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Sample records for continental subduction-zone metamorphism

  1. UHP impure marbles from the Dabie Mountains: Metamorphic evolution and carbon cycling in continental subduction zones

    NASA Astrophysics Data System (ADS)

    Liu, Penglei; Wu, Yao; Chen, Yi; Zhang, Junfeng; Jin, Zhenmin

    2015-01-01

    Impure marbles from ultra-high pressure (UHP) metamorphic belts bear significant information on the metamorphic evolution and carbon cycling during continental subduction and exhumation. In this study, detailed petrological data are presented and a P-T-X(CO2) path is constructed for the impure marbles from the Dabie UHP terrane. Coesite relicts are discovered as inclusions within dolomite from the selected samples, which have a peak assemblage of dolomite, aragonite, garnet, omphacite, phengite, coesite, allanite and rutile. Estimated with the compositions of peak minerals, a P-T condition of 4.05-4.45 GPa at 740-820 °C is obtained by conventional geothermobarometry. The modeled fluid compositions have a low X(CO2) (0.01-0.02) at the peak conditions, while the X(CO2) firstly increased during isothermal exhumation and then decreased at later retrogression. The discovery of coesite within dolomite underscores the role of the "pressure vessel" models and highlights the significance of fluid unavailability in preserving coesite in UHP rocks. Neither petrological evidence nor independent peak P-T estimations support the breakdown of dolomite in the studied marbles, which contests recent suggestions. Analysis on the phase relations in the CaO-MgO-SiO2-H2O-CO2 system shows that the bulk rock compositions have a large control on the stable UHP carbonate associations in carbonate-bearing rocks. The low X(CO2) in the peak fluids indicates a weak decarbonation of the impure marbles under sub-arcs. In the last, a large fraction of CO2 is shown to be sequestrated during regional retrogression of clinopyroxene marbles, which has a profound influence and must be considered for the global carbon cycling.

  2. Metamorphic Perspectives of Subduction Zone Volatiles Cycling

    NASA Astrophysics Data System (ADS)

    Bebout, G. E.

    2008-12-01

    Field study of HP/UHP metamorphic rocks provides "ground-truthing" for experimental and theoretical petrologic studies estimating extents of deep volatiles subduction, and provides information regarding devolatilization and deep subduction-zone fluid flow that can be used to reconcile estimates of subduction inputs and arc volcanic outputs for volatiles such as H2O, N, and C. Considerable attention has been paid to H2O subduction in various bulk compositions, and, based on calculated phase assemblages, it is thought that a large fraction of the initially structurally bound H2O is subducted to, and beyond, subarc regions in most modern subduction zones (Hacker, 2008, G-cubed). Field studies of HP/UHP mafic and sedimentary rocks demonstrate the impressive retention of volatiles (and fluid-mobile elements) to depths approaching those beneath arcs. At the slab-mantle interface, high-variance lithologies containing hydrous phases such as mica, amphibole, talc, and chlorite could further stabilize H2O to great depth. Trench hydration in sub-crustal parts of oceanic lithosphere could profoundly increase subduction inputs of particularly H2O, and massive flux of H2O-rich fluids from these regions into the slab-mantle interface could lead to extensive metasomatism. Consideration of sedimentary N concentrations and δ15N at ODP Site 1039 (Li and Bebout, 2005, JGR), together with estimates of the N concentration of subducting altered oceanic crust (AOC), indicates that ~42% of the N subducting beneath Nicaragua is returned in the corresponding volcanic arc (Elkins et al., 2006, GCA). Study of N in HP/UHP sedimentary and basaltic rocks indicates that much of the N initially subducted in these lithologies would be retained to depths approaching 100 km and thus available for addition to arcs. The more altered upper part of subducting oceanic crust most likely to contribute to arcs has sediment-like δ15NAir (0 to +10 per mil; Li et al., 2007, GCA), and study of HP/UHP eclogites

  3. Earthquakes, fluid pressures and rapid subduction zone metamorphism

    NASA Astrophysics Data System (ADS)

    Viete, D. R.

    2013-12-01

    High-pressure/low-temperature (HP/LT) metamorphism is commonly incomplete, meaning that large tracts of rock can remain metastable at blueschist- and eclogite-facies conditions for timescales up to millions of years [1]. When HP/LT metamorphism does take place, it can occur over extremely short durations (<<1 Myr) [1-2]. HP/LT metamorphism must be associated with processes that allow large volumes of rock to remain unaffected over long periods of time, but then suddenly undergo localized metamorphism. Existing models for HP/LT metamorphism have focussed on the role of fluids in providing heat for metamorphism [2] or catalyzing metamorphic reactions [1]. Earthquakes in subduction zone settings can occur to depths of 100s of km. Metamorphic dehydration and the associated development of elevated pore pressures in HP/LT metamorphic rocks has been identified as a cause of earthquake activity at such great depths [3-4]. The process of fracturing/faulting significantly increases rock permeability, causing channelized fluid flow and dissipation of pore pressures [3-4]. Thus, deep subduction zone earthquakes are thought to reflect an evolution in fluid pressure, involving: (1) an initial increase in pore pressure by heating-related dehydration of subduction zone rocks, and (2) rapid relief of pore pressures by faulting and channelized flow. Models for earthquakes at depth in subduction zones have focussed on the in situ effects of dehydration and then sudden escape of fluids from the rock mass following fracturing [3-4]. On the other hand, existing models for rapid and incomplete metamorphism in subduction zones have focussed only on the effects of heating and/or hydration with the arrival of external fluids [1-2]. Significant changes in pressure over very short timescales should result in rapid mineral growth and/or disequilibrium texture development in response to overstepping of mineral reaction boundaries. The repeated process of dehydration-pore pressure development

  4. Dating Subduction Zone Metamorphism with Garnet and Lawsonite Geochronology

    NASA Astrophysics Data System (ADS)

    Mulcahy, S. R.; Vervoort, J. D.

    2013-12-01

    Lawsonite [CaAl2Si2O7(OH)2 H2O] is a critical index mineral for high- to ultrahigh-pressure metamorphism associated with subduction. Lawsonite is an important carrier of water into the mantle, a likely contributor to subduction zone seismicity, and a bearer of trace elements that link metamorphism to arc magmatism. Due to its limited pressure-temperature stability, lawsonite can serve as a powerful petrogenetic indicator of specific metamorphic events. Lu-Hf dating of lawsonite, therefore provides a potentially powerful new tool for constraining subduction zone processes in a pressure-temperature window where few successful geochronometers exist. Broad application of lawsonite Lu-Hf geochronology requires constraining the role of pressure-temperature path, lawsonite forming reactions, and the Lu and Hf systematics within lawsonite and other blueschist facies minerals. We are working to address the role of the metamorphic path on the applicability of lawsonite Lu-Hf geochronology within the Franciscan Complex of California. The Franciscan Complex preserves mafic high-grade exotic blocks in melange that underwent a counterclockwise pressure-temperature path wherein garnet, which strongly partitions heavy rare-earth elements, formed prior to lawsonite. Coherent mafic rocks within the Franciscan Complex, however, underwent a clockwise pressure-temperature path and lawsonite growth occurred prior to garnet. We sampled exotic blocks of garnet-hornblendite, garnet-epidote amphibolite, garnet-epidote blueschist, and lawsonite blueschist from the Berkeley Hills and Tiburon Peninsula of California. We collected four samples from coherent lawsonite blueschist across the lawsonite-pumpellyite-epidote isograds in Ward Creek, near Cazadero California. High-grade blocks give ages similar to existing Franciscan geochronology: multi-stage garnet in hornblendite gives the following ages: 171×1.3 Ma (MSWD 2.8) for the core and 159.4×0.9 Ma (MSWD 2.0) for the corresponding rim; 166

  5. Chlorine Behavior in Metasedimentary Rocks during Subduction Zone Metamorphism

    NASA Astrophysics Data System (ADS)

    Barnes, J.; Selverstone, J.; Bebout, G. E.; Penniston-Dorland, S.

    2014-12-01

    Chlorine concentrations and isotopic compositions were measured in two well-characterized metasedimentary suites from the Catalina Schist and the Western Alps to determine Cl behavior during prograde metamorphism. The Catalina Schist is a subduction zone metamorphic complex in California, USA containing lawsonite-albite (LA) to amphibolite grade rocks recording temperatures of 350 to 750°C and depths of 15 to 45 km. Previous work has shown a decrease in N, B, Cs, As, and Sb concentrations from the LA to the epidote-blueschist facies, with relatively little loss at higher metametamorphic grade [1], and limited Li loss across all grades [2]. Metapelitic rocks from the Western Alps (Schistes Lustres and Lago di Cignana) record temperatures of 350 to 550°C and depths up to 90 km. In contrast to Catalina, N, B, Cs, Ba, and Rb concentrations are relatively uniform across grade [3]. In the Catalina Schist, Cl concentration shows a pattern of loss similar to B and N, from ~100-500 ppm Cl in the LA facies to ~100 ppm in the lawsonite-blueschist facies to relatively uniform concentrations of ~10-25 ppm at higher grades. This loss is likely not due to the breakdown of apatite as P2O5 concentrations remain constant across grade. In the Alps, Cl concentrations are overall lower and show moderate loss from ~10 ppm in the lowest grade to <5 ppm in the highest grade. δ37Cl values range from -1 to +1.6‰ and -1.7 to -0.7‰ in Catalina and the Alps, respectively. Both suites show significant isotopic heterogeneities within a single metamorphic grade and no systematic change in δ37Cl value with increasing grade. We interpret these heterogeneities to be inherited from the protolith. Despite large Cl losses, limited Cl isotope fractionation at high temperatures minimizes variations in δ37Cl value with increasing metamorphic grade. [1] Bebout et al, 1999, EPSL, 171, 53-81 [2] Penniston-Dorland et al, 2012, GCA, 77, 530-545 [3] Bebout et al, 2013, Chem Geol, 342, 1-20

  6. Does subduction zone magmatism produce average continental crust

    NASA Technical Reports Server (NTRS)

    Ellam, R. M.; Hawkesworth, C. J.

    1988-01-01

    The question of whether present day subduction zone magmatism produces material of average continental crust composition, which perhaps most would agree is andesitic, is addressed. It was argued that modern andesitic to dacitic rocks in Andean-type settings are produced by plagioclase fractionation of mantle derived basalts, leaving a complementary residue with low Rb/Sr and a positive Eu anomaly. This residue must be removed, for example by delamination, if the average crust produced in these settings is andesitic. The author argued against this, pointing out the absence of evidence for such a signature in the mantle. Either the average crust is not andesitic, a conclusion the author was not entirely comfortable with, or other crust forming processes must be sought. One possibility is that during the Archean, direct slab melting of basaltic or eclogitic oceanic crust produced felsic melts, which together with about 65 percent mafic material, yielded an average crust of andesitic composition.

  7. Anatexis of garnet amphibolites from a subduction zone metamorphic terrane

    SciTech Connect

    Sorensen, S.S.; Barton, M.D.; Ernst, W.G.

    1985-01-01

    Concomitant rehydration, metasomatism and amphibolitization of eclogite blocks from a mafic/ultramafic complex of the Catalina Schist terrane, southern California, at estimated metamorphic P approx. 8-12 kb, T approx. 600/sup 0/-700/sup 0/C was apparently accompanied by partial melting of some blocks. Mobilizates of An approx./sub 10-20/ plagioclase (PL) +/- zoisite (ZO) + quartz (QZ) + celadonitic (Si approx. 3.3 p.f.u.) white mica (WM) +/- tourmaline range from stringers and dikelets (approx. 1 cm-0.5 m) in migmatitic amphibolite blocks to dikes approx. 30 m x 3 m which intrude the surrounding, locally enstatite + chlorite +/- talc +/- aluminous actinolite +/- anthophyllite-bearing ultramafic matrix. The uniform phase proportions and the coarse-grained (PL to approx. 20 cm) pegmatitic, graphic, and myrmekitic textures displayed by the dikes and dikelets suggest that they crystallized from silicate melts. WM and ZO appear to be magmatic phases. Fe-rich GT is migmatitic portions of blocks exhibits higher Mg/(Mg + Ca) p.f.u. than GT in restitic portions of blocks; rims are richer in Mg than cores. Field relations, microprobe mineral chemistry, and bulk compositions suggest the pegmatites are low fractions of amphibolite-derived partial melt. Abundant fluid inclusions occur in GT, QZ, PL and clinopyroxene. T/sub h/ for primary H/sub 2/O-rich, low salinity L + V inclusions in GT and QZ from a migmatite range from 136-169/sup 0/C; estimates of T limits for entrapment are 530-640/sup 0/C at 8 kb, 650-780/sup 0/C at 10 kb. H/sub 2/O-rich fluids evidently enabled metasomatism, amphibolitization, and anatexis of (originally) eclogitic rocks at the P-T conditions reflected by the metamorphic mineral assemblages.

  8. Elemental responses to subduction-zone metamorphism: Constraints from the North Qilian Mountain, NW China

    NASA Astrophysics Data System (ADS)

    Xiao, Yuanyuan; Niu, Yaoling; Song, Shuguang; Davidson, Jon; Liu, Xiaoming

    2013-02-01

    Subduction zone metamorphism (SZM) and behaviors of chemical elements in response to this process are important for both arc magmatism and mantle compositional heterogeneity. In this paper, we report the results of our petrographic and geochemical studies on blueschist and eclogite facies rocks of sedimentary and basaltic protoliths from two metamorphic sub-belts with different metamorphic histories in the North Qilian Mountain, Northwest China. The protolith of low-grade blueschists is basaltic in composition and is most likely produced in a back-arc setting, while the protoliths of high-grade blueschists/eclogites geochemically resemble the present-day normal and enriched mid-oceanic ridge basalts plus some volcanic arc rocks. The meta-sedimentary rocks, including meta-graywacke, meta-pelite, meta-chert and marble, show geochemical similarity to global oceanic (subducted) sediments. Assuming that high field strength elements (HFSEs) are relatively immobile, the correlated variations of rare earth elements (REEs) and Th with HFSEs suggest that all these elements are probably also immobile, whereas Pb and Sr are mobile in rocks of both basaltic and sedimentary protoliths during SZM. Ba, Cs and Rb are immobile in rocks of sedimentary protoliths and mobile in rocks of basaltic protolith. The apparent mobility of U in rocks of basaltic protolith may be inherited from seafloor alterations rather than caused by SZM. On the basis of in situ mineral compositional analysis (both major and trace elements), the most significant trace element storage minerals in these subduction-zone metamorphic rocks are: lawsonite, pumpellyite, apatite, garnet and epidote group minerals for REEs, white micas (both phengite and paragonite) for large ion lithophile elements, rutile and titanite for HFSEs. The presence and stability of these minerals exert the primary controls on the geochemical behaviors of most of these elements during SZM. The immobility of REEs, Th and U owing to their

  9. Growth of early continental crust by water-present eclogite melting in subduction zones

    NASA Astrophysics Data System (ADS)

    Laurie, A.; Stevens, G.

    2011-12-01

    The geochemistry of well preserved Paleo- to Meso-Archaean Tonalite-Trondhjemite-Granodiorite (TTG) suite rocks, such as the ca 3.45 Ga trondhjemites from the Barberton greenstone belt in South Africa, provides insight into the origins of Earth's early felsic continental crust. This is particularly well demonstrated by the high-Al2O3 variety of these magmas, such as the Barberton rocks, where the geochemistry requires that they are formed by high pressure (HP) melting of a garnet-rich metamafic source. This has been interpreted as evidence for the formation of these magmas by anatexis of the upper portions of slabs within Archaean subduction zones. Most of the experimental data relevant to Archaean TTG genesis has been generated by studies of fluid-absent melting of metabasaltic sources. However, water drives arc magmatism within Phanerozoic subduction zones and thus, understanding the behaviour of water in Archaean subduction zones, may have considerable value for understanding the genesis of these TTG magmas. Consequently, this study investigates the role of HP water-present melting of an eclogite-facies starting material, in the production of high-Al2O3 type TTG melts. Water-saturated partial melting experiments were conducted between 1.9 and 3.0GPa; and, 870°C and 900°C. The melting reaction is characterized by the breakdown of sodic Cpx, together with Qtz and H2O, to form melt in conjunction with a less sodic Cpx: Qtz + Cpx1 + Grt1 + H2O = Melt + Cpx2 + Grt2. In many of the experimental run products, melt segregated efficiently from residual crystals, allowing for the measurement of a full range of trace elements via Laser Ablation Inductively Coupled Plasma Mass Spectroscopy. The experimental glasses produced by this study have the compositions of peraluminous trondhjemites; and they are light rare earth element, Zr and Sr enriched; and heavy rare earth element, Y and Nb depleted. The compositions of the experimental glasses are similar to high-Al2O3 type

  10. Growth of early continental crust controlled by melting of amphibolite in subduction zones.

    PubMed

    Foley, Stephen; Tiepolo, Massimo; Vannucci, Riccardo

    2002-06-20

    It is thought that the first continental crust formed by melting of either eclogite or amphibolite, either at subduction zones or on the underside of thick oceanic crust. However, the observed compositions of early crustal rocks and experimental studies have been unable to distinguish between these possibilities. Here we show a clear contrast in trace-element ratios of melts derived from amphibolites and those from eclogites. Partial melting of low-magnesium amphibolite can explain the low niobium/tantalum and high zirconium/samarium ratios in melts, as required for the early continental crust, whereas the melting of eclogite cannot. This indicates that the earliest continental crust formed by melting of amphibolites in subduction-zone environments and not by the melting of eclogite or magnesium-rich amphibolites in the lower part of thick oceanic crust. Moreover, the low niobium/tantalum ratio seen in subduction-zone igneous rocks of all ages is evidence that the melting of rutile-eclogite has never been a volumetrically important process. PMID:12075348

  11. A continuum model of continental deformation above subduction zones - Application to the Andes and the Aegean

    NASA Technical Reports Server (NTRS)

    Wdowinski, Shimon; O'Connell, Richard J.; England, Philip

    1989-01-01

    A continuum model of continental deformation above subduction zones was developed that combines the viscous sheet and the corner flow models; the continental lithosphere is described by a two-dimensional sheet model that considers basal drag resulting from the viscous asthenosphere flow underneath, and a corner flow model with a deforming overlying plate and a rigid subducting plate is used to calculate the shear traction that acts on the base of the lithosphere above a subduction zone. The continuum model is applied to the Andes and the Aegean deformations, which represent, respectively, compressional and extensional tectonic environments above subduction zones. The models predict that, in a compressional environment, a broad region of uplifted topography will tend to develop above a more steeply dippping slab, rather than above a shallower slab, in agreement with observations in the various segments of the central Andes. For an extensional environment, the model predicts that a zone of compression can develop near the trench, and that extensional strain rate can increase with distance from the trench, as is observed in the Aegean.

  12. U-Pb SHRIMP geochronology of zircon in garnet peridotite from the Sulu UHP terrane, China: Implications for mantle metasomatism and subduction-zone UHP metamorphism

    USGS Publications Warehouse

    Zhang, R.Y.; Yang, J.S.; Wooden, J.L.; Liou, J.G.; Li, T.F.

    2005-01-01

    We studied the Zhimafang ultrahigh-pressure metamorphic (UHP) peridotite from pre-pilot drill hole PP-1 of Chinese Continental Scientific Drilling project in the Sulu UHP terrane, eastern China. The peridotite occurs as lens within quartofeldspathic gneiss, and has an assemblage of Ol + Opx + Cpx + Phl + Ti-clinohumite (Ti-Chu) + Grt (or chromite) ?? magnesite (Mgs). Zircons were separated from cores at depths of 152 m (C24, garnet lhezolite), 160 m (C27, strongly retrograded phlogopite-rich peridotite) and 225 m (C50, banded peridotite), and were dated by SHRIMP mass spectrometer. Isometric zircons without inherited cores contain inclusions of olivine (Fo91-92), enstatite (En91-92), Ti-clinohumite, diopside, phlogopite and apatite. The enstatite inclusions have low Al2O3 contents of only 0.04-0.13 wt.%, indicating a UHP metamorphic origin. The weighted mean 206Pb/238U zircon age for garnet lherzolite (C24) is 221 ?? 3 Ma, and a discordia lower intercept age for peridotite (C50) is 220 ?? 2 Ma. These ages are within error and represent the time of subduction-zone UHP metamorphism. A younger lower intercept age of 212 ?? 3 Ma for a foliated wehrlite (C27) was probably caused by Pb loss during retrograde metamorphism. The source of zirconium may be partially attributed to melt/fluid metasomatism within the mantle wedge. Geochronological and geochemical data confirm that the mantle-derived Zhimafang garnet peridotites (probably the most representative type of Sulu garnet peridotites) were tectonically inserted into a subducting crustal slab and subjected to in situ Triassic subduction-zone UHP metamorphism. ?? 2005 Elsevier B.V. All rights reserved.

  13. Controls on continental strain partitioning above an oblique subduction zone, Northern Andes

    NASA Astrophysics Data System (ADS)

    Schütt, Jorina M.; Whipp, David M., Jr.

    2016-04-01

    Strain partitioning is a common process at obliquely convergent plate margins dividing oblique convergence into margin-normal slip on the plate-bounding fault and horizontal shearing on a strike-slip system parallel to the subduction margin. In subduction zones, strain partitioning in the upper continental plate is mainly controlled by the shear forces acting on the plate interface and the strength of the continental crust. The plate interface forces are influenced by the subducting plate dip angle and the obliquity angle between the normal to the plate margin and the convergence velocity vector, and the crustal strength of the continent is strongly affected by the presence or absence of a volcanic arc, with the presence of the volcanic arcs being common at steep subduction zones. Along the ˜7000 km western margin of South America the convergence obliquity, subduction dip angles and presence of a volcanic arc all vary, but strain partitioning is only observed along parts of it. This raises the questions, to what extent do subduction zone characteristics control strain partitioning in the overriding continental plate, and which factors have the largest influence? We address these questions using lithospheric-scale 3D numerical geodynamic experiments to investigate the influence of subduction dip angle, convergence obliquity, and weaknesses in the crust owing to the volcanic arc on strain partitioning behavior. We base the model design on the Northern Volcanic Zone of the Andes (5° N - 2° S), characterized by steep subduction (˜ 35°), a convergence obliquity between 31° -45° and extensive arc volcanism, and where strain partitioning is observed. The numerical modelling software (DOUAR) solves the Stokes flow and heat transfer equations for a viscous-plastic creeping flow to calculate velocity fields, thermal evolution, rock uplift and strain rates in a 1600 km x 1600 km box with depth 160 km. Subduction geometry and material properties are based on a

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

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

  15. Prograde and retrograde metamorphic processes in high-pressure subduction zone serpentinites from East Thessaly, Greece

    NASA Astrophysics Data System (ADS)

    Koutsovitis, Petros

    2016-04-01

    The East Thessaly region, Central Greece, includes metaophiolitic mélange formations which extend from the eastern foothills of Mt. Olympus and Ossa, throughout the Agia basin, Mt. Mavrovouni (Sklithro region), South Pelion and reaching up to northeast Othris (regions of Aerino and Velestino). They appear in the form of dispersed and deformed thrust sheets having been variably emplaced onto Mesozoic platform series rocks of the Pelagonian tectonostratigraphic zone[1]. These formations consist mainly of serpentinites, as well as metasediments, metagabbros, metadolerites, rodingites, ophicalcites, talc-schists and chromitites. Based upon petrographic observations, mineral chemistry data and XRD patterns, the subduction zone-related serpentinites from the regions of Potamia, Anavra, Aetolofos and Kalochori-Chasanbali (Agia basin), as well as from the regions of Aerino and Velestino, are characterized by the progressive transformation of lizardite to antigorite and are distinguished into two groups. The first group includes serpentinites from the metaophiolitic formations of Potamia, Anavra, Aerino and Velestino, which are marked by destibillization of lizardite to antigorite, mostly along the grain boundaries of the lizardite mesh textured relics. The presence of lizardite and antigorite in almost equal amounts indicates medium-temperature blueschist facies metamorphic conditions (˜340-370 ° C; P≈10-11 kbar)[2,3,4]. The second serpentinite group appears in the regions of Aetolofos and Kalochori, characterized by the predominance of antigorite, the minor occurrence of lizardite and the complete replacement of spinel by Cr-magnetite. The absence of metamorphic olivine suggests that these serpentinites were most likely formed at slightly higher temperature and pressure conditions compared to the first serpentinite group, corresponding to medium or high temperature blueschist facies metamorphism (˜360-380 ° C; P≈12 kbar)[2,3,4]. These metamorphic conditions are

  16. Continental margin deformation along the Andean subduction zone: Thermo-mechanical models

    NASA Astrophysics Data System (ADS)

    Gerbault, Muriel; Cembrano, J.; Mpodozis, C.; Farias, M.; Pardo, M.

    2009-12-01

    The Chilean Andes extend north-south for about 3000 km over the subducting Nazca plate, and show evidence of local rheological controls on first-order tectonic features. Here, rheological parameters are tested with numerical models of a subduction driven by slab-pull and upper plate velocities, and which calculate the development of stress and strain over a typical period of 4 Myr. The models test the effects of subduction interface strength, arc and fore-arc crust rheology, and arc temperature, on the development of superficial near-surface faulting as well as viscous shear zones in the mantle. Deformation geometries are controlled by the intersection of the subduction interface with continental rheological heterogeneities. Upper plate shortening and trench advance are both correlated, and favored, to a first-order by upper plate weakness, and to a second-order by interface strength. In cases of a strong interface, a weak fore-arc crust is dragged downward by “tectonic erosion”, a scenario for which indications are found along the northern Chilean margin. In contrast for a resistant fore-arc, the slab-pull force transmits to the surface and produces topographic subsidence. This process may explain present-day subsidence of the Salar de Atacama basin and/or the persistence of a Central Depression. Specific conditions for northern Chile produce a shear zone that propagates from the subduction zone in the mantle, through the Altiplano lower crust into the Sub-Andean crust, as proposed by previous studies. Models with a weak interface in turn, allow buoyant subducted material to rise into the continental arc. In case of cessation of the slab-pull, this buoyant material may rise enough to change the stress state in the continental crust, and lead to back-arc opening. In a case of young and hydrated oceanic plate forced by the slab-pull to subduct under a resistant continent, this plate is deviated and indented by the continental mantle, and stretches horizontally

  17. Fractionation of trace elements by subduction-zone metamorphism — effect of convergent-margin thermal evolution

    NASA Astrophysics Data System (ADS)

    Bebout, Gray E.; Ryan, Jeffrey G.; Leeman, William P.; Bebout, Ann E.

    1999-08-01

    Differential chemical/isotopic alteration during forearc devolatilization can strongly influence the cycling of volatile components, including some trace elements, in subduction zones. The nature and magnitude of this devolatilization effect are likely to be strongly dependent on the thermal structure of individual convergent margins. A recent model for metamorphism of the Catalina Schist, involving progressive underplating (at ≤45 km depths) of rock packets metamorphosed along successively lower- T prograde P-T paths in a rapidly cooling, newly initiated subduction zone, affords a unique evaluation of the effects of varying prograde P-T paths on the magnitudes of devolatilization and chemical/isotopic alteration of subducting rocks. In the Catalina Schist, the most extensive devolatilization occurred in metasedimentary rocks which experienced prograde P-T paths encountering the epidote-blueschist facies (>350°C at 9 to 12 kbar) or higher- T conditions; such rocks are depleted in 'fluid-mobile' elements such as N, B, Cs, As, and Sb relative to protoliths. Removal of these elements resulted in changes in B/(Be, Li, La, Zr), Cs/Th, Rb/Cs, As/Ce, Sb/Ce, and C reduced/N, and increases in δ 15N and δ 13C. The relative susceptibilities of the "fluid-mobile" elements to loss along increasingly higher- T P-T paths can be categorized. Boron and Cs show the greatest susceptibility to low- T removal by fluids, showing >50% depletion in even lawsonite-blueschist-facies metasedimentary rocks which experienced relatively low- T prograde metamorphic paths. In rocks which experienced higher- T paths, As and Sb (likely in sulfides) show the greatest depletions (>90%); N, Cs, and B (largely in micas) occur at ˜25% of protolith contents in even partially melted amphibolite-facies rocks. Variations in B/Be, Cs/Th, As/Ce, and Sb/Ce among arcs from differing convergent-margin thermal regimes, and conceivably some cross-arc declines in these ratios, are compatible with evidence

  18. Three-Dimensional Thermal Structure of the Middle-America Subduction Zone: Along-margin mantle flow and slab metamorphism

    NASA Astrophysics Data System (ADS)

    Rosas, J. C.; Currie, C. A.; He, J.

    2013-12-01

    Temperature is the primary control parameter of several processes occurring at subduction zones, such as slab metamorphism and dehydration, arc volcanism and the rupture width of megathrust earthquakes. The thermal state depends on the temperature of the oceanic slab and the flow pattern of the overlying mantle wedge. In most previous studies, mantle flow was modeled as two-dimensional (2D) corner flow, driven by the subducting plate. However, recent studies have shown the limitations of the 2D corner flow scheme, as a three-dimensional (3D) oceanic plate structure can generate along-strike pressure gradients, producing a trench-parallel flow component. One region where 3D effects may be important is the Middle America Subduction Zone (MASZ). Here, the dip of the oceanic plate varies from 0 to 70 degrees along the margin, with abrupt changes in slab dip in Central Mexico and Costa Rica-Nicaragua. Seismic anisotropy and arc magma geochemistry variations suggest a significant along-margin component of flow in these areas. Further, offshore surface heat flow measurements show that there may be along-margin variations in the temperature of the subducting oceanic plate, due to variations in plate age and hydrothermal circulation. In this study, we quantify the changes in the thermal structure of a subduction zone that result from along-margin variations in oceanic plate structure. We use 3D numerical models that consist of kinematically-defined subducting and overriding plates, and a flowing mantle wedge driven by drag exerted by the subducting plate. The finite-element code PGCtherm-3D is used to solve the steady-state governing equations for mantle wedge flow and the 3D thermal structure of the subduction zone. The models employ an oceanic plate that smoothly dips into the mantle and has along-margin variations in the deep dip of 40 and 70 degrees over a distance of 50km to 300km, as observed in some regions of the MASZ. Using an isoviscous mantle wedge, our

  19. Subduction-Zone Metamorphic Pathway for Deep Carbon Cycling: Evidence from the Italian Alps and the Tianshan

    NASA Astrophysics Data System (ADS)

    Bebout, G. E.; Collins, N.; Cook-Kollars, J.; Angiboust, S.; Agard, P.; Scambelluri, M.; John, T.; Kump, L. R.

    2013-12-01

    Depending on the magnitude of the poorly constrained C flux in ultramafic rocks, on a global basis, sediments and altered oceanic crust (AOC) together deliver 70-95% of the C currently entering subduction zones. We are investigating extents of retention and metamorphic release of C in deeply subducted AOC and carbonate-rich sediment represented by HP/UHP meta-ophiolitic and metasedimentary rocks in the Italian Alps and in the Tianshan. Study of metapelite devolatilization in the same W. Alps suite (Bebout et al., 2013, Chem. Geol.) provides a geochemical framework for study of C behavior along prograde P-T paths similar to those experienced in forearcs of most modern subduction margins. Study of veins in the Tianshan affords examination of C mobility in UHP fluids, in later stages as metabasaltic rocks were fragmented in the subduction channel. Our results for sediments and AOC indicate impressive retention of oxidized C (carbonate) and reduced C (variably metamorphosed organic matter) to depths approaching those beneath arc volcanic fronts. In metasedimentary rocks, extensive isotopic exchange between the oxidized and reduced C resulted in shifts in both reservoirs toward upper mantle compositions. Much of the carbonate in metabasalts has C and O isotopic compositions overlapping with those for carbonate in AOC, with some HP/UHP metamorphic veins showing greater influence of organic C signatures from metasedimentary rocks. Calculations of prograde devolatilization histories using Perple-X demonstrate that, in most forearcs, very little decarbonation occurs in the more carbonate-rich rocks unless they are flushed by H2O-rich fluids from an external source, for example, from the hydrated ultramafic section of subducting slabs (cf. Gorman et al., 2006; G3) or from more nearby rocks experiencing dehydration (e.g., metapelites). A comparison of the most recently published thermal models for modern subduction zones (van Keken et al., 2011, JGR) with calculated and

  20. High-pressure mafic oceanic rocks from the Makbal Complex, Tianshan Mountains (Kazakhstan & Kyrgyzstan): Implications for the metamorphic evolution of a fossil subduction zone

    NASA Astrophysics Data System (ADS)

    Meyer, Melanie; Klemd, Reiner; Konopelko, Dmitry

    2013-09-01

    The Makbal Complex in the western Tianshan Mountains of Kazakhstan and Kyrgyzstan consists of HP/UHP metasedimentary host rocks which enclose various HP mafic blocks or boudins. These mafic rocks comprise rare eclogites (sensu stricto and sensu lato), garnet amphibolites (retrograded eclogites) and a newly discovered glaucophanite (glaucophane-garnet-omphacite bearing rock). So far the Makbal Complex has been interpreted to predominantly consist of continental lithologies and the mafic rocks were considered as dismembered dikes intruding continental metasediments. This interpretation is mainly based on the geological relationship and bulk rock chemistry of the different rock types. It was further suggested that the continental lithologies of the Makbal Complex underwent eclogite-facies metamorphism in a former subduction zone. In the present study we combined conventional geothermometry, P-T pseudosection modeling and major and trace element whole rock geochemistry for different mafic samples (glaucophanite and eclogites (sensu lato)) in order to shed light on both the metamorphic evolution and the protoliths of the mafic HP rocks in the Makbal Complex. Prograde to peak-pressure clockwise P-T paths of glaucophanite and eclogites (sensu lato) were modeled using garnet isopleth thermobarometry. The results show that the glaucophanite and eclogite (sensu lato) samples experienced similar prograde P-T paths and slightly different peak metamorphic conditions at ~ 560 °C at 2.4 GPa for the former and between ~ 520 °C at 2.2 GPa and ~ 555 °C at ~ 2.5 GPa for the latter, corresponding to burial depths between 70 and 85 km. Whole rock major and trace element analyses and petrological evidence imply that the various rock types at the Makbal Complex most likely originated from different precursor rocks. Eclogites (sensu lato) are believed to represent strongly retrogressed former eclogite-facies rocks that had never been eclogites (sensu stricto, i.e. > 70 vol.% garnet and

  1. Lu Hf and Ar Ar chronometry supports extreme rate of subduction zone metamorphism deduced from geospeedometry

    NASA Astrophysics Data System (ADS)

    Philippot, Pascal; Blichert-Toft, Janne; Perchuk, Alexei; Costa, Sylvie; Gerasimov, Vladimir

    2001-12-01

    Recent diffusion modeling of eclogitic garnets from the Great Caucasus, Russia, and Yukon, Canada, have shown that the preservation of garnet growth zoning in rocks that have equilibrated at high temperature (680-700 °C) is possible only if rates of pressure and temperature change on the burial and/or exhumation paths are in the order of several cm/year and several hundreds of °C/Ma. In order to confirm this observation, we performed Lu-Hf and Sm-Nd dating of garnet and Ar-Ar dating of mica on the same samples that were used for geospeedometry measurements in an earlier study. In both localities, garnet grew during prograde metamorphism at 690±40 °C and >1.5 GPa (Yukon) and 680±40 °C and >1.6 GPa (Great Caucasus). In contrast, phengite formed soon after the main eclogitic foliation at 520±50 °C (Yukon) and 600±40 °C (Great Caucasus). Garnet of the Yukon samples yielded Lu-Hf ages of 252±7, 255±7, 257±6 and 264±6 Ma that fall within error of phengite Ar-Ar integrated ages of 261±2 (laser spot date) and 256±3 Ma (age of mineral separates). No Sm-Nd ages were measured on the Yukon samples. For Great Caucasus samples, all Sm-Nd ages with the exception of one garnet-whole rock pair yielding a Sm-Nd age of 311±22 Ma are poorly constrained. In contrast, the Lu-Hf garnet chronometer yields ages of 322±14, 316±5 and 296±11 Ma that again fall within error of the phengite Ar-Ar mean age of 303±5 Ma. Because the geospeedometry approach provides information on cooling rates, information on the closure temperature of a given isotopic system can be extracted from the analytical solution of Dodson [Contrib. Mineral. Petrol. 40 (1973) 259] using appropriate sets of experimentally determined diffusion data. The results of these calculations indicate that uncertainties of more than 200 °C are to be expected for the Sm-Nd and Lu-Hf closure temperatures for both the Great Caucasus (750±150 °C) and Yukon samples (710±120 °C). In all cases, calculated closure

  2. Insights into a fossil plate interface of an erosional subduction zone: a tectono-metamorphic study of the Tianshan metamorphic belt.

    NASA Astrophysics Data System (ADS)

    Bayet, Lea; Moritz, Lowen; Li, Jilei; Zhou, Tan; Agard, Philippe; John, Timm; Gao, Jun

    2016-04-01

    Subduction zone seismicity and volcanism are triggered by processes occurring at the slab-wedge interface as a consequence of metamorphic reactions, mass-transfer and deformation. Although the shallow parts of subduction zones (<30-40 km) can be partly accessed by geophysical methods, the resolution of these techniques is insufficient to characterize and image the plate interface at greater depths (>60km). In order to better understand the plate interface dynamics at these greater depths, one has to rely on the rock record from fossil subduction zones. The Chinese Tianshan metamorphic belt (TMB) represents an ideal candidate for such studies, because structures are well exposed with exceptionally fresh high-pressure rocks. Since previous studies from this area focused on fluid-related processes and its metamorphic evolution was assessed on single outcrops, the geodynamic setting of this metamorphic belt is unfortunately heavily debated. Here, we present a new geodynamic concept for the TMB based on detailed structural and petrological investigations on a more regional scale. A ~11km x 13km area was extensively covered, together with E-W and N-S transects, in order to produce a detailed map of the TMB. Overall, the belt is composed of two greenschist-facies units that constitute the northern and southern border of a large high-pressure (HP) to ultra high-pressure (UHP) unit in the center. This HP-UHP unit is mainly composed of metasediments and volcanoclastic rocks, with blueschist, eclogite and carbonate lenses. Only the southern part of the HP-UHP unit is composed of the uppermost part of an oceanic crust (e.g., pillow basalts and deep-sea carbonates). From south to north, the relative abundance and size of blueschist massive boudins and layers (as well as eclogite boudins) decreases and the sequence is increasingly interlayered with metasedimentary and carbonate-rich horizons. This indicates that the subducted material was dominated by trench filling made of

  3. Stress states in subduction zones: Extrapolation of flow laws and piezometric relations of quartz to high-P/low-T metamorphic conditions

    NASA Astrophysics Data System (ADS)

    Shimizu, I.; Ueda, T.

    2015-12-01

    Strength profiles across the continental lithosphere have been extensively discussed based on laboratory rock-mechanics data but little is known about the stress states in subduction zones, which are characterized by the presence of aqueous fluids and siliceous sedimentary rocks. Here we focus on the deep parts of the interplate megathrusts that consist of high-P/low-T metamorphic rocks. For example, the depth range from 45 to 60 km of the interplate megathrust in NE Japan, which caused the 2011 M9.0 Tohoku-oki earthquake, corresponds to the blueschist facies zone with temperatures ranging from 300 to 400 C (Shimizu, 2014). To evaluate the stress states during interseismic periods, we used the dislocation creep flow law of wet quartz proposed by Luan and Paterson (1992). We applied the flow law without making a water fugacity correction, because the strength of quartz in high-P/low-T conditions would be governed by concentrations of water-related species within the crystals rather than water fugacity. Thus the present model gives conservative estimates. The result of calculation indicates the shear strength over 200 MPa at the depth of 45 km and 50-100 MPa at 60 km depth. Recrystallized grain size in quartz rocks has been considered as an indicator of paleo-stress but physical basis of the piezometric relations is still in debate. In the theoretical models of grain size piezometers, formation of new grains and grain growth associated with strain-energy or surface-energy driven grain-boundary migration (ρGBM or γGBM) have been considered as elementary processes of dynamic recrystallization (DRX). Recent dynamic equilibrium models predict negative temperature dependence of the mean grain size (e.g., Shimizu, 2008); however, Platt and Behr (2011) pointed out that the processes of ρGBM and γGBM are locally antagonistic and questioned the model assumptions. Here we evaluate relative importance of surface energy in DRX of quartz and proposed a new model that includes

  4. Petrofabrics and Water Contents of Peridotites from the Western Gneiss Region (Norway): Implications for Fabric Transition of Olivine in Continental Subduction Zones

    NASA Astrophysics Data System (ADS)

    Wang, Q.; Xia, Q.; O'Reilly, S.; Griffin, W. L.; Beyer, E.

    2010-12-01

    significantly affect the fabric development of peridotites during the continental collision. Combined with field observations in the WGR and recent deformation experiments on olivine, we propose that the B- and C-type fabrics of olivine were formed during the subduction of the Baltic plate in fluid-limited conditions. The combination of UHP and low temperature plays a more important role than water to promote [001] slip in continental subduction zones. The spatial distribution of olivine fabrics in the WGR could be related with the increasing pressure from south to north, i.e., the HP to UHP metamorphism transition. It is probable that in continental subduction zones, the B- and C-type fabrics will predominate over the A-type fabric with increasing depths of the subducting lithospheric mantle, and the C-type fabric is more easily to activated at pressure higher than 4 GPa on low geothermal gradients. Therefore the olivine C-type fabric may be a marker of ultradeep subduction.

  5. Partial melting, fluid supercriticality and element mobility in ultrahigh-pressure metamorphic rocks during continental collision

    NASA Astrophysics Data System (ADS)

    Zheng, Yong-Fei; Xia, Qiong-Xia; Chen, Ren-Xu; Gao, Xiao-Ying

    2011-08-01

    Partial melting at continental lithosphere depths plays an important role in generating geochemical variations in igneous rocks. In particular, dehydration melting of ultrahigh-pressure (UHP) metamorphic rocks during continental collision provides a petrological link to intracrustal differentiation with respect to the compositional evolution of continental crust. While island arc magmatism represents one end-member of fluid-induced large-scale melting in the mantle wedge during subduction of the oceanic crust, the partial melting of UHP rocks can be viewed as the other end-member of fluid-induced small-scale anatexis during exhumation of the deeply subducted continental crust. This latter type of melting is also triggered by metamorphic dehydration in response to P-T changes during the continental collision. It results in local occurrences of hydrous melts (even supercritical fluids) as felsic veinlets between boundaries of and multiphase solid inclusions in UHP metamorphic minerals as well as local accumulation of veinlet-like felsic leucosomes in foliated UHP metamorphic rocks and metamorphically grown zircons in orogenic peridotites. Thus, very low-degree melts of UHP rocks provide a window into magmatic processes that operated in continental subduction zones. This article presents a review on available results from experimental petrology concerning the possibility of partial melting under conditions of continental subduction-zone metamorphism, and petrological evidence for the occurrence of dehydration-driven in-situ partial melting in natural UHP rocks during the continental collision. Although the deeply subducted continental crust is characterized by a relative lack of aqueous fluids, the partial melting in UHP rocks commonly takes place during decompression exhumation to result in local in-situ occurrences of felsic melts at small scales. This is caused by the local accumulation of aqueous fluids due to the breakdown of hydrous minerals and the exsolution

  6. Modeling the effects of geological heterogeneity and metamorphic dehydration on slow slip and shallow deformation in subduction zones

    NASA Astrophysics Data System (ADS)

    Skarbek, Robert M.

    Slow slip and tectonic tremor in subduction zones take place at depths (˜20 - 50 km) where there is abundant evidence for distributed shear over broad zones (˜10 - 103 m) composed of rocks with marked differences in mechanical properties and for near lithostatic pore pressures along the plate interface where the main source of fluids must be attributed to chemical dehydration reactions. In Chapter II, I model quasi-dynamic rupture along faults composed of material mixtures characterized by different rate-and-state-dependent frictional properties to determine the parameter regime capable of producing slow slip in an idealized subduction zone setting. Keeping other parameters fixed, the relative proportions of velocity-weakening (VW) and velocity-strengthening (VS) materials control the sliding character (stable, slow, or dynamic) along the fault. The stability boundary between slow and dynamic is accurately described by linear analysis of a double spring-slider system with VW and VS blocks. In Chapter III, I model viscoelastic compaction of material subducting through the slow slip and tremor zone in the presence of pressure and temperature-dependent dehydration reactions. A dehydration fluid source is included using 1) a generalized basalt dehydration reaction in subducting oceanic crust or 2) a general nonlinear kinetic reaction rate law parameterized for an antigorite dehydration reaction. Pore pressures in excess of lithostatic values are a robust feature of simulations that employ parameters consistent with the geometry of the Cascadia subduction margin. Simulations that include viscous deformation uniformly generate traveling porosity waves that transport increased fluid pressures within the slow slip region. Slow slip and tremor also occur in shallow (< 10 km depth) accretionary prism sections of subduction zones. In Chapter IV, I examine how geologic heterogeneities affect the mechanics of accretionary prisms in subduction zones by showing how spatial

  7. Fluid processes in subduction zones.

    PubMed

    Peacock, S A

    1990-04-20

    Fluids play a critical role in subduction zones and arc magmatism. At shallow levels in subduction zones (<40 kilometers depth), expulsion of large volumes of pore waters and CH(4)-H(2)O fluids produced by diagenetic and low-grade metamorphic reactions affect the thermal and rheological evolution of the accretionary prism and provide nutrients for deep-sea biological communities. At greater depths, H(2)O and CO(2) released by metamorphic reactions in the subducting oceanic crust may alter the bulk composition in the overlying mantle wedge and trigger partial melting reactions. The location and conse-quences of fluid production in subduction zones can be constrained by consideration of phase diagrams for relevant bulk compositions in conjunction with fluid and rock pressure-temperature-time paths predicted by numerical heat-transfer models. Partial melting of subducting, amphibole-bearing oceanic crust is predicted only within several tens of million years of the initiation of subduction in young oceanic lithosphere. In cooler subduction zones, partial melting appears to occur primarily in the overlying mantle wedge as a result of fluid infiltration. PMID:17784486

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  9. Mineralogical Evidence for the Bulk Transformation of Continental Crust to Ultrahigh-Pressure Conditions in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Peterman, E. M.; Hacker, B. R.; Kylander-Clark, A. R.

    2005-12-01

    Evidence for (ultra)high-pressure --(U)HP-- metamorphism in modern orogenic belts and the preservation of exhumed (U)HP terranes around the world suggest that subduction and exhumation of continental crust plays an important role in Phanerozoic plate tectonics. The Western Gneiss region (WGR) of Norway, a major (U)HP province extending over 60,000 km2, provides an excellent opportunity to study how subduction to depths >100 km affects continental crust. By studying a ~60 km wide transect bounded to the north by Vartdalsfjorden and Rovdefjorden and the south by the Möre og Romsdal county boundary, we are able to examine mineralogical changes that occurred during subduction and exhumation within a rock composed predominantly of orthogneiss and variably transformed mafic bodies, which indicate the depths to which these rocks were subducted. Previous studies (e.g. Hacker et al., 2005) have suggested that Caledonian deformation in WGR host gneisses is primarily limited to brittle-ductile fabrics characterized by greenschist to lower-amphibolite facies metamorphism; the majority of the deformation in the rocks, including the pervasive foliation and foliation-parallel isoclinal folds, occurred between 1200 and 900 Ma. On the northern half of our study area, however, locally occurring neoblastic garnet crosscuts the foliation in the gneiss. The boundary of this garnet zone coincides with the local HP-UHP boundary, as determined by the presence of coesite in eclogite. Because garnet can retain information about changes in pressure and temperature, as well as the availability of water within the crust to catalyze chemical reactions, our findings suggest that 1) portions of the orthogneiss did transform at high pressures, 2) the presence of garnet within the orthogneiss may indicate conditions that approximate UHP and can therefore be useful in defining the boundaries between UHP and HP conditions, and 3) the growth of garnet during (U)HP metamorphism may be controlled by

  10. Evolution of a Subduction Zone

    NASA Astrophysics Data System (ADS)

    Noack, Lena; Van Hoolst, Tim; Dehant, Veronique

    2014-05-01

    The purpose of this study is to understand how Earth's surface might have evolved with time and to examine in a more general way the initiation and continuance of subduction zones and the possible formation of continents on an Earth-like planet. Plate tectonics and continents seem to influence the likelihood of a planet to harbour life, and both are strongly influenced by the planetary interior (e.g. mantle temperature and rheology) and surface conditions (e.g. stabilizing effect of continents, atmospheric temperature), but may also depend on the biosphere. Employing the Fortran convection code CHIC (developed at the Royal Observatory of Belgium), we simulate a subduction zone with a pre-defined weak zone (between oceanic and continental crust) and a fixed plate velocity for the subducting oceanic plate (Quinquis et al. in preparation). In our study we first investigate the main factors that influence the subduction process. We simulate the subduction of an oceanic plate beneath a continental plate (Noack et al., 2013). The crust is separated into an upper crust and a lower crust. We apply mixed Newtonian/non-Newtonian rheology and vary the parameters that are most likely to influence the subduction of the ocanic plate, as for example density of the crust/mantle, surface temperature, plate velocity and subduction angle. The second part of our study concentrates on the long-term evolution of a subduction zone. Even though we model only the upper mantle (until a depth of 670km), the subducted crust is allowed to flow into the lower mantle, where it is no longer subject to our investigation. This way we can model the subduction zone over long time spans, for which we assume a continuous inflow of the oceanic plate into the investigated domain. We include variations in mantle temperatures (via secular cooling and decay of radioactive heat sources) and dehydration of silicates (leading to stiffening of the material). We investigate how the mantle environment influences

  11. H2O-fluid-saturated melting of subducted continental crust facilitates exhumation of ultrahigh-pressure rocks in continental subduction zones

    NASA Astrophysics Data System (ADS)

    Labrousse, L.; Duretz, T.; Gerya, T.

    2015-10-01

    We present two-dimensional numerical models of plate subduction and collision inspired by the Scandinavian Caledonian orogeny to investigate the possible impact of continental crust partial melting on the exhumation of ultra-high pressure metamorphic rocks. Three possible reactions were tested: low temperature solidus representing H2O-fluid-saturated partial melting, and two end-member reaction curves for dehydration melting. Thermo-mechanical effects of partial melting were implemented as (1) a viscosity decrease as a determined rheologically critical melt percentage was reached (here 0.1), (2) a change in effective heat capacity and adiabatic heating/cooling accounting for a latent heat term in the heat equation. Among the 3 tested reactions, only H2O-fluid-saturated partial melting drastically modifies the collision dynamics from the non-melting reference model holding all other parameters constant. A substantially low general viscosity truncation (here 1017 Pa s) is needed to properly resolve the effect of partial melting on deep collision processes. Low temperature melting indeed induces the development of a low viscosity buoyant plume prior to slab detachment, where migmatites exhume from UHP conditions at rates and with pressure-temperature paths similar to the natural values acknowledged for the Norwegian Caledonides. High temperature melting has no drastic influence on early collision dynamics. While positive buoyancy remains the first order driver for the exhumation of buried continental rocks, exhumation initiates in these cases with eduction subsequent to slab detachment. Melting and formation of a migmatite plume can later occur along decompression path while continental crust undergoes thermal reequilibration at temperatures above 900 °C. Some of the partially molten material can also relaminate in the overriding plate rather than exhume within the collision zone. Even if minor in terms of amount of magma produced, H2O-fluid-saturated partial melting

  12. Late Triassic alkaline complex in Sulu UHP terrane: Implications for post-collisional magmatism along the continental subduction zone

    NASA Astrophysics Data System (ADS)

    Xu, H.; Song, Y.; Liu, Q.

    2014-12-01

    In order to insight into crust-mantle interaction triggered by partial melting of the subudcted continental crust during its exhumation, we carried out a combined study on Shidao alkaline complex in the Sulu ultrahigh pressure (UHP) terrane. The alkaline complex is composed of shoshonitic to ultrapotassic gabbro, pyroxene syenite, amphibole syenite, quartz syenite, and granite. Field researches suggest that the mafic rocks are earlier than the felsic ones in sequence. LA-ICPMS zircon U-Pb dating on them gives Late Triassic ages of 214 ± 2 to 200 ± 3 Ma from mafic to felsic rocks. These ages are a bit younger than Late Triassic ages for partial melting of the Sulu UHP terrane during exhumation, indicating syn-exhumation magmatism during continental collision. The alkaline rocks have wide ranges of SiO2 (49.7 - 76.7 wt.%), MgO (8.25 - 0.03 wt.%),total Fe2O3 (9.23 - 0.47 wt.%), CaO (8.39 - 0.39 wt.%), Ni (126.0 - 0.07 ppm), and Cr (182.0 - 0.45 ppm) contents. Other major oxides are regularly changed with SiO2. The alkaline rocks have characteristics of arc-like patterns in the trace element distribution, e.g., enrichment of LREE and LILE (Rb, Ba, Th and U), depletion of HFSE (Nb, Ta, P and Ti), and positive Pb anomalies. From the mafic to felsic rocks, (La/Yb)N ratios and contents of the total REE, Sr and Ba are decreased but Rb contents are increased. The alkaline rocks also display features of A2-type granitoids, suggesting a post-collisional magmatism. They have high initial 87Sr/86Sr ratios (0.70575 and 0.70927) and negative ɛNd(t) values (-18.6 to -15.0) for whole-rock. The homogeneous initial 87Sr/86Sr ratios and ɛNd(t) values of the alkaline rocks are almost unchanged with SiO2 and MgO contents, suggesting a fractional crystallization (FC) process from a same parental magma. Our studies suggest a series of crust-mantle interaction processes along the continental subduction interface as follows: (1) melts from partial melting of the subducted continental

  13. Geodynamic models of terrane accretion: Testing the fate of island arcs, oceanic plateaus, and continental fragments in subduction zones

    NASA Astrophysics Data System (ADS)

    Tetreault, J. L.; Buiter, S. J. H.

    2012-08-01

    Crustal growth at convergent margins can occur by the accretion of future allochthonous terranes (FATs), such as island arcs, oceanic plateaus, submarine ridges, and continental fragments. Using geodynamic numerical experiments, we demonstrate how crustal properties of FATs impact the amount of FAT crust that is accreted or subducted, the type of accretionary process, and the style of deformation on the overriding plate. Our results show that (1) accretion of crustal units occurs when there is a weak detachment layer within the FAT, (2) the depth of detachment controls the amount of crust accreted onto the overriding plate, and (3) lithospheric buoyancy does not prevent FAT subduction during constant convergence. Island arcs, oceanic plateaus, and continental fragments will completely subduct, despite having buoyant lithospheric densities, if they have rheologically strong crusts. Weak basal layers, representing pre-existing weaknesses or detachment layers, will either lead to underplating of faulted blocks of FAT crust to the overriding plate or collision and suturing of an unbroken FAT crust. Our experiments show that the weak, ultramafic layer found at the base of island arcs and oceanic plateaus plays a significant role in terrane accretion. The different types of accretionary processes also affect deformation and uplift patterns in the overriding plate, trench migration and jumping, and the dip of the plate interface. The resulting accreted terranes produced from our numerical experiments resemble observed accreted terranes, such as the Wrangellia Terrane and Klamath Mountain terranes in the North American Cordilleran Belt.

  14. Ultrahigh-pressure metamorphism: tracing continental crust into the mantle

    NASA Astrophysics Data System (ADS)

    Chopin, Christian

    2003-07-01

    More and more evidence is being discovered in Phanerozoic collision belts of the burial of crustal rocks to previously unsuspected (and ever increasing) depths, presently on the order of 150-200 km, and of exhumation from such depths. This extends by almost one order of magnitude the depth classically ascribed to the metamorphic cycling of continental crust, and demonstrates its possible subduction. The pieces of evidence for this new, ultrahigh-pressure (UHP) metamorphism exclusively occur in the form of relics of high-pressure minerals that escaped back-transformation during decompression. The main UHP mineral indicators are the high-pressure polymorphs of silica and carbon, coesite and microdiamond, respectively; the latter often demonstrably precipitated from a metamorphic fluid and is completely unrelated to kimberlitic diamond or any shock event. Recent discoveries of pyroxene exsolutions in garnet and of coesite exsolutions in titanite suggest a precursor garnet or titanite containing six-fold coordinated silicon, therefore still higher pressures than implied by diamond stability, on the order of 6 GPa. The UHP rocks raise a formidable geological problem: that of the mechanisms responsible for their burial and, more pressingly, for their exhumation from the relevant depths. The petrological record indicates that large tracts of UHP rocks were buried to conditions of low T/ P ratio, consistent with a subduction-zone context. Decompression occurred in most instances under continuous cooling, implying continuous heat loss to the footwall and hangingwall of the rising body. This rise along the subduction channel - an obvious mechanical discontinuity and weak zone - may be driven by buoyancy up to mid-crustal levels as a result of the lesser density of the acidic crustal rocks (even if completely re-equilibrated at depth) after delamination from the lower crust, in a convergent setting. Chronological studies suggest that the rates involved are typical plate

  15. Crustal electrical conductivity of the Indian continental subduction zone: New data from the profile in the Garhwal Himalaya

    NASA Astrophysics Data System (ADS)

    Sokolova, E. Yu.; Israil, M.; Gupta, P.; Koshurnikov, A. V.; Smirnov, M. Yu.; Cherevatova, M. V.

    2016-03-01

    We present the results of studying the geoelectrical structure of the zone of continental subduction of the Indian lithospheric plate within the Gahrwal Himalaya. In the framework of the Russian-Indian project, the data of the broadband magnetotelluric soundings conducted by the Indian Institute of Technology Roorkee on the regional profile across the structures of the orogen were expanded, processed, and interpreted by the new program tools adapted for the measurements in the mountain conditions and for the presence of industrial noise. The constructed model of the deep electrical conductivity cross section for Garhwal revealed its two-dimensional (2D) features and more accurately delineated the location of the midcrustal conductor associated with the ramp structure of the detachment plane. The correlations with the regional distribution of the earthquake hypocenters and the seismotomographic images suggest a common, fluid-related nature of the seismic and geoelectrical anomalies in the crust of the Garhwal Tectonic Corridor and enabled the identification of the seismogenerating zones. Among the data of the expanded profile set of magnetotelluric and magnetovariational transfer functions, the response of a poorly explored deep conductive body is revealed. This object is located east of the profile and is probably associated with the activation of the ancient trans-Himalayan cratonic structures which prepares the segmentation of the Himalayan arc.

  16. Seafloor morphology of the continental slope in front the Petacalco Bay and its tsunamigenic relationship at the Mexican sector of the Middle American subduction zone

    NASA Astrophysics Data System (ADS)

    Mortera-Gutierrez, C. A.; Bandy, W. L.; Millan-Motolinia, C.; Ponce-Nuñez, F.; Ortega-Ramirez, J.

    2014-12-01

    The recent occurrence of offshore, large, earthquake ruptures in the western limit of the Guerrero Seismic Gap and the scattered data of seafloor morphology of the continental slope along this sector at the Mexican Mid American subduction zone have encouraged the UNAM marine geophysical group to initiate a mapping program at the Guerrero margin, from the shelf break to the Middle American Trench. The main objective of this initiative is to have a complete cover of the seafloor morphology of the Guerrero slope as the background data for comparative studies of the seafloor deformation in case of future offshore earthquake ruptures in this region. At he first stage of this initiative, we have mapped the continental slope in front the Petacalco Bay, west of the Guerrero Seismic Gap, where three important large earthquakes occurred and caused great damages in Mexico City: Petatlán earthquake (Mw=7.6) at 1979, Michoacán earthquake (Mw=8.1) and its aftershock (Mw=7.9) at 1985. Geophysical results of two campaigns carry in 2012 (MAMRIV12) and 2013 (BABPET13) on board the BO EL PUMA are presented which include multibeam data and subbottom profiles. These data sets cover an area between 101°W and 103°W, and from the shelf-slope break to the trench. The multibeam chart shows details of the hydrological erosion induced by many submarine cannons at the upper slope, whereas the seafloor relief in the lower slope is dominated by tectonic structures. The subbottom profiles and the seafloor morphology evidence zones of big slumps and faults. For first time the Rio Balsas submarine cannon is completed chart, reaching the trench basin. The river course is deflected, possibly by shear faulting. There are slump sites near the trench that probably one is associated to the 1925 tsunami at Zihuatanejo, Guerrero. The 1985 Michoacán aftershock was accompany by a small Tsunami. At that time, the lack of morphology data in this slope inhibited further studies of seafloor-deformation and

  17. Modeling the migration of fluids in subduction zones

    NASA Astrophysics Data System (ADS)

    Spiegelman, M.; Wilson, C. R.; van Keken, P. E.; Hacker, B. R.

    2010-12-01

    Fluids play a major role in the formation of arc volcanism and the generation of continental crust. Progressive dehydration reactions in the downgoing slab release fluids to the hot overlying mantle wedge, causing flux melting and the migration of melts to the volcanic front. While the qualitative concept is well established the quantitative details of fluid release and especially that of fluid migration and generation of hydrous melting in the wedge is still poorly understood. Here we present new models of the fluid migration through the mantle wedge for subduction zones that span the spectrum of arcs worldwide. We focus on the flow of water and use an existing set of high resolution thermal and metamorphic models (van Keken et al., JGR, in review) to predict the regions of water release from the sediments, upper and lower crust, and upper most mantle. We use this water flux as input for the fluid migration calculation based on new finite element models built on advanced computational libraries (FEniCS/PETSc) for efficient and flexible solution of coupled multi-physics problems. The first generation of these models solves for the evolution of porosity and fluid-pressure/flux throughout the slab and wedge given solid flow, viscosity and thermal fields from the existing thermal models. Fluid flow in the new models depends on both permeability and the rheology of the slab-wedge system as interaction with rheological variability can induce additional pressure gradients that affect the fluid flow pathways. We will explore the sensitivity of fluid flow paths for a range of subduction zones and fluid flow parameters with emphasis on variability of the location of the volcanic arc with respect to flow paths and expected degrees of hydrous melting which can be estimated given a variety of wet-melting parameterizations (e.g. Katz et al, 2003, Kelley et al, 2010). The current models just include dehydration reactions but work continues on the next generation of models which

  18. Building a Subduction Zone Observatory

    USGS Publications Warehouse

    Gomberg, Joan S.; Bodin, Paul; Bourgeois, Jody; Cashman, Susan; Cowan, Darrel; Creager, Kenneth C.; Crowell, Brendan; Duvall, Alison; Frankel, Arthur; Gonzalez, Frank; Houston, Heidi; Johnson, Paul; Kelsey, Harvey; Miller, Una; Roland, Emily C.; Schmidt, David; Staisch, Lydia; Vidale, John; Wilcock, William; Wirth, Erin

    2016-01-01

    Subduction zones contain many of Earth’s most remarkable geologic structures, from the deepest oceanic trenches to glacier-covered mountains and steaming volcanoes. These environments formed through spectacular events: Nature’s largest earthquakes, tsunamis, and volcanic eruptions are born here.

  19. Simulation of convective heat exchange in the subduction zone

    NASA Astrophysics Data System (ADS)

    Solov'ev, S. V.

    2013-09-01

    Results of the mathematical simulation of the convective heat exchange in the process of movement of a lithospheric plate colliding with a continental plate and submerging into the mantle in the subduction zone under conditions where the free fall acceleration in the mantle changes by the linear law are presented.

  20. Why subduction zones are curved

    NASA Astrophysics Data System (ADS)

    Mahadevan, L.; Bendick, R.; Liang, Haiyi

    2010-12-01

    We give an explanation for the polarity, localization, shape, size, and initiation of subduction zones on Earth. By considering a soft, thin, curved lithospheric cap with either elastic or viscous rheology supported by a thick, nearly incompressible mantle, we find two different characteristic subduction geometries arise depending on boundary conditions: (1) plate boundaries where subduction results primarily from the gravitational body force (free subduction) have characteristic plate lengths and form arc-shaped dimpled segments resulting from the competition between bending and stretching in edge buckling modes of thin spherical shells, and (2) subduction zones due to localized applied loads that push one slab of thin, positively buoyant lithosphere beneath an overriding plate (forced subduction) form localized straight segments, consistent with the deformation of indented spherical shells. Both types of subduction are nonlinear subcritical instabilities, so small perturbations in the mechanical properties of the lithosphere have pronounced effects on subduction initiation and evolution. Yet in both cases, geometric relationships determined by the shape of the Earth itself play the most critical role in controlling the basic morphology and characteristic length scales of subduction zones.

  1. Lithification facilitates frictional instability in argillaceous subduction zone sediments

    NASA Astrophysics Data System (ADS)

    Trütner, Sebastian; Hüpers, Andre; Ikari, Matt J.; Yamaguchi, Asuka; Kopf, Achim J.

    2015-12-01

    Previous work suggests that in subduction zones, the onset of large earthquake nucleation at depths > ~ 5-10 km is likely driven by a combination of factors associated with the process of lithification. At these depths, lithification processes affect the entire fault system by modifying the mechanical properties of both the plate boundary fault zone and the wall-rock. To test the hypothesis that lithification of subduction zone sediments produces rocks capable of earthquake nucleation via diagenesis and low-grade metamorphism, we conducted friction experiments on fossil subduction zone sediments recovered from exposures in the Shimanto Belt in SW Japan. These meta-sediments represent accreted and subducted material which has experienced maximum temperatures of 125 to 225 °C, which are representative of seismogenic depths along the active Nankai subduction megathrust in the foreland of the Shimanto Belt. We find that intact Shimanto rock samples, which preserve the influence of diagenetic and metamorphic processes, exhibit the potential for unstable slip under in-situ pressure conditions. Powdered versions of the same samples tested under the same conditions exhibit only velocity-strengthening friction, thus demonstrating that destroying the lithification state also removes the potential for unstable slip. Using advanced porosity loss to quantify the lithification process, we demonstrate that increased velocity weakening correlates with increasingly advanced lithification. In combination with documented frictionally stable behavior of subduction zone sediments from shallower depths, our results provide evidence that the sediment lithification hypothesis can explain the depth-dependent onset of large earthquake nucleation along subduction zone megathrusts.

  2. Modeling the Migration of Fluids in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Wilson, C. R.; Spiegelman, M.; Van Keken, P. E.; Vrijmoed, J. C.; Hacker, B. R.

    2011-12-01

    Fluids play a major role in the formation of arc volcanism and the generation of continental crust. Progressive dehydration reactions in the downgoing slab release fluids to the hot overlying mantle wedge, causing flux melting and the migration of melts to the volcanic front. While the qualitative concept is well established, the quantitative details of fluid release and especially that of fluid migration and generation of hydrous melting in the wedge is still poorly understood. Here we present new models of the fluid migration through the mantle wedge for subduction zones. We use an existing set of high resolution metamorphic models (van Keken et al, 2010) to predict the regions of water release from the sediments, upper and lower crust, and upper most mantle. We use this water flux as input for the fluid migration calculation based on new finite element models built on advanced computational libraries (FEniCS/PETSc) for efficient and flexible solution of coupled multi-physics problems. The first generation of one-way coupled models solves for the evolution of porosity and fluid-pressure/flux throughout the slab and wedge given solid flow, viscosity and thermal fields from separate solutions to the incompressible Stokes and energy equations in the mantle wedge. These solutions are verified by comparing to previous benchmark studies (van Keken et al, 2008) and global suites of thermal subduction models (Syracuse et al, 2010). Fluid flow depends on both permeability and the rheology of the slab-wedge system as interaction with rheological variability can induce additional pressure gradients that affect the fluid flow pathways. These non-linearities have been shown to explain laboratory-scale observations of melt band orientation in labratory experiments and numerical simulations of melt localization in shear bands (Katz et al 2006). Our second generation of models dispense with the pre-calculation of incompressible mantle flow and fully couple the now compressible

  3. Nature of the plate contact and subduction zones diversity

    NASA Astrophysics Data System (ADS)

    De Franco, Roberta; Govers, Rob; Wortel, Rinus

    2008-07-01

    In recent studies we showed that the nature of the plate contact in subduction zones is an important physical feature in both oceanic lithospheric subduction and continental collision. We investigated two fundamental states of the plate contact: one based on a fault and the other based on a subduction channel. Using geodynamic modeling, we determined the specific signatures of both states of the subduction contact. We established that the nature of the plate contact influences the dynamic response of the overriding and subducting plate, and is one of the controlling factors whether back-arc extension develops or not. In the present study, we combine results of our previous numerical experiments with a re-analysis of published observations. Overall, our synthesis connects seismic moment release with back-arc deformation and tectonic processes at the margin. It leads us to identify four classes of subduction zones. The first two classes result directly from our numerical experiments. In class 1, subduction zones are characterized by a plate contact that is largely fault-like with an accretionary margin. In class 2, the plate contacts are largely channel-type and have an erosive margin. Class 3, where the plate contact is entirely channel-like, consists of accretionary margins with a high sediment supply. Subduction zones of class 4, mostly characterized by an erosive convergent margin (northern Chile, Peru, Honshu and Kuril), are more complicated. They can be explained by incorporating regional observations.

  4. Deformation Along the Western Hellenic Subduction Zone From Continuous GPS

    NASA Astrophysics Data System (ADS)

    Floyd, M. A.; Nocquet, J.; Billiris, H.; Paradissis, D.; England, P.; Parsons, B.

    2005-12-01

    In respect of the great Sumatra-Andaman earthquake and subsequent tsunami of 26 December 2004, the study of coupling at subduction zones is imperative to understanding the seismic---or otherwise---hazard posed to a region. The Mediterranean Sea includes the Hellenic subduction zone, which accommodates the ~35 mm/yr of convergence between the oceanic lithosphere of the African plate and the extending continental lithosphere of the Aegean region. How the convergence is accommodated through earthquakes here remains controversial. For example, Jackson & McKenzie [1988] concluded that earthquakes can account for ~10% of the relative motion, implying that the subduction zone is in a stable-sliding state. On the contrary, Pirazzoli et al. [1982], using the Holocene geological record, inferred that large earthquakes (M>8) have occurred along the arc. In the last years, the development of permanent GPS networks near subduction zones has provided new information on the mechanisms of strain accumulation and release along the plate interface, highlighting the existence of transient slow slip events that may account for a significant part of convergence. We present results from a permanent network, in operation since early 2003, covering the western Hellenic Arc. Initial results indicate that no deformation perpendicular to the trench can be detected, supporting the hypothesis that the trench is currently in a stable-sliding state. Comparisons to 1992--2000 campaign data (McClusky et al. [2000]) indicate similar results. However, significant (~1m) extension is found in a comparison between historic triangulation and recent campaign GPS studies (Davies, et al. [1997]), averaged over 100 years, yet no significant earthquake can be invoked to explain such a large trench-perpendicular extension of the overriding plate. The existence of transient slip events elsewhere, however, presents a potential explanation for this discrepency in that the subduction zone does undergo strain

  5. Density model of the Cascadia subduction zone

    USGS Publications Warehouse

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

    2001-01-01

    The main goal of this work is to construct self-consistent density models along two profiles crossing the northern and central Cascadia subduction zone that have been comprehensively studied on the basis of geological, geophysical, etc. data.

  6. Serpentine in active subduction zones

    NASA Astrophysics Data System (ADS)

    Reynard, Bruno

    2013-09-01

    Serpentinization is a key phenomenon for understanding the geodynamics of subduction zones in the 10-200 km depth range. Serpentines are a major water carrier, and their rheological properties have a strong influence on deformation partitioning and seismicity at depths. I review experimental investigations that have been conducted on serpentines, with emphasis on the large body of data acquired over the past decade. Determinations of physical properties at the pressure and temperature conditions of subductions allow interpreting geophysical data in active subduction in terms of mineralogy and petrology, and to link the presence of serpentinites with deformation and fluid circulation. The fluid budget can be partially constrained from geophysical data. Elasticity data provide a quantitative basis for mapping serpentinization in the mantle wedge and slab from seismic tomography. Anisotropy suggests the existence of thin serpentinite channels above the plate interface, that account for mechanical decoupling inferred from down-dip limit of the seismogenic zone and heat flow. Strain-rate dependent rheology of antigorite serpentine is consistent with stable deformation of this thin layer or channel over timescales ranging from those of the seismic cycle to those of thermal equilibration and exhumation of high-pressure rocks, and with the geological record of subduction-related deformation. Circulation of serpentinizing fluids depends on the permeability structure, and is imaged by electrical conductivity tomography. It could be controlled by fracturing in the undeformed cold nose of the mantle wedge, and by plastic deformation along the plate interface. Fluid migration mechanisms are similar to those inferred from petrological and geochemical data on exhumed serpentinites. Estimation of the fluid budget associated with serpentine formation will rely on numerical simulations for which coupling of kinetics of hydration and dehydration at scales ranging from grain size up

  7. Central Cascadia subduction zone creep

    NASA Astrophysics Data System (ADS)

    Schmalzle, Gina M.; McCaffrey, Robert; Creager, Kenneth C.

    2014-04-01

    Cascadia between 43°N and 46°N has reduced interseismic uplift observed in geodetic data and coseismic subsidence seen in multiple thrust earthquakes, suggesting elevated persistent fault creep in this section of the subduction zone. We estimate subduction thrust "decade-scale" locking and crustal block rotations from three-component continuous Global Positioning System (GPS) time series from 1997 to 2013, as well as 80 year tide gauge and leveling-derived uplift rates. Geodetic observations indicate coastal central Oregon is rising at a slower rate than coastal Washington, southern Oregon and northern California. Modeled locking distributions suggest a wide locking transition zone that extends inland under central Oregon. Paleoseismic records of multiple great earthquakes along Cascadia indicate less subsidence in central Oregon. The Cascade thrust under central Oregon may be partially creeping for at least 6500 years (the length of the paleoseismic record) reducing interseismic uplift and resulting in reduced coseismic subsidence. Large accretions of Eocene age basalt (Siletzia terrane) between 43°N and 46°N may be less permeable compared to surrounding terranes, potentially increasing pore fluid pressures along the fault interface resulting in a wide zone of persistent fault creep. In a separate inversion, three-component GPS time series from 1 July 2005 to 1 January 2011 are used to estimate upper plate deformation, locking between slow-slip events (SSEs), slip from 16 SSEs and an earthquake mechanism. Cumulative SSEs and tectonic tremor are weakest between 43°N and 46°N where partial fault creep is increased and Siletzia terrane is thick, suggesting that surrounding rock properties may influence the mode of slip.

  8. Earthquake hazards on the cascadia subduction zone

    SciTech Connect

    Heaton, T.H.; Hartzell, S.H.

    1987-04-10

    Large subduction earthquakes on the Cascadia subduction zone pose a potential seismic hazard. Very young oceanic lithosphere (10 million years old) is being subducted beneath North America at a rate of approximately 4 centimeters per year. The Cascadia subduction zone shares many characteristics with subduction zones in southern Chile, southwestern Japan, and Colombia, where comparably young oceanic lithosphere is also subducting. Very large subduction earthquakes, ranging in energy magnitude (M/sub w/) between 8 and 9.5, have occurred along these other subduction zones. If the Cascadia subduction zone is also storing elastic energy, a sequence of several great earthquakes (M/sub w/ 8) or a giant earthquake (M/sub w/ 9) would be necessary to fill this 1200-kilometer gap. The nature of strong ground motions recorded during subduction earthquakes of M/sub w/ less than 8.2 is discussed. Strong ground motions from even larger earthquakes (M/sub w/ up to 9.5) are estimated by simple simulations. If large subduction earthquakes occur in the Pacific Northwest, relatively strong shaking can be expected over a large region. Such earthquakes may also be accompanied by large local tsunamis. 35 references, 6 figures.

  9. Earthquake hazards on the cascadia subduction zone.

    PubMed

    Heaton, T H; Hartzell, S H

    1987-04-10

    Large subduction earthquakes on the Cascadia subduction zone pose a potential seismic hazard. Very young oceanic lithosphere (10 million years old) is being subducted beneath North America at a rate of approximately 4 centimeters per year. The Cascadia subduction zone shares many characteristics with subduction zones in southern Chile, southwestern Japan, and Colombia, where comparably young oceanic lithosphere is also subducting. Very large subduction earthquakes, ranging in energy magnitude (M(w)) between 8 and 9.5, have occurred along these other subduction zones. If the Cascadia subduction zone is also storing elastic energy, a sequence of several great earthquakes (M(w) 8) or a giant earthquake (M(w) 9) would be necessary to fill this 1200-kilometer gap. The nature of strong ground motions recorded during subduction earthquakes of M(w) less than 8.2 is discussed. Strong ground motions from even larger earthquakes (M(w) up to 9.5) are estimated by simple simulations. If large subduction earthquakes occur in the Pacific Northwest, relatively strong shaking can be expected over a large region. Such earthquakes may also be accompanied by large local tsunamis. PMID:17789780

  10. Seismic coupling and uncoupling at subduction zones

    NASA Technical Reports Server (NTRS)

    Ruff, L.; Kanamori, H.

    1983-01-01

    Some of the correlations concerning the properties of subduction zones are reviewed. A quantitative global comparison of many subduction zones reveals that the largest earthquakes occur in zones with young lithosphere and fast convergence rates. Maximum earthquake size is directly related to the asperity distribution on the fault plane. This observation can be translated into a simple model of seismic coupling where the horizontal compressive stress between two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface. Plate age and rate can control asperity distribution directly through the horizontal compressive stress associated with the vertical and horizontal velocities of subducting slabs. The basalt to eclogite phase change in the down-going oceanic crust may be largely responsible for the uncoupling of subduction zones below a depth of about 40 km.

  11. Seismicity, metamorphism and rheology of the lower continental crust

    NASA Astrophysics Data System (ADS)

    Austrheim, Håkon

    2014-05-01

    Seismological data document that both normal earthquakes and tremors occur in the lower continental crust. Pseudotachylytes (frictional melts and ultracommunited rocks) have been described from several high grade metamorphic terrains and may be the geological manifestation of this seismicity. The Grenville (c. 930Ma) granulite facies complex (T: 800 °C; P: ≤10kbar) of the Lindås Nappe in the Bergen Arcs, W-Norway underwent a fluid induced partial eclogite (T: 600-650 °C; P: 15-20 kbar) and amphibolite facies metamorphism during the Caledonian (c.400-430 Ma) continent collision. Pseudotachylyte fault and injection veins formed in the dry granulites at or close to the reaction fronts both in the eclogitized (western parts) and the amphibolitized (eastern parts) of the Nappe. They are locally recrystalized with the development of amphibolite and eclogite facies assemblages demonstrating that they formed pre or syn the Caledonian metamorphism. The pseudotachylytes transect lithologies ranging from peridotite to anorthosite and consequently the influence of the seismic energy release on a range of granulite facies minerals including garnet, pyroxenes, olivine, plagioclase, hornblende and scapolite can be observed. The seismic energy released promotes the Caledonian metamorphism and change the petrophysical properties of the lower crust in the following ways: The melting and the ultracommunition of the granulite facies minerals increased the reactive surface area and produce local pathways for fluid. S-rich scapolite, a common mineral in granulities play a key role in this process by releasing S and C to form sulfides and carbonates. Small sulfide grains impregnate the pseudotachylyte veins which may lead to an increased electrical conductivity of the deep crust. The pseudotachylyte veins impose inhomogeneities in the massive rocks through grain size reduction and lead to strain localization with development of amphibolite and eclogite facies shear zones. Formation

  12. The global systematics of subduction zones

    NASA Astrophysics Data System (ADS)

    Syracuse, Ellen M.

    To better understand the global systematics of subduction zones, a series of studies investigates their variability taking advantage of comprehensive and accurate seismicity catalogs and advances in computing for wave propagation and geodynamic modeling. Every subduction zone with sufficient intermediate-depth earthquakes (IDE) from a global teleseismic catalog is analyzed and the top of the IDE, presumably the top of the slab, is digitized. Subduction zones are separated into a total of 52 segments 500-km-long. Parameters such as dip, age, convergence velocity, and slab depth beneath arc volcanoes (H) are compiled, resulting in a comprehensive, complete suite of subduction zone descriptions. Surprisingly, H ranges from 70-190 km globally and varies smoothly between arc segments, even though most models and textbooks assume constant H for all arcs, placing new constraints on magma generation models at arcs. To assess regional biases in earthquake location due to large-scale structure, IDEs in each arc segment are relocated in a three-dimensional global velocity model. Although the absolute position of slab surfaces shifts 0-25 km regionally, global variations in H persist. These variations in geometry, as well as slab age, convergence velocity, sediments, the overlying plate, and the location of the transition from localized slip and distributed flow create a large range in the thermal states of subduction zones. Two dimensional thermal kinematic-flow models using these slab geometries for each 500-km segment indicate that slab crust and sediments dehydrate before reaching beneath the arc, whereas slab mantle may still be hydrated, for all slabs and a variety of assumptions. To test these inferences, a temporary array of seismographs was deployed in Central America, sampling the slab and sub-arc mantle. P and S arrival times were inverted for earthquake locations and high-resolution regional velocity structure. Hypocenters confirm high H here. The velocity models

  13. Nonvolcanic tremors in the Mexican subduction zone

    NASA Astrophysics Data System (ADS)

    Payero, J. S.; Kostoglodov, V.; Mikumo, T.; Perez-Campos, X.; Iglesias, A.; Clayton, R.

    2007-05-01

    Nonvolcanic low frequency tremors (NVT) have been discovered and studied recently in Japan and Cascadia subduction zones and deep beneath the San Andreas Fault. The tremors activity is increasing during so-called silent earthquakes (SQ) in Japan and Cascadia. NVT clusters also migrate following the propagation of the SQ. The origin of the NVT is still unclear. The studies of NVT and SQ in different subduction zones are required to understand the cause for these phenomena. We discovered a number of NVT from daily spectrograms of continuous broad band records at seismic stations of Servicio Seismológico Nacional (SSN) an MASE project. The analyzed data cover a period of 2001-2004 (SSN) when in 2002 a large SQ has occurred in the Guerrero- Oaxaca region, and a steady-state interseismic epoch of 2005 and a new large SQ in 2006 (MASE). NVT occurred in the central part of the Mexican subduction zone (Guerrero) at approximately 200 km from the coast. We can not accurately localize the tremors because of sparse station coverage in 2001-2004. The MASE data of 2005-2006 show that NVT records in Mexico are very similar to those obtained in Cascadia subduction zone. The tremors duration is of 10-60 min, and they appear to travel at S-wave velocities. More than 100 strong NVT were recorded by most of the MASE stations with the epicenters clustered in the narrow band of ~40x150 km to the south of Iguala city and parallel to the coast line. NVT depths are poorly constrained but seem to be less than 40 km deep. We noticed a some increase of NVT activity during the 2001-2002 and 2006 SQs compared with an NVT activity for the "SQ quiet" period of 2003-2004 nevertheless. A lack of NVT for the period of 2-3 months after the SQ is apparent in 2002 and 2006.

  14. Subduction-zone cycling of nitrogen in serpentinized mantle rocks

    NASA Astrophysics Data System (ADS)

    Halama, R.; Bebout, G. E.; John, T.; Scambelluri, M.

    2010-12-01

    Nitrogen (N) has shown great potential as a geochemical tracer of volatiles recycling, in part because of large differences in the N isotope composition of the various Earth reservoirs. The subduction flux of N in serpentinized oceanic mantle could be as important as N input flux in oceanic crust and even sediment because, although its N concentrations are lower, its volume is potentially far greater than that of the crust/sediment. However, recycling of oceanic mantle rocks is still poorly constrained for the N cycle, and N isotope data for subduction-related ultramafic rocks are scarce [1]. The primary goal of this study is to characterize the subduction flux of N in subducting altered oceanic mantle by documenting concentrations and isotopic compositions of N in mantle rocks that reflect different stages of the metamorphic subduction zone cycle. The results are crucial to assess the composition of N recycled into the mantle, to determine the extent to which N can be retained in subducted mantle rocks to depths approaching those beneath arcs, and to balance N subduction-zone inputs with outputs in arc volcanic gases. Moreover, information has been gained regarding the redistribution and isotope fractionation of N via ultramafic dehydration and metamorphic fluid-rock interaction. The samples analyzed in this study are ultramafic rocks from shallow oceanic environments to increasing P-T conditions up to depths of ~70 km. Three distinct metamorphic grades, reflecting seafloor fluid uptake, water release due to brucite breakdown and the final antigorite breakdown, were investigated: 1. Pre-subduction serpentinized mantle peridotite from non-subducted ophiolite sequences from the Northern Apennines, Italy (Monte Nero). 2. Eclogite-facies antigorite serpentinites from the Ligurian Alps, Italy (Erro Tobbio). 3. Eclogite-facies chlorite harzburgites derived from dehydration of serpentinites from the Betic Cordillera, Spain (Cerro de Almirez). The pre

  15. Fluid migration in the subduction zone: a coupled fluid flow approach

    NASA Astrophysics Data System (ADS)

    Wang, Hongliang; Huismans, Ritske; Rondenay, Stéphane

    2016-04-01

    Subduction zone are the main entry point of water into earth's mantle and play an important role in the global water cycle. The progressive release of water by metamorphic dehydration induce important physical-chemical process in the subduction zone, such as hydrous melting, hydration and weakening of the mantle wedge, creation of pore fluid pressures that may weaken the subduction interface and induce earthquakes. Most previous studies on the role of fluids in subduction zones assume vertical migration or migration according to the dynamic pressure in the solid matrix without considering the pore fluid pressure effect on the deformation of the solid matrix. Here we investigate this interaction by explicitly modeling two-phase coupled poro-plastic flow during subduction. In this approach, the fluid migrates by compaction and decompaction of the solid matrix and affects the subduction dynamics through pore fluid pressure dependent frictional-plastic yield. Our preliminary results indicate that: 1) the rate of fluid migration depends strongly on the permeability and the bulk viscosity of the solid matrix, 2) fluid transfer occurs preferentially along the slab and then propagates into the mantle wedge by viscous compaction driven fluid flow, 3) fluid transport from the surface to depth is a prerequisite for producing high fluid pore pressures and associated hydration induced weakening of the subduction zone interface.

  16. Satellite magnetic anomalies over subduction zones - The Aleutian Arc anomaly

    NASA Technical Reports Server (NTRS)

    Clark, S. C.; Frey, H.; Thomas, H. H.

    1985-01-01

    Positive magnetic anomalies seen in MAGSAT average scalar anomaly data overlying some subduction zones can be explained in terms of the magnetization contrast between the cold subducted oceanic slab and the surrounding hotter, nonmagnetic mantle. Three-dimensional modeling studies show that peak anomaly amplitude and location depend on slab length and dip. A model for the Aleutian Arc anomaly matches the general trend of the observed MAGSAT anomaly if a slab thickness of 7 km and a relatively high (induced plus viscous) magnetization contrast of 4 A/m are used. A second source body along the present day continental margin is required to match the observed anomaly in detail, and may be modeled as a relic slab from subduction prior to 60 m.y. ago.

  17. Three-dimensional Thermal Model of the Mexican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Rosas, J. C.; Pimentel, F. D. C.; Currie, C. A.; He, J.; Harris, R. N.

    2015-12-01

    Along the Mexican section of the Middle America Trench (MAT), the Cocos plate subducts beneath the North American plate. The most important feature of this subduction zone is the flat-slab section below central Mexico, extending approximately 250 km landward from the trench at a depth of 50 km. Further west, the dip changes to 45-50º. This particular geometry has several unique consequences, such as a volcanic arc that is not aligned with the trench and very shallow slab seismicity. For the mantle wedge, the abrupt change in slab geometry could lead to a three-dimensional (3D) mantle wedge flow that departs from the classical 2D subduction-driven corner flow. Evidence of 3D flow in the region comes from seismic anisotropy studies, which show that olivine fast-direction axes have a component that is parallel to the MAT. In other subduction zones, such as Costa Rica-Nicaragua and Japan, 3D flow has been observed to increase temperatures by >50º C relative to corner flow models.For this study, we have created the first 3D finite-element model of the Mexican subduction zone in order to analyze its thermal structure. Our objective is to assess the effects of 3D mantle flow and hydrothermal circulation (HC) in the subducting slab. In this region, low surface heat flow values near the trench indicate that HC may remove heat from the oceanic plate. Our model incorporates the effect of HC through conductivity proxies in the subducting crust and a 2D oceanic geotherm that includes the age variations of the Cocos plate along the MAT. For an isoviscous mantle, our model shows that the slab dip variations induce a flow that departs from 2D corner flow near the transition between the flat-slab and normal-dipping sections. The mantle flows in eastward direction toward the flat slab, and its orientation is consistent with seismic anisotropy studies. The maximum along-margin flow rate is nearly 2 cm/yr, which is >30% of the convergence rate. Temperatures at the location of this

  18. The redox budget of subduction zones

    NASA Astrophysics Data System (ADS)

    Evans, K. A.

    2012-06-01

    Elements that can occur in more than one valence state, such as Fe, C and S, play an important role in Earth's systems at all levels, and can drive planetary evolution as they cycle through the various geochemical reservoirs. Subduction introduces oxidised Fe, C and S in sediments, altered ocean crust, and partially serpentinised lithospheric mantle to the relatively reduced mantle, with short- and long-term consequences for the redox state of the mantle. The distribution of redox-sensitive elements in the mantle controls the redox state of mantle-derived material added to the lithosphere and atmosphere, such as arc volcanic gases and the magmas that form arc-related ore deposits. The extent of mantle oxidation induced by subduction zone cycling can be assessed, albeit with large uncertainties, with redox budget calculations that quantify the inputs and outputs to subduction zones. Literature data are augmented by new measurements of the chemical composition of partially serpentinised lithospheric mantle from New Caledonia and ODP 209. Results indicate that there is a net addition of Fe (55 ± 13 × 1012 mol year- 1), C (4.6 ± 4.0 × 1012 mol year- 1), S (2.4 ± 0.9 × 1012 mol year- 1), and redox budget (5-89 × 1012 mol year- 1) at subduction zones. Monte Carlo calculations of redox budget fluxes indicate that fluxes are 46 ± 12 × 1012 mol year- 1 entering subduction zones, if input and output parameters are assumed to be normally distributed, and 46-58 × 1012 mol year- 1 if input and output parameters are assumed to be log-normally distributed. Thus, inputs into subduction zones for Fe, C, S and redox budget are in excess of subduction zone outputs. If MORB and plume-related fluxes are taken into account then Fe, C and S fluxes balance, within error. However, the redox budget does not balance, unless the very lowest estimates for the extent of slab oxidation are taken. Thus it is likely that subduction continuously increases the redox budget of the mantle

  19. Teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs: A review

    NASA Astrophysics Data System (ADS)

    Audet, Pascal; Kim, YoungHee

    2016-02-01

    More than a decade after the discovery of deep episodic slow slip and tremor, or slow earthquakes, at subduction zones, much research has been carried out to investigate the structural and seismic properties of the environment in which they occur. Slow earthquakes generally occur on the megathrust fault some distance downdip of the great earthquake seismogenic zone in the vicinity of the mantle wedge corner, where three major structural elements are in contact: the subducting oceanic crust, the overriding forearc crust and the continental mantle. In this region, thermo-petrological models predict significant fluid production from the dehydrating oceanic crust and mantle due to prograde metamorphic reactions, and their consumption by hydrating the mantle wedge. These fluids are expected to affect the dynamic stability of the megathrust fault and enable slow slip by increasing pore-fluid pressure and/or reducing friction in fault gouges. Resolving the fine-scale structure of the deep megathrust fault and the in situ distribution of fluids where slow earthquakes occur is challenging, and most advances have been made using teleseismic scattering techniques (e.g., receiver functions). In this paper we review the teleseismic structure of six well-studied subduction zones (three hot, i.e., Cascadia, southwest Japan, central Mexico, and three cool, i.e., Costa Rica, Alaska, and Hikurangi) that exhibit slow earthquake processes and discuss the evidence of structural and geological controls on the slow earthquake behavior. We conclude that changes in the mechanical properties of geological materials downdip of the seismogenic zone play a dominant role in controlling slow earthquake behavior, and that near-lithostatic pore-fluid pressures near the megathrust fault may be a necessary but insufficient condition for their occurrence.

  20. Opening and closing slab windows in congested subduction zones

    NASA Astrophysics Data System (ADS)

    Moresi, Louis

    2013-04-01

    Subduction zones often try to swallow buoyant material which is embedded in the oceanic lithosphere: plume material or hotspot residues, oceanic plateaux, and fragments of continental material. This often results in the formation of a slab window and it has been shown (Mason et al, 2010; Betts et al, 2012) that this window strongly influences the subsequent evolution of the slab and the advance/retreat rate of the trench. The buoyant material typically pushes the trench into a local state of advance, and the creation of the slab window allows the rest of the trench to retreat as the mantle behind the slab flows in through the window. This situation is inherently unstable: if the buoyancy anomaly is finite in size, then the retreating trench will soon move behind the anomaly and juxtapose negatively buoyant oceanic lithosphere with active subduction. This creates the potential to close the slab window and, in doing so, transfer the buoyant material to the over-riding plate. Models show that this closure of the window initially occurs through a lateral rollback process followed by a catastrophic re-initiation of subduction behind the colliding buoyant anomaly. This rollback leaves a characteristic, tightly rolled remnant in the mantle and significant rotation in the over-riding plate and the newly-docked block. The over-riding plate is thrown into extension perpendicular to the original orientation of the trench. This same situation applies at the late-stages of a closing ocean due to the passive margin geometry and the presence of debris collected from the closing ocean floor and it seems likely that these models can also be applied to the complicated geometry of subduction in such environments. Mason, W. G.; Moresi, L.; Betts, P. G. & Miller, M. S. Three-dimensional numerical models of the influence of a buoyant oceanic plateau on subduction zones Tectonophysics, 2010, 483, 71-79 P. Betts, W. Mason, L. Moresi, The influence of mantle plumes on subduction zone

  1. Overriding Plate Controls on Subduction Zone Evolution

    NASA Astrophysics Data System (ADS)

    Sharples, W. K.; Jadamec, M. A.; Moresi, L. N.; Capitanio, F. A.

    2014-12-01

    Seismic data, rock deformation experiments, and geochemical studies indicate variability in the thickness, buoyancy, and strength of the lithosphere at plate boundaries. However, geodynamic models of subduction commonly either omit an overriding plate or do not investigate role of the variation in overriding plate properties on the subduction evolution. We present time-dependent numerical models of subduction that vary the overriding plate thickness, strength, and density and allow for a plate interface that evolves with time via an anisotropic brittle failure rheology. We examine the emergence of (a) asymmetric versus symmetric subduction, (b) trench retreat versus advance, (c) subduction zone geometry, (d) slab stagnation versus penetration into the lower mantle, and (e) flat slab subduction. The majority of the models result in sustained asymmetric subduction. The models demonstrate that trench retreat is correlated with a thin overriding plate, whereas, trench advance is correlated with a thick and/or strong overriding plate. Slab dip, measured at a depth below the plate boundary interface, has a negative correlation with an increase in overriding plate thickness. Overriding plate thickness exerts a first order control over slab penetration into the lower mantle, with penetration most commonly occurring in models with a thick overriding plate. Periods of flat slab subduction occur with thick, strong overriding plates producing strong plate boundary interface coupling. The results provide insight into how the overriding plate plays a role in establishing advancing and retreating subduction, as well as providing an explanation for the variation of slab geometry observed in subduction zones on Earth.

  2. Acceleration spectra for subduction zone earthquakes

    USGS Publications Warehouse

    Boatwright, J.; Choy, G.L.

    1989-01-01

    We estimate the source spectra of shallow earthquakes from digital recordings of teleseismic P wave groups, that is, P+pP+sP, by making frequency dependent corrections for the attenuation and for the interference of the free surface. The correction for the interference of the free surface assumes that the earthquake radiates energy from a range of depths. We apply this spectral analysis to a set of 12 subduction zone earthquakes which range in size from Ms = 6.2 to 8.1, obtaining corrected P wave acceleration spectra on the frequency band from 0.01 to 2.0 Hz. Seismic moment estimates from surface waves and normal modes are used to extend these P wave spectra to the frequency band from 0.001 to 0.01 Hz. The acceleration spectra of large subduction zone earthquakes, that is, earthquakes whose seismic moments are greater than 1027 dyn cm, exhibit intermediate slopes where u(w)???w5/4 for frequencies from 0.005 to 0.05 Hz. For these earthquakes, spectral shape appears to be a discontinuous function of seismic moment. Using reasonable assumptions for the phase characteristics, we transform the spectral shape observed for large earthquakes into the time domain to fit Ekstrom's (1987) moment rate functions for the Ms=8.1 Michoacan earthquake of September 19, 1985, and the Ms=7.6 Michoacan aftershock of September 21, 1985. -from Authors

  3. Molybdenum isotope systematics in subduction zones

    NASA Astrophysics Data System (ADS)

    König, Stephan; Wille, Martin; Voegelin, Andrea; Schoenberg, Ronny

    2016-08-01

    This study presents Mo isotope data for arc lavas from different subduction zones that range between δ 98 / 95 Mo = - 0.72 and + 0.07 ‰. Heaviest isotope values are observed for the most slab fluid dominated samples. Isotopically lighter signatures are related to increasing relevance of terrigenous sediment subduction and sediment melt components. Our observation complements previous conclusions that an isotopically heavy Mo fluid flux likely mirrors selective incorporation of isotopically light Mo in secondary minerals within the subducting slab. Analogue to this interpretation, low δ 98 / 95 Mo flux that coincides with terrigenous sediment subduction and sediment melting cannot be simply related to a recycled input signature. Instead, breakdown of the controlling secondary minerals during sediment melting may release the light component and lead to decreasing δ 98 / 95 Mo influx into subarc mantle sources. The natural range between slab dehydration and hydrous sediment melting may thus cause a large spread of δ 98 / 95 Mo in global subduction zone magmas.

  4. The fate of the downgoing oceanic plate: Insight from the Northern Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Piana Agostinetti, Nicola; Miller, Meghan S.

    2014-12-01

    In this study, we use teleseismic receiver function analysis to image the seismic structure of the Juan de Fuca oceanic plate during its subduction beneath the North American plate. Seismic data have been recorded at 58 seismic stations deployed along the northern Cascadia subduction zone. Harmonic decomposition of the receiver function data-set along a trench-normal profile allows us to image both the isotropic and the anisotropic structure of the plate (slab). Our images highlight the presence of a highly anisotropic region at 40-70 km depths across the Cascadia subduction zone. The detected seismic anisotropy is interpreted to be related to both metamorphic facies (e.g. blueschists) and fluid released during the dehydration of the subducting mantle. The processes of dehydration and metamorphism produce the variations of the seismic properties within each lithologic unit that constitutes the subducted slab, i.e. basalts, gabbro layer and upper mantle, as the oceanic plate sinks in the upper mantle. Such variations make it almost impossible to recognize the “plate boundary” as a characteristic “velocity-jump” at depth (neither positive nor negative) along the Cascadia subduction zone. Based on the comparative interpretation of both the isotropic and the anisotropic structures retrieved, we propose a 4-stage model of the evolution of the Juan de Fuca oceanic plate during its subduction beneath the North American plate.

  5. Improving Seismic Constraints on Subduction Zone Geometry

    NASA Astrophysics Data System (ADS)

    Syracuse, E. M.; Abers, G. A.; Fischer, K. M.; van Keken, P. E.; Kneller, E. A.; Rychert, C. A.

    2007-12-01

    Accurate slab geometries are necessary for 3D flow modeling, and for understanding the variations in temperature and melting geometry between different subduction zones. Recent studies have shown that the depth to slab beneath arc volcanoes varies by as much as a factor of two between subduction zones, but these results are based on teleseismic earthquake catalogs with potentially large errors. When available, local seismic arrays provide better constraints. The TUCAN array (Tomography Under Costa Rica and Nicaragua) deployed 48 three component broadband PASSCAL instruments for 18 months with station spacing of 10-50 km across the Central America arc. This dataset provides some of the best control anywhere for ground-truth comparison of teleseismic catalogs in steeply dipping subduction zones. Joint inversion of TUCAN arrival times for velocity and hypocenters illuminate a 10-15 km thick Wadati-Benioff zone (WBZ), with absolute hypocenter uncertainties of 1-5 km. Besides providing accurate hypocenters, the tomographic images provide independent constraints on melting and temperature, through the imaging of low Vp (7.5-7.8 km/s) and highly attenuating (40

  6. The earthquake cycle in subduction zones

    NASA Technical Reports Server (NTRS)

    Melosh, H. J.; Fleitout, L.

    1982-01-01

    A simplified model of a subduction zone is presented, which incorporates the mechanical asymmetry induced by the subducted slab to anchor the subducting plate during post-seismic rebound and thus throw most of the coseismic stream release into the overthrust plate. The model predicts that the trench moves with respect to the deep mantle toward the subducting plate at a velocity equal to one-half of the convergence rate. A strong extensional pulse is propagated into the overthrust plate shortly after the earthquake, and although this extension changes into compression before the next earthquake in the cycle, the period of strong extension following the earthquake may be responsible for extensional tectonic features in the back-arc region.

  7. Strain accumulation along the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Murray, Mark H.; Lisowski, Michael

    2000-11-01

    We combine triangulation, trilateration, and GPS observations to determine horizontal strain rates along the Cascadia subduction zone from Cape Mendocino to the Strait of Juan de Fuca. Shear-strain rates are significantly greater than zero (95% confidence) in all forearc regions (26-167 nanoradians/yr), and are not significant in the arc and backarc regions. The deformation is primarily uniaxial contraction nearly parallel to Juan de Fuca-North America plate convergence (N55°-80°E). The strain rates are consistent with an elastic dislocation model for interseismic slip with a shallow 100-km wide locked zone and a deeper 75-km transition zone along the entire megathrust, except along the central Oregon coast where relatively lower strain rates are consistent with 30-40 km wide locked and transition zones.

  8. Imaging segmentation along the Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Allen, R. M.; Hawley, W. B.; Martin-Short, R.

    2015-12-01

    As we learn more about the Cascadia subduction zone, there is clear evidence for segmentation in the character of the many physical processes along its 1000 km length. There is segmentation in the arc magmas, in the seismicity, episodic tremor and slip, crustal structure and mantle structure all the way down to ~400 km depth. What is striking is the fact that the segment boundaries for these processes at depths of a few kilometers to hundreds of kilometers align. We must determine if this is coincidence, or if not, what the causative process is. The seismic deployments of the Cascadia Initiative onshore and offshore allow us to image the structure of the subduction zone, including the incoming Juan de Fuca plate, with unprecedented resolution. We use data from three one-year deployments of 70 ocean bottom seismometers across the Juan de Fuca plate, along with hundreds of onshore stations from the Pacific Northwest Seismic Network, the Berkeley Digital Seismic Network, the Earthscope Transportable Array, and smaller temporary seismic deployments. Our 3D tomographic models show significant variation in the structure of the subducting slab along its length. It extends deepest in the south (the Gorda section) where the plate is youngest, and shallows to the north across southern Oregon. There is a gap in the slab beneath northern Oregon, which appears to correlate with the geochemistry of the arc magmas. The slab is then visible again beneath Washington. We also constrain mantle flow paths using shear-wave splitting measurements at the offshore and onshore seismic stations. Beneath the Juan de Fuca plate the flow is sub-parallel to the motion of the plate. However, beneath the Gorda section of the Juan de Fuca place the flow is sub-parallel to the motion of the Pacific plate, not the Juan de Fuca plate. We are thus beginning to image a complex mantle flow pattern that may also play a role in the observed segmentation.

  9. Cyclic stressing and seismicity at strongly coupled subduction zones

    USGS Publications Warehouse

    Taylor, M.A.J.; Zheng, G.; Rice, J.R.; Stuart, W.D.; Dmowska, R.

    1996-01-01

    We use the finite element method to analyze stress variations in and near a strongly coupled subduction zone during an earthquake cycle. Deformation is assumed to be uniform along strike (plane strain on a cross section normal to the trench axis), and periodic earthquake slip is imposed consistent with the long-term rate of plate convergence and degree of coupling. Simulations of stress and displacement rate fields represent periodic fluctuations in time superimposed on an average field. The oceanic plate, descending slab, and continental lithosphere are assumed here to respond elastically to these fluctuations, and the remaining mantle under and between plates is assumed to respond as Maxwell viscoelastic. In the first part of the analysis we find that computed stress fluctuations in space and time are generally consistent with observed earthquake mechanism variations with time since a great thrust event. In particular, trench-normal extensional earthquakes tend to occur early in the earthquake cycle toward the outer rise but occur more abundantly late in the cycle in the subducting slab downdip of the main thrust zone. Compressional earthquakes, when they occur at all, have the opposite pattern. Our results suggest also that the actual timing of extensional outer rise events is controlled by the rheology of the shallow aseismic portion of the thrust interface. The second part of the analysis shows the effects of mantle relaxation on the rate of ground surface deformation during the earthquake cycle. Models without relaxation predict a strong overall compressional strain rate in the continental plate above the main thrust zone, with the strain rate constant between mainshocks. However with significant relaxation present, a localized region of unusually low compressional, or even slightly extensional, strain rate develops along the surface of the continental plate above and somewhat inland from the downdip edge of the locked main thrust zone. The low strain rate

  10. Permeability anisotropy of serpentinite and fluid migration in subduction zones

    NASA Astrophysics Data System (ADS)

    Kawano, S.; Katayama, I.; Okazaki, K.

    2010-12-01

    Subduction zones are the place where water is transported into the Earth's interior and causes arc volcanism and seismic activities. Subducting slabs release most of the water to the mantle wedge by the dehydration reactions, and the expelled water reacts with mantle rocks, forming serpentinite at the plate interface. The existence of hydrous layer has been detected by low- velocity anomaly and high-Poison's ratio in several subduction zones (Kamiya and Kobayashi 2000 ; Brocher et al. 2003). The migration of water is generally considered to move upward by buoyancy in the mantle. However, if the hydrous layer is extensively deformed, the migration of water can be controlled by the deformation plane within such layer. In order to test this hypothesis, we analyzed the permeability anisotropy of serpentinite with a strongly-developed schistosity and discuss fluid migration in the subduction systems. Serpentinite samples were collected from Nishisonogi metamorphic terrane in Nagasaki, which schistosity is well-defined developed. Two types of experimental samples were prepared: one is parallel to schistosity and the other is perpendicular. We used intra-vessel deformation and fluid- flow apparatus (IVA) in Hiroshima University to measure the permeability. In this study, we measured gas permeability using nitrogen gas and water permeability under isotropic pressure. Gas permeability was measured using the constant flow method, and water permeability was similar to gas and the transient pulse method was also used. The experiments were conducted at confining pressures up to 50 MPa, pore pressures up to 8 MPa at room temperature. We converted gas permeability to intrinsic permeability with Klinkenberg effect. The permeability decreased with increasing confining pressure, and intrinsic permeability of samples parallel to schistosity were about 10^-20 m2 at confining pressure of 50 MPa. We observed two types of pressure effect: one is significant decline due to crack filling

  11. Dehydration-driven topotaxy in subduction zones

    NASA Astrophysics Data System (ADS)

    Padrón-Navarta, José Alberto; Tommasi, Andréa; Garrido, Carlos J.

    2014-05-01

    Mineral replacement reactions play a fundamental role in the chemistry and the strength of the lithosphere. When externally or internally derived fluids are present, interface-coupled dissolution-precipitation is the driving mechanism for such reactions [1]. One of the microstructural features of this process is a 3D arrangement of crystallographic axes across internal interfaces (topotaxy) between reactant and product phases. Dehydration reactions are a special case of mineral replacement reaction that generates a transient fluid-filled porosity. Among others, the dehydration serpentinite is of special relevance in subduction zones because of the amount of fluids involved (potentially up to 13 wt.%). Two topotatic relationships between olivine and antigorite (the serpentine mineral stable at high temperature and pressure) have been reported in partially hydrated mantle wedge xenoliths [2]. Therefore, if precursor antigorite serpentine has a strong crystallographic preferred orientation (CPO) its dehydration might result in prograde peridotite with a strong inherited CPO. However for predicting the importance of topotactic reactions for seismic anisotropy of subduction zones we also need to consider the crystallization orthopyroxene + chlorite in the prograde reaction and, more importantly, the fact that this dehydration reaction produces a transient porosity of ca. 20 % vol. that results in local fluctuations of strain during compaction and fluid migration. We address this issue by a microstructural comparison between the CPO developed in olivine, orthopyroxene and chlorite during high-pressure antigorite dehydration in piston cylinder experiments (at 750ºC and 20 kbar and 1000ºC and 30 kbar, 168 h) and that recorded in natural samples (Cerro del Almirez, Betic Cordillera, Spain). Experimentally developed CPOs are strong. Prograde minerals show a significant inheritance of the former antigorite foliation. Topotactic relations are dominated by (001)atg//(100)ol

  12. Subduction zone guided waves in Northern Chile

    NASA Astrophysics Data System (ADS)

    Garth, Thomas; Rietbrock, Andreas

    2016-04-01

    Guided wave dispersion is observed in subduction zones as high frequency energy is retained and delayed by low velocity structure in the subducting slab, while lower frequency energy is able to travel at the faster velocities associated with the surrounding mantle material. As subduction zone guided waves spend longer interacting with the low velocity structure of the slab than any other seismic phase, they have a unique capability to resolve these low velocity structures. In Northern Chile, guided wave arrivals are clearly observed on two stations in the Chilean fore-arc on permanent stations of the IPOC network. High frequency (> 5 Hz) P-wave arrivals are delayed by approximately 2 seconds compared to the low frequency (< 2 Hz) P-wave arrivals. Full waveform finite difference modelling is used to test the low velocity slab structure that cause this P-wave dispersion. The synthetic waveforms produced by these models are compared to the recorded waveforms. Spectrograms are used to compare the relative arrival times of different frequencies, while the velocity spectra is used to constrain the relative amplitude of the arrivals. Constraining the waveform in these two ways means that the full waveform is also matched, and the low pass filtered observed and synthetic waveforms can be compared. A combined misfit between synthetic and observed waveforms is then calculated following Garth & Rietbrock (2014). Based on this misfit criterion we constrain the velocity model by using a grid search approach. Modelling the guided wave arrivals suggest that the observed dispersion cannot be solely accounted for by a single low velocity layer as suggested by previous guided wave studies. Including dipping low velocity normal fault structures in the synthetic model not only accounts for the observed strong P-wave coda, but also produces a clear first motion dispersion. We therefore propose that the lithospheric mantle of the subducting Nazca plate is highly hydrated at intermediate

  13. Monitoring transient changes within overpressured regions of subduction zones using ambient seismic noise

    PubMed Central

    Chaves, Esteban J.; Schwartz, Susan Y.

    2016-01-01

    In subduction zones, elevated pore fluid pressure, generally linked to metamorphic dehydration reactions, has a profound influence on the mechanical behavior of the plate interface and forearc crust through its control on effective stress. We use seismic noise–based monitoring to characterize seismic velocity variations following the 2012 Nicoya Peninsula, Costa Rica earthquake [Mw (moment magnitude) 7.6] that we attribute to the presence of pressurized pore fluids. Our study reveals a strong velocity reduction (~0.6%) in a region where previous work identified high forearc pore fluid pressure. The depth of this velocity reduction is constrained to be below 5 km and therefore not the result of near-surface damage due to strong ground motions; rather, we posit that it is caused by fracturing of the fluid-pressurized weakened crust due to dynamic stresses. Although pressurized fluids have been implicated in causing coseismic velocity reductions beneath the Japanese volcanic arc, this is the first report of a similar phenomenon in a subduction zone setting. It demonstrates the potential to identify pressurized fluids in subduction zones using temporal variations of seismic velocity inferred from ambient seismic noise correlations. PMID:26824075

  14. Monitoring transient changes within overpressured regions of subduction zones using ambient seismic noise.

    PubMed

    Chaves, Esteban J; Schwartz, Susan Y

    2016-01-01

    In subduction zones, elevated pore fluid pressure, generally linked to metamorphic dehydration reactions, has a profound influence on the mechanical behavior of the plate interface and forearc crust through its control on effective stress. We use seismic noise-based monitoring to characterize seismic velocity variations following the 2012 Nicoya Peninsula, Costa Rica earthquake [M w (moment magnitude) 7.6] that we attribute to the presence of pressurized pore fluids. Our study reveals a strong velocity reduction (~0.6%) in a region where previous work identified high forearc pore fluid pressure. The depth of this velocity reduction is constrained to be below 5 km and therefore not the result of near-surface damage due to strong ground motions; rather, we posit that it is caused by fracturing of the fluid-pressurized weakened crust due to dynamic stresses. Although pressurized fluids have been implicated in causing coseismic velocity reductions beneath the Japanese volcanic arc, this is the first report of a similar phenomenon in a subduction zone setting. It demonstrates the potential to identify pressurized fluids in subduction zones using temporal variations of seismic velocity inferred from ambient seismic noise correlations. PMID:26824075

  15. Tomography and Dynamics of Western-Pacific Subduction Zones

    NASA Astrophysics Data System (ADS)

    Zhao, D.

    2012-01-01

    the Japan Sea and the East Asia margin may be related to a metastable olivine wedge in the subducting Pacific slab. The Pacific slab becomes stagnant in the mantle transition zone under East Asia, and a big mantle wedge (BMW) has formed above the stagnant slab. Convective circulations and fluid and magmatic processes in the BMW may have caused intraplate volcanism (e.g., Changbai and Wudalianchi), reactivation of the North China craton, large earthquakes, and other active tectonics in East Asia. Deep subduction and dehydration of continental plates (such as the Eurasian plate, Indian plate and Burma microplate) are also found, which have caused intraplate magmatism (e.g., Tengchong) and geothermal anomalies above the subducted continental plates. Under Kamchatka, the subducting Pacific slab shortens toward the north and terminates near the Aleutian-Kamchatka junction. The slab loss was induced by friction with the surrounding asthenosphere, as the Pacific plate rotated clockwise 30 Ma ago, and then it was enlarged by the slab-edge pinch-off by the asthenospheric flow. The stagnant slab finally collapses down to the bottom of the mantle, which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle. Suggestions are also made for future directions of the seismological research of subduction zones.

  16. Subduction-zone magnetic anomalies and implications for hydrated forearc mantle

    USGS Publications Warehouse

    Blakely, R.J.; Brocher, T.M.; Wells, R.E.

    2005-01-01

    Continental mantle in subduction zones is hydrated by release of water from the underlying oceanic plate. Magnetite is a significant byproduct of mantle hydration, and forearc mantle, cooled by subduction, should contribute to long-wavelength magnetic anomalies above subduction zones. We test this hypothesis with a quantitative model of the Cascadia convergent margin, based on gravity and aeromagnetic anomalies and constrained by seismic velocities, and find that hydrated mantle explains an important disparity in potential-field anomalies of Cascadia. A comparison with aeromagnetic data, thermal models, and earthquakes of Cascadia, Japan, and southern Alaska suggests that magnetic mantle may be common in forearc settings and thus magnetic anomalies may be useful in mapping hydrated mantle in convergent margins worldwide. ?? 2005 Geological Society of America.

  17. Magnetotelluric imaging of a fossil paleozoic intraoceanic subduction zone in western Junggar, NW China

    NASA Astrophysics Data System (ADS)

    Xu, Yixian; Yang, Bo; Zhang, Sheng; Liu, Ying; Zhu, Lupei; Huang, Rong; Chen, Chao; Li, Yongtao; Luo, Yinhe

    2016-06-01

    The fate of subducted oceanic slabs can provide important clues to plate reconstruction through Earth history. Since oceanic slabs in continental collision zones are typically not well preserved, ancient subduction zones have rarely been imaged by geophysical techniques. Here we present an exception from the Darbut belt in the Junggar accretionary collage in the southern Altaids of Asia. We deployed a 182 km long magnetotelluric (MT) profile including 60 broadband sounding sites across the belt. Quality off-diagonal impedances were inverted by a three-dimensional scheme to image resistivities beneath the profile. The resistivity model along with MT impedance phase ellipses and induction vectors were tested and interpreted in detail. Combining geological and geophysical observations, mineral physical experiment, and geodynamic modeling results, the MT transect suggests a fossil intraoceanic subduction zone during the Late Paleozoic in the western Junggar that has been well preserved due to lack of significant subsequent tecto-thermal events.

  18. Permo-Triassic and Paleoproterozoic metamorphism related to continental collision in Yangpyeong, South Korea

    NASA Astrophysics Data System (ADS)

    Oh, Chang Whan; Imayama, Takeshi; Lee, Seung Yeol; Yi, Sang-Bong; Yi, Keewook; Lee, Byung Choon

    2015-02-01

    Gneisses and migmatites exposed in the Yangpyeong area in the northern Gyeonggi Massif provide insight into the Paleoproterozoic and Triassic metamorphic events in South Korea. Garnet-biotite gneiss and sillimanite-garnet-biotite gneiss in the western part of the area reveal Paleoproterozoic metamorphism (1888-1871 Ma) at P-T conditions of 760-820 °C and 8-10 kbar and 710-750 °C and 5-7 kbar, respectively. These rocks were overprinted by low-P/T type metamorphism (590-650 °C, 3-4 kbar) during the Triassic (ca. 237 Ma). In contrast, a cordierite-rich migmatite near the post-collisional Triassic igneous complex in the eastern part of the area was strongly metamorphosed during the Triassic (ca. 235 Ma) at 750-790 °C and 7-8 kbar. The similar Triassic ages in the western and eastern areas suggest that low-P/T type metamorphism occurred as a second stage of regional metamorphism, which is characterized by the production of cordierite with an irregularly shaped garnet. The metamorphic grade of the Triassic metamorphism decreases spatially towards the west from granulite facies to amphibolite facies metamorphic conditions, and the Paleoproterozoic metamorphism is well preserved in the western part with low grade Triassic metamorphism. The new discovery of the Triassic metamorphic event in the Yangpyeong area, in addition to the previously reported Triassic post-collision igneous event, supports the idea that the continental collision belt between the North and South China blocks extends from the Hongseong area into the Odesan area through the Yangpyeong area in South Korea.

  19. Experimental study of boron geochemistry: implications for fluid processes in subduction zones

    NASA Astrophysics Data System (ADS)

    You, C. F.; Spivack, A. J.; Gieskes, J. M.; Rosenbauer, R.; Bischoff, J. L.

    1995-06-01

    A comprehensive experimental study, utilizing an autoclave hydrothermal apparatus with a 10B isotopic tracer, has been conducted to monitor the geochemical behavior of sediment B during early subduction zone processes. The partition coefficient of exchangeable B ( K D) was determined over a temperature range of 25-350°C, at 800 bars and a water/rock ratio of 3-1.5 w/w. These K D are shown to be a complex function of temperature, pH, and possibly mineralogy. At low temperatures, K D is significantly high at ˜4 in contrast to the value of essentially zero at temperatures higher than ˜100°C. A K D of zero represents no B adsorption, implying efficient mobilization of exchangeable B at shallow depths during sediment subduction. Our experimental results demonstrate high mobilization of bulk B in sediments (both exchangeable and lattice bound) at elevated temperatures (200-350°C), in good agreement with previous observations of B in metasediments indicating progressive depletion during metamorphism. In addition, this study emphasizes the importance of a possible water/rock ratio dependence of B mobilization. In other words, the degree of sedimentary B mobilization in subduction zones strongly depends on the local thermal structure and porosity distribution. In low geothermal gradient areas, large amounts of porewater are expelled before significant B mobilization has occurred, so that some sedimentary B will survive and get into the deeper parts of the subduction zone. Our results imply that efficient mobilization of B from the subducted slab must occur and that arc magmatism recycles most of the remaining subducted B back to surface reservoirs. A reconsideration of the B budget in subduction zones provides critical information with respect to B sources and sinks in the ocean.

  20. Metamorphic belts of Anatolia

    NASA Astrophysics Data System (ADS)

    Oberhänsli, Roland; Prouteau, Amaury; Candan, Osman; Bousquet, Romain

    2015-04-01

    şanlı-Afyon realm. However the differences in time and P-T conditions (eclogite- vs. blueschist-facies units) in the Bitlis Massif indicate that the different metamorphic peak conditions were reached at different times in a single subduction zone. Exhumation from approx. 65 to 35 km depth occurred within <10 myr. The special relations between eclogite-blueschist are due to the fact that collision with the Arabian plate was and still is on going in the Bitlis area. The Bitlis HP rocks represent a subduction realm that separated the Bitlis-Pütürge(-Bistun?) continental block from the South-Armenian (Tauride?) block, further north. Post-Eocene blueschists south of the Bitlis Massif witness the separation of the Bitlis-Pütüre block from the Arabian plate, and the southward migration of the subduction zone from the Late Cretaceous to the Oligocene. Continuous convergence of Africa and Eurasia engendered the simultaneous consumption of several, separated branches of the Neotethys Ocean and amalgamation of different terranes. The rise of the Eastern Anatolia Plateau is related to this complex geodynamic setting. Reduced seismic velocities inferred from geophysical observations, which are interpreted as complete replacement of lithospheric- by asthenospheric mantle, can be explained by thermodynamic modelling as partial hydration of the lithospheric mantle wedge during protracted subduction. Hydrated lithospheric mantle is interpreted as result of the complex geodynamic setting in Anatolia with multiple simultaneous subduction zones.

  1. Deformation Processes in Great Subduction Zone Earthquake Cycles

    NASA Astrophysics Data System (ADS)

    Hu, Yan

    This dissertation consists of two parts and investigates the crustal deformation associated with great subduction zone earthquake at two different spatial scales. At the small scale, I investigate the stress transfer along the megathrust during great earthquakes and its effects on the forearc wedge. At the large scale, I investigate the viscoelastic crustal deformation of the forearc and the back arc associated with great earthquakes. Part I: In a subduction zone, the frontal region of the forearc can be morphologically divided into the outer wedge and the inner wedge. The outer wedge which features much active plastic deformation has a surface slope angle generally larger than that of the inner wedge which hosts stable geological formations. The megathrust can be represented by a three-segment model, the updip zone (velocity-strengthening), seismogenic zone (velocity-weakening), and downdip zone (velocity-strengthening). Our dynamic Coulomb wedge theory postulates that the outer wedge overlies the updip zone, and the inner wedge overlies the seismogenic zone. During an earthquake, strengthening of the updip zone may result in compressive failure in the outer wedge. The inner wedge undergoes elastic deformation. I have examined the geometry and mechanical processes of outer wedges of twenty-three subduction zones. The surface slope of these wedges is generally too high to be explained by the classical critical taper theory but can be explained by the dynamic Coulomb wedge theory. Part II: A giant earthquake produces coseismic seaward motion of the upper plate and induces shear stresses in the upper mantle. After the earthquake, the fault is re-locked, causing the upper plate to move slowly landward. However, parts of the fault will undergo continuous aseismic afterslip for a short duration, causing areas surrounding the rupture zone to move seaward. At the same time, the viscoelastic relaxation of the earthquake-induced stresses in the upper mantle causes prolonged

  2. The Sulfur Cycle at Subduction Zones

    NASA Astrophysics Data System (ADS)

    de Moor, M. J.; Fischer, T. P.; Sharp, Z. D.

    2013-12-01

    We present sulfur (S) isotope data for magmatic gases emitted along the Central American (CA) Arc (oxidizing conditions ΔQFM ~+ 1.5) and at the East African Rift (reduced conditions ΔQFM ~0). The results are interpreted through mass balance calculations to characterize the S cycle through subduction zones with implications for the redox conditions of arc magmas. Voluminous gas emissions from Masaya, an open vent basaltic volcano in Nicaragua, represent >20% of the SO2 flux from the CA arc [1]. Samples from the Masaya plume have S isotope compositions of + 4.8 × 0.4 ‰ [2]. Degassing fractionation modeling and assessment of differentiation processes in this oxidized volcano suggest that this value is close to that of the source composition. High T gas samples from other CA volcanoes (Momotombo, Cerro Negro, Poas, Turrialba) range from + 3 ‰ (Cerro Negro) to + 7 ‰ (Poas; [3]). The high δ34S values are attributed to recycling of subducted oxidized sulfur (sulfate ~ + 20 ‰) through the CA arc. The δ34S values of the more reduced samples from East African Rift volcanoes, Erta Ale - 0.5 × 0.6 ‰ [3] and Oldoinyo Lengai -0.7 ‰ to + 1.2 ‰) are far lower and are probably sourced directly from ambient mantle. The subduction of oxidized material at arcs presents a likely explanation for the oxidized nature of arc magmas relative to magmas from spreading centers. We observe no distinguishable change in melt fO2 with S degassing and attribute these differences to tectonic setting. Monte Carlo modeling suggests that subducted crust (sediments, altered oceanic crust, and serpentinized lithospheric mantle) delivers ~7.7 × 2.2 x 1010 mols of S with δ34S of -1.5 × 2.3‰ per year into the subduction zone. The total S output from the arc is estimated to be 3.4 × 1.1 x 1010 mols/yr with a δ34S value similar to that of Masaya gas (+5 × 0.5 ‰). Considering δ34S values for ambient upper mantle (0 ‰ [4]) and slab-derived fluids (+14 ‰ [5]) allows calculation

  3. Carbonation by fluid-rock interactions at high-pressure conditions: Implications for carbon cycling in subduction zones

    NASA Astrophysics Data System (ADS)

    Piccoli, Francesca; Vitale Brovarone, Alberto; Beyssac, Olivier; Martinez, Isabelle; Ague, Jay J.; Chaduteau, Carine

    2016-07-01

    Carbonate-bearing lithologies are the main carbon carrier into subduction zones. Their evolution during metamorphism largely controls the fate of carbon, regulating its fluxes between shallow and deep reservoirs. Recent estimates predict that almost all subducted carbon is transferred into the crust and lithospheric mantle during subduction metamorphism via decarbonation and dissolution reactions at high-pressure conditions. Here we report the occurrence of eclogite-facies marbles associated with metasomatic systems in Alpine Corsica (France). The occurrence of these marbles along major fluid-conduits as well as textural, geochemical and isotopic data indicating fluid-mineral reactions are compelling evidence for the precipitation of these carbonate-rich assemblages from carbonic fluids during metamorphism. The discovery of metasomatic marbles brings new insights into the fate of carbonic fluids formed in subducting slabs. We infer that rock carbonation can occur at high-pressure conditions by either vein-injection or chemical replacement mechanisms. This indicates that carbonic fluids produced by decarbonation reactions and carbonate dissolution may not be directly transferred to the mantle wedge, but can interact with slab and mantle-forming rocks. Rock-carbonation by fluid-rock interactions may have an important impact on the residence time of carbon and oxygen in subduction zones and lithospheric mantle reservoirs as well as carbonate isotopic signatures in subduction zones. Furthermore, carbonation may modulate the emission of CO2 at volcanic arcs over geological time scales.

  4. Earthquake Production by Subduction Zones is Not Linear in Relative Plate Velocity

    NASA Astrophysics Data System (ADS)

    Bird, P.; Kagan, Y. Y.; Jackson, D. D.; Schoenberg, F. P.; Werner, M. J.

    2007-12-01

    The ratio of \\{long-term-average seismic moment production per unit length of plate boundary\\} to \\{relative plate velocity\\} is determined by the "coupled thickness" of seismogenic lithosphere, and also by elastic moduli and geometric factors that are fairly well known. It is generally assumed that coupled thickness is constant within a given class of plate boundary, such as Bird's [2003, G3]: CCB Continental Convergent Boundary, CRB Continental Rift Boundary, CTF Continental Transform Fault, OCB Oceanic Convergent Boundary, OSR Oceanic Spreading Ridge, OTF Oceanic Transform Fault, or SUB Subduction zone. However, Bird et al. [2002, Geodyn. Ser.] and Bird & Kagan [2004, BSSA] found two exceptions: OSR and OTF both have greater coupled thickness at low relative plate velocities. We test for variation of coupled thickness with relative plate velocity in each of the 7 classes of plate boundary. We use shallow (<70 km) earthquakes from the Harvard CMT catalog, 1982.01.01-2007.03.31, above magnitude MW threshold of 5.51 or 5.66. In order to reduce the influence of aftershock swarms, we estimate the probability of independence of each earthquake according to the likelihood stochastic declustering method of Kagan & Jackson [1991; GJI] and use this as a weight. We use the algorithm of Bird & Kagan [2004, BSSA] to assign 95% of shallow earthquakes to plate boundary steps and plate boundary classes, rejecting all earthquakes that fall into one of the 13 orogens of Bird [2003, G3]. We order the plate-boundary steps outside orogens in each class by relative plate velocity according to the PB2002 model of Bird [2003]. Then, we plot cumulative earthquake count as a function of cumulative model tectonic moment (assuming constant coupled thickness and other parameters within each plate boundary class). The null hypothesis is a linear relation; we use 2 measures (Kolmogorov-Smirnov, and Cramer-von Mises) to quantify departures from this line. We use 10,000 simulations of each

  5. Chromium isotope signature during continental crust subduction recorded in metamorphic rocks

    NASA Astrophysics Data System (ADS)

    Shen, Ji; Liu, Jia; Qin, Liping; Wang, Shui-Jiong; Li, Shuguang; Xia, Jiuxing; Ke, Shan; Yang, Jingsui

    2015-11-01

    The chromium isotope compositions of 27 metamorphic mafic rocks with varying metamorphic degrees from eastern China were systematically measured to investigate the Cr isotope behavior during continental crust subduction. The Cr isotope compositions of all samples studied were Bulk Silicate Earth (BSE) like, with δ53CrNIST979 of greenschists, amphibolites, and eclogites ranging from -0.06‰ to -0.17‰, -0.05‰ to -0.27‰, and -0.01‰ to -0.24‰, respectively. The lack of resolvable isotopic variability among the metamorphic rocks from different metamorphic zones indicated that no systematic Cr isotope fractionation was associated with the degree of metamorphism. However, the Cr isotopic variability among homologous samples may have reflected effects induced by metamorphic dehydration with a change of redox state, rather than protolith heterogeneity (i.e., magma differentiation). In addition, the differences in δ53Cr (Δ53CrCpx-Gt) between coexisting clinopyroxene (Cpx) and garnet (Gt) from two garnet pyroxenites were 0.06‰ and 0.34‰, respectively, indicating that significant inter-mineral Cr isotope disequilibria could occur during metamorphism. To provide a basis for comparison with metamorphic rocks and to provide further constraints on the potential Cr isotope heterogeneity in the mantle and in the protolith of some metamorphic rocks, we analyzed mantle-derived chromites and the associated peridotites from Luobusa, and we obtained the following general order: chromite-free peridotites (-0.21‰ to -0.11‰) < chromite-bearing peridotite (-0.07‰) < chromite (-0.06‰). These findings imply potential mantle heterogeneity as a result of partial melting or fractional crystallization associated with chromite.

  6. Seismicity of the eastern Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

    Bruestle, A.; Kueperkoch, L.; Rische, M.; Meier, T.; Friederich, W.; Egelados Working Group

    2012-04-01

    The Hellenic Subduction Zone (HSZ) is the seismically most active region of Europe. The African plate is subducting beneath the Aegean lithosphere with a relative velocity of 4 cm per year. A detailed picture of the microseismicity of the eastern HSZ was obtained by the recordings of the temporary networks CYCNET (September 2002 - September 2005) and EGELADOS (October 2005 - March 2007). In total, nearly 7000 earthquakes were located with a location uncertainty of less than 20 km. The SE Aegean is dominated by (1) shallow intraplate seismicity within the Aegean plate, by (2) interplate seismicity at the plate contact and by (3) intermediate deep seismicity along the subducting African slab. Strong shallow seismicity in the upper plate is observed along the Ptolemy graben south of Crete extending towards the Karpathos Basin, indicating intense recent deformation of the forearc. In contrary, low shallow seismicity around Rhodes indicates only minor seismic crustal deformation of the upper plate. An almost NS-striking zone of microseismicity has been located, running from the Karpathos basin via the Nisyros volcanic complex towards the EW striking Gökova graben. In the SE Aegean the geometry of the Wadati-Benioff-Zone (WBZ) within the subducting African plate is revealed in detail by the observed microseismicity. Between about 50 to 100 km depth a continuous band of intermediate deep seismicity describes the strongly curved geometry of the slab. From the central to the eastern margin of the HSZ, the dip direction of the WBZ changes from N to NW with a strong increase of the dip angle beneath the eastern Cretan Sea. The margin of the dipping African slab is marked by an abrupt end of the observed WBZ beneath SW Anatolia. Below 100 km depth, the WBZ of the eastern HSZ is dominated by an isolated cluster of intense intermediate deep seismicity (at 100-180 km depth) beneath the Nisyros volcanic complex. It has an extension of about 100x80 km and is build up of 3 parallel

  7. Anisotropy in the subducted oceanic crust and the overlying continental crust explain the existence of a double tectonic tremor zone in the flat portion of the Mexican subduction zone.

    NASA Astrophysics Data System (ADS)

    Husker, A. L.; Castillo, J. A.; Perez-Campos, X.; Frank, W.; Kostoglodov, V.

    2015-12-01

    Tectonic tremor (TT) in Mexico has a complicated behavior due to the shape of the subducted plate. In the flat section the slab dives from the trench to a depth of 40 km at 150 km from the trench where it turns to be flat. It remains at 40 km depth till about 290 - 300 km from the trench where it continues to steeply dive into the mantle. All TT activity is within the flat slab section. An LFE catalog and the vertically averaged shear wave anisotropy observed from receiver functions at the slab interface are used to divide the region into 4 zones. (1) The Transient Zone located at the corner of the slab when it first arrives at 40 km depth (~130 km - 165 km from the trench) where the majority of LFE's are seen in small bursts that produce TT. (2) The Buffer Zone has almost no LFE and is located ~165 km - 190 km from the trench. (3) The sub-Sweet Spot is located ~190 - 204 km from the trench and seems to share many characteristics of the Sweet Spot, but has less than half the LFE activity observed in the Sweet Spot in addition to different anisotropy. (4) The Sweet Spot has the overwhelming majority of LFE and is located ~204 km - 245 km from the trench. No LFE is found from 245 km to 300 km from the trench despite the plate still being at 40 km depth. The anisotropy percentage in the continental crust drops significantly above the Transient Zone and Sweet Spot suggesting the crust acts as a seal in those two zones permitting trapped fluids to generate TT/LFE activity there as has been observed in other zones. The Buffer Zone coincides with a region of high fluid flow in the crust (Jodicke et al., 2005) suggesting that there is no seal in this zone allowing fluids to escape thereby limiting TT/LFE generation. The convergence of the zone would imply that the anisotropy preferred orientation at the plate interface should be perpendicular to the trench as much of it is. However, the fast azimuth direction rotates to be trench parallel in the region of the large SSE

  8. Dynamic topography in subduction zones: insights from laboratory models

    NASA Astrophysics Data System (ADS)

    Bajolet, Flora; Faccenna, Claudio; Funiciello, Francesca

    2014-05-01

    The topography in subduction zones can exhibit very complex patterns due to the variety of forces operating this setting. If we can deduce the theoretical isostatic value from density structure of the lithosphere, the effect of flexural bending and the dynamic component of topography are difficult to quantify. In this work, we attempt to measure and analyze the topography of the overriding plate during subduction compared to a pure shortening setting. We use analog models where the lithospheres are modeled by thin-sheet layers of silicone putty lying on low-viscosity syrup (asthenosphere). The model is shorten by a piston pushing an oceanic plate while a continental plate including a weak zone to localize the deformation is fixed. In one type of experiments, the oceanic plate bends and subducts underneath the continental one; in a second type the two plates are in contact without any trench, and thus simply shorten. The topography evolution is monitored with a laser-scanner. In the shortening model, the elevation increases progressively, especially in the weak zone, and is consistent with expected isostatic values. In the subduction model, the topography is characterized, from the piston to the back-wall, by a low elevation of the dense oceanic plate, a flexural bulge, the trench forming a deep depression, the highly elevated weak zone, and the continental upper plate of intermediate elevation. The topography of the upper plate is consistent with isostatic values for very early stages, but exhibits lower elevations than expected for later stages. For a same amount of shortening of the continental plate, the thickening is the same and the plate should have the same elevation in both types of models. However, comparing the topography at 20, 29 and 39% of shortening, we found that the weak zone is 0.4 to 0.6 mm lower when there is an active subduction. Theses values correspond to 2.6 to 4 km in nature. Although theses values are high, there are of the same order as

  9. The global range of subduction zone thermal structures from exhumed blueschists and eclogites: Rocks are hotter than models

    NASA Astrophysics Data System (ADS)

    Penniston-Dorland, Sarah C.; Kohn, Matthew J.; Manning, Craig E.

    2015-10-01

    The maximum-pressure Psbnd T conditions (Pmax- T) and prograde Psbnd T paths of exhumed subduction-related metamorphic rocks are compared to predictions of Psbnd T conditions from computational thermal models of subduction systems. While the range of proposed models encompasses most estimated Pmax- T conditions, models predict temperatures that are on average colder than those recorded by exhumed rocks. In general, discrepancies are greatest for Pmax < 2 GPa, where only a few of the highest-T model paths overlap petrologic observations and model averages are 100-300 °C colder than average conditions recorded by rocks. Prograde Psbnd T paths similarly indicate warmer subduction than typical models. Both petrologic estimates and models have inherent biases. Petrologic analysis may overestimate temperatures at Pmax where overprinting occurs during exhumation, although Psbnd T paths suggest that relatively warm conditions are experienced by rocks on the prograde subduction path. Models may underestimate temperatures at depth by neglecting shear heating, hydration reactions and fluid and rock advection. Our compilation and comparison suggest that exhumed high-P rocks provide a more accurate constraint on Psbnd T conditions within subduction zones, and that those conditions may closely represent the subduction geotherm. While exhumation processes in subduction zones require closer petrologic scrutiny, the next generation of models should more comprehensively incorporate all sources of heat. Subduction-zone thermal structures from currently available models appear to be inaccurate, and this mismatch has wide-reaching implications for our understanding of global geochemical cycles, the petrologic structure of subduction zones, and fluid-rock interactions and seismicity within subduction zones.

  10. Electromagnetic Precursors Leading to Triangulation of Future Earthquakes and Imaging of the Subduction Zone

    NASA Astrophysics Data System (ADS)

    Heraud, J. A.; Centa, V. A.; Bleier, T.

    2015-12-01

    During several sessions in past AGU meetings, reports on the progress of analysis of magnetometer data have been given, as our research moved from a one dimensional geometry, to two and finally to a three dimensional image. In the first case, we learned how to extract one coordinate, azimuth information, on the occurrence of an earthquake based on the processing of mono-polar pulses received at a single station. A two dimensional geometry was implemented through triangulation and we showed the use of this technique to find out where a future epicenter would occur. Recently, we have obtained compelling evidence that the pressure points leading to the determination of future epicenters originate at a plane, inclined with the same angle as the subduction zone, a three-dimensional position of the future hypocenter. Hence, an image of the subduction zone or interface between the Nazca plate and the continental plate in the northern area of Lima, Peru, has been obtained, corresponding to the subduction zone obtained by traditional seismic methods. Our work with magnetometers deployed along part of the Peruvian coast since 2009, has shown that it is possible to measure, with significant precision, the azimuth of electromagnetic pulses propagating from stress points in the earth's crust due to the subduction of tectonic plates, as to be able to determine precisely the origin of the pulses. The occurrence of earthquakes approximately 11 to 18 days after the appearance of the first pulses and the recognition of grouping of such pulses, has allowed us to determine accurately the direction and the timing of future seismic events. Magnetometers, donated by Quakefinder and Telefonica del Peru were then strategically installed in different locations in Peru with the purpose of achieving triangulation. During two years since 2013, about a dozen earthquakes have been associated with future seismic activity in a pre or post occurrence way. Our presentation will be based on animated

  11. On subduction zone earthquakes and the Pacific Northwest seismicity

    SciTech Connect

    Chung, Dae H.

    1991-12-01

    A short review of subduction zone earthquakes and the seismicity of the Pacific Northwest region of the United States is provided for the purpose of a basis for assessing issues related to earthquake hazard evaluations for the region. This review of seismotectonics regarding historical subduction zone earthquakes and more recent seismological studies pertaining to rupture processes of subduction zone earthquakes, with specific references to the Pacific Northwest, is made in this brief study. Subduction zone earthquakes tend to rupture updip and laterally from the hypocenter. Thus, the rupture surface tends to become more elongated as one considers larger earthquakes (there is limited updip distance that is strongly coupled, whereas rupture length can be quite large). The great Aleutian-Alaska earthquakes of 1957, 1964, and 1965 had rupture lengths of greater than 650 km. The largest earthquake observed instrumentally, the M{sub W} 9.5, 1960 Chile Earthquake, had a rupture length over 1000 km. However, earthquakes of this magnitude are very unlikely on Cascadia. The degree of surface shaking has a very strong dependency on the depth and style of rupture. The rupture surface during a great earthquake shows heterogeneous stress drop, displacement, energy release, etc. The high strength zones are traditionally termed asperities and these asperities control when and how large an earthquake is generated. Mapping of these asperities in specific subduction zones is very difficult before an earthquake. They show up more easily in inversions of dynamic source studies of earthquake ruptures, after an earthquake. Because seismic moment is based on the total radiated-energy from an earthquake, the moment-based magnitude M{sub W} is superior to all other magnitude estimates, such as M{sub L}, m{sub b}, M{sub bLg}, M{sub S}, etc Probably, just to have a common language, non-moment magnitudes should be converted to M{sub W} in any discussions of subduction zone earthquakes.

  12. Diapiric flow at subduction zones: a recipe for rapid transport.

    PubMed

    Hall, P S; Kincaid, C

    2001-06-29

    Recent geochemical studies of uranium-thorium series disequilibrium in rocks from subduction zones require magmas to be transported through the mantle from just above the subducting slab to the surface in as little as approximately 30,000 years. We present a series of laboratory experiments that investigate the characteristic time scales and flow patterns of the diapiric upwelling model of subduction zone magmatism. Results indicate that the interaction between buoyantly upwelling diapirs and subduction-induced flow in the mantle creates a network of low-density, low-viscosity conduits through which buoyant flow is rapid, yielding transport times commensurate with those indicated by uranium-thorium studies. PMID:11431563

  13. Subduction-related metamorphism beneath ophiolites (Oman) and during early stages of continental collision (Himalaya)

    NASA Astrophysics Data System (ADS)

    Searle, Mike; Waters, David; Cowan, Robert; Cherry, Alan; Cooper, Charles

    2014-05-01

    Subduction-related metamorphism occurs beneath ophiolites (Oman), beneath island arcs (Kohistan) and during the early stages of continental collision (Kaghan, Tso Morari; Himalaya). Ophiolite obduction necessarily involves subduction of first oceanic, then continental crust to mantle depths beneath the ophiolite. In Oman an inverted pressure and temperature profile is exposed beneath the Semail ophiolite from garnet+clinopyroxene-bearing granulite to hornblende+plagioclase amphibolite down through epidote amphibolite and a variety of greenschist facies meta-sediments, dominantly cherts, marbles and quartzites. Thermobarometry on Grt+Cpx-bearing amphibolites immediately beneath the contact with mantle sequence harzburgites shows that the upper sole rocks formed at PT conditions of 770-900°C and 11-13 kbar, equivalent to depths of 30-40 km in oceanic lithosphere. Heat for metamorphism can only have been derived from the overlying mantle peridotites. Pressures are higher than can be accounted for by the thickness of the preserved ophiolite (15-20 km). Timing of peak metamorphism was synchronous with formation of the ophiolite gabbroic - trondhjemite crustal sequence and eruption of the pillow lavas (Cenomanian; 96-95 Ma). During the later stages of obduction the continental margin was dragged down to depths of nearly 100 km and basaltic sills within calc-schists were converted to eclogites (20-25 kbar; 500-560oC; 79.1 Ma), then exhumed back up the same subduction channel. Apparent 'extensional' fabrics throughout the HP units are related to upward flow of deeply buried rocks in a wholly compressional environment. Eclogites in a similar structural position occur along the Himalaya in the northernmost exposures of Indian plate rocks. These eclogites formed either during the latest stage of ophiolite obduction or the earliest stage of continental collision.

  14. Three-dimensional thermal structure of subduction zones: effects of obliquity and curvature

    NASA Astrophysics Data System (ADS)

    Bengtson, A. K.; van Keken, P. E.

    2012-11-01

    Quantifying the precise thermal structure of subduction zones is essential for understanding the nature of metamorphic dehydration reactions, arc volcanism, and intermediate depth seismicity. High resolution two-dimensional (2-D) models have shown that the rheology of the mantle wedge plays a critical role and establishes strong temperature gradients in the slab. The influence of three-dimensional (3-D) subduction zone geometry on thermal structure is however not yet well characterized. A common assumption for 2-D models is that the cross-section is taken normal to the strike of the trench with a corresponding velocity reduction in the case of oblique subduction, rather than taken parallel to velocity. A comparison between a full 3-D Cartesian model with oblique subduction and selected 2-D cross-sections demonstrates that the trench-normal cross-section provides a better reproduction of the slab thermal structure than the velocity-parallel cross-section. An exception is found in the case of a strongly curved trench, such as in the Marianas, where strong 3-D flow in the mantle wedge is generated. In this case it is shown that the full 3-D model should be evaluated for an accurate prediction of the slab thermal structure. The models demonstrate that the use of a dynamic slab and wedge, separated by a kinematic boundary, yields good results for describing slab velocities in 3-D.

  15. Three-dimensional thermal structure of subduction zones: effects of obliquity and curvature

    NASA Astrophysics Data System (ADS)

    Bengtson, A. K.; van Keken, P. E.

    2012-07-01

    Quantifying the precise thermal structure of subduction zones is essential for understanding the nature of metamorphic dehydration reactions, arc volcanism, and intermediate depth seismicity. High resolution two-dimensional (2-D) models have shown that the rheology of the mantle wedge plays a critical role and establishes strong temperature gradients in the slab. The influence of three-dimensional (3-D) subduction zone geometry on thermal structure is however not yet well characterized. A common assumption for 2-D models is that the cross-section is taken normal to the strike of the trench with a corresponding velocity reduction in the case of oblique subduction, rather than taken parallel to velocity. A comparison between a full 3-D Cartesian model with oblique subduction and selected 2-D cross-sections demonstrates that the trench-normal cross-section provides a better reproduction of the slab thermal structure than the velocity-parallel cross-section. An exception is found in the case of strongly curved subduction, such as in the Marianas, where strong 3-D flow in the mantle wedge is generated. In this case it is shown that the full 3-D model should be evaluated for an accurate prediction of the slab thermal structure.

  16. 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.

  17. GPS constraints on interplate locking within the Makran subduction zone

    NASA Astrophysics Data System (ADS)

    Frohling, E.; Szeliga, W.

    2016-04-01

    The Makran subduction zone is one of the last convergent margins to be investigated using space-based geodesy. While there is a lack of historical and modern instrumentation in the region, a sparse sampling of continuous and campaign measurements over the past decade has allowed us to make the first estimates of convergence rates. We combine GPS measurements from 20 stations located in Iran, Pakistan and Oman along with hypocentral locations from the International Seismological Centre to create a preliminary 3-D estimate of the geometry of the megathrust, along with a preliminary fault-coupling model for the Makran subduction zone. Using a convergence rate which is strongly constrained by measurements from the incoming Arabia plate along with the backslip method of Savage, we find the Makran subduction zone appears to be locked to a depth of at least 38 km and accumulating strain.We also find evidence for a segmentation of plate coupling, with a 300 km long section of reduced plate coupling. The range of acceptable locking depths from our modelling and the 900 km along-strike length for the megathrust, makes the Makran subduction zone capable of earthquakes up to Mw = 8.8. In addition, we find evidence for slow-slip-like transient deformation events on two GPS stations. These observations are suggestive of transient deformation events observed in Cascadia, Japan and elsewhere.

  18. The relationship between continental collision process and metamorphic pattern in the Himalayan collision belts

    NASA Astrophysics Data System (ADS)

    Oh, Chang-Whan

    2015-04-01

    Both UHP and HP eclogites are reported from the Kaghan Valley and Tso Morari Massif in the western part of the Himalayan collision belt (Ghazanfar and Chaudhry, 1987; Thakur, 1983). UHP eclogites in the Kaghan record peak metamorphic conditions of 770 °C and 30 kbar (O'Brien et al., 2001) and was retrograded into the epidote-amphibolite or blueschist (580-610 °C, 10-13 kbar; Lombardo and Rolfo, 2000). Sensitive high-resolution ion microprobe dating of zircon reveals that the UHP eclogite formed at ca. 46 Ma (Kaneko et al., 2003; Parrish et al., 2006). The Tso Morari UHP eclogite had formed at 750 °C, > 39 kbar (Mukheerjee et al., 2003; Bundy, 1980) and underwent amphibolite facies retro-grade metamorphism (580 °C, 11 kbar) during uplift (Guillot et al., 2008). Peak metamorphism of the Tso Morari Massif was dated at ca. 53-55 Ma (Leech et al., 2005). Only HP eclogites have been reported from the mid-eastern part of the Himalayan collision belt (Lombardo and Rolfo, 2000; Corrie et al., 2010). The HP eclogite in the mid-eastern part may have formed at ca. > 780 °C and 20 kbar and was overprinted by high-pressure granulite facies metamorphism (780-750°C, 12-10 kbar) at ca. 30 Ma (Groppo et al. 2007; Corrie et al., 2010). HP granulite (890 °C, 17-18 kbar) is reported from the NBS, at the eastern terminus of the Himalayan collision belt; the granulite was subjected to retrograde metamorphism to produce lower-pressure granulite (875-850°C, 10-5 kbar), representing near-isothermal decompression (Liu and Zhong, 1997). The HP granulite metamorphism may have occurred at ca. 22-25 Ma. Along the Himalayan collision belt, peak metamorphism changes eastward from UHP eclogite facies through HP eclogite facies to high-pressure granulite facies, indicating a progressive eastwards decrease in the depth of subduction of continental crust and an eastwards increase in the geothermal gradient. The peak metamorphic ages also decrease from 53-46 Ma in the west to 22-25 Ma in the

  19. Three-Dimensional Thermal Model of the Costa Rica-Nicaragua Subduction Zone

    NASA Astrophysics Data System (ADS)

    Rosas, Juan Carlos; Currie, Claire A.; He, Jiangheng

    2015-10-01

    The thermal structure of a subduction zone controls many key processes, including subducting plate metamorphism and dehydration, the megathrust earthquake seismogenic zone and volcanic arc magmatism. Here, we present the first three-dimensional (3D), steady-state kinematic-dynamic thermal model for the Costa Rica-Nicaragua subduction zone. The model consists of the subducting Cocos plate, the overriding Caribbean Plate, and a viscous mantle wedge in which flow is driven by interactions with the downgoing slab. The Cocos plate geometry includes along-strike variations in slab dip, which induce along-strike flow in the mantle wedge. Along-strike flow occurs primarily below Costa Rica, with a maximum magnitude of 4 cm/year (~40 % of the convergence rate) for a mantle with a dislocation creep rheology; an isoviscous mantle has lower velocities. Along-margin flow causes temperatures variations of up to 80 °C in the subducting slab and mantle wedge at the volcanic arc and backarc. The 3D effects do not strongly alter the shallow (<35 km) thermal structure of the subduction zone. The models predict that the megathrust seismogenic zone width decreases from ~100 km below Costa Rica to just a few kilometers below Nicaragua; the narrow width in the north is due to hydrothermal cooling of the oceanic plate. These results are in good agreement with previous 2D models and with the rupture area of recent earthquakes. In the models, along-strike mantle flow is induced only by variations in slab dip, with flow directed toward the south where the dip angle is smallest. In contrast, geochemical and seismic observations suggest a northward flow of 6-19 cm/year. We do not observe this in our models, suggesting that northward flow may be driven by additional factors, such as slab rollback or proximity to a slab edge (slab window). Such high velocities may significantly affect the thermal structure, especially at the southern end of the subduction zone. In this area, 3D models that

  20. Limits on great earthquake size at subduction zones

    NASA Astrophysics Data System (ADS)

    McCaffrey, R.

    2012-12-01

    Subduction zones are where the world's greatest earthquakes occur due to the large fault area available to slip. Yet some subduction zones are thought to be immune from these massive events, where quake size is limited by some physical processes or properties. Accordingly, the size of the 2011 Tohoku-oki Mw 9.0 earthquake caught some in the earthquake research community by surprise. The expectations of these massive quakes have been driven in the past by reliance on our short, incomplete history of earthquakes and causal relationships derived from it. The logic applied is that if a great earthquake has not happened in the past, that we know of, one cannot happen in the future. Using the ~100-year global earthquake seismological history, and in some cases extended with geologic observations, relationships between maximum earthquake sizes and other properties of subduction zones are suggested, leading to the notion that some subduction zones, like the Japan Trench, would never produce a magnitude ~9 event. Empirical correlations of earthquake behavior with other subduction parameters can give false positive results when the data are incomplete or incorrect, of small numbers and numerous attributes are examined. Given multi-century return times of the greatest earthquakes, ignorance of those return times and our relatively limited temporal observation span (in most places), I suggest that we cannot yet rule out great earthquakes at any subduction zones. Alternatively, using the length of a subduction zone that is available for slip as the predominant factor in determining maximum earthquake size, we cannot rule out that any subduction zone of a few hundred kilometers or more in length may be capable of producing a magnitude 9 or larger earthquake. Based on this method, the expected maximum size for the Japan Trench was 9.0 (McCaffrey, Geology, p. 263, 2008). The same approach indicates that a M > 9 off Java, with twice the population density as Honshu and much lower

  1. Water cycling beneath subduction zones in 2D and 3D numerical models (Invited)

    NASA Astrophysics Data System (ADS)

    Rupke, L.; Iyer, K. H.; Hasenclever, J.; Morgan, J.

    2013-12-01

    Tracing the cycling of fluids and volatiles through subduction zones continues to be a challenging task with budgets still having large error bars attached to them. In this contribution we show how numerical models can help to integrate various geological, geophysical, and geochemical datasets and how they can be used to put better bounds on the likely amounts of water being subducted, released into the arc and back-arc melting regions, and recycled to the deeper mantle. To achieve this task we use a suite of numerical models. Bending related faulting and hydration of the incoming lithosphere is resolved using a reactive flow model that solves for crustal scale fluid flow and mantle serpentinization using reaction kinetics. Seismic tomography studies from offshore Chile and Central America are used to evaluate and constrain the effective reaction rate. These rates are then used to assess the contribution of serpentinization to the water budget at subduction zones. The pattern of hydration is controlled by the reaction kinetics and serpentinization is most intense around the 270°C isotherm. The depth of this isotherm correlates well with the dominant spacing of double seismic zones observed globally. Comparison of the results with heat flow data suggests that observed seafloor temperature gradients in the bend-fault region are too low to be caused by ';one-pass' downward water flow into the serpentinizing lithosphere, but rather suggest that bend-faults are areas of active hydrothermal circulation. This implies that serpentine-sourced vents and chemosynthetic vent communities should be found in this deep-sea environment as well. Dehydration reactions are resolved with a 2D kinematic subduction zone model that computes the temperature field and the likely locations and volumes of slab fluid release due to metamorphic dehydration reactions. Here we find that up to 1/3 of the subducted water may be transported into the deeper mantle for the coldest subduction zones

  2. Thermal implications of metamorphism in greenstone belts and the hot asthenosphere-thick continental lithoshere paradox

    NASA Technical Reports Server (NTRS)

    Morgan, P.

    1986-01-01

    From considerations of secular cooling of the Earth and the slow decay of radiogenic heat sources in the Earth with time, the conclusion that global heat loss must have been higher in the Archean than at present seems inescapable. The mechanism by which this additional heat was lost and the implications of higher heat low for crustal temperatures are fundamental unknowns in our current understanding of Archean tectonics and geological processes. Higher heat loss implies that the average global geothermal gradient was higher in the Archean than at present, and the restriction of ultramafic komatiites to the Archean and other considerations suggests that the average temperature of the mantle was several hundred degrees hotter during the Archean than today. In contrast, there is little petrologic evidence that the conditions of metamorphism or crustal thickness (including maximum crustal thickness under mountains) were different in archean continental crust from the Phanerozoic record. Additionally, Archean ages have recently been determined for inclusions in diamonds from Cretaceous kimeberlites in South Africa, indicating temperatures of 900 to 1300 at depths of 150 to 215 km (45 to 65 kbar) in the Archean mantle, again implying relatively low geothermal gradients at least locally in the Archean. The thermal implications of metamorphism are examined, with special reference to greenstone belts, and a new thermal model of the continental lithosphere is suggested which is consistent with thick continental lithosphere and high asthenosphere temperatures in the Archean.

  3. Influence of paired subduction zones: insight into Central Mediterranean tectonics

    NASA Astrophysics Data System (ADS)

    Miller, Meghan Samantha; Moresi, Louis; Faccenna, Claudio; Funiciello, Francesca

    2015-04-01

    The Hellenic and Calabrian slabs are subducting the last remnant of the Ionian oceanic lithosphere into the deep mantle beneath the Central Mediterranean. Seismic tomography studies have provided clear images of the present day morphology of the subducted lithosphere [1]. Tectonic studies have shown that the Calabrian slab has rolled back into its current geometry with episodes of back-arc spreading that have now ceased [2]. Conversely, GPS observations along with tectonic reconstructions show that the Hellenic slab is currently rolling back and appears to have accelerated in the past ~15 My [3], which has resulted in the only region of backarc spreading still active in the Mediterranean. Observations of seismic anisotropy from SKS splitting [4] indicate toroidal flow patterns at the edges of the subducted slabs, which lead to interpretations of mantle convection and flow. Rollback in a confined setting has allowed the two slabs to become a plate-tectonic pushmi-pullyu [5]. The evolution of each slab is necessarily dependent on the other as they are both subducting the same lithosphere in opposite directions and are sufficiently close together that their induced mantle flow patterns must interact strongly. Although this seems to be an oddity in the classical picture of plate tectonics, we note that rollback-dominated subduction is more likely to be important in the highly-confined setting of a closing ocean where the oceanic lithosphere is not always able to develop into a freely-moving plate. Under such conditions, back-to-back pairings of subducting slabs are potentially more common. To investigate this setting, we present preliminary numerical models of paired subduction zones that we have developed using Underworld. We include variations in the strength and buoyancy of the surrounding (over-riding) plates and account for the presence of continentally-derived basement in the Adriatic sea. The geodynamic models allow for exploration into the timing, mechanics

  4. Mantle Flow in the Rivera-Cocos Subduction Zone

    NASA Astrophysics Data System (ADS)

    Leon Soto, G.; Ni, J. F.; Grand, S. P.; Sandvol, E. A.; Valenzuela Wong, R.; Guzman-Speziale, M.; Gomez Gonzalez, J. M.; Dominguez Reyes, T.

    2009-12-01

    Western Mexico, where the young and small Rivera plate and the adjacent large Cocos plate are subducting beneath the North American plate, is a unique region on Earth where tearing of subducting oceanic plates, as well as fragmentation of the overriding continental plate, is occurring today. Characterizing the mantle flow field that accompanies the subduction of the Rivera and adjacent Cocos plates can help to clarify the tectonics and magma genesis of this young plate boundary. Here we report observations of seismic anisotropy, as manifested by shear wave splitting derived from local S and teleseismic SKS data collected by the MARS (Mapping Rivera Subduction zone) array that was deployed from January, 2006, through June, 2007, in southwestern Mexico, and from data collected by two of Mexico's Servicio Sismológico Nacional stations. SKS and local S wave splitting parameters indicate that the fast directions of the split SKS waves for stations that lie on the central and southern Jalisco Block are approximately trench-normal, following the convergence direction between the Rivera plate and Jalisco Block. S-wave splitting from slab events show a small averaged delay time of ~0.2 sec for the upper 60 km of the crust and mantle. Therefore, the main source of anisotropy must reside in the entrained mantle below the young and thin Rivera plate. Trench-oblique fast SKS split directions are observed in the western edge of the Rivera plate and the western parts of the Cocos slab. The curved pattern of fast SKS split directions in the western Jalisco block and beneath the Rivera-Cocos slab gap indicates 3-D toroidal mantle flow, around the northwestern edge of the Rivera slab and the Rivera-Cocos gap, which profoundly affect the finite strain field in the northwestern edge of the Rivera slab and the mantle wedge. Both the tomographic images and shear wave splitting results support the idea that the Rivera and western Cocos plates not only moved in a down-dip direction but

  5. The 2004 Sumatra Earthquake and Tsunami: Lessons Learned in Subduction Zone Science and Emergency Management for the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Cassidy, John F.

    2015-03-01

    The 26 December 2004, Mw 9.3 Sumatra earthquake and tsunami was a pivotal turning point in our awareness of the dangers posed by subduction zone earthquakes and tsunamis. This earthquake was the world's largest in 40 years, and it produced the world's deadliest tsunami. This earthquake ruptured a subduction zone that has many similarities to the Cascadia Subduction Zone. In this article, I summarize lessons learned from this tragedy, and make comparisons with potential rupture characteristics, slip distribution, deformation patterns, and aftershock patterns for Cascadia using theoretical modeling and interseismic observations. Both subduction zones are approximately 1,100-1,300 km in length. Both have similar convergence rates and represent oblique subduction. Slip along the subduction fault during the 26 December earthquake is estimated at 15-25 m, similar to values estimated for Cascadia. The width of the rupture, ~80-150 km estimated from modeling seismic and geodetic data, is similar to the width of the "locked and transition zone" estimated for Cascadia. Coseismic subsidence of up to 2 m along the Sumatra coast is also similar to that predicted for parts of northern Cascadia, based on paleoseismic evidence. In addition to scientific lessons learned, the 2004 tsunami provided many critical lessons for emergency management and preparedness. As a result of that tragedy, a number of preparedness initiatives are now underway to promote awareness of earthquake and tsunami hazards along the west coast of North America, and plans are underway to develop prototype tsunami and earthquake warning systems along Cascadia. Lessons learned from the great Sumatra earthquake and tsunami tragedy, both through scientific studies and through public education initiatives, will help to reduce losses during future earthquakes in Cascadia and other subduction zones of the world.

  6. Two-dimensional numerical modeling of tectonic and metamorphic histories at active continental margins

    NASA Astrophysics Data System (ADS)

    Gerya, Taras; Stöckhert, Bernhard

    2006-04-01

    : (a) an accretionary complex of low-grade metamorphic sedimentary material; (b) a wedge of mainly continental crust, with medium-grade HP metamorphic overprint, wound up and stretched in a marble cake fashion to appear as nappes with alternating upper and lower crustal provenance, and minor oceanic or hydrated mantle interleaved material; (c) a megascale melange composed of high-pressure and ultrahigh-pressure metamorphic oceanic and continental crust, and hydrated mantle, all extruded from the subduction channel; (d) zone represents the upward tilted frontal part of the remaining upper plate lid in the case of a weak upper crust. The shape of the P T paths and the time scales correspond to those typically recorded in orogenic belts. Comparison of the numerical results with the European Alps reveals some similarities in their gross structural and metamorphic pattern exposed after collision. A similar structure may be developed at depth beneath the forearc of the Andes, where the importance of subduction erosion is well documented, and where a strong upper crust forms a stable lid.

  7. Assessing the Seismic Potential Hazard of the Makran Subduction Zone

    NASA Astrophysics Data System (ADS)

    Frohling, E.; Szeliga, W. M.; Melbourne, T. I.; Abolghasem, A.; Lodi, S. H.

    2013-12-01

    Long quiescent subduction zones like the Makran, Sunda, and Cascadia, which have long recurrence intervals for large (> Mw 8) earthquakes, often have poorly known seismic histories and are particularly vulnerable and often ill-prepared. The Makran subduction zone has not been studied extensively, but the 1945 Mw 8.1 earthquake and subsequent tsunami, as well as more recent mid magnitude, intermediate depth (50-100 km) seismicity, demonstrates the active seismic nature of the region. Recent increases in regional GPS and seismic monitoring now permit the modeling of strain accumulations and seismic potential of the Makran subduction zone. Subduction zone seismicity indicates that the eastern half of the Makran is presently more active than the western half. It has been hypothesized that the relative quiescence of the western half is due to aseismic behavior. However, based on GPS evidence, the entire subduction zone generally appears to be coupled and has been accumulating stress that could be released in another > 8.0 Mw earthquake. To assess the degree of coupling, we utilize existing GPS data to create a fault coupling model for the Makran using a preliminary 2-D fault geometry derived from ISC hypocenters. Our 2-D modeling is done using the backslip approach and defines the parameters in our coupling model; we forego the generation of a 3-D model due to the low spatial density of available GPS data. We compare the use of both NUVEL-1A plate motions and modern Arabian plate motions derived from GPS station velocities in Oman to drive subduction for our fault coupling model. To avoid non-physical inversion results, we impose second order smoothing to eliminate steep strain gradients. The fit of the modeled inter-seismic deformation vectors are assessed against the observed strain from the GPS data. Initial observations indicate that the entire subduction zone is currently locked and accumulating strain, with no identifiable gaps in the interseismic locking

  8. Results of trench perpendicular wide angle seismic transects across the Manila subduction zone offshore southern Taiwan

    NASA Astrophysics Data System (ADS)

    Eakin, D. H.; McIntosh, K. D.; Van Avendonk, H. J.

    2011-12-01

    Multi-channel seismic reflection and wide-angle seismic data collected in 2009 aboard the R/V Marcus Langseth as part of the TAIGER program delineate the crustal structure of the Manila subduction zone in the northern South China Sea. As part of that project, we recorded marine seismic data using a deployment of ocean-bottom-seismometers (OBS) from the U.S. instrument pool and National Taiwan Ocean University. The region between northern Luzon and southern Taiwan evolves from oceanic subduction to incipient arc-continent collision. This presentation focuses on results of 2 offshore transects across the Manila subduction zone offshore southern Taiwan. Our goal here is to document the transition from pure oceanic subuction in the south to incipient arc-continent collision in the north, an understanding of which is integral for future geodynamic modeling of the advanced arc-continent collision in the north. The northern transect, line T2 is located at 21.4° N and used 30 OBSs. Line T1 was located at 20.5° N and used 27 OBSs across the Manila subduction zone. Data quality is extremely variable due to the local geology and quality of seafloor coupling at each instrument. Preliminary travel-time tomography of transect T2 shows a 10-15 km thick Eurasian crust with crustal velocities of 5-7.5 km/sec entering the Manila trench suggesting thinned continental crust, serpentinized upper mantle, or both in this region. The model shows the accretionary prism to be cored by high velocity material (6-7 km/sec) that may be the result of accretion of crustal material from the subducting Eurasian slab. We also observe asymmetric crustal thickening beneath the Gagua Ridge that is potentially a result of failed subduction of the Philippine Sea Plate westward along the Gagua Ridge. The wide-angle data is complimented by MCS reflection data to constrain sediment thickness, top of the crystalline basement, and moho. Preliminary work is in progress with transect T1 which will be

  9. Global correlations between maximum magnitudes of subduction zone interface thrust earthquakes and physical parameters of subduction zones

    NASA Astrophysics Data System (ADS)

    Schellart, W. P.; Rawlinson, N.

    2013-12-01

    The maximum earthquake magnitude recorded for subduction zone plate boundaries varies considerably on Earth, with some subduction zone segments producing giant subduction zone thrust earthquakes (e.g. Chile, Alaska, Sumatra-Andaman, Japan) and others producing relatively small earthquakes (e.g. Mariana, Scotia). Here we show how such variability might depend on various subduction zone parameters. We present 24 physical parameters that characterize these subduction zones in terms of their geometry, kinematics, geology and dynamics. We have investigated correlations between these parameters and the maximum recorded moment magnitude (MW) for subduction zone segments in the period 1900-June 2012. The investigations were done for one dataset using a geological subduction zone segmentation (44 segments) and for two datasets (rupture zone dataset and epicenter dataset) using a 200 km segmentation (241 segments). All linear correlations for the rupture zone dataset and the epicenter dataset (|R| = 0.00-0.30) and for the geological dataset (|R| = 0.02-0.51) are negligible-low, indicating that even for the highest correlation the best-fit regression line can only explain 26% of the variance. A comparative investigation of the observed ranges of the physical parameters for subduction segments with MW > 8.5 and the observed ranges for all subduction segments gives more useful insight into the spatial distribution of giant subduction thrust earthquakes. For segments with MW > 8.5 distinct (narrow) ranges are observed for several parameters, most notably the trench-normal overriding plate deformation rate (vOPD⊥, i.e. the relative velocity between forearc and stable far-field backarc), trench-normal absolute trench rollback velocity (vT⊥), subduction partitioning ratio (vSP⊥/vS⊥, the fraction of the subduction velocity that is accommodated by subducting plate motion), subduction thrust dip angle (δST), subduction thrust curvature (CST), and trench curvature angle (

  10. Magma-derived CO2 emissions in the Tengchong volcanic field, SE Tibet: Implications for deep carbon cycle at intra-continent subduction zone

    NASA Astrophysics Data System (ADS)

    Zhang, Maoliang; Guo, Zhengfu; Sano, Yuji; Zhang, Lihong; Sun, Yutao; Cheng, Zhihui; Yang, Tsanyao Frank

    2016-09-01

    Active volcanoes at oceanic subduction zone have long been regard as important pathways for deep carbon degassed from Earth's interior, whereas those at continental subduction zone remain poorly constrained. Large-scale active volcanoes, together with significant modern hydrothermal activities, are widely distributed in the Tengchong volcanic field (TVF) on convergent boundary between the Indian and Eurasian plates. They provide an important opportunity for studying deep carbon cycle at the ongoing intra-continent subduction zone. Soil microseepage survey based on accumulation chamber method reveals an average soil CO2 flux of ca. 280 g m-2 d-1 in wet season for the Rehai geothermal park (RGP). Combined with average soil CO2 flux in dry season (ca. 875 g m-2 d-1), total soil CO2 output of the RGP and adjacent region (ca. 3 km2) would be about 6.30 × 105 t a-1. Additionally, we conclude that total flux of outgassing CO2 from the TVF would range in (4.48-7.05) × 106 t a-1, if CO2 fluxes from hot springs and soil in literature are taken into account. Both hot spring and soil gases from the TVF exhibit enrichment in CO2 (>85%) and remarkable contribution from mantle components, as indicated by their elevated 3He/4He ratios (1.85-5.30 RA) and δ13C-CO2 values (-9.00‰ to -2.07‰). He-C isotope coupling model suggests involvement of recycled organic metasediments and limestones from subducted Indian continental lithosphere in formation of the enriched mantle wedge (EMW), which has been recognized as source region of the TVF parental magmas. Contamination by crustal limestone is the first-order control on variations in He-CO2 systematics of volatiles released by the EMW-derived melts. Depleted mantle and recycled crustal materials from subducted Indian continental lithosphere contribute about 45-85% of the total carbon inventory, while the rest carbon (about 15-55%) is accounted by limestones in continental crust. As indicated by origin and evolution of the TVF

  11. Detection of Deep Fluid Flow in Subduction Zones with Magnetotelluric Monitoring

    NASA Astrophysics Data System (ADS)

    Ritter, O.; Araya, J.

    2014-12-01

    After the 1995 Mw 8 Antofagasta earthquake, Husen and Kissling (2001) interpreted alterations observed in the seismic velocity structure as large-scale fluid distribution changes, deep within the subduction zone. Such large scale fluid relocation would cause similar modifications of the associated deep electrical resistivity structure. In this paper, we examine feasibility to detect such changes in the deep hydraulic system with magnetotelluric monitoring. Continuous magnetotelluric (MT) data have been recorded above the subduction zone in northern Chile as part of the Integrated Plate Boundary Observatory Chile (IPOC) with an array of 9 stations since 2007. With the MT method, electrical resistivity and lateral changes of the resistivity structure are estimated from so called transfer functions (TF). If the subsurface resistivity structure is stable, these TFs vary only within their statistical significance intervals over time. Any statistically significant deviations, particularly when observed over the network of sites, must be originated from a change in the subsurface resistivity structure. We simulate the effects of such changes on the TFs using 3D forward modelling studies. The background model is based on 3D inversion of the IPOC MT stations. The results show that detectable differences in the TFs are obtained if the resistivity decreases by 5 times of its original value in the lower continental crust over the rupture zone. The implications of these results are compared with observed changes in the TFs after the 2007 Mw 7.7 Tocopilla and 2014 Mw 8.2 Pisagua earthquakes.

  12. Supra-subduction zone tectonic setting of the Muslim Bagh Ophiolite, northwestern Pakistan: Insights from geochemistry and petrology

    NASA Astrophysics Data System (ADS)

    Kakar, Mohammad Ishaq; Kerr, Andrew C.; Mahmood, Khalid; Collins, Alan S.; Khan, Mehrab; McDonald, Iain

    2014-08-01

    The geology of the Muslim Bagh area comprises the Indian passive continental margin and suture zone, which is overlain by the Muslim Bagh Ophiolite, Bagh Complex and a Flysch Zone of marine-fluvial successions. The Muslim Bagh Ophiolite has a nearly-complete ophiolite stratigraphy. The mantle sequence of foliated peridotite is mainly harzburgite with minor dunite and contains podiform chromite deposits that grade upwards into transition zone dunite. The mantle rocks (harzburgite/dunite) resulted from large degrees of partial melting of lherzolite and have also been affected by melt-peridotite reaction. The Muslim Bagh crustal section has a cyclic succession of ultramafic-mafic cumulate with dunite at the base, that grades into wehrlite/pyroxenite with gabbros (olivine gabbro, norite and hornblende gabbro) at the top. The sheeted dykes are immature in nature and are rooted in crustal gabbros. The dykes are mainly metamorphosed dolerites, with minor intrusions of plagiogranites. The configuration of the crustal section indicates that the crustal rocks were formed over variable time periods, in pulses, by a low magma supply rate. The whole rock geochemistry of the gabbros, sheeted dykes and the mafic dyke swarm suggests that they formed in a supra-subduction zone tectonic setting in Neo-Tethys during the Late Cretaceous. The dykes of the mafic swarm crosscut both the ophiolite and the metamorphic sole rocks and have a less-marked subduction signature than the other mafic rocks. These dykes were possibly emplaced off-axis and can be interpreted to have been generated in the spinel peridotite stability zone i.e., < 50-60 km, and to have risen through a slab window. The Bagh Complex is an assemblage of Triassic-Cretaceous igneous and sedimentary rocks, containing tholeiitic, N-MORB-like basalts and alkali basalts with OIB-type signatures. Nb-Ta depletion in both basalt types suggests possible contamination from continental fragments incorporated into the opening Tethyan

  13. Subduction Zone Diversity and Nature of the Plate Contact

    NASA Astrophysics Data System (ADS)

    Defranco, R.; Govers, R.; Wortel, R.

    2008-12-01

    We recently showed that the overall dynamics of subduction and initial collision depends on whether the plate contact is a fault or a channel. Here, we combine results of our numerical experiments with a re-analysis of published observations. Overall, our synthesis connects seismic moment release with back-arc deformation and tectonic processes at the margin. It leads us to identify four classes of subduction zones. The first two classes results directly from our numerical experiments. In class 1, subduction zones are characterized by a plate contact that is largely fault-like with an accretionary margin. In class 2, the plate contacts are largely channel-type and have an erosive margin. Class 3, where the plate contact is entirely channel-like, consists of accretionary margins with a high sediment supply. Subduction zones of class 4, mostly characterized by an erosive convergent margin (northern Chili, Peru, Honshu and Kuril), are more complicated. They can be explained by incorporating regional observations.

  14. Earth's rotation variability triggers explosive eruptions in subduction zones

    NASA Astrophysics Data System (ADS)

    Sottili, Gianluca; Palladino, Danilo M.; Cuffaro, Marco; Doglioni, Carlo

    2015-12-01

    The uneven Earth's spinning has been reported to affect geological processes, i.e. tectonism, seismicity and volcanism, on a planetary scale. Here, we show that changes of the length of day (LOD) influence eruptive activity at subduction margins. Statistical analysis indicates that eruptions with volcanic explosivity index (VEI) ≥3 alternate along oppositely directed subduction zones as a function of whether the LOD increases or decreases. In particular, eruptions in volcanic arcs along contractional subduction zones, which are mostly E- or NE-directed, occur when LOD increases, whereas they are more frequent when LOD decreases along the opposite W- or SW-directed subduction zones that are rather characterized by upper plate extension and back-arc spreading. We find that the LOD variability determines a modulation of the horizontal shear stresses acting on the crust up to 0.4 MPa. An increase of the horizontal maximum stress in compressive regimes during LOD increment may favour the rupture of the magma feeder system wall rocks. Similarly, a decrease of the minimum horizontal stress in extensional settings during LOD lowering generates a larger differential stress, which may enhance failure of the magma-confining rocks. This asymmetric behaviour of magmatism sheds new light on the role of astronomical forces in the dynamics of the solid Earth.

  15. Ups and downs in western Crete (Hellenic subduction zone).

    PubMed

    Tiberti, Mara Monica; Basili, Roberto; Vannoli, Paola

    2014-01-01

    Studies of past sea-level markers are commonly used to unveil the tectonic history and seismic behavior of subduction zones. We present new evidence on vertical motions of the Hellenic subduction zone as resulting from a suite of Late Pleistocene - Holocene shorelines in western Crete (Greece). Shoreline ages obtained by AMS radiocarbon dating of seashells, together with the reappraisal of shoreline ages from previous works, testify a long-term uplift rate of 2.5-2.7 mm/y. This average value, however, includes periods in which the vertical motions vary significantly: 2.6-3.2 mm/y subsidence rate from 42 ka to 23 ka, followed by ~7.7 mm/y sustained uplift rate from 23 ka to present. The last ~5 ky shows a relatively slower uplift rate of 3.0-3.3 mm/y, yet slightly higher than the long-term average. A preliminary tectonic model attempts at explaining these up and down motions by across-strike partitioning of fault activity in the subduction zone. PMID:25022313

  16. Ups and downs in western Crete (Hellenic subduction zone)

    PubMed Central

    Tiberti, Mara Monica; Basili, Roberto; Vannoli, Paola

    2014-01-01

    Studies of past sea-level markers are commonly used to unveil the tectonic history and seismic behavior of subduction zones. We present new evidence on vertical motions of the Hellenic subduction zone as resulting from a suite of Late Pleistocene - Holocene shorelines in western Crete (Greece). Shoreline ages obtained by AMS radiocarbon dating of seashells, together with the reappraisal of shoreline ages from previous works, testify a long-term uplift rate of 2.5–2.7 mm/y. This average value, however, includes periods in which the vertical motions vary significantly: 2.6–3.2 mm/y subsidence rate from 42 ka to 23 ka, followed by ~7.7 mm/y sustained uplift rate from 23 ka to present. The last ~5 ky shows a relatively slower uplift rate of 3.0–3.3 mm/y, yet slightly higher than the long-term average. A preliminary tectonic model attempts at explaining these up and down motions by across-strike partitioning of fault activity in the subduction zone. PMID:25022313

  17. Episodic Tremor and Slip in the Cascadia Subduction Zone: A Story of Discovery

    NASA Astrophysics Data System (ADS)

    Dragert, H.

    2003-12-01

    migration velocity of slip for the GPS-determined Cascadia slip events, to the depth and migration velocity of the Japanese tremors triggered the search for seismic signatures for the Cascadia slip events. An examination of seismic records from 1996 to 2002 for sites on Vancouver Is. revealed that what had previously been deemed surface noise was signal from seismic tremors that accompanied slip events. The Cascadia tremors were found to be similar in character to the Japanese deep tremors. In addition, their source region was found to coincide with, or directly overlie, the region of the subducting slab interface where transient slip occurs. The close correlation of tremors with slip coined the naming of the phenomenon as Episodic Tremor and Slip (ETS). The physical processes which give rise to this dynamic behavior on the deeper plate interface are not yet well understood. To date, only the Nankai and Cascadia subduction zones have been observed to share aspects of this behavior, suggesting that this phenomenon may be restricted to young subduction zones. The release of fluids, contact with a hydrated mantle wedge, and episodic changes in shear strength or mechanical coupling may all play a part in governing this behavior. Possible connections of ETS with the development of "E-zone" reflector bands, basal erosion, and pulsating metamorphism await further research. In the context of seismic hazard, the ETS zone may mark the down-dip limit of coseismic rupture of the next megathrust earthquake. Also, since it is conceivable for a slip event to trigger a large subduction thrust earthquake, the onset of ETS activity could identify times of higher probability for the occurrence of megathrust earthquakes.

  18. Protolith age and deformation history of high grade metamorphic rocks from the roots of a continental magmatic arc: the Central Gneiss Complex, British Columbia, Canada

    NASA Astrophysics Data System (ADS)

    MacLeod, D.; Pearson, D. M.

    2014-12-01

    deformation, and whether the shear zone is a folded low angle normal fault or instead related to regional thrust burial. This will yield insight into sediment burial and metamorphism at subduction zones and its potential influence on magmatic arc productivity and middle and lower crustal deformation processes.

  19. Accessory minerals and subduction zone metasomatism: a geochemical comparison of two mélanges (Washington and California, U.S.A.)

    USGS Publications Warehouse

    Sorensen, Sorena S.; Grossman, Jeffrey N.

    1993-01-01

    Data from the Gee Point and Catalina mélanges suggest that the accessory minerals titanite, rutile, apatite, zircon and REE-rich epidote play a significant role in the enrichment of trace elements in both mafic and ultramafic rocks during subduction-related fluid-rock interaction. Mobilization of incompatible elements, and deposition of such elements in the accessory minerals of mafic and ultramafic rocks may be fairly common in fluid-rich metamorphic environments in subduction zones.

  20. Subduction zone guided waves: 3D modelling and attenuation effects

    NASA Astrophysics Data System (ADS)

    Garth, T.; Rietbrock, A.

    2013-12-01

    Waveform modelling is an important tool for understanding complex seismic structures such as subduction zone waveguides. These structures are often simplified to 2D structures for modelling purposes to reduce computational costs. In the case of subduction zone waveguide affects, 2D models have shown that dispersed arrivals are caused by a low velocity waveguide, inferred to be subducted oceanic crust and/or hydrated outer rise normal faults. However, due to the 2D modelling limitations the inferred seismic properties such as velocity contrast and waveguide thickness are still debated. Here we test these limitations with full 3D waveform modelling. For waveguide effects to be observable the waveform must be accurately modelled to relatively high frequencies (> 2 Hz). This requires a small grid spacing due to the high seismic velocities present in subduction zones. A large area must be modelled as well due to the long propagation distances (400 - 600 km) of waves interacting with subduction zone waveguides. The combination of the large model area and small grid spacing required means that these simulations require a large amount of computational resources, only available at high performance computational centres like the UK National super computer HECTOR (used in this study). To minimize the cost of modelling for such a large area, the width of the model area perpendicular to the subduction trench (the y-direction) is made as small as possible. This reduces the overall volume of the 3D model domain. Therefore the wave field is simulated in a model ';corridor' of the subduction zone velocity structure. This introduces new potential sources of error particularly from grazing wave side reflections in the y-direction. Various dampening methods are explored to reduce these grazing side reflections, including perfectly matched layers (PML) and more traditional exponential dampening layers. Defining a corridor model allows waveguide affects to be modelled up to at least 2

  1. Obduction of western Anatolian ophiolites: from birth to steady state of a subduction zone

    NASA Astrophysics Data System (ADS)

    Plunder, Alexis; Agard, Philippe; Chopin, Christian; Whitechurch, Hubert; Okay, Aral

    2015-04-01

    were found and attest to high-pressure and low-temperature conditions. As OC2, OC3 exhibit a clear blueschist facies metamorphism, but slightly higher PT conditions. Both OC2 and 3 were only found in the northern area close to the suture zone. Combining these data, available radiometric and palaeogeographic data and recent themomechanical modelling a tentative reconstruction of the subduction-zone evolution through time during the emplacement of a large-scale ophiolite is presented. We show that the cooling of the subduction must occur very quickly (~<15 My) after subduction inception and investigate the implications for early subduction and obduction dynamics.

  2. Receiver function images of the Hellenic subduction zone and comparison to microseismicity

    NASA Astrophysics Data System (ADS)

    Sodoudi, F.; Brüstle, A.; Meier, T.; Kind, R.; Friederich, W.; Egelados Working Group

    2015-02-01

    New combined P receiver functions and seismicity data obtained from the EGELADOS network employing 65 seismological stations within the Aegean constrained new information on the geometry of the Hellenic subduction zone. The dense network and large data set enabled us to estimate the Moho depth of the continental Aegean plate across the whole area. Presence of a negative contrast at the Moho boundary indicating the serpentinized mantle wedge above the subducting African plate was seen along the entire forearc. Furthermore, low seismicity was observed within the serpentinized mantle wedge. We found a relatively thick continental crust (30-43 km) with a maximum thickness of about 48 km beneath the Peloponnese Peninsula, whereas a thinner crust of about 27-30 km was observed beneath western Turkey. The crust of the overriding plate is thinning beneath the southern and central Aegean and reaches 23-27 km. Unusual low Vp / Vs ratios were estimated beneath the central Aegean, which most likely represent indications on the pronounced felsic character of the extended continental Aegean crust. Moreover, P receiver functions imaged the subducted African Moho as a strong converted phase down to a depth of about 100 km. However, the converted Moho phase appears to be weak for the deeper parts of the African plate suggesting nearly complete phase transitions of crustal material into denser phases. We show the subducting African crust along eight profiles covering the whole southern and central Aegean. Seismicity of the western Hellenic subduction zone was taken from the relocated EHB-ISC catalogue, whereas for the eastern Hellenic subduction zone, we used the catalogues of manually picked hypocentre locations of temporary networks within the Aegean. Accurate hypocentre locations reveal a significant change in the dip angle of the Wadati-Benioff zone (WBZ) from west (~ 25°) to the eastern part (~ 35°) of the Hellenic subduction zone. Furthermore, a zone of high deformation can

  3. Aseismic Slip on the Northern Cascadia Subduction Zone: A Regular but Unique Process?

    NASA Astrophysics Data System (ADS)

    Dragert, H.; Wang, K.; James, T. S.; Schmidt, M.

    2002-12-01

    Our closer re-examination of 1994 to 2002 data from continuous GPS sites in southwestern British Columbia and northern Washington State has confirmed the occurrence of seven aseismic slip events on the deeper subduction interface underlying the inner margin of the northern Cascadia Subduction Zone (CSZ). At any given site in the region of detection, the transient surface displacements observed for each event are strikingly similar in amplitude, direction, and duration, indicating a repetitive process within a confined location. The areal patterns of total surface displacements that accompany each slip suggest that this location is centered beneath southern Vancouver Is. and the eastern Olympic Plateau. The augmented rates of strain accumulation between slip events also appear uniform from one inter-slip period to the next, again suggesting a recurring process that is spatially confined. To date, this pronounced "sawtooth" displacement pattern caused by elevated stress accumulation for a period of about 60 to 70 weeks followed by a two-week period of aseismic stress reduction has not been observed in other subduction zones or even in the southern CSZ. It is possible that the arch in the subducting Juan de Fuca plate and the contact with hydrated mantle material on the deeper (25 to 45 km) subduction interface are two factors contributing to this (possibly) unique behaviour. At these depths, temperatures exceed 550° C and it is conceivable that the release of fluids from metamorphic reactions involving hydrated minerals corrodes inter-granular shear strength, ultimately resulting in aseismic slip, followed by an escape of fluids, a resulting rapid cooling, and a subsequent recovery of shear strength.

  4. Rheological control in subduction zones: slab dynamics, fluid flow and seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Van Keken, P. E.; Spiegelman, M.; Wilson, C. R.

    2011-12-01

    The strong temperature, strain-rate and compositional gradients in subduction zones provide extreme challenges to the modeling of their dynamics. Major questions remain regarding the state of stress in the slab, the nature of the downdip transition of the seismogenic zone, the rheological control on the release of fluids and the role of fluids in magma generation, earthquake source processes and the formation of seismic anisotropy. We use high resolution finite element models of convergent margins to address some of these topics. First, we explore the role of fluids on wedge rheology and how the presence or absence of fluids changes the dynamics of the wedge. The presence of fluids in the cold fore-arc may be the cause of trench-parallel anisotropy due to B-type olivine fabric or due to shape preferred orientation by the alignment of serpentinite filled cracks. Second, The cold fore-arc generally extends to where the slab is at ~80 km depth. This depth has a strong sensitivity to the depth where the overriding wedge couples with the slab. Weak phases such as chlorite, serpentinite and talc may play a critical control on the region where the slab remains decoupled, but the strong non-linearities involved make it difficult to determine a single process for all subduction zones. Finally, the increasing metamorphic grade of rocks in the subducting slab cause progressive fluid production, which has been linked to intermediate depth seismicity. The fluids also are considered the primary cause for arc volcanism, but it is not yet clear how the fluids escape from the slab and travel to the zones of arc magmatism. We use a new set of coupled solid state and porous flow models to determine how fluids affect the shear and bulk viscosity of these rocks and how this in turn controls the flow of fluids from the slab.

  5. H2O and CO2 devolatilization in subduction zones: implications for the global water and carbon cycles (Invited)

    NASA Astrophysics Data System (ADS)

    van Keken, P. E.; Hacker, B. R.; Syracuse, E. M.; Abers, G. A.

    2010-12-01

    Subduction of sediments and altered oceanic crust functions as a major carbon sink. Upon subduction the carbon may be released by progressive metamorphic reactions, which can be strongly enhanced by free fluids. Quantification of the CO2 release from subducting slabs is important to determine the provenance of CO2 that is released by the volcanic arc and to constrain the flux of carbon to the deeper mantle. In recent work we used a global set of high resolution thermal models of subduction zones to predict the flux of H2O from the subducting slab (van Keken, Hacker, Syracuse, Abers, Subduction factory 4: Depth-dependent flux of H2O from subducting slabs worldwide, J. Geophys. Res., under review) which provides a new estimate of the dehydration efficiency of the global subducting system. It was found that mineralogically bound water can pass efficiently through old and fast subduction zones (such as in the western Pacific) but that warm subduction zones (such as Cascadia) see nearly complete dehydration of the subducting slab. The top of the slab is sufficiently hot in all subduction zones that the upper crust dehydrates significantly. The degree and depth of dehydration is highly diverse and strongly depends on (p,T) and bulk rock composition. On average about one third of subducted H2O reaches 240 km depth, carried principally and roughly equally in the gabbro and peridotite sections. The present-day global flux of H2O to the deep mantle translates to an addition of about one ocean mass over the age of the Earth. We extend the slab devolatilization work to carbon by providing an update to Gorman et al. (Geochem. Geophys. Geosyst, 2006), who quantified the effects of free fluids on CO2 release. The thermal conditions were based on three end-member subduction zones with linear interpolation to provide a global CO2 flux. We use the new high resolution and global set of models to provide higher resolution predictions for the provenance and pathways of CO2 release to

  6. Gravity anomalies, forearc morphology and seismicity in subduction zones

    NASA Astrophysics Data System (ADS)

    Bassett, D.; Watts, A. B.; Das, S.

    2012-12-01

    We apply spectral averaging techniques to isolate and remove the long-wavelength large-amplitude trench-normal topographic and free-air gravity anomaly "high" and "low" associated with subduction zones. The residual grids generated illuminate the short-wavelength structure of the forearc. Systematic analysis of all subduction boundaries on Earth has enabled a classification of these grids with particular emphasis placed on topography and gravity anomalies observed in the region above the shallow seismogenic portion of the plate interface. The isostatic compensation of these anomalies is investigated using 3D calculations of the gravitational admittance and coherence. In the shallow region of the megathrust, typically within 100 km from the trench, isolated residual anomalies with amplitudes of up to 2.5 km and 125 mGal are generally interpreted as accreted/subducting relief in the form of seamounts and other bathymetric features. While most of these anomalies, which have radii < 50km, are correlated with areas of reduced seismicity, several in regions such as Japan and Java appear to have influenced the nucleation and/or propagation of large magnitude earthquakes. Long-wavelength (500 - >1000 km) trench-parallel forearc ridges with residual anomalies of up to 1.5 km and 150 mGal are identified in approximately one-third of the subduction zones analyzed. Despite great length along strike, these ridges are less than 100 km wide and several appear uncompensated. A high proportion of arc-normal structure and the truncation/morphological transition of trench-parallel forearc ridges is explained through the identification and tracking of pre-existing structure on the over-riding and subducting plates into the seismogenic portion of the plate boundary. Spatial correlations between regions with well-defined trench-parallel forearc ridges and the occurrence of large magnitude interplate earthquakes, in addition to the uncompensated state of these ridges, suggest links

  7. Numerical Modelling of Subduction Zones: a New Beginning

    NASA Astrophysics Data System (ADS)

    Ficini, Eleonora; Dal Zilio, Luca; Doglioni, Carlo; Gerya, Taras V.

    2016-04-01

    Subduction zones are one of the most studied although still controversial geodynamic process. Is it a passive or an active mechanism in the frame of plate tectonics? How subduction initiates? What controls the differences among the slabs and related orogens and accretionary wedges? The geometry and kinematics at plate boundaries point to a "westerly" polarized flow of plates, which implies a relative opposed flow of the underlying Earth's mantle, being the decoupling located at about 100-200 km depth in the low-velocity zone or LVZ (Doglioni and Panza, 2015 and references therein). This flow is the simplest explanation for determining the asymmetric pattern of subduction zones; in fact "westerly" directed slabs are steeper and deeper with respect to the "easterly or northeasterly" directed ones, that are less steep and shallower, and two end members of orogens associated to the downgoing slabs can be distinguished in terms of topography, type of rocks, magmatism, backarc spreading or not, foredeep subsidence rate, etc.. The classic asymmetry comparing the western Pacific slabs and orogens (low topography and backarc spreading in the upper plate) and the eastern Pacific subduction zones (high topography and deep rocks involved in the upper plate) cannot be ascribed to the age of the subducting lithosphere. In fact, the same asymmetry can be recognized all over the world regardless the type and age of the subducting lithosphere, being rather controlled by the geographic polarity of the subduction. All plate boundaries move "west". Present numerical modelling set of subduction zones is based on the idea that a subducting slab is primarily controlled by its negative buoyancy. However, there are several counterarguments against this assumption, which is not able to explain the global asymmetric aforementioned signatures. Moreover, petrological reconstructions of the lithospheric and underlying mantle composition, point for a much smaller negative buoyancy than predicted

  8. Geophysical evidence for the evolution of the California Inner Continental Borderland as a metamorphic core complex

    USGS Publications Warehouse

    ten Brink, Uri S.; Zhang, Jie; Brocher, Thomas M.; Okaya, David A.; Klitgord, Kim D.; Fuis, Gary S.

    2000-01-01

    We use new seismic and gravity data collected during the 1994 Los Angeles Region Seismic Experiment (LARSE) to discuss the origin of the California Inner Continental Borderland (ICB) as an extended terrain possibly in a metamorphic core complex mode. The data provide detailed crustal structure of the Borderland and its transition to mainland southern California. Using tomographic inversion as well as traditional forward ray tracing to model the wide-angle seismic data, we find little or no sediments, low (≤6.6 km/s) P wave velocity extending down to the crust-mantle boundary, and a thin crust (19 to 23 km thick). Coincident multichannel seismic reflection data show a reflective lower crust under Catalina Ridge. Contrary to other parts of coastal California, we do not find evidence for an underplated fossil oceanic layer at the base of the crust. Coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge, which represents the transition to the western Transverse Ranges. On the shelf the Palos Verdes Fault merges downward into a landward dipping surface which separates "basement" from low-velocity sediments, but interpretation of this surface as a detachment fault is inconclusive. The seismic velocity structure is interpreted to represent Catalina Schist rocks extending from top to bottom of the crust. This interpretation is compatible with a model for the origin of the ICB as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks. The basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until ∼13 m.y. ago. Subduction of the young and hot Monterey and Arguello microplates under the Continental Borderland, followed by rotation and translation of the western Transverse Ranges, may have provided the necessary thermomechanical conditions for this extension and crustal inflow.

  9. Volatile (Li, B, F and Cl) mobility during amphibole breakdown in subduction zones

    NASA Astrophysics Data System (ADS)

    Debret, Baptiste; Koga, Kenneth T.; Cattani, Fanny; Nicollet, Christian; Van den Bleeken, Greg; Schwartz, Stephane

    2016-02-01

    Amphiboles are ubiquitous minerals in the altered oceanic crust. During subduction, their breakdown is governed by continuous reactions up to eclogitic facies conditions. Amphiboles thus contribute to slab-derived fluid throughout prograde metamorphism and continuously record information about volatile exchanges occurring between the slab and the mantle wedge. However, the fate of volatile elements and especially halogens, such as F and Cl, in amphibole during subduction is poorly constrained. We studied metagabbros from three different localities in the Western Alps: the Chenaillet ophiolite, the Queyras Schistes Lustrés and the Monviso meta-ophiolitic complexes. These samples record different metamorphic conditions, from greenschist to eclogite facies, and have interacted with different lithologies (e.g. sedimentary rocks, serpentinites) from their formation at mid-oceanic ridge, up to their devolatilization during subduction. In the oceanic crust, the initial halogen budget is mostly stored in magmatic amphibole (F = 300-7000 ppm; Cl = 20-1200 ppm) or in amphibole corona (F = 100-7000 ppm; Cl = 80-2000 ppm) and titanite (F = 200-1500 ppm; Cl < 200 ppm) formed during hydrothermal seafloor alteration. It is thus the fate of these phases that govern the halogen fluxes between the crust and the overlying mantle and/or the plate interface in subduction zones. Li and B are poorly stored in the oceanic crust (< 5 ppm). In subduction zones, prograde metamorphism of metagabbros is first marked by the crystallization of glaucophane at the expense of magmatic and amphibole coronas. This episode is accompanied with a decrease of halogen concentrations in amphiboles (< 200 ppm of F and Cl) suggesting that these elements can be transferred to the mantle wedge by fluids. In the Queyras Schistes Lustrés complex, the intense deformation and the abundant devolatilization of metasedimentary rocks produce large fluid flows that promote rock chemical hybridization (metasomatic

  10. Large earthquake processes in the northern Vanuatu subduction zone

    NASA Astrophysics Data System (ADS)

    Cleveland, K. Michael; Ammon, Charles J.; Lay, Thorne

    2014-12-01

    The northern Vanuatu (formerly New Hebrides) subduction zone (11°S to 14°S) has experienced large shallow thrust earthquakes with Mw > 7 in 1966 (MS 7.9, 7.3), 1980 (Mw 7.5, 7.7), 1997 (Mw 7.7), 2009 (Mw 7.7, 7.8, 7.4), and 2013 (Mw 8.0). We analyze seismic data from the latter four earthquake sequences to quantify the rupture processes of these large earthquakes. The 7 October 2009 earthquakes occurred in close spatial proximity over about 1 h in the same region as the July 1980 doublet. Both sequences activated widespread seismicity along the northern Vanuatu subduction zone. The focal mechanisms indicate interplate thrusting, but there are differences in waveforms that establish that the events are not exact repeats. With an epicenter near the 1980 and 2009 events, the 1997 earthquake appears to have been a shallow intraslab rupture below the megathrust, with strong southward directivity favoring a steeply dipping plane. Some triggered interplate thrusting events occurred as part of this sequence. The 1966 doublet ruptured north of the 1980 and 2009 events and also produced widespread aftershock activity. The 2013 earthquake rupture propagated southward from the northern corner of the trench with shallow slip that generated a substantial tsunami. The repeated occurrence of large earthquake doublets along the northern Vanuatu subduction zone is remarkable considering the doublets likely involved overlapping, yet different combinations of asperities. The frequent occurrence of large doublet events and rapid aftershock expansion in this region indicate the presence of small, irregularly spaced asperities along the plate interface.

  11. Unraveling topography around subduction zones from laboratory models

    NASA Astrophysics Data System (ADS)

    Husson, Laurent; Guillaume, Benjamin; Funiciello, Francesca; Faccenna, Claudio; Royden, Leigh H.

    2012-03-01

    The relief around subduction zones results from the interplay of dynamic processes that may locally exceed the (iso)static contributions. The viscous dissipation of the energy in and around subduction zones is capable of generating kilometer scale vertical ground movements. In order to evaluate dynamic topography in a self-consistent subduction system, we carried out a set of laboratory experiments, wherein the lithosphere and mantle are simulated by means of Newtonian viscous materials, namely silicone putty and glucose syrup. Models are kept in their most simple form and are made of negative buoyancy plates, of variable width and thickness, freely plunging into the syrup. The surface of the model and the top of the slab are scanned in three dimensions. A forebulge systematically emerges from the bending of the viscous plate, adjacent to the trench. With a large wavelength, dynamic pressure offsets the foreside and backside of the slab by ~ 500 m on average. The suction, that accompanies the vertical descent of the slab depresses the surface on both sides. At a distance equal to the half-width of the slab, the topographic depression amounts to ~ 500 m on average and becomes negligible at a distance that equals the width of the slab. In order to explore the impact of slab rollback on the topography, the trailing edge of the plates is alternatively fixed to (fixed mode) and freed from (free mode) the end wall of the tank. Both the pressure and suction components of the topography are ~ 30% lower in the free mode, indicating that slab rollback fosters the dynamic subsidence of upper plates. Our models are compatible with first order observations of the topography around the East Scotia, Tonga, Kermadec and Banda subduction zones, which exhibit anomalous depths of nearly 1 km as compared to adjacent sea floor of comparable age.

  12. Modelling Subduction Zone Magmatism Due to Hydraulic Fracture

    NASA Astrophysics Data System (ADS)

    Lawton, R.; Davies, J. H.

    2014-12-01

    The aim of this project is to test the hypothesis that subduction zone magmatism involves hydraulic fractures propagating from the oceanic crust to the mantle wedge source region (Davies, 1999). We aim to test this hypothesis by developing a numerical model of the process, and then comparing model outputs with observations. The hypothesis proposes that the water interconnects in the slab following an earthquake. If sufficient pressure develops a hydrofracture occurs. The hydrofracture will expand in the direction of the least compressive stress and propagate in the direction of the most compressive stress, which is out into the wedge. Therefore we can calculate the hydrofracture path and end-point, given the start location on the slab and the propagation distance. We can therefore predict where water is added to the mantle wedge. To take this further we have developed a thermal model of a subduction zone. The model uses a finite difference, marker-in-cell method to solve the heat equation (Gerya, 2010). The velocity field was prescribed using the analytical expression of cornerflow (Batchelor, 1967). The markers contained within the fixed grid are used to track the different compositions and their properties. The subduction zone thermal model was benchmarked (Van Keken, 2008). We used the hydrous melting parameterization of Katz et.al., (2003) to calculate the degree of melting caused by the addition of water to the wedge. We investigate models where the hydrofractures, with properties constrained by estimated water fluxes, have random end points. The model predicts degree of melting, magma productivity, temperature of the melt and water content in the melt for different initial water fluxes. Future models will also include the buoyancy effect of the melt and residue. Batchelor, Cambridge UP, 1967. Davies, Nature, 398: 142-145, 1999. Gerya, Cambridge UP, 2010. Katz, Geochem. Geophys. Geosy, 4(9), 2003 Van Keken et.al. Phys. Earth. Planet. In., 171:187-197, 2008.

  13. Evolution and diversity of subduction zones controlled by slab width.

    PubMed

    Schellart, W P; Freeman, J; Stegman, D R; Moresi, L; May, D

    2007-03-15

    Subducting slabs provide the main driving force for plate motion and flow in the Earth's mantle, and geodynamic, seismic and geochemical studies offer insight into slab dynamics and subduction-induced flow. Most previous geodynamic studies treat subduction zones as either infinite in trench-parallel extent (that is, two-dimensional) or finite in width but fixed in space. Subduction zones and their associated slabs are, however, limited in lateral extent (250-7,400 km) and their three-dimensional geometry evolves over time. Here we show that slab width controls two first-order features of plate tectonics-the curvature of subduction zones and their tendency to retreat backwards with time. Using three-dimensional numerical simulations of free subduction, we show that trench migration rate is inversely related to slab width and depends on proximity to a lateral slab edge. These results are consistent with retreat velocities observed globally, with maximum velocities (6-16 cm yr(-1)) only observed close to slab edges (<1,200 km), whereas far from edges (>2,000 km) retreat velocities are always slow (<2.0 cm yr(-1)). Models with narrow slabs (< or =1,500 km) retreat fast and develop a curved geometry, concave towards the mantle wedge side. Models with slabs intermediate in width ( approximately 2,000-3,000 km) are sublinear and retreat more slowly. Models with wide slabs (> or =4,000 km) are nearly stationary in the centre and develop a convex geometry, whereas trench retreat increases towards concave-shaped edges. Additionally, we identify periods (5-10 Myr) of slow trench advance at the centre of wide slabs. Such wide-slab behaviour may explain mountain building in the central Andes, as being a consequence of its tectonic setting, far from slab edges. PMID:17361181

  14. Can slabs melt beneath forearcs in hot subduction zones?

    NASA Astrophysics Data System (ADS)

    Ribeiro, J.; Maury, R.; Gregoire, M.

    2015-12-01

    At subduction zones, thermal modeling predict that the shallow part of the downgoing oceanic crust (< 80 - 100 km depth to the slab) is usually too cold to cross the water-rich solidus and melts beneath the forearc. Yet, the occasional occurrence of adakites, commonly considered as slab melts, in the forearc region challenges our understanding of the shallow subduction processes. Adakites are unusual felsic rocks commonly associated with asthenospheric slab window opening or fast subduction of young (< 25 Ma) oceanic plate that enable slab melting at shallow depths; but their genesis has remained controversial. Here, we present a new approach that provides new constraints on adakite petrogenesis in hot subduction zones (the Philippines) and above an asthenospheric window (Baja California, Mexico). We use amphibole compositions to estimate the magma storage depths and the composition of the parental melts to test the hypothesis that adakites are pristine slab melts. We find that adakites from Baja California and Philippines formed by two distinct petrogenetic scenarios. In Baja California, hydrous mantle melts mixed/mingled with high-pressure (HP) adakite-type, slab melts within a lower crustal (~30 km depth) magma storage region before stalling into the upper arc crust (~7-15 km depth). In contrast, in the Philippines, primitive mantle melts stalled and crystallized within lower and upper crustal magma storage regions to produce silica-rich melts with an adakitic signature. Thereby, slab melting is not required to produce an adakitic geochemical fingerprint in hot subduction zones. However, our results also suggest that the downgoing crust potentially melted beneath Baja California.

  15. Slab anisotropy from subduction zone guided waves in Taiwan

    NASA Astrophysics Data System (ADS)

    Chen, K. H.; Tseng, Y. L.; Hu, J. C.

    2014-12-01

    Frozen-in anisotropic structure in the oceanic lithosphere and faulting/hydration in the upper layer of the slab are expected to play an important role in anisotropic signature of the subducted slab. Over the past several decades, despite the advances in characterizing anisotropy using shear wave splitting method and its developments, the character of slab anisotropy remains poorly understood. In this study we investigate the slab anisotropy using subduction zone guided waves characterized by long path length in the slab. In the southernmost Ryukyu subduction zone, seismic waves from events deeper than 100 km offshore northern Taiwan reveal wave guide behavior: (1) a low-frequency (< 1 Hz) first arrival recognized on vertical and radial components but not transverse component (2) large, sustained high-frequency (3-10 Hz) signal in P and S wave trains. The depth dependent high-frequency content (3-10Hz) confirms the association with a waveguide effect in the subducting slab rather than localized site amplification effects. Using the selected subduction zone guided wave events, we further analyzed the shear wave splitting for intermediate-depth earthquakes in different frequency bands, to provide the statistically meaningful shear wave splitting parameters. We determine shear wave splitting parameters from the 34 PSP guided events that are deeper than 100 km with ray path traveling along the subducted slab. From shear wave splitting analysis, the slab and crust effects reveal consistent polarization pattern of fast directions of EN-WS and delay time of 0.13 - 0.27 sec. This implies that slab anisotropy is stronger than the crust effect (<0.1 s) but weaker than the mantle wedge and sub-slab mantle effect (0.3-1.3 s) in Taiwan.

  16. Methane generation in subduction zones: A cause for fluid overpressures?

    NASA Astrophysics Data System (ADS)

    Raimbourg, Hugues; Disnar, Jean-Robert; Thiery, Regis; Ramboz, Claire; Yamaguchi, Asuka; Kimura, Gaku

    2013-04-01

    The nature of the fluids involved in the deep plate interface in subduction zones is difficult to constrain, as it incorporates many potential sources (sea water trapped in pores, water from dehydration reactions, fluid from the depths of the subduction channel or from the slab). Using Raman analysis of fluid inclusions in quartz veins from the deep domains of the Shimanto paleo-accretionary complex, Japan, we first show that at temperatures of ~250°C, the fluid is a mixture of water and methane, in agreement with literature on similar terranes. In most of the studied area, we could observe only one, water-rich, kind of inclusion, while in a restricted region a second, methane-rich, kind of inclusion was also present, suggesting in the first case the circulation at depth of a single fluid and in the second case the coexistence of two fluid phases. We used then isochores of the methane-rich fluid inclusions to constrain the paleo- fluid pressure. In the present case, methane-rich inclusions are distributed as planes, i.e. along healed microcracks, hence they provide a record of the conditions that prevailed during a short period of time. Within a single plane of inclusions, homogeneization temperatures of the methane phase show large variations between inclusions, which we interpret as the record of large and rapid variations in fluid pressure. To account for this diversity in the fluid state (single- vs. two-phased) as well as for the rapid variations in pressure, we developed a model of methane generation by thermal cracking of organic matter during burial. In spite of the low average organic matter content of subducted sediments, the porosity, hence the water content of deep sediments is sufficiently low for the oversaturation of the water in methane, hence unmixing of a free, methane-rich phase, to be a realistic scenario. Predicted overpressures resulting from rapid unmixing of methane can be significant with respect to ambient fluid pressure and constitute

  17. Estimates of radiated energy from global shallow subduction zone earthquakes

    NASA Astrophysics Data System (ADS)

    Bilek, S. L.; Lay, T.; Ruff, L.

    2002-12-01

    Previous studies used seismic energy to moment ratios for datasets of large earthquakes as a useful discriminant for tsunami earthquakes. We extend this idea of a "slowness" discriminant to a large dataset of subduction zone underthrusting earthquakes. We determined estimates of energy release in these shallow earthquakes using a large dataset of source time functions. This dataset contains source time functions for 418 shallow (< 70 km depth) earthquakes ranging from Mw 5.5 - 8.0 from 14 circum-Pacific subduction zones. Also included are tsunami earthquakes for which source time functions are available. We calculate energy using two methods, a substitution of a simplified triangle and integration of the original source time function. In the first method, we use a triangle substitution of peak moment and duration to find a minimum estimate of energy. The other method incorporates more of the source time function information and can be influenced by source time function complexity. We examine patterns in source time function complexity with respect to the energy estimates. For comparison with other earthquake parameters, it is useful to remove the effect of seismic moment on the energy estimates. We use the seismic energy to moment ratio (E/Mo) to highlight variations with depth, moment, and subduction zone. There is significant scatter in this ratio using both methods of energy calculation. We observe a slight increase in E/Mo with increasing Mw. There is not much variation in E/Mo with depth seen in entire dataset. However, a slight increase in E/Mo with depth is apparent in a few subduction zones such as Alaska, Central America, and Peru. An average E/Mo of 5x10e-6 roughly characterizes this shallow earthquake dataset, although with a factor of 10 scatter. This value is within about a factor of 2 of E/Mo ratios determined by Choy and Boatwright (1995). Tsunami earthquakes suggest an average E/Mo of 2x10e-7, significantly lower than the average for the shallow

  18. Improved Teleseismic Locations of Shallow Subduction Zone Earthquakes

    NASA Astrophysics Data System (ADS)

    Bisrat, S. T.; Deshon, H. R.; Engdahl, E. R.; Bilek, S. L.

    2009-12-01

    Improved precision teleseismic earthquake locations in subduction zones are being used to better understand shallow megathrust frictional conditions and determine the global distribution of tsunami earthquakes. Most global teleseismic catalogs fail to accurately locate shallow subduction zone earthquakes, especially mid-magnitude events, leading to increased error in determining source time functions useful for identifying tsunami earthquakes. The Engdahl, van der Hilst and Buland (EHB) method had addressed this problem in part by including the teleseismic depth phases pP, pwP and sP in the relocation algorithm. The EHB catalog relies on phase times reported to the ISC and NEIC, but additional high quality depth phase onsets can be incorporated in the relocation procedure to enhance the robustness of individual locations. We present improvements to an automated frequency-based picker that identifies depth phases not reported in the standard catalogs. The revised autopicker uses abrupt amplitude changes of the power spectral density (PSD) function calculated at optimized frequencies for each waveform. It is being used to pick onsets for P and depth phases pP, pwP or sP for inclusion in the EHB phase catalog. In the case of events with an emergent P-wave onset or with a complex waveform consisting of sub-events, the autopicker may either overlook a relatively small change in frequency of the first arrival or misidentify the onset arrival time of associated later arrivals, leading to erroneous results. We track those waveforms by comparing the difference of the P-wave arrival time from ISC/NEIC and the autopicker. The phase arrivals can then be adjusted manually as they usually make up a few percent of the whole data. Epicentral changes following relocation using additional depth phases are generally small (<5 km). Changes in depth may be on the order of 10s of km for some events, though the standard deviation of depth changes within each subduction zone is ~5 km. We

  19. High-pressure veins in eclogite from New Caledonia and their significance for fluid migration in subduction zones

    NASA Astrophysics Data System (ADS)

    Spandler, Carl; Hermann, Jörg

    2006-06-01

    This paper describes the petrology and geochemistry of high-pressure veins and associated metasomatic selvages that are hosted by eclogite in the Pouébo Eclogite Melange of northern New Caledonia. Isotopic and geochemical evidence indicates that the mafic host rock represents seafloor-altered fractionated MORB that underwent eclogite-facies metamorphism in a subduction zone. Within the host rock are cm-thick garnet-quartz-phengite veins that are enveloped by garnet-poor, omphacite-rich selvages or bleach zones. Petrography, thermometry, oxygen isotope characteristics, and mass balance calculations are used to show that the veins largely formed by fluid-mediated mass transfer from the bleach zones during prograde metamorphism. Minerals in the veins are free of inclusions, but the vein garnets preserve complex chemical zoning features that are not present in the host rock garnets. Vein garnets have Mn and HREE zoning patterns that are indicative of progressive garnet growth during prograde metamorphism, whereas Mg and Ca contents reveal prominent sector zoning and fine-scale intergrowth features. We propose that the veins formed over a prolonged period during subduction by local circulation of fluid that was sourced from prograde dehydration of minerals in the host rock. Fluid circulation may have been driven by episodic microcracking/sealing around garnet porphyroblasts, which led to significant mass transfer and progressive vein growth. Mass balance calculations and phengite trace element compositions also require the additional of pelite-derived components to the veins. These components were probably introduced into the veins at conditions close to peak metamorphism via a relatively small external fluid flux. This model for vein formation is consistent with previous studies that suggest fluid flow in deeply subducted oceanic crust is highly restricted in many cases. The delay of fluid migration after hydrous mineral breakdown may provide an important source of

  20. Fore- and Back-Arc Structures Along the Hikurangi-Kermadec Subduction Zone

    NASA Astrophysics Data System (ADS)

    Scherwath, M.; Kopp, H.; Flueh, E. R.; Henrys, S. A.; Sutherland, R.

    2009-04-01

    The Hikurangi-Kermadec subduction zone northeast of New Zealand represents an ideal target to study lateral variations of subduction zone processes. The incoming Pacific plate changes from being a large igneous province, called the Hikurangi Plateau, in the south to normal oceanic plate north of the Rapuhia Scarp. The overriding Australian plate is continental in the south, forming the North Island of New Zealand, and changes to an island arc in the north. Further lateral variability exists in changes in volcanic and hydro-thermal activity, transitions from accretion to subduction erosion, backarc spreading and rifting, and is accompanied by northward increasing seismicity. As part of the MANGO project (Marine Geoscientific Investigations on the Input and Output of the Kermadec Subduction Zone), four marine geophysical transects of largely seismic reflection and refraction data provide constraints on the upper lithospheric structures across the Hikurangi-Kermadec Trench between 29-38 degrees South. On MANGO profile 1 in the south, the initially shallow subduction of the incoming plateau coincides with crustal underplating beneath the East Cape ridge. To the west lies the 100 km wide and over 10 km deep Raukumara Basin. Seismic velocities of the upper arc mantle are around 8 km/s and are considered normal. In contrast, on MANGO profile 4, about 1000 km to the north around the volcanically active Raoul Island, the incoming oceanic crust appears to bend considerably steeper and thus causes a 50 km narrower forearc with a smaller forearc basin. Furthermore, the upper mantle velocities in both plates are relatively low (7.4-7.7 km/s), likely indicating strong bending related deformation of the incoming plate and thermal activity within the arc possibly due to spreading. Here, arc volcanism is relatively active, with many large volcanoes directly on the ridge. The central two transects MANGO 2 and 3, though without data coverage of the structure of the incoming plate

  1. Chronology of historical tsunamis in Mexico and its relation to large earthquakes along the subduction zone

    NASA Astrophysics Data System (ADS)

    Suarez, G.; Mortera, C.

    2013-05-01

    The chronology of historical earthquakes along the subduction zone in Mexico spans a time period of approximately 400 years. Although the population density along the coast of Mexico has always been low, relative to that of central Mexico, several of the large subduction earthquakes reports include references to the presence of tsunamis invading the southern coast of Mexico. Here we present a chronology of historical tsunamis affecting the Pacific coast of Mexico and compare this with the historical record of subduction events and to the existing Mexican and worldwide catalogs of tsunamis in the Pacific basin. Due to the geographical orientation of the Pacific coat of Mexico, tsunamis generated on the other subduction zones of the Pacific have not had damaging effects in the country. Among the tsunamis generated by local earthquakes, the largest one by far is the one produced by the earthquake of 28 March 1787. The reported tsunami has an inundation area that reaches for over 6 km inland. The length of the coast where the tsunami was reported extends for over 450 km. In the last 100 years two large tsunamis have been reported along the Pacific coast of Mexico. On 22 June 1932 a tsunami with reported wave heights of up to 11 m hit the coast of Jalisco and Colima. The town of Cuyutlan was heavily damaged and approximately 50 people lost their lives do to the impact of the tsunami. This unusual tsunami was generated by an aftershock (M 6.9) of the large 3 June 1932 event (M 8.1). The main shock of 3 June did not produce a perceptible tsunami. It has been proposed that the 22 June event is a tsunami earthquake generated on the shallow part of the subduction zone. On 16 November 1925 an unusual tsunami was reported in the town of Zihuatanejo in the state of Guerrero, Mexico. No earthquake on the Pacific rim occurs at the same time as this tsunami and the historical record of hurricanes and tropical storms do not list the presence of a meteorological disturbance that

  2. Water and the Oxidation State of Subduction Zone Magmas

    SciTech Connect

    Kelley, K.; Cottrell, E

    2009-01-01

    Mantle oxygen fugacity exerts a primary control on mass exchange between Earth's surface and interior at subduction zones, but the major factors controlling mantle oxygen fugacity (such as volatiles and phase assemblages) and how tectonic cycles drive its secular evolution are still debated. We present integrated measurements of redox-sensitive ratios of oxidized iron to total iron (Fe{sup 3+}/{Sigma}Fe), determined with Fe K-edge micro-x-ray absorption near-edge structure spectroscopy, and pre-eruptive magmatic H{sub 2}O contents of a global sampling of primitive undegassed basaltic glasses and melt inclusions covering a range of plate tectonic settings. Magmatic Fe{sup 3+}/{Sigma}Fe ratios increase toward subduction zones (at ridges, 0.13 to 0.17; at back arcs, 0.15 to 0.19; and at arcs, 0.18 to 0.32) and correlate linearly with H{sub 2}O content and element tracers of slab-derived fluids. These observations indicate a direct link between mass transfer from the subducted plate and oxidation of the mantle wedge.

  3. Modeled Temperatures and Fluid Source Distributions for the Mexico Subduction Zone: Effects of Hydrothermal Cooling and Implications for Plate Boundary Seismic Processes

    NASA Astrophysics Data System (ADS)

    Perry, M. R.; Spinelli, G. A.; Wada, I.

    2014-12-01

    In subduction zones, spatial variations in pore fluid pressure are hypothesized to control the distribution and nature of slip behavior (e.g., "normal" earthquakes, slow slip events, non-volcanic tremor, very low frequency earthquakes) on the plate boundary fault. A primary control on the pore fluid pressure distribution in subduction zones is the distribution of fluid release from hydrous minerals in the subducting sediment and rock. The distributions of these diagenetic and metamorphic fluid sources are controlled by the pressure-temperature paths that the subducting material follows. Thus, constraining subduction zone thermal structure is required to inform conceptual models of seismic behavior. Here, we present results of thermal models for the Mexico subduction zone, a system that has received recent attention due to observations of slow-slip events and non-volcanic tremor. We model temperatures in five margin-perpendicular transects from 96 ˚W to 104 ˚W. In each transect, we examine the potential thermal effects of vigorous fluid circulation in a high permeability aquifer within the basaltic basement of the oceanic crust. In the transect at 100˚W, hydrothermal circulation cools the subducting material by up to 140 ˚C, shifting peak slab dehydration landward by ~100 km relative to previous estimates from models that do not include the effects of fluid circulation. The age of the subducting plate in the trench increases from ~3 Ma at 104 ˚W to ~18 Ma at 96 ˚W; hydrothermal circulation redistributes the most heat (and cools the system the most) where the subducting plate is youngest. For systems with <20 Ma subducting lithosphere, hydrothermal circulation in oceanic crust should be considered in estimating subduction zone temperatures and fluid source distributions.

  4. Subduction zone deformation and geometry: Influence on megathrust seismicity

    NASA Astrophysics Data System (ADS)

    Kopp, H.

    2011-12-01

    The fundamental concept of plate tectonics has been established decades ago, along with the observation that the largest earthquakes on the planet occur along the megathrust fault of subduction zones. In that timespan, however, our understanding of what governs the magnitude, source region and recurrence interval of megathrust events has not advanced sufficiently to provide robust answers to open problems. This must be attributed to the fact that large parts of the seismogenic zone and forearc are commonly submerged in deep water and difficult to access at the majority of margins. Marine geophysical techniques, which are able to image the complex structures in these settings with sufficient coherency and depth penetration, have only evolved in recent years. And while satellite altimetry provides images of the seafloor and its large-scale structures on a global scale, local tectonic features may only be identified in ship-based high- resolution bathymetric maps. It is thus crucial for the advancement of our scientific knowledge to expand the marine observational basis of convergent margins and to literally overcome the shoreline- an effort that is certainly not completed. The 2004 Sumatra earthquake sparked many of the questions addressed here: why, along a single convergent margin, do some segments produce large megathrust events whereas other portions of the very same margin only nucleate earthquakes of moderate magnitude? How and why are devastating tsunamis generated in both segments? These observations implicate the notion that individual subduction zones or segments thereof differ in their structure and geometry to induce such diverse behavior. Marine geophysical techniques help to unravel the structural diversity of convergent margins and between individual subduction zone segments. Seismic images and multibeam bathymetry offer a detailed view into the shallow and deeper portions of subduction zones. Field data from Indonesia's Sunda Margin have provided an

  5. Earthquake mechanisms and active tectonics of the Hellenic subduction zone

    NASA Astrophysics Data System (ADS)

    Shaw, Beth; Jackson, James

    2010-05-01

    We use improved focal mechanisms and centroid depth estimates of earthquakes, combined with GPS velocities, to examine the tectonics of the Hellenic subduction zone, and in particular the processes occurring at both ends of the Hellenic Arc. Nubia-Aegean convergence is accommodated by shallowly dipping thrust-faulting along the subduction-zone interface, as well as by steeper splay faults in the overriding material. From a comparison of observed and expected seismic moment release over the last 100 yr, combined with existing knowledge of the longer-term documented historical record, we confirm earlier suggestions that most (80 per cent) of this convergence is accommodated aseismically, that is, that the subduction zone is uncoupled. This conclusion is robust, even allowing for rare very large earthquakes on splay faults, such as that of AD 365, and also allowing for the contribution of small earthquakes. The downgoing Nubian plate deforms by arc-parallel contraction at all depths, from 200 km seaward of Crete to at least 100 km within the subducting slab. Extensional (T) axes of earthquakes are aligned downdip within the descending slab suggesting that, even if the aseismic prolongation of the slab has reached the 670 km mantle discontinuity, it does not transmit stresses to shallower depths. Shallow thrust-faulting earthquakes on the subduction interface show a divergence of slip vectors round the arc, and GPS measurements show that this is accommodated mainly by E-W extension on normal faults in the overriding Aegean material. The eastern end of the subduction zone, south of Rhodes, displays distributed deformation in the overriding material, including a mixture of strike-slip and splay-thrust faulting, and probably involves rotations about a vertical axes. Here slip on the interface itself is by thrust faulting with slip vectors oblique to the arc but parallel to the overall Nubia-Aegean convergence: there is no evidence for slip-partitioning in the traditional

  6. Long-Term Hydrogeochemical Records from Ocean Drilling Program Borhehole Observatories in the Costa Rica Subduction Zone

    NASA Astrophysics Data System (ADS)

    Kastner, M.; Solomon, E. A.; Wheat, C. G.; Jannasch, H. W.

    2010-12-01

    The dynamic hydrogeology of subduction zones makes them important regions for geochemical cycling between the major reservoirs: seawater, oceanic crust, continental crust, and the mantle. The distillation and loss of some volatiles and fluid-soluble elements from the shallow slab not only affect reactions and processes within the seismogenic zone, but they also support the deep biosphere and play a central role in the longer-term global cycle of volatiles, such as the return of water and carbon dioxide to the ocean and atmosphere, to the depths of magmatism beneath volcanic arcs, and ultimately the mantle. Examples of key questions that have been addressed at these tectonic regimes through ocean drilling, are: what is the role of fluids in earthquake cycles; what are the global chemical and isotopic fluxes at subduction zones; how does fluid flow in the upper oceanic basement influence these global cycles? Motivated by these questions, sealed borehole hydrologic observatories (CORKs) were developed in 1989, with long-term instrumentation, to record background in-situ values of physical, chemical, and biological properties and transients. Two were deployed at the Costa Rica subduction zone; one in the upper oceanic basement ~0.3 km from the trench, the second along the décollement fault zone ~0.4 km arcward of the prism toe. Both were instrumented to continuously and simultaneously measure formation temperature, pressure, fluid chemistry and flow rate. The results of the first 7 years of deployment (2002-2009) in the oceanic basement, and 2 years in the décollement, constitute the first co-recorded hydrological, chemical, and physical databases and provided the first in-situ fluids from basement. These data have placed constraints on key questions, such as (1) How does fluid flow and chemistry vary spatially and temporally, and how do they change in response to tectonic events; (2) Can in-situ pressure, temperature, fluid flow, and chemistry be used to understand

  7. Storage of fluids and melts at subduction zones detectable by seismic tomography

    NASA Astrophysics Data System (ADS)

    Luehr, B. G.; Koulakov, I.; Rabbel, W.; Brotopuspito, K. S.; Surono, S.

    2015-12-01

    During the last decades investigations at active continental margins discovered the link between the subduction of fluid saturated oceanic plates and the process of ascent of these fluids and partial melts forming a magmatic system that leads to volcanism at the earth surface. For this purpose the geophysical structure of the mantle and crustal range above the down going slap has been imaged. Information is required about the slap, the ascent paths, as well as the reservoires of fluids and partial melts in the mantle and the crust up to the volcanoes at the surface. Statistically the distance between the volcanoes of volcanic arcs down to their Wadati Benioff zone results of approximately 100 kilometers in mean value. Surprisingly, this depth range shows pronounced seismicity at most of all subduction zones. Additionally, mineralogical laboratory investigations have shown that dehydration of the diving plate has a maximum at temperature and pressure conditions we find at around 100 km depth. The ascent of the fluids and the appearance of partial melts as well as the distribution of these materials in the crust can be resolved by seismic tomographic methods using records of local natural seismicity. With these methods these areas are corresponding to lowered seismic velocities, high Vp/Vs ratios, as well as increased attenuation of seismic shear waves. The anomalies and their time dependence are controlled by the fluids. The seismic velocity anomalies detected so far are within a range of a few per cent to more than 30% reduction. But, to explore plate boundaries large and complex amphibious experiments are required, in which active and passive seismic investigations should be combined to achieve best results. The seismic station distribution should cover an area from before the trench up to far behind the volcanic chain, to provide under favorable conditions information down to 150 km depth. Findings of different subduction zones will be compared and discussed.

  8. Seismic evidence for deep fluid circulation in the overriding plate of subduction zones

    NASA Astrophysics Data System (ADS)

    Tauzin, B.; Reynard, B.; Bodin, T.; Perrillat, J. P.; Debayle, E.

    2015-12-01

    In subduction zones, non-volcanic tremors are associated with fluid circulations (Obara, 2002). Their sources are often located on the interplate boundary (Rogers and Dragert, 2003; Shelly et al, 2006; La Rocca, 2009), consistent with fluids released by the dehydration of subducted plates (Hacker et al., 2003). Reports of tremors in the overriding continental crust of several subduction zones in the world (Kao et al., 2005; Payero et al., 2008; Ide, 2012) suggest fluid circulation at shallower depths but potential fluid paths are poorly documented. Here we obtained seismic observations from receiver functions that evidence the close association between the shallow tremor zone, electrical conductivity, and tectonic features of the Cascadia overriding plate. A seismic discontinuity near 15 km depth in the crust of the overriding North American plate is attributed to the Conrad discontinuity. This interface is segmented, and its interruption is spatially correlated with conductive regions and shallow swarms of seismicity and non-volcanic tremors. These observations suggest that shallow fluid circulation, tremors and seismicity are controlled by fault zones limiting blocks of accreted terranes in the overriding plate (Brudzinski and Allen, 2007). These zones constitute fluid "escape" routes that may contribute unloading fluid pressure on the megathrust. Obara, K. (2002). Science, 296, 1679-1681. Rogers, G., & Dragert, H. (2003). Science, 300, 1942-1943. Shelly, D. R., et al. (2006). Nature, 442, 188-191. La Rocca, M., et al. (2009). Science, 323, 620-623. Kao, H., et al. (2005). Nature, 436, 841-844. Payero, J. S., et al. (2008). Geophysical Research Letters, 35. Ide, S. (2012). Journal of Geophysical Research: Solid Earth, 117. Brudzinski, M. R., & Allen, R. M. (2007). Geology, 35, 907-910.

  9. Mantle wedge peridotites: Fossil reservoirs of deep subduction zone processes: Inferences from high and ultrahigh-pressure rocks from Bardane (Western Norway) and Ulten (Italian Alps)

    NASA Astrophysics Data System (ADS)

    Scambelluri, Marco; Van Roermund, Herman L. M.; Pettke, Thomas

    2010-11-01

    The garnet websterites from Bardane (Western Gneiss Region, Norway) derive from cold Archean subcontinental lithosphere involved in Scandian continental subduction to ultrahigh-pressures. Subduction zone metamorphism was promoted by slab fluid infiltration into the cold overlying mantle wedge. The earliest subduction transformation (M3-1) consists of garnet/clinopyroxene exsolution from old pre-subduction orthopyroxene. This stage was likely coeval with fluid input and formation of phlogopite and dolomite rods in the exsolution structures. Magnesite formation after dolomite and entrapment of fluid-related diamond-bearing polyphase inclusions in corona structures around the exsolved orthopyroxenes point to pressure increase to 4.5 GPa (M3-2). Peak pressures of 6.5-7 GPa (c.a. 200 km depth) are witnessed by crystallization of majoritic garnet (M3-3), mostly in veins cutting all the above microstructures. When such veins infiltrate the corona domains, formation of majoritic garnet in coronas is enhanced. This multistage evolution thus envisages episodic fluid influx, favouring rock recrystallization and formation of microdiamond-bearing inclusions and of majoritic garnet veins. These mantle rocks thus record fluid circulation along grain boundaries and microfractures down to 200 km depth in subduction environments. The Ulten Zone peridotites are slices of Variscan mantle wedge. Infiltration of metasomatic subduction fluids favoured transition from spinel-facies to garnet + amphibole ± dolomite parageneses at pressures below 3 GPa. Formation of metasomatized garnet-bearing peridotite mylonites suggest channelled influx of subduction fluids. The incompatible element-enriched signature of all subduction minerals in Bardane indicate that previously depleted websterites have been refertilized by COH subduction fluids. Comparison with the Ulten Zone garnet + amphibole ± dolomite peridotites outlines striking similarities in the metasomatic style and in the COH fluid phase

  10. High-pressure amphibolite facies dynamic metamorphism and the Mesozoic tectonic evolution of an ancient continental margin, east- central Alaska

    USGS Publications Warehouse

    Dusel-Bacon, C.; Hansen, V.L.; Scala, J.A.

    1995-01-01

    Ductilely deformed amphibolite facies tectonites comprise two adjacent terranes in east-central Alaska: the northern, structurally higher Taylor Mountain terrane and the southern, structurally lower Lake George subterrane of the Yukon-Tanana terrane. The pressure, temperature, kinematic and age data are interpreted to indicate that the metamorphism of the Taylor Mountain terrane and Lake George subterrane took place during different phases of a latest Palaeozoic through early Mesozoic shortening episode resulting from closure of an ocean basin now represented by klippen of the Seventymile-Slide Mountain terrane. High- to intermediate-pressure metamorphism of the Taylor Mountain terrane took place within a SW-dipping (present-day coordinates) subduction system. High- to intermediate-pressure metamorphism of the Lake George subterrane and the structural contact zone occurred during NW-directed overthrusting of the Taylor Mountain, Seventymile-Slide Mountain and Nisutlin terranes, and imbrication of the continental margin in Jurassic time. -from Authors

  11. Boron Isotope Evidence for Shallow Fluid Transfer Across Subduction Zones by Serpentinized Mantle

    NASA Astrophysics Data System (ADS)

    Scambelluri, M.; Tonarini, S.; Agostini, S.; Cannaò, E.

    2012-12-01

    Boron Isotope Evidence for Shallow Fluid Transfer Across Subduction Zones by Serpentinized Mantle M. Scambelluri (1), S. Tonarini (2), S. Agostini (2), E. Cannaò (1) (1) Dipartimento di Scienze della Terra, Ambiente e vita, University of Genova, Italy (2) Istituto di Geoscienze e Georisorse-CNR, Pisa, Italy In subduction zones, fluid-mediated chemical exchange between slabs and mantle dictates volatile and incompatible element cycles and influences arc magmatism. Outstanding issues concern the sources of water for arc magmas and its slab-to-mantle wedge transport. Does it occur by slab dehydration beneath arc fronts, or by hydration of fore-arc mantle and subsequent subduction of the hydrated mantle? So far, the deep slab dehydration hypothesis had strong support, but the hydrated mantle wedge idea is advancing supported by studies of fluid-mobile elements in serpentinized wedge peridotites and their subducted high-pressure (HP) equivalents. Serpentinites are volatile and fluid-mobile element reservoirs for subduction: their dehydration causes large fluid and element flux to the mantle.However, direct evidence for their key role in arc magmatism and identification of dehydration environments has been elusive and boron isotopes can trace the process. Until recently, the altered oceanic crust (AOC) was considered the 11B reservoir for arcs, which largely display positive δ11B. However, shallow slab dehydration transfers 11B to the fore-arc mantle and leaves the residual AOC very depleted in 11B below arcs. Here we present high positive δ11B of HP serpentinized peridotites from Erro Tobbio (Ligurian Alps), recording subduction metamorphism from hydration at low-grade to eclogite-facies dehydration. We show a connection among serpentinite dehydration, release of 11B-rich fluids and arc magmatism. The dataset is completed by B isotope data on other HP Alpine serpentinites from Liguria and Lanzo Massif. In general, the δ11B of these rocks is heavy (16 to + 30 permil

  12. Mechanism of décollement formation in subduction zones

    NASA Astrophysics Data System (ADS)

    Hori, Takane; Sakaguchi, Hide

    2011-12-01

    The mechanism of décollement formation was investigated through a particle-based simulation model assuming homogeneity (e.g. no weak layer or pore fluid). A décollement-like structure appeared as a spontaneously localized shear deformation near the bottom of the sediment when the thickness of the sediment was sufficient to balance the gravitational force and tectonic loading. In contrast, no such décollement-like structure was formed when the sediment was too thin; in this case, the entire prism was deformed because of plate motion. These results are consistent with various observations in real subduction zones. A precise analysis of the stress state evolution during accretion reveals that the formation of a décollement-like structure is controlled by the spatio-temporal distribution of isotropic compression states.

  13. Resolution experiments for NW Pacific subduction zone tomography

    NASA Technical Reports Server (NTRS)

    Spakman, Wim; Van Der Hilst, Rob; Wortel, Rinus; Stein, Seth

    1989-01-01

    Results are reported from an investigation of the resolving power of ISC/NEIC P travel-time data in tomographic inversions for the geometry of the subduction zones in the NW Pacific. From thermal models for the Kurile, Janan, Izu-Bonin, Mariana, and Ryukyu slabs, three-dimensional synthetic velocity anomalies for subducting slabs are generated and projected onto a cell model for the uppermost 1400 km of the mantle. These synthetic models are used to compute synthetic delay times for ray paths corresponding to the source and receiver locations used for the actual data, add Gaussian noise, invert the synthetic data, and compare the resulting velocity structure to the initial synthetic models. This comparison is illustrated for sections through the Kuriles and the Mariana arcs. A variety of resolution artifacts are observed, which in many cases resemble features visible in the tomographic results obtained from inverting the actual ISC/NEIC data.

  14. Late holocene tectonics and paleoseismicity, southern cascadia subduction zone.

    PubMed

    Clarke, S H; Carver, G A

    1992-01-10

    Holocene deformation indicative of large subduction-zone earthquakes has occurred on two large thrust fault systems in the Humboldt Bay region of northern California. Displaced stratigraphic markers record three offsets of 5 to 7 meters each on the Little Salmon fault during the past 1700 years. Smaller and less frequent Holocene displacements have occurred in the Mad River fault zone. Elsewhere, as many as five episodes of sudden subsidence of marsh peats and fossil forests and uplift of marine terraces are recorded. Carbon-14 dates suggest that the faulting, subsidence, and uplift events were synchronous. Relations between magnitude and various fault-offset parameters indicate that earthquakes accompanying displacements on the Little Salmon fault had magnitudes of at least 7.6 to 7.8. More likely this faulting accompanied rupture of the boundary between the Gorda and North American plates, and magnitudes were about 8.4 or greater. PMID:17756070

  15. Late Holocene Tectonics and Paleoseismicity, Southern Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Clarke, Samuel H., Jr.; Carver, Gary A.

    1992-01-01

    Holocene deformation indicative of large subduction-zone earthquakes has occurred on two large thrust fault systems in the Humboldt Bay region of northern California. Displaced stratigraphic markers record three offsets of 5 to 7 meters each on the Little Salmon fault during the past 1700 years. Smaller and less frequent Holocene displacements have occurred in the Mad River fault zone. Elsewhere, as many as five episodes of sudden subsidence of marsh peats and fossil forests and uplift of marine terraces are recorded. Carbon-14 dates suggest that the faulting, subsidence, and uplift events were synchronous. Relations between magnitude and various fault-offset parameters indicate that earthquakes accompanying displacements on the Little Salmon fault had magnitudes of at least 7.6 to 7.8. More likely this faulting accompanied rupture of the boundary between the Gorda and North American plates, and magnitudes were about 8.4 or greater.

  16. Quantifying potential tsunami hazard in the Puysegur subduction zone, south of New Zealand

    USGS Publications Warehouse

    Hayes, G.P.; Furlong, K.P.

    2010-01-01

    Studies of subduction zone seismogenesis and tsunami potential, particularly of large subduction zones, have recently seen a resurgence after the great 2004 earthquake and tsunami offshore of Sumatra, yet these global studies have generally neglected the tsunami potential of small subduction zones such as the Puysegur subduction zone, south of New Zealand. Here, we study one such relatively small subduction zone by analysing the historical seismicity over the entire plate boundary region south of New Zealand, using these data to determine the seismic moment deficit of the subduction zone over the past ~100 yr. Our calculations indicate unreleased moment equivalent to a magnitude Mw 8.3 earthquake, suggesting this subduction zone has the potential to host a great, tsunamigenic event. We model this tsunami hazard and find that a tsunami caused by a great earthquake on the Puysegur subduction zone would pose threats to the coasts of southern and western South Island, New Zealand, Tasmania and southeastern Australia, nearly 2000 km distant. No claim to original US government works Geophysical Journal International ?? 2010 RAS.

  17. Dynamic modelling of the subduction zone of central Mexico

    NASA Astrophysics Data System (ADS)

    Gardi, A.; Cocco, M.; Negredo, A. M.; Sabadini, R.; Singh, S. K.

    2000-12-01

    In central Mexico some significant normal faulting events have occurred within the subducted oceanic Cocos plate, just below or near the down-dip edge of the strongly coupled interface. These normal faulting shocks followed large shallow thrust earthquakes. In other subduction zones such events generally precede the up-dip thrust events. A vertical 2-D finite element modelling has been used to simulate the subduction of the Cocos plate beneath the North American plate when the slab is driven by an active convergence velocity or slab pull. We find that the latter mechanism plays only a minor role due to shallow subduction. The modelling results show that the stress pattern is very sensitive to the geometry of the plates. In particular, normal faulting earthquakes that follow large thrust events can be explained on the basis of the flexural response of the overriding and subducting plates to the peculiar geometry of this subduction zone, where the subducting slab becomes horizontal at about 100km from the trench. This horizontal part of the subducting plate, down-dip with respect to the main thrust zone, is under an extensional stress field. This provides an alternative explanation to the slab pull for the occurrence of normal faulting intraplate earthquakes. In order for normal faulting earthquakes to occur in the early part of the seismic cycle, it is necessary that the large up-dip thrust events have a partial stress drop. We find that for small fractional stress drop, a wide region of extension remains below the down-dip edge of the main fault plane following a large thrust earthquake. Thus, the main thrust earthquakes do not invert the polarity of the active stress field, which is compressional and extensional up-dip and down-dip, respectively, with respect to the main thrust fault. Larger fractional stress drops result in larger delays in the occurrence of normal faulting events after the main thrust events.

  18. High interseismic coupling in the Eastern Makran (Pakistan) subduction zone

    NASA Astrophysics Data System (ADS)

    Lin, Y. N.; Jolivet, R.; Simons, M.; Agram, P. S.; Martens, H. R.; Li, Z.; Lodi, S. H.

    2015-06-01

    Estimating the extent of interseismic coupling along subduction zone megathrusts is essential for quantitative assessments of seismic and tsunami hazards. Up to now, quantifying the seismogenic potential of the eastern Makran subduction zone at the northern edge of the Indian ocean has remained elusive due to a paucity of geodetic observations. Furthermore, non-tectonic processes obscure the signature of accumulating elastic strain. Historical earthquakes of magnitudes greater than 7 have been reported. In particular, the 1945 Mw 8.1 earthquake resulted in a significant tsunami that swept the shores of the Arabian Sea and the Indian Ocean. A quantitative estimate of elastic strain accumulation along the subduction plate boundary in eastern Makran is needed to confront previous indirect and contradictory conclusions about the seismic potential in the region. Here, we infer the distribution of interseismic coupling on the eastern Makran megathrust from time series of satellite Interferometric Synthetic Aperture Radar (InSAR) images acquired between 2003 and 2010, applying a consistent series of corrections to extract the low amplitude, long wavelength deformation signal associated with elastic strain on the megathrust. We find high interseismic coupling (i.e. the megathrust does not slip and elastic strain accumulates) in the central section of eastern Makran, where the 1945 earthquake occurred, while lower coupling coincides spatially with the subduction of the Sonne Fault Zone. The inferred accumulation of elastic strain since the 1945 earthquake is consistent with the future occurrence of magnitude 7+ earthquakes and we cannot exclude the possibility of a multi-segment rupture (Mw 8+). However, the likelihood for such scenarios might be modulated by partitioning of plate convergence between slip on the megathrust and internal deformation of the overlying, actively deforming, accretionary wedge.

  19. Second critical endpoints and their bearing on subduction zone magmatism

    NASA Astrophysics Data System (ADS)

    Mibe, K.

    2011-12-01

    Understanding the phase relations in silicate-H2O systems is fundamental for clarifying the physical and chemical evolution of the Earth, because H2O affects melting temperature of rocks, composition of magmas generated, and rheology of rocks. Under high pressure and high temperature conditions, it is known that the solubility of both water in silicate melt and silicate in aqueous fluid increases with increasing pressure. As a result, silicate melt and aqueous fluid in the Earth's interior is expected to become supercritical fluid and the hydrous solidus of the system can no longer be defined beyond a certain critical condition. This condition is called the second critical endpoint and is the point of intersection between the critical curve and hydrous solidus. In recent years, the second critical endpoints in the systems peridotite-H2O and basalt-H2O have been determined using high-pressure and high-temperature X-ray radiography technique [Mibe et al., 2007, JGR; 2011, PNAS]. In these studies, it was concluded that the second critical endpoints in the systems peridotite-H2O and basalt-H2O occurred at around 3.8 and 3.4 GPa, respectively. These results suggest that the aqueous fluid and silicate melt becomes indistinguishable at the depths deeper than ~120 km in the mantle wedge peridotite and ~100 km in the subducting basaltic oceanic crust in subduction zones. The melting temperature of the subducting oceanic crust can no longer be defined beyond this critical condition. The fluid released from subducting oceanic crust at depths deeper than 100 km under volcanic arcs are supercritical fluid rather than aqueous fluid and/or hydrous melts. It is suggested that the position of the second critical endpoint explains why there is a limitation of slab depth (~90 km) where Adakitic magmas are produced and also explains the origin of across-arc geochemical variations of trace elements in volcanic rocks in subduction zones.

  20. Coastline uplift in Oregon and Washington and the nature of Cascadia subduction-zone tectonics

    SciTech Connect

    West, D.O.; McCrumb, D.R.

    1988-02-01

    Coastline deformation resulting from great shallow thrust earthquakes can provide information concerning the paleoseismicity of a subduction zone and thus information on the nature of potential seismicity. The Cascadia subduction zone is different from most other subduction zones in that it has been quiescent with respect to great earthquakes for at least the past 200 yr. The Washington-Oregon coastline also differs from most other coastlines associated with subduction zones in its lack of uplifted Holocene shoreline features and low overall rate of late Quaternary uplift (0.2-0.6 mm/yr). The uplift differences suggest that repeated great earthquakes have not occurred along the Cascadia subduction zone at least during the late Holocene. Alternatively, if the plate interface has generated earthquakes, the differences may be explained by longer recurrence intervals for great earthquakes, smaller magnitude earthquakes, or a mechanism that does not result in uplift of the coastline where expected.

  1. Three-dimensional electrical resistivity image of the South-Central Chilean subduction zone

    NASA Astrophysics Data System (ADS)

    Kapinos, Gerhard; Montahaei, Mansoureh; Meqbel, Naser; Brasse, Heinrich

    2016-01-01

    Based on isotropic 3-D inversion, we re-interpret long-period magnetotelluric data collected across the geotectonic structures of the South-Central Chilean continental margin at latitudes 38°-41°S and summarize results of long-period magnetotelluric (MT) investigations performed between 2000 and 2005. The new 3-D conductivity image of the South-Central Chilean subduction zone basically confirms former 2-D inversion models along three profiles and complete the previous results. The models show good electrical conductors in the tip of the continental crustal beneath the Pacific Ocean, the frequently observed forearc conductor at mid-crustal levels, a highly-conductive zone at similar levels slightly offset from the volcanic arc and a - not well-resolved - conductor in the Argentinian backarc. The subducted Nazca Plate generally appears as a resistive but discontinuous feature. Unlike before, we are now able to resolve upper crustal conductors (interpreted as magma reservoirs) beneath active Lonquimay, Villarrica, and Llaima volcanoes which were obscured in 2-D inversion. Data fit is rather satisfactory but not perfect; we attribute this to large-scale crustal anisotropy particularly beneath the Coastal Cordillera, which we cannot include into our solution for the time being.

  2. The Aeolian Volcanic Arc: New Insights From Subduction Zone Thermal Models and Mineral Solubility Scaling Relationships

    NASA Astrophysics Data System (ADS)

    Creamer, J.; van Keken, P.; Engdahl, E. R.; Spera, F. J.; Bohrson, W. A.

    2007-12-01

    The Calabrian subduction zone, situated southeast of the Italian 'boot' in the Ionian Sea, is the latest manifestation of African-Eurasian plate interaction. This plate interaction has been remarkably dynamic since the Mesozoic, hosting episodes of mountain belt and volcanic arc formation including, for example, the Alpine, Carpathian and Apennine orogenic belts and Hellanic and, most recently, Aeolian volcanic arcs. Subduction of cold oceanic lithosphere beneath Europe initiated around 80 Ma, and the last 30 Ma have been characterized by alternating episodes of rapid back-arc rifting and back-arc spreading (up to 6-8 cm/yr) mediated by dip-parallel and/or trench-parallel tears in the descending slab resulting from differential trench rollback (Wortel and Spakman 2000). Backarc extension effectively moved the plate boundary from the European continental margin in the north to the African continental margin in the south, creating the modern Western Mediterranean basins. The Tyrrhenian oceanic basin was opened during the latest episode of trench rollback, from 5-2 Ma, followed by initiation of the subduction-related Aeolian volcanism by 1.3 Ma (Beccaluva et al. 1982) and complete cessation of extension of the overriding plate around 0.8-0.5 Ma (Goes et al. 2004). The seven subaerial volcanoes of the Aeolian volcanic arc sit atop thin (16-30 km) continental crust, and collectively tap a heterogeneous mantle source. Slab geometry in the depth range of 150 to 500 km has been refined using the hypocenter relocation procedure of Engdahl et al 1998 for teleseismic events beneath the Tyrrhenian Sea, in conjunction with recent tomographic results. The thermal state of the Calabrian subduction zone at depths relevant to dehydration and magma genesis has been investigated using a 2-dimensional time-dependent thermal model of the descending slab and convecting mantle wedge based on seismic, geologic and geodetic observational data. Modeling methodology follows van Keken et al

  3. Electrical structure of the central Cascadia subduction zone: The EMSLAB Lincoln Line revisited

    NASA Astrophysics Data System (ADS)

    Evans, Rob L.; Wannamaker, Philip E.; McGary, R. Shane; Elsenbeck, Jimmy

    2014-09-01

    The EMSLAB experiment was an ambitious onshore-offshore magnetotelluric (MT) transect of the Cascadia subduction zone. When completed (1985-1988), it was the largest experiment of its kind. Modeling and inversion capabilities at the time were, however, not sufficiently sophisticated to handle a fully regularized inversion of the data, including the seafloor data and bathymetric constraints, with the main final model presented based on trial and error forward modeling of the responses. Moreover, new data collected as part of the Earthscope USArray program are of higher quality due to improvements in instrument technology, and augment the original EMSLAB data set, presenting an opportunity to revisit the structure in this part of the subduction system. We have integrated the original wide-band MT data as well as several long-period stations from the original EMSLAB data set and invert these in conjunction with EMSLAB seafloor responses and new Earthscope data on land. This new composite data set has been analyzed in several ways, within a two-dimensional geometry in which conductivity is assumed to be invariant along a strike direction roughly coincident with that of the subduction zone. We have solved for fully smooth regularized models, as well as solutions that allow discontinuities in conductivity along the top surface of the descending slab. Finally, we have tested specific features in the EMSLAB model, notably a moderately shallow (~30 km depth) forearc conductor. A feature similar to this shallow conductor is a consistent and required feature in our new inversion models, but the new models highlight the connection between the slab and what is interpreted to be an accumulation of aqueous fluids in the deep crust. The depth (~40 km) at which the conductor intersects the slab suggests that the fluids are released by the transition of hydrous basalt to eclogite at upper greenschist facies and higher metamorphic grade. The nose of the mantle wedge has a

  4. Garnet growth as a proxy for progressive dehydration in subduction zones

    NASA Astrophysics Data System (ADS)

    Caddick, M. J.; Baxter, E. F.

    2012-12-01

    The release of volatiles from subducting lithologies is a crucial triggering process for arc magmatism, seismicity, the growth and maturation of continents, and the global geological H2O-CO2 cycle. While numerous models have been developed to predict slab volatile release, it has proven challenging to reconstruct and test these fluid fluxes released from specific lithologies in the rock record. Here we show that the growth of garnet may be used as a proxy for progressive devolatilization at blueschist to eclogite facies conditions in subduction zones. Generally, as garnet grows in a subducting rock, fluid is produced due to metamorphic dehydration reactions. Using rigorous thermodynamic analysis, which includes the crucial effects of phase fractionation, we model the proportional relationship between garnet and water production in common lithologies (pelitic sediment and hydrated MORB) along three representative subduction geotherms. The results show that several dehydration reactions contribute to garnet growth especially within a crucial span of the subduction zone (~1.5 to 2.5 GPa) within which slab-mantle decoupling has been predicted to occur in some models and volatile fluxes may be focused. The water:garnet production ratio varies during garnet growth, constrained by the specific hydrous reactant phases that are breaking down, but the average water:garnet production ratios are surprisingly consistent regardless of composition and geotherm. Over the garnet growth interval ~400 to 700 C (and corresponding depths for each geotherm) the average production ratio for altered MORB compositions is 0.52 (wt % water per vol % garnet) in cooler geotherms (Honshu and Nicaragua) and 0.27 in hotter (Cascadia) geotherms, with predictably lower ratios if the input basalt previously experienced less hydrous alteration. Over the same interval the water production ratios are approximately 50 % lower for pelite (0.24 and 0.13, respectively). Lower temperature water release is

  5. Frictional behavior of carbonate-rich sediments in subduction zones

    NASA Astrophysics Data System (ADS)

    Rabinowitz, H. S.; Savage, H. M.; Carpenter, B. M.; Collettini, C.

    2015-12-01

    Carbonate-rich layers make up a significant component of subducting sediments around the world and may impact the frictional behavior of subduction zones. In order to investigate the effect of carbonate subduction, we conducted biaxial deformation experiments within a pressure vessel using the Brittle Rock deformAtion Versatile Apparatus (BRAVA) at INGV. We obtained input sediments for two subduction zones, the Hikurangi trench, New Zealand (ODP Site 1124) and the Peru trench (DSDP Site 321), which have carbonate/clay contents of ~40/60 wt% and ~80/20 wt%, respectively. Samples were saturated with distilled water mixed with 35g/l sea salt and deformed at room temperature. Experiments were conducted at σN = 1-50 MPa with sliding velocities of 1-300 μm/s and hold times of 1-1000 s. Frictional strength of Hikurangi gouge is 0.35-0.55 and Peru gouge is 0.55-0.65. Velocity-stepping tests show that the Hikurangi gouge is consistently velocity strengthening (friction rate parameter (a-b) > 0). The Peru gouge is mostly velocity strengthening but exhibits a minimum in a-b at the 3-10 μm/s velocity step (with velocity weakening behavior at 25 MPa, indicating the potential for earthquake nucleation). Slide-hold-slide tests show that the healing rate (β) of the Hikurangi gouge is 1x10-4-1x10-3 /decade which is comparable to that of clays (β~0.002 /decade) while the healing rate of Peru gouge (β~6x10-3-7x10-3 /decade) is closer to that of carbonate gouge (β~0.01 /decade). The mechanical results are complemented by microstructural analysis. In lower stress experiments, there is no obvious shear localization. At 25 and 50 MPa, pervasive boundary-parallel shears become dominant, particularly in the Peru samples. Degree of microstructural localization appears to correspond with the trends observed in velocity-dependence. Our preliminary results indicate that carbonate/clay compositions could have a significant impact on the frictional behavior of subducting sediments.

  6. Stability and dynamics of serpentinite layer in subduction zone

    NASA Astrophysics Data System (ADS)

    Hilairet, Nadege; Reynard, Bruno

    2009-02-01

    The hydrous phyllosilicate serpentines have a strong influence on subduction zone dynamics because of their high water content and low strength at shallow and intermediate depths. In the absence of data, Newtonian rheology of serpentinites has been assumed in numerical models yet experimental data show that serpentine rheology is best described by a power law rheology recently determined in subduction zone conditions [Hilairet, N., et al., 2007. High-pressure creep of serpentine, interseismic deformation, and initiation of subduction. Science, 318(5858): 1910-1913]. Using a simple 1D model of a serpentinized channel and - as opposed to previous models - in this power law rheology, we examine the influence of channel thickness, temperature and subduction angle on serpentine flow driven by density contrast (serpentinization degree) with the surroundings. At temperatures of 200-500 °C relevant to intermediate depths a fully serpentinized channel is unlikely to be thicker than 2-3 km. For channel thicknesses of 2 km upward velocities are comparable to those using a constant viscosity of 10 18 Pa s. The velocity profile using power law rheology shows shear zones at the edges of the channel and a low strain rate region at its centre consistent with the frequent observation of weakly deformed HP-rocks. Upward velocities estimated for channels 1 to 3 km thick are comparable to the serpentinization rates for maximum estimates of fluid velocities within shear zones in the literature. Competition between the upward flow and serpentinization may lead to intermittent behavior with alternating growth periods and thinning by exhumation. At shallower levels the thickness allowed for a channel may be up to ~ 8-10 km if the rheology has a higher dependence on stress. We therefore propose that the exhumation of HP oceanic units in serpentinite channels is organized in two levels, the deepest and fastest motion being driven by density contrast with the surrounding mantle and the

  7. Melt Inclusions as Windows on Subduction Zone Processes - A Retrospective

    NASA Astrophysics Data System (ADS)

    Sisson, T. W.

    2002-12-01

    A.T. (Fred) Anderson, in a series of papers in the interval 1972-1984, presented evidence from melt inclusions for high dissolved water and Cl concentrations in many subduction zone basalts through andesites. His observations, subsequently shown to be correct, were not widely accepted because (1) phase equilibrium experiments on Paricutin and Mount Hood andesites indicated moderate water concentrations, and some workers reasoned that potentially parental basalts would have been drier still, (2) common basalts lack hydrous phenocrysts, and (3) water content estimates were indirect (water-by-difference) or involved difficult, unfamiliar measurements (single inclusion manometry) and thus were discounted. Subsequent development of techniques for the direct and precise measurement of water and CO2 in melt inclusions (SIMS, FTIR), new hydrous phase-equilibrium studies on arc basalts through rhyolites, and wider appreciation of the diversity of arc magmatic suites changed this situation. Melt inclusion evidence shows that subduction zone basalts can have pre-eruptive dissolved water concentrations as high as ~6 wt% (Sisson and Layne 1993 EPSL; Roggensack et al. 1997 Science), confirming predictions from phase-equilibrium experiments (Sisson and Grove 1993a,b CMP), and supporting the now standard model of water-fluxed melting to drive arc magmatism. An important discovery, presaged in the original Anderson data, is that there is a wide range of pre-eruptive water contents in arc basalts, with some as dry as MORB (Sisson and Bronto 1998 Nature). Nearly dry arc basalts can erupt at the volcanic front (Galunggung, Java) and sporadically along the arc axis over distances of hundreds of km (Cascades, USA), in some cases in proximity to demonstrably water-rich magmatic centers (Mt. Shasta, Crater Lake). To produce dry primitive basalts requires upwelling and pressure-release melting of peridotite in the mantle wedge at temperatures (~1300° C) well above those predicted by

  8. Anisotropy and Attenuation in a Retreating Subduction Zone: Southern Italy

    NASA Astrophysics Data System (ADS)

    Baccheschi, P.; Margheriti, L.; Steckler, M. S.; de Gori, P.; Boschi, E.

    2010-12-01

    We present a collection of high quality S-wave splitting measurements in the Southern Italy subduction system. We analyzed deep earthquakes that occurred within the descending slab to determine the splitting parameters φ and δt. The local deep earthquakes allowed us to analyze ray-paths primarily sampling the slab and the wedge above it. Mainland Calabria is a forearc high, enabling us to sample rays that propagate up the slab. S splitting parameters show a complex pattern of anisotropy with variable fast directions across the subduction zone and delay times ranging from 0.1 sec to 2.2 sec. Measurements at single stations are quite variable excluding the overriding plate as main source of anisotropy. The S wave splitting parameters also show frequency-dependent behaviour that we attribute to the presence of small-scale anisotropic heterogeneities. Comparison of the S splitting measurements to the P-wave velocity anomaly at 100-200 km depth shows that where the rays primarily sample the slab the delay times are small. In contrast, where the S rays sample the mantle wedge, the delay times are quite high. This δt pattern depicts the slab as a weakly anisotropic region and suggests that the main source of anisotropy in the subduction zone is the surrounding asthenosphere. We also determined the attenuation structure of the slab and of the surrounding regions by the inversion of high quality S-waves t* from slab earthquakes. We obtained high-resolution Qs model down to 300 km depth. The results indicate low values of Qs (values down to 200) corresponding to crustal layers (down to 25 km depth), while the slab is characterized by higher but heterogeneous Qs structure (Qs values up to 1100). At 100 km depth the high Qs body is well reconstructed beneath the Calabrian Arc and at 200 km depth it is extended offshore the Southern Tyrrhenian Basin beneath the Aeolian Islands. These preliminary attenuation results allowed us to better define the geometry and the boundary of

  9. Modeling Diverse Pathways to Age Progressive Volcanism in Subduction Zones.

    NASA Astrophysics Data System (ADS)

    Kincaid, C. R.; Szwaja, S.; Sylvia, R. T.; Druken, K. A.

    2015-12-01

    One of the best, and most challenging clues to unraveling mantle circulation patterns in subduction zones comes in the form of age progressive volcanic and geochemical trends. Hard fought geological data from many subduction zones, like Tonga-Lau, the Cascades and Costa-Rica/Nicaragua, reveal striking temporal patterns used in defining mantle flow directions and rates. We summarize results from laboratory subduction models showing a range in circulation and thermal-chemical transport processes. These interaction styles are capable of producing such trends, often reflecting apparent instead of actual mantle velocities. Lab experiments use a glucose working fluid to represent Earth's upper mantle and kinematically driven plates to produce a range in slab sinking and related wedge transport patterns. Kinematic forcing assumes most of the super-adiabatic temperature gradient available to drive major downwellings is in the tabular slabs. Moreover, sinking styles for fully dynamic subduction depend on many complicating factors that are only poorly understood and which can vary widely even for repeated parameter combinations. Kinematic models have the benefit of precise, repeatable control of slab motions and wedge flow responses. Results generated with these techniques show the evolution of near-surface thermal-chemical-rheological heterogeneities leads to age progressive surface expressions in a variety of ways. One set of experiments shows that rollback and back-arc extension combine to produce distinct modes of linear, age progressive melt delivery to the surface through a) erosion of the rheological boundary layer beneath the overriding plate, and deformation and redistribution of both b) mantle residuum produced from decompression melting and c) formerly active, buoyant plumes. Additional experiments consider buoyant diapirs rising in a wedge under the influence of rollback, back-arc spreading and slab-gaps. Strongly deflected diapirs, experiencing variable rise

  10. Modeling the effects of 3-D slab geometry and oblique subduction on subduction zone thermal structure

    NASA Astrophysics Data System (ADS)

    Wada, I.; Wang, K.; He, J.

    2013-12-01

    In this study, we revisit the effects of along-strike variation in slab geometry and oblique subduction on subduction zone thermal structures. Along-strike variations in slab dip cause changes in the descending rate of the slab and generate trench-parallel pressure gradients that drive trench-parallel mantle flow (e.g., Kneller and van Keken, 2007). Oblique subduction also drives trench-parallel mantle flow. In this study, we use a finite element code PGCtherm3D and examine a range of generic subduction geometries and parameters to investigate the effects of the above two factors. This exercise is part of foundational work towards developing detailed 3-D thermal models for NE Japan, Nankai, and Cascadia to better constrain their 3-D thermal structures and to understand the role of temperature in controlling metamorphic, seismogenic, and volcanic processes. The 3-D geometry of the subducting slabs in the forearc and arc regions are well delineated at these three subduction zones. Further, relatively large compilations of surface heat flow data at these subduction zones make them excellent candidates for this study. At NE Japan, a megathrust earthquake occurred on March 11, 2011; at Nankai and Cascadia, there has been a great effort to constrain the scale of the next subduction thrust earthquake for the purpose of disaster prevention. Temperature influences the slip behavior of subduction faults by (1) affecting the rheology of the interface material and (2) controlling dehydration reactions, which can lead to elevated pore fluid pressure. Beyond the depths of subduction thrust earthquakes, the thermal structure is affected strongly by the pattern of mantle wedge flow. This flow is driven by viscous coupling between the subducting slab and the overriding mantle, and it brings in hot flowing mantle into the wedge. The trench-ward (up-dip) extent of the slab-mantle coupling is thus a key factor that controls the thermal structure. Slab-mantle decoupling at shallow

  11. Tectonic evolution of early Paleozoic HP metamorphic rocks in the North Qilian Mountains, NW China: New perspectives

    NASA Astrophysics Data System (ADS)

    Song, Shuguang; Niu, Yaoling; Zhang, Lifei; Wei, Chunjing; Liou, June G.; Su, Li

    2009-07-01

    eclogite and carpholite meta-pelite. Geochemical data suggest that protoliths of mafic blueschist and eclogite in both belts have features of present-day MORB or OIB. These observations indicate that the North Qilian HP metamorphic rocks have experienced a history of seafloor subduction in a cold subduction-zone with a geotherm of ˜6-7 °C/km in the early Paleozoic before exhumed at the Late Silurian and Devonian in response to continental collision.

  12. Geoid anomalies in the vicinity of subduction zones

    NASA Technical Reports Server (NTRS)

    Mcadoo, D. C.

    1980-01-01

    The regional geoid of the southwest Pacific is matched reasonably well by results from a model of the upper mantle density structure (including slabs) associated with subduction zones of the region. Estimates of the geoid are obtained from Geos-3 and Seasat radar altimeter data. These data are very well suited to the task of detecting intermediate wavelength (600-4000 km) geopotential variations. Actually, subducting slabs can be expected to produce primarily intermediate and longer wavelength variations. Gravimetric profiles across trench/island arc complexes resolve primarily short wavelengths. The model represents subducting slabs as thin surfaces of anomalous mass per unit area. These surfaces are positioned using published seismicity results which detail the configuration of the Benioff zones. Crustal effects are ignored. Effects due to the contrast between the young thermal lithosphere of the behind-arc regions (marginal basins) and the older lithosphere seaward of the trench are modelled. Results indicate that the New Hebrides slab possesses an average areal density anomaly of about 300,000 gm/sq cm. This is about three times that which is estimated for the Tonga-Kermadec slab. Additional modelling suggests that slabs worldwide may be an important source of large, long wavelength gravity highs; i.e., they may contribute substantially to geopotential power of harmonic degree as low as three or four up to twenty or more.

  13. A new source of water in seismogenic subduction zones

    NASA Astrophysics Data System (ADS)

    Kameda, Jun; Yamaguchi, Asuka; Saito, Saneatsu; Sakuma, Hiroshi; Kawamura, Katsuyuki; Kimura, Gaku

    2011-11-01

    Seismogenic plate-boundary faults at accretionary margins (e.g., the Nankai margin, southwest Japan) may occur where the uppermost part of subducting oceanic crust, composed of basaltic rocks, is in contact with the overriding plate of a lithified accretionary prism. The plate-boundary faults in ancient accretionary complexes typically record high-velocity slip under fluid-rich conditions. Although previous studies have emphasized the mechanical significance of fluids in terms of dynamic slip-weakening, the source of fluid in seismogenic subduction zones remains poorly constrained. In this work, we focus on the hydrous smectite in the uppermost oceanic crust, an alteration product of intact basalt before arrival at the trench axis. A comparison between (1) new mineralogical data on basalt drillcore recovered by Integrated Ocean Drilling Program (IODP) Expedition 322 at site C0012, a reference site for subduction input to the Nankai Trough, and (2) mineralogical data on basalt within ancient oceanic crust embedded in a fossil accretionary complex of the Shimanto Belt, southwest Japan, suggests that progressive smectite-chlorite conversion would liberate bound fluids at a rate of 0.34 to 0.65 × 10-14 s-1 along the plate interface. This rate of fluid production appears to be more than an order of magnitude greater than that from other possible sources, including from overlying sediments via smectite-illite conversion and the expulsion of pore fluids, and may facilitate seismic slip along plate-boundary faults.

  14. Slab melting versus slab dehydration in subduction-zone magmatism

    PubMed Central

    Mibe, Kenji; Kawamoto, Tatsuhiko; Matsukage, Kyoko N.; Fei, Yingwei; Ono, Shigeaki

    2011-01-01

    The second critical endpoint in the basalt-H2O system was directly determined by a high-pressure and high-temperature X-ray radiography technique. We found that the second critical endpoint occurs at around 3.4 GPa and 770 °C (corresponding to a depth of approximately 100 km in a subducting slab), which is much shallower than the previously estimated conditions. Our results indicate that the melting temperature of the subducting oceanic crust can no longer be defined beyond this critical condition and that the fluid released from subducting oceanic crust at depths greater than 100 km under volcanic arcs is supercritical fluid rather than aqueous fluid and/or hydrous melts. The position of the second critical endpoint explains why there is a limitation to the slab depth at which adakitic magmas are produced, as well as the origin of across-arc geochemical variations of trace elements in volcanic rocks in subduction zones. PMID:21536910

  15. Teleseismic shear wave tomography of the Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Asamori, Koichi; Zhao, Dapeng

    2015-12-01

    We present a high-resolution shear wave tomography of the Japan subduction zone down to a depth of 700 km, which is determined by inverting a large number of high-quality S-wave arrival-time data from local earthquakes and teleseismic events. The subducting Pacific and Philippine Sea (PHS) slabs are revealed clearly as high-velocity (high-V) zones, whereas low-velocity (low-V) anomalies are revealed in the mantle wedge above the two slabs. The PHS slab has subducted aseismically down to a depth of 480 km under the Japan Sea and to a depth of 540 km under the Tsushima Strait. A window is revealed within the aseismic PHS slab, being consistent with P-wave tomography. Prominent low-V and high-Poisson's ratio (σ) anomalies exist below the PHS slab and above the Pacific slab, which reflect hot and wet mantle upwelling caused by the joint effect of deep dehydration of the Pacific slab and convective circulation process in the mantle wedge above the Pacific slab. The hot and wet mantle upwelling has caused the complex geometry and structure of the PHS slab in SW Japan, and contributed to the Quaternary volcanism along the Japan Sea coast. In eastern Japan, low-V zones are revealed at depths of 200-700 km below the Pacific slab, which may reflect hot upwelling from the lower mantle or even the core-mantle boundary.

  16. Radiocarbon test of earthquake magnitude at the Cascadia subduction zone

    USGS Publications Warehouse

    Atwater, B.F.; Stuiver, M.; Yamaguchi, D.K.

    1991-01-01

    THE Cascadia subduction zone, which extends along the northern Pacific coast of North America, might produce earthquakes of magnitude 8 or 9 ('great' earthquakes) even though it has not done so during the past 200 years of European observation 1-7. Much of the evidence for past Cascadia earthquakes comes from former meadows and forests that became tidal mudflats owing to abrupt tectonic subsidence in the past 5,000 years2,3,6,7. If due to a great earthquake, such subsidence should have extended along more than 100 km of the coast2. Here we investigate the extent of coastal subsidence that might have been caused by a single earthquake, through high-precision radiocarbon dating of coastal trees that abruptly subsided into the intertidal zone. The ages leave the great-earthquake hypothesis intact by limiting to a few decades the discordance, if any, in the most recent subsidence of two areas 55 km apart along the Washington coast. This subsidence probably occurred about 300 years ago.

  17. Slab melting versus slab dehydration in subduction-zone magmatism.

    PubMed

    Mibe, Kenji; Kawamoto, Tatsuhiko; Matsukage, Kyoko N; Fei, Yingwei; Ono, Shigeaki

    2011-05-17

    The second critical endpoint in the basalt-H(2)O system was directly determined by a high-pressure and high-temperature X-ray radiography technique. We found that the second critical endpoint occurs at around 3.4 GPa and 770 °C (corresponding to a depth of approximately 100 km in a subducting slab), which is much shallower than the previously estimated conditions. Our results indicate that the melting temperature of the subducting oceanic crust can no longer be defined beyond this critical condition and that the fluid released from subducting oceanic crust at depths greater than 100 km under volcanic arcs is supercritical fluid rather than aqueous fluid and/or hydrous melts. The position of the second critical endpoint explains why there is a limitation to the slab depth at which adakitic magmas are produced, as well as the origin of across-arc geochemical variations of trace elements in volcanic rocks in subduction zones. PMID:21536910

  18. Electrical image of subduction zone beneath northeastern Japan

    NASA Astrophysics Data System (ADS)

    Ichiki, Masahiro; Ogawa, Yasuo; Kaida, Toshiki; Koyama, Takao; Uyeshima, Makoto; Demachi, Tomotsugu; Hirahara, Satoshi; Honkura, Yoshimori; Kanda, Wataru; Kono, Toshio; Matsushima, Masaki; Nakayama, Takashi; Suzuki, Syuichi; Toh, Hiroaki

    2015-12-01

    We conducted long-period magnetotelluric observations in northeastern Japan from 2010 to 2013 to investigate the three-dimensional electrical resistivity distribution of the subduction zone. Incorporating prior information of the subducting slab into the inversion scheme, we obtained a three-dimensional resistivity model in which a vertically continuous conductive zone is imaged from the subducting slab surface to the lower crust beneath the Ou Backbone Range. The conductive body indicates a saline fluid and/or melt pathway from the subducting slab surface to the lower crust. The lower crust conductor is less than 10 Ω m, and we estimate a saline fluid and/or melt fraction of at least 0.7 vol. %. Other resistivity profiles in the across-arc direction reveal that the conductive body segregates from the subducting slab surface at 80-100 km depth and takes an overturned form toward the back arc. The head of the conducting body reaches the lower crust just beneath Mt. Gassan, one of the prominent back-arc volcanoes in the system.

  19. Possible emplacement of crustal rocks into the forearc mantle of the Cascadia Subduction Zone

    USGS Publications Warehouse

    Calvert, A.J.; Fisher, M.A.; Ramachandran, K.; Trehu, A.M.

    2003-01-01

    Seismic reflection profiles shot across the Cascadia forearc show that a 5-15 km thick band of reflections, previously interpreted as a lower crustal shear zone above the subducting Juan de Fuca plate, extends into the upper mantle of the North American plate, reaching depths of at least 50 km. In the extreme western corner of the mantle wedge, these reflectors occur in rocks with P wave velocities of 6750-7000 ms-1. Elsewhere, the forearc mantle, which is probably partially serpentinized, exhibits velocities of approximately 7500 ms-1. The rocks with velocities of 6750-7000 ms-1 are anomalous with respect to the surrounding mantle, and may represent either: (1) locally high mantle serpentinization, (2) oceanic crust trapped by backstepping of the subduction zone, or (3) rocks from the lower continental crust that have been transported into the uppermost mantle by subduction erosion. The association of subparallel seismic reflectors with these anomalously low velocities favours the tectonic emplacement of crustal rocks. Copyright 2003 by the American Geophysical Union.

  20. Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone.

    PubMed

    Calvert, Andrew J

    2004-03-11

    At convergent continental margins, the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single, thin interface known as a megathrust. Great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together. Earthquakes of lower magnitude occur within the underlying oceanic plate, and have been linked to geochemical dehydration reactions caused by the plate's descent. Here I present deep seismic reflection data from the northern Cascadia subduction zone that show that the inter-plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a >5-km-thick, approximately 110-km-wide region of imbricated crustal rocks. Earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter-plate boundary zone. This implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate. Slip on the shallower megathrust shear zone, which may occur by aseismic slow slip, will transport crustal rocks into the upper mantle above the subducting oceanic plate and may, in part, provide an explanation for the unusually low seismic wave speeds that are observed there. PMID:15014496

  1. New seismic images of the cascadia subduction zone from cruise SO 108-ORWELL

    USGS Publications Warehouse

    Flueh, E.R.; Fisher, M.A.; Bialas, J.; Childs, J. R.; Klaeschen, D.; Kukowski, Nina; Parsons, T.; Scholl, D. W.; ten Brink, U.; Trehu, A.M.; Vidal, N.

    1998-01-01

    In April and May 1996, a geophysical study of the Cascadia continental margin off Oregon and Washington was conducted aboard the German R/V Sonne. This cooperative experiment by GEOMAR and the USGS acquired wide-angle reflection and refraction seismic data, using ocean-bottom seismometers (OBS) and hydrophones (OBH), and multichannel seismic reflection (MCS) data. The main goal of this experiment was to investigate the internal structure and associated earthquake hazard of the Cascadia subduction zone and to image the downgoing plate. Coincident MCS and wide-angle profiles along two tracks are presented here. The plate boundary has been imaged precisely beneath the wide accretionary wedge close to shore at c13km depth. Thus, the downgoing plate dips more shallowly than previously assumed. The dip of the plate changes from 2?? to 4?? at the eastern boundary of the wedge on the northern profile, whereas approximately 3km of sediment is entering the subduction zone. On the southern profile, where the incoming sedimentary section is about 2.2km thick, the plate dips about 0.5?? to 1.5?? near the deformation front and increases to 3.5?? further landwards. On both profiles, the deformation of the accretionary wedge has produced six ridges on the seafloor, three of which represent active faulting, as indicated by growth folding. The ridges are bordered by landward verging faults which reach as deep as the top of the oceanic basement. Thus, the entire incoming sediment package is being accreted. At least two phases of accretion are evident, and the rocks of the older accretionary phase(s) forms the backstop for the younger phase, which started around 1.5 Ma ago. This documents that the 30 to 50km wide frontal part of the accretionary wedge, which is characterized by landward vergent thrusts, is a Pleistocene feature which was formed in response to the high input of sediment building the fans during glacial periods. Velocities increase quite rapidly within the wedge, both

  2. Rock strength and fault rheology at the Nankai subduction zone: Insight from laboratory experiments

    NASA Astrophysics Data System (ADS)

    Kitajima, H.; Saffer, D. M.; Takahashi, M.; Otsubo, M.

    2015-12-01

    Understanding the wide spectrum of fault slip behaviors observed in continental and subduction faults zones requires investigation of absolute fault strength and fault rheology that are controlled by in-situ stress, strain, pressure, temperature, and chemical conditions. Because the deformation mode and strength of porous rocks are dependent on porosity and effective stress, rapid reduction of sediment porosity from 60-70% to less than 5% should strongly affect deformation style in the shallow portion of subduction zones. Here, we present the results of laboratory experimental studies on modern clay-rich sediments subducting at the Nankai Trough and rocks from the exhumed Shimanto accretionary complex near the Nobeoka thrust, which is an ancient out-of-sequence-thrust, to constrain porosity, rock strength and fault rheology in the Nankai subduction zone. Based on uniaxial and triaxial experiments on mudstones, combined with seismic velocity data from geophysical surveys in the Nankai Trough, estimated porosity along the plate boundary decreases from ~40% at the trench, to ~5% at 50 km from the trench (~8 km depth). This porosity reduction is faster than that expected in a basinal environment, and can be attributed to the effects of horizontal tectonic loading. The phyllites and cataclasites (2-5% porosity) across the Nobeoka thrust fault deform brittlely with a peak strength of 80-90 MPa, followed by strain weakening to residual strengths of 40-60 MPa at effective pressure of 20 MPa and temperature of 250°C. At effective pressure of 120 MPa, on the other hand, they exhibit strain hardening and their strength at 10% strain reaches ~300 MPa. The extrapolation of a brittle-ductile transition for the Nankai mudstones as a function of porosity and confining stress is consistent with the behavior of the ancient prism samples, which serve as an analog for the mechanical behavior of rocks at seismogenic depth in the modern Nankai Trough. These results suggest that as the

  3. Active deformation along the Andaman-Nicobar subduction zone from seismic reflection studies

    NASA Astrophysics Data System (ADS)

    Moeremans, R. E.; Singh, S. C.

    2013-12-01

    The Andaman-Sumatra subduction zone is one of the most seismically active regions on Earth and is a prime example of oblique subduction. It is the result of the oblique convergence between the downgoing Indo-Australian and the overriding Eurasian plates, leading to slip partitioning into a trench-normal thrust component along the plate interface and a trench-subparallel strike-slip component along a sliver fault. The direction of convergence is 90° with respect to the trench near Java, reduces to 45° off of northern Sumatra, and becomes almost parallel to the trench along the Andaman-Nicobar portion of the subduction. Rates of subduction vary from 63 mm/yr off of Java, 50 mm/yr near Nias Island, 45 mm/yr northwest of Sumatra, and 39 mm/yr near the Andaman Islands. After the great December 2004 earthquake, the Sumatran section of the subduction zone was heavily investigated using marine geophysical studies, but the deformation processes in the Andaman-Nicobar region remain poorly understood due to the lack of data. Here, we present seismic reflection profiles from the Andaman-Nicobar region that cover the deformation front, the forearc high, and the forearc basin. We find that the presence of thick (> 3 s TWT) sediments lead to slip taking place predominantly along landward vergent frontal faults. The frontal fault vergence changes to seaward due to the thinning (< 2 s TWT) of the sediments in the region where the Ninetyeast ridge subducts. The presence of a thick (> 3 s TWT) 20 km-long unit of undeformed sediments, possibly resulting from the landward vergence of the frontal thrusts, suggests that ~40 km of the Ninetyeast ridge has subducted beneath the Andaman forearc. The forearc is widest between the Andaman and Nicobar Islands, likely due to the subduction of thick sediments. The forearc basin is bounded in the west by a series of backthrusts and is underlain by a continental crust, which was once a part of the Malay Peninsula. The forearc basin is crescent

  4. Comparing the Gibraltar and Calabrian subduction zones (central western Mediterranean) based on seismic tomography

    NASA Astrophysics Data System (ADS)

    Argnani, Andrea; Battista Cimini, Giovanni; Frugoni, Francesco; Monna, Stephen; Montuori, Caterina

    2016-04-01

    The Central Western Mediterranean (CWM) was shaped by a complex tectonic and geodynamic evolution. Deep seismicity and tomographic studies point to the existence, under the Alboran and Tyrrhenian Seas, of lithospheric slabs extending down to the bottom of the mantle transition zone, at 660 km depth. Two narrow arcs correspond to the two slabs, the Gibraltar and Calabrian Arcs (e.g., Monna et al., 2013; Montuori et al., 2007). Similarities in the tectonic and mantle structure of the two areas have been explained by a common subduction and roll-back mechanism for the opening of the CWM, in which the two arcs are symmetrical end products. In spite of this unifying model, a wide amount of literature from different disciplines shows that many aspects of the two areas are still controversial. We present a new 3-D tomographic model at mantle scale for the Calabrian Arc and compare it with a recently published 3-D tomographic model for the Gibraltar Arc by Monna et al (2013). The two models are based on non-linear inversion of teleseismic phase arrivals, and have scale and parametrization that allow for a direct comparison. Unlike previous studies the tomographic models here presented include Ocean Bottom Seismometer broadband data, which improved the resolution of the mantle structures in the marine areas surrounding the arcs. We focus on key features of the two models that constrain reconstructions of the geodynamic evolution of the CWM (e.g., Monna et al., 2015). At Tortonian time the opening of the Tyrrhenian basin was in its initial stage, and the Calabrian arc formed subsequently; on the contrary, the Gibraltar arc was almost completely defined. We hypothesize that the complexity of the continental margin approaching the subduction zone played a key role during the final stages of the arc formation. References Monna, S., G. B. Cimini, C. Montuori, L. Matias, W. H. Geissler, and P. Favali (2013), New insights from seismic tomography on the complex geodynamic evolution

  5. Slow-slip events hiding in low-coupled areas of the Chilean subduction zone ?

    NASA Astrophysics Data System (ADS)

    Métois, Marianne; Vigny, Christophe; Socquet, Anne

    2014-05-01

    The recent expansion of dense GPS networks over plate boundaries allows for remarkably precise mapping of interseismic coupling along active faults. The coupling coefficient is linked to the ratio between slipping velocity on the fault during the interseismic period and the long-term plates velocity. The coupling coefficient is a phenomenological parameter representing the kinematic state of the system, but a physical quantitative description of that parameter is needed for seismic hazard assessment. In other words, which amount of coupling or decoupling is needed to allow for earthquake to nucleate, propagate or stop, would be of great help to build rupture scenarios. Here, we investigate the link between coupling and present-day seismicity over the Chilean subduction zone. We combine recent GPS data acquired over the 2000 km long margin (38-18°S) with older data acquired at continental scale to get a nearly continuous picture of the interseismic coupling variations on the interface. We identify at least six zones where the coupling decreases dramatically, dividing individual highly coupled segments. These low-coupled areas often behave as barriers to past megathrust ruptures and experience high rates of seismicity during the interseismic period, including swarm-like sequences. We suggest that in these regions, the subduction interface is a patchwork of small velocity-weakening patches surrounded by velocity-strengthening material that would slide during the interseimic period. This relationship is consistent with observations over other subduction zones, notably in Ecuador where shallow aseismic transients have been observed near low coupled swarm-prone areas (Vallée et al. 2013). However for now, no transient event has been recorded yet all over the Chilean megathrust, preventing clear identification of creeping portions of the interface. Here, we test the hypothesis supposing that, similar to the Ecuador 2010 swarm episode, significant slow-slip events

  6. Subduction Zone Science - Examples of Seismic Images of the Central Andes and Subducting Nazca Slab

    NASA Astrophysics Data System (ADS)

    Beck, S. L.; Zandt, G.; Scire, A. C.; Ward, K. M.; Portner, D. E.; Bishop, B.; Ryan, J. C.; Wagner, L. S.; Long, M. D.

    2015-12-01

    Subduction has shaped large regions of the Earth and constitute over 55,000 km of convergent plate margin today. The subducting slabs descend from the surface into the lower mantle and impacts earthquake occurrence, surface uplift, arc volcanism and mantle convection as well as many other processes. The subduction of the Nazca plate beneath the South America plate is one example and constitutes the largest present day ocean-continent convergent margin system and has built the Andes, one of the largest actively growing mountain ranges on Earth. This active margin is characterized by along-strike variations in arc magmatism, upper crustal shortening, crustal thickness, and slab geometry that make it an ideal region to study the relationship between the subducting slab, the mantle wedge, and the overriding plate. After 20 years of portable seismic deployments in the Central Andes seismologists have combined data sets and used multiple techniques to generate seismic images spanning ~3000 km of the South American subduction zone to ~800 km depth with unprecedented resolution. For example, using teleseismic P- waves we have imaged the Nazca slab penetrating through the mantle transition zone (MTZ) and into the uppermost lower mantle. Our tomographic images show that there is significant along-strike variation in the morphology of the Nazca slab in the upper mantle, MTZ, and the lower mantle, including possible tears, folding, and internal deformation. Receiver function studies and surface wave tomography have revealed major changes in lithospheric properties in the Andes. Improved seismic images allow us to more completely evaluate tectonic processes in the formation and uplift of the Andes including: (1) overthickened continental crust driven by crustal shortening, (2) changes in slab dip and coupling with the overlying plate (3) localized lithospheric foundering, and (4) large-scale mantle and crustal melting leading to magmatic addition and/or crustal flow. Although

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

    PubMed Central

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

    2013-01-01

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

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

    PubMed

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

    2013-01-01

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

  9. A Computer-Based Subduction-Zone-Earthquake Exercise for Introductory-Geology Classes.

    ERIC Educational Resources Information Center

    Shea, James Herbert

    1991-01-01

    Describes the author's computer-based program for a subduction-zone-earthquake exercise. Instructions for conducting the activity and obtaining the program from the author are provided. Written in IBM QuickBasic. (PR)

  10. On the mechanism of seismic decoupling and back are spreading at subduction zones

    SciTech Connect

    Scholz, C.H.; Campos, J.

    1995-11-10

    This report discusses a force model for the mechanics of seismic decoupling and back arc spreading at subduction zones. This model predicts three regimes: seismically coupled compressional arcs; seismically decoupled extensional arcs; and strongly extensional arcs with back arc spreading.

  11. Interplate coupling at oblique subduction zones: influence on upper plate erosion.

    NASA Astrophysics Data System (ADS)

    Malatesta, Cristina; Gerya, Taras; Crispini, Laura; Federico, Laura; Scambelluri, Marco; Capponi, Giovanni

    2014-05-01

    In active subduction zones, when the converging plates cannot slip freely past each other, "plate coupling" occurs. The moving subducting slab and therefore the coupling/decoupling relationship between plates control both short- and long-term deformation of the upper plate. Short-term deformation is dominantly elastic, occurs at human timescales and can be directly associated with earthquakes. Long-term deformation is cumulative, permanent and prevails at the geological timescale (Hoffman-Rothe et al., 2006, Springer Berlin Heidelberg). Here we used 3D numerical simulations to test oblique subduction zones and to investigate: 1) how long-term deformation and coupling relationship vary along the trench-axis; 2) how this relationship influences erosion and down-drag of upper plate material. Our models are based on thermo-mechanical equations solved with finite differences method and marker-in-cell techniques combined with a multigrid approach (Gerya, 2010, Cambridge Univ. Press). The reference model simulates an intraoceanic subduction close to the continental margin (Malatesta et al., 2013, Nature Communications, 4:2456 DOI:10.1038/ncomms3456). The oceanic crust is layered with a 5-km-thick layer of gabbro overlain by a 3-km-thick layer of basalt. The ocean floor is covered by 1-km-thick sediments. Plates move with a total velocity of 3.15 cm/yr; the oblique convergence is obtained using velocity vectors that form an angle of 45° with the initial starting point of subduction (weak zone in the lithosphere). After initiation of plate convergence, part of sediments on top of the incoming plate enters the subduction zone and is buried; another part is suddenly transferred along strike at shallow depths and along the subducting slab according to the direction of the along-trench velocity component of subduction. The lateral migration of sediment causes the evolution of the trench along its strike from sediment-poor to sediment-rich. As soon as subduction starts, where

  12. Fluid Flow in Subduction Zones and Mountain Belts: The Importance of Permeability Heterogeneity and Anisotropy

    NASA Astrophysics Data System (ADS)

    Ague, J. J.

    2004-12-01

    Fluids are generally expected to be driven upward in the deep parts of orogens, but permeability heterogeneity and anisotropy must also be considered to properly interpret fluid infiltration and kinetic reaction histories preserved in the rock record. This paper focuses on new 2-D models of Darcian fluid flow incorporating permeability contrasts between rock units, the permeability tensor, and reactive fluid sources (e.g., dehydration). Factor of ten contrasts between the minimum and maximum permeability values in anisotropic rocks can strongly divert flow, but contrasts of as little as a factor of two still influence flow behavior. The first example considers fluid flow in subduction zone mélange, Syros, Greece. Geochemical evidence suggests that the interiors of meta-mafic blocks of oceanic crust in the mélange underwent limited fluid-rock reaction, despite extensive dehydration and decarbonation of the subduction complex. Modeling shows that if the blocks have lower permeability than the surrounding serpentine-rich matrix, then flow is diverted around the blocks resulting in little infiltration except at block margins, consistent with field relations. In this way, the subducted oceanic crust could preserve little evidence of fluid infiltration, even though considerable flow occurred through the mélange. The largest fluid fluxes are concentrated in matrix where blocks are in close proximity, and this effect increases as the anisotropy of the matrix increases. The lack of fluid infiltration into blocks could account for the observed limited metamorphism and strong kinetic overstepping of reactions that in some cases allowed preservation of ocean-floor mineral assemblages even at blueschist-eclogite facies conditions. The second example examines fluid flow through a folded sequence in which the direction of maximum permeability is parallel to the folded layering, and is based on field relations of Barrovian metamorphic sequences in CT, USA, and Scotland. As the

  13. A sulfur isotope perspective of fluid transport across subduction zones

    NASA Astrophysics Data System (ADS)

    Shimizu, N.; Mandeville, C. W.

    2011-12-01

    While there is a broad consensus that mantle melting in subduction zones occurs as a result of transport of aqueous fluid (or H2O-rich components) from the subducting slab to the mantle wedge, how and where the transport occurs is still one of the outstanding questions. We report recent SIMS-based sulfur isotope data of input to (pyrites in eclogites) and output from (un-degassed olivine-hosted primitive melt inclusions from arcs) subduction zones, and argue, on the basis of sulfur isotope mass balance, that our results do not support a widely held view of deep fluid transfer from slab to wedge. We suggest, instead, that hydration of the mantle wedge occurs at shallow levels with subsequent subduction and dehydration as the likely source of H2O-rich components for magma generation. Our data from olivine-hosted un-degassed primitive melt inclusions from Galunggung (δ34S ranging from -3 to +10 %, average = +2.9% with 1000 - 2000 ppm S), Krakatau (+1.6 - +8.7 %, av = +4.2%, 1200 - 2400 ppm S), and Augustine (+11 - +17%, 2500 - 5200 ppm S) clearly show that mantle wedge (δ34S ~0%, ~250 ppm S) has been significantly modified by slab-derived fluid (e.g., seawater with +21%, ~900 ppm S). On the other hand, eclogitic pyrites from the Western Gneiss Region, Norway (2 - 2.5 GPa, 700 - 850°C: Kylander-Clark et al., 2007) range in δ34S from -3.4 to +2.8%, similar to that for altered oceanic crust (e.g., Alt, 1995). Fluid in equilibrium with the eclogitic pyrites could have δ34S up to +10% (Ohmoto and Rye, 1979) and could contain up to ~1000 ppm S, based on the solubility data of Newton and Manning (2005). Mass balance calculations show that more than 10 wt.% of this fluid would be needed for modifying δ34S of the mantle wedge with ~250 ppm S from 0% to +5%, at least an order of magnitude greater than predicted by trace element-based arguments. For fluids with more seawater-like salinity, much more would be necessary for modifying the sulfur isotopic composition of the

  14. A possible source of water in seismogenic subduction zones

    NASA Astrophysics Data System (ADS)

    Kameda, J.; Yamaguchi, A.; Kimura, G.; Iodp Exp. 322 Scientists

    2010-12-01

    Recent works on the subduction megathrusts have emphasized the mechanical function of fluids contributing dynamic slip-weakening. Basalt-hosting fault zones in on-land accretionary complexes present several textures of seismic slip under fluid-assisted condition such as implosion breccia with carbonate matrix and decrepitation of fluid inclusion. In order to clarify initiation and evolution processes of such fault zones as well as possible source of fluid in the seismogenic subduction zone, we examined a mineralogical/geochemical feature of basaltic basement recovered by IODP Exp. 322 at C0012, that is a reference site for subduction input in the Nankai Trough. A total of 10 samples (about 4 m depth interval from the basement top) were analyzed in this study. XRD analyses indicate that all of the samples contain considerable amount of smectite. The smectite does not appear as a form of interstratified phase with illite or chlorite. Preliminary chemical analyses by EDS in TEM suggest that the smectite is trioctahedral saponite with Ca as a dominant interlayer cation. To determine the saponite content quantitatively, cation exchange capacity (CEC) of bulk samples was measured. The samples show almost similar CEC of around 30 meq/100g, implying that bulk rock contains about 30 wt% of saponite, considering a general CEC of 100 meq/100g for monomineralic saponite. Such abundance of saponite might be a result from intense alteration of oceanic crust due to sea water circulation at low temperature. Previous experimental work suggests that saponite might be highly hydrated (two to three water layer hydration form) at the seismogenic P-T condition. Hence, altered upper oceanic crust is a possible water sink in the seismogenic zone. The water stored in the smectite interlayer region will be expelled via smectite to chlorite transition reaction, that might contribute to the dynamic weakening of the seimogenic plate boundary between the basement basalt and overlying

  15. Carbon in, Carbon out: Reevaluating Carbon Fluxes in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Manning, C. E.; Kelemen, P. B.

    2015-12-01

    Subduction zones exert a fundamental control on the deep carbon cycle. We reevaluated carbon inputs and outputs in convergent margins considering new estimates of C concentration in subducting mantle peridotites, carbonate solubility in aqueous fluids along subduction geotherms, melting and diapirism of carbon-bearing metasediments, and diffuse degassing from arcs. Our updated estimate of carbon inputs to the global subduction system, which includes estimates for C in altered peridotite, is 40-66 megatons carbon/year (MtC/y). We find that estimates of C lost from slabs (14-66 MtC/y) must take into account the high CaCO3 solubility in aqueous fluids, which contributes significant C that must be added to that derived from mineral decarbonation reactions. When taken together with hydrous silicate and carbonatite melts and metasediment diapirs, nearly all C can be scavenged from subducting lithosphere. The return of C to the atmosphere via arc-volcano degassing is only 18-43 MtC/y, but consideration deep volatile saturation of arc magmas, magma ponding in the middle and deep arc crust, and CO2 venting in forearcs can account for the remaining C lost from the slab. Thus, whereas previous studies concluded that about half the subducting carbon is returned to the convecting mantle, we find that relatively little carbon may be recycled. If so, substantial quantities of carbon are stored in the mantle lithosphere and crust and the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing, at least over the last 5-10 My. This is consistent with inferences from noble gas data. Recycled carbon in diamonds is a small fraction of the global carbon inventory.

  16. Constraints on Subduction Zone Processes from Low Frequency Earthquakes

    NASA Astrophysics Data System (ADS)

    Bostock, M. G.

    2015-12-01

    The discovery of tectonic tremor and constituent low-frequency earthquakes (LFEs) offers seismologists new opportunities to study both deformational processes and structure within the subduction zone forearc. This assertion is especially true for northern Cascadia where i) regular seismicity is sparse, and ii) a relatively transparent overriding plate inflicts minimal distortion upon direct P and S wave arrivals from LFEs. Despite low signal-to-noise ratios, LFEs are highly repetitive and signal can be enhanced through construction of stacked templates. Studies in both Cascadia and Nankai reveal an association between LFE hypocenters and a high Vp/Vs, low-velocity zone (LVZ) that is inferred to represent overpressured upper oceanic crust. Scattered signals within Vancouver Island templates, interpreted to originate at boundaries of the LVZ, place LFEs within the LVZ and suggest that this structure may define a distributed (several km) zone of deformation. A recent analysis of LFE magnitudes indicates that LFEs exhibit scaling relations distinct from both regular earthquakes and longer period (10's of seconds to days) phenomena associated with slow slip. Regular earthquakes generally obey a scaling of moment proportional to duration cubed consistent with self similarity, whereas long period slow slip phenomena exhibit a linear scaling between moment and duration that can be accommodated through constant slip or constant stress drop models. In contrast, LFE durations are nearly constant suggesting that moment is governed by slip alone and that asperity size remains approximately constant. The implied dimensions (~1 km2), the persistance of LFEs in time and their stationarity in space point to structural heterogeneity, perhaps related to pockets of upper oceanic crust impervious to hydrothermal circulation, as a fundamental control.

  17. Permeability-porosity relationships of subduction zone sediments

    USGS Publications Warehouse

    Gamage, K.; Screaton, E.; Bekins, B.; Aiello, I.

    2011-01-01

    Permeability-porosity relationships for sediments from the northern Barbados, Costa Rica, Nankai, and Peru subduction zones were examined based on sediment type, grain size distribution, and general mechanical and chemical compaction history. Greater correlation was observed between permeability and porosity in siliciclastic sediments, diatom oozes, and nannofossil chalks than in nannofossil oozes. For siliciclastic sediments, grouping of sediments by percentage of clay-sized material yields relationships that are generally consistent with results from other marine settings and suggests decreasing permeability as percentage of clay-sized material increases. Correction of measured porosities for smectite content improved the correlation of permeability-porosity relationships for siliciclastic sediments and diatom oozes. The relationship between permeability and porosity for diatom oozes is very similar to the relationship in siliciclastic sediments, and permeabilities of both sediment types are related to the amount of clay-size particles. In contrast, nannofossil oozes have higher permeability values by 1.5 orders of magnitude than siliciclastic sediments of the same porosity and show poor correlation between permeability and porosity. More indurated calcareous sediments, nannofossil chalks, overlap siliciclastic permeabilities at the lower end of their measured permeability range, suggesting similar consolidation patterns at depth. Thus, the lack of correlation between permeability and porosity for nannofossil oozes is likely related to variations in mechanical and chemical compaction at shallow depths. This study provides the foundation for a much-needed global database with fundamental properties that relate to permeability in marine settings. Further progress in delineating controls on permeability requires additional carefully documented permeability measurements on well-characterized samples. ?? 2010 Elsevier B.V.

  18. Numerical modeling of fluid migration in subduction zones

    NASA Astrophysics Data System (ADS)

    Walter, M. J.; Quinteros, J.; Sobolev, S. V.

    2015-12-01

    It is well known that fluids play a crucial role in subduction evolution. For example, mechanical weakening along tectonic interfaces, due to high fluid pressure, may enable oceanic subduction. Hence, the fluid content seems to be a critical parameter for subduction initiation. Studies have also shown a correlation between the location of slab dehydration and intermediate seismic activity. Furthermore, expelled fluids from the subduction slab affect the melting temperature, consequently, contributing to partial melting in the wedge above the down-going plate and extensive volcanism. In summary, fluids have a great impact on tectonic processes and therefore should be incorporated into geodynamic numerical models. Here we use existing approaches to couple and solve fluid flow equations in the SLIM-3D thermo-mechanical code. SLIM-3D is a three-dimensional thermo-mechanical code capable of simulating lithospheric deformation with elasto-visco-plastic rheology. It has been successfully applied to model geodynamic processes at different tectonic settings, including subduction zones. However, although SLIM-3D already includes many features, fluid migration has not been incorporated into the model yet. To this end, we coupled solid and fluid flow assuming that fluids flow through a porous and deformable solid. Thereby, we introduce a two-phase flow into the model, in which the Stokes flow is coupled with the Darcy law for fluid flow. Ultimately, the evolution of porosity is governed by a compaction pressure and the advection of the porous solid. We show the details of our implementation of the fluid flow into the existing thermo-mechanical finite element code and present first results of benchmarks and experiments. We are especially interested in the coupling of subduction processes and the evolution of the magmatic arc. Thereby, we focus on the key factors controlling magma emplacement and its influence on subduction processes.

  19. Thermal Evolution of Juvenile Subduction Zones ' New Constraints from Lu-Hf Geochronology on HP oceanic rocks (Halilbaǧi, Central Anatolia)

    NASA Astrophysics Data System (ADS)

    Pourteau, Amaury; Scherer, Erik; Schmidt, Alexander; Bast, Rebecca

    2015-04-01

    The thermal structure of subduction zones plays a key role on mechanical and chemical processes taking place along the slab-mantle interface. Until now, changes through time of this thermal structure have been explored mostly by the means of numerical simulations. However, both "warm" (i.e., epidote-bearing), and "cold" (i.e., lawsonite-bearing) HP oceanic rocks have been reported in some fossil subduction complexes exposed at the Earth's surface (e.g., Franciscan Complex, California; Rio San Juan Complex, Hispañola; Halilbağı Unit, Central Anatolia). These a-priori "incompatible" rocks witness different thermal stages of ancient subduction zones and their study might provide complementary constraints to numerical models. To decipher the meaning of these contrasting metamorphic rocks in the Halilbağı Unit, we are carrying out Lu-Hf geochronology on garnet (grt) and lws from a variety of HP oceanic rocks, as well as the metamorphic sole of the overlying ophiolite. We selected five samples that are representative of the variety of metamorphic evolutions (i.e. peak conditions and P-T paths) encountered in this area. Preliminary analyses yielded 110 Ma (grt-hbl isochron) for a sub-ophiolitic grt amphibolite; 92 Ma (grt-omp) for an eclogite with prograde and retrograde ep; 90 Ma (grt-omp) for an eclogitic metabasite with prograde ep and retrograde ep+lws; 87 Ma (grt-gln) for a lws eclogite with prograde ep; and 86 Ma (grt-gln) for a blueschist with prograde and retrograde lws. These ages are mainly two-point isochrons. Further-refined data will be presented at the EGU General Assembly 2015, in Vienna. The consistent younging trend from "warm" to "cold" metamorphic rocks revealed by these first-order results points to metamorphic-sole formation during the initiation of intra-oceanic subduction at ~110 Ma, and subsequent cooling of the slab-mantle interface between 92 and 86 Ma. Therefore, the contrasting metamorphic evolutions encountered in the Halilbağı Unit

  20. Deep observation and sampling of the earth's continental crust (DOSECC): Continental scientific drilling workshop

    SciTech Connect

    Not Available

    1985-01-01

    Research summaries are presented of ongoing or proposed deep drilling programs to explore hydrothermal systems, buried astroblemes, continental crust, magma systems, mountain belt tectonics, subduction zones, and volcanoes. Separate abstracts have been prepared for individual papers. (ACR)

  1. Key role of Upper Mantle rocks in Alpine type orogens: some speculations derived from extensional settings for subduction zone processes and mountain roots

    NASA Astrophysics Data System (ADS)

    Müntener, Othmar

    2016-04-01

    Orogenic architecture and mountain roots are intrinsically related. Understanding mountain roots largely depends on geophysical methods and exhumed high pressure and high temperature rocks that might record snapshots of the temporal evolution at elevated pressure, temperatures and/or fluid pulses. If such high pressure rocks represent ophiolitic material they are commonly interpreted as exhumed remnants of some sort of 'mid-ocean ridge' processes. Mantle peridotites and their serpentinized counterparts thus play a key role in understanding orogenic architecture as they are often considered to track suture zones or ancient plate boundaries. The recognition that some mantle peridotites and their serpentinized counterparts are derived from ocean-continent transition zones (OCT's) or non-steady state (ultra-)slow plate separation systems question a series of 'common beliefs' that have been applied to understand Alpine-type collisional orogens in the framework of the ophiolite concept. Among these are: (i) the commonly held assumption of a simple genetic link between mantle melting and mafic (MORB-type) magmatism, (ii) the commonly held assumption that mélange zones represent deep subduction zone processes at the plate interface, (iii) that pre-collisional continental crust and oceanic crust can easily be reconstructed to their original thickness and used for reconstructions of the size of small subducted oceanic basins as geophysical data from rifted margins increasingly indicate that continental crust is thinned to much less than the average 30-35 kilometers over a large area that might be called the 'zone of hyperextension', and (iv) the lack of a continuous sheet of mafic oceanic crust and the extremely short time interval of formation results in a lack of 'eclogitization potential' during convergence and hence a lack of potential for subsequent slab pull and, perhaps, a lack of potential for 'slab-breakoff'. Here we provide a synopsis of mantle rocks from the

  2. Magmatism and metamorphism linked to the accretion of continental blocks south of the Hindu Kush, Afghanistan

    NASA Astrophysics Data System (ADS)

    Faryad, Shah Wali; Collett, Stephen; Petterson, Mike; Sergeev, Sergey A.

    2013-08-01

    Metamorphic basement rocks in the southern part of the Western Hindu Kush at contact with the Kabul and Helmand crustal blocks were investigated to elucidate pressure-temperature variation and relative time relations among different metamorphic rocks. The rocks are represented by Proterozoic amphibolite facies para-/orthogneisses and migmatites with low-grade Paleozoic volcano-sedimentary sequences. Major- and trace-element geochemistry from two orthogneiss bodies and geochronological data, including new SHRIMP analyses on zircon from one of these bodies shows that they are derived from granitic rocks that related to two different magmatic arcs of Triassic and Cretaceous ages. The Triassic granites are common in the Western Hindu Kush where they intrude basement units; the Cretaceous granitic belt crosses the Afghan Central blocks south of the Hindu Kush Mountains. Three different metamorphic events have been distinguished in the southern part of Western Hindu Kush. Based on an unconformity between basement units and Carboniferous cover sequences, the first two amphibolite and greenschist facies metamorphic events are Proterozoic and Pre-Carboniferous in age respectively. The third metamorphism was recognized in Triassic and Cretaceous granitic rocks near to contact with the Kabul Block. It is of Eocene age and reached medium pressure amphibolite facies conditions. This event is genetically linked to the collision of India and Eurasia which produced a series of trans-Afghan Central block magmatic arcs and crustal scale deformation.

  3. Permeability of the continental crust: Implications of geothermal data and metamorphic systems

    USGS Publications Warehouse

    Manning, C.E.; Ingebritsen, S.E.

    1999-01-01

    In the upper crust, where hydraulic gradients are typically 10 MPa km-1, the mean permeabilities required to accommodate the estimated metamorphic fluid fluxes decrease from ~10-16 m2 to ~10-18 m2 between 5- and 12-km depth. Below ~12 km, which broadly corresponds to the brittle-plastic transition, mean k is effectively independent of depth at ~10(-18.5??1) m2. Consideration of the permeability values inferred from thermal modeling and metamorphic fluxes suggests a quasi-exponential decay of permeability with depth of log k ~ -3.2 log z - 14, where k is in meters squared and z is in kilometers. At mid to lower crustal depths this curve lies just below the threshold value for significant advection of heat. Such conditions may represent an optimum for metamorphism, allowing the maximum transport of fluid and solute mass that is possible without advective cooling.

  4. Transdimensional imaging of random velocity inhomogeneities in Nankai subduction zone

    NASA Astrophysics Data System (ADS)

    Takahashi, T.; Obana, K.; Yamamoto, Y.; Kaiho, Y.; Nakanishi, A.; Kodaira, S.; Kaneda, Y.

    2014-12-01

    The Nankai trough in southwestern Japan is a convergent margin where the Philippine Sea plate is subducting beneath the Eurasian plate. We have conducted five seismic observations with ocean bottom seismograms (OBSs) from 2008 to 2012 to elucidate detailed seismic structures and its relations with fault segments of large earthquakes. These observations covered the entire area of the Nankai trough, but quantity and quality of data are not spatially uniform because of different observing lengths and various noises. Waveform data of OBSs suggests variously-sized anomalies of random velocity inhomogeneity (i.e., scattering strength) in this subduction zone. To clarify details of random inhomogeneity structures, we conducted a transdimensional imaging of random inhomogeneities by means of the reversible jump Markov Chain Monte Carlo (rjMCMC) without assuming smooth spatial distributions of unknown parameters. We applied the rjMCMC for the inversion of peak delay times of S-wave envelopes at 4-8, 8-16, and 16-32 Hz, where the peak delay time is defined as the time lag from the S-wave onset to its maximal amplitude arrival. This delay time mainly reflects the accumulated multiple forward scattering effect due to random inhomogeneities. We assumed the von Karman type power spectral density function (PSDF) for random velocity fluctuation, and estimated two parameters related with the PSDF at large wavenumber. Study area is partitioned by discrete Voronoi cells of which number and spatial sizes are variable. Estimated random inhomogeneities show clear lateral variations along the Nankai trough. The strongest inhomogeneity on the Nankai trough was found near the subducted Kyushu-Palau ridge that is located at the western margin of the fault segments. We also find a horizontal variation of inhomogeneity along the non-volcanic tremor zone. Relatively strong inhomogeneities in this tremor zone were imaged beneath west Shikoku and Kii-Peninsula. These anomalies were not clearly

  5. Numerical modeling of fluid migration in subduction zones

    NASA Astrophysics Data System (ADS)

    Walter, Marius J.; Quinteros, Javier; Sobolev, Stephan V.

    2015-04-01

    It is well known that fluids play a crucial role in subduction evolution. For example, excess mechanical weakening along tectonic interfaces, due to excess fluid pressure, may enable oceanic subduction. Hence, the fluid content seems to be a critical parameter for subduction initiation. Studies have also shown a correlation between the location of slab dehydration and intermediate seismic activity. Furthermore, expelled fluids from the subduction slab affect the melting temperature, consequently, contributing to partial melting in the wedge above the downgoing plate, and resulting in chemical changes in earth interior and extensive volcanism. In summary, fluids have a great impact on tectonic processes and therefore should be incorporated into geodynamic numerical models. Here we use existing approaches to couple and solve fluid flow equations in the SLIM-3D thermo-mechanical code. SLIM-3D is a three-dimensional thermo-mechanical code capable of simulating lithospheric deformation with elasto-visco-plastic rheology. It incorporates an arbitrary Lagrangian Eulerian formulation, free surface, and changes in density and viscosity, due to endothermic and exothermic phase transitions. It has been successfully applied to model geodynamic processes at different tectonic settings, including subduction zones. However, although SLIM-3D already includes many features, fluid migration has not been incorporated into the model yet. To this end, we coupled solid and fluid flow assuming that fluids flow through a porous and deformable solid. Thereby, we introduce a two-phase flow into the model, in which the Stokes flow is coupled with the Darcy law for fluid flow. This system of equations becomes, however, nonlinear, because the rheology and permeability are depended on the porosity (fluid fraction of the matrix). Ultimately, the evolution of porosity is governed by the compaction pressure and the advection of the porous solid. We show the details of our implementation of the

  6. Multiscale seismic imaging of the Western-Pacific subduction zone

    NASA Astrophysics Data System (ADS)

    Zhao, D.

    2011-12-01

    We used multiscale seismic tomography to determine the detailed 3-D structure of the crust and mantle under the Western-Pacific subduction zone. The subducting Pacific and Philippine Sea (PHS) slabs are imaged clearly from their entering the mantle at the oceanic trenches to their reaching the mantle transition zone and finally to the core-mantle boundary (CMB). High-resolution local tomography of Northeast Japan has imaged the shallow portion of the slab from the Japan Trench down to about 200 km depth under Japan Sea. The 3-D Vp and Vs structures of the forearc region under the Pacific Ocean are constrained by locating suboceanic events precisely with sP depth phases. Strong structural heterogeneity is revealed in the megathrust zone under the forearc region, and there is a good correlation between the heterogeneity and the distribution of large thrust earthquakes including the great 2011 Tohoku-oki earthquake (Mw 9.0). A joint inversion of local and teleseismic data imaged the subducting Pacific slab down to 670 km depth under the Japan Islands and the Japan Sea. The PHS slab is detected down to 500 km depth under SW Japan. A mantle upwelling is found under SW Japan that rises from about 400 km depth right above the Pacific slab up to the PHS slab. Regional and global tomography revealed the Pacific slab that is stagnant in the mantle transition zone under Eastern China. A big mantle wedge (BMW) has formed in the upper mantle above the stagnant slab. Convective circulations in the BMW and deep dehydration of the stagnant slab may have caused the intraplate volcanoes in NE Asia, such as the Changbai and Wudalianchi volcanoes. The active Tengchong volcanism in SW China is caused by a similar process in the BMW above the subducting Burma (or Indian) slab. Global tomography shows pieces of fast anomalies in the middle and lower mantle as well as in the D" layer above the CMB, suggesting that the stagnant slab finally collapses down to the lower mantle and CMB as a

  7. Links between fluid circulation, temperature, and metamorphism in subducting slabs

    USGS Publications Warehouse

    Spinelli, G.A.; Wang, K.

    2009-01-01

    The location and timing of metamorphic reactions in subducting lithosph??re are influenced by thermal effects of fluid circulation in the ocean crust aquifer. Fluid circulation in subducting crust extracts heat from the Nankai subduction zone, causing the crust to pass through cooler metamorphic faci??s than if no fluid circulation occurs. This fluid circulation shifts the basalt-to-eclogite transition and the associated slab dehydration 14 km deeper (35 km farther landward) than would be predicted with no fluid flow. For most subduction zones, hydrothermal cooling of the subducting slab will delay eclogitization relative to estimates made without considering fluid circulation. Copyright 2009 by the American Geophysical Union.

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

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

  9. 15 Years Of Ecuadorian-French Research Along The Ecuadorian Subduction Zone

    NASA Astrophysics Data System (ADS)

    Charvis, P.

    2015-12-01

    The Ecuadorian segment of the Nazca/South America subduction zone is an outstanding laboratory to study the seismic cycle. Central Ecuador where the Carnegie ridge enters the subduction marks a transition between a highly coupled segment that hosted one of the largest seismic sequence during the 20thcentury and a ~1200-km long weakly coupled segment encompassing southern Ecuador and northern Peru. A shallow dipping subduction interface and a short trench-coast line distance ranging from 45 to 80 km, together with La Plata Island located only 33 km from the trench axis, allow to document subduction processes in the near field with an exceptional resolution. Since 2000, a close cooperation between the Institute of Geophysics (Quito), INOCAR (Oceanographic Institute of the Ecuadorian Navy) with French groups allowed us to conduct up to 6 marine geophysics cruises to survey the convergent margin and jointly develop dense GPS and seismological networks. This fruitful collaboration now takes place in the framework of an International Joint Laboratory "Earthquakes and Volcanoes in the Northern Andes" (LMI SVAN), which eases coordinating research projects and exchanges of Ecuadorian and French scientists and students. This long-term investigation has already provided a unique view on the structure of the margin, which exhibits a highly variable subduction channel along strike. It allowed us to evidence the contrast between creeping and coupled segments of subduction at various scale, and the existence of large continental slivers whose motion accommodates the obliquity of the Nazca/South America convergence. Finally, we could evidence the first Slow Slip Events (SSE) that oppositely to most SSE documented so far, are accompanied with intense micro-seismicity. The recent support of the French National Research Agency and the Ecuadorian Agency for Sciences and Technology (Senescyt) will enable us to integrate the already obtained results, in an attempt to develop an

  10. Structure and Deformation of the Hikurangi-Kermadec Subduction Zone - Transitions Revealed by Seismic Wide-angle Data

    NASA Astrophysics Data System (ADS)

    Scherwath, M.; Kopp, H.; Flueh, E. R.; Henrys, S. A.; Sutherland, R.

    2008-12-01

    The Hikurangi-Kermadec subduction zone northeast of New Zealand represents an ideal target to study lateral variations of subduction zone processes. The incoming Pacific plate changes from being a large igneous province, called the Hikurangi Plateau, in the south to normal oceanic plate north of the Rapuhia Scarp. The overriding Australian plate of continental character in the south, forming the North Island of New Zealand, and changes to an island arc in the north. Further lateral variability exists in changes in volcanic and hydro-thermal activity, transitions from accretion to subduction erosion, backarc spreading and rifting, and is accompanied by northward increasing seismicity. As part of the MANGO project (Marine Geoscientific Investigations on the Input and Output of the Kermadec Subduction Zone), four marine geophysical transects of largely seismic reflection and refraction data provide constraints on the upper lithospheric structures across the Hikurangi-Kermadec Trench between 29-38 deg S. On MANGO profile 1 in the south, the initially shallow subduction of the incoming plateau coincides with crustal underplating beneath the East Cape ridge. To the west lies the 100 km wide and over 10 km deep Raukumara Basin. Seismic velocities of the upper mantle of both plates are around 8 km/s and are considered normal. In contrast, on MANGO profile 4, about 1000 km to the north around the volcanically active Raoul Island, the incoming oceanic crust appears to bend considerably steeper and thus causes a 50 km narrower forearc with a smaller forearc basin. Furthermore, the upper mantle velocities in both plates are relatively low (7.4-7.7 km/s), likely indicating strong bending related deformation of the incoming plate and thermal activity within the arc possibly due to spreading. The central two transects MANGO 2 and 3, though without data coverage of the structure of the incoming plate, are more similar to MANGO 4. The arc regions appear to be strongly affected by

  11. Structure and seismic activity of the Lesser Antilles subduction zone

    NASA Astrophysics Data System (ADS)

    Evain, M.; Galve, A.; Charvis, P.; Laigle, M.; Ruiz Fernandez, M.; Kopp, H.; Hirn, A.; Flueh, E. R.; Thales Scientific Party

    2011-12-01

    Several active and passive seismic experiments conducted in 2007 in the framework of the European program "Thales Was Right" and of the French ANR program "Subsismanti" provided a unique set of geophysical data highlighting the deep structure of the central part of the Lesser Antilles subduction zone, offshore Dominica and Martinique, and its seismic activity during a period of 8 months. The region is characterized by a relatively low rate of seismicity that is often attributed to the slow (2 cm/yr) subduction of the old, 90 My, Atlantic lithosphere beneath the Caribbean Plate. Based on tomographic inversion of wide-angle seismic data, the forearc can clearly be divided into an inner forearc, characterised by a high vertical velocity gradient in the igneous crust, and an outer forearc with lower crustal velocity gradient. The thick, high velocity, inner forearc is possibly the extension at depth of the Mesozoic Caribbean crust outcropping in La Désirade Island. The outer forearc, up to 70 km wide in the northern part of the study area, is getting narrower to the south and disappears offshore Martinique. Based on its seismic velocity structure with velocities higher than 6 km/s the backstop consists, at least partly, of magmatic rocks. The outer forearc is also highly deformed and faulted within the subducting trend of the Tiburon Ridge. With respect to the inner forearc velocity structure the outer forearc basement could either correspond to an accreted oceanic terrane or made of highly fractured rocks. The inner forearc is a dense, poorly deformable crustal block, tilted southward as a whole. It acts as a rigid buttress increasing the strain within both the overriding and subducting plates. This appears clearly in the current local seismicity affecting the subducting and the overriding plates that is located beneath the inner forearc. We detected earthquakes beneath the Caribbean forearc and in the Atlantic oceanic plate as well. The main seismic activity is

  12. Uncertainty in turbidite correlations along the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Atwater, B. F.

    2012-12-01

    of sandy layers. (c) The sandy sequence correlates among core sites hundreds of kilometers apart along the length of the subduction zone. (d) This stratigraphic similarity, evidenced mainly by logs of density and magnetic susceptibility, enables a full-length rupture of magnitude 9 to be distinguished geologically from a series of shorter ruptures. Open questions include: Do the density and magnetic signatures of a sandy sequence have enough complexity in shape and reproducibility among adjacent cores to justify long-distance correlation of individual sandy layers? Do the initial mass movements respond less to individual pulses than to cumulative shaking, and do they commonly begin or continue after the mainshock has finished? Are the pulses of shaking likely to vary along strike, as in strong-motion records from the 2010 Maule and 2011 Tohoku earthquakes? An unknown fraction of the so-called full-length ruptures represents series of shorter ruptures, and solitary short ruptures may sometimes break the plate boundary offshore southern British Columbia and northern Washington.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  14. Anatexis and metamorphism in tectonically thickened continental crust exemplified by the Sevier hinterland, western North America

    NASA Technical Reports Server (NTRS)

    Patino Douce, Alberto E.; Humphreys, Eugene D.; Johnston, A. Dana

    1990-01-01

    This paper presents a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening exemplified by the Sevier hinterland in western North America, and uses the model to examine the geological and physical processes leading to crustally derived magmatism. The results of numerical experiments show that anatexis was an inevitable end-product of Barrovian metamorphism in the thickened crust of the late Mesozoic Sevier orogenic belt and that the advection of heat across the lithosphere, in the form of mantle-derived mafic magmas, was not required for melting of metasedimentary rocks. It is suggested that, in the Sevier belt, as in other intracontinental orogenic belts, anatexis occurred in the midcrust and not at the base of the crust.

  15. Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up

    PubMed Central

    Kelemen, Peter B.; Manning, Craig E.

    2015-01-01

    Carbon fluxes in subduction zones can be better constrained by including new estimates of carbon concentration in subducting mantle peridotites, consideration of carbonate solubility in aqueous fluid along subduction geotherms, and diapirism of carbon-bearing metasediments. Whereas previous studies concluded that about half the subducting carbon is returned to the convecting mantle, we find that relatively little carbon may be recycled. If so, input from subduction zones into the overlying plate is larger than output from arc volcanoes plus diffuse venting, and substantial quantities of carbon are stored in the mantle lithosphere and crust. Also, if the subduction zone carbon cycle is nearly closed on time scales of 5–10 Ma, then the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing. Such an increase is consistent with inferences from noble gas data. Carbon in diamonds, which may have been recycled into the convecting mantle, is a small fraction of the global carbon inventory. PMID:26048906

  16. Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up

    NASA Astrophysics Data System (ADS)

    Kelemen, Peter B.; Manning, Craig E.

    2015-07-01

    Carbon fluxes in subduction zones can be better constrained by including new estimates of carbon concentration in subducting mantle peridotites, consideration of carbonate solubility in aqueous fluid along subduction geotherms, and diapirism of carbon-bearing metasediments. Whereas previous studies concluded that about half the subducting carbon is returned to the convecting mantle, we find that relatively little carbon may be recycled. If so, input from subduction zones into the overlying plate is larger than output from arc volcanoes plus diffuse venting, and substantial quantities of carbon are stored in the mantle lithosphere and crust. Also, if the subduction zone carbon cycle is nearly closed on time scales of 5-10 Ma, then the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing. Such an increase is consistent with inferences from noble gas data. Carbon in diamonds, which may have been recycled into the convecting mantle, is a small fraction of the global carbon inventory.

  17. Stress orientations in subduction zones and the strength of subduction megathrust faults

    USGS Publications Warehouse

    Hardebeck, Jeanne L.

    2015-01-01

    Subduction zone megathrust faults produce most of the world’s largest earthquakes. Although the physical properties of these faults are difficult to observe directly, their frictional strength can be estimated indirectly by constraining the orientations of the stresses that act on them. A global investigation of stress orientations in subduction zones finds that the maximum compressive stress axis plunges systematically trenchward, consistently making a 45°-60° angle to the subduction megathrust fault. These angles indicate that the megathrust fault is not substantially weaker than its surroundings. Together with several other lines of evidence, this implies that subduction zone megathrusts are weak faults in a low-stress environment. The deforming outer accretionary wedge may decouple the stress state along the megathrust from the constraints of the free surface.

  18. Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up.

    PubMed

    Kelemen, Peter B; Manning, Craig E

    2015-07-28

    Carbon fluxes in subduction zones can be better constrained by including new estimates of carbon concentration in subducting mantle peridotites, consideration of carbonate solubility in aqueous fluid along subduction geotherms, and diapirism of carbon-bearing metasediments. Whereas previous studies concluded that about half the subducting carbon is returned to the convecting mantle, we find that relatively little carbon may be recycled. If so, input from subduction zones into the overlying plate is larger than output from arc volcanoes plus diffuse venting, and substantial quantities of carbon are stored in the mantle lithosphere and crust. Also, if the subduction zone carbon cycle is nearly closed on time scales of 5-10 Ma, then the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing. Such an increase is consistent with inferences from noble gas data. Carbon in diamonds, which may have been recycled into the convecting mantle, is a small fraction of the global carbon inventory. PMID:26048906

  19. Processes of subduction and exhumation of continental blocks in collisional orogeny

    NASA Astrophysics Data System (ADS)

    Tirel, C.; Brun, J.; Burov, E. B.; Wortel, M. J.; Lebedev, S.

    2009-12-01

    Understanding the mechanism of accretion, subduction and exhumation of rocks during orogeny is a fundamental issue for plate tectonics. Numerous models have been proposed in order to explain the tectonic events that accompany continental subduction. Here, in order to examine the different processes in a simple, self-consistent manner, the fully coupled thermo-mechanical numerical code PARAFLAM is used to perform a parametric study on the subduction mode and, particularly, on the continental subduction processes. A wide range of parameters including slab-pull magnitude, temperature and viscosity distribution, initial geometry of the subduction zone and rheology, is explored to understand the dynamics of accretion, the different processes of exhumation, the deformation of the slab and the deformation of the overriding plate. The first results of this study show the evolution of one or two small continental blocks (terranes) reaching the subduction-zone continental margin. A set of experiments demonstrates that the progressive incorporation of a continental block into the over-riding plate is governed by the dynamics of subduction. Initially, the continental block is partly or entirely subducted, which increases its buoyancy and induces its detachment from the subducting slab. By the time the emplacement of the block into the continental margin is completed, exhumation is likely to have occurred, depending mainly on the slab pull magnitude and the rheology of the continental block itself. As an example, some experiments show the continuous subduction of a continental block followed by an UHP-HP exhumation and a metamorphic-core-complex type of exhumation. These cases can be compared with the pattern of deformation observed in the Aegean-Sea domain or in the Tyrrhenian one.

  20. Tomography, Dynamical Modeling and the Geologic History of the Subduction Zone Around the Japanese Islands

    NASA Astrophysics Data System (ADS)

    Honda, S.

    2014-12-01

    Since the subduction zone is one of the most geologically active regions in the world, it has attracted much attention from the various fields of the earth science. In this presentation, we try to combine the results and knowledge of seismic tomography, geodynamic modeling and the geologic history of the subduction zone around the Japanese Islands to constrain the nature of the subduction zone there. For this purpose, first, we estimate the cold temperature anomaly by converting the fast velocity anomaly of GAP_P4 model [Fukao & Obayashi, 2013] to the cold temperature anomaly using the recent estimate of d(ln Vp)/dT by Karato [2008]. The magnitude of the anomaly is constrained by the work on the relation between the theoretical estimate of temperature and the seismicity in the subducting slab [Emmerson & McKenzie, 2007]. We find that, although the velocity anomaly itself does not show a significant high velocity anomaly just below the stagnated slab, the estimated temperature shows rather continuous cold anomaly from the upper to the lower mantle. This continuous feature is consistent with the recent results of geodynamic modeling of the subduction zone. However, we still see a significant thinning or an absence of the slab just below the stagnated slab in the transition zone. This is more evident in other tomographic models. Geodynamical modeling of subduction, especially, the stagnation of the slab in the transition zone shows that the slab behavior strongly depends on the geological settings of subduction zone such as the rollback of trench. To understand the present feature of the slab revealed by the seismic tomography, we construct a simple half-kinematic model of subduction zone by taking into account the geological settings, that is, the opening of the Japan Sea. We find that the slab similar to the present image is obtained in terms of disruption of the slab suggesting that it occurred during the opening of the Japan Sea.

  1. Effects of 3D Velocity and Attenuation in the Tonga-Fiji Subduction Zone

    NASA Astrophysics Data System (ADS)

    Savage, B.; Wiens, D. A.; Tromp, J.

    2005-12-01

    The current understanding of a subduction zone's temperature and composition is limited. Much of our recent knowledge of subduction zones comes from earthquake locations, geochemical measurements, and lab based experiments. Recently, two studies of the Tonga-Fiji subduction zone have presented tomographic images of velocity and attenuation (Roth et al., 1999; Zhao et al., 1997). Roth et al. (2000) then combined these two tomographic models of the Tonga-Fiji subduction zone to derive an empirical relationship between changes in velocity and attenuation. This relationship agrees well with two independent, experimental data sets (Jackson et al., 1992; Sato et al., 1989). Using the tomographic velocity model and the empirical relationship between velocity and attenuation we create synthetic seismograms for the Tonga-Fiji subduction zone to test whether a simple increase in velocity accurately depicts this subduction zone. To construct the model we use the tomographic model of Zhao et al. (1997) to create a shear velocity model using a simple Vs/Vp ratio. Following Roth et al. (2000) these tomographic models are combined with the empirical relation between velocity and attenuation to create an attenuation model. The resulting synthetics are compared to recorded data to validate the tomographic velocity model and the empirical relation between velocity and attenuation. Any mismatch in this comparison will provide a basis for further refinement of the tomographic models and the velocity-attenuation relation. The synthetics are created using the SPECFEM3D global code (Komatitsch et al., 2002) with the new addition of a three-dimensional attenuation operator. Attenuation is simulated by a set of standard linear solids over the desired frequency range as described in Liu et al. (1976). Our initial results at a minimum period of 3.3 seconds suggest that the attenuation structure plays a minor role for the present source-receiver geometry. The addition of the 3D attenuation

  2. Teaching about Subduction Zone Magmagenesis using MARGINS Subduction Factory Focus Site Geochemical Compilations and ABS3 (Invited)

    NASA Astrophysics Data System (ADS)

    Stern, R. J.; Jordan, E.; Raye, U.; Carr, M. J.; Feigenson, M.; Gill, J. B.; Hacker, B. R.; Kimura, J.; Lehnert, K. A.; Tamura, Y.; van Keken, P. E.

    2010-12-01

    Processes and inputs involved in generating arc magmas are reasonably well known but quantitative modeling is often overlooked when teaching about subduction zone magmagenesis. In order to appreciate these complexities, students need to be able to explore subduction zone magmagenetic processes using trace element compositions of igneous rocks. The MARGINS Subduction Factory experiment selected two endmember convergent margins, the Izu-Bonin-Mariana (IBM) arc, which subducts old, cold, dense seafloor, and the Central American (CentAm) arc, which subducts young, hot, buoyant seafloor. We have compiled high-quality trace element and isotopic data for young, fresh lavas from along the magmatic fronts of these endmember arcs, using the EarthChem database. Comparing data for primitive magmas from the two arc systems allows for first-order distinctions, including the greater relative abundances of fluid-mobile elements (e.g., K, Sr, U) in IBM lavas and greater relative abundances of elements requiring sediment melting (e.g., Th, LREE, Zr) in CentAm lavas. These differences can be explored quantitatively using the Arc Basalt Simulator version 3 (ABS3). ABS3 is a free Excel-based spreadsheet forward model that allows the user to control compositions of subducted sediment and altered oceanic crust in tandem with realistic thermal models to predict metamorphic conditions in the subducted slab, using simplified results from Perple_X, and to understand when sediment- and slab-melting is likely (See J.-I. Kimura et al, this meeting "V15 The Subduction Filter" session for more information about ABS3). Prograde metamorphism along with experimentally-determined partition coefficients are used to predict hydrous fluid compositions; experimental results along with mineral-melt distribution coefficients are used to predict slab melt compositions. Hydrous fluid or melt is allowed to rise into and metasomatize overlying mantle, and the modified fluid allowed to trigger mantle melting

  3. Hf-Nd input flux in the Izu-Mariana subduction zone and recycling of subducted material in the mantle

    NASA Astrophysics Data System (ADS)

    Chauvel, Catherine; Marini, Jean-Christophe; Plank, Terry; Ludden, John N.

    2009-01-01

    In subduction zones, two major mass fluxes compete: the input flux of altered oceanic crust and sediments subducted into the mantle and the output flux of magma that forms the volcanic arc. While the composition and the amount of material erupted along volcanic arcs are relatively well known, the chemical and isotopic composition of the subducted material (altered oceanic crust and sediments) is poorly constrained and is an important factor in the mass balance calculation. Ocean Drilling Program Leg 185 in the Western Pacific used systematic sampling of the altered basaltic basement and sediment pile and the creation of composite mixtures to quantify the total chemical flux subducted at the Izu-Mariana margin. Here, we report Hf and Nd isotopic compositions of materials recovered from this Leg. The Hf and Nd isotopic compositions of altered basalts from Hole 801C are indistinguishable from those of recent unaltered Pacific mid-ocean ridge basalt, suggesting that hydrothermal alteration had no effect on either isotopic systems. The complete Site 1149 sedimentary pile has a weighted average ɛNd of -5.9 and ɛHf of +4.4, values similar to those of Fe-Mn crusts and nodules. Therefore, the Hf and Nd isotopic compositions of the sediments collected at Site 1149 indicate minimal contributions from continental detrital material to the rare earth elements and high field strength elements. However, the Hf isotopic budget of the oldest sediments is more influenced by continental material than the younger sediments, despite the large distances to continental masses 130 Ma ago. In the Izu subduction zone, we calculate a sedimentary input of less than about 2% in the volcanic lava source. In contrast, at least 85% of the sedimentary Nd and Hf are recycled into the mantle to affect its general composition. Assuming that sediments have been recycled in a similar manner into the mantle for millions of years, large chemical heterogeneities must be produced in the mantle. In

  4. Simulation of tsunamis from great earthquakes on the cascadia subduction zone.

    PubMed

    Ng, M K; Leblond, P H; Murty, T S

    1990-11-30

    Large earthquakes occur episodically in the Cascadia subduction zone. A numerical model has been used to simulate and assess the hazards of a tsunami generated by a hypothetical earthquake of magnitude 8.5 associated with rupture of the northern sections of the subduction zone. Wave amplitudes on the outer coast are closely related to the magnitude of sea-bottom displacement (5.0 meters). Some amplification, up to a factor of 3, may occur in some coastal embayments. Wave amplitudes in the protected waters of Puget Sound and the Strait of Georgia are predicted to be only about one fifth of those estmated on the outer coast. PMID:17829212

  5. Microstructures, Chemical Composition, and Viscosities of Fault-generated Friction Melts in the Shimanto Accretionary Complex, Southwest Japan: Implication for Dynamics of Earthquake Faulting in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Ujiie, K.; Yamaguchi, H.

    2004-12-01

    The pseudotachylytes (PT) were recently found in the Cretaceous Shimanto accretionary complex of eastern and western Shikoku, southwest Japan, but their microstructures under a backscattered electron image, chemical composition, and effects of frictional melting on co-seismic slip in the accretionary prism remains poorly understood. The PT bearing fault is the 1-2 m thick roof thrust of a duplex structure, which bounds the off-scraped coherent turbidites above from the imbricated melange below without a thermal inversion across the fault. The fault zone consists of foliated cataclasite of sandstone-shale melange in origin and dark veins. The PT commonly occurs as brecciated fragments in dark veins. The PT matrix is transparent under plane-polarized light and is optically homogeneous under cross-polarized light, similar to glass matrix. Under a backscattered electron image, the PT clearly shows the evidences for frictional melting and subsequent rapid cooling: rounded and irregularly shaped grains and vesicles in matrix and fracturing associated with grain margins. These textural features of the PT are very similar to those of experimentally generated PT. The EPMA analysis indicates that chemical composition of the PT matrix corresponds to illite with 5.7-9.9 wt% H2O and that partially melted grains are dominated by orthoclase and quartz. This indicates that the temperatures of the PT melt could reach the breakdown temperatures of orthoclase (1150 C) and quartz (1730 C), greater than the maximum temperature recorded in host rocks (170-200 C). We calculated the viscosity of friction melt, based on the chemical composition of the PT matrix and the volume fraction and aspect ratio of grains in the PT. We considered both Arrhenian and non-Arrhenian models for viscosity calculation. Our result demonstrates that the melt viscosity is much lower than PT in continental plutonic and metamorphic rocks: 10^3 Pa s (Arrhenian model) and 10^2 Pa s (non-Arrhenian model) even at

  6. Along-strike complex geometry of subduction zones - an experimental approach

    NASA Astrophysics Data System (ADS)

    Midtkandal, I.; Gabrielsen, R. H.; Brun, J.-P.; Huismans, R.

    2012-04-01

    Recent knowledge of the great geometric and dynamic complexity insubduction zones, combined with new capacity for analogue mechanical and numerical modeling has sparked a number of studies on subduction processes. Not unexpectedly, such models reveal a complex relation between physical conditions during subduction initiation, strength profile of the subducting plate, the thermo-dynamic conditions and the subduction zones geometries. One rare geometrical complexity of subduction that remains particularly controversial, is the potential for polarity shift in subduction systems. The present experiments were therefore performed to explore the influence of the architecture, strength and strain velocity on complexities in subduction zones, focusing on along-strike variation of the collision zone. Of particular concern were the consequences for the geometry and kinematics of the transition zones between segments of contrasting subduction direction. Although the model design to some extent was inspired by the configuration along the Iberian - Eurasian suture zone, the results are also of significance for other orogens with complex along-strike geometries. The experiments were set up to explore the initial state of subduction only, and were accordingly terminated before slab subduction occurred. The model wasbuilt from layers of silicone putty and sand, tailored to simulate the assumed lithospheric geometries and strength-viscosity profiles along the plate boundary zone prior to contraction, and comprises two 'continental' plates separated by a thinner 'oceanic' plate that represents the narrow seaway. The experiment floats on a substrate of sodiumpolytungstate, representing mantle. 24 experimental runs were performed, varying the thickness (and thus strength) of the upper mantle lithosphere, as well as the strain rate. Keeping all other parameters identical for each experiment, the models were shortened by a computer-controlled jackscrew while time-lapse images were

  7. Zagros blueschists: Episodic underplating and long-lived cooling of a subduction zone

    NASA Astrophysics Data System (ADS)

    Angiboust, Samuel; Agard, Philippe; Glodny, Johannes; Omrani, Jafar; Oncken, Onno

    2016-04-01

    Pressure-Temperature-time (P-T-t) trajectories of high-pressure rocks provide important constraints to understand the tectonic evolution of convergent margins. New field observations and P-T-t constraints for the evolution of the only known blueschist-facies exposure along the Zagros suture zone in Southern Iran are reported here. These blueschists, now exposed in tectonic windows under the Sanandaj-Sirjan zone (upper plate), constitute accreted fragments of the Tethyan domain during N-vergent Cretaceous subduction. Three units were identified in the field: from top to bottom, the Ashin unit (mafic and felsic gneisses), the Seghin complex (mafic tuffs and ultramafics) and the Siah Kuh massif (coherent volcanic edifice). Microstructural observations, P-T estimates and Rb-Sr deformation ages indicate that the Ashin unit possibly underwent burial down to 30-35 km and 550°C along a relatively warm P-T gradient (c. 17°/km) and was ultimately deformed between 85 and 100 Ma. The Seghin complex exhibits remarkably well-preserved HP-LT assemblages comprising lawsonite, glaucophane, aragonite, omphacite and garnet. P-T-t reconstruction indicates that this slice was subducted down to c. 50 km at temperatures of c. 500°C along a very cold subduction gradient (c. 7°/km). Deformation in the Seghin complex stopped at around 65 Ma, close to peak metamorphic conditions. Field relationships and estimates of the P-T trajectory followed by the Siah Kuh volcanic edifice indicate that this massif was lately subducted down to 15 km depth along the same very cold gradient. This slice-stack represents a well-preserved field example (i) highlighting the existence of transient underplating processes juxtaposing pluri-kilometric tectonic slices along the subduction channel and (ii) imaging the discontinuous down-stepping of the active main subduction thrust with ongoing accretion. The Zagros blueschists also record an apparent cooling of the Zagros subduction zone between 90 and 65 Ma

  8. Zagros blueschists: Episodic underplating and long-lived cooling of a subduction zone

    NASA Astrophysics Data System (ADS)

    Angiboust, Samuel; Agard, Philippe; Glodny, Johannes; Omrani, Jafar; Oncken, Onno

    2016-06-01

    Pressure-temperature-time (P- T- t) trajectories of high-pressure rocks provide important constraints to understand the tectonic evolution of convergent margins. New field observations and P- T- t constraints for the evolution of the only known blueschist-facies exposure along the Zagros suture zone in Southern Iran are reported here. These blueschists, now exposed in tectonic windows under the Sanandaj-Sirjan zone (upper plate), constitute accreted fragments of the Tethyan domain during N-directed Cretaceous subduction. Three units were identified in the field: from top to bottom, the Ashin unit (mafic and felsic gneisses), the Seghin complex (mafic tuffs and ultramafics) and the Siah Kuh massif (coherent volcanic edifice). Microstructural observations, P- T estimates and Rb-Sr deformation ages indicate that the Ashin unit possibly underwent burial down to 30-35 km and 550 °C along a relatively warm P- T gradient (c. 17°/km) and was ultimately deformed between 85 and 100 Ma. The Seghin complex exhibits remarkably well-preserved HP-LT assemblages comprising lawsonite, glaucophane, aragonite, omphacite and garnet. P- T- t reconstruction indicates that this slice was subducted down to c. 50 km at temperatures of c. 500 °C along a very cold subduction gradient (c. 7°/km). Deformation in the Seghin complex stopped at around 65 Ma, close to peak metamorphic conditions. Field relationships and estimates of the P- T trajectory followed by the Siah Kuh volcanic edifice indicate that this massif was lately subducted down to 15 km depth along the same very cold gradient. This slice-stack represents a well-preserved field example (i) highlighting the existence of transient underplating processes juxtaposing pluri-kilometric tectonic slices along the subduction channel and (ii) imaging the discontinuous down-stepping of the active main subduction thrust with ongoing accretion. The Zagros blueschists also record an apparent cooling of the Zagros subduction zone between 90

  9. Earthquake faulting in subduction zones: insights from fault rocks in accretionary prisms

    NASA Astrophysics Data System (ADS)

    Ujiie, Kohtaro; Kimura, Gaku

    2014-12-01

    Subduction earthquakes on plate-boundary megathrusts accommodate most of the global seismic moment release, frequently resulting in devastating damage by ground shaking and tsunamis. As many earthquakes occur in deep-sea regions, the dynamics of earthquake faulting in subduction zones is poorly understood. However, the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) and fault rock studies in accretionary prisms exhumed from source depths of subduction earthquakes have greatly improved our understanding of earthquake faulting in subduction zones. Here, we review key advances that have been made over the last decade in the studies of fault rocks and in laboratory experiments using fault zone materials, with a particular focus on the Nankai Trough subduction zone and its on-land analog, the Shimanto accretionary complex in Japan. New insights into earthquake faulting in subduction zones are summarized in terms of the following: (1) the occurrence of seismic slip along velocity-strengthening materials both at shallow and deep depths; (2) dynamic weakening of faults by melt lubrication and fluidization, and possible factors controlling coseismic deformation mechanisms; (3) fluid-rock interactions and mineralogical and geochemical changes during earthquakes; and (4) geological and experimental aspects of slow earthquakes.

  10. Non-elastic Plate Weakening at Tonga, Costa Rica and Japanese Subduction Zones

    NASA Astrophysics Data System (ADS)

    Arredondo, K.; Billen, M. I.

    2010-12-01

    Traditionally studies of plate bending in subduction zones have utilized elastic, viscous or elastic-plastic rheologies to model the deformation of subducting plates, yet they are based on averaged plate properties and do not take into account variations in plate strength. Direct measurements of plate strength at subduction zones could permit more detailed models of how plates deform during subduction and may allow differentiation between the elastic and viscous or plastic rheologies. Additionally, weakening of the subducting plate is important for understanding the degree of coupling of the surface plate to the negative buoyancy of descending slabs. To obtain quantitative measurements of changes in plate strength along profiles parallel to the trench, we use analysis of the gravity-topography admittance in three subduction zones: Tonga, Costa Rica and Japan. We show that the plate flexural rigidity decreases near and inside the trench of the Tonga and Japan subduction zones, in agreement with previous results for the Kermadec subduction zone (1). Near the trench the flexural rigidity values are consistently smaller than those predicted by an elastic rheology and the plate age (2). This degree of weakening, by up to 3 orders magnitude, suggests that the plate does not act elastically as it is subducted, possibly due to lithospheric-scale weakening by extensional faulting and plastic yielding at depth. In contrast lithospheric-scale weakening in the Costa Rica subduction zone is less clear. This may be due to the younger age of the subducting plate and the small age difference between the seamounts and surrounding plate, which limits the sensitivity of the gravity field to changes in the non-isostatic support of topographic feature. These results suggest that this technique is only applicable to older plates with large seamounts that are appreciably younger than the subducting plate. Comparison of the flexural rigidity results to the tectonic characteristics of all

  11. Seismic Wave Attenuation Estimated from Tectonic Tremor and Radiated Energy in Tremor for Various Subduction Zones

    NASA Astrophysics Data System (ADS)

    Yabe, S.; Baltay, A.; Ide, S.; Beroza, G. C.

    2013-12-01

    Ground motion prediction is an essential component of earthquake hazard assessment. Seismic wave attenuation with distance is an important, yet difficult to constrain, factor for such estimation. Using the empirical method of ground motion prediction equations (GMPEs), seismic wave attenuation with distance, which includes both the effect of anelastic attenuation and scattering, can be estimated from the distance decay of peak ground velocity (PGV) or peak ground acceleration (PGA) of ordinary earthquakes; however, in some regions where plate-boundary earthquakes are infrequent, such as Cascadia and Nankai, there are fewer data with which to constrain the empirical parameters. In both of those subduction zones, tectonic tremor occurs often. In this study, we use tectonic tremor to estimate the seismic wave attenuation with distance, and in turn use the attenuation results to estimate the radiated seismic energy of tremor. Our primary interest is in the variations among subduction zones. Ground motion attenuation and the distribution of released seismic energy from tremors are two important subduction zone characteristics. Therefore, it is very interesting to see whether there are variations of these parameters in different subduction zones, or regionally within the same subduction zone. It is also useful to estimate how much energy is released by tectonic tremor from accumulated energy to help understand subduction dynamics and the difference between ordinary earthquakes and tremor. We use the tectonic tremor catalog of Ide (2012) in Nankai, Cascadia, Mexico and southern Chile. We measured PGV and PGA of individual tremor bursts at each station. We assume a simple GMPE relationship and estimate seismic attenuation and relative site amplification factors from the data. In the Nankai subduction zone, there are almost no earthquakes on the plate interface, but intra-slab earthquakes occur frequently. Both the seismic wave attenuation with distance and the site

  12. Mantle plume-subduction zone interactions over the past 60 Ma

    NASA Astrophysics Data System (ADS)

    Fletcher, Michael; Wyman, Derek A.

    2015-09-01

    A variety of mantle plume types have been proposed and there is a wide range of ways that these plumes might interact with subduction zone arcs. This study looks at the frequency of interaction between previously catalogued plumes and subduction zones while also assessing the potential role of slab windows to either generate false plume signals or contribute to genuine examples of mantle plume-subduction zone interactions. Of the plumes included in several widely cited catalogues, 29% have moved within 1000 km of a subduction zone and 17% have moved within 500 km of a subduction zone over the past 60 Ma, assuming that the plume life span extended over this period. Of the plumes that moved within 1000 km of a subduction zone, 56% are rated as either a deep or mid-mantle plume by an author of at least one of the catalogues. The 44% of interacting plumes that are not rated as mid-mantle or deep by at least one author are the most likely to be related to "top-down" plate tectonic processes. This study shows that they were never coincident with a slab window, although they have often interacted within distances of 1000 km. The manner of interaction between plumes and slab windows depends on the relative positions of the plume, ridge, and slab window. Of the plumes that interact over a 1000 km circular "Zone of Potential Interaction" (ZPI), 28% are no longer interacting today, but have survived that process. While most plumes interact in the form of a ZPI moving over a trench from either behind or in front of the trench, several plumes do cross the trench, demonstrating that plumes can survive even that interaction. Plume-trench interaction occurs in clusters in the northeast and southwest Pacific with limited events in the northwest and southeast Pacific. The presence of clusters in the northeast and southwest Pacific may be caused by the closer proximity of mid-mantle and deep plumes to a subduction zone in these areas. Whereas some deeper plumes may be modified by

  13. Guatemala jadeitites and albitites were formed by deuterium-rich serpentinizing fluids deep within a subduction zone

    USGS Publications Warehouse

    Johnson, C.A.; Harlow, G.E.

    1999-01-01

    Jadeitites and albitites from the Motagua Valley, Guatemala, are high-pressure-low-temperature metasomatic rocks that occur as tectonic inclusions in serpentinite-matrix melange. Metasomatism was driven by a fluid with a ??18OH2O value of 6???, and a ??DH2O value that is high in comparison with metamorphic fluids at other high-pressure-low-temperature localities of similar grade. We infer that the fluid was originally seawater that was entrained during subduction either as mineral-bound H2O or as free pore waters. The fluid drove serpentinization reactions in ultramafic rocks, possibly leading to deuterium enrichment of H2O, prior to forming the Jadeitites and albitites at a depth of 29 ?? 11 km. There are isotopic and fluid-inclusion similarities to rodingites, which are Ca-rich metasomatites found at other serpentinite localities. Our results suggest that the serpentinization process, whether it occurs within subduction zones or on the flanks of oceanic spreading ridges, may produce residual fluids that are H2O rich, have 1-8 wt% equivalent NaCl, and have high, perhaps sea water-like, ??D values.

  14. Elevation of volcanoes and their edifice heights at subduction zones

    SciTech Connect

    Ben-Avraham, Z.; Nur, A.

    1980-08-10

    The elevation above sea level of circum-Pacific volcanoes situated on continental crust varies greatly, not only between various chains but also within chains. Their edifice heights, however, are essentially constant with each chain. This pattern is reversed for oceanic volcanoes: The elevation circum-Pacific volcanoes situated on oceanic curst is constant within arcs, while edifice heights are greatly variable. In continents the depth to the root zones of volcanoes may be within the elastic part of the lithosphere, whereas in the oceans it may be well below the elastic part of the lithosphere. We suggest that melting, or the onset of the volcanic uprising, may be controlled in both cases primarily by pressure: in the continental lithosphere by the overburden pressure determined by depth below the local surface and in the oceanic lithosphere by the isostatically compensated pressure zone controlled by depth below sea level. The pattern seems to hold even in complex geological regions and may be used to identify the nature of the crust in such regions.

  15. Unrevealing the History of Earthquakes and Tsunamis of the Mexican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Ramirez-Herrera, M. T.; Castillo-Aja, M. D. R.; Cruz, S.; Corona, N.; Rangel Velarde, V.; Lagos, M.

    2014-12-01

    The great earthquakes and tsunamis of the last decades in Sumatra, Chile, and Japan remind us of the need for expanding the record of history of such catastrophic events. It can't be argued that even countries with extensive historical documents and tsunami sand deposits still have unsolved questions on the frequency of them, and the variables that control them along subduction zones. We present here preliminary results of a combined approach using historical archives and multiple proxies of the sedimentary record to unrevealing the history of possible great earthquakes and their tsunamis on the Mexican Subduction zone. The Mexican subduction zone extends over 1000 km long and little is known if the entire subduction zone along the Middle American Trench behaves as one enormous unit rather than in segments that rupture at different frequencies and with different strengths (as the short instrumental record shows). We searched on historical archives and earthquake databases to distinguish tsunamigenic events registered from the 16th century to now along the Jalisco-Colima and Guerrero-Oaxaca coastal stretches. The historical data referred are mostly from the 19th century on since the population on the coast was scarce before. We found 21 earthquakes with tsunamigenic potential, and of those 16 with doubtful to definitive accompanying tsunami on the Jalisco-Colima coast, and 31 tsunamigenic earthquakes on the Oaxaca-Guerrero coast. Evidence of great earthquakes and their tsunamis from the sedimentary record are scarce, perhaps due poor preservation of tsunami deposits in this tropical environment. Nevertheless, we have found evidence for a number of tsunamigenic events, both historical and prehistorical, 1932 and 1400 AD on Jalisco, and 3400 BP, 1789 AD, 1979 ad, and 1985 AD on Guerrero-Oaxaca. We continue working and a number of events are still to be dated. This work would aid in elucidating the history of earthquakes and tsunamis on the Mexican subduction zone.

  16. Characterizing Mega-Earthquake Related Tsunami on Subduction Zones without Large Historical Events

    NASA Astrophysics Data System (ADS)

    Williams, C. R.; Lee, R.; Astill, S.; Farahani, R.; Wilson, P. S.; Mohammed, F.

    2014-12-01

    Due to recent large tsunami events (e.g., Chile 2010 and Japan 2011), the insurance industry is very aware of the importance of managing its exposure to tsunami risk. There are currently few tools available to help establish policies for managing and pricing tsunami risk globally. As a starting point and to help address this issue, Risk Management Solutions Inc. (RMS) is developing a global suite of tsunami inundation footprints. This dataset will include both representations of historical events as well as a series of M9 scenarios on subductions zones that have not historical generated mega earthquakes. The latter set is included to address concerns about the completeness of the historical record for mega earthquakes. This concern stems from the fact that the Tohoku Japan earthquake was considerably larger than had been observed in the historical record. Characterizing the source and rupture pattern for the subduction zones without historical events is a poorly constrained process. In many case, the subduction zones can be segmented based on changes in the characteristics of the subducting slab or major ridge systems. For this project, the unit sources from the NOAA propagation database are utilized to leverage the basin wide modeling included in this dataset. The length of the rupture is characterized based on subduction zone segmentation and the slip per unit source can be determined based on the event magnitude (i.e., M9) and moment balancing. As these events have not occurred historically, there is little to constrain the slip distribution. Sensitivity tests on the potential rupture pattern have been undertaken comparing uniform slip to higher shallow slip and tapered slip models. Subduction zones examined include the Makran Trench, the Lesser Antilles and the Hikurangi Trench. The ultimate goal is to create a series of tsunami footprints to help insurers understand their exposures at risk to tsunami inundation around the world.

  17. Metamorphic sole formation, emplacement and blueschist overprint: early obduction dynamics witnessed by W. Turkey ophiolites

    NASA Astrophysics Data System (ADS)

    Plunder, Alexis; Agard, Philippe; Chopin, Christian; Soret, Mathieu; Okay, Aral; Whitechurch, Hubert

    2016-04-01

    Western Turkey, with a >200 km long-belt of unmetamorphosed ophiolite overlying continental lithosphere is one or even the largest obducted ophiolite on Earth and therefore a key example to study obduction and early subduction dynamics. All Western Turkish ophiolite fragments are considered as part of the same Neotethyan branch resulting of a long-lived continental subduction (or underthrusting). Synchronous (ca. ~ 93 Ma) metamorphic sole formation and preservation at the base of most of the Turkish ophiolite fragments support this single event and place a strong constraint on the age of subduction initiation. Metamorphic soles are indeed generally considered to have formed during the early and hot subduction zone at 25 ± 10 km depths and welded to the overriding oceanic lithosphere. In Western Turkey however (as for most places worldwide) a systematic study of the pressure-temperature conditions with modern thermobarometric tools is generally lacking, and fundamental mechanisms of formation or accretion to the upper plate are poorly (if at all) constrained. We herein reappraise Western Turkish metamorphic soles focusing on the following points and issues: (i) detailed structures of metamorphic sole and other subduction derived units, petrological evolution and refined pressure-temperature conditions; peak pressure-temperature conditions of metamorphic sole were estimated using garnet, clinopyroxene, amphibole and plagioclase as the peak paragenesis at 10.5 ± 2 kbar and 800 ± 50°C based on pseudosections using the Theriak/Domino package (ii) the rather unique (and enigmatic) blueschist facies overprint found in places was investigated in terms of structural position and pressure-temperature conditions. Conditions of overprint were estimated around 12 kbar and 425 °C from the presence of glaucophane, lawsonite, jadeite and garnet overgrowing the amphibolite-facies assemblage. This field-based study provides clues to mechanisms of metamorphic sole underplating

  18. Turbidite event history--Methods and implications for Holocene paleoseismicity of the Cascadia subduction zone

    USGS Publications Warehouse

    Goldfinger, Chris; Nelson, C. Hans; Morey, Ann E.; Johnson, Joel E.; Patton, Jason R.; Karabanov, Eugene; Gutierrez-Pastor, Julia; Eriksson, Andrew T.; Gracia, Eulalia; Dunhill, Gita; Enkin, Randolph J.; Dallimore, Audrey; Vallier, Tracy; Kayen, Robert, (Edited By)

    2012-01-01

    Turbidite systems along the continental margin of Cascadia Basin from Vancouver Island, Canada, to Cape Mendocino, California, United States, have been investigated with swath bathymetry; newly collected and archive piston, gravity, kasten, and box cores; and accelerator mass spectrometry radiocarbon dates. The purpose of this study is to test the applicability of the Holocene turbidite record as a paleoseismic record for the Cascadia subduction zone. The Cascadia Basin is an ideal place to develop a turbidite paleoseismologic method and to record paleoearthquakes because (1) a single subduction-zone fault underlies the Cascadia submarine-canyon systems; (2) multiple tributary canyons and a variety of turbidite systems and sedimentary sources exist to use in tests of synchronous turbidite triggering; (3) the Cascadia trench is completely sediment filled, allowing channel systems to trend seaward across the abyssal plain, rather than merging in the trench; (4) the continental shelf is wide, favoring disconnection of Holocene river systems from their largely Pleistocene canyons; and (5) excellent stratigraphic datums, including the Mazama ash and distinguishable sedimentological and faunal changes near the Pleistocene-Holocene boundary, are present for correlating events and anchoring the temporal framework. Multiple tributaries to Cascadia Channel with 50- to 150-km spacing, and a wide variety of other turbidite systems with different sedimentary sources contain 13 post-Mazama-ash and 19 Holocene turbidites. Likely correlative sequences are found in Cascadia Channel, Juan de Fuca Channel off Washington, and Hydrate Ridge slope basin and Astoria Fan off northern and central Oregon. A probable correlative sequence of turbidites is also found in cores on Rogue Apron off southern Oregon. The Hydrate Ridge and Rogue Apron cores also include 12-22 interspersed thinner turbidite beds respectively. We use 14C dates, relative-dating tests at channel confluences, and

  19. Finite element model predictions of static deformation from dislocation sources in a subduction zone: Sensitivities to homogeneous, isotropic, Poisson-solid, and half-space assumptions

    USGS Publications Warehouse

    Masterlark, Timothy

    2003-01-01

    Dislocation models can simulate static deformation caused by slip along a fault. These models usually take the form of a dislocation embedded in a homogeneous, isotropic, Poisson-solid half-space (HIPSHS). However, the widely accepted HIPSHS assumptions poorly approximate subduction zone systems of converging oceanic and continental crust. This study uses three-dimensional finite element models (FEMs) that allow for any combination (including none) of the HIPSHS assumptions to compute synthetic Green's functions for displacement. Using the 1995 Mw = 8.0 Jalisco-Colima, Mexico, subduction zone earthquake and associated measurements from a nearby GPS array as an example, FEM-generated synthetic Green's functions are combined with standard linear inverse methods to estimate dislocation distributions along the subduction interface. Loading a forward HIPSHS model with dislocation distributions, estimated from FEMs that sequentially relax the HIPSHS assumptions, yields the sensitivity of predicted displacements to each of the HIPSHS assumptions. For the subduction zone models tested and the specific field situation considered, sensitivities to the individual Poisson-solid, isotropy, and homogeneity assumptions can be substantially greater than GPS. measurement uncertainties. Forward modeling quantifies stress coupling between the Mw = 8.0 earthquake and a nearby Mw = 6.3 earthquake that occurred 63 days later. Coulomb stress changes predicted from static HIPSHS models cannot account for the 63-day lag time between events. Alternatively, an FEM that includes a poroelastic oceanic crust, which allows for postseismic pore fluid pressure recovery, can account for the lag time. The pore fluid pressure recovery rate puts an upper limit of 10-17 m2 on the bulk permeability of the oceanic crust. Copyright 2003 by the American Geophysical Union.

  20. High Resolution Thermal Model and Heat Flow along the Washington Margin of the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Salmi, M.; Harris, R. N.; Johnson, H. P.; Solomon, E. A.

    2015-12-01

    Understanding the temperature distribution along an active subducting plate interface improves our understanding of subduction zone dynamics and seismic hazards. The temperature dependence of the locked zone is an important tool in identifying the region of stress accumulation along the subducting plate. The temperature at the up-dip limit of the seismic zone typically ranges from 100-150°C and the down-dip limit is a transition zone between 350°C and 450°C. In July 2012, Multi-Channel Seismic (MCS) data was collected using the R/V Langseth along nine profiles perpendicular to the accretionary wedge offshore Grays Harbor, Washington. The MCS lines extend from seaward of the deformation front to the continental shelf. In August 2013, we made seafloor heat flow measurements using a violin bow probe, thermal blankets and the Jason heat flow probe. These data show mean heat flow values of 110 mW/m2 over the incoming plate, 30 mW/m2 at the first deformation ridge, and mean of 100 mW/m2 over the lower accretionary wedge terrace. These measurements were co-located with two MCS profiles allowing for direct comparison with Bottom Simulating Reflectors (BSRs) that provide heat flow along all MCS lines from the deformation front to the methane hydrate stability depth at roughly 500 m. BSR-derived heat flow decreases from 90 mW/m2 at the deformation front to 60 mW/m2 beyond 60 km landward of the deformation front lower than consistent with our heat flow measurements, implying active upward diffuse fluid flow. Seismic velocities from MCS data provide an estimate of porosity and thermal conductivity of the underlying sediments providing the thermal parameters for a 2D model. Local but substantial heat flow anomalies likely reflect advective heat transfer within the shallow portion of the accretionary wedge. Preliminary modeling results indicate an incoming oceanic plate temperature of 215°C, potentially placing the up-dip limit of the seismogenic zone at the deformation

  1. High-resolution seismic imaging of the western Hellenic subduction zone using teleseismic scattered waves

    NASA Astrophysics Data System (ADS)

    Suckale, J.; Rondenay, S.; Sachpazi, M.; Charalampakis, M.; Hosa, A.; Royden, L. H.

    2009-08-01

    The active Hellenic subduction system has long been considered an ideal setting for studying subduction dynamics because it is easily accessible and of limited spatial extent. It has been the focus of numerous seismological studies over the last few decades but, nonetheless, the detailed structure of both the slab and the surrounding mantle remain poorly constrained in an intermediate depth range from 30 to 150 km. The objective of this paper is to fill this gap. The intermediate depth regime is of particular interest because it is pivotal for improving our understanding of the dynamic interaction between subducting lithosphere and the surrounding mantle. An interdisciplinary effort aimed at addressing this challenge is currently undertaken by the `Multidisciplinary Experiments for Dynamic Understanding of Subduction under the Aegean Sea' (MEDUSA) project. As part of the MEDUSA initiative, a temporary array consisting of 40 densely spaced broad-band seismometers from the IRIS-PASSCAL pool has been deployed in southern Greece. We process the teleseismic data recorded by this array with a migration algorithm based on the generalized radon transform to obtain high-resolution images of the subduction zone in 2-D. The images reveal a sharp Mohorovičić discontinuity (Moho) at depths ranging from 30 km beneath the western margin of the Aegean Sea to 40 km beneath the central Peloponnesus, where it outlines the crustal root of the Hellenides. To the west of the Hellenides, the continental Moho is not identified, but we interpret a pronounced discontinuity imaged at ~20 km depth as the contact between low-velocity sediments and high-velocity crystalline basement. The images also show the subducted oceanic crust as a low-velocity layer that plunges at a constant angle of 21° from west to east. The oceanic crust exhibits low velocities to at least 90 km depth, indicating that the bulk of fluid transfer from the subducted slab into the mantle wedge occurs below this depth

  2. Imaging the Locked Zone of the Cascadia Subduction Zone Using Receiver Functions from the Cascadia Initiative

    NASA Astrophysics Data System (ADS)

    Janiszewski, H. A.; Abers, G. A.; Gaherty, J. B.; Carton, H. D.

    2014-12-01

    The Cascadia subduction zone is a hot end-member system that is characterized by the subduction of young, thickly sedimented lithosphere. Previous receiver function studies have observed a low velocity zone (LVZ) with strong contrasts along the thrust up to 40 km depth. It is hypothesized that this may be created by a channel of either near-lithostatic pressure fluids or stronger metasediments, implying a weak thrust zone. These studies have been limited to data from onshore stations, and thus have not imaged the shallower, geodetically locked portion of the thrust zone, which is located offshore. The ocean bottom seismometers (OBS) from the Cascadia Initiative (CI), which are among the first broadband instruments successfully deployed in shallow water using low-profile Trawl-Resistant-Mounts (TRM), offer the opportunity to extend receiver function studies of the LVZ offshore. Calculation of receiver functions from OBS data is difficult due to water column noise. Fortunately, the TRM housing yields quieter horizontal-component signals, and with proper application of tilt and compliance corrections receiver functions are calculated at all of the successfully deployed TRM OBS from CI Year 1, as well as at some deep water stations. We use velocity models from the previous onshore receiver function studies to generate synthetic receiver functions to compare with our data. Several of the stations on the continental margin have consistent arrivals at 3-4 s lag that match predicted depths for the subduction interface. The shallow-water stations deployed off the coast of Grays Harbor, Washington record a high-amplitude asymmetric arrival consistent with reverberations off the top and bottom of the LVZ. This high-amplitude arrival is not as evident at other stations along the margin region. This along strike variation may be evidence for segmentation along the thrust zone; however, a careful analysis of these complex signals will be needed to determine the extent of the LVZ

  3. FAST TRACK PAPER: Mantle flow in the Rivera-Cocos subduction zone

    NASA Astrophysics Data System (ADS)

    Soto, Gerardo León; Ni, James F.; Grand, Stephen P.; Sandvol, Eric; Valenzuela, Raúl W.; Speziale, Marco Guzmán; González, Juan M. Gómez; Reyes, Tonatiuh Domínguez

    2009-11-01

    Western Mexico, where the young and small Rivera Plate and the adjacent large Cocos Plate are subducting beneath the North American Plate, is a unique region on Earth where tearing of subducting oceanic plates, as well as fragmentation of the overriding continental plate, is occurring today. Characterizing the mantle flow field that accompanies the subduction of the Rivera and adjacent Cocos plates can help to clarify the tectonics and magma genesis of this young plate boundary. Here we report observations of seismic anisotropy, as manifested by shear wave splitting derived from local S and teleseismic SKS data collected by the Mapping Rivera Subduction zone array that was deployed from 2006 January to 2007 June, in southwestern Mexico, and from data collected by two of Mexico's Servicio Sismológico Nacional stations. SKS and local S-wave splitting parameters indicate that the fast directions of the split SKS waves for stations that lie on the central and southern Jalisco Block are approximately trench-normal, following the convergence direction between the Rivera Plate and Jalisco Block. S-wave splitting from slab events show a small averaged delay time of ~0.2 s for the upper 60 km of the crust and mantle. Therefore, the main source of anisotropy must reside in the entrained mantle below the young and thin Rivera Plate. Trench-oblique fast SKS split directions are observed in the western edge of the Rivera Plate and the western parts of the Cocos slab. The curved pattern of fast SKS split directions in the western Jalisco block and beneath the Rivera-Cocos slab gap indicates 3-D toroidal mantle flow, around the northwestern edge of the Rivera slab and the Rivera-Cocos gap, which profoundly affect the finite strain field in the northwestern edge of the Rivera slab and the mantle wedge. Both the tomographic images and shear wave splitting results support the idea that the Rivera and western Cocos plates not only moved in a downdip direction but also have recently

  4. Cenozoic volcanic rocks of North Kamchatka: In search of subduction zones

    NASA Astrophysics Data System (ADS)

    Shapiro, M. N.; Solov'ev, A. V.

    2011-05-01

    Two belts of subaerial volcanic rocks—the Eocene Kinkil belt and the Neogene belt of the Sredinny Range—extend along the Kamchatka Isthmus. It is suggested that their formation is related to subduction of the oceanic lithosphere beneath the continental margin of North Kamchatka. The oceanic lithosphere consumed in the subduction zones could have been formed as a result of active spreading in the Komandorsky Basin. In the simplest case, both spreading and subduction reflect the northwestward motion of the lithosphere of the Komandorsky Plate relative to Kamchatka, the Shirshov Ridge, and the Aleutian Basin combined into one relatively immobile plate conventionally called the North American Plate. The authors perform a simulation of conjugate spreading and subduction. The most important parameter determining the regional geodynamics—the velocity of the Komandorsky Plate moving relative to the North American Plate—is taken as 2.5, 5.0, and 7.5 cm/yr. The calculated ages of the onset and end of volcanic activity in the aforementioned belts are compared with the dates obtained with the isotopic and paleontological methods. For the Eocene Kinkil belt, where volcanism started 44 Ma ago, the model age of the onset of subduction depends on the accepted velocity of the motion of the Komandorsky Plate and varies from 54 Ma at the velocity of 2.5 cm/yr to 47.5 Ma at the velocity of 7.5 cm/yr. It can be assumed that the model of fast subduction in this age interval is most consistent with the geological data. For the Miocene-Pliocene belt of the Sredinny Range, assuming the velocity of the motion of the Komandorsky Plate at 5.0 and 7.5 cm/yr, multiple rifting at the boundary with the Shirshov Ridge should be assumed. Therefore, for the end of the Neogene, a model with low velocity (2.5-5.0 cm/yr, i.e., about 4.0 cm/yr) is preferable.

  5. Subduction Zone Concepts and the 2010 Chile Earthqake (Arthur Holmes Medal Lecture)

    NASA Astrophysics Data System (ADS)

    von Huene, Roland

    2010-05-01

    Knowledge of convergent margin systems evolved from hypothesis testing with marine geophysical technology that improved over decades. Wegener's drift hypothesis, Holmes mantle convection, and marine magnetic anomaly patterns were integrated into an ocean spreading concept that won wide acceptance after ocean drilling confirmed the crustal younging trend toward the Mid-Atlantic ridge. In contrast, the necessary disposal of oceanic and trench sediment at convergent margins remained largely hypothetical. Fresh interpretations of some coastal mountains as exposing ancient convergent margin rock assemblages and the seismologist's "Wadati-Benioff" zone were combined into a widely-accepted hypothesis. A convergent margin upper plate was pictured as an imbricate fan of ocean sediment thrust slices detached from the lower plate. During the 1980s ocean drilling to test the hypothesis revealed what then were counter-intuitive processes of sediment subduction and subduction erosion. Rather than the proposed seaward growth by accretion, many margins had lost material from erosion. In current concepts, individual margins are shaped by the net consequences of subduction accretion, sediment subduction, and subduction erosion. Similarly, recently acquired age data from ancient subduction complexes reveal periods dominated by accretion separated by periods dominated by tectonic erosion. Globally, the recycling of continental crustal material at subduction zones appears largely balanced by magmatic addition at volcanic arcs. The longevity of the original imbricate fan model in text books confirms its pictorial simplicity, because geophysical images and drill core evidence show that it commonly applies to only a relatively small frontal prism. A better understanding of convergent margin dynamics is of urgent societal importance as coastal populations increase rapidly and as recent disastrous earthquakes and tsunamis verify. The shift in convergent margin concepts has developed through

  6. Deformation of Lawsonite at High Pressure and High Temperature - Implications for Low Velocity Layers in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Amiguet, E.; Hilairet, N.; Wang, Y.; Gillet, P.

    2014-12-01

    During subduction, the hydrated oceanic crust undergoes a series of metamorphic reactions and transform gradually to blueschists and eclogite at depths of 20-50 km. Detailed seismic observations of subduction zones suggest a complex layered structure with the presence of a Low Velocity Layer (LVL) related to the oceanic crust [1] persisting to considerable depths (100- 250 km).While the transformation from blueschist to eclogite [2] and the presence of glaucophane up to 90-100 km [3] could explain some of these observations, the presence of LVL at greater depths could be related to the presence of the hydrous mineral lawsonite (CaAl2(Si2O7)(OH)2 H2O). Its stability field extends to 8.5 GPa and 1100K corresponding to depths up to 250 km in cold hydrous part of subducting slabs [4]. Because these regions undergo large and heterogeneous deformation, lawsonite plasticity and crystal preferred orientation (CPOs) may strongly influence the dynamic of subduction zones and the seismic properties. We present a deformation study at high presssure and high temperature on lawsonite. Six samples were deformed at 4-10 GPa and 600K to 1000K using a D-DIA apparatus [5] at 13-BMD at GSECARS beamline, APS, in axial compression up to 30% deformation with strain rates of 3.10-4s-1 to 6.10-6s-1. We measured in-situ lattice strains (a proxy for macroscopic stress), texture and strain using synchrotron radiations and calculated the macroscopic stress using lawsonite elastic properties [6]. Results from lattice strain analysis show a dependence of flow stress with temperature and strain rate. Texture analysis coupled with transmission electron microscopy showed that dislocation creep is the dominant deformation mechanism under our deformation conditions. [1] Abers, Earth and Planetary Science Letters, 176, 323-330, 2000 [2] Helffrich et al., Journal of Geophysical Research, 94, 753-763, 1989 [3] Bezacier et al., Tectonophysics, 494, 201-210, 2010 [4] Schmidt & Poli, Earth and Planetary

  7. Strength of megathrust faults and its control on subduction-zone seismotectonics

    NASA Astrophysics Data System (ADS)

    Tassara, A.; Hackney, R.

    2009-04-01

    Predicting where devastating subduction-zone earthquakes could occur requires identification of key fault parameters from geophysical observations. Global and regional-scale comparisons of subduction-zone seismicity against gravity and bathymetry anomalies show that a large percentage of the moment released by co-seismic slip during megathrust earthquakes concentrates over regions characterized by gravity and bathymetry lows. This has been interpreted after assuming that high interplate friction implies high mechanical coupling between the subducting slab and the overriding forearc and, therefore, a depressed forearc topography leading to low gravity anomalies. This interpretation supports the accepted paradigm as to that seismic asperities (areas of large co-seismic slip) are strong patches of the plate interface characterized by high friction. Attempting to test this idea from a physically compelling perspective, we apply a wavelet formulation of the classical spectral isostatic analysis to invert grids of gravity anomalies and bathymetry/topography into maps of flexural rigidity for several subduction zones worldwide. Flexural rigidity explicitly links those surface observables with the integrated mechanical strength of the lithosphere. For the special case of subduction zones where two tectonic plates are in contact along a low-angle fault, it can be argued that local- to regional-scale lateral variations of flexural rigidity are mostly due to spatial changes of the shear strength along the interplate fault caused by the physical conditions of the subduction channel, which supersede large-scale variations of thermo-mechanical properties of both converging plates. In our interpretation, high/low flexural rigidity along subduction zones means high/low shear stress supported by the megathrust fault and, therefore, high/low friction and/or low/high pore pressure along the subduction channel. We use the gravity model EIGEN-GL04C, which combine data from satellite

  8. Probing the Detailed Seismic Velocity Structure of Subduction Zones Using Advanced Seismic Tomography Methods

    NASA Astrophysics Data System (ADS)

    Zhang, H.; Thurber, C. H.

    2005-12-01

    Subduction zones are one of the most important components of the Earth's plate tectonic system. Knowing the detailed seismic velocity structure within and around subducting slabs is vital to understand the constitution of the slab, the cause of intermediate depth earthquakes inside the slab, the fluid distribution and recycling, and tremor occurrence [Hacker et al., 2001; Obara, 2002].Thanks to the ability of double-difference tomography [Zhang and Thurber, 2003] to resolve the fine-scale structure near the source region and the favorable seismicity distribution inside many subducting slabs, it is now possible to characterize the fine details of the velocity structure and earthquake locations inside the slab, as shown in the study of the Japan subduction zone [Zhang et al., 2004]. We further develop the double-difference tomography method in two aspects: the first improvement is to use an adaptive inversion mesh rather than a regular inversion grid and the second improvement is to determine a reliable Vp/Vs structure using various strategies rather than directly from Vp and Vs [see our abstract ``Strategies to solve for a better Vp/Vs model using P and S arrival time'' at Session T29]. The adaptive mesh seismic tomography method is based on tetrahedral diagrams and can automatically adjust the inversion mesh according to the ray distribution so that the inversion mesh nodes are denser where there are more rays and vice versa [Zhang and Thurber, 2005]. As a result, the number of inversion mesh nodes is greatly reduced compared to a regular inversion grid with comparable spatial resolution, and the tomographic system is more stable and better conditioned. This improvement is quite valuable for characterizing the fine structure of the subduction zone considering the highly uneven distribution of earthquakes within and around the subducting slab. The second improvement, to determine a reliable Vp/Vs model, lies in jointly inverting Vp, Vs, and Vp/Vs using P, S, and S

  9. Geometry and thermal structure of the Menderes Massif Core Complex (Western Turkey), implications for thermal evolution of Hellenic subduction zone

    NASA Astrophysics Data System (ADS)

    Roche, Vincent; Jolivet, Laurent; Guillou-Frottier, Laurent; Tuduri, Johann; Bouchot, Vincent; Beccaletto, Laurent; Lahfid, Abdeltif

    2016-04-01

    The eastern Mediterranean region is one of the most promising geothermal areas, with more than 250 geothermal fields discovered in Turkey (Parlaktuna, 2013), in a region of active tectonics and volcanism. Although the potential of these deep geothermal resources has not been systematically investigated yet, the geothermal activity of the western Turkey area is the most recent signature of the high heat flow (120-140 mW/m²; Aydin, 2005, from Teczan, 1995). Based on Turkish data, 2084 MWt are being utilized for direct applications and most of the energy originates from the Menderes Massif (Baba et al., 2015). This large-scale thermal anomaly at the surface is correlated to a long wavelength east-west increase of surface heat flow that could reflect the thermal state of Aegean subduction zone at depth. In order to better understand and characterize the possible connections between large-scale mantle dynamics and surface processes in space and time, we study the structure and thermal evolution of the Menderes Massif. Both the acceleration of the Aegean extension in the Middle Miocene and the recent escape of Anatolia have been proposed to result from several slab tearing events, the first one being located below western Turkey and the Eastern Aegean Sea. These events have triggered the formation of metamorphic complexes with contrasted exhumation P-T paths. While the extension in the Aegean domain is well-characterized with high-temperature domes in the center and east, the succession of several metamorphic events in the Menderes Massif and their significance in terms of geodynamics is still debated. Hence, the exhumation history is key to understanding the temporal and spatial distribution of the thermal signature of the Hellenic slab and its tearing/detachment. The Menderes Massif displays a large variety of metamorphic facies, from the Barrovian type metamorphism in the Eocene (the Main Menderes Metamorphism) to the coeval (?) HP-LT metamorphism on the southernmost

  10. Middle Cambrian to Late Ordovician evolution of the Appalachian margin: Foundering of a passive margin to form a subduction zone and volcanic arc

    SciTech Connect

    Washington, P.A. , Southern Pines, NC )

    1994-03-01

    From late Middle Cambrian to early Late Ordovician time, the Appalachian passive margin experienced a series of orogenic events culminating in the Taconic orogeny. Most of these events are generally viewed as enigmatic and isolated, but they can be viewed as a coherent tectonic sequence of events. The early stages involved broad uplifts and localized extension, especially of internal shelf and adjacent continental interiors. Later stages involved increased subsidence rates of the outer shelf, resulting in retreat of the outer margin of the carbonate platform.The beginning of volcanic activity coincides with, or immediately follows, the rapid subsidence. Onset of compressional orogenesis is often temporally separated from the initial rapid subsidence. These events can be integrated into a tectonic model in which the passive margin is converted into an active Andean margin. Early uplift and extension events represented the surface expression of the beginning of deep-seated downward mantle convection. Subsequent rapid subsidence events represented the mechanical failure of the lithosphere as the convection reaches maturity. Failure of the lithosphere resulted in a subduction zone that quickly created arc volcanism. The compressive Taconic orogenesis occurred when the arc was thrust back onto the shelf margin as the subduction zone migrated continentward in response to progressively channeled convective flow.