexural strength of the system and may also lead to important horizontal flow of the ductile material. In particular, the commonly inferred concept of level of necking (that assumes the existence of a stationary horizontal stretching level during rifting) looses its sense if necking occurs at several distinct levels. In this case, due to different mechanical strength of the rheological layers, several necking levels develop and switch from one depth to another resulting in step-like variations of rifting style and accelerations/decelerations of subsidence during the active phase of rifting. During the post-rifting phase, initially decoupled

  8. Lake Bonneville - Constraints on lithospheric thickness and upper mantle viscosity from isostatic warping of Bonneville, Provo, and Gilbert stage shorelines

    NASA Technical Reports Server (NTRS)

    Bills, Bruce G.; May, Glenn M.

    1987-01-01

    Data collected from three deformed shorelines of Lake Bonneville (the Bonneville, Provo, and Gilbert shorelines) are used to constrain the effective elastic lithospheric thickness to 23 + or - 2 km, the mantle viscosity to (1.2 + or - 0.2) x 10 to the 20th Pa sec, and the depth to a significant viscosity increase to no less than 300 km. A modification of the earth model of Nakiboglu and Lambeck (1982, 1983) is used for the calculations, and the water load is computed at each time step from a digital terrain model and a specified lake elevation. Differences noted between the observed and computed shoreline elevations indicate a regional tilt down to the NE of about 6 x 10 to the -5th, which is suggested to be due to collapse of the peripheral bulge formed by the Laurentide ice sheet.

  9. Hafnium isotope evidence from Archean granitic rocks for deep-mantle origin of continental crust

    NASA Astrophysics Data System (ADS)

    Guitreau, Martin; Blichert-Toft, Janne; Martin, Hervé; Mojzsis, Stephen J.; Albarède, Francis

    2012-07-01

    Combined whole-rock and zircon MC-ICP-MS Lu-Hf isotope data are reported for a large collection of Archean granitoids belonging to typical tonalite-trondhjemite-granodiorite (TTG) suites. Our data demonstrate that the time-integrated Lu/Hf of the mantle source of TTGs has not significantly changed over the last 4 Gy. Continents therefore most likely grew from nearly primordial unfractionated material extracted from the deep mantle via rising plumes that left a depleted melt residue in the upper mantle. The deep mantle could retain its primitive relative element abundances over time because sinking plates are largely stripped barren of their oceanic and continental crust components at subduction zones; this process results in only small proportions (<15-25%) of present-day continental mass getting recycled to great depths. Zircon populations extracted from the analyzed TTGs have Hf isotopic compositions broadly consistent with those of their host whole-rocks, whereas the U-Pb system in the same grains is often disturbed, causing a discrepancy that creates spurious initial ɛHf values. This problem is endemic to the Archean detrital zircon record and consistent with experimental results bearing on the relative retentivity of Hf vs. U and Pb in zircon. We argue that this behavior biases the Archean zircon record toward negative ɛHf values, which are at odds with the present TTG data set. If Hadean Jack Hills zircons are considered in light of these results, the mantle source of continents has remained unchanged for the last 4.3 Gy.

  10. Postseismic response of the Kashmir region and the role of lateral mechanical heterogeneity in the continental lithosphere (Invited)

    NASA Astrophysics Data System (ADS)

    Bendick, R. O.; Khan, F.; Burgmann, R.; Banerjee, P.; Khan, M.; Bilham, R. G.

    2013-12-01

    Most plausible continental mechanical configurations respond to instantaneous perturbations to the state of stress (by earthquakes or surface loads) with transient changes to the surface velocity field. Both the magnitude and characteristic time constants of these velocity transients contain information about the material properties of the crustal architecture that is not accessible in the steady state velocity field. Therefore, the best test for the presence of lateral mechanical heterogeneity in the continental lithosphere is to use a large seismic perturbation as a starting point and explore lateral variations in the postseismic surface response. We use postseismic GPS observations surrounding the 2005 Kashmir, Pakistan earthquake rupture, which crossed the Hazara Syntaxis, a prominent crustal suture, to compare the response of dominantly sedimentary and dominantly crystalline crustal packages. We also use forward numerical simulations to estimate the effective viscosities for these different crustal terranes and therefore the magnitude of lateral heterogeneity in effective viscosity.

  11. Upper mantle structures beneath the Carpathian-Pannonian region: Implications for the geodynamics of continental collision

    NASA Astrophysics Data System (ADS)

    Ren, Y.; Stuart, G. W.; Houseman, G. A.; Dando, B.; Ionescu, C.; Hegedus, E.; Radovanovic, S.; South Carpathian Project Working Group

    2012-04-01

    The Carpathian-Pannonian system of Eastern and Central Europe represents a unique opportunity to study the interaction between surface tectonic processes involving convergence and extension and convective processes in the upper mantle. Here, we present high-resolution images of upper mantle structure beneath the region obtained using P- and S-wave finite-frequency teleseismic tomography to help constrain the geodynamical interpretation of the region. We have selected earthquakes with magnitude greater than 5.5 in the distance range 30°-95°, which occurred between 2006 and 2011. The data were recorded on 54 temporary stations deployed in the South Carpathian Project (2009-2011), 56 temporary stations deployed in the Carpathian Basins Project (2005-2007), and 131 permanent broadband stations of national networks. The relative arrival times are measured in high, intermediate and low frequency bands (0.5-2.0 Hz, 0.1-0.5 Hz and 0.03-0.1 Hz for both P-waves, 0.1-0.5 Hz, 0.05-0.1 Hz and 0.02-0.05 Hz for S-waves), and are inverted using a 3-D finite-frequency formulation to produce P- and S-wave velocity maps at depths between 75 and 600 km in the mantle. Our images show a sub-vertical slab of fast material beneath the eastern Alps which extends eastward across the Pannonian basin at depths below ~300 km. The fast material extends down into the mantle transition zone (MTZ), where it spreads out beneath the entire basin. Above ~300 km, the upper mantle below the Pannonian basin is dominated by relatively slow velocities, the most dominant of which extends down to nearly 200 km and underlies the >7km thick sediments of the Mako-Békés rift basins. We suggest that cold mantle lithospheric downwelling occurred below the Pannonian Basin before detaching in the mid-Miocene. In the Vrancea Zone, intermediate depth seismicity occurs at the NE end of a tabular upper mantle high velocity structure that trends SW along the southern edge of the South Carpathians and extends to

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

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

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

  13. Lithosphere and upper-mantle structure of the southern Baltic Sea estimated from modelling relative sea-level data with glacial isostatic adjustment

    NASA Astrophysics Data System (ADS)

    Steffen, H.; Kaufmann, G.; Lampe, R.

    2014-06-01

    During the last glacial maximum, a large ice sheet covered Scandinavia, which depressed the earth's surface by several 100 m. In northern central Europe, mass redistribution in the upper mantle led to the development of a peripheral bulge. It has been subsiding since the begin of deglaciation due to the viscoelastic behaviour of the mantle. We analyse relative sea-level (RSL) data of southern Sweden, Denmark, Germany, Poland and Lithuania to determine the lithospheric thickness and radial mantle viscosity structure for distinct regional RSL subsets. We load a 1-D Maxwell-viscoelastic earth model with a global ice-load history model of the last glaciation. We test two commonly used ice histories, RSES from the Australian National University and ICE-5G from the University of Toronto. Our results indicate that the lithospheric thickness varies, depending on the ice model used, between 60 and 160 km. The lowest values are found in the Oslo Graben area and the western German Baltic Sea coast. In between, thickness increases by at least 30 km tracing the Ringkøbing-Fyn High. In Poland and Lithuania, lithospheric thickness reaches up to 160 km. However, the latter values are not well constrained as the confidence regions are large. Upper-mantle viscosity is found to bracket [2-7] × 1020 Pa s when using ICE-5G. Employing RSES much higher values of 2 × 1021 Pa s are obtained for the southern Baltic Sea. Further investigations should evaluate whether this ice-model version and/or the RSL data need revision. We confirm that the lower-mantle viscosity in Fennoscandia can only be poorly resolved. The lithospheric structure inferred from RSES partly supports structural features of regional and global lithosphere models based on thermal or seismological data. While there is agreement in eastern Europe and southwest Sweden, the structure in an area from south of Norway to northern Germany shows large discrepancies for two of the tested lithosphere models. The lithospheric

  14. Role of mantle flow in Nubia-Somalia plate divergence

    NASA Astrophysics Data System (ADS)

    Stamps, D. S.; Iaffaldano, G.; Calais, E.

    2015-01-01

    Present-day continental extension along the East African Rift System (EARS) has often been attributed to diverging sublithospheric mantle flow associated with the African Superplume. This implies a degree of viscous coupling between mantle and lithosphere that remains poorly constrained. Recent advances in estimating present-day opening rates along the EARS from geodesy offer an opportunity to address this issue with geodynamic modeling of the mantle-lithosphere system. Here we use numerical models of the global mantle-plates coupled system to test the role of present-day mantle flow in Nubia-Somalia plate divergence across the EARS. The scenario yielding the best fit to geodetic observations is one where torques associated with gradients of gravitational potential energy stored in the African highlands are resisted by weak continental faults and mantle basal drag. These results suggest that shear tractions from diverging mantle flow play a minor role in present-day Nubia-Somalia divergence.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  16. 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

    Our recent studies (Puziewicz et al. 2015, IJES 104:1913-1924, and references therein) show that the subcontinental lithospheric mantle (SCLM) beneath Lower Silesia (SW Poland) and neighbouring part of Upper Lusatia (SE Germany) is dominated by harzburgites. Part of them contain small amounts of clinopyroxene which, despite its primary textural appearance, is a late addition to the protoliths which are residues after extensive (up to 30 %) partial melting. This clinopyroxene was added to the harzburgites in Cenozoic times by alkaline basaltic melts migrating upwards from their asthenospheric sources during rifting in the Variscan foreland of the Alpine-Carpathian chain. The pre-rifting history of the SCLM beneath the region is thus recorded in the olivine and orthopyroxene. The forsterite content in olivine divides the Lower Silesian harzburgites into two groups: A (olivine Fo 90.5 - 92.0), and B (olivine Fo 84.0 - 90.0; for data see Puziewicz et al. 2015, op. cit.). The Al content in orthopyroxene is low and similar in both A and part of B harzburgites, called B1 in the following. The orthopyroxene occurring in the B1 harzburgites contains typically 0.05 - 0.10 atoms of Al per formula unit (corresponding to 0.5 - 2.5 wt. % Al2O3), although slightly lower (down to 0.02 a pfu) and slightly higher (up to 0.13 a pfu) Al contents occur in subordinate number of samples. The Al content in the B1 orthopyroxene is not correlated with forsterite content in coexisting olivine. The B2 harzburgites occur only in one site (Księginki). They contain orthopyroxene which Al content exhibits negative correlation with forsterite content in coexisting olivine. The most Al -rich orthopyroxene (0.24 atoms of Al pfu, corresponding to ca. 5.7 wt % Al2O3) coexists with olivine Fo 86.5 in Księginki. The low contents of Al in orthopyroxene is specific for the Lower Silesian/Upper Lusatian domain of European lithospheric mantle. The Al-poor mantle domain below Lower Silesia and upper

  17. Characterization of hydration in the mantle lithosphere: Peridotite xenoliths from the Ontong Java Plateau as an example

    NASA Astrophysics Data System (ADS)

    Demouchy, Sylvie; Ishikawa, Akira; Tommasi, Andréa; Alard, Olivier; Keshav, Shantanu

    2015-01-01

    We report concentrations of hydrogen (H) in upper mantle minerals of peridotites (olivine and pyroxenes) transported by alnöitic lavas, which erupted on the southwestern border of the Ontong Java Plateau (Malaita, Solomon Islands, West Pacific). Unpolarized FTIR analyses show that olivine, orthopyroxene, and diopside contain 2-32 ppm, 162-362 ppm and 159-459 ppm wt H2O, respectively. In the studied lherzolites, garnets are anhydrous. The concentration of hydrogen within individual olivine and pyroxene grains is almost homogeneous, indicating no evidence of dehydration or hydration by ionic diffusion. In the lherzolite, the concentration of hydrogen in olivine tends to increase weakly with depth (based on geothermobarometry), consistent with the increase of water solubility with increasing water fugacity as a function of pressure, but concentrations remain well below water-saturation values determined experimentally. The highest concentration of H in olivine (32 ppm wt H2O) is, however, found in refractory spinel harzburgites, which equilibrated at depths of 85 km., while deeper specimens as the high-temperature spinel harzburgites, and some of the garnet lherzolites, contain less hydrogen in olivine. Olivines from pyroxene- or pargasite-rich peridotites have also lower hydrogen concentrations. We interpret the high hydrogen concentrations in olivine from the refractory spinel harzburgites as due to (1) simultaneous hydration and metasomatism of the lithospheric mantle by a water-rich silicate melt/fluid, during which hydrogen follows MREE and where spinel harzburgite have experienced 'stealth' metasomatism, and/or (2) to a late 'fleeting' hydrogen metasomatism, which would hydrate the rock after this first 'stealth' metasomatism event. In the second case, the composition of the 'fleeting' percolating fluid (small volume fraction of very evolved fluids, with high volatiles concentration and transient properties) is likely to be linked to the decrease of the plume

  18. Recrystallisation, phase mixing and strain localisation in peridotite during rapid extrusion of sub-arc mantle lithosphere

    NASA Astrophysics Data System (ADS)

    Czertowicz, T. A.; Toy, V. G.; Scott, J. M.

    2016-07-01

    The Anita Peridotite, in southwestern New Zealand, is a ∼1 × 20 km ultramafic massif that was rapidly extruded from beneath a Cretaceous arc within the 4 km wide mylonitic Anita Shear Zone. The peridotitic body contains a spectacular array of textures that preserve evidence for changing temperature, stress, and deformation mechanisms during the exhumation process. Olivine and orthopyroxene microstructures and lattice-preferred orientations (LPO) record a three-phase deformation history. Dislocation glide on the C- and E-type slip systems is recorded by coarse pre-mylonitised olivine grains, and occurred under hydrous conditions at T ∼650 °C, stress ∼200-700 MPa and strain rate ∼10-15 s-1, probably within hydrated sub-arc mantle lithosphere. Rare protomylonite pods record deformation by dislocation creep in porphyroclasts and dislocation-accommodated grain boundary sliding in the matrix on {0kl}[100] in olivine and (100)[001] in orthopyroxene, under conditions of T ∼730-770 °C, stress ∼52-700 MPa and strain rate ∼10-15 s-1. The massif, however, is dominated by mylonite and ultramylonite that wrap the protomylonite pods, comprising mostly fine-grained olivine neoblasts that lack internal distortions and have uniform LPOs. These textures indicate deformation occurred by grain-size sensitive (GSS) creep at T ∼650 °C, stress ∼69-137 MPa and strain rate ∼10-15 s-1, and thus during conditions of cooling and decreasing stress. GSS creep became more dominant with time, as the proportion of randomly-oriented neoblasts increased and formed interlinked networks that accommodated much of the strain. Grain boundary pinning allowed GSS creep to be maintained in polyphase regions, following mixing of olivine and orthopyroxene, which may have occurred by grain boundary transport in a fluid phase during a "creep cavitation" process. The results indicate that the Anita Peridotite recrystallised and underwent rheological weakening at a constant strain rate

  19. 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

    The study of Silicon (Si) isotopes in Ocean Island Basalts (OIB) has the potential to elucidate between possible heterogeneities in the mantle. Relatively large (~several per mil per atomic mass unit) Si isotope fractionation occurs in low-temperature environments during biochemical and geochemical precipitation of dissolved Si, where the precipitate is preferentially enriched in the lighter isotopes [1]. In contrast, only a limited range (~tenths of a per mil) of Si isotope fractionation has been observed in high-temperature igneous processes [2]. Therefore, Si isotopes may be useful as tracers for the presence of crustal material (derived from low-temperature surface processes) in OIB source regions in a manner similar to more conventional stable isotope systems, such as O. Here we present the first comprehensive set of high-precision Si isotope data obtained by MC-ICP-MS for a diverse suite of OIBs, including new data for the Canary Islands. Samples represent the Pacific, Atlantic, and Indian Ocean basins and include representative end-members for the EM-1, EM-2, and HIMU mantle components. Average δ30Si values for OIBs representing the EM-1 (-0.32 ± 0.06‰, 2 sd), EM-2 (-0.30 ± 0.01‰, 2 sd), and HIMU (-0.34 ± 0.09‰, 2 sd) mantle components are all in general agreement with previous estimates for the δ30Si value of Bulk Silicate Earth [3]. However, small systematic variations are present; HIMU (Mangaia, Cape Verde, La Palma) and Iceland OIBs are enriched in the lighter isotopes of Si (δ30Si values lower than MORB). Further, the difference in Si isotope composition between La Palma and El Heirro (Canary Islands) has previously been observed for O isotopes [4], suggesting a relationship between the Si and O isotope mantle systematics. The Si isotope variations among OIBs may be explained by the sampling of a primitive mantle reservoir enriched in the light isotopes of Si, as suggested by [5], but most likely reflects the incorporation of recycled

  20. Distribution and transport of hydrogen in the lithospheric mantle: A review

    NASA Astrophysics Data System (ADS)

    Demouchy, Sylvie; Bolfan-Casanova, Nathalie

    2016-01-01

    The minerals constituting the Earth's upper mantle are nominally anhydrous silicates (NAMs). However they do contain hydrogen as a trace element, decorating point defects in their crystalline structure. Experimental petrology and mineralogy have quantified the maximum concentration under several compositional and thermodynamic conditions, but systematic studies on the hydrogen concentration in minerals from mantle-derived rocks have only recently been carried out. Here, we have compiled the distribution of hydrogen in upper mantle peridotite xenoliths, from which several conclusions can be drawn. NAMs from peridotite xenoliths contain a few ppm wt. H2O in their structure. From the current database, the hydrogen concentrations in olivine regularly increase with increasing depth. The amount of hydrogen in NAMs from peridotite xenoliths from subduction contexts is not higher than in other geological context for similar temperature and pressure conditions. The highest hydrogen concentrations is found in peridotitic olivines from cratonic mantle, and are likely due to the depth of origin. The increasing hydrogen concentration in olivine with increasing depth is likely controlled by the increase of H partitioning into olivine at the expense of orthopyroxene as imposed by a decrease in Al content in opx with depth. However, the sparse data could also indicate that the bulk hydrogen concentration slightly increases with depth > 150 km. In this case, it would suggest, locally (Udachnaya for example), a possible increase in water fugacity due to fluid saturation. Even if the most abundant mineral in mantle rocks is olivine, the bulk hydrogen concentration in peridotites is controlled by the amount of hydrogen stored in pyroxenes. However, hydrogen concentration in olivine remains crucial for consequences on physical properties such as rheology and electrical conductivity. Kinetics of hydrogen transport is reviewed and hydrous melt/fluid percolation appears necessary to

  1. The geochemical and temporal evolution of the continental lithosphere and its relationship to continental-scale faulting: The Karakoram Fault, eastern Karakoram, NW Himalayas

    NASA Astrophysics Data System (ADS)

    Phillips, Richard J.; Searle, Michael P.; Parrish, Randall R.

    2013-03-01

    New laser ablation multicollector-inductively coupled plasma-mass spectrometry and isotope dilution-thermal ionization mass spectrometry U-Pb ages, coupled with Sm-Nd isotope and geochemical analysis, define the temporal and geochemical evolution of the continental lithosphere in the eastern Karakoram, India, NW Himalaya. Our analysis demonstrates that magmatism occurred between ~108 and 69 Ma and ~22 and 13 Ma. The new age data, coupled with geochemical examination of the granitoids, confirm a parallel evolution with the western Karakoram in Pakistan and supports a model of regional continental crustal thickening and related metamorphism. Middle to Late Cretaceous magmatism immediately adjacent to the Karakoram fault suggests that crustal melting and associated metamorphism are unrelated to shearing along the fault. Miocene leucogranite magmatism occurred almost exactly concomitant with the emplacement of the Baltoro batholith in Pakistan. These trans-Karakoram leucogranites also display similar geochemical evolution trends. Our new data clearly link the leucogranites along the fault to the regional Baltoro batholith and related metamorphic complexes to the west. This supports previous work suggesting that magmatism and metamorphism were not syn-kinematic with continental-scale faulting. The data demonstrate that the Karakoram fault could not have accommodated lateral offset in this region prior to ~16 Ma, limiting the long-term averaged slip rate to a maximum of ~10 mm/yr.

  2. The influence of mantle internal heating on lithospheric mobility: Implications for super-Earths

    NASA Astrophysics Data System (ADS)

    Stein, C.; Lowman, J. P.; Hansen, U.

    2013-01-01

    Super-Earths, a recently discovered class of exoplanets, have been inferred to be of a similar rock and metal composition to the Earth. As a result, the possibility that they are characterised by the presence of plate tectonics has been widely debated. However, as the super-Earths have higher masses than Earth, it is assumed that they will also have higher Rayleigh numbers and non-dimensional heating rates. Accordingly, we conduct a systematic 2D study to investigate the influence of these parameters on the surface behaviour of mantle convection. The main focus of our work considers the response of surface motion to the mantle's internal heating. However, we also include an analysis of other parameters scaling with planet mass, such as viscosity. In agreement with the findings of Valencia and O'Connell (2009) and van Heck and Tackley (2011) we find plate-like surface mobilisation for increased Rayleigh numbers. But increasing the internal heating leads to the formation of a strong stagnant-lid because the mantle heating effects thermally activated viscosity. Additionally, viscosity is affected by the increased pressures and temperatures of super-Earths. In total, our findings indicate that surface mobility will likely be reduced on super-sized Earths. Our numerical models show that the interior temperature of the convecting system is of vital importance. In planets with a hotter interior plate tectonics is less likely.

  3. The influence of mantle internal heating on lithospheric mobility: implications for super-Earths

    NASA Astrophysics Data System (ADS)

    Stein, C.; Lowman, J.; Hansen, U.

    2013-09-01

    Super-Earths, a recently discovered class of exoplanets, have been inferred to be of a similar rock and metal composition to the Earth. As a result, the possibility that they are characterised by the presence of plate tectonics has been widely debated. However, as the super-Earths have higher masses than Earth, it is assumed that they will also have higher Rayleigh numbers and non-dimensional heating rates. Accordingly, we conduct a systematic 2D study to investigate the influence of these parameters on the surface behaviour of mantle convection. The main focus of our work considers the response of surface motion to the mantle's internal heating. However, we also include an analysis of other parameters scaling with planet mass, such as viscosity. In agreement with the findings of Valencia & O'Connell (2009) and van Heck & Tackley (2011) we find plate-like surface mobilisation for increased Rayleigh numbers. But increasing the internal heating leads to the formation of a strong stagnant-lid because the mantle heating effects thermally activated viscosity. Additionally, viscosity is affected by the increased pressures and temperatures of super- Earths. In total, our findings indicate that surface mobility will likely be reduced on super-sized Earths.

  4. Interrelation between rifting, faulting, sedimentation, and mantle serpentinization during continental margin formation—including examples from the Norwegian Sea

    NASA Astrophysics Data System (ADS)

    Rüpke, Lars H.; Schmid, Daniel W.; Perez-Gussinye, Marta; Hartz, Ebbe

    2013-10-01

    The conditions permitting mantle serpentinization during continental rifting are explored within 2-D thermotectonostratigraphic basin models, which track the rheological evolution of the continental crust, account for sediment blanketing effects, and allow for kinetically controlled mantle serpentinization processes. The basic idea is that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur. The isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions. A systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs. Increased sedimentation rates shift this critical stretching factor to higher values as sediment blanketing effects result in higher crustal temperatures. Sediment supply has therefore, through the temperature-dependence of the viscous flow laws, strong control on crustal strength and mantle serpentinization reactions are only likely when sedimentation rates are low and stretching factors high. In a case study for the Norwegian margin, we test whether the inner lower crustal bodies (LCB) imaged beneath the Møre and Vøring margin could be serpentinized mantle. Multiple 2-D transects have been reconstructed through the 3-D data set by Scheck-Wenderoth and Maystrenko (2011). We find that serpentinization reactions are possible and likely during the Jurassic rift phase. Predicted thicknesses and locations of partially serpentinized mantle rocks fit to information on LCBs from seismic and gravity data. We conclude that some of the inner LCBs beneath the Norwegian margin may be partially serpentinized mantle.

  5. Conserving mass and energy in cooling models of oceanic lithosphere requires upper mantle origins for trends in subsidence and heat flux and indicates global power of 30 TW

    NASA Astrophysics Data System (ADS)

    Criss, R. E.; Hofmeister, A. M.; Hamza, V. N.

    2008-12-01

    One-dimensional conductive cooling models of ocean lithosphere fail to predict the lateral variation in oceanic heat flux and provide problematic calculations of subsidence, for reasons enumerated below. Our new model follows conservation laws and shows that bathymetric trends are tied to upper mantle temperature variations, given realistic values for thermal expansivity. Heat flux increases towards mid-ocean ridges due to (1) flux varying across upper mantle convection cells and (2) redistribution of mantle heat (Qmtl) by moving magma, and also by (3) hydrothermal circulation. Foremost, widespread, lateral, uptake of Qmtl as latent heat occurs during deep lithospheric melting but this energy is released near ridges through dike emplacement during seafloor spreading. Redistribution and energy conservation account for the local heat flux maximum near x=1200 km, heretofore unexplained. We show that the trend Qmtl(x) far from the ridge is consistent with behavior near the ridge and measured global power of <30 TW , which is compatible with quasi-steady-state conditions and an enstatite chondrite model for the Earth. Observables, such as the pattern of mid-ocean ridges on the globe, point to layered convection and lack of vigor, and gross characteristics of the Earth are supported by an enstatite chondrite model. Our analysis circumvents problems associated with 1-d conductive cooling models of the lithosphere: (1) Existing models replaced conservation of rock-mass with isostatic balance, which unwittingly created subsidence by converting lithosphere to ocean. (2) Half-space models incorrectly cancelled infinities. (3) Plate models omitted latent heat which is immense. (4) 1-d models only permit vertical contraction. These faulty constructs fitted seafloor depths through erroneous use of volumetric (αV=3αL) thermal expansivity coupled with great leeway in cross-multiplied parameters. The underlying premise that thermal aspects of lithosphere can be separately

  6. Interpretation of geoid anomalies in the contact zone between the East European Craton and the Palaeozoic Platform-II: Modelling of density in the lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Świeczak, M.; Kozlovskaya, E.; Majdański, M.; Grad, M.

    2009-05-01

    We present analysis of lateral variations of density in the upper mantle in the area of contact of the precambrian East European Craton (EEC) and the Palaeozoic Platform (PP) in Poland, obtained by analysis of the gravimetric geoid undulations. A precise 3-D density model of the crust in the study area down to a depth of 50 km, discussed in the first part of this paper (Majdański et al., in press) did not explain all features of the observed geoid. This suggests that these features can be due to density inhomogeneities in the upper mantle. To estimate them, we performed inversion of a residual between the observed geoid and undulations caused by the 3-D density distribution in the crust. Basing on the assumption of local isostatic compensation and Pratt-Hayford isostasy model, the density distribution in the upper mantle was parametrized as a 40-km-thick layer located above the assumed compensation depth of 140 km and subdivided into irregular blocks. The boundaries of the blocks were defined according to boundaries of major tectonic units in the study area and position and shape of the most pronounced anomalies in the residual geoid. A series of sensitivity tests calculated for such density heterogeneities in the upper mantle showed that they can produce geoid undulations of the order of several metres. The density values in each unit were taken as model parameters for the inversion procedure, and inverse problem was solved using global optimization with constraints. The density variations in the upper mantle in the final model correlate well with the surface heat flow. This suggests that these variations can be due to diversity in mantle temperature. The Trans-European Suture Zone (TESZ), which is a major suture separating the EEC from the PP, is not observed as a distinct unit in the mantle. Instead, our study suggests continuation of the lithosphere of the EEC beneath the PP and confirms subdivision of the TESZ into terranes with distinctly different evolution

  7. Crustal Anisotropy of the Archean Continental Lithosphere in the Minnesota River Valley Subprovince

    NASA Astrophysics Data System (ADS)

    Ferre, E. C.; Gebelin, A.; Teyssier, C.

    2005-12-01

    The Minnesota River Valley [MRV] Subprovince belongs to the Superior Province and consists of four Archean crustal blocks separated by E-W north-dipping shear zones. These blocks, with contrasted aeromagnetic signatures, are characterized by a gradational, layered crust-mantle transition at a depth of 45-51 km. The northermost block hosts tonalites, quartz diorites, and granodiorites. This block is separated from the southern blocks by a Penokean age [2.45-1.75 Ga] shear zone. The 3 southern blocks consist of amphibolite- to granulite-grade migmatites with tonalitic, granodioritic, dioritic, and pelitic types, interlayered and grading into each other. Our petrofabric and microstructural investigations focused on the Morton Block, one of the three southern blocks. The Morton migmatites exhibit compositional layering formed by alternating layers (cm to 10's of cm thick) of pervasive pink Kfs-rich granitic leucosome, gray Plag-rich tonalitic leucosome and dark mafic melanosome at a cm scale. The layering and the foliation are parallel in these migmatites and dip about 20° to the east. The layering of the migmatite originated from solid-state mineral segregation, under high grade conditions, and from melt segregation.Amphibolite layers, a few tens of centimeters in thickness, occur as elongate boudins sub-parallel to the migmatitic layering and are interpreted as boudinaged tholeiitic basalt sills. Mineral stretching lineations are shallow plunging to sub-horizontal and trend ~N075°. Lineations could be measured in the field at six outcrops only. The scarcity of macroscopically visible lineations in the field was one of the main motivations to undertake a magnetic fabric study on the Morton Block. Multidomain to pseudo-single domain magnetite grains dominate the magnetic susceptibility and the AMS. The AMS is formed mostly by the magnetostatic (shape) anisotropy of magnetite grains, acquired at high temperature (600-800°C) by dislocation creep. The high temperature

  8. 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 elas