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Sample records for enriched subcontinental lithosphere

  1. Southward Ejection of Subcontinental Lithosphere and large-scale Asthenospheric Enrichment beneath central Chile resulting from Flat Subduction

    NASA Astrophysics Data System (ADS)

    Jacques, G.; Hoernle, K.; Schaefer, B. F.; Hauff, F.; Gill, J.; Holm, P. M.; Bindeman, I. N.; Folguera, A.; Lara, L.; Ramos, V. A.

    2015-12-01

    Proterozoic subcontinental lithospheric mantle and 2) squeezed the eroded lithosphere radially away from the region of flat subduction. Therefore, flat subduction could be a major mechanism for transferring continental lithosphere into the upper mantle asthenosphere, and for distributing this enriched material laterally over 1000s of kilometers.

  2. On the recent enrichment of subcontinental lithosphere: A detailed UPb study of spinel lherzolite xenoliths, Yukon, Canada

    NASA Astrophysics Data System (ADS)

    Carignan, Jean; Ludden, John; Francis, Don

    1996-11-01

    Lead strontium, and osmium isotopic data have been obtained for whole rocks and mineral separates (olivine, orthopyroxene, clinopyroxene, and spinel) for spinel lherzolite xenoliths hosted by lavas of the Quaternary Alligator Lake volcanic centre, southern Yukon. Whole-rock xenolith samples display a large variation of lead concentrations, from 16 ppb for a harzburgite to up to 400 ppb for a lherzolite. However, their lead isotope ratios are relatively homogeneous with 206Pb /204Pb of 19.07 ± 0.08, 207Pb /204Pb of 15.65 ± 0.07, and 208Pb /204Pb of 38.67 ± 0.17 ( n = 7). However, the 238U /204Pb ratios display a large variation, from 12.2 to 46.5, and do not correlate with indices of fertility such as calcium or aluminum content. Mineral separates yield even larger variations in lead isotopic composition and lead and uranium concentrations. Some olivine fractions have both the lowest radiogenic compositions ( 206Pb /204Pb = 18.75 ) and the lowest 238U /204Pb ratios (˜3.1). Clinopyroxenes (cpx) display the highest lead and uranium concentrations (up to 1277 ppb and 195 ppb, respectivelly) and generally similar or more radiogenic lead isotopic composition and higher 238U /204Pb ratios than their whole-rock compositions. Orthopyroxene and spinel fractions yield intermediate compositions between olivine and cpx. Although whole rocks and cpx for individual samples yield almost identical 87Sr /86Sr , the xenoliths ( n = 5) display a large variation of strontium isotopic compositions ( 87Sr /86Sr from 0.07033 to 0.7050), lead and strontium isotope ratios of cpx and the distribution of the data in a UPb isochron diagram suggest that the subcontinental lithosphere under the Yukon was affected by a recent (< ˜30 Ma) enrichment in uranium, lead, and strontium. The metasomatic fluid/magma might have had an isotopic composition close to that of some sediments in the northern Pacific Ocean. When compared to K d values reported in the literature, olivine is enriched in

  3. Sr, Nd, and Pb isotopes of ultramafic xenoliths in volcanic rocks of Eastern China: enriched components EMI and EMII in subcontinental lithosphere

    USGS Publications Warehouse

    Tatsumoto, M.; Basu, A.R.; Wankang, H.; Junwen, W.; Guanghong, X.

    1992-01-01

    The UThPb, SmNd, and RbSr isotopic systematics of mafic and ultramafic xenolithic rocks and associated megacrystic inclusions of aluminous augite and garnet, that occur in three alkalic volcanic suites: Kuandian in eastern Liaoning Province, Hanluoba in Hebei Province, and Minxi in western Fujian Province, China are described. In various isotopic data plots, the inclusion data invariably fall outside the isotopic ranges displayed by the host volcanic rocks, testifying to the true xenolithic nature of the inclusions. The major element partitioning data on Ca, Mg, Fe, and Al among the coexisting silicate minerals of the xenoliths establish their growth at ambient mantle temperatures of 1000-1100??C and possible depths of 70-80 km in the subcontinental lithosphere. Although the partitioning of these elements reflects equilibrium between coexisting minerals, equilibria of the Pb, Nd, and Sr isotopic systems among the minerals were not preserved. The disequilibria are most notable with respect to the 206Pb 204Pb ratios of the minerals. On a NdSr isotopic diagram, the inclusion data plot in a wider area than that for oceanic basalts from a distinctly more depleted component than MORB with higher 143Nd 144Nd and a much broader range of 87Sr 86Sr values, paralleling the theoretical trajectory of a sea-water altered lithosphere in NdSr space. The garnets consistently show lower ?? and ?? values than the pyroxenes and pyroxenites, whereas a phlogopite shows the highest ?? and ?? values among all the minerals and rocks studied. In a plot of ??207 and ??208, the host basalts for all three areas show lower ??207 and higher ??208 values than do the xenoliths, indicating derivation of basalts from Th-rich (relative to U) sources and xenoliths from U-rich sources. The xenolith data trends toward the enriched mantle components, EMI and EMII-like, characterized by high 87Sr 86Sr and ??207 values but with slightly higher 143Nd 144Nd. The EMI trend is shown more distinctly by the host

  4. 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

  5. Extreme heterogeneity in North Lanzo peridotite: insights on mantle processes in the sub-continental lithosphere

    NASA Astrophysics Data System (ADS)

    Guarnieri, L.; Piccardo, G. B.; Nakamura, E.; Shimizu, N.; Vannucci, R.; Zanetti, A.

    2009-04-01

    The North Lanzo peridotite body (Western Alps, NW Italy) represents a sector of sub-continental lithospheric mantle that was exhumed and exposed at the sea-floor of the Jurassic Ligurian Tethys. Structural and compositional features indicate the extreme heterogeneity of these mantle rocks. The field mutual relationships between the different rock types indicate that the oldest mantle protoliths mostly consist of Sp harzburgites preserving structural relics (Opx+Sp clusters) of pristine mantle garnet(Gnt). They are diffusely veined by Sp pyroxenite bands and pods which show widespread structural relics of pre-existing Gnt. Peridotites and pyroxenites were equilibrated at Sp-peridotite facies conditions. Widespread subsolidus structures (i.e. Plg+Opx exsolutions in Cpx, Ol+Plg reaction rims between Px's and Sp) indicate that pristine Sp peridotites were exhumed to Plg-facies conditions. Peridotites and pyroxenites locally underwent significant structural-compositional modifications suggesting reactive melt-rock interaction (pyroxene dissolution and olivine precipitation) by silica undersaturated melts. On a decametric-hectometric scale, they were strongly enriched of magmatic Plg and mm-size gabbroic pods, indicating melt impregnation. All these rocks types are locally replaced by channels and pods of Plg-free Sp peridotites in places enriched of interstitial magmatic Cpx, suggesting reactive depletion/enrichment by melt-rock interaction. The strong compositional heterogeneity of the Lanzo mantle is well documented by the Cpx trace element composition: (1) Cpx of the mantle protoliths show LREE and HREE fractionated patterns documenting variable Gnt- and Sp-facies melting processes; (2) Cpx of Sp pyroxenites show negatively fractionated LREE patterns, suggesting equilibration with MORB melts, and very high HREE (and Sc) contents that are reminiscent of a precursor Gnt-bearing assemblage; (3) Cpx of Plg-enriched peridotites and pyroxenites show relatively low (La

  6. 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

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  8. 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

  9. 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

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

    NASA Astrophysics Data System (ADS)

    Wittig, Nadine

    2014-05-01

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

  11. 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

  12. Growth of subcontinental lithosphere: evidence from repeated dike injections in the Balmuccia lherzolite massif, Italian Alps

    NASA Astrophysics Data System (ADS)

    Mukasa, Samuel B.; Shervais, John W.

    1999-09-01

    The Balmuccia alpine lherzolite massif is a fragment of subcontinental lithospheric mantle emplaced into the lower crust 251 Ma ago during the final, extensional phase of the Hercynian orogeny. The Balmuccia massif consists largely of lherzolite, with subordinate harzburgite and dunite, and an array of dike rocks formed in the mantle before crustal emplacement. Dike rocks include websterite and bronzitite of the Cr-diopside suite, spinel clinopyroxenite and spinel-poor websterite of the Al-augite suite, gabbro and gabbronorite of the late gabbro suite, and hornblendite of the hydrous vein suite. The dike rocks display consistent intrusive relationships with one another, such that Cr-diopside suite dikes are always older than dikes and veins of the Al-augite suite, followed by dikes of the late gabbro suite and veins of the hydrous vein suite. Phlogopite (phl) veinlets that formed during interaction with the adjacent crust are the youngest event. There are at least three generations of Cr-diopside suite dikes, as shown by crosscutting relations. Dikes of the Al-augite suite form a polybaric fractionation series from spinel clinopyroxenite to websterite and feldspathic websterite, which crystallized from aluminous alkaline magmas at relatively high pressures. The late gabbro suite of dikes intruded at lower pressures, where plagioclase saturation occurred before significant mafic phase fractionation. Hornblendite veins have distinct compositional and isotopic characteristics, which show that they are not related to either the Al-augite suite or to the late gabbro dike suite. Cr-diopside suite dikes have Nd and Sr isotopic compositions similar to those of the host lherzolite and within the range of compositions defined by ocean-island basalts. The Al-augite dikes and the hornblendite veins have Sr and Nd isotopic compositions similar to those of Cr-diopside suite lherzolite and websterite. The late gabbro dikes have Nd and Sr isotopic compositions similar to mid

  13. 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.

  14. The thinning of subcontinental lithosphere: The roles of plume impact and metasomatic weakening

    NASA Astrophysics Data System (ADS)

    Wang, Hongliang; van Hunen, Jeroen; Pearson, D. Graham

    2015-04-01

    Geologically rapid (tens of Myr) partial removal of thick continental lithosphere is evident beneath Precambrian terranes, such as North China Craton, southern Africa, and the North Atlantic Craton, and has been linked with thermomechanical erosion by mantle plumes. We performed numerical experiments with realistic viscosities to test this hypothesis and constrain the most important parameters that influence cratonic lithosphere erosion. Our models indicate that the thermomechanical erosion by a plume impact on typical Archean lithospheric mantle is unlikely to be more effective than long-term erosion from normal plate-mantle interaction. Therefore, unmodified cratonic roots that have been stable for billions of years will not be significantly disrupted by the erosion of a plume event. However, the buoyancy and strength of highly depleted continental roots can be modified by fluid-melt metasomatism, and our models show that this is essential for the thinning of originally stable continental roots. The long-term but punctuated history of metasomatic enrichment beneath ancient continents makes this mode of weakening very likely. The effect of the plume impact is to speed up the erosion significantly and help the removal of the lithospheric root to occur within tens of Myr if affected by metasomatic weakening.

  15. Subcontinental lithosphere reactivation beneath the Hoggar swell (Algeria): Localized deformation, melt channeling and heat advection

    NASA Astrophysics Data System (ADS)

    Kourim, Fatna; Vauchez, Alain; Bodinier, Jean-Louis; Alard, Olivier; Bendaoud, Abderrahmane

    2015-05-01

    In the Tahalgha district (southwestern Hoggar, Algeria), the Cenozoic volcanism has sampled subcontinental mantle beneath two crustal terranes that collided during the Pan-African orogeny: the "Polycyclic Central Hoggar" to the east and the "Western Hoggar" to the west. Two major lithospheric shear zones separate these terranes: the "4°35" and the "4°50" faults. Mantle xenoliths were collected between the two faults and across the 4°35 fault. In addition to a range in equilibrium temperatures and chemical compositions reported elsewhere, the samples show variations in their microstructures and crystallographic preferred orientations. Equilibrium temperatures and geochemical characteristics allow dividing them into low - (LT; 700-900 °C), intermediate - (IT; 900-1000 °C), and high-temperature (HT; 1000-1100 °C) xenoliths. The LT and IT peridotites occur on both sides of the 4°35 fault; they are usually coarse-grained. HT xenoliths are present only east of the 4°35 fault, in the narrow domain stuck between the two faults; they are fine-grained and extensively affected by annealing and melt-rock reactions. Microstructures and crystallographic textures indicate that deformation in the LT- and IT-xenoliths occurred through dislocation creep under relatively high-temperature, low-pressure conditions, followed by post-kinematic cooling. The fine-grained HT-xenoliths were deformed under relatively high-stress conditions before being annealed. Combining microstructural and CPO data with petrological and geochemical informations suggests that: (1) the LT xenoliths are remnants of the Neoproterozoic lithospheric mantle that preserved microstructural and chemical characteristics inherited from the Pan-African orogeny, and (2) the HT xenoliths record localized Cenozoic deformation associated with melt channeling through feed-back processes that culminated in the formation of high-permeability porous-flow conduits. Limited grain-growth in HT xenoliths suggests that

  16. 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

  17. 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

  18. Geochemical composition of subcontinental lithospheric mantle in the westernmost Mediterranean: constrains from peridotite xenoliths in Plio-Pleistocene alkali basalts (eastern Betic Cordillera, SE Spain)

    NASA Astrophysics Data System (ADS)

    Marchesi, Claudio; Konc, Zoltán; Bosch, Delphine; Garrido, Carlos J.; Hidas, Károly; Varas-Reus, María Isabel

    2016-04-01

    Peridotite xenoliths in Plio-Pleistocene alkali basalts from the eastern Betic Cordillera (Murcia, SE Spain) provide key information on Alpine tectono-magmatic processes that affected the subcontinental lithospheric mantle beneath the westernmost Mediterranean. Here we present a detailed geochemical study comprising whole-rock and mineral major- and trace-element, as well as Sr-Nd-Pb isotopic compositional data of spinel ± plagioclase lherzolite, spinel ± plagioclase harzburgite and spinel wehrlite xenoliths from Tallante and Los Perez volcanic centers. The whole-rock major element compositions and mineral chemistry of the studied xenoliths reflect increasing fertility from clinopyroxene-poor peridotites (Group I; Mg# up to 91.5), to common lherzolites (Group II; Mg# up to 90.6), fertile lherzolites (Group III; Mg# = 86.8-88.9) and wehrlites (Mg# = 86.7-87.4). The mineral major element chemistry records the geochemical imprint of maximum 10-12 % partial melting in the most depleted Group I peridotites. However, trace element and isotopic data attest for various degrees of metasomatic enrichment that overprinted the previously depleted lithospheric mantle. Interaction with melts produced enrichment of LREE in Group II and Group III peridotites, as well as in wehrlites. In contrast to major and trace elements, Sr-Nd-Pb radiogenic isotope systematic is unrelated to compositional groups and shows isotopic variations between DMM and EM2 end-members and contribution of an Atlantic sediment-like component. Different whole-rock trace element compositions coupled to similar isotopic signatures indicate that metasomatism was caused by external melt(s) issued from a common source not before the Tertiary. These geochemical evidences attest for the percolation of slab-derived, SiO2-undersaturated melts (and hydrous fluids) with carbonate sediment affinity in the pre-Miocene supra-subduction continental lithospheric mantle beneath the Alboran Basin.

  19. 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

  20. Re Os isotope constraints on subcontinental lithospheric mantle evolution of southern South America

    NASA Astrophysics Data System (ADS)

    Schilling, Manuel Enrique; Carlson, Richard Walter; Conceição, Rommulo Vieira; Dantas, Celine; Bertotto, Gustavo Walter; Koester, Edinei

    2008-04-01

    We present Re-Os isotopic data for widely dispersed mantle xenoliths carried to the surface of southern South America (36°-52° S) by Eocene to recent alkaline magmatism. Our hypothesis is that the lithospheric mantle sections formed as the roots of southern South America reflect the history of crust formation and amalgamation at different periods of time and so present a complimentary picture of continent growth in South America by sampling deeper sections of continental lithosphere than provided by crustal rocks from the area. The Re-Os isotopic system gives unique chronological information about the time of mantle depletion that is associated with lithosphere formation. Our data show coherent model ages for the lithospheric mantle that can be correlated with some hypotheses for crustal evolution of this region. Most samples show Os isotopic values similar to the present suboceanic mantle, suggesting a relatively recent lithospheric mantle formation from the convecting mantle. Xenoliths from Agua Poca and Prahuaniyeu represent fragments of an ancient depleted lithosphere, probably corresponding to the roots of the Cuyania terrane inferred to be a fragment derived from Laurentia and formed during the Mesoproterozoic. Alternatively, all or parts of the recognized ancient lithosphere are relicts of other known ancient continental blocks, such as the Pampia terrane or the Río de la Plata craton. Samples erupted in the southwest corner of the Deseado Massif give Proterozoic depletion ages (1.34 to 2.11 Ga) that are considerably older than previous radiogenic formation ages obtained for the very few basements rocks of this continental block. We propose that Deseado Massif is Proterozoic in age, probably related to the Malvinas/Falkland Islands and plateau and so should be considered for the reconstruction of the supercontinent of Rodinia.

  1. Spinel peridotite xenoliths from the Tariat Depression, Mongolia. II: Geochemistry and Nd and Sr isotopic composition and their implications for the evolution of the subcontinental lithosphere

    NASA Astrophysics Data System (ADS)

    Stosch, H.-G.; Lugmair, G. W.; Kovalenko, V. I.

    1986-12-01

    A suite of spinel peridotite xenoliths from the Shavaryn-Tsaram volcano, Tariat Depression (central Mongolia) represents (for major elements) fertile to moderately depleted subcontinental lithosphere. Part of the variation of moderately incompatible trace elements is ascribed to small-scale mineralogical heterogeneities caused by processes like metamorphic differentiation accompanying partial melting or by mechanical segregation. Several bulk lherzolites show a high relative enrichment of the LREE over HREE which can be traced to a grain boundary phase genetically linked to, but not directly representing, the host basanitoid. In Nd and Sr isotopic composition the anhydrous peridotites cover the field of oceanic basalts (143Nd/144Nd = 0.5128-0.5133, 87Sr/86Sr = 0.7020-0.7039). In contrast, a phlogopite peridotite has a high 87Sr/86Sr and also a less radiogenic 143Nd/144Nd. The majority of "dry" lherzolites have Nd and Sr "bulk earth" model ages around 2 Ga. They may be interpreted as dating a small-degree (< ~5%) melting event which would not have severely affected the major element chemistry of the xenoliths. The ~2 Ga model ages may indicate a genetic relation between the lithospheric mantle and the stabilization of the continental crust in Mongolia at that time. Alternatively, if the peridotites are unrelated to the overlying crust, they may be pieces of a young asthenospheric diapir. Coexisting ortho-and clinopyroxenes are in Nd isotopic equilibrium for Iherzolites having equilibrated at temperatures around 950°C at mantle pressures. Disequilibrium melting models of mantle rocks are not supported by our data because for medium to coarse-grained mantle spinel peridotite the Rb-Sr and Sm-Nd isotopic systems close with respect to diffusional exchange at temperatures around 900°C, as indicated by recently published diffusion experiment results and supported by our data.

  2. 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).

  3. Hard processing vs. episodic underplating of a terrain: isotopic signatures of mantle and crustal magmatic sources from the sub-continental lithosphere

    NASA Astrophysics Data System (ADS)

    Rasskazov, S.; Chuvashova, I.

    2012-04-01

    Hypothesese on origin of sub-continental lithosphere are tested, in this presentation, by isotopic data on magmatic liquids from crustal and mantle sources that might be genetically related or unrelated to each other. A common origin of the components reflects a radical recycling of a terrain resulted in separation of crustal and mantle constituents, characterized by a common inherited isochron of melt portions in U-Pb, Rb-Sr, and other isotope systems. A different origin assumes episodic underplating of growing sub-continental lithosphere that is reflected in contrast compositions of crustal and mantle sources, each of which yields melt potions with specific inherited isochrons. The lithospheric terrain of the former type produced 1) Late Tertiary volcanic rocks in the Shandong Peninsula, China with the inherited Pb-Pb isochron corresponding to the age of the eastern block of the North China craton (~2.57 Ga) (data of Zartman et al. [1991]), 2) Late Tertiary volcanic rocks in the Rungwe Province, Tanzania with the inherited Rb-Sr isochron corresponding to the end of the Pan-African orogeny (~0.46 Ga), and 3) Neoproterozoic (~0.9 Ga) dikes in the Gargan block of Eastern Siberia, Russia with the inherited Pb-Pb isochron corresponding to the age of the block basement (~2.7 Ga). The lithospheric terrain of the latter type yielded Cretaceous-Paleogene volcanic rocks in the Tien Shan, Kyrgyzstan and adjacent China with the inherited crustal and the newly formed mantle Rb-Sr isochrons of ~340 and ~50 Ma, respectively.

  4. 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

    In order to understand the dynamic response of the subcontinental lithospheric mantle (SCLM) to rifting, it is important to be able to distinguish the geochemical signatures of SCLM vs. asthenosphere. Recent work demonstrates that unradiogenic Os isotope ratios can indicate old depletion events in the convecting upper mantle (e.g. Rudnick & Walker, 2009), and allow us to make these distinctions. Thus, if SCLM can be traced across a rifted margin, its fate during rifting can be established. The Western Ross Sea provides favorable conditions to test the dynamic response of SCLM to rifting. Re-Os measurements from 8 locations extending from the rift shoulder to 200 km into the rift basin reveal 187Os/188Os ranging from 0.1056 at Foster Crater on the shoulder, to 0.1265 on Ross Island within the rift. While individual sample model ages vary widely throughout the margin, 'aluminochron' ages (Reisberg & Lorand, 1995) reveal a narrower range of lithospheric stabilization ages. Franklin Island and Sulfur Cones show a range of Re-depletion ages (603-1522 Ma and 436-1497 Ma) but aluminochrons yield Paleoproterozoic stabilization ages of 1680 Ma and 1789 Ma, respectively. These ages coincide with U-Pb zircon ages from Transantarctic Mountain (TAM) crustal rocks, in support of SCLM stabilization at the time of crust formation along the central TAM. The Paleoproterozoic stabilization age recorded at Franklin Island is especially significant, since it lies 200km off of the rift shoulder. The similar ages beneath the rift shoulder and within the rift suggests stretched SCLM reaches into the rift and thus precludes replacement by asthenospheric mantle. The persistence of thinned Paleoproterozoic SCLM into the rifted zone in WARS suggests that it represents a 'type I' margin of Huismans and Beaumont (2011), which is characterized by crustal breakup before loss of lithospheric mantle. The Archean Re-depletion age of 3.2 Ga observed on the rift shoulder suggests that cratonic

  5. Effects of melt percolation on highly siderophile elements and Os isotopes in subcontinental lithospheric mantle: A study of the upper mantle profile beneath Central Europe

    NASA Astrophysics Data System (ADS)

    Ackerman, Lukáš; Walker, Richard J.; Puchtel, Igor S.; Pitcher, Lynnette; Jelínek, Emil; Strnad, Ladislav

    2009-04-01

    The effects of melt percolation on highly siderophile element (HSE) concentrations and Re-Os isotopic systematics of subcontinental lithospheric mantle are examined for a suite of spinel peridotite xenoliths from the 4 Ma Kozákov volcano, Bohemian Massif, Czech Republic. The xenoliths have previously been estimated to originate from depths ranging from ˜32 to 70 km and represent a layered upper mantle profile. Prior petrographic and lithophile trace element data for the xenoliths indicate that they were variably modified via metasomatism resulting from the percolation of basaltic melt derived from the asthenosphere. Chemical and isotopic data suggest that lower sections of the upper mantle profile interacted with melt characterized by a primitive, S-undersaturated composition at high melt/rock ratios. The middle and upper layers of the profile were modified by more evolved melt at moderate to low melt/rock ratios. This profile permits an unusual opportunity to examine the effects of variable melt percolation on HSE abundances and Os isotopes. Most HSE concentrations in the studied rocks are significantly depleted compared to estimates for the primitive upper mantle. The depletions, which are most pronounced for Os, Ir and Ru in the lower sections of the mantle profile, are coupled with strong HSE fractionations (e.g., Os N/Ir N ratios ranging from 0.3 to 2.4). Platinum appears to have been removed from some rocks, and enriched in others. This enrichment is coupled with lithophile element evidence for the degree of percolating melt fractionation (i.e., Ce/Tb ratio). Osmium isotopic compositions vary considerably from subchondritic to approximately chondritic ( γOs at 5 Ma from -6.9 to +2.1). The absence of correlations between 187Os/ 188Os and indicators of fertility, as is common in many lithospheric mantle suites, may suggest significant perturbation of the Os isotopic compositions of some of these rocks, but more likely reflect the normal range of isotopic

  6. Redox state of subcontinental lithospheric mantle and relationships with metasomatism: insights from spinel peridotites from northern Victoria Land (Antarctica)

    NASA Astrophysics Data System (ADS)

    Perinelli, Cristina; Andreozzi, Giovanni B.; Conte, Aida M.; Oberti, Roberta; Armienti, Pietro

    2012-12-01

    Rift-related Cenozoic alkaline mafic lavas from northern Victoria Land (Antarctica) carry abundant mantle xenoliths whose oxygen fugacities ( fO2) were determined to assess how the metasomatism, related to Cenozoic magmatism, affected the state of oxidation of the lithospheric mantle. The xenoliths used for this study are anhydrous spinel peridotites sampled in two localities, Greene Point and Baker Rocks, that show different extents of metasomatism: these are limited to incompatible element enrichments in Greene Point and to enrichments in major, minor and trace elements at Baker Rocks. The data set includes a composite xenolith from Baker Rocks, formed by a depleted lherzolite crosscut by an amphibole-bearing vein. Mössbauer spectroscopy was used to accurately determine the Fe3+/Fetot ratios in spinel and amphibole minerals. Amphiboles were also characterized by Single-Crystal X-ray Diffraction, and the crystallographic data were used to calculate the dehydrogenation. The oxidation state recorded by the xenoliths ranges from 0.2 to 1.5 log-bar units below the fayalite-magnetite-quartz (FMQ) buffer (Δlog fO2) with the highest values observed in the metasomatized samples from Greene Point. For the vein of composite Baker Rocks xenolith, Δlog fO2 was estimated on the basis of the amphibole in -1.7 log-bar units, a value close to those calculated for all Baker Rocks xenoliths (Δlog fO2 = -1.5 to -1.1 log-bar units). These results indicate a similar oxidation state for lithospheric mantle prior to the metasomatic event at Greene Point and Baker Rocks (Δlog fO2 ~ -1.3 log-bar units). Metasomatism produced different effects in the shallow mantle at the two sites. At Greene Point, an oxidizing metasomatic melt caused the rise of fO2 in peridotite portions close to melt conduits up to FMQ. In contrast, at Baker Rocks, a metasomatizing melt with fO2 similar to that of the peridotite matrix produced chemical changes in the surrounding mantle rocks and amphibole

  7. 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.

  8. 1.8 Ga magmatism in the Fennoscandian Shield; lateral variations in subcontinental mantle enrichment

    NASA Astrophysics Data System (ADS)

    Andersson, U. B.; Eklund, O.; Fröjdö, S.; Konopelko, D.

    2006-01-01

    The ca. 1.8 Ga post-collisional magmatism in the southern part of the 2.1-1.86 Ga Svecofennian domain of the Fennoscandian Shield have been studied with particular reference to the character and sources of the mafic rocks in a traverse from the Archaean craton margin in east, across the juvenile Svecofennian domain to its western margin. For this purpose three key areas were selected in the eastern, central and western parts of the domain: (i) in the western part of the domain, the Tjällmo-Vättern zone (southern Sweden) of the Transscandinavian Igneous Belt (TIB) consists of extensive areas of dominantly alkali-calcic granitoids associated with calc-alkaline to tholeiitic mafic rocks. Initial ɛNd for the mafic rocks vary from around 0 to above +3. (ii) In the central part of the domain (SW Finland), the post-collisional rocks are represented by small intrusions consisting of calc-alkaline high-Ba-Sr granites associated with shoshonitic lamprophyres and their plutonic equivalents. Initial ɛNd for the rocks in this series vary between 0 and +1. (iii) In the easternmost part of the Svecofennian domain (Russian Karelia and SE Finland) shoshonitic associations occur, comprising lamprophyres and their plutonic equivalents (apatite and magnetite-rich monzodiorites), which are related to syenites and high-Ba-Sr granites by fractional crystallization. All the rock types in this shoshonitic association have strongly elevated contents of P 2O 5, LREE and LILE. Initial ɛNd for all rocks in Karelia fall between 0 and -1. Geochemical and isotopic results indicate that the post-collisional rocks in the central and eastern part of the domain stem from lithospheric mantle sources that were enriched during the preceding Svecofennian orogeny. The HFSE depletion, combined with the strong Sr, LILE and LREE enrichment, recall signatures of increasingly carbonate-dominated metasomatism of the mantle eastwards towards the craton margin. In contrast, the mainly LILE enriched mafic

  9. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-05-01

    We report gabbroic dikes in the plagioclase tectonite domains of the Ojén and Ronda massifs (Betic Cordillera, southern Spain), which record crystallization at low-pressure syn-, or slightly postkinematic to the late ductile history of the Betic Peridotite in the westernmost Mediterranean. We present mineral major and trace element compositional data of discordant gabbroic dikes in the Ojén massif and gabbroic patches in the Ronda massif, complemented by the whole rock and electron backscattered diffraction (EBSD) data of the Ojén occurrence. In the Ojén massif, gabbro occurs as 1-3 centimeter wide discordant dikes that crosscut the plagioclase tectonite foliation at high angle. These dikes are composed of cm-scale igneous plagioclase and clinopyroxene crystals that show shape preferred orientations subparallel to the lineation of the host peridotite and oblique to the trend of the dike. Intrusion of Ojén gabbro dikes is coherent with the stress field that formed the high temperature, ductile plagioclase tectonite foliation and then attests for a mantle igneous event prior to the intracrustal emplacement of the massif. In the Ronda massif, gabbroic rocks crystallized in subcentimeter wide anastomozing veins, or as interstitial patches in the host dunite. They are mostly composed of plagioclase and clinopyroxene. Plagioclase composition is bytownitic in the Ojén, and andesinic in the Ronda massif. Clinopyroxene in both places shows identical, light Rare-Earth Element (LREE) depleted trace element patterns. The calculated trace element composition of melts in exchange equilibrium with the studied igneous clinopyroxenes reflects LREE-enriched character coupled with negative Eu anomaly, and indicates that gabbro-forming melts in Ronda and Ojén share a common melt source with an island arc tholeiitic affinity. Geothermobarometric data and liquidus mineralogy indicate that gabbro crystallization occurred at shallow depths (0.2-0.5 GPa) in a 7-16 km thick

  11. 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.

  12. Mid-Tertiary (25-21 Ma) lamprophyres in NW Mexico derived from subduction-modified subcontinental lithospheric mantle in an extensional backarc environment following steepening of the Benioff zone

    NASA Astrophysics Data System (ADS)

    Orozco-Garza, Alberto; Dostal, Jaroslav; Keppie, J. Duncan; Paz-Moreno, Francisco A.

    2013-04-01

    The mid-Tertiary lamprophyre dike swarm (~ 8 km × 2.5 km in size) from Hermosillo (Sonora, NW Mexico) has calc-alkaline characteristics and includes NNW-striking, amphibole-phyric spessartite (~ 85% of the swarm) and NNE-striking, phlogopite-phyric kersantite. The 40Ar/39Ar geochronology of amphibole and phlogopite gives overlapping plateau ages ranging from 25 to 21 Ma. Although all the lamprophyres are enriched in incompatible elements and display negative Nb-Ta and Ti anomalies on the primitive mantle-normalized plots, kersantite has higher K/Na, La/Yb, P, Ti and incompatible trace elements (e.g., Zr) compared to spessartite. The lamprophyres have radiogenic Sr and Nd isotopic signatures (87Sr/86Sr ~ 0.7057-0.7065 and ɛNd ~- 1 to - 2.3) suggesting derivation from the subcontinental lithospheric mantle that was previously modified by subduction-related fluids. This mantle is similar to that beneath the southern Grenvillian orogen, which has younger TDM ages than the 1.6-1.7 Ga TDM ages of the Caborca block. The lamprophyric magmas were generated at various mantle depths at the southwestern edge of North America. Intrusion of the lamprophyres was synchronous with extension that produced normal faults and core complexes with WSW-vergence. Extension occurred immediately following steepening of the Benioff zone, during which the magmatic arc migrated from east to west of Hermosillo, and the lamprophyres were intruded just behind the contemporaneous arc.

  13. 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

  14. Mineral associations and major element compositions of base metal sulphides from the subcontinental lithospheric mantle of NE Spain

    NASA Astrophysics Data System (ADS)

    Galán, Gumer; Cruz, Erzika; Fernández-Roig, Mercè; Martínez, Francisco J.; Oliveras, Valentí

    2016-02-01

    This study deals with textural types and major element compositions of Cu-Ni-Fe sulphides from spinel lherzolite, harzburgite and olivine websterite xenoliths found in alkali basaltic rocks of the Neogene-Quaternary volcanic zone of Catalonia (NE Spain). Sulphides in harzburgites and websterites are scarce. Four textural types have been distinguished: inclusions in silicates and spinel, trails of small droplets often radiating from inclusions, interstitial grains, and grains related to pyrometamorphic textures. The mineral associations are dominated by one or two low-temperature monosulphide solid solutions: mss1, mss2, occasionally accompanied by pyrrhotite, pentlandite and Cu-rich sulphides. Compositions of mss1 are more Fe-enriched in inclusions and interstitial grains than in grains related to pyrometamorphism. Compositions of mss2 are Ni-rich very close to pentlandite. Sulphide bulk compositions correspond to high-temperature monosulphide solid solution equilibrated with a relatively Cu-Ni enriched sulphide melt at 1100-1000 °C. The breakdown products of these earlier compositions could have been either equilibrated below 600, 300 °C or being at disequilibrium. A restitic origin is consistent with the main sulphide mineral associations, the estimated melt extraction for peridotites (<30 %) and with the fact that lherzolites are less affected by cryptic metasomatism than harzburgites . However, Ni exchange coefficients between olivine and the high-temperature monosulphide solid solution underestimate equilibrium values. This suggests that some lherzolites could derive from pervasive refertilization. The scarcity of sulphides in websterites is explained by S incompatible behaviour during the formation of earlier cumulates from the mafic alkaline magmas which caused the cryptic metasomatism.

  15. 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

  16. 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

  17. 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

    represents a global mechanism in subduction zone, a portion of oceanic lithospheric mantle is likely to be metasomatized; recycling of these enriched domains into the convecting mantle is fundamental to understand the generation of small scale mantle isotopic and volatile heterogeneities sampled by OIBs and MORBs.

  18. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  20. 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.

  1. Geochemistry of the Quaternary alkali basalts of Garrotxa (NE Volcanic Province, Spain): a case of double enrichment of the mantle lithosphere

    NASA Astrophysics Data System (ADS)

    Cebriá, J. M.; López-Ruiz, J.; Doblas, M.; Oyarzun, R.; Hertogen, J.; Benito, R.

    2000-11-01

    The area of Garrotxa (also known as the Olot area) represents the most recent (700,000-11,500 y) and better preserved area of magmatic activity in the NE Volcanic Province of Spain (NEVP). This region comprises a suite of intracontinental leucite basanites, nepheline basanites and alkali olivine basalts, which in most cases represent primary or nearly primary liquids. The geochemical characteristics of these lavas are very similar to the analogous petrologic types of other Cenozoic volcanics of Europe, which are intermediate between HIMU, DM and EM1. Quantitative trace element modeling, suggests derivation from an enriched mantle source by degrees of melting that progressively increased from the leucite basanites (˜4%) to the olivine basalts (˜16%). However, the relatively more variable Sr-Nd-Pb isotope signature of the magmas suggests the participation of at least two distinct components in the mantle source: (1) a sublithospheric one with a geochemical signature similar to the magmas of Calatrava (Central Spain) and other basalts of Europe; and (2) an enriched lithospheric component with a K-bearing phase present. The geochemical model proposed here involves the generation of a hybrid mantle lithosphere source produced by the infiltration of the sublithospheric liquids into enriched domains of the mantle lithosphere, shortly before the melting event that generated the Garrotxa lavas. The available geological data suggest that the first enrichment event of the mantle lithosphere under the NEVP could be the result of Late Variscan mantle upwelling triggered by the extensional collapse of the Variscan orogen during the Permo-Carboniferous. By Jurassic/Cretaceous time, large-scale NNE-directed sublithospheric mantle channeling of thermally and chemically anomalous plume material was placed under the Iberian Peninsula and Central Europe. However, the geodynamic conditions in the NEVP did not favor magmatism, which could not take place until the Cenozoic after

  2. Evidence from mantle xenoliths for lithosphere removal beneath the central Rio Grande Rift

    NASA Astrophysics Data System (ADS)

    Byerly, Benjamin L.; Lassiter, John C.

    2012-11-01

    Seismic tomography beneath the Central Rio Grande Rift (RGR) at ˜34°N shows a low P and S wave velocity zone in the mantle that extends up the base of the Moho. This low-velocity region has been interpreted by (Gao et al., 2004) to be the result of convective removal of a portion of the once >100 km thick Proterozoic lithosphere. The amount of extension in the central RGR is thought to be low (˜25%) and thus cannot account for the amount of lithosphere thinning suggested by seismic tomography. We measured whole rock and mineral major element, trace element, and isotopic compositions of spinel-peridotite xenoliths erupted along the central axis of the rift (Elephant Butte) and the eastern margin of the Colorado Plateau (Cerro Chato) to determine their depth of origin and mantle provenance and to test the delamination hypothesis. If lithosphere removal has not occurred and the low P and S wave velocities are instead the result of hydration or melt infiltration in the lithosphere, then xenoliths erupted on the rift axis should have geochemical compositions similar to Proterozoic sub-continental lithospheric mantle (SCLM). At Cerro Chato, on the margin of the Colorado Plateau, xenoliths were derived from ˜60 km depth and have geochemical signatures similar to Proterozoic sub-continental lithospheric mantle (e.g. refractory major element compositions, LREE-enrichment, enriched Sr and Nd isotopes, unradiogenic Os isotopes). At Elephant Butte, along the central rift axis, two distinct groups of xenoliths are present. The majority of xenoliths from Elephant Butte are LREE-depleted and have fertile major element compositions. Additionally, these xenoliths have isotopic signatures similar to the range for DMM (e.g. 87Sr/86Sr ranging from 0.7018 to 0.7023, ɛNd ranging from 7 to 21, and 187Os/188Os ranging from 0.122 to 0.130). We interpret this group of xenoliths to be derived from asthenospheric mantle. A less-abundant group of xenoliths at Elephant Butte are LREE

  3. 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.

  4. Origin of low δ26Mg basalts with EM-I component: Evidence for interaction between enriched lithosphere and carbonated asthenosphere

    NASA Astrophysics Data System (ADS)

    Tian, Heng-Ci; Yang, Wei; Li, Shu-Guang; Ke, Shan; Chu, Zhu-Yin

    2016-09-01

    This study presents stable Mg isotopic data for Cenozoic potassic basalts from Wudalianchi and Erkeshan in northeastern China to determine the interactions between upwelling carbonated asthenosphere and enriched lithospheric mantle. Although the Wudalianchi and Erkeshan basalts have variable MgO contents of 4.45 to 9.47 wt.%, they exhibit a homogeneous Mg isotopic composition with δ26Mg values ranging from -0.57‰ to -0.46‰ and averaging -0.51 ± 0.06‰ (2SD, n = 18). This Mg isotopic composition is lighter than that of the average mantle (δ26Mg = -0.25 ± 0.07‰) but similar to late Cretaceous (<110 Ma) and Cenozoic basalts from the North China Craton and the South China Block (δ26Mg = -0.60 to -0.35‰). The high CaO/Al2O3 and Ba/Rb, and low Hf/Hf∗ ratios of the Wudalianchi and Erkeshan basalts are typical characteristics of carbonatitic metasomatism, suggesting that the light Mg isotopic composition could derive from involvement of recycled sedimentary carbonates in the mantle source. The high Dy/Er ratios (2.55 to 2.75) and excess of 230Th (230Th/238U = 1.24 to 1.33) suggest presence of garnet in a relatively deep mantle source. Additionally, the seismic tomographic observations show the existence of the stagnant Pacific slab in the mantle transition zone (410-660 km) under eastern China. This carbonated mantle source should be located in the asthenosphere. However, compared to other low δ26Mg basalts from eastern China with MORB-like Sr-Nd-Pb isotopic compositions and OIB-like trace element features, the Wudalianchi and Erkeshan basalts exhibit EM-I Sr-Nd-Pb isotopic compositions combined with high SiO2, Ba, K, Pb and LREE contents, high Ba/Th and low Ce/Pb ratios. These geochemical features require a contribution from another mantle source, most likely an EM-I lithospheric mantle. Therefore, an asthenosphere-lithosphere interaction model is proposed for determine the origin of the Wudalianchi and Erkeshan basalts. The original melt was derived from

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

    NASA Astrophysics Data System (ADS)

    Piccardo, G. B.

    2009-04-01

    The Monte Maggiore peridotite body, cropping out within the Alpine Corsica metamorphic belt, is an ophiolite massif derived from the more internal setting of the Jurassic Ligurian Tethys basin. It is mostly composed by spinel and plagioclase peridotites that are cut by MORB gabbroic dykes. The spinel peridotites, similarly to other ophiolitic peridotites from the Internal Ligurides, have been considered, on the basis of their low abundance of fusible components, low Si and high Mg contents, as refractory residua after MORB-type partial melting related to the formation of the Jurassic basin (e.g. Rampone et al., 1997). Recent studies (e.g. Müntener & Piccardo 2003; Rampone et al. 2008) have evidenced that these depleted spinel peridotites show diffuse melt-rock interaction micro-textures and contrasting bulk vs. mineral chemistry features which cannot be simply reconciled with partial melting. Accordingly, these peridotites have been recognized as reactive peridotites, formed by interaction of pristine peridotites with melts percolating by porous flow. Geochemical data have evidenced the depleted MORB signature of the percolating melts. Recent field studies at Monte Maggiore (Piccardo, 2007; Piccardo & Guarnieri, 2009), have revealed: 1) the presence and local abundance of pyroxenite-bearing, cpx-rich spinel lherzolites and 2) the replacement relationships of the reactive peridotites on the pyroxenite-bearing lherzolite rock-types. The pyroxenite-veined spinel lherzolites record a composite history of subsolidus evolution under lithospheric P-T conditions, thus indicating their provenance from the sub-continental lithospheric mantle. Accordingly, the pristine sub-continental mantle protoliths were infiltrated by MORB melts and transformed by melt-rock interaction to reactive spinel peridotites and refertilized by melt impregnation to plagioclase-enriched peridotites. Available isotopic data on the Mt. Maggiore spinel and plagioclase peridotites and gabbroic rocks

  6. Contrasting lithospheric mantle domains beneath the Massif Central (France) revealed by geochemistry of peridotite xenoliths

    NASA Astrophysics Data System (ADS)

    Lenoir, Xavier; Garrido, Carlos J.; Bodinier, Jean-Louis; Dautria, Jean-Marie

    2000-09-01

    We report major and trace element analyses for 82 coarse-grained peridotite xenoliths from 25 Cenozoic volcanic centres throughout the Massif Central (France). These data cover a region of about 150×150 km, allowing an investigation of large scale compositional variations in the subcontinental lithospheric mantle (SCLM). In agreement with textural variations, geochemical data define two contrasting lithospheric domains, situated north and south of latitude 45°30'. Peridotites of the northern domain show protogranular textures, characterised by clustered pyroxene-spinel distributions. They are rather refractory and depleted in MREE relative to HREE, but pervasively enriched in LREE and other highly incompatible elements. The samples show mantle-normalised patterns with negative anomalies of Nb, Ta, Zr and Hf, similar to enriched mantle xenoliths ascribed to carbonatitic metasomatism. In contrast, the peridotites of the southern domain are devoid of pyroxene-spinel clusters and are therefore referred to as coarse-granular. They are distinguished from the northern suite by more fertile compositions and relatively flat MREE-HREE patterns. In addition, only the harzburgites and a few lherzolites are enriched in LREE. Most southern domain lherzolites are depleted in these elements and the average composition of the southern suite is comparable to that of depleted MORB-source mantle (DMM). The main compositional differences between the two domains cannot be accounted for by a secular evolution of the Massif Central SCLM caused by Cenozoic plume upwelling. Instead, these differences record the existence of distinct lithospheric blocks assembled during the Variscan orogeny. To some degree, the northern and southern domains are reminiscent of cratonic and circumcratonic SCLM domains. Being relatively refractory and pervasively enriched in LREE, the northern domain displays similarities with cratonic SCLM. It is interpreted as a relatively ancient (pre

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

    NASA Astrophysics Data System (ADS)

    Wittig, Nadine; Pearson, D. Graham

    2010-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2009-11-01

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

  9. Petrogenesis of Mafic Volcanic Rocks from the Pribilof Islands, Alaska, by Melting of Metasomatically Enriched Depleted Lithosphere, Crystallization Differentiation, and Magma Mixing

    NASA Astrophysics Data System (ADS)

    Feeley, T. C.; Chang, J. M.; Deraps, M. R.

    2008-12-01

    The Pribilof Islands, Alaska, are located in the Bering Sea in a continental intraplate setting. In this study we examine the petrology and geochemistry of mafic volcanic rocks from St. Paul (0.54 to 0.003 Ma) and St. George (2.9 to 1.4 Ma) Islands, the two largest Pribilof Islands. Together these islands offer an opportunity to simultaneously investigate an active and extinct Bering Sea basaltic volcanic field in a setting where features such as lithospheric thickness and composition, distance from the Aleutian arc front, and other tectonic factors are virtually constant. Rocks from St. George can be divided into three groups. Group 1 contains high MgO, low SiO2 rocks that are primarily basanites. Group 2 contains high MgO, high SiO2 rocks that predominantly alkali basalts. Group 3 contains intermediate to low MgO rocks that include alkali basalts and trachybasalts with high modal plagioclase contents. Major and trace element compositions indicate that Groups 1 and 2 formed by partial melting (2-4%) of amphibole-bearing, garnet peridotite. Group 1 rocks were produced from the most hydrous parts of the mantle, as they show the strongest signature of amphibole in their source. Rocks from St. Paul inlcude alkali basalts and basanites with MgO contents from 4.2 to 14.4 wt% at relatively constant SiO2 contents (43.1 to 47.3 wt%). The most primitive St. Paul rocks are interpreted as mixtures between magmas with compositions similar to Groups 1 and 2 from St. George Island. These magmas subsequently fractionated olivine, clinopyroxene, and spinel to form more evolved, plagioclase-rich rocks. Plagioclase-rich Group 3 rocks from St. George can be modeled as mixtures between an evolved St. Paul end-member and a fractionated Group 2 end-member from St. George. Mantle potential temperatures estimated for primitive basanites and alkali basalts average 1370°C and are similar to those calculated for mid-ocean ridge basalts (MORB). Similarly, 87Sr/86Sr and 143Nd/144Nd values for

  10. 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

  11. Lithosphere-asthenosphere boundary: Where and why?

    NASA Astrophysics Data System (ADS)

    Aryasova, Olga; Khazan, Yakov

    2015-04-01

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

  12. Lithospheric processes

    SciTech Connect

    Baldridge, W.

    2000-12-01

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

  13. Majorite Garnet and Lithosphere Evolution: Kaapvaal Craton

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  14. Yellowstone hotspot-continental lithosphere interaction

    NASA Astrophysics Data System (ADS)

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

    2014-03-01

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

  15. 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

  16. Petrogenesis of Cenozoic, alkalic volcanic lineages at Mount Morning, West Antarctica and their entrained lithospheric mantle xenoliths: Lithospheric versus asthenospheric mantle sources

    NASA Astrophysics Data System (ADS)

    Martin, Adam P.; Cooper, Alan F.; Price, Richard C.

    2013-12-01

    Two volcanic lineages are identified at Mount Morning, a Cenozoic to recent, eruptive centre in the Ross Sea, West Antarctica, which is part of the McMurdo Volcanic Group. Both the older (at least 18.7-11.4 Ma), mildly alkalic, nepheline- or quartz-normative Mason Spur Lineage, and the younger (at least 6-0.02 Ma), nepheline normative, strongly alkalic Riviera Ridge Lineage evolved by fractional crystallization from nominally anhydrous (<0.5 wt% H2O) parental magmas. Both lineages are analogous to other, relatively anhydrous lineages in the McMurdo Volcanic Group and distinctly different from those in which kaersutite is present on the liquid line of descent. Sub-continental lithospheric mantle (SCLM) xenoliths entrained in Riviera Ridge Lineage rocks show trace element and isotopic Sr-Nd-Pb variation that is consistent with four-component mixing whereby depleted mantle has been refertilised by enriched, HIMU-like and Nb-enriched (carbonatite) components. Refertilization may have occurred c. 530-490 Ma ago when fluids derived from subduction associated with Gondwanaland amalgamation infiltrated the SCLM. Similar trace element and isotope variation (Sr-Nd-Pb) in Mount Morning basaltic rocks and entrained xenoliths suggests that the source for the basaltic magmas lies (at least in part) in the lithospheric mantle. It has long been recognized that Cenozoic volcanic rocks in Antarctica (Victoria Land - including Mount Morning - and Marie Byrd Land), Zealandia and eastern Australia share common chemical and isotopic source characteristics and they have been argued to collectively constitute a single diffuse alkaline magmatic province (DAMP). Source characteristic similarities suggest DAMP volcanic rocks inherit at least some of their trace element and isotopic characteristics from the lithospheric mantle. Super-chondritic Nb/Ta values measured in some SCLM xenoliths and volcanic rocks at Mount Morning, and in volcanic rocks across the DAMP, can be explained by addition

  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

    conjugate margin of the Atlantic. Its distribution completely overlaps with the distribution of EAR rocks. We therefore propose that the previously termed European Asthenospheric Reservoir (EAR) is actually the Pan-African Lithospheric Mantle (PALM), which is a direct source of alkalic-basaltic melts. A mechanism for the generation of melts from an ancient, veined sub-continental lithospheric mantle is the advection of heat by melts generated in the asthenosphere as a result of extensional decompression which infiltrate or underplate the lithosphere, or alternatively heating by advection of hotter mantle such as by a plume. Cebria, J., and Wilson, M., 1995, Cenozoic mafic magmatism in Western/Central Europe: a common European asthenospheric reservoir: Terra Nova, v. 7, p. 162. Médard, E., Schmidt, M. W., Schiano, P., and Ottolini, L., 2006, Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts: Journal of Petrology, v. 47, no. 3, p. 481-504. Pilet, S., Baker, M. B., Müntener, O., and Stolper, E. M., 2011, Monte Carlo simulations of metasomatic enrichment in the lithosphere and implications for the source of alkaline basalts: Journal of Petrology, v. 52, no. 7-8, p. 1415-1442. Zindler, A., and Hart, S., 1986, Chemical geodynamics: Annual review of earth and planetary sciences, v. 14, p. 493-571.

  18. Cenozoic magmatism in the South China Basin: Decompression melting and implications of an enriched mantle source

    SciTech Connect

    Flower, M.F.J.; Kan Tu; Ming Zhang ); Guanghong Xie )

    1990-06-01

    A widespread eposide of interplate volcanism followed the cessation of seafloor spreading in the South China Basin (SCB), affecting the South China Sea, and fringing areas of southern China and Indochina. Geochemical data for basalts from South China Sea islands and seamounts, Hainan Island, and Taiwan define an enriched (Dupal-like) mantle domain yielding oceanic island basalt (OIB) suites with {Delta}7/4Pb = 2-13, {Delta}8/4Pb = 45-73, {sup 87}Sr/{sup 86}Sr > {approximately}0.70325, Th/Ta > 2, and Th/Ba > 0.02. Opening of the SCB resulted from disaggregation of the South China block in response to the Indo-Eurasian collision, a process involving at least one seafloor spreading episode, terminated by collision of microcontinents with the Philippines and Borneo. The lack of precursive flood basalt suggests that active mantle upwelling was not involved and that melting was a passive effect of lithosphere stretching. However, while mantle decompression at ambient stretching factors ({approximately}1.7-2.5) appears to permit melting on the observed scale, the enriched source may preclude such a simple mantle dynamic. Three alternatives are considered: (1) passive melting of a mature metasomatised boundary layer, (2) active melting of thermally eroded subcontinental lithosphere (deep enrichment) or metasomatised boundary layer (shallow enrichment), and (3) relict diapirs of pre-SCB and/or Java trench subduction slabs (intermediate/deep enrichment). These models are evaluated in terms of chemical and isotopic mass balances associated with the generation and movement of small melt fractions in depleted, nondepleted, and enriched mantle.

  19. Lithospheric processes

    SciTech Connect

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

    1997-11-01

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

  20. Petrogenesis of basaltic volcanic rocks from the Pribilof Islands, Alaska, by melting of metasomatically enriched depleted lithosphere, crystallization differentiation, and magma mixing

    USGS Publications Warehouse

    Chang, J.M.; Feeley, T.C.; Deraps, M.R.

    2009-01-01

    trace element characteristics are similar to those of ocean island basalts (OIB), including enrichment in alkalis and incompatible trace elements. These characteristics are interpreted to indicate that their mantle source experienced an ancient melt-removal event that is reflected in depleted radiogenic isotopic compositions and was then re-enriched by metasomatism that elevated incompatible trace element contents, but was too young to produce a time-integrated change in radiogenic isotopic ratios. Evidence suggests that the Pribilof Island basalts did not form in either a plume or a back-arc basin tectonic setting. Rather, they were produced by melting of metasomatically hydrated upper mantle peridotite at relatively low temperatures and were able to erupt at the surface through extensional or transtensional faults that served as conduits for the magmas. ?? The Author 2009. Published by Oxford University Press.

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

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

  2. The giant Carlin gold province: a protracted interplay of orogenic, basinal, and hydrothermal processes above a lithospheric boundary

    NASA Astrophysics Data System (ADS)

    Emsbo, Poul; Groves, David I.; Hofstra, Albert H.; Bierlein, Frank P.

    2006-09-01

    Northern Nevada hosts the only province that contains multiple world-class Carlin-type gold deposits. The first-order control on the uniqueness of this province is its anomalous far back-arc tectonic setting over the rifted North American paleocontinental margin that separates Precambrian from Phanerozoic subcontinental lithospheric mantle. Globally, most other significant gold provinces form in volcanic arcs and accreted terranes proximal to convergent margins. In northern Nevada, periodic reactivation of basement faults along this margin focused and amplified subsequent geological events. Early basement faults localized Devonian synsedimentary extension and normal faulting. These controlled the geometry of the Devonian sedimentary basin architecture and focused the discharge of basinal brines that deposited syngenetic gold along the basin margins. Inversion of these basins and faults during subsequent contraction produced the complex elongate structural culminations that characterize the anomalous mineral deposit “trends.” Subsequently, these features localized repeated episodes of shallow magmatic and hydrothermal activity that also deposited some gold. During a pulse of Eocene extension, these faults focused advection of Carlin-type fluids, which had the opportunity to leach gold from gold-enriched sequences and deposit it in reactive miogeoclinal host rocks below the hydrologic seal at the Roberts Mountain thrust contact. Hence, the vast endowment of the Carlin province resulted from the conjunction of spatially superposed events localized by long-lived basement structures in a highly anomalous tectonic setting, rather than by the sole operation of special magmatic or fluid-related processes. An important indicator of the longevity of this basement control is the superposition of different gold deposit types (e.g., Sedex, porphyry, Carlin-type, epithermal, and hot spring deposits) that formed repeatedly between the Devonian and Miocene time along the

  3. 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

  4. 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

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

    NASA Astrophysics Data System (ADS)

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

    2014-02-01

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

  6. 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.

  7. Arctic lithosphere - A review

    NASA Astrophysics Data System (ADS)

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

    2014-07-01

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

  8. Formation and metasomatism of continental lithospheric mantle in intra-plate and subduction-related tectonic settings

    NASA Astrophysics Data System (ADS)

    Ionov, Dmitri

    2010-05-01

    Our knowledge of the origin and evolution of the continental lithospheric mantle (CLM) remains fragmentary and partly controversial in spite of recent advances in petrologic, geochemical and geophysical studies of the deep Earth and experimental work. Debate continues on a number of essential topics, like relative contributions of partial melting, metasomatism and ‘re-fertilisation' as well as the timing, conditions and tectonic settings of those processes. These topics can be addressed by studies of ultramafic xenoliths in volcanic rocks which arguably provide the least altered samples of modern and ancient CLM. The subcontinental lithosphere is thought to be a mantle region from which melts have been extracted, thus making the lithosphere more refractory. Melting degrees can be estimated from Al contents while the depth of melt extraction can be assessed from Al-Fe (Mg#) relations in unmetasomatized melting residues in comparison with experimental data, e.g. [1]. High silica and opx in the residues may indicate melting in water-rich conditions. High-precision Mg# and Mn for olivine may constrain degrees and conditions of partial melting and/or metasomatism, tectonic settings, modal compositions (e.g. presence of garnet) and equilibration conditions of mantle peridotites [2]. These estimates require both adequate sampling and high-quality major element and modal data; sampling and analytical uncertainties in published work may contribute substantially to chemical heterogeneities (and different origins) inferred for CLM domains [3]. Very fertile peridotite xenolith suites are rare worldwide [3]. They were initially viewed as representing mantle domains that experienced only very small degrees of melt extraction but are attributed by some workers to ‘refertilization' of refractory mantle by percolating asthenospheric melts. Such alternative mechanisms might be valid for some rare hybrid and Fe-enriched peridotites but they fail to comprehensively explain modal

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

    NASA Astrophysics Data System (ADS)

    Gibson, Sally

    2014-05-01

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

  10. Lithosphere mapping beneath the North American plate

    NASA Astrophysics Data System (ADS)

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

    2004-09-01

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

  11. Silica- and LREE-enriched spinel peridotite xenoliths from the Quaternary intraplate alkali basalt, Jeju Island, South Korea: Old subarc fragments?

    NASA Astrophysics Data System (ADS)

    Woo, Yonghoon; Yang, Kyounghee; Kil, Youngwoo; Yun, Sung-Hyo; Arai, Shoji

    2014-11-01

    wedge, the upper mantle beneath proto-Jeju Island was transformed from a subarc environment to an intraplate environment. The Jeju peridotites, representing old subarc fragments, were subsequently transported to the surface, incorporated into ascending Quaternary intraplate alkali basalt. The result of this study implies that long term material transfer in the transformation of geotectonic setting from a subarc to intraplate may have played a significant role in the evolution of the subcontinental lithospheric mantle, resulting in the enriched mantle domains, such as EMI or EMII.

  12. Tracing the thermal evolution of continental lithosphere through depth-dependent extension

    NASA Astrophysics Data System (ADS)

    Smye, A.; Lavier, L. L.; Stockli, D. F.; Zack, T.

    2015-12-01

    Rifting of continental lithosphere requires a mechanism to reduce lithospheric thickness from 100-150 kilometers to close to zero kilometers at the point of rupture. At magma-poor continental margins, this has long-thought to be caused by uniform stretching and thinning of the lithosphere accompanied by passive upwelling of the asthenosphere [1]. For the last thirty years depth-dependent thinning has been proposed as an alternative to this model to explain the anomalously shallow environment of deposition along many continental margins [2, 3]. A critical prediction of this modification is that the lower crust and sub-continental lithospheric mantle undergo a phase of increased heat flow, potentially accompanied by heating, during thinning of the lithospheric mantle. Here, we test this prediction by applying recently developed U-Pb age depth profiling techniques [4] to lower crustal accessory minerals from the exhumed Alpine Tethys and Pyrenean margins. Inversion of diffusion-controlled U-Pb age profiles in rutile affords the opportunity to trace the thermal evolution of the lower crust through the rifting process. Resultant thermal histories are used to calculate thinning factors of the crust and lithospheric mantle by 2D thermo-kinematic models of extending lithosphere. Combined, we use the measured and modeled thermal histories to propose a mechanism to explain the initiation and growth of lithospheric instabilities that lead to depth-dependent thinning at magma-poor continental margins. [1] McKenzie, D. (1978) EPSL 40, 25-32; [2] Royden, L. & Keen, C. (1980) EPSL 51, 343-361; [3] Huismans, R. & Beaumont, C. (2014) EPSL, 407, 148-162; [4] Smye, A. and Stockli, D. (2014) EPSL, 408, 171-182.

  13. Hydrologic sensitivity of Indian sub-continental river basins to climate change

    NASA Astrophysics Data System (ADS)

    Mishra, Vimal; Lilhare, Rajtantra

    2016-04-01

    Climate change may pose profound implications for hydrologic processes in Indian sub-continental river basins. Using downscaled and bias corrected future climate projections and the Soil Water Assessment Tool (SWAT), we show that a majority of the Indian sub-continental river basins are projected to shift towards warmer and wetter climate in the future. During the monsoon (June to September) season, under the representative concentration pathways (RCP) 4.5 (8.5), the ensemble mean air temperature is projected to increase by more than 0.5 (0.8), 1.0 (2.0), and 1.5 (3.5) °C in the Near (2010-2039), Mid (2040-2069), and End (2070-2099) term climate, respectively. Moreover, the sub-continental river basins may face an increase of 3-5 °C in the post-monsoon season under the projected future climate. While there is a large intermodel uncertainty, robust increases in precipitation are projected in many sub-continental river basins under the projected future climate especially in the Mid and End term climate. A sensitivity analysis for the Ganges and Godavari river basins shows that surface runoff is more sensitive to change in precipitation and temperature than that of evapotranspiration (ET). An intensification of the hydrologic cycle in the Indian sub-continental basins is evident in the projected future climate. For instance, for Mid and End term climate, ET is projected to increase up to 10% for the majority of the river basins under both RCP 4.5 and 8.5 scenarios. During the monsoon season, ensemble mean surface runoff is projected to increase more than 40% in 11 (15) basins under the RCP 4.5 (8.5) scenarios by the end of the 21st century. Moreover, streamflow is projected to increase more than 40% in 8 (9) basins during the monsoon season under the RCP 4.5 (8.5) scenarios. Results show that water availability in the sub-continental river basins is more sensitive towards changes in the monsoon season precipitation rather than air temperature. While in the majority

  14. 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

  15. The international lithosphere program

    NASA Astrophysics Data System (ADS)

    Flinn, Edward A.

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

  16. 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.

  17. 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.

  18. 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

  19. 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

  20. The giant Carlin gold province: A protracted interplay of orogenic, basinal, and hydrothermal processes above a lithospheric boundary

    USGS Publications Warehouse

    Emsbo, P.; Groves, D.I.; Hofstra, A.H.; Bierlein, F.P.

    2006-01-01

    Northern Nevada hosts the only province that contains multiple world-class Carlin-type gold deposits. The first-order control on the uniqueness of this province is its anomalous far back-arc tectonic setting over the rifted North American paleocontinental margin that separates Precambrian from Phanerozoic subcontinental lithospheric mantle. Globally, most other significant gold provinces form in volcanic arcs and accreted terranes proximal to convergent margins. In northern Nevada, periodic reactivation of basement faults along this margin focused and amplified subsequent geological events. Early basement faults localized Devonian synsedimentary extension and normal faulting. These controlled the geometry of the Devonian sedimentary basin architecture and focused the discharge of basinal brines that deposited syngenetic gold along the basin margins. Inversion of these basins and faults during subsequent contraction produced the complex elongate structural culminations that characterize the anomalous mineral deposit "trends." Subsequently, these features localized repeated episodes of shallow magmatic and hydrothermal activity that also deposited some gold. During a pulse of Eocene extension, these faults focused advection of Carlin-type fluids, which had the opportunity to leach gold from gold-enriched sequences and deposit it in reactive miogeoclinal host rocks below the hydrologic seal at the Roberts Mountain thrust contact. Hence, the vast endowment of the Carlin province resulted from the conjunction of spatially superposed events localized by long-lived basement structures in a highly anomalous tectonic setting, rather than by the sole operation of special magmatic or fluid-related processes. An important indicator of the longevity of this basement control is the superposition of different gold deposit types (e.g., Sedex, porphyry, Carlin-type, epithermal, and hot spring deposits) that formed repeatedly between the Devonian and Miocene time along the trends

  1. Magnetotelluric deep soundings, gravity and geoid in the south São Francisco craton: Geophysical indicators of cratonic lithosphere rejuvenation and crustal underplating

    NASA Astrophysics Data System (ADS)

    Pinto, Luis Gustavo Rodrigues; de Pádua, Marcelo Banik; Ussami, Naomi; Vitorello, Ícaro; Padilha, Antonio Lopes; Braitenberg, Carla

    2010-09-01

    In the south São Francisco craton a circular and 8-m amplitude geoid anomaly coincides with the outcropping terrain of an Archean-Paleoproterozoic basement. Broadband magnetotelluric (MT) data inversions of two radial profiles within the positive geoid and Bouguer gravity anomaly yield geo-electrical crustal sections, whereby the lower crust is locally more conductive (10 to 100 Ωm) in spatial coincidence with a denser lower crust modeled by the gravity data. This anomalous lower crust may have resulted from magmatic underplating, associated with Mesoarchean and Proterozoic episodes of tholeiitic dike intrusion. Long-period MT soundings reveal a low electrical resistivity mantle (20 to 200 Ωm) from depths beyond 120 km. Forward geoid modeling, using the scope of the low electrical resistivity region within the mantle as a constraint, entails a density increase (40 to 50 kg/m 3) possibly due to Fe enrichment of mantle minerals. However, this factor alone does not explain the observed resistivity. A supplemented presence of small amounts of percolated carbonatite melting (~ 0.005 vol.%), dissolved water and enhanced oxygen fugacity within the peridotitic mantle are viable agents that could explain the less resistive upper mantle. We propose that metasomatic processes confined in the sub-continental lithospheric mantle foster the conditions for a low degree melting with variable CO 2, H 2O and Fe content. Even though the precise age of this metasomatism is unknown it might be older than the Early Cretaceous based on the evidence that a high-degree of melting in a lithospheric mantle impregnated with carbonatites originated the tholeiitic dike intrusions dispersed from the southeastern border of the São Francisco craton, during the onset of the lithosphere extension and break-up of the western Gondwana. The proxies are the NE Paraná and Espinhaço (130 Ma, Ar/Ar ages) tholeiitic dikes, which contain (~ 3%) carbonatites in their composition. The occurrence of a

  2. Lithospheric and crustal thinning

    NASA Technical Reports Server (NTRS)

    Moretti, I.

    1985-01-01

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

  3. Evolution of Mojavian mantle lithosphere influenced by Farallon plate subduction: Evidence from Hf and Nd isotopes in peridotite xenoliths from Dish Hill, CA

    NASA Astrophysics Data System (ADS)

    Armytage, Rosalind M. G.; Brandon, Alan D.; Andreasen, Rasmus; Lapen, Thomas J.

    2015-06-01

    A major issue in the assembly of continents is the role of subduction in building and reworking the continental mantle lithosphere. Spinel lherzolite xenoliths from Dish Hill, CA represent Mojavian sub-continental lithospheric mantle (SCLM) that existed along an off-craton continental edge during late Cretaceous Farallon plate subduction. The Dish Hill locale is well situated for recording any Farallon plate influence, be it as oceanic lithosphere accretion or for its role in providing metasomatic agents to the Mojavian SCLM. The 176Hf/177Hf and 143Nd/144Nd isotopic compositions of clinopyroxenes from these xenoliths are radiogenic with εHf from +12.9 to +134.4 and εNd from +2.2 to +26.1, indicative of ancient Proterozoic melt depletion. Four out of the sixteen samples lie on a 2.1 Ga reference line for melt extraction from primitive mantle for both 176Hf/177Hf and 143Nd/144Nd, confirming their position on the 2.1 Ga 187Os/188Os aluminachron from previous work on these peridotites (Armytage et al., 2014). A second Re-depletion age obtained from an 187Os/188Os aluminachron of 1.3 Ga is also observed in the 176Lu-176Hf and 147Sm-143Nd systematics. The 176Hf/177Hf-143Nd/144Nd data from Dish Hill do not provide strong evidence for the existence of a duplex of oceanic lithosphere and SCLM, or for these peridotites being sourced from modern asthenospheric mantle upwelling after lithospheric removal. However, subchondritic 176Lu/177Hf and 147Sm/144Nd ratios and trace element compositions in some of the peridotites point to the influence of metasomatic processes. In seven of the peridotites 176Hf/177Hf ratios are not complemented by similarly radiogenic 143Nd/144Nd ratios. Such decoupling, relative to the mantle array, indicates that the 176Hf/177Hf record in these peridotites is more robust to resetting by these local metasomatic processes than 143Nd/144Nd. The 87Sr/86Sr ratios measured in these samples fall into two distinct groups based on (Ce/Yb)PM, with the less

  4. Lithospheric structure of North America imaged using waveform inversion of global and USArray data

    NASA Astrophysics Data System (ADS)

    Schaeffer, Andrew; Lebedev, Sergei

    2015-04-01

    velocities between the Great Bear Arc and Beaufort Sea provide convincing evidence for the recently proposed 'MacKenzie Craton', unexposed at the surface. Within the continental interior, the lithosphere surrounding the 1 Ga failed Mid-Continental Rift shows a reduction in wavespeeds compared to the surrounding craton, likely indicating thermo-chemical alteration of the sub-continental lithospheric mantle, in agreement with results from geochemical and petrological analyses of diamondiferous kimberlites and peridotites. We examine the spatial extent of the lithospheric mantle root and LAB variations across the continent, and compare them with respect to the spatial location of diamondiferous kimberlites. Finally, we discuss potential lithospheric control on the distribution crustal seismicity.

  5. Helium, neon, and argon systematics of the European subcontinental mantle: Implications for its geochemical evolution

    NASA Astrophysics Data System (ADS)

    Dunai, T. J.; Baur, H.

    1995-07-01

    the uncontaminated 40Ar/36Ar-ratio of the European subcontinental mantle is ⩾30,000. The4He/40Ar ratios have been fractionated, since with values around 0.25 they are below both the current production ratio and the MORB ratio. They cannot be explained by atmospheric contamination, as the samples with high 40Ar/36Ar ratios also have low 4He/40Ar ratios. Helium and Ar were probably fractionated during initial closed system degassing of the host magma. We calculate the original unfractionated 4He/40Ar-ratios of the Massif Central and Eifel magma source which are 2.56 ± .18 and 2.71 ± .17, respectively. The noble gas systematics are compatible with the notion that addition of crustal material has modified a former MORB-source signature. We use our mean 3He/4 He ratios and reconstructed 4He/40Ar-ratios to model the time and degree of contamination of a MORB-like source with crustal material. Combining our model with evidence from published oxygen, strontium, neodymium, and lead isotope data we judge that the source of the Massif Central and Eifel magmas became enriched in K, U, and Th shortly prior or during the Variscan orogeny (pre-collisional/collisional). In this case the enrichment in these elements is therefore most probably related to subduction of crustal material in a destructive continental margin setting.

  6. Geochemical and Isotopic Variations of Three Basalt Groups in the Early Permian Tarim Large Igneous Province (NW China): Implications for Plume-Lithosphere Interaction

    NASA Astrophysics Data System (ADS)

    Li, Y.; Li, Z.; Langmuir, C. H.; Yang, S.; Chen, H.; Yu, X.; Zou, S.

    2014-12-01

    Several lines of geological, petrological and geochemical evidence have supported that the Early Permian Tarim Large Igneous Province (LIP) in the Tarim cratonic block of northwestern China were generated by a mantle plume. However, the over 200,000 km2 Tarim continental flood basalts, as the dominant part of the Tarim LIP, show little geochemical and isotopic features similar to those plume-derived intrusive rocks in this region. This is mainly because that their parental magmas were more or less contaminated by the thick crust during ascending. Modeling by trace element and Nd isotopic compositions further suggest that the three basalt groups (Groups 1a, 1b and 2) in the Tarim LIP have experienced variable degree of crustal contamination (i.e., Group 1b > Group 1a > Group 2). After eliminating the effect of crustal contamination, the widespread Group 1 basalts (including both Groups 1a and 1b) would have relatively uniform ɛNd(t) values of ca. -1.7. This indicates that they were more likely to be produced by partial melting of some enriched mantle components in the sub-continental lithospheric mantle (SCLM) beneath the Tarim block, probably due to conductive heating that resulted from an incubating mantle plume. The Group 2 basalts, on the other hand, were only found in a small region but display a relatively higher and wider range of ɛNd(t) values roughly between -1.7 and 0.8 (if without crustal contamination). This may suggest that during the generation of Group 2 basalts, the upwelling mantle plume not only provided an enormous amount of heat, but also continuously injected isotopically depleted plume components into the isotopically enriched magma source region in the SCLM. The source isotopic heterogeneity of three basalt groups and other various Tarim LIP rocks (e.g., picrites, ultramafic-mafic intrusive rocks and syenitic rocks), with their ɛNd(t) values varying between ca. -5 and 5, may correlate with the plume-lithosphere interaction during the

  7. Development of a real time streamflow monitoring system for the Indian sub-continental basins

    NASA Astrophysics Data System (ADS)

    Shah, H. L.; Mishra, V.

    2015-12-01

    Real-time streamflow monitoring is essential in the Indian sub-continental river basins as a large population is affected by floods. Moreover, streamflow monitoring may help in managing the water resources in the agriculture dominated region. In the Indian sub-continental basins, it is challenging to obtain the real time information of streamflow, which is valuable for reservoir operations, water management, and flood forecasts. We setup the Variable Infiltration Capacity (VIC) hydrological model at daily temporal resolution and 0.25◦ spatial resolution using the bias corrected satellite precipitation product from the Tropical rainfall Measurement Mission Real Time (TRMM-3B42RTV7) and bias corrected temperature product from the Global Ensemble Forecast System (GEFS), version 2. Near-real-time precipitation and temperatures are bias corrected using the historic precipitation and temperature data from the India Meteorological Department (IMD). Moreover, we evaluated data assimilation approaches to improve the real-time monitoring of streamflow in the sub-continental basins.

  8. Magmatic expression of lithospheric thinning across continental rifts

    NASA Astrophysics Data System (ADS)

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

    1994-05-01

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

  9. SEASAT observations of lithospheric flexure

    NASA Technical Reports Server (NTRS)

    Mcadoo, D. C.

    1984-01-01

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

  10. Pyroxenite-derived Early Cretaceous lavas in the Liaodong Peninsula: Implication for metasomatism and thinning of the lithospheric mantle beneath North China Craton

    NASA Astrophysics Data System (ADS)

    Pang, Chong-Jin; Wang, Xuan-Ce; Xu, Yi-Gang; Wen, Shu-Nv; Kuang, Yong-Sheng; Hong, Lu-Bing

    2015-06-01

    The Xiaoling lavas, erupted at ca. 110 Ma in the Liaodong Peninsula, North China, provide vital constraints on the thermochemical state of subcontinental lithospheric mantle (SCLM) during the destruction of the craton. The Xiaoling lavas comprise basalt, andesite and dacite. They are characterized by depletion of high field strength elements (HFSE), enrichment of large ion lithophile elements (LILE) and EM1-like Sr-Nd isotopic compositions (εNd(t) = - 8.7--16.0; 87Sr/86Sri = 0.7046-0.7054), consistent with a derivation from the SCLM. With the exception of TiO2, the studied samples have major element compositions similar to those of experimentally determined partial melts of volatile-free Mid-Ocean-Ridge Basalt (MORB)-like eclogite at 3-5 GPa, but differ from anhydrous peridotite-derived melts. The olivine phenocrysts of the basaltic samples have high Ni and Fe/Mn, and low Ca contents, which are typical of the olivines crystallized from melts derived from a garnet pyroxenitic mantle source. This suggests that the Xiaoling lavas were derived from a pyroxenitic mantle source, which may have been formed by the solid-state reaction between recycled crustal materials and their surrounding peridotites. The presence of abundant amphibole phenocrysts in the Xiaoling lavas suggests a highly hydrated SCLM in this region. The high Rb/Sr but low initial 87Sr/86Sr ratios of the Xiaoling samples require a recent metasomatism in the mantle source, which is most likely related to the Pacific subduction. The genesis of the Xiaoling lavas therefore highlights the important role of water and Pacific subduction in the destruction of the North China Craton.

  11. Are Archean lithospheric keels inverted?

    NASA Astrophysics Data System (ADS)

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

    2007-02-01

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

  12. Evolution of the Precambrian Crust and Sub-Continental Lithsophere in Eastern Canada: Constraints from Probabilistic Inversion and H-k Stacking of Receiver Functions

    NASA Astrophysics Data System (ADS)

    Petrescu, L.; Bastow, I. D.; Darbyshire, F. A.; Levin, V. L.; Menke, W. H.

    2014-12-01

    Cratons are continental nuclei that have been tectonically quiescent for at least a billion years. They show distinct geological and geophysical signatures from younger continental regions. Some Precambrian crustal generation models suggest a change in tectonic processes at the end of the Archean, marking the onset of modern-style plate tectonics and different lithospheric growth mechanisms. Eastern Canada comprises Archean, Proterozoic and Phanerozoic terranes, recording tectonic events spanning ~3 Ga of Earth history. It is a natural laboratory to test hypotheses concerning craton genesis and Precambrian plate tectonics. To constrain structural variations across this region, a new broadband seismograph network has been deployed from southernmost Hudson Bay to coastal Nova Scotia. The main profile crosses major tectonic boundaries between the Superior craton, the Proterozoic Grenville and Phanerozoic Appalachian provinces. A profile of crustal structure across the major tectonic terranes was constructed from transdimensional Bayesian receiver function inversions. 1D structure beneath individual stations is described in terms of two parameters: shear wavespeed variation with depth, and likelihood of discontinuity, both defined probabilistically. A clear Moho can be identified at about 36-42 km with variable transition width. Moreover, a persistent sub-continental lithospheric impedance contrast is detected at 52-57 km beneath most stations. The ubiquity of this feature across a profile spanning 3 Ga of lithospheric processes could imply that post-cratonization chemical modification may have homogenized the uppermost mantle. Small age-dependent variations in mean bulk crustal Vp/Vs ratios are revealed from receiver function H-k stacking: ~1.71, ~1.77 and ~1.75 for the Archean, Proterozoic and Phanerozoic respectively. Crustal thickness also varies systematically with age, with Moho depths of ~35 km for the Archean and Phanerozoic but up to 10 km thicker in some

  13. Correlation between mobile continents and elevated temperatures in the subcontinental mantle

    NASA Astrophysics Data System (ADS)

    Jain, Charitra; Rozel, Antoine; Tackley, Paul

    2016-04-01

    Rolf et al. (EPSL, 2012) and Coltice et al. (Science, 2012) have previously shown that continents exert a first order influence on Earth's mantle flow by affecting convective wavelength and surface heat flow. With stationary continents, Heron and Lowman (JGR, 2014) highlighted the decreasing role of continental insulation on subcontinental temperatures with higher Rayleigh number (Ra). However, the question whether there exists a correlation between mobile continents and elevated temperatures in the subcontinental mantle or not remains to be answered. By systematically varying parameters like core-mantle boundary (CMB) temperature, continental size, and mantle heating modes (basal and internal); we model thermo-chemical mantle convection with 2D spherical annulus geometry (Hernlund and Tackley, PEPI 2008) using StagYY (Tackley, PEPI 2008). Starting with a simple incompressible model having mobile continents, we observe this correlation. Furthermore, this correlation still holds when the model complexity is gradually increased by introducing internal heating, compressibility, and melting. In general, downwellings reduce the mantle temperature away from the continents, thereby resulting in correlation between mobile continents and elevated temperatures in the subcontinental mantle. For incompressible models (Boussinesq approximation), correlation exists and the dominant degree of convection varies with the continental distribution. When internal heating is switched on, correlation is observed but it is reduced as there are less cold regions in the mantle. Even for compressible models with melting, big continents are able to focus the heat underneath them. The dominant degree of convection changes with continental breakup. Additionally, correlation is observed to be higher in the upper mantle (300 - 1000 km) compared to the lower mantle (1000 - 2890 km). At present, mobile continents in StagYY are simplified into a compositionally distinct field drifting at the top of

  14. 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

  15. Lithospheric thickness controlled compositional variations in potassic basalts of Northeast China by melt-rock interactions

    NASA Astrophysics Data System (ADS)

    Liu, Jian-Qiang; Chen, Li-Hui; Zeng, Gang; Wang, Xiao-Jun; Zhong, Yuan; Yu, Xun

    2016-03-01

    Melt-rock interaction is a common mantle process; however, it remains unclear how this process affects the composition of potassic basalt. Here we present a case study to highlight the link between compositional variations in the potassic basalts and melt-rock interaction in cold lithosphere. Cenozoic potassic basalts in Northeast China are strongly enriched in incompatible elements and show EM1-type Sr-Nd-Pb isotopes, suggesting an enriched mantle source. These rocks show good correlations between 87Sr/86Sr and K2O/Na2O and Rb/Nb. Notably, these ratios decrease with increasing lithospheric thickness, which may reflect melt-lithosphere interaction. Phlogopite precipitated when potassic melts passed through the lithospheric mantle, and K and Rb contents of the residual melts decreased over time. The thicker the lithosphere, the greater the loss of K and Rb from the magma. Therefore, the compositions of potassic basalts were controlled by both their enriched sources and reactions with lithospheric mantle.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  17. Lithospheric delamination underneath Far East Russia

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  18. Lithospheric Structure of the North American Craton Imaged Using Waveform Inversion of Global and Usarray Data

    NASA Astrophysics Data System (ADS)

    Schaeffer, A. J.; Lebedev, S.

    2014-12-01

    The North American Craton, now forming the stable core of North America, has a long, eventful tectonic history. The assembly of the craton, collisions and accretion at its current boundaries, major rifting episodes within it, and the loss of ancient lithosphere beneath parts of it are type examples of these key components of cratonic dynamics and evolution. Seismic tomography offers rich evidence on the structure and evolution of the cratonic lithosphere. With the deployment of the USArray during the last decade, much of the North American continent has been densely sampled with broadband seismic data. The resolution of regional-scale imaging, however, remains uneven, with important questions regarding deep structure, lateral extent and evolution difficult to answer. Here we present a new high-resolution model of the upper mantle beneath North America constrained by waveform fits of over 700,000 vertical-component broadband seismograms. Automated multimode waveform inversion was used to extract structural information from surface and S waveforms, yielding resolving power from the crust down to the transition zone, and improved resolution for a variety of features in North America. The internal structure of the Craton is resolved in detail. The lithosphere surrounding the 1 Ga failed Mid-Continental Rift shows reduced wavespeeds compared to the surrounding craton, likely indicating thermo-chemical alteration of the sub-continental lithospheric mantle. The sharp northern boundaries of the cratonic lithosphere closely follow the coastlines, with North America's and Greenland's lithospheric roots clearly separated. Strong lateral velocity gradients at depth observed in western Canada indicate the transition from cratonic lithosphere to Cordillera closely follows the surface trace of the Deformation Front. On the eastern margin of the continent, where multiple episodes of continental rifting are superimposed, the craton boundary coincides with the western extent of the

  19. Lithospheric architecture beneath Hudson Bay

    NASA Astrophysics Data System (ADS)

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

    2015-07-01

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

  20. Chromatographic metasomatism of the Arabian—Nubian lithosphere

    NASA Astrophysics Data System (ADS)

    Stein, Mordechai; Navon, Oded; Kessel, Ronit

    1997-11-01

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

  1. Chromatographic metasomatism of the Arabian-Nubian lithosphere

    NASA Astrophysics Data System (ADS)

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

    1997-11-01

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

  2. Lithospheric control on basaltic magma compositions within a long-lived monogenetic magmatic province: the Cainozoic basalts of eastern Victoria, south-eastern Australia

    NASA Astrophysics Data System (ADS)

    Price, R. C.; Nicholls, I. A.; Maas, R.

    2012-12-01

    Palaeozoic lithospheric blocks are present beneath southern Victoria and the lowest 87Sr/86Sr ratios are observed in basalts erupted above the Proterozoic (Selwyn) block. The inference is that there is a lithospheric control on basaltic magma chemistry and since a substantial proportion of Older Volcanics have the geochemical characteristics of primary magmas, this could indicate that magmas have been sourced from regionally heterogeneous sub-continental lithospheric mantle. References Price, RC, Gray, CM, Frey, FA. (1997). Strontium isotopic and trace element heterogeneity in the plains basalts of the Newer Volcanic Province, Victoria, Australia. Geochimica et Cosmochimica Acta 61, 171-192. Price RC, Nicholls, IA, Gray, CM. (2003). Cainozoic igneous activity: widespread volcanism resulting from long-term mantle instability and rifting. In: Birch, WD (ed.). Geology of Victoria, Geological Society of Australia Special Publication 23, 360-375.

  3. Start of the Wilson cycle at 3 Ga shown by diamonds from subcontinental mantle.

    PubMed

    Shirey, Steven B; Richardson, Stephen H

    2011-07-22

    Mineral inclusions encapsulated in diamonds are the oldest, deepest, and most pristine samples of Earth's mantle. They provide age and chemical information over a period of 3.5 billion years--a span that includes continental crustal growth, atmospheric evolution, and the initiation of plate tectonics. We compiled isotopic and bulk chemical data of silicate and sulfide inclusions and found that a compositional change occurred 3.0 billion years ago (Ga). Before 3.2 Ga, only diamonds with peridotitic compositions formed, whereas after 3.0 Ga, eclogitic diamonds became prevalent. We suggest that this resulted from the capture of eclogite and diamond-forming fluids in subcontinental mantle via subduction and continental collision, marking the onset of the Wilson cycle of plate tectonics. PMID:21778395

  4. Magmatic expressions of continental lithosphere removal

    NASA Astrophysics Data System (ADS)

    Wang, Huilin; Currie, Claire A.

    2015-10-01

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

  5. Correlation Between Mobile Continents and Elevated Temperatures in the Subcontinental Mantle

    NASA Astrophysics Data System (ADS)

    Jain, C.; Rozel, A. B.; Tackley, P.

    2015-12-01

    Rolf et al. (EPSL, 2012) and Coltice et al. (Science, 2012) have previously shown that continents exert a first order influence on Earth's mantle flow by affecting convective wavelength and surface heat flow. With stationary continents, Heron and Lowman (JGR, 2014) highlighted the decreasing role of continental insulation on subcontinental temperatures with higher Rayleigh number (Ra). However, the question whether there exists a correlation between mobile continents and elevated temperatures in the subcontinental mantle or not remains to be answered. Continental motion is attributed to the viscous stresses imparted by the convecting mantle and the extent of this motion depends on the heat budget of the mantle. Core-mantle boundary (CMB) heat flux, internal heating from decay of radioactive elements, and mantle cooling contribute to this heat budget. Out of these sources, CMB heat flux is not well defined. However, the recent determination of core's high thermal conductivity requires a CMB heat flow of at least 12 TW (de Koker et al., PNAS 2012; Pozzo et al., Nature 2012; Gomi et al., PEPI 2013). Thus it is necessary to characterize the impact of basal heating on mantle dynamics with mobile continents and self-consistent plate tectonics. By systematically varying parameters such as CMB temperature, continental size, mantle heating modes, and Rayleigh number; we model Boussinesq, incompressible, thermo-chemical mantle convection with 2D spherical annulus geometry using StagYY (Tackley, PEPI 2008). We observe the aforementioned correlation irrespective of the variations in basal heating and continental size (except for very small continents). Moreover, we see episodicity between correlation-anticorrelation with increasing convective vigour. Furthermore, the effect of radioactivity in the continental crust on this correlation is investigated. At present, mobile continents in StagYY are simplified into a compositionally distinct field drifting at the top of the mantle

  6. Subcontinental-scale crustal velocity changes along the Pacific-North America plate boundary.

    PubMed

    Davis, J L; Wernicke, B P; Bisnath, S; Niemi, N A; Elósegui, P

    2006-06-29

    Transient tectonic deformation has long been noted within approximately 100 km of plate boundary fault zones and within active volcanic regions, but it is unknown whether transient motions also occur at larger scales within plates. Relatively localized transients are known to occur as both seismic and episodic aseismic events, and are generally ascribed to motions of magma bodies, aseismic creep on faults, or elastic or viscoelastic effects associated with earthquakes. However, triggering phenomena and systematic patterns of seismic strain release at subcontinental (approximately 1,000 km) scale along diffuse plate boundaries have long suggested that energy transfer occurs at larger scale. Such transfer appears to occur by the interaction of stresses induced by surface wave propagation and magma or groundwater in the crust, or from large-scale stress diffusion within the oceanic mantle in the decades following clusters of great earthquakes. Here we report geodetic evidence for a coherent, subcontinental-scale change in tectonic velocity along a diffuse approximately 1,000-km-wide deformation zone. Our observations are derived from continuous GPS (Global Positioning System) data collected over the past decade across the Basin and Range province, which absorbs approximately 25 per cent of Pacific-North America relative plate motion. The observed changes in site velocity define a sharp boundary near the centre of the province oriented roughly parallel to the north-northwest relative plate motion vector. We show that sites to the west of this boundary slowed relative to sites east of it by approximately 1 mm yr(-1) starting in late 1999. PMID:16810252

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

    PubMed

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

    2005-07-28

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

  8. Lithospheric structure of the Canadian Shield as characterised by its seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Snyder, D. B.

    2011-12-01

    Distinct domains or layers of seismic anisotropy are increasingly recognized within Precambrian shields worldwide. Several relatively well-defined examples are now known from the Canadian Shield in North America. These benefit from a significantly large public suite of xenoliths produced during diamond exploration that provide calibration as to rock types found within these layers as well as increasingly resolved paleogeotherms. Independent parts of the seismic wave field provide consistent results; SKS, Ps conversions and Rayleigh waves were used to define sub-horizontal layers and to characterize deep lateral transitions associated with the edges of cratons previously defined by surface geological domain boundaries. The Slave and Rae cratons have received more intense study to date and can be shown to have formed by wedge tectonics that accreted smaller ancient blocks over time to first form the cratons in Archean times and the Canadian Shield in Proterozoic times. Differing mantle domains have distinct seismic anisotropy polarizations, inferred to represent both mineral fabrics (LPO) and larger scale dyke stockworks (SPO). Boundaries between these domains and layers appear as seismic discontinuities, primarily on sections constructed using transverse components of the seismic wavefield. This suggests that structures themselves, rather than greatly diverse physical properties and rock types, are key to imaging the sub-continental mantle lithosphere architecture and why gradual transitions such as the lithosphere-asthenosphere boundary may be difficult to characterize beneath shields.

  9. Origin of enriched components in the South Atlantic: Evidence from 40 Ma geochemical zonation of the Discovery Seamounts

    NASA Astrophysics Data System (ADS)

    Schwindrofska, Antje; Hoernle, Kaj; Hauff, Folkmar; van den Bogaard, Paul; Werner, Reinhard; Garbe-Schönberg, Dieter

    2016-05-01

    Spatial geochemical zonation is being increasingly recognized in Pacific and Atlantic hotspot tracks and is believed to reflect zonation within plumes upwelling from the margins of the Large Low Shear Velocity Provinces (LLSVPs) at the base of Earth's mantle. We present new 40Ar/39Ar age data for the Discovery Rise (South Atlantic Ocean) that show an age progression in the direction of plate motion from 23 Ma in the southwest to 40 Ma in the northeast of the Rise, consistent with formation of the Rise above a mantle plume. The lavas have incompatible element and Sr-Nd-Pb-Hf radiogenic isotope characteristics similar to the enriched DUPAL anomaly occurring in the southern hemisphere. The northern chain of seamounts is compositionally similar to the adjacent Gough subtrack of the bilaterally-zoned Tristan-Gough hotspot track, whereas the southern chain has some of the most extreme DUPAL compositions found in South Atlantic intraplate lavas thus far. The nearby southern Mid-Atlantic Ridge, believed to interact with the Discovery hotspot, shows a similar spatial geochemical distribution, consistent with the Discovery hotspot being zoned over its entire 40 Ma history. Our study implies a deep origin for the DUPAL anomaly, suggesting recycling of subcontinental lithospheric mantle (± lower crust) and oceanic crust through the lower mantle. The presence of an additional (Southern Discovery) DUPAL-like component, in addition to the Tristan and Gough/Northern Discovery components, in long-term zoned South Atlantic hotspots, points to the presence of a third lower mantle reservoir and thus is not consistent with the simple model that bilaterally-zoned plumes sample a chemically distinct LLSVP and the ambient mantle outside of the LLSVP.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  11. 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

    blocks of finite size that became unstable due to cooling from above and describe the peculiar horizontal planform that developed. Dynamical behaviour depends on three dimensionless numbers, a Rayleigh number for the unstable block, a buoyancy number that scales the intrinsic density contrast to the thermal one and the aspect ratio of the block. Within the block, instability develops in two different ways in an outer annulus and in an inner region. In the outer annulus, upwellings and downwellings take the form of radial rolls spaced regularly. In the interior region, the planform adopts the more familiar form of polygonal cells. Translated to geological conditions, such instabilities should manifest themselves as linear rifts striking at a right angle to the continent-ocean boundary and an array of domal uplifts, volcanic swells and basins in the continental interior. The laboratory data lead to simple scaling laws for the dimensions and spacings of the convective structures. For the sub-continental lithospheric mantle, these dimensions and distances take values in the 500-1000 km range, close to geological examples. The large intrinsic buoyancy of Archean lithospheric roots prevents this type of instability, which explains why the widespread volcanic activity that currently affects Western Africa is confined to post-Archean domains.

  12. Petrogenesis of the amphibole-rich veins from the Lherz orogenic lherzolite massif (Eastern Pyrenees, France): a case study for the origin of orthopyroxene-bearing amphibole pyroxenites in the lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Fabriès, J.; Lorand, J.-P.; Guiraud, M.

    The Lherz orogenic lherzolite massif (Eastern French Pyrenees) displays one of the best exposures of subcontinental lithospheric mantle containing veins of amphibole pyroxenites and hornblendites. A reappraisal of the petrogenesis of these rocks has been attempted from a comprehensive study of their mutual structural relationships, their petrography and their mineral compositions. Amphibole pyroxenites comprise clinopyroxene, orthopyroxene and spinel as early cumulus phases, with garnet and late-magmatic K2O-poor pargasite replacing clinopyroxene, and subsolidus exsolution products (olivine, spinel II, garnet II, plagioclase). The original magmatic mineralogy and rock compositions were partly obscured by late-intrusive hornblendites and over a few centimetres by vein-wallrock exchange reactions which continued down to subsolidus temperatures for Mg-Fe. Thermobarometric data and liquidus parageneses indicate that amphibole pyroxenites started to crystallize at P>=13kbar and recrystallized at P<12kbar. The high AlVI/AlIV ratio (>1) of clinopyroxenes, the early precipitation of orthopyroxene and the late-magmatic amphibole are arguments for parental melts richer in silica but poorer in water than alkali basalts. Their modelled major element compositions are similar to transitional alkali basalt with about 1-3wt% H2O. In contrast to amphibole pyroxenites, hornblendites only show kaersutite as liquidus phase, and phlogopite as intercumulus phase. They are interpreted as crystalline segregates from primary basanitic magmas (mg=0.6; 4-6wt% H2O). These latter cannot be related to the parental liquids of amphibole pyroxenites by a fractional crystallization process. Rather, basanitic liquids mostly reused pre-existing pyroxenite vein conduits at a higher structural level (P<=10kbar). A continuous process of redox melting and/or alkali melt/peridotite interaction in a veined lithospheric mantle is proposed to account for the origin of the Lherz hydrous veins. The

  13. Lithospheric dynamics near plate boundaries

    NASA Astrophysics Data System (ADS)

    Solomon, Sean C.

    1992-09-01

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

  14. Lithospheric dynamics near plate boundaries

    NASA Technical Reports Server (NTRS)

    Solomon, Sean C.

    1992-01-01

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

  15. The strength of Miranda's lithosphere

    NASA Technical Reports Server (NTRS)

    Pappalardo, Robert; Greeley, Ronald

    1991-01-01

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

  16. Isotopic and trace element compositions of upper mantle and lower crustal xenoliths, Cima volcanic field, California: Implications for evolution of the subcontinental lithospheric mantle

    USGS Publications Warehouse

    Mukasa, S.B.; Wilshire, H.G.

    1997-01-01

    Ultramafic and mafic xenoliths from the Cima volcanic field, southern California, provide evidence of episodic modification of the upper mantle and underplating of the crust beneath a portion of the southern Basin and Range province. The upper mantle xenoliths include spinel peridotite and anhydrous and hydrous pyroxenite, some cut by igneous-textured pyroxenite-gabbro veins and dikes and some by veins of amphibole ?? plagioclase. Igneous-textured pyroxenites and gabbros like the dike rocks also occur abundantly as isolated xenoliths inferred to represent underplated crust. Mineral and whole rock trace element compositions among and within the different groups of xenoliths are highly variable, reflecting multiple processes that include magma-mantle wall rock reactions, episodic intrusion and it filtration of basaltic melts of varied sources into the mantle wall rock, and fractionation. Nd, Sr, and Pb isotopic compositions mostly of clinopyroxene and plagioclase mineral separates show distinct differences between mantle xenoliths (??Nd = -5.7 to +3.4; 87Sr/86Sr = 0.7051 - 0.7073; 206Pb/204Pb = 19.045 - 19.195) and the igneous-textured xenoliths (??Nd = +7.7 to +11.7; 87Sr/86Sr = 0.7027 - 0.7036 with one carbonate-affected outlier at 0.7054; and 206Pb/204Pb = 18.751 - 19.068), so that they cannot be related. The igneous-textured pyroxenites and gabbros are similar in their isotopic compositions to the host basaltic rocks, which have ??Nd of+5.1 to +9.3; 87Sr/86Sr of 0.7028 - 0.7050, and 206Pb/204Pb of 18.685 - 21.050. The igneous-textured pyroxenites and gabbros are therefore inferred to be related to the host rocks as earlier cogenetic intrusions in the mantle and in the lower crust. Two samples of peridotite, one modally metasomatized by amphibole and the other by plagioclase, have isotopic compositions intermediate between the igneous-textured xenoliths and the mantle rock, suggesting mixing, but also derivation of the metasomatizing magmas from two separate and distinct sources. Sm-Nd two-mineral "isochrons" yield apparent ages for petrographically identical rocks believed to be coeval ranging from -0 to 113 ?? 26 Ma, indicating the unreliability of dating these rocks with this method. Amphibole and plagioclase megacrysts are isotopically like the host basalts and probably originate by mechanical breakup of veins comagmatic with the host basaltic rocks. Unlike other Basin and Range localities, Cima Cr-diopside group isotopic compositions do not overlap with those of the host basalts. Copyright 1997 by the American Geophysical Union.

  17. Spatial analysis of toxic emissions in LCA: a sub-continental nested USEtox model with freshwater archetypes.

    PubMed

    Kounina, Anna; Margni, Manuele; Shaked, Shanna; Bulle, Cécile; Jolliet, Olivier

    2014-08-01

    This paper develops continent-specific factors for the USEtox model and analyses the accuracy of different model architectures, spatial scales and archetypes in evaluating toxic impacts, with a focus on freshwater pathways. Inter-continental variation is analysed by comparing chemical fate and intake fractions between sub-continental zones of two life cycle impact assessment models: (1) the nested USEtox model parameterized with sub-continental zones and (2) the spatially differentiated IMPACTWorld model with 17 interconnected sub-continental regions. Substance residence time in water varies by up to two orders of magnitude among the 17 zones assessed with IMPACTWorld and USEtox, and intake fraction varies by up to three orders of magnitude. Despite this variation, the nested USEtox model succeeds in mimicking the results of the spatially differentiated model, with the exception of very persistent volatile pollutants that can be transported to polar regions. Intra-continental variation is analysed by comparing fate and intake fractions modelled with the a-spatial (one box) IMPACT Europe continental model vs. the spatially differentiated version of the same model. Results show that the one box model might overestimate chemical fate and characterisation factors for freshwater eco-toxicity of persistent pollutants by up to three orders of magnitude for point source emissions. Subdividing Europe into three archetypes, based on freshwater residence time (how long it takes water to reach the sea), improves the prediction of fate and intake fractions for point source emissions, bringing them within a factor five compared to the spatial model. We demonstrated that a sub-continental nested model such as USEtox, with continent-specific parameterization complemented with freshwater archetypes, can thus represent inter- and intra-continental spatial variations, whilst minimizing model complexity. PMID:24815341

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

    NASA Astrophysics Data System (ADS)

    Zhang, Kai-Jun

    2012-01-01

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

  19. Despotic, high-impact species and the subcontinental scale control of avian assemblage structure.

    PubMed

    MacNally, Ralph; Bowen, Michiala; Howes, Alison; McAlpine, Clive A; Maron, Martine

    2012-03-01

    Some species have disproportionate influence on assemblage structure, given their numbers or biomass. Most examples of such "strong interactors" come from small-scale experiments or from observations of the effects of invasive species. There is evidence that entire avian assemblages in open woodlands can be influenced strongly by individual species over very large areas in eastern Australia, with small-bodied species (< 50 g) being adversely affected. We used data from repeated surveys in 371 sites in seven districts across a region from Victoria to Queensland (> 2000 km). A series of linked Bayesian models was used to identify large-bodied (> or = 50 g) bird species that were associated with changes in occurrence and abundance of small-bodied species. One native species, the Noisy Miner (Manorina melanocephala; family Meliphagidae), was objectively identified as the sole large-bodied species having similar detrimental effects in all districts, depressing occurrence of 57 of 71 small-bodied species. Adverse effects on abundances of small-bodied species were profound when the Noisy Miner occurred with mean site abundances > or = 1.6 birds/2 ha. The Noisy Miner may be the first species to have been shown to influence whole-of-avifauna assemblage structure through despotic aggressiveness over subcontinental scales. These substantial shifts in occurrence rates and abundances of small-bodied species flow on to alter species abundance distributions of entire assemblages over much of eastern Australia. PMID:22624220

  20. Lithospheric instabilities. [associated with mantle geoid

    NASA Technical Reports Server (NTRS)

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

    1977-01-01

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

  1. Lithospheric influences on magma compositions of late Mesozoic and Cenozoic intraplate basalts (the Older Volcanics) of Victoria, south-eastern Australia

    NASA Astrophysics Data System (ADS)

    Price, Richard C.; Nicholls, Ian A.; Day, Arthur

    2014-10-01

    distinctive convex upwards patterns but are characterised by strong depletions of K, Rb and Ba relative to Nb. In both groups there is additional subtle variation with some samples having patterns with relative enrichments in Nb, Sr and Eu and/or depletions in Pb. Group 1 basalt compositions can be approximated by quantitative models involving 2 to 10% partial melting of an originally depleted mantle composition that has been metasomatised by the addition of 2 to 3% of an enriched component with a composition similar to EM1 intraplate basalt. The trace element patterns of Group 2 basalts can be modelled by 2 to 10% partial melting of an originally depleted mantle metasomatised by the addition of 1% of a calci-carbonatite composition. When Sr isotope data for Older Volcanics are projected onto an east-west profile across the state of Victoria, they outline distinctive discontinuities in isotopic composition that appear to be related to surface and subsurface structural features within the basement. One such discontinuity has previously been identified using data for the Newer Volcanics of the Western District Province of Victoria. Lithospheric blocks present beneath southern Victoria range in age from NeoProterozoic or Cambrian to Palaeozoic and some of the lowest 87Sr/86Sr ratios are observed in basalts erupted above an older basement unit (the Selwyn Block). The inference is that there is some form of lithospheric control on basaltic magma chemistry and since a substantial proportion of Older Volcanics have the geochemical characteristics of primary magmas (high Mg# and moderate to high abundances of Ni and Cr), this could indicate that magmas have been sourced from regionally heterogeneous, variably metasomatised, sub-continental lithospheric mantle. Neither the temporal and spatial relationships of the magmatic activity that followed continental breakup nor the uplift history of the south-eastern Australian passive margin are readily explained in terms of deep mantle plume

  2. Evidence for the Mesozoic and Cenozoic Evolution of the Lithosphere in the Trans-European Suture Zone from Surface Wave Tomography

    NASA Astrophysics Data System (ADS)

    Meier, Thomas; Soomro, Riaz; Lebedev, Sergei; Weidle, Christian; Viereck, Lothar; Behrmann, Jan; Cristiano, Luigia; Hanemann, Ricarda

    2016-04-01

    The Trans-European Suture Zone marks the transition between the East European Craton and Phanerozoic central Europe. Subduction of the Thor ocean and collision of Avalonia resulted in a Caledonian terrane assemblage that has been strongly affected by Permian volcanism, sedimentation in Post-Permian basins and recently by Cenozoic inversion tectonics. Whereas the structure of the crust in the area has been extensively studied by Deep Seismic Soundings, properties of the subcontinental mantle lithosphere are less well known. Surface waves are well suited to study the structure of the lithosphere and the sublithospheric mantle being mainly sensitive to the S-wave velocity structure at those depths. It has been shown before that the Tornquist-Teisseyre Zone representing the boundary to the East-European Craton in the southwest of the Trans-European Suture Zone is associated with a sharp transition between thick cratonic lithosphere in the northeast and thinner lithosphere to the southwest. Here we present results of a tomographic surface wave study based on automated broad-band measurements of average inter-station Rayleigh wave phase velocities providing higher resolution especially at lithospheric depths. All available broad-band recordings including data of temporary deployments like the TOR and PASSEQ experiments have been processed. At shorter periods phase velocities are sensitive to the sedimentary basins providing a 3D image of average shear-wave velocities. At intermediate periods differences in the crustal thickness and the structure of the uppermost mantle in the regions of the North German Basin and the Polish Basin become obvious. The latter is characterized by larger crustal thickness and rather low sub-Moho S-wave velocities. Also, lithospheric thickness varies along the Trans-European Suture Zone. In the region of the Sorgenfrei-Tornquist Zone a rather gradual decrease of lithospheric thickness towards central Europe is observed, whereas a shallow

  3. 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

    that the lithospheric mantle beneath Mount Morning, Pipecleaner Glacier, and White Island stabilized between 1.6-1.7 Ga, while Sulfur Cones and Franklin Island stabilized between 1.9-2.0 Ga. Conical Hill stabilized at ~2.3 Ga. The 2.0 Ga aluminachron stabilization age at Franklin Island supports the persistence of thinned subcontinental lithosphere 200 km into the rift basin. Based on our findings, we propose a Paleoproterozoic stabilization of the lithosphere now located beneath the western WARS in the study area, which may be coeval with the formation of crust along the central Transantarctic Mountains [8], although it is older than the directly overlying crust. We attribute the 3.2 and 3.3 Ga Re-depletion ages of the lithospheric mantle beneath Foster Crater to the formation of the East Antarctic Shield in the Archean and suggest the persistence of the Archean lithosphere through the Pan-African and Ross orogenies. [1] Behrendt, 1999 Global Planet Change (23) 25-44, [2] Bannister et al., 2003 Geophys J Int (155) 870-884, [3] Baranov 2011 Izv Phys Earth (47)1058-1070, [4] Ritzwoller et al 2001 JGR (106) 30645-30670, [5] Handler et al. 1997 EPSL (151) 61-75, [6] Janney et al. 1997 J Petrol (51) 1849-1890, [7] Walker et al. 1989 GCA (53) 1583-1595, [8] Goodge 1999 Geology (27) 1007-1010

  4. Seismic, Structural and Petrological Models of the Subcrustal Lithosphere in Southern Germany: A Quantitative Revaluation

    NASA Astrophysics Data System (ADS)

    Weiss, T.; Siegesmund, S.; Bohlen, T.

    1999-09-01

    Anisotropy in the subcontinental lithosphere becomes increasingly important, because it is observed in many seismic studies especially for P n -waves. Typical rocks of the uppermost mantle are peridotites, which predominantly exhibit a pronounced elastic anisotropy. This anisotropy is mainly caused by the anisotropic elastic properties and the lattice preferred orientation (here referred to as texture) of olivine. To evaluate the elastic anisotropy of peridotites from the subcontinental lithosphere, specimens of the Northern Hessian Depression (Germany) and the Balmuccia Ultramafic Massif (Northern Italy) have been used. They comprise four olivine texture types, which are characteristic for olivine textures observed worldwide. The bulk rock elastic properties have been calculated using olivine and orthopyroxene textures, their single-crystal elastic constants at ambient pressure/temperature conditions and their volume fraction. Clinopyroxene and spinel are assumed to be randomly distributed. The effect of four different orientations of the foliation within the uppermost mantle has been evaluated, since this orientation is usually unknown.¶Two of the olivine textures have a pronounced azimuthal dependence of compressional waves when a horizontal foliation within the uppermost mantle is presumed. These variations cause significant azimuthal variations of the P-wave reflections coefficients at the Moho. Primarily, we predict a significant azimuthal dependence of the critical points where the reflected amplitude increases from approximately 15% to 95%. Possibly, these azimuthal variations can be detected by seismic reflection measurements carried out at earth surface.¶The remaining two texture types only manifest a small directional dependence. When anisotropy of compressional waves is observed in seismic studies, these latter types can only be of subordinate importance. However, all of the peridotites investigated are able to explain the seismically observed

  5. 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

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

    NASA Astrophysics Data System (ADS)

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

    2007-10-01

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

  7. Anatomy of lithosphere necking during orthogonal rifting

    NASA Astrophysics Data System (ADS)

    Nestola, Yago; Cavozzi, Cristian; Storti, Fabrizio

    2013-04-01

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

  8. 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

  9. Petrology of exhumed mantle rocks at passive margins: ancient lithosphere and rejuvenation processes

    NASA Astrophysics Data System (ADS)

    Müntener, Othmar; McCarthy, Anders; Picazo, Suzanne

    2014-05-01

    Mantle peridotites from ocean-continent transition zones (OCT's) and ultraslow spreading ridges question the commonly held assumption of a simple link between mantle melting and MORB. 'Ancient' and partly refertilized mantle in rifts and ridges illustrates the distribution of the scale of chemical and isotopic upper mantle heterogeneity even on a local scale. Field data and petrology demonstrates that ancient, thermally undisturbed, pyroxenite-veined subcontinental mantle blobs formed parts of the ocean floor next to thinned continental crust. These heterogeneities might comprise an (ancient?) subduction component. Upwelling of partial melts that enter the conductive lithospheric mantle inevitably leads to freezing of the melt and refertilization of the lithosphere and this process might well be at the origin of the difference between magma-poor and volcanic margins. Similar heterogeneity might be created in the oceanic lithosphere, in particular at slow to ultra-slow spreading ridges where the thermal boundary layer (TBM) is thick and may be veined with metasomatic assemblages that might be recycled in subduction zones. In this presentation, we provide a summary of mantle compositions from the European realm to show that inherited mantle signatures from previous orogenies play a key role on the evolution of rift systems and on the chemical diversity of peridotites exposed along passive margins and ultra-slow spreading ridges. Particularly striking is the abundance of plagioclase peridotites in the Alpine ophiolites that are interpreted as recorders of refertilization processes related to thinning and exhumation of mantle lithosphere. Another important result over the last 20 years was the discovery of extremely refractory Nd-isotopic compositions with highly radiogenic 147Sm/144Nd which indicates that partial melting processes and Jurassic magmatism in the Western Thetys are decoupled. Although the isotopic variability might be explained by mantle heterogeneities

  10. 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

  11. Is there any correlation between continents and elevated temperatures in the subcontinental mantle?

    NASA Astrophysics Data System (ADS)

    Jain, Charitra; Rozel, Antoine; Tackley, Paul

    2015-04-01

    Rolf et al. (EPSL, 2012) and Coltice et al. (Science, 2012) have previously shown that continents exert a first order influence on Earth's mantle flow by affecting convective wavelength and surface heat flow. However, how continents influence the development and location of mantle plumes (upwellings) remains a topic of considerable debate. While Heron and Lowman (GRL, 2010; Tectonophysics, 2011) propose regions where downwelling has ceased (irrespective of overlying plate) as the preferred location for plumes, O'Neill et al. (Gondwana Research, 2009) show an anti-correlation between the average positions of subducting slabs at continental margins, and mantle plumes at continental/oceanic interiors. Continental motion is attributed to the viscous stresses imparted by the convecting mantle and the extent of this motion depends on the heat budget of the mantle. Core-mantle boundary (CMB) heat flux, internal heating from decay of radioactive elements, and mantle cooling contribute to this heat budget. Out of these sources, CMB heat flux is not well defined. However, the recent determination of core's high thermal conductivity requires a CMB heat flow of at least 12 TW (de Koker et al., PNAS 2012; Pozzo et al., Nature 2012; Gomi et al., PEPI 2013). Thus it is necessary to characterize the impact of basal heating on mantle dynamics with continents and self-consistent plate tectonics. By systematically varying parameters like CMB temperature, continental size, mantle heating modes (basal and internal), and Rayleigh number; we model Boussinesq, incompressible, thermo-chemical mantle convection in 2D spherical annulus geometry using StagYY (Tackley, PEPI 2008). We observe correlation between continents and elevated temperatures in the subcontinental mantle irrespective of the variations in basal heating and continental size (except for very small continents). Moreover, we see episodicity between correlation and anti-correlation with increasing Rayleigh number. Furthermore

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

    NASA Astrophysics Data System (ADS)

    Wallner, Herbert; Schmeling, Harro

    2016-04-01

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

  13. Anisotropic tomography of the European lithospheric structure from surface wave studies

    NASA Astrophysics Data System (ADS)

    Nita, Blanka; Maurya, Satish; Montagner, Jean-Paul

    2016-06-01

    We present continental-scale seismic isotropic and anisotropic imaging of shear wave upper-mantle structure of tectonically diversified terranes creating the European continent. Taking into account the 36-200 s period range of surface waves enables us to model the deep subcontinental structure at different vertical scale-lengths down to 300 km. After very strict quality selection criteria, we have obtained phase wave speeds at different periods for fundamental Rayleigh and Love modes from about 9000 three-component seismograms. Dispersion measurements are performed by using Fourier-domain waveform inversion technique named "roller-coaster-type" algorithm. We used the reference model with a varying average crustal structure for each source-station path. That procedure led to significant improvement of the quality and number of phase wave speed dispersion measurements compared to the common approach of using a reference model with one average crustal structure. Surface wave dispersion data are inverted at depth for retrieving isotropy and anisotropy parameters. The fast axis directions related to azimuthal anisotropy at different depths constitute a rich database for geodynamical interpretations. Shear wave anomalies of the horizontal dimension larger than 200 km are imaged in our models. They correlate with tectonic provinces of varying age-provenance. Different anisotropy patterns are observed along the most distinctive feature on our maps-the bordering zone between the Palaeozoic and Precambrian Europe. We discuss the depth changes of the lithosphere-asthenosphere boundary along the profiles crossing the chosen tectonic units of different origin and age: Fennoscandia, East European Craton, Anatolia, Mediterranean subduction zones. Within the flat and stable cratonic lithosphere, we find traces of the midlithospheric discontinuity.

  14. The Formation of Non-Volcanic Rifted Margins by the Progressive Extension of the Continental Lithosphere

    NASA Astrophysics Data System (ADS)

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

    2003-12-01

    Rifted margins include two main end-members: those termed "Volcanic Rifted Margins - VRMs" where magmatism is much more voluminous than predicted by passive asthenospheric upwelling (e.g. White et al., 1989), and those where magmatism is consistent or even less than the same predictions. The latter are termed "Non-Volcanic Rifted Margins - NVRMs" to emphasise the contrast with the VRMs: the name does not exclude the presence of minor amounts of magmatic activity. The NVRMs are typified by the North Biscay, south Australian, SW Greenland, and the West Iberian margins, which share a number of common characteristics: - extreme crustal thinning, increasing towards the ocean; - presence of well-defined rotated fault blocks. However at the feather edge of the continent there is an extension discrepancy: the amount that can be inferred from the geometry of these faults is far less than that indicated by the crustal thinning observed; - presence in places of a detachment fault at the base of the fault blocks; - little evidence for synrift magmatism; - the presence of a broad zone of partially serpentinised mantle (Boillot et al., 1988; Whitmarsh et al., 1996; Krawczyk et al., 1996; Pickup et al., 1996), both occurring beneath the highly thinned and faulted continental crust, and as a zone of exhumed continental mantle, now largely buried by postrift sediments. We show that such margins are the logical result of progressive extension of continental lithosphere above cool sub-lithospheric mantle. The key factors controlling the development of the margin are the rheological evolution of the crust (explaining the serpentinisation of the mantle), the occurrence of multiple phases of faulting (explaining the apparent extension discrepancy), and the temperature structure of the sub-continental mantle (explaining the lack of magmatism).

  15. 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.

  16. 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

  17. Rifting Thick Lithosphere - Canning Basin, Western Australia

    NASA Astrophysics Data System (ADS)

    Czarnota, Karol; White, Nicky

    2016-04-01

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

  18. Rheology of the lithosphere: selected topics.

    USGS Publications Warehouse

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

    1987-01-01

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

  19. Lithospheric Discontinuities Beneath North America (Invited)

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

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

  20. Relationship between Famatinian Arc Magmatism and Recent Mafic Volcanism in Northwest Argentina: Implications for Lithospheric Composition and Evolution Beneath the Puna Plateau

    NASA Astrophysics Data System (ADS)

    Drew, S.; Schoenbohm, L.; Ducea, M.

    2008-12-01

    The tectonic and magmatic evolution of the Puna Plateau (NW Argentina) has generated much debate over the past two decades. This study focuses on the young (< 7 Ma), mafic magmatism that led to the creation of monogenetic and simple polygenetic volcanoes throughout the plateau. These volcanics provide a means to evaluate the recent petro-tectonic development of the plateau and, in combination with Ordovician intrusive rocks, determine the isotopic composition and long term evolution of the sub-continental lithospheric mantle (SCLM) beneath the Andean back-arc domain. Here we present new whole rock major and trace element data and isotopic values for volcanic samples collected from the Antofagasta and Pasto Ventura basins in the southern Puna Plateau. Major element chemistry shows most of our samples are basalt, trachybasalt, basaltic andesite and basaltic trachyandesites, some with < 50.0 wt% SiO2 and > 8.0 wt% MgO, which is indicative of a strong mantle component. The more primitive lavas likely have a sub-crustal origin and experienced minimal interaction with overlying crust during transport to the surface. Two of our samples with low wt% MgO, a silicic andesite and a dacite, indicate an extensive crustal component and possibly a lower crust origin for evolved magmas. All samples have light trace element enrichment compared to NMORB and elevated abundances of LIL and LRE elements compared to HFS and HRE elements, indicating the magmas originated from a metasomatized source region. The samples also have variable (low and high) Nb, Ta and Ti negative anomalies, which are interpreted to be a signature of the source region. Our samples do not have a lithospheric delamination (~OIB) trace element signature as proposed by previous workers in support of a delamination model. Additionally, the samples have isotopic values (e.g. 87Sr/86Sr >0.7055 and ɛNd <0) that are not comparable to depleted asthenosphere. It is impossible for asthenospheric magma to obtain these

  1. Geochronology and geochemistry of Eocene-aged volcanic rocks around the Bafra (Samsun, N Turkey) area: Constraints for the interaction of lithospheric mantle and crustal melts

    NASA Astrophysics Data System (ADS)

    Temizel, İrfan; Arslan, Mehmet; Yücel, Cem; Abdioğlu, Emel; Ruffet, Gilles

    2016-08-01

    40Ar-39Ar age, whole-rock chemical, and Sr-Nd isotope data are presented for the post-collisional, Eocene (51.3-44.1 Ma)-aged volcanic rocks from the Bafra (Samsun) area in the western part of the Eastern Pontides (N Turkey) aiming to unravel their sources and evolutionary history. The studied Eocene volcanic rocks can be divided into two groups: analcime-bearing (tephritic lava flows and dykes) and analcime-free (basaltic to trachytic lava flows and basaltic dykes). The analcime-bearing volcanic rocks have a fine-grained porphyritic texture with clinopyroxene phenocrysts, whereas analcime-free volcanic rocks show a variety of textures including hyalo-microlitic microgranular porphyritic, intersertal, trachytic, fluidal, and glomeroporphyritic. The volcanic rocks also show evidence of mineral-melt disequilibrium textures such as sieved, rounded, and corroded plagioclases, partially melted and dissolved clinopyroxenes and poikilitic texture. Petrochemically, the parental magmas of the volcanic rocks evolved from alkaline to calc-alkaline lava suites and include high-K and shoshonitic compositions. They display enrichments in light rare earth and large ion lithophile elements such as Sr, K, and Rb, as well as depletions in high field strength elements such as Nb, Ta, Zr, and Ti, resembling subduction-related magmas. The analcime-bearing and -free volcanic rocks share similar incompatible element ratios and chondrite-normalised rare rearth element patterns, indicating that they originated from similar sources. They also have relatively low to moderate initial 87Sr/86Sr (0.7042-0.7051), high positive εNd(t) values (+ 0.20 to + 3.32), and depleted mantle Nd model ages (TDM1 = 0.63-0.93 Ga, TDM2 = 0.58-0.84 Ga). The bulk-rock chemical and Sr-Nd isotope features as well as the high Rb/Y and Th/Zr, but low Nb/Zr and Nb/Y ratios, indicate that the volcanic rocks were derived from a lithospheric mantle source that had been metasomatised by slab-derived fluids. Trace element

  2. Relationship between oxidation state and texture and the influence of metasomatism in the lithospheric mantle beneath the Massif Central, France

    NASA Astrophysics Data System (ADS)

    Uenver-Thiele, Laura; Woodland, Alan; Downes, Hilary; Altherr, Rainer

    2013-04-01

    Investigating regional variations in structure and composition of the subcontinental lithospheric mantle (SCLM) is usually hampered by the lack of samples. However, the numerous volcanic centers of the French Massif Central have brought samples of the SCLM to the surface over a geographic area of ~20.000km2. Lenoir et al. (2000) and Downes et al. (2003) identified textural and geochemical differences between two domains lying north and south of 45° 30' latitude, respectively. The northern domain is relatively refractory, but has experienced pervasive enrichment of LREE. The southern domain is generally more fertile, although the peridotites are LREE depleted. Many xenolith suites have undergone variable degrees of metasomatism. Variations in oxidation state might be expected from the differing histories of these two domains. We have undertaken an extensive study to determine the oxidation state of the SCLM beneath the Massif Central over the largest geographical area possible, including 140 peridotite xenoliths from 45 localities. All xenoliths are spinel peridotites and vary in composition from lherzolites to harzburgites. Using the nomenclature of Mercier and Nicolas (1975) the xenoliths are mostly protogranular or protogranular-porphyroclastic, although some are porphyroclastic or equigranular. Small amounts of amphibole or biotite occur in some xenoliths, particularly in the southern domain, reflecting modal metasomatism. These metasomatic phases are found predominantly in peridotites with protogranular textures. Oxidation state was determined using the equilibrium between the Fe-bearing components of olivine and pyroxene and the magnetite component in spinel (i.e. Wood et al. 1990). Major element compositions of the individual minerals were determined by microprobe. Ferric iron contents of spinel were determined by Mössbauer spectroscopy and gave values of Fe3+/ Fetot from 0.159 to 0.459, with a conservative uncertainty of ±0.02. Oxygen fugacity (fO2) of

  3. 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

  4. Deformation and instability of underthrusting lithospheric plates

    NASA Technical Reports Server (NTRS)

    Liu, H.

    1972-01-01

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

  5. The structure of the Ionian lithosphere

    NASA Technical Reports Server (NTRS)

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

    1984-01-01

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

  6. Lithospheric layering in the North American craton.

    PubMed

    Yuan, Huaiyu; Romanowicz, Barbara

    2010-08-26

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

  7. 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

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

    NASA Astrophysics Data System (ADS)

    Wallner, Herbert; Schmeling, Harro

    2016-06-01

    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

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

    NASA Astrophysics Data System (ADS)

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

    2011-09-01

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

  10. Two Lithologies in Lithospheric Mantle Beneath Nothern Margin of the Bohemian Massif (e Germany and SW Poland).

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    The subcontinental lithospheric mantle (SCLM) occurring beneath Bohemian Massif in Central Europe has been sampled in Cenozoic times by numerous lavas. Recent studies (Puziewicz et al. 2011 and references therein) show that mantle in this region is mostly anhydrous, harzburgitic, and was subjected to various kinds of metasomatic events. Two major mantle lithologies characterized by different major element composition of peridotite- forming minerals occur in the SCLM Lower Silesia and Lusatia (op. cit. and unpublished results, 9 sites). Lithology "A" (minimal temperatures from 900 to 1000ºC or no equilibrium between cpx and opx) contains olivine Fo90.5 -92.0. Part of the population "A" peridotites contain clinopyroxene of mg# 94 - 95, typical for low temperatures of equilibration. The lithology "B" (equilibration temperatures close to 900 ºC) contains olivine Fo87.5-90.0. Elevated contents of LREE in clinopyroxene from both the lithologies "A" and "B" suggest their equilibration with one of the two metasomatic agents stated in this area: anhydrous silicate alkaline melt or carbonatite-silicate melt. Action of hydrous alkaline melts in the mantle in the region is recorded only locally (e.g. Wilcza Góra). In some sites (e.g. Krzeniów) the trace element patterns show that decreasing mg# of clinopyroxene in the "A" peridotites is due to gradual replacement of primary lower-temperature mineral assemblage by the later higher-temperature one. This suggests that the variation of mineral chemistry is rather due to chromatographic fractionation of metasomatic agents than due to vertical variation in lithospheric mantle temperatures (Christensen et al.,2001). The "B" peridotites originated due to "Fe-metasomatism" of more magnesian peridotites by silicate melts percolating through lithospheric mantle. The peridotites belonging to lithology "A" might have been partly the protolith of the lithology "B". The data on Central European lithospheric mantle are equivocal and thus

  11. Complexity of the Fennoscandian lithosphere

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  12. 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

  13. 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

  14. Permeability Barrier Generation in the Martian Lithosphere

    NASA Astrophysics Data System (ADS)

    Schools, Joe; Montési, Laurent

    2015-11-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    The PICASSO (Program to Investigate Convective Alboran Sea System Overturn) project and the concomitant TopoMed (Plate re-organization in the western Mediterranean: Lithospheric causes and topographic consequences - an ESF EUROSCORES TOPO-EUROPE project) project were designed to collect high resolution, multi-disciplinary lithospheric scale data in order to understand the tectonic evolution and lithospheric structure of the western Mediterranean. The over-arching objectives of the magnetotelluric (MT) component of the projects are (i) to provide new electrical conductivity constraints on the crustal and lithospheric structure of the Atlas Mountains, and (ii) to test the hypotheses for explaining the purported lithospheric cavity beneath the Middle and High Atlas inferred from potential-field lithospheric modeling. We present the results of an MT experiment we carried out in Morocco along two profiles: an approximately N-S oriented profile crossing the Middle Atlas, the High Atlas and the eastern Anti-Atlas to the east (called the MEK profile, for Meknes) and NE-SW oriented profile through western High Atlas to the west (called the MAR profile, for Marrakech). Our results are derived from three-dimensional (3-D) MT inversion of the MT data set employing the parallel version of Modular system for Electromagnetic inversion (ModEM) code. The distinct conductivity differences between the Middle-High Atlas (conductive) and the Anti-Atlas (resistive) correlates with the South Atlas Front fault, the depth extent of which appears to be limited to the uppermost mantle (approx. 60 km). In all inverse solutions, the crust and the upper mantle show resistive signatures (approx. 1,000 Ωm) beneath the Anti-Atlas, which is the part of stable West African Craton. Partial melt and/or exotic fluids enriched in volatiles produced by the melt can account for the high middle to lower crustal and uppermost mantle conductivity in the Folded Middle Atlas, the High Moulouya Plain and the

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

    NASA Technical Reports Server (NTRS)

    Morgan, P.

    1984-01-01

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

  17. Slow Mid-Lithosphere Beneath the Mid-continent, USA

    NASA Astrophysics Data System (ADS)

    Bedle, H.; van der Lee, S.

    2005-12-01

    The Illinois basin is one of several basins within the North American craton. Despite years of study, the formational mechanisms of the basin remain largely unknown. While the crustal structure of the basin is well-defined from seismic studies, it remains unclear whether the basin is expressed in the sub-crustal structure; and, if the mantle structure provides clues towards an understanding the basin. We study the S-velocity structure of the upper-mantle using seismic tomography beneath the Illinois basin and its surrounding area. Since this area of the North American craton has been relatively stable since 1.4 Gy, significant deviations in S-velocities within the uppermost mantle are not expected. Nevertheless, a slower S-velocity in the upper-mantle of the Illinois basin was previously modeled in the 3D model NA04 (van der Lee and Frederiksen, 2005). To improve resolution and verify the existence of this seismically slow region, we fit seismic waveforms from 11 mid-continent events, using the methodology of Partitioned Waveform Inversion (Nolet, 1990). In doing so, we also incorporate constraints that are representative of the Illinois basin's crustal and sedimentary layering. The resultant 3D model, IL05, confirms the existence of a slow S-velocity structure in the uppermost mantle beneath this region. This anomalously slow region exists from the base of the crust to depths of ~100 km, and is slower than a cratonic average by about 200 m/s. The slow uppermost-mantle beneath the Illinois basin is then underlain by a faster lithosphere typical of the North American craton to depths of ~200 km. The coherency of this deeper cratonic lithosphere rules out a lithospheric delamination origin of the anomaly. The model IL05 points to a heterogeneous mantle beneath the mid-continent. Our imaging agrees with evidence of a heterogeneous mantle recently found in the form of sub-Moho reflectors from reprocessed seismic reflection surveys (McBride and Okure, 2004). The

  18. Lithospheric buoyancy and continental intraplate stresses

    USGS Publications Warehouse

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

    2003-01-01

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

  19. 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.

  20. Insolation driven variations of Mercury's lithospheric strength

    NASA Astrophysics Data System (ADS)

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

    2011-01-01

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

  1. The hydrothermal power of oceanic lithosphere

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

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

  2. A Physical Description of the Lithosphere and Seismic Low-Velocity Zone beneath Oahu: Perspectives from Hawaiian Mantle Xenoliths

    NASA Astrophysics Data System (ADS)

    Sen, G.; Bizimis, M.; Keshav, S.

    2003-12-01

    Mantle xenoliths provide a random sampling of a significant portion of the uppermost mantle beneath Oahu, including all of the lithosphere and the lithosphere-asthenosphere transition (LAT). We combine all the petrological, geochemical, and isotopic information on these xenoliths and evaluate existing models of the lithosphere and LAT beneath Oahu. The xenoliths are principally of two kinds - spinel peridotites and garenet clinopyroxenites; and they occur within the post-erosional Honolulu Volcanics (HV). Plagioclase peridotites are rare, and plagioclases in them are exotic, having crystallized from Koolau-like (enriched) magma. Trace element and Nd-Hf isotope ratios indicate recent metasomatic enrichment of the depleted peridotitic lithosphere by HV-like fluids. The pyroxenite suite xenoliths are dominantly composed of clinopyroxene and garnet with variable proportions of olivine, minor spinel and orthopyroxene, and rare phlogopite, carbonate, amphibole, and melt pockets. Phase equilibrium considerations indicate that these rocks form accumulates at the LAT and along the walls of fracture-conduits within the deep lithosphere. Nd-Sr isotope data on whole rocks and garnet and cpx separates show them to be depleted as well, partially overlapping with the host HV lavas and lithospheric spinel peridotite xenoliths. We view the Oahu lithosphere (below the crustal part) as follows: 20-30 km - depleted harzburgite with veins of plagioclase; 30 - 90 km - spinel peridotite layer (we model the vertical modal variation), 90 -130 km - cumulate garnet clinopyroxenites and "piclogites" with small amounts of (5 percent) intergranular H2O+CO2-rich fluids that may be of kimberlitic-carbonatite affinity. Our inferred compsition of the lithosphere and LAT are generally similar to geophysical models but predict a 200-250oC lower temperature within the LVZ beneath Oahu than the latter. In our model, Hawaiian plume-derived shield-stage magmas have little interaction with the lithosphere

  3. Os, Sr, Nd, and Pb isotope systematics of southern African peridotite xenoliths - Implications for the chemical evolution of subcontinental mantle

    NASA Technical Reports Server (NTRS)

    Walker, R. J.; Carlson, R. W.; Shirey, S. B.; Boyd, F. R.

    1989-01-01

    Isotope analyses of Os, Sr, Nd, and Pb elements were caried out on twelve peridotite xenoliths from the Jagersfontein, Letseng-la-terae, Thaba Patsoa, Mothae, and Premier kimberlites of southern Africa, to investigate the timing and the nature of melt extraction from the continental lithosphere and its relation to the continent formation and stabilization. The distinct Os and Pb isotopic characteristics found in these samples suggested that both the low- and the high-temperature peridotites reside in an ancient stable lithospheric 'keel' to the craton that has been isolated from chemical exchange with the sublithospheric mantle for time periods in excess of 2 Ga.

  4. Lithospheric stress patterns: A global view

    NASA Astrophysics Data System (ADS)

    Zoback, Mary Lou; Burke, Kevin

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

  5. Viscosity of the lithosphere of Enceladus

    NASA Technical Reports Server (NTRS)

    Passey, Q. R.

    1983-01-01

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

  6. Lithospheric structure in the Pacific geoid

    NASA Technical Reports Server (NTRS)

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

    1985-01-01

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

  7. 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

  8. Uppermantle anisotropy and the oceanic lithosphere

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.; Regan, J.

    1983-01-01

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

  9. A composite, isotopically-depleted peridotite and enriched pyroxenite source for Madeira magmas: Insights from olivine

    NASA Astrophysics Data System (ADS)

    Gurenko, Andrey A.; Geldmacher, Jörg; Hoernle, Kaj A.; Sobolev, Alexander V.

    2013-06-01

    relatively young (< 1 Ga) recycled oceanic crust. The isotopically depleted, DMM-like (peridotitic) component sampled by the younger post-erosional stage magmas is thought to reflect the ultramafic portion of the recycled oceanic lithosphere, whereas the HIMU-type pyroxenite of similar age to the peridotite is derived from recycled oceanic crust. Combining our new results with the data from the Canary hotspot, we conclude that the < 1 Ga recycled crustal component is common to both plume sources. The Canary volcanism also reflects the presence of older (> 1 Ga) recycled crust and a contribution from African subcontinental lithosphere.

  10. Lithospheric thinning beneath rifted regions of Southern California.

    PubMed

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

    2011-11-11

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

  11. Lithospheric Thickness Modeled From Long Period Surface Wave Dispersion

    NASA Astrophysics Data System (ADS)

    Pasyanos, M. E.

    2007-12-01

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

  12. Job Enrichment

    ERIC Educational Resources Information Center

    Sanders, Rick

    1970-01-01

    Job enrichment means giving people more decision-making power, more responsibility, more grasp of the totality of the job, and a sense of their own importance in the company. This article presents evidence of the successful working of this approach (Donnelly Mirrors), and the lack of success with an opposing approach (General Motors). (NL)

  13. Imaging Lithospheric Structure beneath the Indian continent

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  14. Magnetic mineralogy of the Mercurian lithosphere

    NASA Astrophysics Data System (ADS)

    Strauss, Becky; Feinberg, Joshua; Johnson, Catherine

    2016-04-01

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

  15. ETHNOPRED: a novel machine learning method for accurate continental and sub-continental ancestry identification and population stratification correction

    PubMed Central

    2013-01-01

    Background Population stratification is a systematic difference in allele frequencies between subpopulations. This can lead to spurious association findings in the case–control genome wide association studies (GWASs) used to identify single nucleotide polymorphisms (SNPs) associated with disease-linked phenotypes. Methods such as self-declared ancestry, ancestry informative markers, genomic control, structured association, and principal component analysis are used to assess and correct population stratification but each has limitations. We provide an alternative technique to address population stratification. Results We propose a novel machine learning method, ETHNOPRED, which uses the genotype and ethnicity data from the HapMap project to learn ensembles of disjoint decision trees, capable of accurately predicting an individual’s continental and sub-continental ancestry. To predict an individual’s continental ancestry, ETHNOPRED produced an ensemble of 3 decision trees involving a total of 10 SNPs, with 10-fold cross validation accuracy of 100% using HapMap II dataset. We extended this model to involve 29 disjoint decision trees over 149 SNPs, and showed that this ensemble has an accuracy of ≥ 99.9%, even if some of those 149 SNP values were missing. On an independent dataset, predominantly of Caucasian origin, our continental classifier showed 96.8% accuracy and improved genomic control’s λ from 1.22 to 1.11. We next used the HapMap III dataset to learn classifiers to distinguish European subpopulations (North-Western vs. Southern), East Asian subpopulations (Chinese vs. Japanese), African subpopulations (Eastern vs. Western), North American subpopulations (European vs. Chinese vs. African vs. Mexican vs. Indian), and Kenyan subpopulations (Luhya vs. Maasai). In these cases, ETHNOPRED produced ensembles of 3, 39, 21, 11, and 25 disjoint decision trees, respectively involving 31, 502, 526, 242 and 271 SNPs, with 10-fold cross validation accuracy of

  16. Carbonatite melt-peridotite interaction at 5.5-7.0 GPa: Implications for metasomatism in lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Sokol, Alexander G.; Kruk, Alexey N.; Chebotarev, Dimity A.; Palyanov, Yury N.

    2016-04-01

    Interaction between carbonatite melt and peridotite is studied experimentally by melting samples of interlayered peridotite-carbonatite-peridotite in graphite containers at 1200-1350 °C and 5.5-7.0 GPa in a split-sphere multianvil apparatus. Starting compositions are lherzolite and harzburgite, as well as carbonatite which may form in the upper part of a slab or in a plume-related source. Most experimental runs were of 150 h duration in order for equilibrium to be achieved. The interaction produced carbonatitic melts with low SiO2 (≤ 7 wt.%) and high alkalis. At 1200 °C, melt-peridotite interaction occurs through Mg-Ca exchange, resulting in elimination of orthopyroxene and crystallization of magnesite and clinopyroxene. At 1350 °C hybridization of the carbonatite and magnesite-bearing peridotite melts occurred with consumption of clinopyroxene and magnesite, and crystallization of orthopyroxene at MgO/CaO ≥ 4.3. The resulting peridotite-saturated melt has Ca# (37-50) depending on primary carbonatite composition. Compositions of silicate phases are similar to those of high-temperature peridotite but are different from megacrysts in kimberlites. CaO and Cr2O3 changes in garnet produced from the melt-harzburgite interaction at 1200 and 1350 °C perfectly match the observed trend in garnet from metasomatized peridotite of the Siberian subcontinental lithospheric mantle. K-rich carbonatite melts equilibrated with peridotite at 5.5-7.0 GPa and 1200-1350 °C correspond to high-Mg inclusions in fibrous diamond. Carbonatite melt is a weak solvent of entrained xenoliths and therefore cannot produce kimberlitic magma if temperatures are ~ 1350 °C on separation from the lithospheric peridotite source and ~ 1000 °C on eruption.

  17. Depth variation of seismic anisotropy and petrology in central European lithosphere: A tectonothermal synthesis from spinel lherzolite

    NASA Astrophysics Data System (ADS)

    Christensen, N. I.; Medaris, L. G., Jr.; Wang, H. F.; Jelínek, E.

    2001-01-01

    Spinel lherzolite xenoliths from the Neogene Kozákov volcano in central Europe, yielding temperatures from 680°C to 1065°C and estimated to originate from depths of 32 to 70 km, provide an exceptionally continuous record of the depth variation in seismic and petrological properties of subcontinental lithospheric mantle. Extraction depths of the xenoliths and thermal history and rheological properties of the mantle have been evaluated from a tectonothermal model for basaltic underplating associated with Neogene rifting. The chemical depletion of sub-Kozákov mantle decreases with depth, the Mg number in olivine decreasing from ˜91.4 to 90.5 and the Cr number in spinel, decreasing from ˜38.9 to 14.7. Texturally, the sampled mantle consists of an equigranular upper layer (32-43 km), an intermediate protogranular layer (43-67 km), and a lower equigranular layer (below 67 km). Olivine petrofabrics show strong axis concentrations, which change with depth from orthorhombic symmetry in the equigranular upper layer to axial symmetry in the lowermost layer. Calculated compressional and shear wave anisotropies, which average 8% and 6%, respectively, show significant depth trends that correlate with variations in depth of olivine fabric strengths and symmetries. Comparisons of the xenolith anisotropies with field observations of Pn anisotropy and SKS shear wave splitting in the region suggest that foliation is horizontal in the upper layer of the lithospheric mantle and vertical in the middle and lower layers. The depth variation in mantle properties and complexity in central Europe is the result of Devonian to Early Carboniferous convergence, continental accretion, and crustal thickening, followed by Late Carboniferous to Permian extension and gravitational collapse and final modification by Neogene rifting, thinning, and magmatic heating.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2005-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2006-12-01

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

  1. The anomalous lithium isotopic signature of Himalayan collisional zone carbonatites in western Sichuan, SW China: Enriched mantle source and petrogenesis

    NASA Astrophysics Data System (ADS)

    Tian, Shihong; Hou, Zengqian; Su, Aina; Qiu, Lin; Mo, Xuanxue; Hou, Kejun; Zhao, Yue; Hu, Wenjie; Yang, Zhusen

    2015-06-01

    Lithium concentrations and isotopic compositions of 38 carbonatites and associated syenites from the Maoniuping, Lizhuang, and Dalucao in western Sichuan, along with previously published and new Pb-Sr-Nd-C-O isotope data and whole-rock analyses, are used to constrain their mantle source and genesis. Carbonatites and syenites are characterized by extremely varying Li concentrations (0.8-120 ppm) and highly variable Li isotopic compositions (-4.5‰ to +10.8‰). Among them, the majority of the carbonatites and syenites have δ7Li values between +0.2‰ and +5.8‰, which overlap with the reported values for MORB and OIB; 3 carbonatites have higher δ7Li values between +8.7‰ and +10.8‰; 5 carbonatites and 4 syenites have lighter δ7Li values between -4.5‰ and -0.3‰. These highly variable δ7Li compositions could not have been produced by diffusive-driven isotopic fractionation of Li and thus may record the isotopic signature of the late Proterozoic subcontinental lithospheric mantle (SCLM). This paper demonstrates the existence of anomalous δ7Li within the late Proterozoic subcontinental lithospheric mantle, suggesting that the ancient SCLM beneath western Sichuan was modified by interaction with fluids derived from the subducted oceanic crust and marine sediments. The modeling curves of fluids derived from a dehydrated slab (ratios: AOC80-SED20 to AOC40-SED60) with a representative mantle composition can account for the majority of lithium compositional variations. Some samples with unusual Pb-Sr-Nd-O isotopic compositions and highly variable δ7Li compositions are affected by significant involvement of marine sediments in their source region, not contaminated by crustal materials. The carbonatites and syenites in western Sichuan were generated by the partial melting of subcontinental lithospheric mantle, which was metasomatized by the Li-rich fluids derived from the subducted oceanic crust and marine sediments. This melting was most likely triggered by a

  2. Mantle lithosphere fabrics around the TESZ

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

  3. Simulating temperature-dependent ecological processes at the sub-continental scale: male gypsy moth flight phenology as an example

    NASA Astrophysics Data System (ADS)

    Régnière, J.; Sharov, Alexei

    We simulated male gypsy moth flight phenology for the location of 1371 weather stations east of 100° W longitude and north of 35° N latitude in North America. The output of these simulations, based on average weather conditions from 1961 to 1990, was submitted to two map-interpolation methods: multiple regression and universal kriging. Multiple regression was found to be as accurate as universal kriging and demands less computing power. A map of the date of peak male gypsy moth flight was generated by universal kriging. This map itself constitutes a useful pest-management planning tool; in addition, the map delineates the potential range of the gypsy moth based on its seasonality at the northern edge of its current distribution in eastern North America. The simulation and map-interpolation methods described in this paper thus constitute an interesting approach to the study and monitoring of the ecological impacts of climate change and shifts in land-use patterns at the sub-continental level.

  4. Geochemistry of peridotite xenoliths in basalt from Hannuoba, eastern China: Implications for subcontinental mantle heterogeneity

    SciTech Connect

    Yan Song; Frey, F.A. )

    1989-01-01

    Based on geochemical studies of six anhydrous spinel peridotite xenoliths in basanite, the upper mantle beneath Hannuoba, eastern China is compositionally heterogeneous. These samples range in Sr and Nd isotopic ratios from MORB-like to near bulk-earth estimates. The low {sup 87}Sr/{sup 86}Sr and high {sup 143}Nd/{sup 144}Nd samples contain the largest amount of a basaltic component, but they are relatively depleted in light rare earth elements compared to chondrites. Other samples have U-shaped chondrite-normalized REE patterns. Trace element and radiogenic isotopic data require enrichment processes acting on depleted mantle. Constraints on these processes are: (a) inverse correlations between basaltic constituents, such as CaO and Al{sub 2}O{sub 3}, and La/Sm; and, (b) samples most depleted in CaO and Al{sub 2}O{sub 3} have the highest {sup 87}Sr/{sup 86}Sr and lowest {sup 143}Nd/{sup 144}Nd. These trends can be explained by a model whereby garnet peridotite zoned in isotopic composition undergoes partial melting. Because of a gradient in degree of melting, e.g., from the wall-rock contact to hotter interior, or as a function of depth in a diapir, melts initially segregate from regions where the degree of melting is high. Subsequently, the recently created residues are infiltrated by slower segregating incipient melts. Preferential mixing of these incipient melts with residues from high degrees of melting can explain the observed complex geochemical trends seen in Hannuoba and many other peridotite xenolith suites. Clinopyroxene-rich veins in some of the peridotites may reflect pathways of ascending melt.

  5. 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

  6. 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.

  7. The electrical Lithosphere of the Alboran Domain

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  8. Flexure and rheology of Pacific oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Hunter, Johnny; Watts, Tony

    2016-04-01

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

  9. 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.

  10. Adakites from collision-modified lithosphere

    NASA Astrophysics Data System (ADS)

    Haschke, M.; Ben-Avraham, Z.

    2005-08-01

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

  11. Fennoscandian lithosphere - electromagnetic and seismic constraints

    NASA Astrophysics Data System (ADS)

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

    2009-04-01

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

  12. Thermal classification of lithospheric discontinuities beneath USArray

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  13. Evolution of the lithosphere beneath Oahu, Hawaii: rare earth element abundances in mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Sen, Gautam; Frey, Frederick A.; Shimizu, Nobumichi; Leeman, William P.

    1993-08-01

    Rare earth element contents of clinopyroxenes in Hawaiian mantle xenoliths from Oahu were determined with an ion microprobe. The analyzed xenoliths are from four vents of the alkali Honolulu Volcanics (HV). Three (Kaau, Pali and Kalihi—KPK) are located close to the caldera of the extinct Koolau shield volcano, and the fourth, Salt Lake Crater (SLC), is on the periphery of the shield volcano. Systematic differences exist in REE contents between clinopyroxenes of the KPK and SLC xenoliths: (1) KPK pyroxenes are typically zoned in REE contents whereas SLC pyroxenes are homogeneous, (2) the LREE-depleted (chondrite-normalized) patterns that characterize many of the KPK xenoliths are not found in SLC xenoliths, and (3) the convex-upward REE patterns that are characteristic of SLC xenoliths are not found in KPK xenoliths. Relative to abyssal peridotites, the LREE-depleted Hawaiian lherzolite pyroxenes (interpreted to be residual oceanic lithosphere) have higher contents of REE, Na 2O, TiO 2 and FeO, and more modal clinopyroxene. These LREE-depleted Hawaiian xenoliths represent deeper, less-depleted parts of the melting column, whereas the abyssal peridotites represent the uppermost, more strongly depleted part of the mantle. The spoon-shaped, LREE-enriched and convex-upward REE patterns in the xenoliths have resulted from metasomatic enrichment of the lithosphere caused by reaction with magmas that formed the Honolulu Volcanics. A model for the evolution of the oceanic lithosphere is presented in which fractures were the main mode of transport of the Honolulu Volcanics. Metasomatic enrichment resulted from interaction between percolating Honolulu Volcanics magmas and wallrock. The differences between SLC and KPK xenoliths are attributed to chromatographic fractionation effects: SLC xenoliths are postulated to have come from a greater depth where they equilibrated to a larger extent with the percolating magmas than the KPK rocks.

  14. Lithospheric Thickness Modeled from Long Period Surface Wave Dispersion

    SciTech Connect

    Pasyanos, M E

    2008-05-15

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

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

    NASA Astrophysics Data System (ADS)

    Pasyanos, Michael E.

    2010-01-01

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

  16. Boron cycling by subducted lithosphere; insights from diamondiferous tourmaline from the Kokchetav ultrahigh-pressure metamorphic belt

    NASA Astrophysics Data System (ADS)

    Ota, Tsutomu; Kobayashi, Katsura; Kunihiro, Takuya; Nakamura, Eizo

    2008-07-01

    Subduction of lithosphere, involving surficial materials, into the deep mantle is fundamental to the chemical evolution of the Earth. However, the chemical evolution of the lithosphere during subduction to depth remains equivocal. In order to identify materials subjected to geological processes near the surface and at depths in subduction zones, we examined B and Li isotopes behavior in a unique diamondiferous, K-rich tourmaline (K-tourmaline) from the Kokchetav ultrahigh-pressure metamorphic belt. The K-tourmaline, which includes microdiamonds in its core, is enriched in 11B relative to 10B (δ 11B = -1.2 to +7.7) and 7Li relative to 6Li (δ 7Li = -1.1 to +3.1). It is suggested that the K-tourmaline crystallized at high-pressure in the diamond stability field from a silicate melt generated at high-pressure and temperature conditions of the Kokchetav peak metamorphism. The heavy isotope signature of this K-tourmaline differs from that of ordinary Na-tourmalines in crustal rocks, enriched in the light B isotope (δ 11B = -16.6 to -2.3), which experienced isotope fractionation through metamorphic dehydration reactions. A possible source of the heavy B-isotope signature is serpentine in the subducted lithospheric mantle. Serpentinization of the lithospheric mantle, with enrichment of heavy B-isotope, can be produced by normal faulting at trench-outer rise or trench slope regions, followed by penetration of seawater into the lithospheric mantle. Serpentine breakdown in the lithospheric mantle subducted in subarc regions likely provided fluids with the heavy B-isotope signature, which was acquired during the serpentinization prior to subduction. The fluids could ascend and cause partial melting of the overlying crustal layer, and the resultant silicate melt could inherit the heavy B-isotope signature. The subducting lithospheric mantle is a key repository for modeling the flux of fluids and associated elements acquired at a near the surface into the deep mantle.

  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. 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.

  19. 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

    of this Mg-rich ancient SCLM relative to the asthenosphere, results in the persistence today of low-density, rheologically coherent Archean domains (including relict blobs in rifted ocean basins and commonly, preservation of old crustal domains (the 'life-raft' model)). Secondly, the enduring (and volumetrically dominating) Archean lithospheric mantle domains represent a reservoir for metasomatic enrichment over their 3.5 billion year history, creating a potentially metallogenicalally ly fertile mantle impregnated with critical elements (e.g. Au, Cu, Ni? and platinum group elements). Thirdly, the formation of Archean cratons provided an architectural lithospheric mantle-scape of regions with contrasting rheology, composition and depth penetration. The cohesive Archean domains control magma and fluid pathways around their margins, and may act as both sinks and sources for element exchange; they may explain the occurrence of basaltic magmas with the geochemical signatures of ancient lithospheric components.

  20. LAB and other lithospheric discontinuities below Cratons

    NASA Astrophysics Data System (ADS)

    Sodoudi, Forough

    2013-04-01

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

  1. Les xénolites ultramafiques du volcanisme alcalin quaternaire d'Oranie (Tell, Algérie occidentale), témoins d'une lithosphère cisaillée et enrichieUltramafic xenoliths from Quaternary alkali volcanism from Oranie (Tell, western Algeria): witnesses of a sheared and enriched lithosphere

    NASA Astrophysics Data System (ADS)

    Zerka, Mohamed; Cottin, Jean-Yves; Grégoire, Michel; Lorand, Jean-Pierre; Megartsi, M'Hamed; Midoun, Mohamed

    Numerous ultramafic xenoliths occur within the Aı̈n-Temouchent volcanic complex (Northwestern Oranie, Algeria). Most of them are type I mantle tectonites (lherzolites and harzburgites) and composite xenoliths (harzburgite/clinopyroxenite) are rare. Only a few samples of spinel lherzolites display relatively fertile compositions when the major part of type I xenoliths have refractory major element compositions but enriched LREE contents showing that they have been affected by mantle metasomatism. The composite xenoliths are witnesses of reactions of alkaline magmas with the upper mantle. An asthenospheric rising, in relation with the large strike slip fault affecting the North African plate margin at Trias time is proposed as a possible geodynamical setting. To cite this article: M. Zerka et al., C. R. Geoscience 334 (2002) 387-394.

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

    Helium isotopic ratios (3He/4He) observed in 25 mineral springs and wells above the Cascadia forearc provide a marker for fluids derived from Juan de Fuca lithosphere. This exploratory study documents a significant component of mantle-derived helium within forearc springs and wells, and in turn, documents variability in helium enrichment across the Cascadia forearc. Sample sites arcward of the forearc mantle corner generally yield significantly higher ratios (˜1.2-4.0 RA) than those seaward of the corner (˜0.03-0.7 RA). 3He detected above the inner forearc mantle wedge may represent a mixture of both oceanic lithosphere and forearc mantle sources, whereas 3He detected seaward of the forearc mantle corner likely has only an oceanic source. The highest ratios in the Cascadia forearc coincide with slab depths (˜40-45 km) where metamorphic dehydration of young oceanic lithosphere is expected to release significant fluid and where tectonic tremor occurs, whereas little fluid is expected to be released from the slab depths (˜25-30 km) beneath sites seaward of the corner. These observations provide independent evidence that tremor is associated with deep fluids, and further suggest that high pore pressures associated with tremor may serve to keep fractures open for 3He migration through the ductile upper mantle and lower crust.

  3. Cr-pyrope garnets in the lithospheric mantle 2. Compositional populations and their distribution in time and space

    NASA Astrophysics Data System (ADS)

    Griffin, W. L.; Fisher, N. I.; Friedman, J. H.; O'Reilly, Suzanne Y.; Ryan, C. G.

    2002-12-01

    Three novel statistical approaches (Cluster Analysis by Regressive Partitioning [CARP], Patient Rule Induction Method [PRIM], and ModeMap) have been used to define compositional populations within a large database (n > 13,000) of Cr-pyrope garnets from the subcontinental lithospheric mantle (SCLM). The variables used are the major oxides and proton-microprobe data for Zn, Ga, Sr, Y, and Zr. Because the rules defining these populations (classes) are expressed in simple compositional variables, they are easily applied to new samples and other databases. The classes defined by the three methods show strong similarities and correlations, suggesting that they are statistically meaningful. The geological significance of the classes has been tested by classifying garnets from 184 mantle-derived peridotite xenoliths and from a smaller database (n > 5400) of garnets analyzed for >20 trace elements by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICPMS). The relative abundances of these classes in the lithospheric mantle vary widely across different tectonic settings, and some classes are absent or very rare in either Archean or Phanerozoic SCLM. Their distribution with depth also varies widely within individual lithospheric sections and between different sections of similar tectonothermal age. These garnet classes therefore are a useful tool for mapping the geology of the SCLM. Archean SCLM sections show high degrees of depletion and varying degrees of metasomatism, and they are commonly strongly layered. Several Proterozoic SCLM sections show a concentration of more depleted material near their base, grading upward into more fertile lherzolites. The distribution of garnet classes reflecting low-T phlogopite-related metasomatism and high-T melt-related metasomatism suggests that many of these Proterozoic SCLM sections consist of strongly metasomatized Archean SCLM. The garnet-facies SCLM beneath Phanerozoic terrains is only mildly depleted

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

    SciTech Connect

    Pasyanos, M E

    2009-11-19

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

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

    PubMed

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

    2004-07-30

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

  6. LA-ICP MS zircon dating, whole-rock and Sr-Nd-Pb-O isotope geochemistry of the Camiboğazı pluton, Eastern Pontides, NE Turkey: Implications for lithospheric mantle and lower crustal sources in arc-related I-type magmatism

    NASA Astrophysics Data System (ADS)

    Kaygusuz, Abdullah; Arslan, Mehmet; Siebel, Wolfgang; Sipahi, Ferkan; İlbeyli, Nurdane; Temizel, İrfan

    2014-04-01

    Late Cretaceous I-type plutons are widespread in the Eastern Pontides, NE Turkey. The studied Camiboğazı pluton is a composite pluton consisting of diorite, tonalite, monzodiorite, monzonite, quartz monzonite, granite, and mafic microgranular enclaves (MMEs). Laser ablation ICP-MS U-Pb dating of zircon yielded crystallization ages of 76.21 ± 0.79 Ma, 75.65 ± 0.50 Ma, 75.04 ± 0.83 Ma, and 74.73 ± 0.86 Ma for diorite, monzodiorite, monzonite, and granite, respectively. The rocks of the pluton have I-type, high-K to shoshonitic and metaluminous character, displaying whole-rock geochemical features of arc-related granites. They are enriched in large-ion lithophile and light rare-earth elements, and depleted in high-field-strength elements. Major element variations can be attributed to fractionation of plagioclase, clinopyroxene, hornblende, and Fe-Ti oxides. The rocks show considerable variation in 87Sr/86Sr(i) (0.70498 to 0.70622), ɛNd(i) (- 2.79 to - 0.36), δ18O values (+ 6.3 to + 11.4) and Nd model ages (TDM) (0.81 Ga to 1.26 Ga). Besides, they have (206Pb/204Pb) = 18.44-19.09, (207Pb/204Pb) = 15.64-15.69, and (208Pb/204Pb) = 38.37-38.89. Although isotope signatures of the mafic microgranular enclaves (MMEs) (87Sr/86Sr(i) = 0.70551 to 0.70622; ɛNd(i) = - 2.9 to - 1.23; δ18O = + 8.3 to + 9.7) are largely similar to the host rocks, MMEs are characterized by relatively high Mg-numbers (32-36), low contents of SiO2 (52-56 wt.%) and low ASI (0.7-0.9). Estimated crystallization temperatures for the rocks of the pluton range from 735 ± 58 °C to 844 ± 24 °C and a shallow intrusion depth (< 10 km) is estimated from Al-in-hornblende thermobarometry. Whole-rock geochemical and isotopic data suggest magma generation by dehydration melting of an amphibolite-type lower crustal component with additional input of a subcontinental lithospheric mantle component. Furthermore, Sr-Nd isotope mixing model reveals ~ 30% to 40% lower crustal magma contribution to the mantle

  7. Deep magmatism alters and erodes lithosphere and facilitates decoupling of Rwenzori crustal block

    NASA Astrophysics Data System (ADS)

    Wallner, Herbert; Schmeling, Harro

    2013-04-01

    The title is the answer to the initiating question "Why are the Rwenzori Mountains so high?" posed at the EGU 2008. Our motivation origins in the extreme topography of the Rwenzori Mountains. The strong, cold proterozoic crustal horst is situated between rift segments of the western branch of the East African Rift System. Ideas of rift induced delamination (RID) and melt induced weakening (MIW) have been tested with one- and two-phase flow physics. Numerical model parameter variations and new observations lead to a favoured model with simple and plausible definitions. Results coincide in the scope of their comparability with different observations or vice versa reduce ambiguity and uncertainties in model input. Principle laws of the thermo-mechanical physics are the equations of conservation of mass, momentum, energy and composition for a two-phase (matrix-melt) system with nonlinear rheology. A simple solid solution model determines melting and solidification under consideration of depletion and enrichment. The Finite Difference Method with markers is applied to visco-plastic flow using the streamfunction in an Eulerian formulation in 2D. The Compaction Boussinesq and the high Prandtl number Approximation are employed. Lateral kinematic boundary conditions provide long-wavelength asthenospheric upwelling and extensional stress conditions. Partial melts are generated in the asthenosphere, extracted above a critical fraction, and emplaced into a given intrusion level. Temperature anomalies positioned beneath the future rifts, the sole specialization to the Rwenzori situation, localize melts which are very effective in weakening the lithosphere. Convection patterns tend to generate dripping instabilities at the lithospheric base; multiple slabs detach and distort uprising asthenosphere; plumes migrate, join and split. In spite of appearing chaotic flow behaviour a characteristic recurrence time of high velocity events (drips, plumes) emerges. Chimneys of increased

  8. 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

  9. 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

  10. Study of the time evolution of the lithosphere

    NASA Technical Reports Server (NTRS)

    Roufosse, M. C.

    1983-01-01

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

  11. Transient creep and convective instability of the lithosphere

    NASA Astrophysics Data System (ADS)

    Birger, Boris I.

    2012-12-01

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

  12. Lithospheric models of the North American continent

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  13. Global equivalent magnetization of the oceanic lithosphere

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

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

    NASA Technical Reports Server (NTRS)

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

    2014-01-01

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

  15. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  16. Lithospheric Mantle Deformation beneath the Indian Cratons.

    PubMed

    Pandey; Agrawal

    1999-11-01

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

  17. Mantle Response to Collision, Slab Breakoff & Lithospheric Tearing in Anatolian Orogenic Belts, and Cenozoic Geodynamics of the Aegean-Eastern Mediterranean Region

    NASA Astrophysics Data System (ADS)

    Dilek, Yildirim; Altunkaynak, Safak

    2010-05-01

    The geochemical and temporal evolution of the Cenozoic magmatism in the Aegean, Western Anatolian and peri-Arabian regions shows that plate tectonic events, mantle dynamics, and magmatism were closely linked in space and time. The mantle responded to collision-driven crustal thickening, slab breakoff, delamination, and lithospheric tearing swiftly, within geologically short time scales (few million years). This geodynamic continuum resulted in lateral mantle flow, whole-sale extension and accompanying magmatism that in turn caused the collapse of tectonically and magmatically weakened orogenic crust. Initial stages of post-collisional magmatism (~45 Ma) thermally weakened the orogenic crust in Tethyan continental collision zones, giving way into large-scale extension and lower crustal exhumation via core complex formation starting around 25-23 Ma. Slab breakoff was the most common driving force for the early stages of post-collisional magmatism in the Tethyan mountain belts in the eastern Mediterranean region. Magmatic rocks produced at this stage are represented by calc-alkaline-shoshonitic to transitional (in composition) igneous suites. Subsequent lithospheric delamination or partial convective removal of the sub-continental lithospheric mantle in collision-induced, overthickened orogenic lithosphere caused decompressional melting of the upwelling asthenosphere that in turn resulted in alkaline basaltic magmatism (<12 Ma). Attendant crustal extension and widespread thinning of the lithosphere facilitated rapid ascent of basaltic (OIB) magmas without much residence time in the crust and hence the eruption of relatively uncontaminated, asthenosphere-derived magmas at the surface (i.e. Kula lavas in SW Anatolia). Subduction of the Tethyan mantle lithosphere northward beneath Eurasia was nearly continuous since the latest Cretaceous, only temporarily punctuated by the collisional accretion of several ribbon continents (i.e. Pelagonia, Sakarya, Tauride-South Armenian

  18. The Subcontinental Surge

    PubMed Central

    CARROLL, JOHN

    2004-01-01

    Biotech is following big pharma to India, where R&D costs are a fraction of those in the U.S. India itself is working hard to make it happen. Ultimately, this could reduce the price of biotech therapies. But what about India’s reputation for disregarding intellectual property rights? PMID:23393438

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

    SciTech Connect

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

    1992-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Zhang, H.; Ravat, D.

    2015-12-01

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

  1. 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

  2. Perennial plate tectonics with lasting mantle lithosphere scars

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Birger, Boris I.

    2013-11-01

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

  4. Cratonic lithosphere: an electrifying view (Invited)

    NASA Astrophysics Data System (ADS)

    Jones, A. G.

    2013-12-01

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

  5. Electromagnetic imaging of lithosphere permeable zones

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

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

    NASA Astrophysics Data System (ADS)

    Chuvashova, Irina; Sun, Yi-min

    2016-04-01

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

  7. Juvenile psittacine environmental enrichment.

    PubMed

    Simone-Freilicher, Elisabeth; Rupley, Agnes E

    2015-05-01

    Environmental enrichment is of great import to the emotional, intellectual, and physical development of the juvenile psittacine and their success in the human home environment. Five major types of enrichment include social, occupational, physical, sensory, and nutritional. Occupational enrichment includes exercise and psychological enrichment. Physical enrichment includes the cage and accessories and the external home environment. Sensory enrichment may be visual, auditory, tactile, olfactory, or taste oriented. Nutritional enrichment includes variations in appearance, type, and frequency of diet, and treats, novelty, and foraging. Two phases of the preadult period deserve special enrichment considerations: the development of autonomy and puberty. PMID:25902270

  8. Lithospheric Instabilities within the Northern Basin and Range Province

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  9. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  10. Limiting depth of magnetization in cratonic lithosphere

    NASA Technical Reports Server (NTRS)

    Toft, Paul B.; Haggerty, Stephen E.

    1988-01-01

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

  11. Moho, seismogenesis, and rheology of the lithosphere

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  12. Moho, Seismogenesis, and Rheoloy of the Lithosphere

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  13. Lithospheric Architecture of the Hudson Bay Region

    NASA Astrophysics Data System (ADS)

    Eaton, D.; Darbyshire, F.

    2009-05-01

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

  14. 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

  15. Composition and structure of the lithospheric mantle beneath NE Iran: Constraints from mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Su, Ben-Xun; Chung, Sun-Lin; Zarrinkoub, Mohammad Hossein; Pang, Kwan-Nang; Chen, Ling; Ji, Wei-Qiang; Brewer, Aaron; Ying, Ji-Feng; Khatib, Mohammad Mahdi

    2014-08-01

    A detailed study on petrology and mineral chemistry of 32 mantle xenoliths has been conducted to decipher the physical and chemical characteristics of the lithosphere beneath NE Iran. Spinel lherzolite, the most abundant xenolith type, is made up of olivine, orthopyroxene, clinopyroxene, and spinel. Clinopyroxenes in the spinel lherzolites display a primitive mantle-like composition, typical of non-cratonic peridotites. Pyroxenite, another major xenolith type, shows equilibrated textures and highly variable compositions including olivine websterite, websterite and clinopyroxenite. These pyroxenites, together with an equigranular dunite, delineate a clear metasomatic trend, characterized by systematic Mg#, Cr#, Al2O3, and TiO2 variations in the constituent minerals, coupled with light rare earth element enrichment and high field strength element depletion in clinopyroxene. The pyroxenites are therefore suggested to have formed by the interaction between garnet-bearing peridotites within the lithospheric mantle and melts from a stagnant slab within the asthenosphere. The lithospheric mantle may have undergone multiple stages of partial melting. The earliest stage, evidenced by the equigranular dunite, resulted in significant NiO depletion in olivine, low Al2O3 and TiO2 coupled with high Mg# and Cr# in clinopyroxene, and high Cr# in spinel. The second stage occurred more widely and gave rise to the large ion lithophile element depletion in clinopyroxenes of all rock types. The extent of melting is lower in the spinel lherzolites than that in the pyroxenites, implying that the partial melting was not caused by decompression and thus most likely related to Tethyan subduction. A third and more recent melting stage, responsible for the spongy texture in some clinopyroxenes, is attributed to the extensional tectonic regime that started in the middle Miocene in the region. Temperature estimates show that both the spinel lherzolites and pyroxenites equilibrated at ~ 900

  16. 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

  17. Tectonic determinations of lithospheric thickness on Ganymede and Callisto

    NASA Technical Reports Server (NTRS)

    Croft, S. K.

    1985-01-01

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

  18. The Lithospheric Structure of Southern Africa from Magnetotelluric Sounding

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  19. SEASAT observations of lithospheric flexure seaward of trenches

    NASA Technical Reports Server (NTRS)

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

    1983-01-01

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

  20. Lithospheric structure of Venus from gravity and topography

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

  1. Rejuvenation of the lithosphere by the Hawaiian plume.

    PubMed

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

    2004-02-26

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

  2. 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.

  3. Metasomatized lithospheric mantle beneath Turkana depression in southern Ethiopia (the East Africa Rift): geochemical and Sr-Nd-Pb isotopic characteristics

    NASA Astrophysics Data System (ADS)

    Meshesha, Daniel; Shinjo, Ryuichi; Matsumura, Risa; Chekol, Takele

    2011-11-01

    Mantle xenoliths entrained in Quaternary alkaline basalts from the Turkana Depression in southern Ethiopia (the East Africa Rift) were studied for their geochemical and Sr-Nd-Pb isotopic compositions to constrain the evolution of the lithosphere. The investigated mantle xenoliths are spinel lherzolites in composition with a protogranular texture. They can be classified into two types: anhydrous and hydrous spinel lherzolites; the latter group characterized by the occurrences of pargasite and phlogopite. The compositions of whole-rock basaltic component (CaO = 3.8-5.6 wt%, Al2O3 = 2.5-4.1 wt%, and MgO = 34.7-38.1 wt%), spinel (Cr# = 0.062-0.117, Al2O3 = 59.0-64.4 wt%) and clinopyroxene (Mg# = 88.4-91.7, Al2O3 = 5.2-6.7 wt%) indicate that the lherzolites are fertile and have not experienced significant partial melting. Both types are characterized by depleted 87Sr/86Sr (0.70180-0.70295) and high 143Nd/144Nd (0.51299-0.51348) with wide ranges of 206Pb/204Pb (17.86-19.68) isotopic compositions. The variations of geochemical and isotopic compositions can be explained by silicate metasomatism induced by different degree of magma infiltrations from ascending mantle plume. The thermobarometric estimations suggest that the spinel lherzolites were derived from depths of 50-70 km (15.6-22.2 kb) and entrained in the alkaline magma at 847-1,052°C. Most of the spinel lherzolites from this study record an elevated geotherm (60-90 mW/m2) that is related to the presence of rising mantle plume in an active tectonic setting. Sm-Nd isotopic systematic gives a mean TDM model age of 0.95 Ga, interpreted as the minimum depletion age of the subcontinental lithosphere beneath the region.

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

    NASA Astrophysics Data System (ADS)

    Darbyshire, F. A.

    2008-12-01

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

  5. Re-Os isotopic evidence for an enriched-mantle source for the Noril'sk-type, ore-bearing intrusions, Siberia

    USGS Publications Warehouse

    Walker, R.J.; Morgan, J.W.; Horan, M.F.; Czamanske, G.K.; Krogstad, E.J.; Fedorenko, V.A.; Kunilov, V.E.

    1994-01-01

    Magmatic Cu-Ni sulfide ores and spatially associated ultramafic and mafic rocks from the Noril'sk I, Talnakh, and Kharaelakh intrusions are examined for Re-Os isotopic systematics. Neodymium and lead isotopic data also are reported for the ultramafic and mafic rocks. The Re-Os data for most samples indicate closed-system behavior since the ca. 250 Ma igneous crystallization age of the intrusions. There are small but significant differences in the initial osmium isotopic compositions of samples from the three intrusions. Ores from the Noril'sk I intrusion have ??Os values that vary from +0.4 to +8.8, but average +5.8. Ores from the Talnakh intrusion have ??Os values that range from +6.7 to +8.2, averaging +7.7. Ores from the Kharaelakh intrusion have ??Os values that range from +7.8 to +12.9, with an average value of +10.4. The osmium isotopic compositions of the ore samples from the Main Kharaelakh orebody exhibit minimal overlap with those for the Noril'sk I and Talnakh intrusions, indicating that these Kharaelakh ores were derived from a more radiogenic source of osmium than the other ores. Combined osmium and lead data for major orebodies in the three intrusions plot in three distinct fields, indicating derivation of osmium and lead from at least three isotopically distinct sources. Some of the variation in lead isotopic compositions may be the result of minor lower-crustal contamination. However, in contrast to most other isotopic and trace element data, Os-Pb variations are generally inconsistent with significant crustal contamination or interaction with the subcontinental lithosphere. Thus, the osmium and lead isotopic compositions of these intrusions probably reflect quite closely the compositions of their mantle source, and suggest that these two isotope systems were insensitive to lithospheric interaction. Ultramafic and mafic rocks have osmium and lead isotopic compositions that range only slightly beyond the compositions of the ores. These rocks also

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

    NASA Astrophysics Data System (ADS)

    Liu, S.; Wang, L.

    2009-04-01

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

  7. Late Mesozoic-Cenozoic Evolution of the North American Cordillera: Lithospheric Response to Plume-Slab Interaction

    NASA Astrophysics Data System (ADS)

    Ihinger, P. D.; Watkins, J. M.; Bernhardt, J. E.; Johnson, B. R.

    2003-12-01

    Throughout the late Mesozoic and Cenozoic eras, western North America experienced widespread deformation and volcanism. The nature and extent of this activity does not fit conveniently into the plate tectonic framework for crustal evolution. To date, there is no satisfying explanation that integrates the Laramide Orogeny, Basin and Range extensional activity, and the rise of the Colorado Plateau with the voluminous Tertiary volcanism including Eocene high-K magmatism, the mid-Tertiary ignimbrite `flare-up' in and around the Great Basin, and the more recent out-pouring of the Columbia River flood basalts (CRB). In fact, the unusual spatial and temporal relationship between the CRB and the on-going time-progressive basaltic and rhyolitic volcanism associated with the Yellowstone hot-spot track has led some researchers to question whether upwelling plumes do, indeed, exist. We note that magmas produced in western North America throughout the Cenozoic were derived from two compositionally distinct source regions: the sub-continental lithosphere (representing >95% of the magmatism) and the OIB source region (<5%). The two magma types occur at the same place and time throughout the province. We propose that the impingement of the positively buoyant plume head of the Yellowstone hot spot with the underside of the negatively buoyant, subducting Farallon plate at 80 My is responsible for the widespread Tertiary deformation and magmatic activity in the North American Cordillera. The dynamics of mantle flow required to accommodate the mutual passing of the two bodies led to a crisis in the normal mantle flow regime. Upwelling of the plume head was accommodated by shallowing both the Farallon slab and the overlying mantle wedge. This activity drove the subsequent tectonics of the over-riding continental lithosphere for nearly 80 My. We argue that this model is consistent with existing geological, geochemical and geophysical observations of the Cordillera; it provides a new

  8. Olympus Mons shaped by lithospheric flexure

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

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

  9. Lithospheric loading and tectonics of the lunar irregular maria

    NASA Technical Reports Server (NTRS)

    Hall, J. Lynn; Solomon, Sean C.

    1987-01-01

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

  10. Application of MAGSAT to lithospheric modeling in South America

    NASA Technical Reports Server (NTRS)

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

    1983-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Kotsiaros, Stavros; Olsen, Nils; Finlay, Christopher

    2016-07-01

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

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

    NASA Technical Reports Server (NTRS)

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

    1976-01-01

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

  13. Lithospheric Loading by the Northern Polar CAP on Mars

    NASA Astrophysics Data System (ADS)

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

    1999-03-01

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

  14. Lithospheric thermal structure and rheology of the eastern China

    NASA Astrophysics Data System (ADS)

    Wang, Yang; Cheng, Su-Hua

    2012-03-01

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

  15. Lithospheric Stress and Deformation (Paper 7R0323)

    NASA Astrophysics Data System (ADS)

    McNutt, Marcia

    1987-07-01

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

  16. The effects of strain heating in lithospheric stretching models

    NASA Technical Reports Server (NTRS)

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

    1985-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-10-01

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

  18. Beyond Job Enrichment to Employment Enrichment

    ERIC Educational Resources Information Center

    Werther, William B., Jr.

    1975-01-01

    Employment enrichment views the total work environment confronting employees as a system consisting of two overlapping areas: worker-job and worker-organization subsystems. Job enrichment has improved the worker-job subsystem. The focus of this article is on methods of improving the worker-organization relationship. (Author/JB)

  19. Swarm magnetic gradients for lithospheric modelling (SLIM)

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  20. Lithospheric imaging via teleseismic scattering tomography

    NASA Astrophysics Data System (ADS)

    Frederiksen, A. W.; Revenaugh, J.

    2004-12-01

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

  1. Influence of sediment deposition on deep lithospheric tectonics

    NASA Astrophysics Data System (ADS)

    Gray, R.; Pysklywec, R. N.

    2012-06-01

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

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

    USGS Publications Warehouse

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

    2001-01-01

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

  3. Derived enriched uranium market

    SciTech Connect

    Rutkowski, E.

    1996-12-01

    The potential impact on the uranium market of highly enriched uranium from nuclear weapons dismantling in the Russian Federation and the USA is analyzed. Uranium supply, conversion, and enrichment factors are outlined for each country; inventories are also listed. The enrichment component and conversion components are expected to cause little disruption to uranium markets. The uranium component of Russian derived enriched uranium hexafluoride is unresolved; US legislation places constraints on its introduction into the US market.

  4. 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.

  5. Lithosphere-Atmosphere-Ionosphere coupling model

    NASA Astrophysics Data System (ADS)

    Kachakhidze, M. K., III

    2015-12-01

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

  6. 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. Lithospheric thermal and strength model of the Arctic region

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  8. Seismically Imaging the Destruction of Continental Lithosphere beneath Afar

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  9. 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

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

    NASA Astrophysics Data System (ADS)

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

    2013-08-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2005-12-01

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

  12. Implications of Melt Intrusion Levels in Heated Lithosphere Under Continental Extension Conditions

    NASA Astrophysics Data System (ADS)

    Wallner, H.; Schmeling, H.

    2012-12-01

    Beneath continents under extension melts are generated in the asthenosphere and emplaced at shallow depth into a given intrusion level. These melts are very effective in weakening the lithosphere. The vertical position of this emplacement zone and its effect on the resulting rift evolution is subject of our numerical exploration. Our motivation roots in the geodynamically extreme situation of the Rwenzori Mountains. The old, stiff and cold crustal horst is located inside the western branch of the East African Rift System and it is conspicuous due to elevations above 5 km relative to its locale size of 50 x 120 km. We proposed RID (rift induced delamination) as an explaining process applying one-phase physics Wallner & Schmeling 2010, 2011). A two-phase approach employing a sophisticated concept of MIW (melt induced weakening, Schmeling & Wallner 2012) provides a more realistic and self-consistent process, according to observations, indicating partial melts and an asthenospheric magma source. Instead of a strong ad hoc initial temperature anomaly, moderate temperatures and anomalies can be used. Thermo-mechanical physics of visco-plastic flow is approximated by Finite Difference Method in an Eulerian formulation in 2D. The equations of conservation of mass, momentum and energy are solved for a multi component (crust-mantle) and two phase (melt-matrix) system. Rheology is temperature-, pressure-, and stress-dependent. In consideration of depletion and enrichment melting and solidification are controlled by a simplified linear binary solid solution model. Extension is modeled by a lateral outflow boundary condition of 1 mm/a (half drift rate). The temperature background level with respect to the melting curve, especially in the asthenosphere, is important; an increase of about 100 K intensifies convection and lets the lithosphere-asthenosphere boundary rise up. Additional temperature anomalies in the asthenosphere focuses uprising melt batches; their amplitudes

  13. Lithospheric structure and compensation mechanisms of the Galapagos Archipelago

    NASA Astrophysics Data System (ADS)

    Feighner, Mark A.; Richards, Mark A.

    1994-04-01

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

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

    SciTech Connect

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

    1989-01-10

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

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

    NASA Astrophysics Data System (ADS)

    Denlinger, Roger P.; Savage, William Z.

    1989-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Grose, Christopher J.; Afonso, Juan Carlos

    2013-09-01

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

  17. Circum-Arctic lithospheric transects from onshore to offshore

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-05-01

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

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

    PubMed

    Rychert, Catherine A; Shearer, Peter M

    2009-04-24

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

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

    NASA Technical Reports Server (NTRS)

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

    1976-01-01

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

  2. Magmatism & lithospheric destruction along the Colorado Plateau margin

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  3. 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.

  4. Plume-lithosphere interaction beneath a fast moving plate

    NASA Astrophysics Data System (ADS)

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

    2006-01-01

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

  5. Understanding lithospheric stresses in Arctic: constraints and models

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  7. Subcontinental rift initiation and ocean-continent transitional setting of the Dinarides and Vardar zone: Evidence from the Krivaja-Konjuh Massif, Bosnia and Herzegovina

    NASA Astrophysics Data System (ADS)

    Faul, Ulrich H.; Garapić, Gordana; Lugović, Boško

    2014-08-01

    The Dinaride and Vardar zone ophiolite belts extend from the south-eastern margins of the Alps to the Albanian and Greek ophiolites. Detailed sampling of the Krivaja-Konjuh massif, one of the largest massifs in the Dinaride belt, reveals fertile compositions and an extensive record of deformation at spinel peridotite facies conditions. High Na2O clinopyroxene and spinel-orthopyroxene symplectites after garnet indicate a relatively high pressure, subcontinental origin of the southern and western part of Krivaja, similar to orogenic massifs such as Lherz, Ronda and the Eastern Central Alpine peridotites. Clinopyroxene and spinel compositions from Konjuh show similarities with fertile abyssal peridotite. In the central parts of the massif the spinel lherzolites contain locally abundant patches of plagioclase, indicating impregnation by melt. The migrating melt was orthopyroxene undersaturated, locally converting the peridotites to massive olivine-rich troctolites. Massive gabbros and more evolved gabbro veins cross-cutting peridotites indicate continued melt production at depth. Overall we infer that the massif represents the onset of rifting and early stages of formation of a new ocean basin. In the south of Krivaja very localized chromitite occurrences indicate that much more depleted melts with supra-subduction affinity traversed the massif that have no genetic relationship with the peridotites. This indicates that volcanics with supra-subduction affinity at the margins of the Krivaja-Konjuh massif record separate processes during closure of the ocean basin. Comparison with published compositional data from other Balkan massifs shows that the range of compositions within the Krivaja-Konjuh massif is similar to the compositional range of the western massifs of the Dinarides. The compositions of the Balkan massifs show a west to east gradient, ranging from subcontinental on the western side of the Dinarides to depleted mid-ocean ridge/arc compositions in the Vardar

  8. Remobilization in the cratonic lithosphere recorded in polycrystalline diamond

    PubMed

    Jacob; Viljoen; Grassineau; Jagoutz

    2000-08-18

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

  9. Extensional and compressional instabilities in icy satellite lithospheres

    SciTech Connect

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

    1990-05-01

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

  10. An elastoviscoplastic finite element model of lithospheric deformation

    NASA Astrophysics Data System (ADS)

    Albert, Richard Alan

    1998-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  12. Seismic Tomography of the Arctic Lithosphere and Asthenosphere

    NASA Astrophysics Data System (ADS)

    Schaeffer, Andrew; Lebedev, Sergei

    2015-04-01

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

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

    NASA Astrophysics Data System (ADS)

    Ebbing, J.

    2014-12-01

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

  14. Extensional and compressional instabilities in icy satellite lithospheres

    NASA Technical Reports Server (NTRS)

    Herrick, David L.; Stevenson, David J.

    1990-01-01

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

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

    PubMed

    Tilmann, Frederik; Ni, James

    2003-05-30

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  17. 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.

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

    NASA Astrophysics Data System (ADS)

    Houseman, Gregory A.

    2015-04-01

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

  19. Triassic granitoids in the eastern Songpan Ganzi Fold Belt, SW China: Magmatic response to geodynamics of the deep lithosphere

    NASA Astrophysics Data System (ADS)

    Yuan, Chao; Zhou, Mei-Fu; Sun, Min; Zhao, Yongjiu; Wilde, Simon; Long, Xiaoping; Yan, Danping

    2010-02-01

    The Songpan Ganzi Fold Belt (SGFB), SW China, was developed from a passive continental margin into an orogenic belt with the consumption of the Paleo-Tethys. During the evolution of the SGFB, numerous Late Triassic granitic plutons formed and exhibited a progressive development from adakite/I-type granite, high Ba-Sr granite, A-type granite and monzonite. Representative Late Triassic plutons were studied to unravel the bewildering evolution of the eastern SGFB. The Menggu Pluton (224 ± 3 Ma) consists of granites with high alkali (K 2O+Na 2O = 7.85-10.4 wt.%) and adakitic characteristics (Sr/Y = 19-38). The ɛNd T values (- 2.77 to - 5.03), initial 87Sr/ 86Sr ratios (0.7050-0.7063) and low Nb/Ta ratios (8-10) are indicative of an origin by partial melting of amphibolitic lower crust. Rocks from the Niuxingou Pluton (215 ± 3 Ma) are richer in K than Na (K 2O/Na 2O = 1.1-1.5) and contain high Sr (1006-1662 ppm) and Ba (1277-2009 ppm), typical of shoshonite and high Ba-Sr granite. They have less enriched ɛNd T values (+ 0.08 to - 2.04) and less radiogenic 87Sr/ 86Sr i ratios (0.7047-0.7048), and formed from a mixed melt derived from upwelling asthenosphere and the overlying metasomatised lithospheric mantle. The Taiyanghe Pluton (205 ± 3 Ma) consists of monzonites, with high Al 2O 3 (> 20 wt.%), but low MgO (0.94-1.39 wt.%). The rocks are richer in Na than K (K 2O/Na 2O < 0.7), contain high large ion lithophile element (LILE) (681-834 ppm Sr and 2142-9453 ppm Ba) and display strongly fractionated REE patterns ((La/Yb) N = 35-63). These features, together with their enriched Nd-Sr isotopic compositions (ɛNd T = - 4.78 to - 6.50; 87Sr/ 86Sr i = 0.7074-0.7090), suggest that the monzonite probably formed from low degrees of partial melting of metasomatised lithospheric mantle. Although a continuous compressional regime during the Mid- and Late Triassic has been invoked for the SGFB, the generation of crustally derived adakitic and shoshonitic plutons reflect

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

    NASA Astrophysics Data System (ADS)

    Liu, S.; Wang, L.

    2006-12-01

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

  1. 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

  2. Horizontal stress in planetary lithospheres from vertical processes

    NASA Technical Reports Server (NTRS)

    Banerdt, W. B.

    1991-01-01

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

  3. Seismological constraints on lithospheric structure beneath rifted margins

    NASA Astrophysics Data System (ADS)

    Fishwick, Stewart

    2014-05-01

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

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

    NASA Astrophysics Data System (ADS)

    Babuška, V.; Plomerová, J.

    1992-06-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2008-12-01

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

  6. Lithospheric and atmospheric interaction on the planet Venus

    NASA Technical Reports Server (NTRS)

    Volkov, Vladislav P.

    1991-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Huismans, Ritske S.

    2014-05-01

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

  8. Extreme Hf-Os Isotope Compositions in Hawaiian Peridotite Xenoliths: Evidence for an Ancient Recycled Lithosphere

    NASA Astrophysics Data System (ADS)

    Bizimis, M.; Lassiter, J. C.; Salters, V. J.; Sen, G.; Griselin, M.

    2004-12-01

    We report on the first combined Hf-Os isotope systematics of spinel peridotite xenoliths from the Salt Lake Crater (SLC), Pali and Kaau (PK) vents from the island of Oahu, Hawaii. These peridotites are thought to represent the Pacific oceanic lithosphere beneath Oahu, as residues of MORB-type melting at a paleo-ridge some 80-100Ma ago. Clinopyroxene mineral separates in these peridotites have very similar Nd and Sr isotope compositions with the post erosional Honolulu Volcanics (HV) lavas that bring these xenoliths to the surface. This and their relatively elevated Na and LREE contents suggest that these peridotites are not simple residues of MORB-type melting but have experience some metasomatic enrichment by the host HV lavas. However, the SLC and PK xenoliths show an extreme range in Hf isotope compositions towards highly radiogenic values (ɛ Hf= 7-80), at nearly constant Nd isotope compositions (ɛ Nd= 7-10), unlike any OIB or MORB basalt. Furthermore, these Oahu peridotites show a bimodal distribution in their bulk rock 187Os/186Os ratios: the PK peridotites have similar ratios to the abyssal peridotites (0.130-0.1238), while the SLC peridotites have highly subchondritic ratios (0.1237-0.1134) that yield 500Ma to 2Ga Re-depletion ages. Hf-Os isotopes show a broad negative correlation whereby the samples with the most radiogenic 176Hf/177Hf have the most unradiogenic 187Os/186Os ratios. Based on their combined Hf-Os-Nd isotope and major element compositions, the PK peridotites can be interpreted as fragments of the Hawaiian lithosphere, residue of MORB melting 80-100Ma ago, that have been variably metasomatized by the host HV lavas. In contrast, the extreme Hf-Os isotope compositions of the SLC peridotites suggest that they cannot be the source nor residue of any kind of Hawaiian lavas, and that Hf and Os isotopes survived the metasomatism or melt-rock reaction that has overprinted the Nd and Sr isotope compositions of these peridotites. The ancient (>1Ga

  9. Enrichment through Creative Arts.

    ERIC Educational Resources Information Center

    Krause, Claire S.

    The CREST (Creative Resources Enriching Student Talents) Project, an enrichment approach for elementary gifted, talented, and creative students, is described. The project is explained to incorporate an interdisciplinary approach to instruction in art and science using resources within the community. Chapter 1 outlines the project philosophy,…

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

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

  11. Anorogenic plateau formation induced by a heated lithosphere

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  12. Rheological implications of the seismological lithosphere-asthenosphere boundary

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  13. 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

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

    NASA Astrophysics Data System (ADS)

    Corti, Giacomo; Dooley, Tim P.

    2015-11-01

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

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

    NASA Astrophysics Data System (ADS)

    Matsuyama, Isamu

    2010-10-01

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

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

    NASA Astrophysics Data System (ADS)

    Matsuyama, I.

    2010-12-01

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

  17. Preseismic Lithosphere-Atmosphere-Ionosphere Coupling

    NASA Astrophysics Data System (ADS)

    Kamogawa, Masashi

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

  18. Helium isotopic evidence for modification of the cratonic lithosphere during the Permo-Triassic Siberian flood basalt event

    NASA Astrophysics Data System (ADS)

    Barry, Peter H.; Hilton, David R.; Day, James M. D.; Pernet-Fisher, John F.; Howarth, Geoffrey H.; Magna, Tomas; Agashev, Aleksey M.; Pokhilenko, Nikolay P.; Pokhilenko, Lyudmila N.; Taylor, Lawrence A.

    2015-02-01

    Major flood basalt emplacement events can dramatically alter the composition of the sub-continental lithospheric mantle (SCLM). The Siberian craton experienced one of the largest flood basalt events preserved in the geologic record - eruption of the Permo-Triassic Siberian flood basalts (SFB) at ~ 250 Myr in response to upwelling of a deep-rooted mantle plume beneath the Siberian SCLM. Here, we present helium isotope (3He/4He) and concentration data for petrologically-distinct suites of peridotitic xenoliths recovered from two temporally-separated kimberlites: the 360 Ma Udachnaya and 160 Ma Obnazhennaya pipes, which erupted through the Siberian SCLM and bracket the eruption of the SFB. Measured 3He/4He ratios span a range from 0.1 to 9.8 RA (where RA = air 3He/4He) and fall into two distinct groups: 1) predominantly radiogenic pre-plume Udachnaya samples (mean clinopyroxene 3He/4He = 0.41 ± 0.30 RA (1σ); n = 7 excluding 1 outlier), and 2) 'mantle-like' post plume Obnazhennaya samples (mean clinopyroxene 3He/4He = 4.20 ± 0.90 RA (1σ); n = 5 excluding 1 outlier). Olivine separates from both kimberlite pipes tend to have higher 3He/4He than clinopyroxenes (or garnet). Helium contents in Udachnaya samples ([He] = 0.13-1.35 μcm3STP/g; n = 6) overlap with those of Obnazhennaya ([He] = 0.05-1.58 μcm3STP/g; n = 10), but extend to significantly higher values in some instances ([He] = 49-349 μcm3STP/g; n = 4). Uranium and thorium contents are also reported for the crushed material from which He was extracted in order to evaluate the potential for He migration from the mineral matrix to fluid inclusions. The wide range in He content, together with consistently radiogenic He-isotope values in Udachnaya peridotites suggests that crustal-derived fluids have incongruently metasomatized segments of the Siberian SCLM, whereas high 3He/4He values in Obnazhennaya peridotites show that this section of the SCLM has been overprinted by Permo-Triassic (plume-derived) basaltic

  19. The Contribution from Hot, Subducted Lithosphere to Mantle Wedge: Melt or Fluid?

    NASA Astrophysics Data System (ADS)

    Grove, T. L.

    2005-12-01

    In the Mt. Shasta region, N. Calif., USA, primitive basaltic andesites and andesites (similar to adakites) preserve a remarkable record of subducted lithosphere and mantle wedge elemental contributions. Estimates of pre-eruptive water contents allow the development of models of magma generation. When combined with a mantle melting model, one can characterize the chemical composition of the subducted slab contribution. Mt. Shasta lies above the young Juan de Fuca plate, where a hot slab environment has been proposed for the origin of the lavas. Melting is modeled as a process where an initial melt is formed in the mantle wedge above the slab by vapor-saturated melting of peridotite, metasomatized and enriched by the slab-derived melt or fluid. Vapor-saturated melting leads to the production of a water-rich melt (25-30 wt. % H2O) that ascends into the overlying mantle and continuously reacts as it encounters hotter, shallower mantle. The melt fraction increases, the water content decreases and the slab contribution is modified and diluted. The result is a flux melt whose major elements are dominantly derived by mantle melting and whose trace elements and isotopic characteristics reflect the subducted oceanic lithosphere. Two distinct sources are indicated by Sr, Nd and Pb isotopic evidence: a MORB and a sediment source. When the major element signature of the mantle wedge is removed, the slab contribution more closely resembles a low degree melt (2 to 5 wt. %) of a garnet + clinopyroxene source. The subducted component is less similar to experimental fluids equilibrated with eclogite. Although the LIL elements are a good match with a fluid, the model abundances of rare earth elements (Ce, Sm and Yb) are low by several orders of magnitude. Thus, a melt seems a better fit at Mt. Shasta based on our current state of understanding of melt vs. fluid equilibrium in the deep subduction environment.

  20. Earthquakes in western Iberia: improving the understanding of lithospheric deformation in a slowly deforming region

    NASA Astrophysics Data System (ADS)

    Custódio, S.; Dias, N. A.; Carrilho, F.; Góngora, E.; Rio, I.; Marreiros, C.; Morais, I.; Alves, P.; Matias, L.

    2015-10-01

    Mainland Portugal, on the southwestern edge of the European continent, is located directly north of the boundary between the Eurasian and Nubian plates. It lies in a region of slow lithospheric deformation (<5 mm yr-1), which has generated some of the largest earthquakes in Europe, both intraplate (mainland) and interplate (offshore). Some offshore earthquakes are nucleated on old and cold lithospheric mantle, at depths down to 60 km. The seismicity of mainland Portugal and its adjacent offshore has been repeatedly classified as diffuse. In this paper, we analyse the instrumental earthquake catalogue for western Iberia, which covers the period between 1961 and 2013. Between 2010 and 2012, the catalogue was enriched with data from dense broad-band deployments. We show that although the plate boundary south of Portugal is diffuse, in that deformation is accommodated along several distributed faults rather than along one long linear plate boundary, the seismicity itself is not diffuse. Rather, when located using high-quality data, earthquakes collapse into well-defined clusters and lineations. We identify and characterize the most outstanding clusters and lineations of epicentres and correlate them with geophysical and tectonic features (historical seismicity, topography, geologically mapped faults, Moho depth, free-air gravity, magnetic anomalies and geotectonic units). Both onshore and offshore, clusters and lineations of earthquakes are aligned preferentially NNE-SSW and WNW-ESE. Cumulative seismic moment and epicentre density decrease from south to north, with increasing distance from the plate boundary. Only few earthquake lineations coincide with geologically mapped faults. Clusters and lineations that do not match geologically mapped faults may correspond to previously unmapped faults (e.g. blind faults), rheological boundaries or distributed fracturing inside blocks that are more brittle and therefore break more easily than neighbour blocks. The seismicity map

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  2. 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.

  3. Lithospheric structure and deformation of the North American continent

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  4. Impact of lithosphere rheology on the dynamic topography

    NASA Astrophysics Data System (ADS)

    Burov, Evgueni; Gerya, Taras; Koptev, Alexander

    2014-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-07-01

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

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

    NASA Astrophysics Data System (ADS)

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

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

  7. Matching Lithosphere velocity changes to the GOCE gravity signal

    NASA Astrophysics Data System (ADS)

    Braitenberg, Carla

    2016-07-01

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

  8. Mesozoic thermal evolution of the southern African mantle lithosphere

    NASA Astrophysics Data System (ADS)

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

    2003-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Kusznir, N. J.

    2009-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Kusznir, Nick; Gozzard, Simon; Alvey, Andy

    2016-04-01

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

  11. Physico-chemical constraints on cratonic lithosphere discontinuities

    NASA Astrophysics Data System (ADS)

    Aulbach, Sonja; Rondenay, Stéphane; Huismans, Ritske

    2014-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

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

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

    NASA Astrophysics Data System (ADS)

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

    1984-12-01

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

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

    NASA Technical Reports Server (NTRS)

    Comer, R. P.

    1983-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Calmant, Stephane

    1987-09-01

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

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

    NASA Astrophysics Data System (ADS)

    Gerya, Taras; Burov, Evgenii

    2016-04-01

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

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

    PubMed

    Schmerr, Nicholas

    2012-03-23

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

  18. Linking petrology and seismology of the southwest Greenland lithosphere

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  19. Analysis of lithospheric magnetization in vector spherical harmonics

    NASA Astrophysics Data System (ADS)

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

    2011-10-01

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

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

    NASA Technical Reports Server (NTRS)

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

    1990-01-01

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

  1. Marine electromagnetic constraints on lithosphere/asthenosphere structure

    NASA Astrophysics Data System (ADS)

    Constable, Steven; Key, Kerry; Naif, Samer

    2014-05-01

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

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

    NASA Astrophysics Data System (ADS)

    Pauselli, Cristina; Ranalli, Giorgio; Federico, Costanzo

    2010-03-01

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

  3. Volcanism and Volatile Recycling on Venus from Lithospheric Delamination

    NASA Technical Reports Server (NTRS)

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

    2005-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Pavlenkova, N. I.

    1995-03-01

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

  5. Horizontal stresses induced by vertical processes in planetary lithospheres

    NASA Technical Reports Server (NTRS)

    Banerdt, W. B.

    1993-01-01

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

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

    NASA Technical Reports Server (NTRS)

    Ravat, Dhananjay

    1993-01-01

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

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

    SciTech Connect

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

    1981-05-10

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

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

    SciTech Connect

    Pasyanos, M

    2006-10-31

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

  9. Towards Full-Waveform Tomography of the Italian Lithosphere

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  10. Oceanic lithosphere and asthenosphere - Thermal and mechanical structure

    NASA Technical Reports Server (NTRS)

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

    1976-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Wang, Xingchen; Ding, Zhifeng; Zhu, Lupei

    2016-08-01

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

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

    NASA Astrophysics Data System (ADS)

    Wang, Xingchen; Ding, Zhifeng; Zhu, Lupei

    2016-04-01

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

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

    NASA Astrophysics Data System (ADS)

    Karato, Shun-ichiro

    2014-05-01

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

  14. The influence of Middle Paleozoic Yakutian plume on the geochemical modification of Siberian craton lithosphere

    NASA Astrophysics Data System (ADS)

    Solovjeva, L.; Goncharov, A.; Kalashnikova, T.

    2012-04-01

    The most promising for diamond potential the Middle Paleozoic kimberlite magmatism on Siberian Craton was due to upwelling of the Yakutian deep plume [Ernst, Buchan, 1997], that resulted to extensive event of the fluid and magma transfer in upper mantle. The plume approached to the base of rigid continental plate produced asthenospheric melts parental to Cr-poor megacrysts and high-temperature deformed peridotites. The melts realized simultaneous magmatic substitution of matter in the upper part of asthenospheric layer and in lower lithosphere [Solovjeva et al., 2008]. The distributions of incompatible trace elements in Grt megacrysts and Grt from high-temperature deformed Grt lherzolites of coarse-porphyroclastic type agree with magmatic trend with maxima of HFSE against REE. Grt from deformed peridotites of fine-porphyroclastic type show the sinusoidal REE curves and minima HFSE against REE. The latter rocks are thought to be lower lithosphere slices trapped by plume. The obtained data fit the model of reworking of asthenosphere and the lowermost lithosphere by the plume melts and agree with the mechanism of percolative fractional melt crystallization [Burgess & Harte, 2004; Harte et al., 1993]. The material of the plume source is belived to be been enriched in majorite and silicate-perovskite in transition zone and lower mantle. Trace elements distribution in garnet and clinopyroxene grains from low-temperature coarse-grained Grt and Sp-Grt peridotites suggests that lithospheric mantle located above the infiltration zone of asthenospheric melts was "washing out" by redox fluids deriving from chambers of asthenospheric liquids [Solovjeva, 2007]. It is verified by sharp depletion of garnet and clinopyroxene in incompatible trace elements from xenoliths with the most low logfO2 calculated by method of Gudmundsson & Wood [1995] (content Fe+3 in garnet). There are pale-green and colorless olivine in rocks with most low contents of incompatible trace elements. On the

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

    NASA Astrophysics Data System (ADS)

    Pastor-Galán, Daniel; Thieulot, Cedric

    2015-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  17. 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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  19. Using natural laboratories and modeling to decipher lithospheric rheology

    NASA Astrophysics Data System (ADS)

    Sobolev, Stephan

    2013-04-01

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

  20. Lithosphere extension and magmatism at volcanic passive margins

    NASA Astrophysics Data System (ADS)

    Geoffroy, Laurent; Gernigon, Laurent; Werner, Philippe

    2014-05-01

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

  1. Origin of garnet peridotites in the lithospheric mantle beneath the Siberian craton

    NASA Astrophysics Data System (ADS)

    Doucet, L. S.; Ionov, D. A.; Brey, G. P.; Golovin, A. V.; Ashchepkov, I. V.

    2012-04-01

    Garnet peridotites represent the largest part of the lithospheric mantle beneath Archean crust, yet the origin of garnet in these rocks continues to be debated. The cratonic mantle is believed to be produced by extensive melt extraction indicated by common low Al and Ca (<1%) and high Mg#WR (≥0.92) of cratonic peridotites [1]. However, even though many garnet peridotites are low in Al and Ca, they usually have lower Mg#WR (<0.92) than spinel harzburgites, which together with common high modal cpx and garnet (>5%) appear to be inconsistent with a residual origin by high degrees of partial melting [2]. To better constraint the origin of garnet in cratonic mantle we report modal, major and trace element compositions for >30 garnet peridotites from the Udachnaya kimberlite in central Siberia (as well as preliminary Nd-isotope data for selected samples). These rocks, unlike many other kimberlite-hosted peridotites worldwide, are unusually fresh, with very low LOI (≤1%) and unaltered minerals [3]. The garnet peridotites in this study are coarse (mostly low-T) to sheared (high-T) harzburgites with Mg#WR of 0.90-0.92 and ≤1% Al2O3 and CaO as well as two lherzolites. Their cpx (2-6%) and garnet (1-9%) have complex REE patterns affected by both melt extraction and various enrichment events. Modal and major oxide compositions of spinel harzburgites from Udachnaya indicate an origin by >35% of partial melting in a broad depth range (2-7 GPa) based on experimental results [4]. By comparison, only 5 out 30 garnet harzburgites in this study plot close to the melting trends defined by spinel harzburgites. The majority of garnet harzburgites in this study (especially high-T) show a range of enrichments in Fe, Si, Ti, HREE etc. relative to pristine melting residues. Moreover, the Nd isotope data for the garnet peridotites (calculated from garnet and cpx analyses) yield an isochron age of 0.8-0.7 Ga, which is much younger than whole-rock Re-Os formation ages (2 Ga [5]). The

  2. Laser and gas centrifuge enrichment

    SciTech Connect

    Heinonen, Olli

    2014-05-09

    Principles of uranium isotope enrichment using various laser and gas centrifuge techniques are briefly discussed. Examples on production of high enriched uranium are given. Concerns regarding the possibility of using low end technologies to produce weapons grade uranium are explained. Based on current assessments commercial enrichment services are able to cover the global needs of enriched uranium in the foreseeable future.

  3. Laser and gas centrifuge enrichment

    NASA Astrophysics Data System (ADS)

    Heinonen, Olli

    2014-05-01

    Principles of uranium isotope enrichment using various laser and gas centrifuge techniques are briefly discussed. Examples on production of high enriched uranium are given. Concerns regarding the possibility of using low end technologies to produce weapons grade uranium are explained. Based on current assessments commercial enrichment services are able to cover the global needs of enriched uranium in the foreseeable future.

  4. 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

  5. Aleutian lead isotopic data: additional evidence for the evolution of lithospheric plumbing systems

    SciTech Connect

    Myers, J.D.; Marsh, B.D.

    1987-07-01

    Lead isotopic ratios and concentrations have been measured in lavas from the Aleutian volcanic centers of Adak (12) and Atka (12). Lead contents in lavas from Atka increase four-fold over the compositional range of the volcanic suite. In contrast, Adak lavas have concentration levels of 0.6-13 ppm and display no simple correlation with SiO/sub 2/. The lead isotopic data alone can be explained by three different processes. Model 1 assigns lead isotopic differences to original magma source heterogeneity. According to Model 2, the isotopic ratios of a primary, non-radiogenic component from the mantle are elevated by the addition of an isotopically enriched slab-derived component. In contrast, Model 3 assumes a primary radiogenic magma produced by melting of the slab is contaminated by a non-radiogenic lithospheric component during conduit formation. Because these models all adequately explain the lead isotopic data, supplementary geologic, petrographic, geochemical and isotopic data must be used to select the most likely model. Careful consideration of the evidence suggests Model 3 best explains their extensive lead isotopic data as well as other characteristics of Aleutian lavas. The authors study suggests detailed isotopic studies of individual volcanic centers can be extremely useful in understanding the complex processes of magma generation, extraction, ascent and evolution.

  6. Late Cretaceous lithospheric extension in SE China: Constraints from volcanic rocks in Hainan Island

    NASA Astrophysics Data System (ADS)

    Zhou, Yun; Liang, Xinquan; Kröner, Alfred; Cai, Yongfeng; Shao, Tongbin; Wen, Shunv; Jiang, Ying; Fu, Jiangang; Wang, Ce; Dong, Chaoge

    2015-09-01

    Petrological, geochemical and in-situ zircon U-Pb dating and Hf-isotope analyses have been carried out on a suite of basalt-andesite-rhyolite volcanic rocks exposed in the Liuluocun area, Hainan Island, SE China. Zircon analyses show that these volcanic rocks crystallized in the Early Cretaceous (ca. 102 Ma). The basalts are characterized by low MgO contents and mg-numbers but high rare earth element, high field strength element and large ion lithophile element contents and Nb-Ta negative anomalies. They have relatively uniform Sr-Nd isotope compositions with εNd(t) values of - 4.09 to - 3.63. The andesites show enrichment of high field strength element and rare earth element with negligible Eu anomalies. They have εNd(t) values of - 2.35 to - 3.88 and εHf(t) values of - 9.73 to - 1.13. The rhyolites have high K2O and SiO2 contents. They are characterized by prominent Eu, P and Ti negative anomalies and enrichment in large ion lithophile element, and show εHf(t) values of - 7.51 to + 0.47 and εNd(t) values of - 2.49 to - 2.69. Petrogenetic analysis indicates that the Liuluocun volcanic rocks were produced by incomplete reaction of the mantle wedge peridotite with felsic melts derived from partial melting of subducted sediment. All these characteristics, combined with geological observations, suggest that their formation was related to regional lithospheric extension in the South China Craton during the Early Cretaceous, which may have been caused by subduction of the Paleo-Pacific plate beneath the continental plate of China.

  7. 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.

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

    NASA Astrophysics Data System (ADS)

    Artyushkov, Eugene

    2013-04-01

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

  9. Science Student Enrichment Opportunities.

    ERIC Educational Resources Information Center

    California State Dept. of Education, Sacramento.

    This document was developed with the intention of increasing California public school students' awareness of and participation in science-related enrichment activities. Some of the activities are intended for participation by individuals, while others are meant for teams of students. These annual events are listed in chronological order for a…

  10. Job Enrichment in Extension.

    ERIC Educational Resources Information Center

    Fourman, Louis S.; Jones, Jo

    1997-01-01

    Interviews with 10 participants in Ohio State University's job enrichment program for midcareer extension agents found that 5 returned to their same jobs after the experience but only 2 felt challenged/renewed. Part-time participation while working made it difficult to balance responsibilities. More information and a structured orientation were…

  11. Designing job enrichment projects.

    PubMed

    Clakeley, G L

    1988-01-01

    This paper describes a management strategy for a job satisfaction program utilized in a large occupational therapy department. The goal of the program is to retain satisfied, productive employees and reduce attrition of therapists and assistants. The use of job enrichment projects for occupational therapy assistants will be presented with brief descriptions of two projects. PMID:23944880

  12. Enrichments in Biology

    ERIC Educational Resources Information Center

    Richard, Paul W.

    1969-01-01

    Emphasizes the need for enrichment materials in addition to laboratory and textbook work, particularly for interested and able students. Discusses the use of filmstrips, loop films, Innovations to Inquiry, BSCS pamphlets, newspapers, magazines and scientific periodicals, television, field trips, library resources, and programed units. (EB)

  13. Enriching the Catalog

    ERIC Educational Resources Information Center

    Tennant, Roy

    2004-01-01

    After decades of costly and time-consuming effort, nearly all libraries have completed the retrospective conversion of their card catalogs to electronic form. However, bibliographic systems still are really not much more than card catalogs on wheels. Enriched content that Amazon.com takes for granted--such as digitized tables of contents, cover…

  14. Enriching Number Knowledge

    ERIC Educational Resources Information Center

    Mack, Nancy K.

    2011-01-01

    Exploring number systems of other cultures can be an enjoyable learning experience that enriches students' knowledge of numbers and number systems in important ways. It helps students deepen mental computation fluency, knowledge of place value, and equivalent representations for numbers. This article describes how the author designed her…

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

    NASA Astrophysics Data System (ADS)

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

    2013-10-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2007-12-01

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

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

    NASA Technical Reports Server (NTRS)

    Stein, M.

    1988-01-01

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

  18. Linking plate reconstructions with deforming lithosphere to geodynamic models

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2000-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-05-01

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

  1. Crustal and Lithospheric Structure at Isidis Planitia, Mars

    NASA Astrophysics Data System (ADS)

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

    2005-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Perry-Houts, J.; Humphreys, G.

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Baes, Marzieh; Govers, Rob; Wortel, Rinus

    2011-12-01

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

  4. Deformation of olivine single crystals under lithospheric conditions

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  5. Rethinking the problem of ionosphere-lithosphere coupling

    NASA Astrophysics Data System (ADS)

    Ruzhin, Yuri; Novikov, Victor

    2014-05-01

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

  6. Melt-Enabled Lithospheric Delamination in the Western US

    NASA Astrophysics Data System (ADS)

    Perry-Houts, J.; Humphreys, E.

    2014-12-01

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

  7. Lithospheric Structure and Seismotectonics of Central East Antarctica

    NASA Astrophysics Data System (ADS)

    Reading, A. M.

    2006-12-01

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

  8. Lithospheric strength variations in Mainland China: tectonic implications

    NASA Astrophysics Data System (ADS)

    Deng, Y.; Tesauro, M.

    2015-12-01

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

  9. Anomalous variations of lithosphere magnetic field before several earthquakes

    NASA Astrophysics Data System (ADS)

    Ni, Z.; Chen, B.

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Pasyanos, Michael E.; Nyblade, Andrew A.

    2007-11-01

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

  11. 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

  12. Lithosphere structure in Northern Canada from receiver function (RF)

    NASA Astrophysics Data System (ADS)

    Barantseva, Olga; Vinnik, Lev; Artemieva, Irina

    2016-04-01

    We present preliminary results of seismic data analysis for Northern Canada (around the Slave craton and NE of the Hudson Bay) in order to infer the lithosphere and asthenosphere structure beneath various Precambrian terrains of the North American craton. Seismic analysis includes data processing for the several stations of the Canadian National Seismic Network, for which P and S-velocity profiles are calculated through the simultaneous inversion of receiver functions. We report variations in the Moho depth and sharpness, as well as the depth to the LAB. The results are compared with regional petrological data for xenoliths.

  13. On searching applicants for mechanism of solar-lithosphere relations

    NASA Astrophysics Data System (ADS)

    Kairatkyzy, Dina; Andreyev, Alexey; Zhumabayeva, Saltanat; Seraliyev, Alibek

    2016-04-01

    It is actively discussed at present a question on possible influence of solar activity (high-speed solar wind streams bearing the "frozen" magnetic field lines of the Sun) on the stress status of the lithosphere and, consequently, on the Earth's seismic activity (e.g. Zhang, 1998, Acta Seismologica Sinica; Khachikyan et al., EGU2016-2754-1; IUGG2015-3132). There are at least two ideas on possible applicants for physical mechanism of solar-lithosphere relations: (i) - the muons of cosmic rays, which can penetrate the Earth's crust to a depth of at least the first ten kilometers and in tense seismic environment generate nuclear-electromagnetic cascade which energy can be a trigger of earthquake (Tsarev and Chechin, 1988, Preprint № 179, Physical Institute after Lebedev, Moscow); (ii) - the geomagnetic storms (Sobolev et al., 1998, Physics of the Earth #7) when the high-frequency oscillations of the geomagnetic field during the main phase of the storm generate significant induction currents which electric energy entering into the crust can be converted into mechanical energy increasing the stress status of the lithosphere (Sobolev and Demin, Mechano-electric phenomena into the Earth. M . Nauka. 1980). Besides this, among the possible cosmogenic factors changing the stress state of the lithosphere, could be the variation of the angular velocity of rotation of the Earth (e.g. Bostrom, 2000. Tectonic consequence of the Earth's Rotation), if it depends on solar activity variations. More of 50 years ago, Munk and Donald (The Rotation of the Earth, Cambrige University Press, 1960) suggested that the interaction between solar wind and geomagnetic field would probably influence the short period variation of angular velocity of the Earth. In this work, we check up this suggestion on the base of very precise data on the length of day (LOD) from 1986 to the present, which are presented by the International Earth Rotation and Reference Systems Service (IERS). Using the methods

  14. Three-dimensional density distributions in the Asian lithosphere

    NASA Astrophysics Data System (ADS)

    Zhang, G.; Li, C.; Wang, X.; Wang, Z.; Fang, J.; Sino-probe-cugb

    2011-12-01

    We have inversed the residual Bouguer gravity anomalies to study the three-dimensional density distributions of the Asian lithosphere (60°~150°E and 15°~60°N). Firstly, we have collected the free-air gravity anomalies (30'×30') and topography data of GTOP030 with 5'×5' grid spacing, and then calculated the Bougouer gravity anomalies by terrain correction and Bougouer correction. We have also collected the depth data of the Moho discontinuity (30'×30') and the discontinuity of sedimentary layer. By using the Oldenburg-Parker formula (Parker, 1972) and the forward modeling method, we calculated the theoretical gravity anomalies which mainly are caused by the Moho discontinuity and the sedimentary layer discontinuity. In our study, the average depths of Moho discontinuity and sedimentary layer discontinuity are 33 km and 4 km, and the density differences are 0.42 g/cm3 and 0.2 g/cm3, respectively. In addition, we have simulated the gravity anomalies of the spherical harmonics with the 2-6 order for the lower mantle by using the formula of Bowin (1983) which represented the relation between the depth of field source and the order of the geopotential spherical harmonics. Using all data mentioned above, we have calculated the residual Bougouer gravity anomalies, which may be caused by anomalous density bodies in the lithosphere. Secondly, we used the calculated residual Bougouer gravity anomalies to inverse the three-dimensional density differences in the Asian lithosphere by using the Algebra Reconstruction Techniques (ART). During the inversion, the densities converted from the P-wave velocity data (with grid spacing of 2°×2°) according to the Birch Law are considered as the initial density model. The grid spacing is set as 2°×2° in the horizontal direction, and it is 25 km, 55 km and 100 km in the vertical direction, respectively. Comparing the density anomalies at the three depths, we can conclude that (1) the density in the lithosphere beneath Asian

  15. Lithospheric origin for Neogene-Quaternary Middle Atlas lavas (Morocco): Clues from trace elements and Sr-Nd-Pb-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Bosch, Delphine; Maury, René C.; El Azzouzi, M'hammed; Bollinger, Claire; Bellon, Hervé; Verdoux, Patrick

    2014-09-01

    This study presents new geochemical data on 26 mafic lavas from the Middle Atlas and Central Morocco volcanic provinces, including Miocene nephelinites and Pliocene-Quaternary (3.9-0.6 Ma) nephelinites, basanites, alkali and subalkaline basalts. Most of them represent near-primary magmas, although some alkali basalts were derived from the minor fractionation of olivine and diopside phenocrysts. These evolved samples and the subalkaline basalt display higher 207Pb/204Pb and Zr/Nb ratios and lower εNd consistent with their contamination by lower crustal granulites during an open fractionation process. The progressive enrichment in incompatible elements observed from alkali basalts to nephelinites suggests their derivation from decreasing partial melting degrees of an enriched mantle source located at the garnet-spinel transition zone. The strong negative spikes observed for K in multielement patterns indicate that this source contained a residual pargasitic amphibole. We propose that partial melting occurred at around 2 GPa, i.e. near the lithosphere-asthenosphere boundary beneath the Middle Atlas (60-80 km). The trace element and isotopic Sr-Nd-Pb-Hf signature of the uncontaminated lavas displays a geochemical flavour intermediate between those of high μ (HIMU), “C”, and enriched mantle components. It is very similar to that of abundant metasomatic amphibole- and clinopyroxene-rich lithospheric peridotites and pyroxenites carried by Middle Atlas lavas, which likely represent an analog of the source of these lavas. It is therefore not necessary to postulate the contribution of a “fresh” asthenospheric mantle to their genesis. We propose that they resulted from the partial melting of the base of a veined lithospheric mantle metasomatised during the late Cretaceous by alkaline melts from the Central Atlantic plume, the ancestor of the Canary plume. Melting was probably triggered by the flux of a hot mantle within a regional SW-NE sub-lithospheric channel, in

  16. Density heterogeneity of lithospheric mantle beneath the Siberian craton: testing geophysical models by petrological data

    NASA Astrophysics Data System (ADS)

    Cherepanova, Yulia; Artemieva, Irina

    2015-04-01

    Using free-board modeling, we examine a vertically-averaged mantle density beneath the Archean-Proterozoic Siberian Craton in the layer from the Moho down to base of the chemical boundary layer (CBL). Two models are tested: in Model 1 the base of the CBL coincides with the LAB, whereas in Model 2 the base of the CBL is at a 180 km depth. The uncertainty of density model is < 0.02 t/m3 or < 0.6% with respect to primitive mantle. The results, calculated at in situ and at room temperature (SPT) conditions, indicate a heterogeneous density structure of the Siberian lithospheric mantle with a strong correlation between mantle density variations and the tectonic setting. Three types of cratonic mantle are recognized from mantle density anomalies. 'Pristine' cratonic regions not sampled by kimberlites have the strongest depletion with density deficit of 1.8-3.0% (and SPT density of 3.29-3.33 t/m3 as compared to 3.39 t/m3 of primitive mantle). Cratonic mantle affected by magmatism (including the kimberlite provinces) has a typical density deficit of 1.0-1.5%, indicative of a metasomatic melt-enrichment. Intracratonic sedimentary basins have a high density mantle (3.38-3.40 t/m3 at SPT) which suggests, at least partial, eclogitization. Moderate density anomalies beneath the Tunguska Basin imply that the source of the Siberian LIP lies outside of the Craton. In situ mantle density is used to test the isopycnic condition of the Siberian Craton. Both CBL thickness models indicate significant lateral variations in the isopycnic state, correlated with mantle depletion and best achieved for the Anabar Shield region and other intracratonic domains with a strongly depleted mantle. A comparison of synthetic Mg# for the bulk lithospheric mantle calculated from density with Mg# from petrological studies of peridotite xenoliths from the Siberian kimberlites suggests that melt migration may produce local patches of metasomatic material in the overall depleted mantle.

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  18. 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).

  19. 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

  20. Lithosphere-Asthenosphere Transition, Mid-Lithosphere Discontinuity and Radial Anisotropy from Multi-mode Surface Wave Tomography

    NASA Astrophysics Data System (ADS)

    Yoshizawa, K.; Kennett, B. L. N.

    2015-12-01

    Seismic surface waves are the major source of information to delineate the lateral heterogeneity and anisotropy in the upper mantle. S-wave radial anisotropy representing the difference between SV and SH velocities is often discussed with the seismic properties of the lithosphere-asthenosphere transition (LAT) and the mid-lithosphere discontinuity (MLD). Such boundaries have been studied well by body-wave receiver functions, which have a good sensitivity to the sharpness of boundaries. Surface waves are rather insensitive to the boundary sharpness, but can delineate the spatial distributions of shear wave speeds and radial anisotropy in the upper mantle, which can be alternative constraints on LAT and MLD. We have recently constructed a radially anisotropic 3-D S wave speed model of the Australian continent from multi-mode Love and Rayleigh waves with enhanced ray coverage. In the inversions for S-wave radial anisotropy, we can use either parameterizations for SH and SV velocities, or for dimensionless radially anisotropic parameter ξ=(Vsh/Vsv)2. Both are theoretically the same, but this difference causes non-negligible effects on the estimated radial anisotropy, mainly due to the different sensitivities of Love-wave phase speeds to the structural parameters. Synthetic experiments and data variance reductions suggest the former is the better choice. The LAT throughout the continent can be estimated by the vertical velocity gradient of the isotropic S-wave model. The radial anisotropy with the suitable model parameterization shows strong anisotropy with faster SH velocity in the asthenosphere, suggesting the influence of strong shear beneath the fast drifting Australian continent. We can also identify the clear vertical changes in the radial anisotropy profiles at the MLD depth estimated from earlier receiver function studies in cratonic regions, which can be a key to elucidate the enigmatic MLD in the continental lithosphere.

  1. Enrichment Strategies in Phosphoproteomics.

    PubMed

    Leitner, Alexander

    2016-01-01

    The comprehensive study of the phosphoproteome is heavily dependent on appropriate enrichment strategies that are most often, but not exclusively, carried out on the peptide level. In this chapter, I give an overview of the most widely used techniques. In addition to dedicated antibodies, phosphopeptides are enriched by their selective interaction with metals in the form of chelated metal ions or metal oxides. The negative charge of the phosphate group is also exploited in a variety of chromatographic fractionation methods that include different types of ion exchange chromatography, hydrophilic interaction chromatography (HILIC), and electrostatic repulsion HILIC (ERLIC) chromatography. Selected examples from the literature will demonstrate how a combination of these techniques with current high-performance mass spectrometry enables the identification of thousands of phosphorylation sites from various sample types. PMID:26584921

  2. 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

  3. Compositional vs. thermal buoyancy and the evolution of subducted lithosphere

    NASA Technical Reports Server (NTRS)

    Gaherty, James B.; Hager, Bradford H.

    1994-01-01

    We formulate 2-D Cartesian finite element models that explore the fate of compositionally defined lithosphere as it encounters a viscosity increase at the boundary between the upper and lower mantle. Subducted lithosphere is represented as a cold, stiff, layered composite of denser eclogite underlain by more buoyant harzburgite. Slabs impinging on a lower mantle 30 and 100 times more viscous than the upper mantle thicken and fold strongly as they penetrate the lower mantle. Approximately a factor of two thickening occurs via pure shear just above the discontinuity, with additional enhancement due to folding by over a factor of two. No separation of the individual slab components occurs at the discontinuity, and direct comparison with models in which compositional buoyancy is explicitly ignored indicates that slab evolution is largely controlled by the thermal buoyancy. These results are at odds with hypotheses about slab evolution in which the compositional buoyancy contributions lead to component separation and the formation of slab megaliths or a compositionally layered upper mantle.

  4. Colorado Plateau magmatism and uplift by warming of heterogeneous lithosphere.

    PubMed

    Roy, Mousumi; Jordan, Thomas H; Pederson, Joel

    2009-06-18

    The forces that drove rock uplift of the low-relief, high-elevation, tectonically stable Colorado Plateau are the subject of long-standing debate. While the adjacent Basin and Range province and Rio Grande rift province underwent Cenozoic shortening followed by extension, the plateau experienced approximately 2 km of rock uplift without significant internal deformation. Here we propose that warming of the thicker, more iron-depleted Colorado Plateau lithosphere over 35-40 Myr following mid-Cenozoic removal of the Farallon plate from beneath North America is the primary mechanism driving rock uplift. In our model, conductive re-equilibration not only explains the rock uplift of the plateau, but also provides a robust geodynamic interpretation of observed contrasts between the Colorado Plateau margins and the plateau interior. In particular, the model matches the encroachment of Cenozoic magmatism from the margins towards the plateau interior at rates of 3-6 km Myr(-1) and is consistent with lower seismic velocities and more negative Bouguer gravity at the margins than in the plateau interior. We suggest that warming of heterogeneous lithosphere is a powerful mechanism for driving epeirogenic rock uplift of the Colorado Plateau and may be of general importance in plate-interior settings. PMID:19536263

  5. Lithosphere-Asthenosphere interactions near the San Andreas fault.

    NASA Astrophysics Data System (ADS)

    Houlie, N.

    2015-12-01

    We decipher the strain history of the upper mantle in California through the comparison of the long-term finite strain field in the mantle and the surface strain-rate field, respectively inferred from fast polarization directions of seismic phases (SKS and SKKS), and Global Positioning System (GPS) surface velocity fields. We show that mantle strain and surface strain-rate fields are consistent in the vicinity of San Andreas Fault (SAF) in California. Such an agreement suggests that the lithosphere and strong asthenosphere have been deformed coherently and steadily since >1 Ma. We find that the crustal stress field rotates (up to 40 degrees of rotation across a 50 km distance from 50 degrees relative to the strike of the SAF, in the near-field of SAF) from San Francisco to the Central Valley. Both observations suggest that the SAF extends to depth, likely through the entire lithosphere. From Central Valley towards the Basin and Range, the orientations of GPS strain-rates, shear wave splitting measurements and seismic stress fields diverge indicating reduced coupling or/and shallow crustal extension and/or presence of frozen anisotropy.

  6. The Cascadia Subduction Zone: two contrasting models of lithospheric structure

    USGS Publications Warehouse

    Romanyuk, T.V.; Blakely, R.; Mooney, W.D.

    1998-01-01

    The Pacific margin of North America is one of the most complicated regions in the world in terms of its structure and present day geodynamic regime. The aim of this work is to develop a better understanding of lithospheric structure of the Pacific Northwest, in particular the Cascadia subduction zone of Southwest Canada and Northwest USA. The goal is to compare and contrast the lithospheric density structure along two profiles across the subduction zone and to interpet the differences in terms of active processes. The subduction of the Juan de Fuca plate beneath North America changes markedly along the length of the subduction zone, notably in the angle of subduction, distribution of earthquakes and volcanism, goelogic and seismic structure of the upper plate, and regional horizontal stress. To investigate these characteristics, we conducted detailed density modeling of the crust and mantle along two transects across the Cascadia subduction zone. One crosses Vancouver Island and the Canadian margin, the other crosses the margin of central Oregon.

  7. Lithospheric thinning associated with rifting in East Africa

    NASA Technical Reports Server (NTRS)

    Wendlandt, R. F.; Morgan, R.

    1982-01-01

    The rate of ascent of the thermal perturbation of eruptive rock types in the Kenya Dome region can be calculated for depths of origin ranging from greater than 170 km to the lower crust (25-30 km) as combined with dated occurrences. An equation is derived and solved iteratively for ascent velocity at 10 km depth intervals from 150-30 km, assuming a thermal diffusivity of 1 sq mm/sec. For radii of 50 and 100 km and constant gradient increases within the range 55-60 C/km over the lithosphere-asthenosphere boundary (LAB) ascent depth range, the calculated rate of upwelling corresponds well with the results of a simple thermal model of lithospheric thinning. A 1 C/km change in gradient results approximately in a 0.55 Myr change in ascent time from 150 to 30 km, due to the increasing differential between the geotherm and the solidus with decreasing depths and the increasing rate of heat loss to the surface. It is suggested by the temporal-spatial-compositional relations of eruptives in the Kenya Dome region that there is a shallowing of magma source regions with time, implying encroachment of a thermal anomaly.

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

    PubMed

    Macgregor, I D; Basu, A R

    1974-09-20

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

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

    SciTech Connect

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

    1995-06-01

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

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

    USGS Publications Warehouse

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

    1997-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    1995-04-01

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

  12. 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.

  13. 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

  14. Why intracontinental basins subside longer: 3-D feedback effects of lithospheric cooling and sedimentation on the flexural strength of the lithosphere

    NASA Astrophysics Data System (ADS)

    Cacace, M.; Scheck-Wenderoth, M.

    2016-05-01

    The aim of this study is to reevaluate the character and evolution of the large-scale subsidence of intracontinental basins using 3-D thermomechanical numerical simulations accounting for the coupling between sedimentation, rheology-dependent lithospheric flexure, and thermal contraction by lithospheric cooling. The flexural rigidity of the lithospheric plate is controlled by elastic-brittle-plastic rheology, enabling the computation of thermal and mechanical feedback processes occurring during basin subsidence. Numerical results show that depending on the sediment loading history, a rheological stratified lithosphere can subside over geological time scales without imposition of ad hoc geometric and kinematic initial conditions. Three-dimensional feedback effects of sedimentation on the thermomechanical structure of the plate result in a weakened lower crust mechanically decoupled from the underlying mantle and therefore easily reactivated even under low background stresses. Our results explain the first-order characteristics of the subsidence in intracontinental basins and reconcile basic observations of their deformation history.

  15. Numerical modelling of lithospheric extension: doming vs. thermal condition

    NASA Astrophysics Data System (ADS)

    Schenker, Filippo Luca; Gerya, Taras; Kaus, Boris; Burg, Jean-Pierre

    2010-05-01

    Structural aspects of extensional doming have been modelled numerically using simplified 2D visco-plastic models (e.g. Huismans et al. 2005, Buiter et al. 2008) concentrating mainly on symmetric/asymmetric doming, fault tectonics and deformation of domes and surrounding rocks. Recent works focus their attention to the influence of geotherms on the rheology (Tirel et al. 2008), even taking into account melting (Rey et al., 2009). However, thermal aspects remain difficult to compute because of the coupled interaction between mechanical forces and temperature. This coupling is fundamental, because it provides a link between modelling and thermochronometry. Indeed, cooling ages of extensional dome flanks can constrain time, size, and patterns of metamorphic overprints simulated in thermo-mechanical models. We treat mechanical and thermal aspects together (including modelling of metamorphic P-T-time paths of crustal rocks), using a visco-elasto-plastic rheology in a four layer setup (upper crust, lower crust, lithospheric mantle and asthenospheric mantle). The asthenospheric mantle is considered in order to predict the bending effect of the lithosphere. We employed I2ELVIS, a numerical 2D computer code designed for conservative finite differences method. The model domain is 300 km wide and 160 km deep. We observed two modes of dome development and geometry, depending on first order parameters such as temperature at the Moho and thickness of the crust: (i) Lower crustal doming: with a hot Moho (TMOHO > 700 °C) and/or a thick crust, strain is localized in the upper crust and distributed in the mantle. At these conditions partial melting in the lower crust forms the core of the dome and maintains a flat Moho. (ii) Asthenospheric-triggered doming: with a cold Moho (TMOHO < 700 °C), strain is distributed in the crust and localized in the lithospheric mantle, which allows upwelling of the asthenosphere. The migmatite "core complexes" develop after the upwelling of the

  16. Satellite-derived geoid for the estimation of lithospheric cooling and basal heat flux anomalies over the northern Indian Ocean lithosphere

    NASA Astrophysics Data System (ADS)

    Rajesh, S.; Majumdar, T. J.

    2015-12-01

    The northern Indian Ocean consists of older Bay of Bengal (BOB) oceanic lithosphere with numerous intra-plate loads; whereas, contrasting elements like active Mid-Ocean ridge divergence and slow spreading ridges are present in the relatively younger (<60 Ma) Arabian Sea oceanic lithosphere. The mechanism of lithospheric cooling of young age oceanic lithosphere from the moderately active and slow spreading Carlsberg Ridge is analysed by considering the hypothesis of near lithospheric convective action or whole upper mantle convection. We addressed these issues by studying the marine geoid at different spatial wavelengths and retrieved and compared their lithospheric cooling signatures, plate spreading and distribution of mass and heat anomalies along with seismicity, bathymetry, gravity and isochron age data. Results show that progressive cooling of young-aged oceanic lithosphere from the Mid-Ocean Carlsberg Ridge is because of conductive cooling and those signals are retrieved in the shorter wavelength band (111 < λ< 1900 km) of constrained residual geoid with mass anomaly sources near to sublithospheric. This shows steadiness in the geoid anomaly decay rate (˜-0.1 m/Ma), consistency in the growth of thermal boundary layer and progressive fall of basal temperature and heat flux (900- 300 K and 100-18 mW m-2) with increase of lithospheric age. The above observations are attributed to the fact that the advective-convective action beneath the Mid-Ocean Carlsberg Ridge is driven by the basal temperature gradient between the lithosphere and the near lithospheric low viscose thin layer. But, for the case of old-aged oceanic lithosphere in the BOB, the residual geoid anomaly cooling signals are not prominently seen in the same band as that of the Arabian Sea because of the Ninetyeast Ridge magmatism. However, its cooling anomaly signatures are retrieved at relatively higher band (1335 ≤ λ≤ 3081 km) having erratic geoid decay rates (-0.3 to 0.2 m/Ma) owing to

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

    PubMed

    Schlindwein, Vera; Schmid, Florian

    2016-07-14

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

  18. Convective Removal of Continental Margin Lithosphere at the Edges of Subducting Oceanic Plates

    NASA Astrophysics Data System (ADS)

    Levander, A.; Bezada, M. J.; Palomeras, I.; Masy, J.; Humphreys, E.; Niu, F.

    2013-12-01

    Although oceanic lithosphere is continuously recycled to the deeper mantle by subduction, the rates and manner in which different types of continental lithospheric mantle are recycled is unclear. Cratonic mantle can be chemically reworked and essentially decratonized, although the frequency of decratonization is unclear. Lithospheric mantle under or adjacent to orogenic belts can be lost to the deeper mantle by convective downwellings and delamination phenomena. Here we describe how subduction related processes at the edges of oceanic plates adjacent to passive continental margins removes the mantle lithosphere from beneath the margin and from the continental interior. This appears to be a widespread means of recycling non-cratonic continental mantle. Lithospheric removal requires the edge of a subducting oceanic plate to be at a relatively high angle to an adjacent passive continental margin. From Rayleigh wave and body wave tomography, and receiver function images from the BOLIVAR and PICASSO experiments, we infer large-scale removal of continental margin lithospheric mantle from beneath 1) the northern South American plate margin due to Atlantic subduction, and 2) the Iberian and North African margins due to Alboran plate subduction. In both cases lithospheric mantle appears to have been removed several hundred kilometers inland from the subduction zones. This type of ';plate-edge' tectonics either accompanies or pre-conditions continental margins for orogenic activity by thinning and weakening the lithosphere. These processes show the importance of relatively small convective structures, i.e. small subducting plates, in formation of orogenic belts.

  19. Uplift of the Colorado Plateau due to lithosphere attenuation during Laramide low-angle subduction

    USGS Publications Warehouse

    Spencer, J.E.

    1996-01-01

    The Colorado Plateau is blanketed by Phanerozoic marine and nonmarine strata as young as Cretaceous that are now exposed at elevations of about 2 km. Crustal thickening due to magmatism and horizontal crustal shortening was far less than necessary to cause this uplift, which is commonly attributed to the consequences of mantle lithosphere thinning and heating. The Colorado Plateau and the midcontinent region around Iowa consist of Precambrian bedrock overlain by a similar amount of Paleozoic platformal strata, and thus both regions once had similar lithospheric buoyancy. Mesozoic sedimentation increased the crustal thickness and lithospheric buoyancy of the Colorado Plateau relative to the midcontinent region. Backstripping calculations yield elevation without these sediments and lead to a calculated elevation difference between the two areas of about 1200 m, which represents unexplained plateau uplift. Review of constraints on uplift timing finds little support for a late Cenozoic uplift age and allows early to middle Cenozoic uplift, which is consistent with uplift mechanisms related to low-angle subduction that ended in the middle Cenozoic. Finite element heat flow calculations of low-angle subduction and lithosphere attenuation, using a range of initial lithosphere thicknesses and degree of attenuation, indicate that required uplift can result from tectonic removal of about 120 km of mantle lithosphere from an initially 200-km-thick lithosphere. This allows for partial preservation of North American mantle lithosphere with its distinctive isotopic signature in some late Cenozoic volcanic rocks and is consistent with normal Pn velocities in the uppermost mantle beneath the plateau.

  20. Trace element characteristics of lithospheric and asthenospheric mantle in the Rio Grande rift region

    SciTech Connect

    Perry, F.V.

    1994-06-01

    Trace element analyses of 10 mafic volcanic rocks from the Colorado Plateau transition zone, Colorado Plateau, Rio Grande rift, and Great Plains were obtained to characterize the trace element characteristics of asthenospheric and lithospheric mantle beneath these regions. Characterization of these mantle reservoirs using the trace element contents of basalts allows one to track the response of the lithosphere to continental rifting and extension.

  1. 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.

  2. Study of Ionospheric TEC from GPS observations and comparisons with IRI and SPIM model predictions in the low latitude anomaly Indian subcontinental region

    NASA Astrophysics Data System (ADS)

    Panda, S. K.; Gedam, S. S.; Rajaram, G.

    2015-04-01

    The present study investigates variation of the ionospheric total electron content (TEC) in the low latitude Indian sub-continental region from the GPS observations and its comparison with the global ionosphere maps (GIMs), standard international reference ionosphere (IRI 2012), and the standard plasmasphere-ionosphere model (SPIM) for the period from November 2011 to October 2012 that corresponds to the progressive phase towards the midst of the solar cycle-24. Observations during quiet period show diurnal maximum of TEC occurring around 14:00-16:00 IST, with relatively broader and longer duration of local maximum at Bangalore and behave reversely towards Delhi. The secondary maximum of TEC was markedly noticeable at Bangalore during the months of March and September, and only in the month of September at Hyderabad and Mumbai. However, the relatively higher TEC during December month than the June is ascribed to the winter anomaly which is more prevalent during the high solar activity periods. The prevailing instability in latitudes of anomaly crest during January 2012 is possibly due to the seasonal variation of lunar tidal effects, modulating the EEJ strength at the equator. The studies covered the period of a strong geomagnetic storm during 6-11 March 2012 (SYM-H: -149 nT) which resulted in positive deviation of GPS-TEC at Bangalore (↑ 20%), Hyderabad (↑ 22%), and Lucknow (↑ 94%) compared to the mean quiet days level. The relatively large deviation of TEC at Lucknow could be attributed to the poleward shifting of the anomaly crest, manifested by enhanced fountain effect at the equator. Studies confirm excellent agreement (80-85%) of GPS-TEC with IGS-GIM at Bangalore and Hyderabad with the exception of the night-time hours (Deviations >50%). However relatively larger deviation of GPS-TEC from GIM-TEC at Delhi could be due to the unavailability of IGS stations in the proximity of the position. Predictions of the SPIM model (extension of IRI up to GPS

  3. Highly Siderophile Elements as Tracers for the Subcontinental Mantle Evolution Beneath the Southwestern USA: The San Carlos and Kilbourne Hole Peridotite Xenoliths Revisited

    NASA Technical Reports Server (NTRS)

    vanAcken, D.; Brandon, A. D.; Peslier, A. H.; Lee, C.-T. A.

    2010-01-01

    Peridotite xenoliths from San Carlos, Arizona, and Kilbourne Hole, New Mexico, have been studied since the 1970 s to give insights into melting and metasomatism in the subcontinental mantle beneath the southwestern USA. More recently, the highly siderophile elements (HSE; Os, Ir, Ru, Rh, Pt, Pd, and Re) and the included Re-Os isotope system have been established as powerful tools for the study of mantle processes because of their range in compatibility during mantle melting and their siderophile and chalcophile geochemical behavior. Model aluminachron Re-Os ages for San Carlos and Kilbourne Hole, as well as for the nearby Dish Hill and Vulcan's Throne sites, give consistent depletion ages of around 2.2 Ga. This age can be interpreted as a single large scale mantle melting event linked to crustal formation and continental growth under the southwestern USA. Highly siderophile elements, however, may be added to depleted peridotites via melt-rock interaction, especially the more incompatible and hence mobile Pt, Pd, and Re. This may result in overprinting of the signature of melt extraction, thus abating the usefulness of Re-Os mantle extraction model ages. A comprehensive characterization of the suite of mantle xenoliths from the SW USA in terms of HSE concentrations is thus necessary to re-assess the Re-Os system for dating purposes. San Carlos peridotites are depleted to moderately fertile, as indicated by their bulk Al2O3 contents between 0.66 wt% and 3.13 wt%. Bulk Os-187/Os-188 in San Carlos peridotites range from 0.1206 to 0.1357. In contrast, Kilbourne Hole peridotites tend to be more fertile with Al2O3 between 2.11 and 3.78 wt%, excluding one extremely depleted sample with 0.30 wt% Al2O3, and have Os-187/Os-188 between 0.1156 and 0.1272, typical for mantle peridotites. No large fractionation between the more compatible HSE Os, Ir, and Ru are observed. The more incompatible HSE Re, Pd, and to a minor extent, Pt, however, are depleted in a number of samples by

  4. Melt-induced weakening of the lithosphere: theory and geodynamic implications

    NASA Astrophysics Data System (ADS)

    Gerya, T.

    2015-12-01

    Melt-induced weakening can play critical role for enabling lithospheric deformation in the areas of intense mantle-derived magmatism, such as mid-ocean ridges, rift zones and hot spots. It implies significant reduction in the long-term brittle strength of the deforming lithosphere subjected to frequent melt percolation episodes. Such weakening corresponds to conditions when shear stress reaches the tensile yield strength of rocks at nearly equal melt and lithostatic pressures. The dominant features of melt transport in this regime are planar, sharply localized zones (dykes) in which melt is transported though the lithosphere from the source region. Mechanical energy dissipation balance shows that the long-term effective strength of the melt-weakened lithosphere is a strain-averaged rather than a time-averaged quantity. Its magnitude is mainly defined by the ratio between melt pressure and lithostatic pressure along dykes during short dyke emplacement episodes, which control most of the lithospheric deformation and mechanical energy dissipation. We quantified the range of expected values of the lithospheric strength by performing 2D numerical hydro-mechanical experiments on melt-bearing rock deformation as well as seismo-mechanical experiments on long-term lithospheric deformation assisted by frequent short-term dyke propagation episodes. These numerical experiments showed that the long-term lithospheric strength in the areas of intense magmatism can be as low as few MPa and is critically dependent on the availability of melt for enabling frequent episodes of dyke propagation through the lithosphere. Short-lived viscous-plastic deformation is localized along propagating weak dykes whereas bulk of the lithosphere only deforms elastically and is subjected to large deviatoric stresses. The experiments suggest that it is not the high strength of the elastically deforming strong lithospheric blocks but the low strength of visco-plastically deforming dykes that define the

  5. Early thermal profiles and lithospheric strength of Ganymede from extensional tectonic features

    SciTech Connect

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

    1986-11-01

    The early thermal profiles and the lithospheric stability and strength of Ganymede are quantitatively determined on the basis of brittle lithosphere thickness estimates derived from the width and spacing of extensional tectonic features, together with lithospheric strength envelopes for ice. Plots of the brittle and ductile yield stress vs. depth for the icy lithosphere of Ganymede exhibit a linear increase in brittle strength with depth to a maximum at the brittle-ductile transition that is followed by an exponential decrease in ductile yield stress with depth. The results obtained imply that the thermal gradient and lithospheric strength have varied laterally by factor as great as 5, and that Ganymede underwent cooling in a highly inhomogeneous fashion with lateral thermal anomalies. The present analysis furnishes reasons for the stability of large cratered terrain remnants. 47 references.

  6. 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 withou