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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 indicates retention of seafloor N signatures and, in some cases, enrichments in sedimentary N due to forearc metamorphic fluid-rock interactions (Halama et al., this session). A global estimate of C cycling, using seafloor inputs (carbonate and organic matter) and estimates of volcanic CO2 outputs, indicates ~40% return (with large uncertainty) of the subducting C in volcanic gases. This imbalance appears plausible, given the evidence for deep carbonate subduction, in UHP marbles, and the preservation of graphite in UHP metasediments, together seemingly indicating that large fractions of subducting C survive forearc-to-subarc metamorphism. Estimates of return efficiency in the Central America arc, based on data for volcanic gases, are lower and variable along strike (12-29%), quite reasonably explained by de Leeuw et al. (2007, EPSL) as resulting from incomplete decarbonation of subducting sediment and AOC, fluid flow patterns expected given sediment section thickness, and varying degrees of forearc underplating. The attempts to mass-balance C and N across individual arc-trench systems demonstrate valuable integration of information from geophysical, field, petrologic, and geochemical observations. Studies of subduction-zone metamorphic suites can yield constraints on the evolution of deeply subducting rocks and the physicochemical characteristics of fluids released in forearcs and contributing to return flux in arc volcanic gases.

  3. Permo-Triassic Collision, Subduction-Zone Metamorphism, and Tectonic Exhumation Along the East Asian Continental Margin

    NASA Astrophysics Data System (ADS)

    Ernst, W. G.; Tsujimori, Tatsuki; Zhang, Ruth; Liou, J. G.

    2007-05-01

    Convergent plate motion at 320-210 Ma generated the Tongbai-Dabie-Sulu (east-central China)-Imjingang-Gyeonggi (central Korea)-Renge-Suo (Southwestern Japan)-Sikhote-Alin orogen along the paleo-Pacific edge of cratonal Asia. This amalgamated belt reflects collision between the Sino-Korean and Yangtze cratons on the SW portion, and accretion of outboard oceanic arcs ± sialic fragments against the NE margin. Subducted Proterozoic-Paleozoic continental and oceanic crustal complexes underwent high- and ultrahigh-pressure metamorphism at low to moderate temperatures. Tectonic slices of sialic crust episodically disengaged from the downgoing plate and, driven by buoyancy, ascended rapidly to midcrustal levels from depths exceeding 90-200 km after continental collision in east-central China plus or minus Korea, and from 30-50 km after arrival of far-traveled oceanic terranes in SW Japan and the Russian Far East. On achieving neutral buoyancy and stalling out at 10-20 km depth, later doming, gravitational collapse, and erosion exposed parts of the high- and ultrahigh-pressure complexes. This curvilinear orogen has been segmented and offset by major and minor transverse faults. Also, regional backarc spreading opened marginal basins behind the Permo-Triassic convergent suture zone, further disturbing portions oceanward.

  4. 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-earthquake-pore pressure relief could conceivably produce a record of episodic HP/LT metamorphism driven by rapid pressure pulses. A new hypothesis is presented for the origins of HP/LT metamorphism: that HP/LT metamorphism is driven by effective pressure pulses caused by localized, earthquake-related modifications to fluid pressures in the subducted slab. In other words, HP/LT metamorphism marks abrupt changes in stress state within the subducted slab, driven by earthquake rupture and fluid flow, and involving a rapid return toward lithostatic pressure from effective pressures well below lithostatic. References: 1. Bjørnerud, MG, Austrheim, H & Lund, MG, 2002. Processes leading to eclogitization (densification) of subducted and tectonically buried crust. Journal of Geophysical Research 107, 2252. 2. Camacho, A, Lee, JKW, Hensen, BJ & Braun, J, 2005. Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids. Nature 435, 1191-1196. 3. Green, HW & Houston, H, 1995. The mechanics of deep earthquakes. Annual Reviews of Earth and Planetary Sciences 23, 169-213. 4. Hacker, BR, Peacock, SM, Abers, GA & Holloway, SD, 2003. Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?. Journal of Geophysical Research 108, 2030.

  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 redistribution into newly formed minerals suggests that subduction-zone dehydration metamorphism will not contribute to the enrichment of these elements in arc magmatism. These observations require the formation and contribution of supercritical fluids or hydrous melts (these can effectively transport the aforementioned incompatible elements) from greater depths to arc magmatism. In addition, the overall sub-chondritic Nb/Ta ratio retained in rutile-bearing eclogites indicates that the subducting/subducted residual ocean crust passing through SZM cannot be responsible for the missing Nb (relative to Ta) in the bulk silicate earth.

  9. 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

  10. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2005-09-01

    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 (Fo 91-92), enstatite (En 91-92), Ti-clinohumite, diopside, phlogopite and apatite. The enstatite inclusions have low Al 2O 3 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.

  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. 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 magmas that otherwise do not show strong evidence for crustal input and imply that crustal recycling rates in some arcs may be higher than hitherto realized.

  14. 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 at 100 km depth. This situation might explain the flat Wadati-Benioff zone of Central Chile.

  15. 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 preliminary results show that the introduction of a 3D oceanic plate geometry causes along-margin variations in slab surface and inslab temperatures, with differences as much as 50C from 2D models. The differences increase as the along-margin transition in slab geometry becomes more abrupt. Future models will explore a non-Newtonian, temperature-dependent mantle wedge rheology and will include a three-dimensional oceanic geotherm that fits the offshore surface heat flow for the MASZ. The combined effect of a 3D oceanic geotherm and temperature-dependent flow in the wedge is expected to have a strong impact on metamorphic reactions within the oceanic plate, such as the transformation of oceanic basalt into eclogite, as well as the width of megathrust seismogenic zone.

  16. Record in Metamorphic Tourmalines of Subduction-Zone Devolatilization and Boron Cycling

    NASA Astrophysics Data System (ADS)

    Bebout, G. E.; Nakamura, E.

    2001-12-01

    Boron partitioning among fluids, tourmaline, and micas may dictate the extent and location, and isotopic composition, of B loss in subducting slabs, but the scarcity of information regarding mineral-fluid fractionation has limited efforts to understand deep B cycling. To clarify tourmaline's role in B redistribution during devolatilization of subducting sediments, we have analyzed tourmaline δ 11B in HP (Catalina, Sambagawa; peak <1.2 GPa) and UHP (Lago di Cignana; peak 2.7-2.9 GPa) metasedimentary rocks employing high-precision (2σ mean = 0.5-0.8‰ ) ion microprobe methods. Sambagawa metasediments show relatively uniform whole-rock B across grade, but an increasing proportion of B in tourmaline and a decreasing proportion in micas at higher grades. In contrast, Catalina Schist metasediments contain less tourmaline, and at higher grades show up to 75 percent decrease in whole-rock B with correlated decreased mica B. In both suites, some higher-grade (~epidote-amphibolite) rocks contain abundant, zoned, dravitic tourmaline. Tourmaline cores have lower Mg/(Mg+Fe+2) and Ca/(Ca+Na) and higher δ 11BSRM951 (up to +1.6‰ , mostly -6 to -2‰ ), and tourmalines show increased Mg/(Mg+Fe+2) and Ca/(Ca+Na) and decreased δ 11B (approaching -15‰ ) toward rims. These variations are consistent with significant prograde tourmaline growth. Some grains have thin outermost zones compositionally similar to cores and attributed to retrogradation. The δ 11B of less-abundant tourmaline in lower-grade rocks (-7 to +4.5‰ ) overlaps with, or is higher than, that of tourmaline cores in higher-grade rocks, and some cores in low- and high-grade rocks are regarded as detrital. For tourmaline in Lago di Cignana metasediments, we associate cores with garnet and rutile inclusions, Mg/(Mg+Fe+2) up to 0.95, and δ 11B as low as -16‰ with prograde, high-P/T metamorphism, and rims with clinozoisite and quartz inclusions, lower Mg/(Mg+Fe+2), and higher δ 11B up to +4.3‰ with overprinting during exhumation. In the absence of tourmaline, devolatilization can reduce whole-rock B (Catalina), whereas tourmaline growth may retain whole-rock B lost from micas and minimize change in whole-rock \\delta11B (Sambagawa). At Lago di Cignana, tourmaline (with \\delta11B = -16 to -9\\permil somewhat lower than that of the initially subducted structurally bound B component in sediment) was stable to depths of ~90 km. Based on our results, and results of other recent studies of B isotope fractionation, it appears that B lost during devolatilization of subducting sedimentary rocks (and mica-bearing altered oceanic crust) is isotopically heavier than residual B in micas in the devolatilizing rocks. This heavier B could be sequestered by tourmaline (particularly in sediments), in which it could be subducted to greater depths, or could be released into and mobilized in metamorphic fluids. Fluids containing B released from micas in subducting, progressively devolatilizing sediment and altered oceanic crust appear capable of imparting relatively high-\\delta11B signatures in arc lavas and producing across-arc trends of decreasing \\delta11B observed in several arcs.

  17. 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

    The Western Gneiss Region (WGR) of Norway lies within the Caledonian collision zone, which was subjected to the eclogites-facies metamorphism during the Silurian-Devonian continental collision of Baltica and Laurentia. Exhumed from depths of 80-200 km, the peridotites enclosed in Proterozoic gneisses in the WGR can be divided into two groups: garnet-free Archean dunites with extremely depleted compositions, and garnet-bearing peridotites that resulted from refertilization of the dunites during the Proterozoic time (Beyer et al., 2006). Therefore peridotites from the WGR provide a unique window to investigate petrofabrics of the upper mantle with different compositions and deformation history. Using the electron backscatter diffraction (EBSD) technique, we measured lattice-preferred orientations of 10 peridotite samples from the WGR. The water contents of olivine were determined by Fourier transform infrared spectroscopy in Patersons calibration (1982). For dunites from Almklovdalen in the southern WGR, one sample with water content of 0.5 ppm H2O displays the [100](010) (the A-type) olivine fabric, while the other two samples with water contents of 5-7 ppm H2O developed [001](010) (the B-type) fabric. In contrast, garnet lherzolites and wehrlites with various water content of 0.2-7 ppm H2O from Almklovdalen and Gursky yield the B-type fabric. The [001](100) (the C-type) fabric of olivine was observed in garnet harzburgite with very low water content (1.4 ppm H2O) from Otry in the northern WGR, where coesite, majoritic garnet and microdiamond were found by previous studies. Compared with the fabric diagram of olivine proposed by Jung and Karato (2001), all 10 peridotite samples came from dry conditions, under which the A-type or the D-type fabric is expected at low or high stress, respectively. Clearly, our results indicate that water is not the key to producing the different fabrics in the WGR peridotites, and the Proterozoic refertilization did not 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.

  18. 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 variations in pore pressure, porosity, and internal friction coefficient affect predictions of basal shear stress, taper angle, and internal slip surface geometry. My results suggest that assuming average porosity throughout the prism may be a good approximation in many cases, but assuming an average value for the pore pressure can cause significant errors. This dissertation includes previously published and unpublished coauthored material.

  19. Atmospheric Ar and Ne trapped in coesite eclogite during Late Miocene (U)HP metamorphism: implications for the recycling of noble gases in subduction zones

    NASA Astrophysics Data System (ADS)

    Baldwin, S.; Das, J. P.

    2013-12-01

    Several isotopic methods, including 40Ar/39Ar dating of phengite, have been used to determine the timing and duration of (U)HP metamorphism. However, in some (U)HP terranes phengite 40Ar/39Ar data , has yielded anomalously old ages interpreted to result from the presence of extraneous Ar (i.e., either inherited or excess Ar). We analyzed Ar and Ne extracted from phengite and omphacite from coesite eclogite in the Papua New Guinea (U)HP terrane to 1) assess the reliability of 40Ar/39Ar phengite ages to record the timing of (U)HP metamorphism in the youngest (U)HP terrane on Earth, and 2) to assess the non-radiogenic trapped Ar and Ne compositions in minerals that crystallized during subduction zone metamorphism. Step heat experiments on irradiated phengite yielded a 40Ar/39Ar weighted mean age of 8.31 +/- 0.32 Ma (2σ) corresponding to ~88% 39Ar released. These results are concordant with previously published 238U/206Pb zircon ages, and nearly concordant with a Lu-Hf garnet isochron age, both obtained on the same sample. Results suggest that phengite reliably records the timing of peak (U)HP metamorphism and that excess 40Ar is not present in this coesite eclogite. Step heat experiments on irradiated phengite and pyroxene yielded 38Ar/36Ar above atmospheric values (>0.1885). These higher 38Ar/36Ar ratios from outgassed irradiated samples results from reactor-produced 38ArCl likely due to the presence of Cl-derived from fluid inclusions (i.e., via the nuclear reaction 37Cl(n,γ)38Cl(β)38Ar). The high temperature release of 38ArCl may result from smaller fluid inclusions (<1-2 μm). To further investigate the composition of non-radiogenic trapped Ar and Ne in coesite eclogite, step heat experiments were performed on multiple unirradiated splits of phengite and omphacite. Both minerals yielded atmospheric 38Ar/36Ar, including for high temperature (>1400°C) steps. The abundance of radiogenic 40Ar corresponds to the respective [K] and ~8 Ma age of minerals also suggesting the absence of excess 40Ar in these samples. Omphacite outgassed at high temperature (>1400°C) also yielded atmospheric 20Ne/22Ne. Results indicate that atmospheric Ar and Ne were trapped when minerals crystallized at ~8 Ma during (U)HP metamorphism. The survival of trapped atmospheric Ar and Ne in minerals formed during (U)HP metamorphism supports models that call for recycling of noble gases from the atmosphere back into the mantle at subduction zones.

  20. 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

  1. 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 hydration of the crust, thus explaining the partial transformation to (U)HP mineral assemblages throughout the WGR.

  2. 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 the subduction of the oceanic crust in terms of subduction velocity and subduction angle over time. We develop scaling laws combining the subduction velocity and angle depending on the mantle environment (and thus time). These laws can then be applied to continental growth simulations with 1D parameterized models (Höning et al., in press) or 2D/3D subduction zone simulations at specific geological times (using the correct subduction zone setting). References: Quinquis, M. et al. (in preparation). A comparison of thermo-mechanical subduction models. In preparation for G3. Noack, L., Van Hoolst, T., Dehant, V., and Breuer, D. (2013). Relevance of continents for habitability and self-consistent formation of continents on early Earth. XIII International Workshop on Modelling of Mantle and Lithosphere Dynamics, Hønefoss, Norway, 31. Aug. - 5. Sept. 2013. Höning, D., Hansen-Goos, H., Airo, A., and Spohn, T. (in press). Biotic vs. abiotic Earth: A model for mantle hydration and continental coverage. Planetary and Space Science.

  3. 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 at UHP conditions could therefore have a dramatic rheological effect and actually limits continental rocks subduction and facilitates their exhumation.

  4. 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 velocities (1-2 cm/yr), especially during early stages of exhumation, and bear no relation to normal erosion rates. Important observations are that: (i) as a result of strain partitioning and fluid channelling, significant volumes of subducted crust may remain unreacted (i.e. metastable) even at conditions as high as 700°C and 3 GPa - with implications as to geophysical modeling; (ii) subducted continental crust shows no isotopic or geochemical evidence of interaction with mantle material. An unknown proportion of subducted continental crust must have escaped exhumation and effectively recycled into the mantle, with geochemical implications still to be explored, bearing in mind the above inefficiency of mixing. The repeated occurrence of UHP metamorphism, hence of continental subduction, through time and space since at least the late Proterozoic shows that it must be considered a common process, inherent to continental collision. Evidence of older, Precambrian UHP metamorphism is to be sought in high-pressure granulite-facies terranes.

  5. 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 crust during its exhumation metasomatized the overlying mantle wedge; (2) partial melting of the enriched lithospheric mantle generated the Late Triassic alkaline complex; and (3) the alkaline magma was successively experienced crystal fractionation mainly dominated by olivine, clinopyroxene, plagioclase and alkali feldspar.

  6. Cascadia Subduction Zone

    USGS Publications Warehouse

    Frankel, Arthur D.; Petersen, Mark D.

    2008-01-01

    The geometry and recurrence times of large earthquakes associated with the Cascadia Subduction Zone (CSZ) were discussed and debated at a March 28-29, 2006 Pacific Northwest workshop for the USGS National Seismic Hazard Maps. The CSZ is modeled from Cape Mendocino in California to Vancouver Island in British Columbia. We include the same geometry and weighting scheme as was used in the 2002 model (Frankel and others, 2002) based on thermal constraints (Fig. 1; Fluck and others, 1997 and a reexamination by Wang et al., 2003, Fig. 11, eastern edge of intermediate shading). This scheme includes four possibilities for the lower (eastern) limit of seismic rupture: the base of elastic zone (weight 0.1), the base of transition zone (weight 0.2), the midpoint of the transition zone (weight 0.2), and a model with a long north-south segment at 123.8? W in the southern and central portions of the CSZ, with a dogleg to the northwest in the northern portion of the zone (weight 0.5). The latter model was derived from the approximate average longitude of the contour of the 30 km depth of the CSZ as modeled by Fluck et al. (1997). A global study of the maximum depth of thrust earthquakes on subduction zones by Tichelaar and Ruff (1993) indicated maximum depths of about 40 km for most of the subduction zones studied, although the Mexican subduction zone had a maximum depth of about 25 km (R. LaForge, pers. comm., 2006). The recent inversion of GPS data by McCaffrey et al. (2007) shows a significant amount of coupling (a coupling factor of 0.2-0.3) as far east as 123.8? West in some portions of the CSZ. Both of these lines of evidence lend support to the model with a north-south segment at 123.8? W.

  7. 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.

  8. Crustal growth in subduction zones

    NASA Astrophysics Data System (ADS)

    Vogt, Katharina; Castro, Antonio; Gerya, Taras

    2015-04-01

    There is a broad interest in understanding the physical principles leading to arc magmatisim at active continental margins and different mechanisms have been proposed to account for the composition and evolution of the continental crust. It is widely accepted that water released from the subducting plate lowers the melting temperature of the overlying mantle allowing for "flux melting" of the hydrated mantle. However, relamination of subducted crustal material to the base of the continental crust has been recently suggested to account for the growth and composition of the continental crust. We use petrological-thermo-mechanical models of active subduction zones to demonstrate that subduction of crustal material to sublithospheric depth may result in the formation of a tectonic rock mélange composed of basalt, sediment and hydrated /serpentinized mantle. This rock mélange may evolve into a partially molten diapir at asthenospheric depth and rise through the mantle because of its intrinsic buoyancy prior to emplacement at crustal levels (relamination). This process can be episodic and long-lived, forming successive diapirs that represent multiple magma pulses. Recent laboratory experiments of Castro et al. (2013) have demonstrated that reactions between these crustal components (i.e. basalt and sediment) produce andesitic melt typical for rocks of the continental crust. However, melt derived from a composite diapir will inherit the geochemical characteristics of its source and show distinct temporal variations of radiogenic isotopes based on the proportions of basalt and sediment in the source (Vogt et al., 2013). Hence, partial melting of a composite diapir is expected to produce melt with a constant major element composition, but substantial changes in terms of radiogenic isotopes. However, crustal growth at active continental margins may also involve accretionary processes by which new material is added to the continental crust. Oceanic plateaus and other crustal units may collide with continental margins to form collisional orogens and accreted terranes in places where oceanic lithosphere is recycled back into the mantle. We use thermomechanical-petrological models of an oceanic-continental subduction zone to analyse the dynamics of terrane accretion and its implications to arc magmatisim. It is shown that terrane accretion may result in the rapid growth of continental crust, which is in accordance with geological data on some major segments of the continental crust. Direct consequences of terrane accretion may include slab break off, subduction zone transference, structural reworking, formation of high-pressure terranes and partial melting (Vogt and Gerya., 2014), forming complex suture zones of accreted and partially molten units. Castro, A., Vogt, K., Gerya, T., 2013. Generation of new continental crust by sublithospheric silicic-magma relamination in arcs: A test of Taylor's andesite model. Gondwana Research, 23, 1554-1566. Vogt, K., Castro, A., Gerya, T., 2013. Numerical modeling of geochemical variations caused by crustal relamination. Geochemistry, Geophysics, Geosystems, 14, 470-487. Vogt, K., Gerya, T., 2014. From oceanic plateaus to allochthonous terranes: Numerical Modelling. Gondwana Research, 25, 494-508

  9. 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.

  10. A 3D Velocity Model of the Central Cascadia Subduction Zone beneath the Oregon Continental Shelf and Coast Range

    NASA Astrophysics Data System (ADS)

    Kenyon, C.; Trehu, A. M.; Toomey, D. R.; Wilcock, W. S.; Carbotte, S. M.; Carton, H. D.; Canales, J.

    2013-12-01

    Signals from the R/V Langseth's tuned airgun array were recorded on 6 ocean-bottom seismometers and 35 EarthScope FlexArray seismometers deployed in the Oregon Coast Range from 43.5° to 45°N as a piggyback project on the 2012 Ridge2Trench experiment to image the structure of the Juan de Fuca plate. This section of the Juan de Fuca/North America plate boundary slipped in 2 moderate low-angle thrust earthquakes in 2004, and continuing seismicity has clustered around these events since that time. It has also been associated with a transition in megathrust recurrence rate based on paleoseismic data and exhibits anomalous locking characteristics in geodetic data. Previously acquired bathymetric, potential field, and seismic data indicate the presence of subducted seamounts in this region and hint at large velocity variations of the overlying forearc crust immediately above the plate boundary [Tréhu et al., 2012]. The sources for this study are ~18,000 airgun shots along 4 lines on the continental shelf and upper slope and a line oblique to the coast that extends across the margin to the trench (and ultimately to the ridge). Data quality is excellent, with strong Pg and PmP arrivals observed at most stations for all shots. A preliminary look at the data supports the presence of large along-strike velocity variations in this region. To date, ~2/3 of the data have been picked. We will present velocity models that merge the new data with constraints from 2D and 3D onshore/offshore data acquired in 1989 and 1996 to improve the resolution and extend the velocity model to the south to include Heceta Bank, a region of uplifted late Miocene and early Pliocene sediments underlain by a previously poorly resolved high density anomaly. [Tréhu, A.M., Blakely, R.J., Williams, M.C., Subducted seamounts and recent earthquakes beneath the central Cascadia forearc, Geology, v. 40, p. 103-106, 2012

  11. The Moho in subduction zones

    NASA Astrophysics Data System (ADS)

    Bostock, M. G.

    2013-12-01

    The Moho in subduction zones exists in two distinct forms, one associated with the subducting oceanic plate and second with the overriding plate. The seismic expression of both forms is linked to the nature of a landward dipping, low-velocity zone (LVZ) that has been detected in a majority of subduction zones about the globe and that approximately coincides with Wadati-Benioff seismicity. We review seismic studies that constrain the properties of the LVZ in Cascadia where it has been extensively studied for over a quarter century. A model in which the LVZ is identified with hydrated pillow basalts and sheeted dikes of oceanic crustal Layer 2, is consistent with available geological and geophysical data, and reconciles previously conflicting interpretations. In this model, the upper oceanic crust is hydrated through intense circulation at the ridge and becomes overpressured upon subduction as a result of metamorphic dehydration reactions combined with an impermeable plate boundary above and a low porosity gabbroic Layer 3 below. The resulting seismic velocity contrast (approaching 50% for S-waves) significantly overwhelms that of a weaker, underlying oceanic Moho. At greater depths, oceanic crust undergoes eclogitization in a top-down sense leading to gradual disappearance of the LVZ. The large volume change accompanying eclogitization is postulated to rupture the plate boundary allowing fluids to penetrate the cooled, forearc mantle wedge. Pervasive serpentinization and free fluids reduce velocities within the wedge, thereby diminishing, erasing or even inverting the seismic contrast associated with the Moho of the overriding plate. This model is tested against observations of LVZs and forearc mantle structure worldwide.

  12. Mass Flux of Continental Material at Cenozoic Subduction Zones--New Global and Trench-sector Calculations Using New Geological and Geophysical Observations

    NASA Astrophysics Data System (ADS)

    Scholl, D. W.; von Huene, R.

    2001-12-01

    INTRODUCTION: A decade ago, then available geophysical and geological data implied that more than 65 percent of ocean floor sediment entering most subduction zones (SZ) accompanied the oceanic crust to the mantle (= sediment subduction or SS). The underthrusting slab also eroded the margin's crustal framework and conveyed this material to the mantle (= subduction erosion or SE). Globally, the mass of continental material recycled to the mantle was estimated at 1.3-1.8 km3 / yr (SS. = 0.7 km3 + SE = 0.6-1.1 km3). SEDIMENT SUBDUCTION: New and enhanced seismic reflection data, new drilling observations, and reevaluation of older information stress that the efficacy of SS is higher than earlier assessed. In detail, it appears that 100 percent SS occurs at non-accreting margins (19,000 km), at least 80 percent at accreting margins (16,000 km) where small to moderate size accretionary prisms (width=5-40 km) are forming, and 40-45 percent where larger prisms are accumulating (8,000 km). At Cenozoic SZs (~43,000 km), it is now estimated that the long-term (i.e., >10 Myr) rate of SS is at least 1.0 km3 / yr (solid volume). SUBDUCTION EROSION: New and reassessed seismic, drilling, submersible, coastal mapping and arc-retreat observations suggest a higher long-term rate of SE than formerly estimated at 30 km3 / Myr / km of trench. We now estimate that, except perhaps where large accretionary bodies are forming, the long-term rate of forearc erosion averages at least 40 km3 / Myr (range = 28-62), which corresponds to a global recycling rate of 1.4 km3 / yr. The matching average rate of landward truncation of the submerged forearc is 2.5 km / Myr (range = 1.8-4.2). SUMMARY: The late Cenozoic rate at which continental crust is recycled at SZs is currently estimated at 2.4 km3 / yr (ss=1+ se=1.4) +/- 25 percent, which is basically that now approximated for arc magmatic additions. It can thus be inferred that at Cenozoic SZs rates of crustal addition and recycling have been in general balance. This quasi-stasis may be applicable to the Phanerozoic.

  13. 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 its tsunamigenic relationship. Funding providing by UNAM-DGAPA-PAPIIT grants: IN115613 and IN115513

  14. Subduction Zone Mass Fluxes

    NASA Astrophysics Data System (ADS)

    Porter, K. A.; White, W.

    2004-12-01

    Recycled crustal material is often cited as a source for various mantle reservoir characteristics. These inferences are often based on ingoing slab compositions, although processes within the subduction zone could significantly change the chemistry of the slab before it is mixed into the mantle. Few studies have examined the composition of "residual" slab material. Using a mass-balance approach, we have estimated the chemical compositions of residual slabs at nine oceanic subduction zones (Aleutians, Central America, Izu-Bonin, Kurile, Mariana, Northern Lesser Antilles, Southern Lesser Antilles, Sunda-Java, and Tonga), representing approximately 30% of the total length of subduction zones worldwide. We remove an inferred mantle wedge component from fractionation-corrected average island arc volcanics (IAV) in order to estimate the composition and flux of the "slab component" in the IAV. We then calculate the residual slab composition by subtracting the "slab component" flux from the ingoing slab flux. We have also calculated slab-to-backarc fluxes at the Marianas and Tonga island arcs in order to more accurately constrain residual slab fluxes. Our results allow us to investigate the feasibility of various mantle evolution models. The HIMU mantle reservoir gives rise to basalts with very high 206Pb/204Pb and somewhat low 87Sr/86Sr. It has often been suggested that this reservoir represent anciently subducted oceanic crust. We use our calculated residual slab U/Pb, Rb/Sr, and Sm/Nd values to estimate the present isotopic composition of material subducted 1.8 Ga ago. Only two of the arcs (Izu-Bonin and the Marianas) have high enough residual slab U/Pb to produce HIMU-like Pb isotope signatures. However, Rb/Sr ratios in these arcs are too high to be consistent with HIMU characteristics, and ɛ Nd is also inconsistent with the HIMU reservoir. Galer and O'Nions (1985) argued that to account for the difference between present 232Th/238U ratios (kappa) and the time-integrated 232Th/238U calculated from Pb isotope ratios (κ Pb) in MORB, the residence time of Pb in the depleted mantle must be short (about 600 Ma). Others have argued instead that low present-day kappa in the depleted mantle reflects preferential recycling of U via subduction of low-Th/U altered crust. Our results indicate that subduction processing does not fractionate Th from U; therefore, a residual slab will only contribute low Th/U material to the mantle if the ingoing slab has a low Th/U ratio. Low crustal Th/U is only expected in oxidizing ocean conditions. However, recent studies indicate that the oceans may have remained fairly anoxic for much of Earth's history (until about 600 Ma). If this was the case, it is unlikely that oceanic crust could have caused the lowering of mantle Th/U ratios. The Pb/Ce ratio has been used as an indicator of recycled material in the mantle because of the large contrast in crust and mantle values (0.30 vs. 0.035). Most basalts derived from mantle plumes have Pb/Ce ratios similar to those of MORB (about 0.04). This observation is apparently inconsistent with the idea that mantle plumes contain recycled crustal material, since subducted slabs tend to have high Pb/Ce (greater than 0.20) due to their sedimentary component. Our results show that, although subduction zone processing can lower slab Pb/Ce ratios, residual slab compositions are still not consistent with the observed mantle plume compositions: the lowest residual slab Pb/Ce we observe is roughly 0.07. It therefore seems likely that other processes must contribute to the lowering of slab Pb/Ce ratios.

  15. 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.

  16. Non-lithostatic pressure in subduction zones

    NASA Astrophysics Data System (ADS)

    Raimbourg, Hugues; Kimura, Gaku

    2008-10-01

    The pressure at depth is not directly observable and no one knows precisely to which extent the pressure conditions in subduction zones, recorded by high-pressure metamorphic rocks, deviate from mantle lithostatic pressure. As an alternative to large-scale complex numerical models of subduction zones, the analytical subduction channel model can give us some insight on the physical processes that control the development of non-lithostatic pressure, as well as some estimation of its amplitude. We propose a new approach coupling the flow of crust within the channel to the deformation of the mantle bounding the channel, occurring as the pressure within the channel deviates from mantle lithostatic values. While for very weak crust within the subduction channel, the channel walls are rigid and channel geometry does not vary, for stronger crust, our coupled approach unravels a new domain of behaviour where the mantle is no longer completely rigid and the deformation of the channel walls prevents arbitrarily large non-lithostatic pressure to develop. This new regime poses an upper bound on the amplitude of non-lithostatic pressure within the channel that depends only on the mantle viscosity. The transition from one regime to another is dependent on an adimensional parameter α={μ}/{μ}{h03}/{L}, incorporating not only mantle and crust viscosity but also the geometry of the channel. The development of larger non-lithostatic pressure in thinner channels than in larger ones, predicted in the rigid channel model, is partly inhibited in the fully coupled model as thinner channels more easily induce channel wall deformation. The lengthscale of the channel width perturbations influences the amplitude of non-lithostatic pressure, as small-scale ones, inducing a more rigid response of the mantle, potentially trigger larger non-lithostatic pressure.

  17. Geochemistry of subduction zone serpentinites: A review

    NASA Astrophysics Data System (ADS)

    Deschamps, Fabien; Godard, Marguerite; Guillot, Stéphane; Hattori, Kéiko

    2013-09-01

    Over the last decades, numerous studies have emphasized the role of serpentinites in the subduction zone geodynamics. Their presence and role in subduction environments are recognized through geophysical, geochemical and field observations of modern and ancient subduction zones and large amounts of geochemical database of serpentinites have been created. Here, we present a review of the geochemistry of serpentinites, based on the compilation of ~ 900 geochemical data of abyssal, mantle wedge and exhumed serpentinites after subduction. The aim was to better understand the geochemical evolution of these rocks during their subduction as well as their impact in the global geochemical cycle. When studying serpentinites, it is essential to determine their protoliths and their geological history before serpentinization. The geochemical data of serpentinites shows little mobility of compatible and rare earth elements (REE) at the scale of hand-specimen during their serpentinization. Thus, REE abundance can be used to identify the protolith for serpentinites, as well as magmatic processes such as melt/rock interactions before serpentinization. In the case of subducted serpentinites, the interpretation of trace element data is difficult due to the enrichments of light REE, independent of the nature of the protolith. We propose that enrichments are probably not related to serpentinization itself, but mostly due to (sedimentary-derived) fluid/rock interactions within the subduction channel after the serpentinization. It is also possible that the enrichment reflects the geochemical signature of the mantle protolith itself which could derive from the less refractory continental lithosphere exhumed at the ocean-continent transition. Additionally, during the last ten years, numerous analyses have been carried out, notably using in situ approaches, to better constrain the behavior of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, U, Th, Sr) incorporated in serpentine phases. The abundance of these elements provides information related to the fluid/rock interactions during serpentinization and the behavior of FME, from their incorporation to their gradual release during subduction. Serpentinites are considered as a reservoir of the FME in subduction zones and their role, notably on arc magma composition, is underestimated presently in the global geochemical cycle.

  18. 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.

  19. 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 accumulation and release cycles, but with a signal that, thus far, has been undetectable by intermittent studies in the region.

  20. Permeabilities of Subduction Zone Sediments

    NASA Astrophysics Data System (ADS)

    Screaton, E.; Gamage, K. R.; Daigle, H.; Harris, R. N.

    2013-12-01

    Permeabilities of subseafloor sediments control fluid expulsion from sediments as they are subducted or accreted and thus, compaction state, fluid overpressures, and deformation. We compare results from Integrated Ocean Drilling Program samples to compare to previously-developed permeability-porosity relationships for subduction zone sediments. Hemipelagic clay samples obtained from the incoming plate Kumano transect of the Nankai Trough (NanTroSEIZE) yield slightly lower permeability for a given porosity than previously reported results from Nankai Trough's Muroto transect and are lower than clay-rich sediments from the upper plate of CRISP offshore the Osa Peninsula of Costa Rica (CRISP). Samples from the Pacific Equatorial Transect (PEAT) and the South Pacific Gyre provide further insight to permeability behavior of sediments deposited in the Pacific basin. South Pacific Gyre sediments consist of slowly deposited pelagic clay with little biogenic or coarse clastic input. Measured permeabilities for given porosities are consistently lower than values reported for clay-rich sediments of Nankai and Costa Rica. PEAT samples comprise biogenic oozes and yield inconsistent results, with some of the highest permeabilities (10-14 m2) as well as some results similar to clay-rich sediments.

  1. Metamorphic geology

    NASA Astrophysics Data System (ADS)

    Peacock, Simon M.

    The complex heat- and mass-transfer processes of terrestrial metamorphism are described in a critical review of U.S. research from the period 1987-1990. Topics examined include metamorphic pressure-temperature-time paths and regional studies (crustal thickening, contact and subduction-zone metamorphism, crustal extension, and granulite facies metamorphism and crustal anatexis); metamorphic fluid-rock interaction in regional, contact, subduction-zone, and granulite terrains; metamorphic thermochemistry, phase relations, and mineral chemistry; metamorphic kinetics; and microstructures in metamorphic rocks. Textbooks and computer programs in metamorphic geology are briefly characterized, and it is concluded in general that a first-order explanation of the basic processes is now being developed. A comprehensive bibliography is provided.

  2. 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.

  3. 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.

  4. A numerical approach to melting in warm subduction zones

    NASA Astrophysics Data System (ADS)

    Bouilhol, Pierre; Magni, Valentina; van Hunen, Jeroen; Kaislaniemi, Lars

    2015-02-01

    The complex feedback between dehydration and melting in hot subduction zones is quantitatively addressed in this study. We present an integrated numerical tool that combines a high-resolution thermo-mechanical subduction model with a thermodynamic database that allows modeling metamorphic devolatilization, and subsequent re-hydration and melting reactions. We apply this tool to quantify how the hydration state of a lithologically layered subducting slab varies during interaction with the hot mantle wedge and how this affects any melting taking place in the subducting crust or the overlying mantle wedge. Total crustal dehydration is achieved before any crustal melting can occur, even in very young subducting slabs. Significant oceanic crust melting is only achieved if the metamorphic fluids from the dehydrating underlying subducting slab mantle are fluxed through the dry eclogites. But our models further demonstrate that even if the oceanic crust can melt in these specific conditions, the preceding crustal dehydration will simultaneously result in extensive mantle wedge melting at lower pressures than for colder slabs. The significant mantle wedge melting implies that also for hot subduction zones, most of the melt feeding the overriding plate is of mantle origin.

  5. 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

  6. 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.

  7. 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.

  8. 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 of eclogite facies breccias where meter size blocks of rotated granulites are enclosed in eclogite may have initiated by the seismic events as indicated by fractures in the relict granulite facies garnet. The seismic events may have been important in large scale transport of fluid required to bring about the metamorphism of the dry granulite facies complex.

  9. Investigating Subduction Zone Processes in Chile

    NASA Astrophysics Data System (ADS)

    Scherwath, Martin; Flueh, Ernst; Grevemeyer, Ingo; Tilmann, Frederik; Contreras-Reyes, Eduardo; Weinrebe, Wilhelm

    2006-07-01

    The highly active subduction zone ofsouthern Chile was the source region of the1960 Valdivia megathrust earthquake (Mw= 9.5),the largest earthquake ever recorded. Thisregion is currently under investigation by themultidisciplinary TIPTEQ (From the IncomingPlate to Mega-Thrust Earthquake Processes)project, which is studying the structure,state, and deformation of thesubduction zone lithosphere.

  10. 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-subduction peridotites have the lowest N concentrations (1.3-2.1 ppm) and highly variable δ15N values (-4 to +3). High-pressure peridotites have higher N contents (up to 20 ppm) and mostly positive δ15N values. Data for veins in the peridotites are suggestive that N isotopes were not significantly fractionated by dehydration. Hence, N recycled through subduction zones via peridotites should show a range in δ15N similar to that of oceanic serpentinized peridotites. The range in δ15N for serpentinized peridotites, from pristine mantle values towards those typical of modern marine sediments, suggests derivation of N from organic-sedimentary sources, incorporated during bending-related faulting of the subducting slab and/or via metasomatic additions during subduction. N is enriched in serpentinized peridotites compared to pristine mantle rocks, and it is retained down to depths of at least 70 km and possibly deep into the mantle. Hence, serpentinized peridotites appear to represent an important reservoir for deep subduction zone N cycling. [1] Philippot et al. (2007), Min. Pet. 91:11-24

  11. 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.

  12. Geochemistry of serpentinites in subduction zones: A review

    NASA Astrophysics Data System (ADS)

    Deschamps, Fabien; Godard, Marguerite; Guillot, Stéphane; Hattori, Kéiko

    2013-04-01

    Over the last decades, numerous studies have emphasized the role of serpentinites in the subduction zones geodynamics. Their presence and effective role in this environment is acknowledged notably by geophysical, geochemical and field observations of (paleo-) subduction zones. In this context, with the increasing amount of studies concerning serpentinites in subduction environments, a huge geochemical database was created. Here, we present a review of the geochemistry of serpentinites, based on the compilation of ~ 900 geochemical analyses of abyssal, mantle wedge and subducted serpentinites. The aim was to better understand the geochemical evolution of these rocks during their subduction history as well as their impact in the global geochemical cycle. When studying serpentinites, it is often a challenge to determine the nature of the protolith and their geological history before serpentinisation. The present-day (increasing) geochemical database for serpentinites indicates little to no mobility of incompatible elements at the scale of the hand-sample in most serpentinized peridotites. Thus, Rare Earth Elements (REE) distribution can be used to identify the initial protolith for abyssal and mantle wedge serpentinites, as well as magmatic processes such as melt/rock interactions taking place before serpentinisation. In the case of subducted serpentinites, the interpretation of trace element data is more difficult due to secondary enrichments independent of the nature of the protolith, notably in (L)REE. We propose that these enrichments reflect complex interactions probably not related to serpentinisation itself, but mostly to fluid/rock or sediment/rock interactions within the subduction channel, as well as intrinsic feature of the mantle protolith which could derive from the continental lithosphere exhumed at the ocean-continent transition. Additionally, during the last ten years, numerous studies have been carried out, notably using in situ approaches, to better constrain the geochemical budget of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, U, Th, Sr) stored in serpentinites and serpentine phases. These elements are good markers of the fluid/rock interactions taking place during serpentinisation. Today, the control of serpentinites on the behaviour of these elements, from their incorporation to their gradually release during subduction, is better understood. Serpentinites must be considered as a component of the FME budget in subduction zones and their role, notably on arc magmas composition, is undoubtedly underestimated presently in the global geochemical cycle.

  13. Seismic structure of the Rivera subduction zone - the MARS experiment

    NASA Astrophysics Data System (ADS)

    Grand, S. P.; Yang, T.; Sudharja, S.; Wilson, D.; Guzman Speziale, M.; Gomez Gonzalez, J.; Leon-Soto, G.; Ni, J.; Dominguez Reyes, T.

    2007-05-01

    The subduction zone of western Mexico is a unique region on Earth where microplate capture and overriding plate disruption are occurring today. The small Rivera plate is subducting beneath western most Mexico primarily beneath Jalisco state while to the east it is the Cocos plate that is subducting. Above the Rivera plate the Jalisco block of Mexico is bounded by the north trending Colima Rift and the northwest trending Tepic-Chapala Rift and may form a microplate in its own right. Magmatism is present throughout the region and is unusual for a subduction zone in that geochemical analyses indicate an ocean island basalt component to some of the lavas. Also, Colima volcano is offset trenchward from other volcanoes in the Mexican Volcanic Belt. Little is known of the subducting Rivera plate geometry due to the paucity of seismicity within the plate yet the geometry of the Rivera and Cocos plates at depth are likely critical for understanding the tectonic evolution of western Mexico. The MARS (MApping the Rivera Subduction zone) project consists of the deployment of 50 broadband seismometers covering the Jalisco block from the coast to the Tepic-Chapala rift in the north and about 150 km to the west of the Colima rift. The instruments were deployed in January, 2006 and will be removed in June, 2007. The goal of the project is to seismically image the subducting Rivera and Cocos plates at depth as well as the mantle wedge above the plates. A number of different analyses of MARS data are underway including teleseismic tomography, receiver function analysis, and shear wave splitting analysis. The preliminary tomography results clearly show both subducting plates with a sharp change in dip to the east of the Colima rift probably indicating a tear between the two plates along a trend more eastward than the trend of the rift. The images also show extremely slow shallow mantle velocities beneath the Tepic-Chapala rift but not beneath the Colima rift. Receiver functions indicate an average Moho depth of roughly 40 km in the continental interior but are very complicated near the coast. Finally, shear wave splitting observations indicate a generally north fast direction with a reduction in magnitude of splits beneath the Colima rift.

  14. 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 dynamics, Geology, 40, 739-742 (2012)

  15. Searching for aseismic slip in subduction zones

    NASA Astrophysics Data System (ADS)

    Reverso, Thomas; Marsan, David; Helmstetter, Agnès

    2013-04-01

    Aseismic slip is known to occur in subduction zones. This phenomenon is of paramount importance for understanding earthquake hazard, and for estimating the potential for future mega-thrust events. Aseismic slip is however difficult to detect exept for the largest cases, for which geodesy is the most useful type of data. Here, we propose a systematic detection of aseismic slip events, based on seismicity data. We search for transient increases in background seismicity rate in subduction zones, that would indicate the presence of aseismic slip. To that purpose, we make use of an ETAS model in space and time, to distinguish earthquakes due to background seismicity from aftershocks. We optimize the model parameters, and test the sensitivity of the results with changes in parameters. Given the 'best' model, we measure the statistical significance of the departure of the local (in time and space) background rate with the 'normal' background rate. Significant departure then indicates the need to temporary increase the background rate in order to explain the observed earthquake occurrences. We thus can single out such episodes of aseismic transients, and characterize their duration and spatial extent.

  16. 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

  17. 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.

  18. Hydrologic control of forearc strength and seismicity in the Costa Rican subduction zone

    NASA Astrophysics Data System (ADS)

    Audet, Pascal; Schwartz, Susan Y.

    2013-10-01

    Subduction zones can exhibit variable seismic behaviour, ranging from great earthquakes to slow slip. This variability may be linked to fault frictional properties, and the rheology and structure of the upper plate. The subduction zone beneath the Nicoya Peninsula, Costa Rica, is characterized by strong variations in fault-slip behaviour and a lateral change in the origin of the subducting plate. In the northwest, the plate interface is locked, and experiences large, infrequent earthquakes, and the subducting plate is formed at the East Pacific Rise. In contrast, in the southeast, slow-slip events occur frequently and the subducting plate is formed at the Cocos-Nazca spreading centre. Here we use seismic receiver-function data to analyse the structure of the subduction zone beneath the Nicoya Peninsula. We find extremely high P-S seismic-velocity ratios within the entire subducting oceanic crust that we interpret as high pore-fluid pressure. Velocity ratios in the overriding continental crust, however, change from lower values in the northwest to higher ones in the southeast, indicating a disparity in fluid accumulation. We infer that this disparity is caused by a higher supply of fluid from the subducting slab in the southeast, owing to the permeability structure of oceanic crust formed at the Cocos-Nazca spreading centre. We suggest that the spatial gradient in fluid content influences upper-plate strength and controls the segmentation of seismogenic behaviour in this subduction zone.

  19. Seismological Investigation of Deep Subduction Zones

    NASA Astrophysics Data System (ADS)

    Lucifredi, I.; Ishii, M.

    2008-12-01

    Models of subduction zones have a potential of impacting thinking about many of the central problems in the structure, dynamics, chemistry, and history of the solid earth, cummulative effects of which would reach across the earth sciences. The long-term goal of this work is to develop adaptive seismic imaging concepts for modeling and understanding of the subduction zone processes, taking into account the modification of the propagating seismic field due to reflection, refraction, and scattering from complex geological structures such as slabs, low velocity layers , scatterers, and topography. Different from the relatively small-scale approaches employed so far in reflection seismology, our approach provides the resultant field from deep geological structures, and surface topography spanning hundreds of kilometers, incorporating the signatures of the large-scale features typical in subduction zones. A part of the objective consists of obtaining a fundamental understanding of the 3D structure such as the high velocity slab, through the development of efficient physically based propagation and scattering models. This includes features such as aspect-dependent scattering objects, geological structures, and topography. Moreover, the analysis addresses the resultant physical mechanisms such as waveguide multipath effects produced by wave-trapping slabs. Additionally, the effort aims to analyze the existing experimental data and develop novel signal processing techniques for spatial, temporal, and spectral identification of different classes of waves, including early slab arrivals. The initial phase consists of the development and testing of the modeling and simulation work to explore the underlying physical mechanisms of seismic propagation through slabs beneath the ocean floor, scattering from complex structure and closed boundary objects, and the prediction of the resultant field to the surface receivers. For the representation of range-dependent propagation, we employ a hybrid coupled wavenumber integration approach to range-dependent seismic modeling based on the OASES environmental modeling framework. The generic model is a suite of modules covering a variety of range-dependent waveguide, topography, and source/receiver representations. Furthermore, the representation and the analysis of scattering from arbitrarily shaped structure as well as from closed boundary scatterers is performed by developing a wave theory based computational model combining a virtual source approach to open and closed boundary scattering with an established range- dependent wavenumber integration model for propagation. The current subduction zone modeling has shown that for specific receivers, early arrivals are a manifestation of the slab-trapped traveling waves reaching the surface first, while the rest of the response follows significantly later - a concept which will be further explored in the actual data. The spaciotemporal and frequency characteristics of data recorded by the Japanese seismic network is analyzed and compared to the numerical model. Time-frequency, array processing, and event classification methods for extracting properties of slab structure and scatterer signatures are developed and implemented on the data as well as on the synthetics. The model and experimental data time-frequency analysis exhibits frequency dependence and intricate time of arrival features. The understanding of these, and other manifestations of the underlying physical processes, are further explored and analyzed using signal processing techniques.

  20. Strain accumulation along the Cascadia subduction zone

    USGS Publications Warehouse

    Murray, M.H.; Lisowski, M.

    2000-01-01

    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.

  1. 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 starts in the middle or late part of the cycle, depending on position. This result suggests that the negligible or small contraction measured on the Shumagin Islands, Alaska, during 1980 to 1991, may not invalidate an interpretation of that region as being a moderately coupled subduction zone. In contrast, mantle relaxation causes only modest temporal nonuniformity of uplift rates in the overriding plate and of extensional stress rates in the subducting plate, even when the Maxwell time is an order of magnitude less than the recurrence interval.

  2. 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 at low-pressure and the other is a gradual decline due to crystal grain boundary consolidation at high-pressure. Intrinsic permeability for sample perpendicular to schistosity was about 100 times lower than that parallel to schistosity. Porosity at atmospheric pressure was estimated about 0.5%. Assumeing a constant pressure derivative for porosity and permeability, flow velocity parallel to the foliation was 44cm/year. This result represent that fluid migration is much faster than the plate subducting rate. These experimental data show that fluid migration was influenced by not only water buoyancy but also by the schistosity of the rock (deformation geometry). In this case, released water from subducting oceanic slab can be migrated along plate interfaces.

  3. 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// (100)opx//(001)chl. The relation [010]atg// [001]ol //[001]opx can also be inferred but it is weaker. Similar topotactic relations are observed in the Cerro del Almirez samples, but the CPOs are weaker and more complex. The complexity arises from constant interfacial angles and systematic low-index interfacial contacts between orthopyroxene-olivine-chlorite (e.g. (001)chl // (100)opx). As a consequence the inheritance from the antigorite serpentinite is partially obliterated. Compaction-related microstructural features are also present including: (1) smooth bending of the former foliation and diffuse olivine veinlets perpendicular to it, (2) gradual crystallographic misorientation (up to 15º) of prismatic enstatite due to buckling, (3) localized orthoenstatite(Pbca)/low clinoenstatite (P21/c) inversion, and (4) brittle fracturing of prismatic enstatite wrapped by plastically deformed chlorite. These observations suggest that topotactic crystrallographic relations are dominant in undrained systems, but that the mechanisms allowing for compaction and fluid draining significantly affect the final texture in drained systems. Because the second case prevails in subduction zones, compaction mechanisms need to be better understood for modelling the development of CPOs after foliated protoliths in the slab and the mantle wedge. [1] Putnis, A., 2009. Reviews in Mineralogy and Geochemistry 70, 87-124. [2] Boudier, F., et al. 2010 J. Petrology 51, 495-512.

  4. 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

  5. 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

  6. Low Poisson Ratios in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Hacker, B. R.; Abers, G. A.

    2010-12-01

    The growing capability to measure seismic velocities in subduction zones has led to an unusual observation: VP/VS ratios as low as 1.65 with VS ~ 4.7 km/s. This is difficult to explain because most minerals have VP/VS ratios exceeding 1.75, and some of the likely alteration phases, like antigorite, clinohumite and chlorite have high VP/VS ratios. Here we explore how these unusually low VP/VS ratios might be produced, using three methods. 1. Velocity anisotropy was calculated for deformed peridotite, using the Christoffel equation and crystal preferred orientations (CPOs) reported in the literature. Peridotite with ‘Type-A’ olivine CPOs can reproduce the observed values for VP/VS1, but not VP/VS2, and only at temperatures <1000°C; other types of olivine CPOs yield VP/VS1 > 1.73. Anisotropy may be effective in situations where measurements of Vs are dominated by the fast, first-arriving signal, as may be the case in some local tomography studies. 2. Isotropic velocities were calculated for rocks other than peridotite. Although the presence of free quartz produces low VP/VS, it requires extensive SiO2 metasomatism, produces rocks too buoyant to remain in the wedge, is stable only over a narrow 1000-1342°C temperature interval at 3 GPa, and yields VS that are too slow. Brucite and talc are also too slow. Less SiO2 metasomatism produces orthopyroxene, and this yields peridotite with appropriate VP/VS ratios, but only at 600-900°C and only for pure enstatite, not the Mg# 93-95 expected. 3. Mineralogy was calculated as a function of P & T for a range of rock compositions (including metasomatized peridotite, igneous rocks, and sediment) using Perple_X, and velocity anisotropy calculations were made using the Christoffel equation and typical crystal preferred orientations for the constituent phases. Metasomatized mantle containing talc can produce the appropriate VP/VS ratios and VS at 600°C as can mantle with mica + amphibole + pyroxene at 800°C. At 1000°C, no calculated velocities were found that match those observed. Thus, the unusually low VP/VS ratios observed in some subduction zones can be explained as the result of metasomatized peridotite with “Type-A” olivine CPOs at temperatures <1000°C.

  7. Mantle convection with strong subduction zones

    NASA Astrophysics Data System (ADS)

    Conrad, Clinton P.; Hager, Bradford H.

    2001-02-01

    Because mantle viscosity is temperature-dependent, cold subducting lithosphere should be strong, which implies that the rapid, localized deformation associated with subduction should strongly resist plate motions. Due to computational constraints, the deformation of a subducting plate cannot be accurately resolved in mantle-scale convection models, so its effect on convection is difficult to investigate. We have developed a new method for implementing subduction that parametrizes the deformation of the oceanic lithosphere within a small region of a finite element grid. By imposing velocity boundary conditions in the vicinity of the subduction zone, we enforce a geometry for subduction, producing a slab with a realistic thermal structure. To make the model dynamically consistent, we specify a rate for subduction that balances the energy budget for convection, which includes an expression for the energy needed to deform the oceanic lithosphere as it subducts. This expression is determined here from a local model of bending for a strong viscous lithosphere. By implementing subduction in this way, we have demonstrated convection with plates and slabs that resemble those observed on Earth, but in which up to 40 per cent of the mantle's total convective resistance is associated with deformation occurring within the subduction zone. This additional resistance slows plate velocities by nearly a factor of two compared to models with a weak slab. For sufficiently strong lithosphere, the bending deformation slows surface plates sufficiently that they no longer actively participate in global-scale convection, which occurs instead beneath a `sluggish lid'. By introducing a low-viscosity asthenosphere beneath the oceanic plate, we demonstrate that small-scale convection at the base of oceanic lithosphere may limit plate thickness, and thus the resistance to bending, and cause plate velocities to depend on the strength of the bending lithosphere rather than on the viscosity of the underlying mantle. For a cooling Earth, the effective lithosphere viscosity should be nearly constant, but the mantle viscosity should increase with time. Thus, subduction-resisted convection should produce nearly constant plate velocities and heat flow over time, which has implications for the Earth's thermal evolution. We estimate that this style of convection should apply if the effective viscosity of the bending lithosphere is greater than about 1023Pas, but only if some mechanism, such as small-scale convection, prevents the bending resistance from stopping plates altogether. Such a mechanism could be fundamental to plate tectonics and Earth's thermal history.

  8. 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.

  9. Tomography and Dynamics of Western-Pacific Subduction Zones

    NASA Astrophysics Data System (ADS)

    Zhao, D.

    2012-01-01

    We review the significant recent results of multiscale seismic tomography of the Western-Pacific subduction zones and discuss their implications for seismotectonics, magmatism, and subduction dynamics, with an emphasis on the Japan Islands. Many important new findings are obtained due to technical advances in tomography, such as the handling of complex-shaped velocity discontinuities, the use of various later phases, the joint inversion of local and teleseismic data, tomographic imaging outside a seismic network, and P-wave anisotropy tomography. Prominent low-velocity (low-V) and high-attenuation (low-Q) zones are revealed in the crust and uppermost mantle beneath active arc and back-arc volcanoes and they extend to the deeper portion of the mantle wedge, indicating that the low-V/low-Q zones form the sources of arc magmatism and volcanism, and the arc magmatic system is related to deep processes such as convective circulation in the mantle wedge and dehydration reactions in the subducting slab. Seismic anisotropy seems to exist in all portions of the Northeast Japan subduction zone, including the upper and lower crust, the mantle wedge and the subducting Pacific slab. Multilayer anisotropies with different orientations may have caused the apparently weak shear-wave splitting observed so far, whereas recent results show a greater effect of crustal anisotropy than previously thought. Deep subduction of the Philippine Sea slab and deep dehydration of the Pacific slab are revealed beneath Southwest Japan. Significant structural heterogeneities are imaged in the source areas of large earthquakes in the crust, subducting slab and interplate megathrust zone, which may reflect fluids and/or magma originating from slab dehydration that affected the rupture nucleation of large earthquakes. These results suggest that large earthquakes do not strike anywhere, but in only anomalous areas that may be detected with geophysical methods. The occurrence of deep earthquakes under 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.

  10. 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.

  11. Tracking slabs beneath northwestern Pacific subduction zones

    NASA Astrophysics Data System (ADS)

    Gu, Yu Jeffrey; Okeler, Ahmet; Schultz, Ryan

    2012-05-01

    This study uses the amplitudes of bottom-side reflected shear waves to constrain the morphology and dynamics of subducted oceanic lithosphere beneath northwestern Pacific subduction zones. Across Honshu arc, the 410- and 660-km seismic discontinuities are detected at the respective depths of 395 5 and 685 5 km within the Wadati-Benioff zone. Their topographies are negatively correlated along slab dip, showing the dominant effect of temperature on the olivine phase changes within the upper mantle transition zone. The base of the upper mantle shows broad depressions as well as localized zones of shallow/average depths beneath Korea and northeast China. The 15 + km peak-to-peak topography west of the Wadati-Benioff zones suggests that the stagnant part of the subducted Pacific plate is not as flat as previously suggested. Eastward slab 'pile-up' is also possible at the base of the upper mantle. Across southern Kuril arc, the shear wave reflection coefficients of major olivine phase boundaries fall below 5% within the Wadati-Benioff zone. The apparent reflection gaps and the spatial connection between a strong reflector at ~ 900 km depth may imply 1) possible compositional variations at the top and bottom of the transition zone and 2) substantial mass/heat flux across the 660-km seismic discontinuity. We also identify strong reflectors within the subducted oceanic lithosphere at mid transition zone depths. The depths and strengths of these reflectors are highly variable between Honshu and southern Kuril islands.

  12. Metamorphic belts of Anatolia

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    Investigating metamorphic rocks from high-pressure/low-temperature (HP/LT) belts that formed during the closure of several oceanic branches, building up the present Anatolia continental micro-plate gives insight to the palaeogeography of the Neotethys Ocean in Anatolia. Two coherent HP/LT metamorphic belts, the Tavşanlı Zone (distal Gondwana margin) and the Ören-Afyon-Bolkardağ Zone (proximal Gondwana margin), parallel their non-metamorphosed equivalent (the Tauride Carbonate Platform) from the Aegean coast in NW Anatolia to southern Central Anatolia. P-T conditions and timing of metamorphism in the Ören-Afyon-Bolkardağ Zone (>70?-65 Ma; 0.8-1.2 GPa/330-420°C) contrast those published for the overlying Tavşanlı Zone (88-78 Ma; 2.4 GPa/500 °C). These belts trace the southern Neotethys suture connecting the Vardar suture in the Hellenides to the Inner Tauride suture along the southern border of the Kirşehir Complex in Central Anatolia. Eastwards, these belts are capped by the Oligo-Miocene Sivas Basin. Another HP/LT metamorphic belt, in the Alanya and Bitlis regions, outlines the southern flank of the Tauride Carbonate Platform. In the Alanya Nappes, south of the Taurides, eclogites and blueschists yielded metamorphic ages around 82-80 Ma (zircon U-Pb and phengite Ar-Ar data). The Alanya-Bitlis HP belt testifies an additional suture not comparable to the northerly Tavşanlı and Ören-Afyon belts, thus implying an additional oceanic branch of the Neotethys. The most likely eastern lateral continuation of this HP belt is the Bitlis Massif, in SE Turkey. There, eclogites (1.9-2.4 GPa/480-540°C) occur within calc-arenitic meta-sediments and in gneisses of the metamorphic (Barrovian-type) basement. Zircon U-Pb ages revealed 84.4-82.4 Ma for peak metamorphism. Carpholite-bearing HP/LT metasediments representing the stratigraphic cover of the Bitlis Massif underwent 0.8-1.2 GPa/340-400°C at 79-74 Ma (Ar-Ar on white mica). These conditions compares to the Tavşanlı-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.

  13. Origin and serpentinization of ultramafic rocks of Manipur Ophiolite Complex in the Indo-Myanmar subduction zone, Northeast India

    NASA Astrophysics Data System (ADS)

    Ningthoujam, P. S.; Dubey, C. S.; Guillot, S.; Fagion, A.-S.; Shukla, D. P.

    2012-05-01

    The Manipur Ophiolite Complex (MOC) is part of the Manipur-Nagaland ophiolite belt (MNOB). The belt is exposed in the eastern margin of the Indo-Myanmar Ranges (IMRs), which formed by the collision between the India and Myanmar continental plates. Several contrasting views were put forward concerning the origin of the MNOB. The complex represents a dismembered ophiolite sequence with serpentinite as the largest litho-unit formed. Petrography and Raman spectroscopy of the serpentinite suggest that they are serpentinized ultramafic cumulate and peridotite. The serpentinization may have occurred at a condition of low pressure and low temperature metamorphism. Geochemical signatures of the rocks and spinel grains revealed that the protolith be an abyssal peridotite, derived from a less depleted fertile mantle melt at a MORB setting after low degree (10-15%) partial melting. The study concluded that the serpentinite may have been created at a slow-spreading ridge, rather than a supra-subduction-zone setting. These rocks were later obducted and incorporated into the IMR of Indo-Myanmar suture zone.

  14. Subduction zone earthquake probably triggered submarine hydrocarbon seepage offshore Pakistan

    NASA Astrophysics Data System (ADS)

    Fischer, David; José M., Mogollón; Michael, Strasser; Thomas, Pape; Gerhard, Bohrmann; Noemi, Fekete; Volkhard, Spiess; Sabine, Kasten

    2014-05-01

    Seepage of methane-dominated hydrocarbons is heterogeneous in space and time, and trigger mechanisms of episodic seep events are not well constrained. It is generally found that free hydrocarbon gas entering the local gas hydrate stability field in marine sediments is sequestered in gas hydrates. In this manner, gas hydrates can act as a buffer for carbon transport from the sediment into the ocean. However, the efficiency of gas hydrate-bearing sediments for retaining hydrocarbons may be corrupted: Hypothesized mechanisms include critical gas/fluid pressures beneath gas hydrate-bearing sediments, implying that these are susceptible to mechanical failure and subsequent gas release. Although gas hydrates often occur in seismically active regions, e.g., subduction zones, the role of earthquakes as potential triggers of hydrocarbon transport through gas hydrate-bearing sediments has hardly been explored. Based on a recent publication (Fischer et al., 2013), we present geochemical and transport/reaction-modelling data suggesting a substantial increase in upward gas flux and hydrocarbon emission into the water column following a major earthquake that occurred near the study sites in 1945. Calculating the formation time of authigenic barite enrichments identified in two sediment cores obtained from an anticlinal structure called "Nascent Ridge", we find they formed 38-91 years before sampling, which corresponds well to the time elapsed since the earthquake (62 years). Furthermore, applying a numerical model, we show that the local sulfate/methane transition zone shifted upward by several meters due to the increased methane flux and simulated sulfate profiles very closely match measured ones in a comparable time frame of 50-70 years. We thus propose a causal relation between the earthquake and the amplified gas flux and present reflection seismic data supporting our hypothesis that co-seismic ground shaking induced mechanical fracturing of gas hydrate-bearing sediments creating pathways for free gas to migrate from a shallow reservoir within the gas hydrate stability zone into the water column. Our results imply that free hydrocarbon gas trapped beneath a local gas hydrate seal was mobilized through earthquake-induced mechanical failure and in that way circumvented carbon sequestration within the sediment. These findings lead to conclude that hydrocarbon seepage triggered by earthquakes can play a role for carbon budgets at other seismically active continental margins. The newly identified process presented in our study is conceivable to help interpret data from similar sites. Reference: Fischer, D., Mogollon, J.M., Strasser, M., Pape, T., Bohrmann, G., Fekete, N., Spieß, V. and Kasten, S., 2013. Subduction zone earthquake as potential trigger of submarine hydrocarbon seepage. Nature Geoscience 6: 647-651.

  15. 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 seaward motion of areas farther landward including the forearc and the back arc. The postseismic and interseismic crustal deformation depends on the interplay of these three primary processes. I have used three-dimensional viscoelastic finite element models to study the contemporary crustal deformation of three margins, Sumatra, Chile, and Cascadia, that are presently at different stages of their great earthquake cycles. Model results indicate that the earthquake cycle deformation of different margins is governed by a common physical process. The afterslip of the fault must be at work immediately after the earthquake. The model of the 2004 Sumatra earthquake constrains the characteristic time of the afterslip to be 1.25 yr. With the incorporation of the transient rheology, the model well explains the near-field and far-field postseismic deformation within a few years after the 2004 Sumatra event. The steady-state viscosity of the continental upper mantle is determined to be 1019 Pa S, two orders of magnitude smaller than that of the global value obtained through global postglacial rebound models.

  16. 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 of the flux of S released from slab into the mantle wedge. Based on these constraints, we calculate that 1.2 × 0.4 x 1010 mols of S/yr is released from the slab. If slab-derived S is in the S6+ oxidation state, this flux is enough to oxidize the entire mantle wedge to the Fe3+/Fe2+ observed in typical arc rocks in ~ 20 million years. [1] Hilton et al. (2002) Noble Gases in Geochemistry and Cosmochemistry. pp. 319-370 [2] de Moor et al., (in review) G-cubed [3] Rowe (1994) Chem. Geol., 236:303-322 [4] Sakai et al. (1984) J. Petrol., 52: 1307-1331 [5] Alt et al. (2012) Earth Plan. Sci. Lett., 327: 50-60

  17. 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.

  18. Accretion of continental extensional zone and development of Rehamna metamorphic dome (Morocco)

    NASA Astrophysics Data System (ADS)

    Chopin, F.; Schulmann, K.; Corsini, M.; El Houicha, M.; Ghienne, J. F.; El Attari, M.

    2009-04-01

    The Rehamna massif is a part of the Morocco Variscan belt where the metamorphic infrastructure has been exhumed in a continental accretionary wedge. The relationships between infra- and supra-structure tectonics originate by thickening of intra-continental Devonian and Carboniferous (Mississipian) basin. Two superposed deformations probably related to plate configuration changes have been identified. The first one corresponds to the development of the Barrovian metamorphism with a climax estimated at 0.5-0.7 Gpa, 500-550°C (litt.). The thickening of thermally preheated crust is related to a SW vergent nappe stacking in the orogenic infrastructure. This event terminates with large scale folding and extrusion of high grade rocks forming a large scale E-W trending metamorphic dome surrounded by un-metamorphosed Lower Palaeozoic rocks of orogenic suprastructre indicating complete decoupling between the two crustal layers. The second E-W shortening along Western Meseta Shear Zone event is responsible for the development of important deformation gradient orthogonal to the metamorphic dome axis. It is marked by an increase of the strain intensity towards the Cambrian rigid buttress located further west. This deformation gradient is marked by mechanical coupling and gentle folding of infra- and supra-structure in the east, development of slaty cleavage in the central part of the dome and intense deformation front close to the western buttress. Here the superposed fold pattern results in accentuation of asymmetrical NE-SW trending metamorphic dome close to the buttress and to further exhumation of deepest rocks associated with subsequent elevation and folding of metamorphic isograds. 40Ar/39Ar dating reveals that the cooling associated with the first thermal event occurred at around 310 Ma (biotite) while the second thermal event yields typical Alleghanian age around 280 Ma (muscovite). This last thermal event is contemporaneous with magmatic intrusions and continental basins opening in a context of Permo-Carboniferous dextral shearing.

  19. Crust and subduction zone structure of Southwestern Mexico

    NASA Astrophysics Data System (ADS)

    Suhardja, Sandy Kurniawan; Grand, Stephen P.; Wilson, David; Guzman-Speziale, Marco; Gomez-Gonzalez, Juan Martin; Dominguez-Reyes, Tonatiuh; Ni, James

    2015-02-01

    Southwestern Mexico is a region of complex active tectonics with subduction of the young Rivera and Cocos plates to the south and widespread magmatism and rifting in the continental interior. Here we use receiver function analysis on data recorded by a 50 station temporary deployment of seismometers known as the MARS (MApping the Rivera Subduction zone) array to investigate crustal structure as well as the nature of the subduction interface near the coast. The array was deployed in the Mexican states of Jalisco, Colima, and Michoacan. Crustal thickness varies from 20 km near the coast to 42 km in the continental interior. The Rivera plate has steeper dip than the Cocos plate and is also deeper along the coast than previous estimates have shown. Inland, there is not a correlation between the thickness of the crust and topography indicating that the high topography in northern Jalisco and Michoacan is likely supported by buoyant mantle. High crustal Vp/Vs ratios (greater than 1.82) are found beneath the trenchward edge of magmatism including below the Central Jalisco Volcanic Lineament and the Michoacan-Guanajuato Volcanic Field implying a new arc is forming closer to the trench than the Trans Mexican Volcanic Belt. Elsewhere in the region, crustal Vp/Vs ratios are normal. The subducting Rivera and Cocos plates are marked by a dipping shear wave low-velocity layer. We estimate the thickness of the low-velocity layer to be 3 to 4 km with an unusually high Vp/Vs ratio of 2.0 to 2.1 and a drop in S velocity of 25%. We postulate that the low-velocity zone is the upper oceanic crust with high pore pressures. The low-velocity zone ends from 45 to 50 km depth and likely marks the basalt to eclogite transition.

  20. 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 class with random Poissonian seismicity in each plate boundary step (with expectations based on the tectonic model) to assess the significance of the measures obtained. Subduction zones have velocity-dependent coupled thickness: P < 0.001 for the null hypothesis. Subduction zones with relative plate velocity <67 mm/a (which would comprise 35% of the model tectonic moment rate, in the null hypothesis) actually produce only 20% of the global subduction zone earthquakes (outside orogens), and thus have a coupled thickness about half that of faster subduction zones (if corner magnitude and spectral slope are constant). This result contradicts the uniform coupling of subduction zones inferred by Kreemer et al. [2002, Geodyn. Ser.]; the difference may be due to their exclusion of several slow subduction zones including Aegean, Cascadia, New Zealand, Caribbean, and South Shetland. Continental CCBs show a similarly strong relation (P < 0.001), with an increase in coupled thickness when velocity exceeds 25 mm/a. OSRs show coupled thickness declining with velocity, as in previous studies. OTFs and OCBs give complex results with significant variations (P < 0.01; P < 0.05) that are not easy to interpret. For CRBs and CTFs we do not reject the null hypothesis of constant coupled thickness.

  1. 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, linear subclusters, dipping along the subducting slab to the NW. The change of crustal deformation and slap dip indicates a significant deformation of the eastern HSZ between Karpathos and Crete.

  2. The Generation of Continents through Subduction Zone Processing of Large Igneous Provinces: A Case Study from the Central American Subduction Zone

    NASA Astrophysics Data System (ADS)

    Harmon, N.; Rychert, C.

    2013-12-01

    Billions of years ago primary mantle magmas evolved to form the continental crust, although no simple magmatic differentiation process explains the progression to average andesitic crustal compositions observed today. A multiple stage process is often invoked, involving subduction and or oceanic plumes, to explain the strong depletion observed in Archean xenoliths and as well as pervasive tonalite-trondhjemite-granodiorite and komatiite protoliths in the greenstone belts in the crust in the cratons. Studying modern day analogues of oceanic plateaus that are currently interacting with subductions zones can provide insights into continental crust formation. Here we use surface waves to image crustal isotropic and radially anisotropic shear velocity structure above the central American subduction system in Nicaragua and Costa Rica, which juxtaposes thickened ocean island plateau crust in Costa Rica with continental/normal oceanic crust in Nicaragua. We find low velocities beneath the active arc regions (3-6% slower than the surrounding region) and up to 6% radially anisotropic structures within the oceanic crust of the Caribbean Large Igneous Province beneath Costa Rica. The low velocities and radial anisotropy suggest the anomalies are due to pervasive deep crustal magma sills. The inferred sill structures correlate spatially with increased silicic outputs in northern Costa Rica, indicating that deep differentiation of primary magmas is more efficient beneath Costa Rica relative to Nicaragua. Subduction zone alteration of large igneous provinces promotes efficient, deep processing of primary basalts to continental crust. This scenario can explain the formation of continental lithosphere and crust, by both providing strongly depleted mantle lithosphere and a means for rapidly generating a silicic crustal composition.

  3. Thermobarometric and fluid expulsion history of subduction zones

    NASA Astrophysics Data System (ADS)

    Ernst, W. G.

    1990-06-01

    Phanerozoic, unmetamorphosed, weathered, and altered lithotectonic complexes subjected to subduction exhibit the prograde metamorphic facies sequence: zeolite → prehnite-pumpellyite → glaucophane schist → eclogite. Parageneses reflect relatively high-P trajectories, accompanied by semicontinuous devolatilization. The thermal evolution of convergent plate junctions results in early production of high-rank blueschists, high-P amphibolites, and eclogues at depth within narrow subduction zones while the hanging wall lithosphere is still hot. Protracted underflow drains heat from the nonsubducted plate and, even at profound depths, generates very low-T/high-P parageneses. Inclusion studies suggest that two-phase immiscible volatiles (liquid H2O, and gaseous high-hydrocarbons, CH4 and CO2) are evolved in turn during progressive metamorphism of the subducted sections. Expulsion of pore fluids and transitions from weathered and altered supracrustal rocks to zeolite facies assemblages release far more fluid than the better understood higher-grade transformations. Many blueschist parageneses, such as those of the internal Western Alps, have been partially overprinted by later greenschist and/or epidote-amphibolite facies assemblages. Alpine-type postblueschist metamorphic paths involved fairly rapid, nearly adiabatic decompression; some terranes even underwent modest continued heating and fluid evolution during early stages of ascent. Uplift probably occurred as a consequence of the underthrusting of low-density island arc or microcontinental crust along the convergent plate junction, resulting in marked deceleration or cessation of lithospheric underflow, decoupling, and nearly isothermal rise of the recrystallized subduction complex. Other, less common blueschist terranes, such as the eastern Franciscan belt of western California, preserve metamorphic aragonite and other high-P minerals, and lack a low-pressure overprint; physical conditions during retrogression approximately retraced the prograde path or, for early formed high-grade blocks, reflect somewhat higher pressures and lower temperatures. Subducted sections constituting portions of the Franciscan-type of metamorphic belt evidently moved slowly back up the inclined lithospheric plate junction during continued convergence and sustained refrigeration. Upward motion due to isostatic forces was produced by tectonic imbrication of fault suces, laminar return flow in melange zones, and lateral extension of the underplated accretionary prism. The ease with which volatiles are expelled from a subduction complex and migrate upward along the plate junction zone is roughly proportional to the sandstone/shale ratio: low-permeability mudstones tend to maintain fluid values approaching lithostatic, lose strength, and deform chaotically (forming melange belts), whereas permeable sandstone-rich sections retain structural/stratigraphic coherence and fail brittlely (forming coherent terranes). Because of substantial updip expulsion of volatiles during prograde recrystallization, only small amounts of H2O and CO2 are available to support hydration and carbonation of the accretionary complex during its return toward the surface; thus limited back reaction takes place and occurs at low Pfluid/Plithostatic ratios, unless an abundance of volatiles is introduced during uplift.

  4. 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 dynamic topography and could represent the dynamic effect of the slab sinking into the asthenosphere and lowering the elevation of the upper plate.

  5. Spatiotemporal evolution of dehydration reactions in subduction zones (Invited)

    NASA Astrophysics Data System (ADS)

    Padron-Navarta, J.

    2013-12-01

    Large-scale deep water cycling takes place through subduction zones in the Earth, making our planet unique in the solar system. This idiosyncrasy is the result of a precise but unknown balance between in-gassing and out-gassing fluxes of volatiles. Water is incorporated into hydrous minerals during seafloor alteration of the oceanic lithosphere. The cycling of volatiles is triggered by dehydration of these minerals that release fluids from the subducting slab to the mantle wedge and eventually to the crust or to the deep mantle. Whereas the loci of such reactions are reasonably well established, the mechanisms of fluid migration during dehydration reactions are still barely known. One of the challenges is that dehydration reactions are dynamic features evolving in time and space. Experimental data on low-temperature dehydration reactions (i.e. gypsum) and numerical models applied to middle-crust conditions point to a complex spatiotemporal evolution of the dehydration process. The extrapolation of these inferences to subduction settings has not yet been explored but it is essential to understand the dynamism of these settings. Here I propose an alternative approach to tackle this problem through the textural study of high-pressure terrains that experienced dehydration reactions. Spatiotemporal evolution of dehydration reactions should be recorded during mineral nucleation and growth through variations in time and space of the reaction rate. Insights on the fluid migration mechanism could be inferred therefore by noting changes in the texture of prograde assemblages. The dehydration of antigorite in serpentinite is a perfect candidate to test this approach as it releases a significant amount of fluid and produces a concomitant porosity. Unusual alternation of equilibrium and disequilibrium textures observed in Cerro del Almirez (Betic Cordillera, S Spain)[1, 2] attest for a complex fluid migration pattern for one of the most relevant reactions in subduction zones. This opens the possibility to correlate textural features recorded in high-pressure terrains with the physical fingerprint of dehydration reactions such as fluid flow rates and eventually seismicity or tremor. References [1] Padrón-Navarta, J. A., Tommasi, A., Garrido, C. J., López Sánchez-Vizcaíno, V., Gómez-Pugnaire, M. T., Jabaloy, A. & Vauchez, A. (2010). Fluid transfer into the wedge controlled by high-pressure hydrofracturing in the cold top-slab mantle. Earth and Planetary Science Letters 297, 271-286. [2] Padrón-Navarta, J. A., López Sánchez-Vizcaíno, V., Garrido, C. J. & Gómez-Pugnaire, M. T. (2011). Metamorphic Record of High-pressure Dehydration of Antigorite Serpentinite to Chlorite Harzburgite in a Subduction Setting (Cerro del Almirez, Nevado-Filábride Complex, Southern Spain). Journal of Petrology 52, 2047-2078.

  6. 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.

  7. Seismicity Within the West Sumatra Subduction Zone

    NASA Astrophysics Data System (ADS)

    Collings, R.; Rietbrock, A.; Lange, D.; Tilmann, F. J.; Natawidjaja, D. H.; Suwargadi, B. W.

    2009-12-01

    Oblique subduction along the western Sunda arc results in strain partitioning along the Sumatra margin. The oblique 65 mm/yr convergence is partitioned into 45 mm/yr of thrust motion accommodated along the megathrust and 11-28 mm/yr of dextral strike slip motion along the Sumatra Fault. Past seismicity along the Sumatra-Andaman margin indicates that previous large earthquakes (1797, 1833, 1861, 2004, 2005) along Sumatra margin, resulted from shallow thrust faulting below the fore-arc. The Sumatra Fault extends for 1900km from the Sunda Strait to the Andaman sea across Sumatra, creating a plate sliver known as the Sunda fore-arc. The obliquity of the subduction zone and increase in slip rate as you move northwest wards along the fault (6mm/yr at the Sunda strait to 25 mm/yr at the equator, increasing to to 50 mm/yr in the Andaman sea) results in stretching of the Sunda fore-arc. As only two thirds of the trench parallel motion is taken up by the Sumatran Fault in southern Sumatra, one third of the motion must be taken up elsewhere. Within the Sunda fore-arc, east of the Mentawai Islands at the boundary between the fore-arc ridge and the fore-arc basin lies the 600km long Mentawai fault. Originally it was suggested that the Mentawai fault was a large strike slip fault but more recent studies indicate that this may not be the case. Here, data is presented from a seismic network located within the Mentawai region of Sumatra between November 2007 and October 2008, recording the the post seismic activity of the Mw 8.5 Bengkulu 2007 earthquake including the Mw 7.2 event in February 2008. From the continuous data, arrival times of compressional (P) and shear waves (S) for 1037 events (12 600 P picks and 6 285 S picks) were manually picked, with a high quality subset of 386 events being selected for a simultaneous inversion to determine accurate hypocentres, a 1D velocity model for the region and station corrections. Hypocentre locations in the first half of the survey (November 2007 to April 2008) exhibit a high level of aftershock activity from the 2007 and 2008 earthquakes focused between the North and South Pagai islands, 100km from the trench at depths of 10-30km. Two large clusters of activity are seen 175km and 200km from the trench, within the offshore fore-arc between the mainland and the islands, which coincide with the SW and NE boundaries of the fore-arc basin. The first cluster probably corresponds to the Mentawai fault but the second cluster that extends from the slab into the overriding crust does not appear to correspond with any previously mapped faults. In the second half of the survey (April 2008 to October 2008) as the aftershock rate declines significantly, seismic activity is more evenly spread on the subduction interface.

  8. 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.

  9. 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 east indicating propagation of collision towards east. The following collision model of the Himalayan collision belt is proposed based on data published in previous studies. Collision between the Indian and Asian blocks started in the west before ca. 55 Ma. In the western part, the amount of oceanic slab subducted prior to continent collision was enough to pull the continental crust down to the depths of UHP metamorphism, as a wide ocean existed between the Asian and Indian blocks prior to collision. Following UHP metamorphism, oceanic slab break-off started at ca. 55~46 Ma in the west due to the very strong buoyancy of the deeply subducted continental block. In contrast, the subduction of continental crust continued at this time in the middle and eastern parts of the belt. The zone of break-off migrated eastward, initiating a change from steep- to low-angle subduction. Final break-off may have occurred in the easternmost part of the belt at ca. 22-25 Ma. The depth of slab break-off decreased toward the east due to the westward decrease of the amount of subducted oceanic crust along the Himalayan collision belt, resulting eastwards decrease of an uplifting rate due to a decrease in buoyancy of the continental slab. The slower uplift resulted in a longer period of thermal relaxation and a higher geothermal gradient. In the west, the high rate of uplift resulted the epidote amphibolite facies (580-610°C) retrograde metamorphic overprint on the UHP eclogites, whereas the relatively slow uplift in the mid-eastern part caused high-grade granulites (850°C) retrograde metamorphic overprint on the HP eclogites. The study indicates that the metamorphic pattern along the collision belt is strongly related to the amount of subducted oceanic crust between continents before collision and the depth of slab break-off. Therefore metamorphic pattern can be used to interpret both the disappeared and ongoing tectonic process during continental collision.

  10. 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.

  11. 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.

  12. 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 include slab rollback and a slab edge are needed to investigate the mantle structure and dynamics.

  13. Geochemical Tracing of Mantle Flow above Subduction Zones

    NASA Astrophysics Data System (ADS)

    Pearce, J. A.; Barry, T. L.; Millar, I. L.; Leat, P. T.; Stern, R. J.

    2004-12-01

    Geochemical tracing may be used to track mantle flow above and behind subduction zones and so provide an independent test of the applicability of seismic anisotropy measurements. The theory is that, if mantle flow is accompanied by decompression, then extraction of small degree melts from multi-component mantle leads to compositional gradients in the mantle in both isotope and trace element space. These gradients may be obtained by inverting geochemical data from the products of mantle melting. If mantle flow is accompanied by addition of a subduction fluid, then simultaneous melting and subduction component addition may also produce compositional gradients. Numerical experiments enable compositional gradients to be quantified in terms of the extent of melt extraction, mantle temperature and other variables. In addition, isotope and trace element systematics provide evidence for the provenance of the mantle entering the subduction system, with Hf isotopes and immobile trace elements providing a means of establishing provenance even from magmas generated directly above the dehydrating subducted plate. This work focuses on a series of geochemical maps which enable mantle flow to be traced for range of oceanic arc basin systems (Izu-Bonin-Mariana, Tonga-Vanuatu, Scotia, Manus) and, provisionally, some continental systems (Japan, Cascades). Using maps based on geochemical proxies for melt extraction (such as Ta/Yb), subduction-addition (such as Th/Ta) and mantle provenance (such as epsilon-Hf v epsilon Nd), it is possible to demonstrate the existence of a wide range of mantle flow regimes. Thus, the Izu and Japan systems appear to be characterised by simple trench-orthogonal flow, the Mariana system by dispersion away from several separate centers of mantle upwelling, the Tonga-Vanuatu system by unidirectional flow from beneath the Pacific plate in the north, and the Scotia system by bi-directional flow from both north and south. In a number of these cases, isotopic fingerprinting using immobile isotope ratios is critical for establishing the ultimate source of the mantle: for example, the mantle entering the Scotia system may be seen to originate from the Atlantic Bouvet domain rather than the Atlantic Tristan or the Pacific domains. These results are broadly in keeping with seismic anisotropy measurements to date while providing greater coverage but, of course, geochemical tracing is restricted to areas of magmatic activity. Integration of geophysical and geochemical methods may therefore be necessary to provide the maximum information on mantle flow.

  14. 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, and evolution of the last remnant of the oldest oceanic lithosphere on Earth being consumed due to the convergence of Africa and Eurasia. References: [1] Piromallo & Morelli, 2004; Lucente et al, 1999 [2] D'Agostino & Selvaggi, 2004; Serpelloni et al., 2007 [3] Royden & Husson 2006 [4] Civello & Margheriti, 2004; Evangelidis et al, 2011 [5] Lofting, 1920

  15. 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.

  16. Experimental Constraints on the Transformation of Granulite-Facies Metapelites to Eclogites and the Role of Brines in Subduction-Zone Processes

    NASA Astrophysics Data System (ADS)

    Tropper, P.; Manning, C. E.

    2007-12-01

    A common feature of HP and UHP terranes is the subduction of lower crustal rocks to great depths. Investigations in the Bergen Area in Norway have shown that this process is triggered by fluids present during eclogite-facies metamorphism (e.g. Austrheim, 1987, EPSL, 81, 221-232). Fluid inclusions in eclogite-facies minerals range from dilute solutions to chloride-rich brines and previous studies have shown that highly saline fluids may occur in subduction zones where continental material is involved. A key example of these processes is exposed at Val Savenca of the Sesia-Lanzo Zone, Italy, where Eo-Alpine eclogite-facies metamorphism and fluid flow led to partial transformation of Hercynian amphibolite-eclogite facies metapelites (garnet + biotite + sillimanite + K-feldspar + plagioclase + quartz) to zoisite + jadeite + kyanite + phengite + quartz with associated jadeite veins. Application of the K-feldspar-jadeite-quartz barometer to the plagioclase domains yields P-T conditions of 1.7-2.1 GPa at 600°C and low a(H2O) of 0.3-0.6 (Tropper et al., 1999, JMG, 17, 195-209). In order to understand the role of brines in the transformation exposed at Val Savenca, we carried out piston- cylinder experiments with a fresh, natural granulite from the Moldanubic Unit in upper Austria with the assemblage garnet + biotite + K-feldspar + plagioclase + quartz. The experiments were conducted in the presence of H2O-NaCl fluids at 600°C and 2 GPa for 2-4 days. The fluids had the compositions X(H2O) = 1.0, 0.90, 0.80 and 0.70, and fluid/rock ratio varied from 1:1 to 1:10. Oxygen fugacity was buffered at NNO and HM in most experiments. Preliminary data from unbuffered experiments show increasing reaction progress with increasing salinity in the fluid. At X(H2O) = 1.0, no visible reaction occurred, whereas at X(H2O) = 0.7, the protolith assemblage is almost completely replaced by large (up to 100 mm long), euhedral crystals of jadeite + phengite + quartz. Biotite is the only relict phase. The experiments also show a strong indication of Na-Ca exchange: the anorthite component of plagioclase goes readily into solution, producing zoisite needles only upon quench. Future experiments will be conducted on fluids in the system H2O-NaCl-CaCl2 to quantify this behavior. These experiments show that brines are highly effective in promoting reaction progress in subduction zone processes.

  17. 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 presented along with transect T2 to provide an along strike comparison of the Manila subduction zone at both latitudes. We suspect that there may be more typical oceanic crust subducting in the south and primarily sedimentary material in the prism along transect T1.

  18. 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 (αT). The results indicate that MW > 8.5 subduction earthquakes occur for rapidly shortening to slowly extending overriding plates (-3.0 ⩽ vOPD⊥ ⩽ 2.3 cm/yr), slow trench velocities (-2.9 ⩽ vT⊥ ⩽ 2.8 cm/yr), moderate to high subduction partitioning ratios (vSP⊥/vS⊥ ⩽ 0.3-1.4), low subduction thrust dip angles (δST ⩽ 30°), low subduction thrust curvature (CST ⩽ 2.0 × 10-13 m-2) and low trench curvature angles (-6.3° ⩽ αT ⩽ 9.8°). Epicenters of giant earthquakes with MW > 8.5 only occur at trench segments bordering overriding plates that experience shortening or are neutral (vOPD⊥ ⩽ 0), suggesting that such earthquakes initiate at mechanically highly coupled segments of the subduction zone interface that have a relatively high normal stress (deviatoric compression) on the interface (i.e. a normal stress asperity). Notably, for the three largest recorded earthquakes (Chile 1960, Alaska 1964, Sumatra-Andaman 2004) the earthquake rupture propagated from a zone of compressive deviatoric normal stress on the subduction zone interface to a region of lower normal stress (neutral or deviatoric tension). Stress asperities should be seen separately from frictional asperities that result from a variation in friction coefficient along the subduction zone interface. We have developed a global map in which individual subduction zone segments have been ranked in terms of their predicted capability of generating a giant subduction zone earthquake (MW > 8.5) using the six most indicative subduction zone parameters (vOPD⊥, vT⊥, vSP⊥/vS⊥, δST, CST and αT). We identify a number of subduction zones and segments that rank highly, which implies a capability to generate MW > 8.5 earthquakes. These include Sunda, North Sulawesi, Hikurangi, Nankai-northern Ryukyu, Kamchatka-Kuril-Japan, Aleutians-Alaska, Cascadia, Mexico-Central America, South America, Lesser Antilles, western Hellenic and Makran. Several subduction segments have a low score, most notably Scotia, New Hebrides and Mariana.

  19. Structure of the subduction zone south of Taiwan constrained by OBS data

    NASA Astrophysics Data System (ADS)

    Lin, Yi-Wei; Peng, Cheng-Chien; Kuo, Ban-Yuan

    2012-04-01

    Along the Manila trench the oceanic part of the Eurasian plate, i.e., the South China Sea (SCS) basin, is subducting eastward under the Philippine Sea plate. The Manila trench terminates in the north offshore southern Taiwan, where the subduction is impeded by the collision between the Luzon arc and the Eurasian continental margin which have built the Taiwan orogeny. The subduction zone structure at the ocean-continent junction south of Taiwan is so far unknown because of the lack of in-situ sampling. We have conducted OBS experiments in this region during 2009 - 2011. Events with M > 2 were relocated to better delineate the crustal and slab structure. We carried out tomographic inversion for Vp, Vs, and Vp/Vs and examine several topics including (1) whether the mantle wedge is warmer because it is exposed laterally to the asthenosphere to the north, (2) whether the forearc wedge is serpentinized, and (3) whether the impact of the collision on the subduction zone is significant.

  20. 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.

  1. 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 oceanic basin during break-up of Gondwana. The lithologies and ages of the Bagh Complex imply that these rocks formed in an area extending from the continental margin over the Neo-Tethyan ocean floor. The Bagh Complex was then juxtaposed with the Muslim Bagh Ophiolite in the final stage of tectonic emplacement.

  2. 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

  3. Earth's Rotation Variability Triggers Explosive Eruptions in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Cuffaro, M.; Sottili, G.; Palladino, D. M.; Doglioni, C.

    2014-12-01

    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 opposite 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 the LOD increases, whereas they are more frequent in backarc extensional settings along W-directed subduction zones when the LOD decreases. We infer that the LOD variability determines a modulation of the horizontal shear stresses acting on the crust-lithosphere. 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 the back arc extensional settings during LOD lowering generates a larger differential stress, which may enhance failure of the magma confining rocks. This asymmetric behaviour of magmatism contributes to the knowledge of the role of astronomical forces on the dynamics of the solid Earth.

  4. 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

  5. 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.

  6. 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.

  7. A Case for CO2-rich Subduction Zone Magmas (Invited)

    NASA Astrophysics Data System (ADS)

    Blundy, J.; Cashman, K. V.; Rust, A.; Witham, F.

    2009-12-01

    Most of what we know about the pre-eruptive volatile budgets of subduction zone magmas comes from studies of glassy melt inclusions. Melt inclusions (MI) from arc magmas, ranging in composition from basalt to rhyolite, testify to the importance of H2O, with contents of up to a few wt% in basalts, increasing to >6 wt% in more evolved compositions. This behaviour is consistent with crystallisation of anhydrous minerals from basalts under H2O-undersaturated conditions to generate andesite and dacites. The role of CO2 is less clear. MI from basalts typically have higher CO2 than those from more evolved compositions, contrary to what would be expected from fractional crystallisation alone. Moreover, suites of MI from a single eruption rarely subscribe to simple open or closed degassing trends. In fact, at many arc volcanoes H2O-CO2 systematics of melt inclusions lie closer to isopleths than degassing paths, suggestive of fluxing by CO2-rich gas. We propose that the observed patterns of H2O and CO2 in MI from andesite and dacite arc magmas are a consequence of these magmas having sufficiently high initial CO2 that they become gas saturated at or close to their inferred deep crustal source regions. The high CO2 of these magmas is a result of differentiation of parental arc basalts with high initial CO2. Once derivative magmas detach from their source regions and ascend through the crust they begin a process of degassing that strips them of most of their original CO2 content with little concomitant loss of H2O. By means of thermodynamic calculations in the well-studied analogue system albite-H2O-CO2 we show that this degassing is unaccompanied by crystallisation in all but the most slowly ascending magmas. As a consequence, magmas are unable to trap MI with CO2 contents anywhere near their original values. The lost CO2 may percolate upwards, fluxing shallower-stored magma bodies. By the time the ascending melt reaches its liquidus and traps MI, some 90% of the original CO2 has been lost. We present petrological modelling of Mount St. Helens dacites to show that initial CO2 contents at the point of generation in the lower crust were of the order 1.5 wt%, while H2O was c. 7 wt%. In contrast, the most volatile-rich MI from the May 18th 1980 plinian eruption contain 6.1 wt% H2O and only 400 ppm CO2. The role of CO2 in magmatic differentiation is often overlooked because in H2O-undersaturated systems at lower crustal pressures it has little effect on phase relations. Deep and sustained degassing of CO2 from ascending arc magmas has important consequences for the behaviour of other volatile species, while the CO2 liberated may also play a role in deep crustal metamorphism, as widely acknowledge as an important factor in the amphibolite to granulite transition.

  8. Deformation processes in great subduction zone earthquake cycles

    NASA Astrophysics Data System (ADS)

    Hu, Y.; Wang, K.; He, J.

    2011-12-01

    Crustal deformation associated with great subduction zone earthquakes yields important information on mantle rheology and slip evolution of the megathrust. We have used three-dimensional viscoelastic finite element models to study the contemporary crustal deformation of three margins, Sumatra, Chile, and Cascadia, that are presently at different stages of their great earthquake cycles. At Sumatra where an Mw 9.2 earthquake occurred in 2004, all the GPS stations are moving seaward. At Chile where an Mw 9.5 earthquake occurred in 1960, coast GPS stations are moving landward, obviously due to the re-locking of the fault, while the inland stations are still moving seaward. At Cascadia where an Mw 9.0 earthquake occurred in 1700, all the GPS stations are moving landward. The earthquake cycle deformation at Alaska where an Mw 9.2 earthquake occurred in 1964 is similar to that of Chile, and the deformation at NE Japan where an Mw 9.0 earthquake occurred in 2011 is similar to that of Sumatra. Model results indicate that the earthquake cycle deformation of different margins is governed by a common physical process. A great earthquake causes the upper plate to move towards the trench and induces shear stresses in the upper mantle. After the earthquake, the fault is re-locked, causing the upper plate to move landward. However, portions of the fault undergo aseismic afterslip for a short duration, causing the overriding areas to move seaward. At the same time, the viscoelastic stress relaxation of the upper mantle causes prolonged seaward motion in inland areas including the forearc and the back arc. After a long time when the earthquake-induced stresses have mostly relaxed, the upper plate moves landward due to the re-locking of the fault. The model of the 2004 Sumatra earthquake indicates that the afterslip must be at work immediately after the earthquake, and the characteristic time of the afterslip is ~1 yr. With the incorporation of the transient (biviscous) rheology, the model well explains the near-field and far-field postseismic deformation within a few years after the 2004 Sumatra event. For all the margins modeled, the steady-state (Maxwell) viscosity of the continental upper mantle is determined to be ~1019 Pa s, two orders of magnitude lower than that of the global value obtained through global postglacial rebound analyses. Based on the model for the 2004 Sumatra earthquake, the transient (Kelvin) viscosity of the continental mantle is one to two orders of magnitude lower than that of the stead-state viscosity. Long-term postseismic deformation is controlled mainly by the steady-state viscosity of the mantle and is relatively better understood. For the short-term postseismic deformation, the interaction of the afterslip of the fault and the transient deformation of the mantle is still poorly understood. Geodetic monitoring following the 2010 Mw 8.8 Maule earthquake and 2011 Mw 9.0 Tohoku earthquakes is expected to improve greatly our understanding the short-term deformation over the next few years.

  9. 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

    The Central Gneiss Complex, located in western British Columbia, preserves mid and lower crustal igneous and metasedimentary rocks that yield insight into deformational and thermal processes at the roots of a continental magmatic arc. The complex primarily consists of gneiss interpreted as a volcanic sequence with calcareous interlayers and lesser clastic metasedimentary rocks. Cretaceous U-Pb zircon ages from granulite-facies equivalent rocks in the core of the complex hint at rapid burial following deposition. However, a Permian or older crinoid fossil found in one locality (Hill, 1985) requires the presence of some late Paleozoic or early Mesozoic material. A new U-Pb zircon age (313±5 Ma; LA-ICPMS) from ~10 km west of this fossil locality is interpreted to record volcanism. We also conducted focused structural and geochronological analysis at higher structural levels in the northeastern Central Gneiss Complex to positively identify sedimentary lithologies with which to document the early structural history of the complex prior to early Cenozoic rapid exhumation. In this area, a subhorizontal shear zone forms the boundary between mainly clastic metasedimentary rocks and the widespread metavolcanic and carbonate rocks where an important stretched pebble conglomerate has been previously documented. In the footwall of the shear zone, flattening fabrics transition structurally upward into E-W trending stretching lineations, lineation-parallel isoclinal fold axes, and boudinage that record E-W stretching and major shear strain near the contact. S-C fabrics and shear bands yield a top to the east sense of shear. Where observed, a shallowly dipping, ~15 m thick zone of cataclasite forms the lithologic contact and overprints the shear zone. Sills and dikes record ongoing but localized magmatism throughout deformation and steep NE striking brittle normal faults crosscut all features. Ongoing work will further constrain the protolith age of these rocks, the timing of 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.

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

    NASA Astrophysics Data System (ADS)

    Dragert, H.

    2003-12-01

    For more than two decades, seismologists at the Pacific Geoscience Centre puzzled over the episodic appearance of simultaneous "noise" on seismographs from stations located over the Cascadia subduction zone in southwestern British Columbia. With the 1992 initiation of continuous GPS monitoring in Victoria, B.C., another puzzle presented itself in an inexplicable 5-mm westward displacement of the then solitary regional continuous GPS monument over a period of about a week in October 1994. These observations remained unexplained (and unrelated) until recently. Beginning in 1996, as a result of improvements in GPS orbits and regional densification of continuous GPS networks, transient aseismic crustal motions lasting from periods of a few days to over a year were starting to be recognized. For the Pacific Northwest, detailed analyses of continuous GPS data successfully resolved a spatially coherent, transient signal that occurred in August 1999. Unrelated to after-slip that can follow great thrust earthquakes or to shallow slow-slip "tsunami" earthquakes, this signal, detected at 7 contiguous GPS sites, was characterized by a change in site positions ranging from 2 to 5 mm over a period of 6 to 15 days in a direction opposite to long-term deformation motions. This brief reversal was modeled by ˜2 cm of slip on the plate interface, providing the first evidence for discrete "silent" slip events occurring on the deeper Cascadia subduction zone. In early 2002, researchers at Central Washington University established the surprising regularity of Cascadia slip events on the plate interface underlying southern Vancouver Is. and northwestern Washington State. Eight slip events were identified between 1992 and 2002, with a recurrence interval of 14.5 +/-2 months. Next, Japanese scientists discovered the episodic occurrence of unique, non-volcanic tremors at average depths of about 30 km along the Nankai Subduction Zone. The similarity of the average depth of slip and the 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.

  11. 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.

  12. 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 Hz (needed for dispersion analysis) for the large model area that is considered. Simulations with a variety of quality factors (Q) at different parts of the subduction zone have been run to investigate how seismic attenuation affects the observed dispersed waveforms. We show that the low Q in the mantle wedge can improve the fit of the dispersed waveforms. A low Q in the low velocity waveguide structure however means that the delayed high frequency energy has very low amplitude, and so is not seen clearly at the surface. The Q of the low velocity crustal waveguide must therefore be greater than 250, suggesting that melting does not occur in the subducted oceanic crust at depths of 220 km or less. The velocity contrast seen at these depths must therefore be due to compositional variations. Benchmarking 2D elastic models with the 3D case shows that 2D models give a good approximation of 3D subduction zone waveguide structure. Visco-elastic simulations show that attenuation in the mantle wedge affects the observed dispersion, but the low velocity waveguide itself does not have significantly reduced Q. This work is an example of how the increasing computing power coupled with well-defined model boundaries can allow high resolution 3D modelling to be applied to specific structures of interest.

  13. 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

    During Cretaceous times, the convergence between the Anatolide Tauride block (following the movement of Africa) and Eurasia lead to the closure of a branch of the Neotethyan ocean and to ophiolite obduction. Obducted ophiolite and their sub-ophiolitic units can be found along a 400 kilometre-long north to south transect in western Anatolia. The aim of this contribution is twofold: (1) (re)-appraise the metamorphic pressure-temperature (PT) conditions and evolution of the sub-ophiolitic units of western Anatolia, by constraining the formation of the metamorphic sole during the first stages of subduction and the unusual accretion of ocean-derived units along a subduction interface in an evolving, cooling thermal regime, and (2) understand the dynamics of a large-scale and long-lived obduction. Directly below the ophiolite (mostly made of mantle-derived rocks) lies a metamorphic sole. The upper part is this sole is made of garnet and garnet clinopyroxene amphibolites, the lower part consisting in amphibolite or green-schist facies metapelites and metabasite suggestive of discrete accretion steps. In the northern part of the section the metamorphic sole is characterised by an important blueschist-facies overprint destabilizing the amphibolite paragenesis. This high-pressure overprint is lacking in the southern area. Using field and petrological observations, three units (namely and from top to bottom, OC1, OC2 and OC3) were distinguished in the accretionary complex with PT conditions ranging from incipient metamorphism to blueschist facies conditions. OC1 represents most of the outcropping unit, is found all along the section and shows only low-grade metamorphism. Metamorphic conditions remains hard to establish in this unit made of a stack of hm-thick tectonic slices showing subtle differences in their metamorphic grade (from pristine pillow basalts and hydrothermalized lavas to lawsonite pumpellyite-lawsonite bearing basalts). In OC2, Fe-Mg carpholite-bearing layers 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.

  14. Reflection Seismic Imaging of the Subduction Zone in Southern Central Chile (Project TIPTEQ)

    NASA Astrophysics Data System (ADS)

    Gross, K.; Buske, S.; Wigger, P.

    2006-12-01

    We present the first results of the active seismic reflection survey of project TIPTEQ (from The Incoming Plate to mega-Thrust EarthQuake processes), which covers the subduction zone in southern central Chile in the area of the 1960 Valdivia earthquake hypocentre. The application of Kirchhoff prestack depth migration as well as two advanced imaging techniques (FVM, RIS) reveal a clear image of the subducted oceanic Nazca plate from the coast down to a depth of about 50 km below the Central Valley. The overriding continental crust is strongly structured showing horizontal, dipping and arching reflectors that indicate basal accretion. The reflectivity varies across the section and appears to be weak around the area of the 1960 earthquake hypocentre. Other specific features can be observed, like for instance a possible subduction channel at the top of the oceanic plate near the coast, a major crustal fault zone (LFZ) as well as a strong west dipping reflector perpendicular to the plate interface.

  15. 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.

  16. 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 the mantle wedge and a more robust prediction of the global CO2 flux in subduction.

  17. 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 between the morphology of the forearc and the peak earthquake stress drop on the subduction megathrust. We present our classification of residual bathymetric and gravitational anomalies using examples from Sumatra, Kuril-Kamchatka, Mariana, Peru-Chile and the Tonga-Kermadec margin. We reassess proposed links between trench-parallel residual topography and gravity anomalies and subduction zone seismicity using global earthquake catalogs and a new compilation of published aftershock locations and distributed slip models from over 200 of the largest subduction zone earthquakes. Our results highlight the role of pre-existing structure in both the over-riding and subducting plates in modulating the along- and across-strike segmentation of subduction zones. Understanding the genesis of long-wavelength trench-parallel forearc ridges may provide further insights into links between forearc morphology, the rheology of the overriding and subducting plates and seismicity in subduction zones.

  18. 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 mixing with hybrid composition between metasedimentary rock and metagabbro) at the metasedimentary rock/metagabbro contacts. Such fluid/rock interactions result in a strong addition of Li in glaucophane (up to 600 ppm) whereas halogen concentrations are unaffected. At eclogite facies conditions, metagabbros display low halogens concentrations (< 20 ppm of F and < 100 ppm of Cl) relative to altered oceanic crust (F = 40-650 ppm; Cl = 40-1400 ppm) suggesting that these elements are continuously released by fluids during the first 30-80 km of subduction whatever the tectonic environment (e.g. slab, plate interface) and the considered fluid/rock interactions.

  19. 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.

  20. 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.

  1. 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.

  2. Influence of deformation mechanisms and metamorphic reactions during strain localization in the continental crust under lower amphibolite facies conditions: an example from the Gotthard massif

    NASA Astrophysics Data System (ADS)

    Oliot, E.; Goncalves, P.; Schulmann, K.; Marquer, D.

    2009-04-01

    Ductile shear zones are the result of the process of strain localization in the continental crust. Depending on the metamorphic conditions during deformation, strain localization is coeval with dramatic changes in microstructures, mineralogy and mass transfers, due to the interactions with externally-derived fluid. Therefore, to accurately model the mechanisms of strain localization, it is critical to identify deformation mechanisms related to the recrystallization of the quartzo-feldspathic assemblages, and to better constrain the role of metamorphic reactions during deformation. The aim of this contribution is to characterize the mineralogical, geochemical, textural and microstructural evolution of a high strain zone from the Fibbia granite, which is located in the Gotthard Massif (External Crystalline Massif, Central Alps). This variscan massif has been affected by Alpine Tertiary metamorphism and deformation under lower amphibolite facies conditions. The strain gradient is approximately a meter width. The rock texture evolves from a weakly deformed granite, toward an orthogneiss, a mylonite and a ~10 cm-wide ultramylonite. The mineralogical assemblage changes from a metastable magmatic assemblage consisting of Qtz + Kspar + Pla + Bio ± Pheng ± Grt ± Ep to a fine banded texture consisting of a quartzo-feldspathic matrix, with metamorphic phyllosilicates (biotite and phengite) and garnet in the ultramylonite. Cathodoluminescence (CL) imaging has been used to quantify the modal proportions of phases in the quartzo-feldspathic matrix in this strain gradient. More specifically, in the orthogneiss and the mylonite, CL imaging reveals a subtle layering consisting of alternating bands of quartz-rich ribbons, K-feldspars and coupled quartz- and plagioclase-rich ribbons. The texture in the ultramylonite is more homogeneous with isolated single quartz and K-feldspar grains. CL imaging has also revealed chemical zoning, as "core and mantle" texture in plagioclases. With increasing strain, modal abundance of K-feldspars decreases from 28% to 16%, whereas both micas increase from 5% to 21%. Similarly, albite evolves from 25% to 8%, whereas oligoclase evolves from 5% to 25%. Deformation mechanisms responsible for these microstructures have been studied by combining a quantitative textural analysis (CSD, SPO, grain boundary frequency and orientation - PolyLX MatlabTM toolbox; Lexa, 2005) and a crystallographic study by EBSD. Deformation mechanisms of quartz, K-feldspar and plagioclase are a combination of SGR and GBM in the orthogneiss and in the mylonite, whereas GBS is active in the ultramylonite. CPO characteristic features are still a matter of debate. Because mass transfers occurred in this shear zone (gains of MnO, CaO, Fe2O3, P2O5 and TiO2) without volume change, thermodynamic modeling of phase relations in such open system must consider the variations of effective bulk rock composition during deformation. In this example, phase relations have been mapped using Perple_X'07 (Connolly, 2005) as a function of P, T, M(H2O) and X(bulk composition), in order to highlight the influence of subtle mass transfers on the syn-deformation stability of mineral assemblages at 500°C and 7.2 kbars. A particular attention has been paid to the role of water content on the stable assemblage and on compositions of metamorphic phyllosilicates. Water under-saturated conditions induce the stability of aluminosilicates, and should increase the Xmg in biotite and decrease the amount of tschermak substitution in phengite. P- and T-M(H20) diagram suggest that the Alpine ductile shear zones occurred under water-saturated conditions. This study reveals that strain localization is related to the metamorphic reactions (breakdowns of K-feldspars to phengites and magmatic plagioclase to albite and oligoclase), which induce a strong decrease in grain size reduction and a switch in deformation mechanism from SGR and GBM to GBS in the ultramylonite. The good agreement between phase diagram section predictions and the observations suggest that high strain zones are in thermodynamic equilibrium and the equilibrium volume is at least at the thin-section scale. Therefore, pseudosections can provide a forward model of the mineralogical evolution of metagranites during shear zone formation for any P-T-H2O conditions and chemical mass transfer. References: Connolly, J. A. D., 2005. Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation. Earth and Planetary Science Letters, 236(1-2), 524. Lexa, O., Stipska, P., Schulmann, K., Baratoux, L. & Kröner, A., 2005. Contrasting textural record of two distinct metamorphic events of similar P-T conditions and different durations. Journal of Metamorphic Geology, 23(8), 649-666.

  3. 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 therefore a potential source for rapid drop in effective pressure, possibly resulting in seismic events. Finally, the evaluation of the potential to generate methane at depth over worldwide subduction zones shows that the classical distinction between aseismic, Marianna-type subduction zones and seismic, Chilean-type subduction zones coincides with a division between methane-poor and methane-rich subducted sediments.

  4. Recurrence of postseismic coastal uplift, Kuril subduction zone, Japan

    USGS Publications Warehouse

    Kelsey, H.; Satake, K.; Sawai, Y.; Sherrod, B.; Shimokawa, K.; Shishikura, M.

    2006-01-01

    Coastal stratigraphy of eastern Hokkaido indicates that decimeters of coastal uplitt occurred repeatedly m the late Holocene. Employing radiocarbon dating and tephrochronology, we identify along a 100 km length of the Kuril subduction zone six uplift events since ???2,800 years B.P. Uplift events occur at the same frequency as unusually high tsunamis. Each coastal uplift event, which occurs on average every 500 years, is the product of decade-long post seismic deep slip on the down dip extension of the seismogenic plate boundary following an offshore multi-segment earthquake that generates unusually high tsunamis. Copyright 2006 by the American Geophysical Union.

  5. Hydration of the incoming plate in the Kuril subduction zone

    NASA Astrophysics Data System (ADS)

    Fujie, G.; Kodaira, S.; Yamashita, M.; Sato, T.; Takahashi, T.; Takahashi, N.; Noguchi, N.

    2010-12-01

    Water supplied from the subducting oceanic plate by dehydration is inferred to cause seismicity and magmatism in subduction zones. It is important, therefore, to reveal the distribution of water within the incoming plate for understanding seismic and volcanic activities in subduction zones. In 2009 and 2010, to reveal the detailed seismic structure and hydration process within the incoming plate, we conducted a wide-angle seismic survey in the Kuril subduction zone, where the old Pacific plate formed in the eastern Pacific ridge is subducting from south to north beneath the island arc of Japan. We designed a north-south 500km-long seismic experimental line to be perpendicular to the Kuril trench. The northern end of our line is located at about 30km south of the trench axis and well-developed horst and graben structure is observed around the northern end. We deployed 80 Ocean Bottom Seismometers (OBSs) at intervals of 6km and shot a large tuned airgun array towed by R/V Kairei. In addition, we obtained MCS reflection data using a 444-channel hydrophone streamer (6km long) along the same line. We modelled both P-wave and S-wave velocity structures by the traveltime inversion using refraction, reflection and PS-conversion traveltimes. Our results show that P-wave velocity beneath the well-developed horst and graben structure is about 5% lower than that in the south of outer rise. This is consistent with a previous structure study in the Chili subduction zone that shows the P-wave velocity in the vicinity of the trench axis is lower than that of normal oceanic plate. More notable feature of our results is the regional variations of Vp/Vs. The S-wave velocity, as well as P-wave velocity, gradually decreases toward the trench axis. However Vp/Vs is not uniform; Vp/Vs immediately beneath the sediments is remarkably high beneath the well-developed horst and graben structure, and Vp/Vs decreases with depth (high Vp/Vs is confined to the top of the oceanic plate). Since the high Vp/Vs implies the high degree of crustal hydration, one plausible explanation for our Vp/Vs model is that normal faults related to the well-developed horst and graben structure provide pathways for water percolation from sediments to oceanic crust, and lead to crustal hydration of the incoming plate.

  6. Background seismicity rate at subduction zones linked to slab-bending-related hydration

    NASA Astrophysics Data System (ADS)

    Nishikawa, Tomoaki; Ide, Satoshi

    2015-09-01

    Tectonic properties strongly control variations in seismicity among subduction zones. In particular, fluid distribution in subduction zones influences earthquake occurrence, and it varies among subduction zones due to variations in fluid sources such as hydrated oceanic plates. However, the relationship between variations in fluid distribution and variations in seismicity among subduction zones is unclear. Here we divide Earth's subduction zones into 111 regions and estimate background seismicity rates using the epidemic type aftershock sequence model. We demonstrate that background seismicity rate correlates to the amount of bending of the incoming oceanic plate, which in turn is related to the hydration of oceanic plates via slab-bending-related faults. Regions with large bending may have high-seismicity rates because a strongly hydrated oceanic plate causes high pore fluid pressure and reduces the strength of the plate interface. We suggest that variations in fluid distribution can also cause variations in seismicity in subduction zones.

  7. Subduction zone plate bending earthquakes and implications for the hydration of the downgoing plate

    NASA Astrophysics Data System (ADS)

    Emry, E. L.; Wiens, D. A.

    2011-12-01

    The greatest uncertainty in the amount of water input into the Earth at subduction zones results from poor constraints on the degree and depth extent of mantle serpentinization in the downgoing slab. The maximum depth of serpentinization is thought to be partly controlled by the maximum depth of tensional earthquakes in the outer rise and trench and is expected to vary from subduction zone to subduction zone or even along-strike for a single subduction zone. We explore the maximum depth of extensional faulting on the incoming plate for various subduction zones in order to gain insight into the possible extent of slab serpentinization. We relocate trench events at island arc subduction zones using hypocentroidal decomposition to determine which earthquakes occurred within the incoming plate. For earthquakes with Mw ~5.5+, we determine accurate depths and refine the CMT focal mechanism by inverting teleseismic P and SH waveforms. Results from the Mariana outer rise indicate that extensional earthquakes occur in the Pacific plate at depths ranging from 10-20 km beneath the top of the crust, with the character of trench seismicity changing significantly between the northern and southern portions of the subduction zone. In comparision, results from the Aleutian subduction zone show extensional trench earthquakes occurring from 5-30 km below the surface of the subducting slab. Compressional incoming plate earthquakes occur only near the Alaskan Peninsula, possibly due to stronger coupling between the slab and overriding plate in this region. Further results from oceanic arc subduction zones will be presented and differences between subduction zones as well as along-strike differences in the character of trench seismicity will be highlighted. If the presence of extensional faulting indicates subducting lithosphere hydration, then we expect that as much as the top 30 km of the slab may be hydrated and that the degree of slab serpentinization may vary significantly between subduction zones, potentially affecting arc geochemistry, intermediate depth seismicity, and the subduction zone water budget.

  8. 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, are more similar to MANGO 4. The arc regions appear to be strongly affected by the activity of the arc. The arc crust of the northern MANGO 3 becomes significantly thinner in the backarc region due to extension, and much reduced volcanism behind the ridge. The structures on MANGO 2, on the other hand, cover strong and densely spaced thermal activity from the adjacent arc volcanism, possibly linked to a recent, fluid-rich passage of the Hikurangi Plateau.

  9. Elasticity of Hydrous Phases in Subduction Zones- Geophysical Implications

    NASA Astrophysics Data System (ADS)

    Mookherjee, M.; Mainprice, D.

    2014-12-01

    Globally, subduction zones are region associated with earthquakes and volcanic activities, both involving risk to local populations. These processes are intimately related to the thermodynamic stability and instabilty of hydrous phases that are subducted with the down going slab. These phases sequestrate several wt % of water in their crystallographic structure and can account for significant proportion of the hydrogen budget of the upper mantle , transition zone and perhaps the top of the lower mantle. In order to quantify the degree of mantle hydration, we need to have a good understanding of the elastic properties of layered hydrous phases, the effects of temperature, and pressure and relate them to seismological observables, such as the velocity and its anisotropy. Using first principle simulations, we have investigated several layered hydrous phases, including the important minerals antigorite, talc, and chlorite. These results are complementary to the recent experimental Brillouin Scattering results at ambient conditions. Based on the full elastic constant tensor we note that these hydrous phases have significant shear wave anisotropy and often have unusual pressure dependence of the anisotropy. Together with elasticity data, thermodynamic predictions of phase stability and experimental plastic deformation studies it is apparent that these layered hydrous phases could account for the large delay times observed in certain subduction zone settings, such as Ryukyu trench. Acknowledgement- MM is supported by the US National Science Foundation grant (EAR-1250477). MM acknowledges computing resources (request # EAR130015) from the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number OCI-1053575.

  10. 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.

  11. Understanding Seismotectonic Aspects of Central and South American Subduction Zones

    NASA Astrophysics Data System (ADS)

    Vargas-Jiménez, Carlos A.; Monsalve-Jaramillo, Hugo; Huérfano, Victor

    2004-10-01

    The Circum-Pacific, and particularly the Central and South America, subduction zones are complex structures that are subject to frequent, large-magnitude earthquakes, volcanic activity, tsunamis, and geological hazards. Among these natural hazards, earthquakes produce the most significant social and economic impacts in Latin America, and the subduction zones therefore demand constant vigilance and intensive study. The American continent has witnessed serveral earthquakes that rank among the most destrive in the world. Earthquakes such as the ones that occurred in Colombia-Ecuador [Mw = 8.9, 1906], Chile [Mw = 9.6, 1960; Mw = 8.9, 1995], Mexico [Mw = 9.6, 1985], and Peru [Mw = 8.0, 2001], as well as a number of destuctive events related to crustal fault systems and volcanic eruptions [e.g., Soufrière, El Ruiz, Galeras, ect.], have produced significant human and economic loss.The latent seismic hazards in the Caribbean, and Central and South America demand from the regional Earth sciences community accurate models to explain the mechanisms of these natural phenomena.

  12. Great Earthquakes of Variable Magnitude at the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Nelson, A. R.; Kelsey, H. M.; Witter, R. C.

    2005-12-01

    Radiocarbon age ranges for evidence of great plate-boundary earthquakes and accompanying tsunamis at seven coastal paleoseismic sites, combined with relative amounts of coseismic subsidence and heights of tsunamis, show a 5000-year history of great earthquakes of variable magnitude at the Cascadia subduction zone. Evidence for rupture of most of the plate boundary (greater than 620 km) is only conclusive for the earthquake of 26 January 1700, but the earthquake about 1.6 ka also had a long rupture, substantial subsidence, and a high tsunami consistent with a magnitude 9 earthquake. Correlations suggest that earthquakes about 1.35 ka, 2.5 ka, and 3.4 ka also may have been close to magnitude 9. A rupture about 0.8 ka was limited to the northern and central parts of the subduction zone, and a northern rupture about 2.9 ka may have been similarly limited. Times of probable short ruptures in southern Cascadia include about 1.1 ka, 1.7 ka, 3.2 ka, 4.2 ka, 4.6 ka, and 4.7 ka. Rupture patterns suggest that the plate boundary in northern Cascadia usually breaks in long ruptures during the greatest earthquakes. Southernmost Cascadia is typified by short as well as long ruptures during great earthquakes of variable magnitude and more irregular recurrence than in northern Cascadia.

  13. Enrichment of trace elements in garnet amphibolites from a paleo-subduction zone: Catalina Schist, southern California

    USGS Publications Warehouse

    Sorensen, Sorena S.; Grossman, J.N.

    1989-01-01

    The abundance, P-T stability, solubility, and element-partitioning behavior of minerals such as rutile, garnet, sphene, apatite, zircon, zoisite, and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones. The influence of these minerals on the composition of subduction-related magmas has been inferred (and disputed) from inverse modelling of the geochemistry of island-arc basalt, or by experiment. Although direct samples of the dehydration + partial-melting region of a mature subduction zone have not been reported from subduction complexes, garnet amphibolites from melanges of circumpacific and Caribbean blueschist terranes reflect high T (>600??C) conditions in shallower regions. Such rocks record geochemical processes that affected deep-seated, high-T portions of paleo-subduction zones. In the Catalina Schist, a subduction-zone metamorphic terrane of southern California, metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta-ultramafic rocks. This mafic and ultramafic complex may represent either slab-derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab-mantle wedge contact, or both. The trace-element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by INAA, XRF, electron microprobe, and SEM. In order of increasing alteration from a probable metabasalt protolith, three common types of garnet amphibolite blocks in the Catalina Schist are: (1) non-migmatitic, clinopyroxene-bearing blocks, which are compositionally similar to MORB that has lost an albite component; (2) garnet-amphibolite blocks, which have rinds that reflect local interaction between metabasite, metaperidotite, and fluid; and (3) migmatites that are extremely enriched in Th, HFSE, LREE, and other trace elements. These trace-element enrichments are mineralogically controlled by rutile, garnet, sphene, apatite, zircon, zoisite, and allanite. Alkali and alkaline earth elements are much less enriched in the solid assemblage, and thus appear to be decoupled from the other elements in the inferred metasomatic process(es). The compositions of migmatitic garnet amphibolite blocks seem to complement that of "average" island-arc tholeiite. Trace-element metasomatism reflects fluid-solid, rather than melt-solid, interaction. The metasomatic effects indicate that H2O-rich fluid, perhaps with a significant component of Na-Al silicate and alkalis, carried Th, U, Sr, REE, and HFSE. Fractionations of LREE in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex. "Exotic" LREE deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment. If the paleo-subduction zone represented by the mafic and ultramafic complex of the Catalina Schist had continued its thermal and fluid evolution, a selvage of similarly enriched rocks might have been generated along the slab-mantle wedge contact between ~30 and 85 km depth. Rocks affected by "subduction-zone metasomatism," although rarely recognized at the surface, could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments. ?? 1989.

  14. Enrichment of trace elements in garnet amphibolites from a paleo-subduction zone: Catalina Schist, southern California

    NASA Astrophysics Data System (ADS)

    Sorensen, S. S.; Grossman, J. N.

    1989-12-01

    The abundance, P- T stability, solubility, and element-partitioning behavior of minerals such as rutile, garnet, sphene, apatite, zircon, zoisite, and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones. The influence of these minerals on the composition of subduction-related magmas has been inferred (and disputed) from inverse modelling of the geochemistry of island-arc basalt, or by experiment. Although direct samples of the dehydration + partial-melting region of a mature subduction zone have not been reported from subduction complexes, garnet amphibolites from melanges of circumpacific and Caribbean blueschist terranes reflect high T (>600°C) conditions in shallower regions. Such rocks record geochemical processes that affected deep-seated, high- T portions of paleo-subduction zones. In the Catalina Schist, a subduction-zone metamorphic terrane of southern California, metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta-ultramafic rocks. This mafic and ultramafic complex may represent either slab-derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab-mantle wedge contact, or both. The trace-element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by INAA, XRF, electron microprobe, and SEM. In order of increasing alteration from a probable metabasalt protolith, three common types of garnet amphibolite blocks in the Catalina Schist are: (1) non-migmatitic, clinopyroxene-bearing blocks, which are compositionally similar to MORB that has lost an albite component; (2) garnet-amphibolite blocks, which have rinds that reflect local interaction between metabasite, metaperidotite, and fluid; and (3) migmatites that are extremely enriched in Th, HFSE, LREE, and other trace elements. These trace-element enrichments are mineralogically controlled by rutile, garnet, sphene, apatite, zircon, zoisite, and allanite. Alkali and alkaline earth elements are much less enriched in the solid assemblage, and thus appear to be decoupled from the other elements in the inferred metasomatic process(es). The compositions of migmatitic garnet amphibolite blocks seem to complement that of "average" island-arc tholeiite. Trace-element metasomatism reflects fluid-solid, rather than melt-solid, interaction. The metasomatic effects indicate that H 2O-rich fluid, perhaps with a significant component of Na-Al silicate and alkalis, carried Th, U, Sr, REE, and HFSE. Fractionations of LREE in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex. "Exotic" LREE deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment. If the paleo-subduction zone represented by the mafic and ultramafic complex of the Catalina Schist had continued its thermal and fluid evolution, a selvage of similarly enriched rocks might have been generated along the slab-mantle wedge contact between ~30 and 85 km depth. Rocks affected by "subduction-zone metasomatism," although rarely recognized at the surface, could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments.

  15. 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 important stepping-stone in resolving margin structure: the concepts outlined here have developed from observations identified in high-quality data (seismic wide-angle refraction and multichannel reflection data, high-resolution bathymetry), which have been analyzed using state- of-the art processing modules in combination with novel methods. Geophysical imaging shows margin segmentation to be far more complex than previously inferred. The Sunda accretionary wedge was a type-example of 'steady-state' accretion forming a uniform body of imbricate thrusts. However, the newly observed segmentation of structure across-strike correlates to mechanics that vary during an earthquake cycle. Furthermore, the recognition of splay faults and their potential role in tsunami excitation represents an important advance for hazard mitigation efforts. In addition, the impact of lower plate structural diversity on along-strike segmentation is documented. Comparative studies from subduction zones around the globe show the identified structures to be consistent in different margin settings. Geophysical imaging may thus provide the observational foundation from which to assess the spatial and temporal variation of interplate seismicity.

  16. 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 sense. In the west, the subduction zone terminates in a distributed zone of parallel NE-SW strike-slip faults, of which the most prominent is the Kefalonia Transform Fault (KTF). A flexural gravity anomaly confirms that the deep bathymetric escarpment of the KTF is a lateral ramp, formed as the Ionian islands are emplaced SW onto the Apulian lithosphere, and enhanced by minor thrust faulting with slip vectors perpendicular to the scarp. Distributed parallel strike-slip faults both SW and NE of mainland central Greece terminate in E-W graben in central Greece, which accommodate the overall NE-SW shear by clockwise block rotation. Central Greece therefore acts as a relay zone between the strike-slip faulting of the NE Aegean and the Ionian Islands-western Peloponnese.

  17. 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 involved. Comparable geochemical fingerprints observed in HP and UHP Ulten and Bardane mantle rocks call for concomitant subduction of the continental crust, that would provide the incompatible element-enriched fluids. For Bardane this implies subduction of crustal slabs to 200 km depth, to release the fluid phases necessary for UHP metasomatism and for majoritic garnet crystallization. Mantle refertilization by crust-derived COH subduction fluids thus operates over a large depth range, down to 200 km depth. The textural features of the Bardane and Ulten Zone mantle rocks indicate that rock recrystallization and refertilization is concomitant with fluid infiltration along channelways, outside which the rocks preserve pre-subduction, long-lasting stories and pristine geochemical characteristics. Comparably with uprising magmas, the subducted continental crust appears to be a quite efficient carrier to the surface of ultradeep mantle tectonic 'xenoliths' representing major observatories of diverse metamorphic and geodynamic histories recorded by the Earth's mantle at variable depths.

  18. 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

  19. Sources of Magmatic Volatiles Discharging from Subduction Zone Volcanoes

    NASA Astrophysics Data System (ADS)

    Fischer, T.

    2001-05-01

    Subduction zones are locations of extensive element transfer from the Earth's mantle to the atmosphere and hydrosphere. This element transfer is significant because it can, in some fashion, instigate melt production in the mantle wedge. Aqueous fluids are thought to be the major agent of element transfer during the subduction zone process. Volatile discharges from passively degassing subduction zone volcanoes should in principle, provide some information on the ultimate source of magmatic volatiles in terms of the mantle, the crust and the subducting slab. The overall flux of volatiles from degassing volcanoes should be balanced by the amount of volatiles released from the mantle wedge, the slab and the crust. Kudryavy Volcano, Kurile Islands, has been passively degassing at 900C fumarole temperatures for at least 40 years. Extensive gas sampling at this basaltic andesite cone and application of CO2/3He, N2/3He systematics in combination with C and N- isotopes indicates that 80% of the CO2 and approximately 60% of the N 2 are contributed from a sedimentary source. The mantle wedge contribution for both volatiles is, with 12% and 17% less significant. Direct volatile flux measurements from the volcano using the COSPEC technique in combination with direct gas sampling allows for the calculation of the 3He flux from the volcano. Since 3He is mainly released from the astenospheric mantle, the amount of mantle supplying the 3He flux can be determined if initial He concentrations of the mantle melts are known. The non-mantle flux of CO2 and N2 can be calculated in similar fashion. The amount of non-mantle CO2 and N2 discharging from Kudryavy is balanced by the amount of CO2 and N2 subducted below Kudryavy assuming a zone of melting constrained by the average spacing of the volcanoes along the Kurile arc. The volatile budget for Kudryavy is balanced because the volatile flux from the volcano is relatively small (75 t/day (416 Mmol/a) SO2, 360 Mmol/a of non-mantle CO2 and 5.4 Mmol/a of non-mantle N2). Other subduction zone volcanoes are currently degassing a much more substantial amount of volatiles. Popocatepetl, Mexico, has degassed approximately 14 Mt of SO2 to the atmosphere over the past 6 years (Witter et al. 2000). Satsuma-Iwojima, Japan, has degassed for longer than 800 years and is currently releasing 500-1000 tones/day (Kazahaya et al. 2000). At these volcanoes CO2 and N2 discharges from the magma should also be balanced by the supply from slab and crustal sources. The rate of subduction off Mexico and Japan, however, is similar to the rate at the Kuriles. Therefore, large amounts of slab derived volatiles must be, in some fashion, stored in the "subduction factory" to supply the large amounts degassing passively from these volcanoes. Kazahaya et al. (2000) Seventh Field Workshop on Volcanic Gases, IAVCEI. Witter et al (2000) Seventh Field Workshop on Volcanic Gases, IAVCEI.

  20. Late Holocene tectonics and paleoseismicity, southern Cascadia subduction zone

    USGS Publications Warehouse

    Clarke, S.H., Jr.; Carver, G.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.

  1. 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.

  2. 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

  3. 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.

  4. 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.

  5. When Boundary Layers Collide: Plumes v. Subduction Zones

    NASA Astrophysics Data System (ADS)

    Moresi, L. N.; Betts, P. G.; Miller, M. S.; Willis, D.; O'Driscoll, L.

    2014-12-01

    Many subduction zones retreat while hotspots remain sufficiently stable in the mantle to provide an approximate reference frame. As a consequence, the mantle can be thought of as an unusual convecting system which self-organises to promote frequent collisions of downgoing material with upwellings. We present three 3D numerical models of subduction where buoyant material from a plume head and an associated ocean-island chain or plateau produce flat slab subduction and deformation of the over-riding plate. We observe transient instabilities of the convergent margin including: contorted trench geometry; trench migration parallel with the plate margin; folding of the subducting slab and orocline development at the convergent margin; and transfer of the plateau to the overriding plate. The presence of plume material beneath the oceanic plateau causes flat subduction above the plume, resulting in a "bowed" shaped subducting slab. In the absence of a plateau at the surface, the slab can remain uncoupled from the over-riding plate during very shallow subduction and hence there is very little shortening at the surface or advance of the plate boundary. In plateau-only models, plateau accretion at the edge of the overriding plate results in trench migration around the edge of the plateau before subduction re-establishes directly behind the trailing edge of the plateau. The plateau shortens during accretion and some plateau material subducts. In a plateau-plus-plume model, accretion is associated with rapid trench advance as the flat slab drives the plateau into the margin. This indentation stops once a new convergent boundary forms close to the original trench location. A slab window formed beneath the accreted plateau allows plume material to flow from beneath the subducting plate to the underside of the overriding plate. In all of these models the subduction zone maintains a relatively stable configuration away from the buoyancy anomalies within the downgoing plate. The models provide a dynamic context for plateau and plume accretion in accretionary orogenic systems.

  6. Great earthquakes of variable magnitude at the Cascadia subduction zone

    USGS Publications Warehouse

    Nelson, A.R.; Kelsey, H.M.; Witter, R.C.

    2006-01-01

    Comparison of histories of great earthquakes and accompanying tsunamis at eight coastal sites suggests plate-boundary ruptures of varying length, implying great earthquakes of variable magnitude at the Cascadia subduction zone. Inference of rupture length relies on degree of overlap on radiocarbon age ranges for earthquakes and tsunamis, and relative amounts of coseismic subsidence and heights of tsunamis. Written records of a tsunami in Japan provide the most conclusive evidence for rupture of much of the plate boundary during the earthquake of 26 January 1700. Cascadia stratigraphic evidence dating from about 1600??cal yr B.P., similar to that for the 1700 earthquake, implies a similarly long rupture with substantial subsidence and a high tsunami. Correlations are consistent with other long ruptures about 1350??cal yr B.P., 2500??cal yr B.P., 3400??cal yr B.P., 3800??cal yr B.P., 4400??cal yr B.P., and 4900??cal yr B.P. A rupture about 700-1100??cal yr B.P. was limited to the northern and central parts of the subduction zone, and a northern rupture about 2900??cal yr B.P. may have been similarly limited. Times of probable short ruptures in southern Cascadia include about 1100??cal yr B.P., 1700??cal yr B.P., 3200??cal yr B.P., 4200??cal yr B.P., 4600??cal yr B.P., and 4700??cal yr B.P. Rupture patterns suggest that the plate boundary in northern Cascadia usually breaks in long ruptures during the greatest earthquakes. Ruptures in southernmost Cascadia vary in length and recurrence intervals more than ruptures in northern Cascadia.

  7. Great earthquakes of variable magnitude at the Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Nelson, Alan R.; Kelsey, Harvey M.; Witter, Robert C.

    2006-05-01

    Comparison of histories of great earthquakes and accompanying tsunamis at eight coastal sites suggests plate-boundary ruptures of varying length, implying great earthquakes of variable magnitude at the Cascadia subduction zone. Inference of rupture length relies on degree of overlap on radiocarbon age ranges for earthquakes and tsunamis, and relative amounts of coseismic subsidence and heights of tsunamis. Written records of a tsunami in Japan provide the most conclusive evidence for rupture of much of the plate boundary during the earthquake of 26 January 1700. Cascadia stratigraphic evidence dating from about 1600 cal yr B.P., similar to that for the 1700 earthquake, implies a similarly long rupture with substantial subsidence and a high tsunami. Correlations are consistent with other long ruptures about 1350 cal yr B.P., 2500 cal yr B.P., 3400 cal yr B.P., 3800 cal yr B.P., 4400 cal yr B.P., and 4900 cal yr B.P. A rupture about 700-1100 cal yr B.P. was limited to the northern and central parts of the subduction zone, and a northern rupture about 2900 cal yr B.P. may have been similarly limited. Times of probable short ruptures in southern Cascadia include about 1100 cal yr B.P., 1700 cal yr B.P., 3200 cal yr B.P., 4200 cal yr B.P., 4600 cal yr B.P., and 4700 cal yr B.P. Rupture patterns suggest that the plate boundary in northern Cascadia usually breaks in long ruptures during the greatest earthquakes. Ruptures in southernmost Cascadia vary in length and recurrence intervals more than ruptures in northern Cascadia.

  8. 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.

  9. High-temperature metamorphism during extreme thinning of the continental crust: a reappraisal of the North Pyrenean passive paleomargin

    NASA Astrophysics Data System (ADS)

    Clerc, C.; Lahfid, A.; Monié, P.; Lagabrielle, Y.; Chopin, C.; Poujol, M.; Boulvais, P.; Ringenbach, J.-C.; Masini, E.; de St Blanquat, M.

    2015-06-01

    An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a data set of more than 100 peak-temperature estimates obtained using Raman spectroscopy of the carbonaceous material (RSCM). This data set is completed by previous PT (pressure and temperature) estimates based on mineral assemblages, and new 40Ar-39Ar (amphibole, micas) and U-Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomargins are discussed. Ages range mainly from 110 to 90 Ma, and no westward or eastward propagation of the metamorphism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key localities of the Mauléon basin, Arguenos-Moncaup, Lherz, Boucheville and the Bas-Agly, we analyze the thermal conditions prevailing during the Cretaceous crustal thinning. The results are synthetized into a series of three regional thematic maps and into two detailed maps of the Arguenos-Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamorphism are intimately associated with the areas where subcontinental mantle rocks have been unroofed or exhumed.

  10. Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America

    USGS Publications Warehouse

    Nelson, A.R.; Shennan, I.; Long, A.J.

    1996-01-01

    Tidal-wetland stratigraphy reveals that great plate boundary earthquakes have caused hundreds of kilometers of coast to subside at the Cascadia subduction zone. However, determining earthquake recurrence intervals and mapping the coastal extent of past great earthquake ruptures in this region are complicated by the effects of many sedimentologic, hydrographic, and oceanographic processes that occur on the coasts of tectonically passive as well as active continental margins. Tidal-wetland stratigraphy at many Cascadia estuaries differs little from that at similar sites on passive-margin coasts where stratigraphic sequences form through nonseismic processes unrelated to coseismic land level changes. Methods developed through study of similar stratigraphic sequences in Europe provide a framework for investigating the Cascadia estuarine record. Five kinds of criteria must be evaluated when inferring regional coastal subsidence due to great plate boundary earthquakes: the suddenness and amount of submergence, the lateral extent of submerged tidal-wetland soils, the coincidence of submergence with tsunami deposits, and the degree of synchroneity of submergence events at widely spaced sites. Evaluation of such criteria at the Cascadia subduction zone indicates regional coastal subsidence during at least two great earthquakes. Evidence for a coseismic origin remains equivocal, however, for the many peat-mud contacts in Cascadia stratigraphic sequences that lack (1) contrasts in lithology or fossils indicative of more than half a meter of submergence, (2) well-studied tsunami deposits, or (3) precise ages needed for regional correlation. Paleoecologic studies of fossil assemblages are particularly important in estimating the size of sudden sea level changes recorded by abrupt peat-mud contacts and in helping to distinguish erosional and gradually formed contacts from coseismic contacts. Reconstruction of a history of great earthquakes for the Cascadia subduction zone will require rigorous application of the above criteria and many detailed investigations.

  11. 3D Lithospheric Density Structure of the Central American Subduction Zone from Gravity Data

    NASA Astrophysics Data System (ADS)

    Lücke, O. H.; Arroyo, I. G.; Linkimer, L.

    2013-12-01

    Data from the EGM2008 Combined Geopotential Model has been interpreted to construct a comprehensive three-dimensional model of the lithospheric density structure along the Central American Isthmus. This is the first time that integration of all geophysical information available for the isthmus has been undertaken. The density model is constrained by seismic velocity models, magnetotelluric cross-sections, receiver functions,and hypocenter data from local seismic networks acquired along the Middle American Subduction Zone by different institutes and projects during the last three decades. The segmentation of the crustal basement of the Caribbean Plate was modeled with separate units for the Chortis Block (2.77 Mg/m^3), the Mesquito Composite Oceanic Terrane / SiunaTerrane (3 Mg/m^3), and the Caribbean Large Igneous Province (2.90 Mg/m^3). Furthermore, first order boundary layers such as the Moho and the Cocos-Caribbean plate interface were modeled and extracted for correlation with tectonic features and dynamic processes. The Costa Rican segment has been the most widely studied along the Central American margin. Here, it is possible to review the slab geometry based on the three-dimensional density model against seismological information from local networks in greater detail. By integrating probabilistic relocated hypocenters with the density model by means of 3D visualization, a joint interpretation of the distribution of seismicity with the density units in the subducted slab was carried out.A change in the depth of intra-plate seismicity is observed reaching 220 km for the northwestern part and becoming shallower toward the southeast where it reaches a maximum depth of 75 km. The changes in the maximum depth registered for the seismicity, correlate with changes in the density structure of the subducted slab which were modeled based on the gravity response of the model. The crust of the oceanic plate was assigned an initial density of 2.80 Mg/m3, deeper than this the density changes within the slab were modeled based on published petrological calculations constrained by thermal and lithostatic pressure conditions and considering metamorphic reactions. In consequence, the initial density for the Cocos Plate crust increases to 3.15 Mg/m^3 corresponding to the downdip extent of the Wadati-Benioff seismicity and then to 3.30 Mg/m^3 for the deepest, aseismic sections of the slab where the subducted crust is interpreted as being anhydrous. The increase in density varies in depth along the subduction zone and its correlation with the terminal depth of intraslab seismicity supports the hypothesis that differences in the state of initial hydration of the oceanic lithosphere affect the depth reached by dehydration reactions.

  12. 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 conductivity consistent with a dry peridotite composition and thermal models of the system. At a depth of around 80 km the mantle intersecting the slab shows a slight increase in conductivity. This increase is not sufficient to require the presence of melt, but a conductor indicative of melt can be inserted into the model at this depth without compromising the fit.

  13. 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 shallowest circulation being driven by forced return flow. The thicknesses estimated here for serpentinized layers at intermediated depths are similar to the precision of seismic studies. The deepest serpentinite channel may thus be difficult to detect by seismic methods, but it will have a strong influence on the mechanical coupling between the slab and mantle wedge.

  14. Subduction zone evolution and low viscosity wedges and channels

    NASA Astrophysics Data System (ADS)

    Manea, Vlad; Gurnis, Michael

    2007-12-01

    Dehydration of subducting lithosphere likely transports fluid into the mantle wedge where the viscosity is decreased. Such a decrease in viscosity could form a low viscosity wedge (LVW) or a low viscosity channel (LVC) on top of the subducting slab. Using numerical models, we investigate the influence of low viscosity wedges and channels on subduction zone structure. Slab dip changes substantially with the viscosity reduction within the LVWs and LVCs. For models with or without trench rollback, overthickening of slabs is greatly reduced by LVWs or LVCs. Two divergent evolutionary pathways have been found depending on the maximum depth extent of the LVW and wedge viscosity. Assuming a viscosity contrast of 0.1 with background asthenosphere, models with a LVW that extends down to 400 km depth show a steeply dipping slab, while models with an LVW that extends to much shallower depth, such as 200 km, can produce slabs that are flat lying beneath the overriding plate. There is a narrow range of mantle viscosities that produces flat slabs (5 to10 × 10 19 Pa s) and the slab flattening process is enhanced by trench rollback. Slab can be decoupled from the overriding plate with a LVC if the thickness is at least a few 10 s of km, the viscosity reduction is at least a factor of two and the depth extent of the LVC is several hundred km. These models have important implications for the geochemical and spatial evolution of volcanic arcs and the state of stress within the overriding plate. The models explain the poor correlation between traditional geodynamic controls, subducting plate age and convergence rates, on slab dip. We predict that when volcanic arcs change their distance from the trench, they could be preceded by changes in arc chemistry. We predict that there could be a larger volatile input into the wedge when arcs migrate toward the trench and visa-versa. The transition of a subduction zone into the flat-lying regime could be preceded by changes in the volatile budget such that the dehydration front moves to shallower depths. Our flat-slab models shed some light on puzzling flat subduction systems, like in Central Mexico, where there is no deformation within the overriding plate above the flat segment. The lack of in-plane compression in Central Mexico suggests the presence of a low viscosity shear zone above the flat slab.

  15. Sublithospheric Triggers for Episodic Silicic Magmatism in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Gerya, T.; Vogt, K.; Schubert, M.

    2014-12-01

    The melt source and ascent mechanisms for crustal-scale silicic magmatism in subduction zones remain a matter of debate. Recent petrological-thermo-mechanical numerical experiments suggest that important physical controls of this process can be of sublithospheric origin. Firstly, deep sources of silicic magma can be related to episodic development of positively buoyant diapiric structures in the mantle wedge originated from deeply subducted rock mélanges (Gerya and Yuen, 2003; Castro and Gerya, 2008). Partial melting of these rapidly ascending lithologically mixed structures can produce silicic magmas with a relatively constant major element composition and variable time-dependent isotopic ratios inherited from the mélange (Vogt et al., 2013). Secondly, episodic injections of subduction-related mantle-derived mafic magmas into a partially molten hot zone of the arc lower crust can drive ascents of pre-existing felsic crustal magmas toward upper crustal levels. The injection of mafic magma induces overpressure in the lower crustal magma reservoir, which increases crustal stresses and triggers development of brittle/plastic fracture zones serving as conduits for the rapid episodic ascent of felsic magmas (Shubert et al., 2013). Our numerical results thus imply that subduction-related sublithospheric magma intrusions into the lower arc crust may both be the prime source for the generation of silicic magmas and the major physical driving mechanism for their episodic ascent toward upper crustal levels. References:Castro, A., and Gerya, T.V., 2008. Magmatic implications of mantle wedge plumes: experimental study. Lithos 103, 138-148. Gerya, T.V., and Yuen, D.A., 2003. Rayleigh-Taylor instabilities from hydration and melting propel "cold plumes" at subduction zones. Earth and Planetary Science Letters 212, 47-62.Schubert, M., Driesner, T., Gerya, T.V., Ulmer, P., 2013. Mafic injection as a trigger for felsic magmatism: A numerical study. Geochemistry, Geophysics, Geosystems, 14, 1910-1928.Vogt, K., Castro, A., and Gerya, T., 2013. Numerical modeling of geochemical variations cause by crustal relamination. Geochemestry Geophysics Geosystems 14, 470-487.

  16. Initiation of the Fiordland-Puysegur subduction zone, New Zealand

    NASA Astrophysics Data System (ADS)

    Mao, X.; Gurnis, M.; May, D.

    2014-12-01

    The Australia-Pacific plate boundary south of New Zealand was an active ridge 45 Ma to 30 Ma, generating oceanic crust between the Resolution Rifted Margin and the Campbell Rifted Margin. Referred to as the Macquarie Ridge Complex (MRC), this boundary progressively evolved into a strike-slip boundary from 30 Ma to 20 Ma; the northern segment, the Fiordland-Puysegur subduction zone (FPSZ), has had a substantial transpressional component. Over the last 20 Myr, 600 km of highly oblique plate motion occurred at the MRC, and resulted in a maximum total convergence of 150-200 km at the FPSZ, which some simple models suggested might be near the threshold for a self-sustaining subduction. The morphology of the Puysegur Ridge shows a diagnostic change from uplift to subsidence expected for the transition of a subduction zone from being forced externally to being internally driven by the negative buoyancy of the slab. The large negative gravity anomalies over the Snares Zone, in the middle of the FPSZ, imply strong vertical forces pulling downward the lithosphere. To better understand these observations, we use a viscous flow forward model with a free surface to simulate the geodynamics of the FPSZ since 20 Ma. The forward model describes the dynamics of an incompressible, Stokes fluid. Brittle-ductile behavior of the material within the crust-asthenosphere is modeled by using a fluid viscosity defined via a composite flow law comprised from an Arrhenius and a Drucker-Prager rheology. The well-constrained relative plate motion between the Australian and Pacific plates is used to define a Dirichlet boundary condition for velocity within the lithosphere. In the mantle, we apply the hydrostatic pressure as a normal stress boundary condition. A simplified surface process model consisting of linear diffusion is applied at the free surface to simulate short-range erosion and sedimentation. Our models show that the topographic variations within the Puysegur Ridges may correspond to different stages of subduction initiation. The Snares Zone may be self-sustaining, while the northern and southern parts of the FPSZ are approaching such a self-sustaining threshold.

  17. 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 depths causes mantle stagnation and a cool condition, which allows serpentinization to occur, whereas coupling at greater depths drives hot flowing mantle, providing the thermal condition required for melt generation in the mantle wedge. The flowing mantle also causes rapid heating of the subducting slab and affects the occurrence of intraslab earthquakes. In the generic model calculations in the study, we also investigate the effect of local fluctuations in the depth of decoupling-coupling transition on the 3-D mantle wedge flow pattern and thermal structure. Kneller, E.A., and P.E. van Keken (2008), Effect of three-dimensional slab geometry on deformation in the mantle wedge: Implications for shear wave anisotropy, Geochem. Geophys. Geosyst., 9, Q01003, doi:10.1029/2007GC001677.

  18. Lithium and its isotopes as tracers of subduction zone fluids and metasomatic processes: Evidence from the Catalina Schist, California, USA

    NASA Astrophysics Data System (ADS)

    Penniston-Dorland, Sarah C.; Bebout, Gray E.; Pogge von Strandmann, Philip A. E.; Elliott, Tim; Sorensen, Sorena S.

    2012-01-01

    The Catalina Schist, well-studied for its tectonometamorphic history and geochemical evolution, provides a field area in which to evaluate the extent of Li mobility in subduction-zone fluids and the degree to which Li isotopes can trace metamorphic fluid-rock interactions. A suite of 63 samples of the Catalina Schist that represent a range of metamorphic grades and diverse lithologies was analyzed for Li concentration and isotope composition. Metasedimentary rocks show no evidence for significant loss of Li, with increasing metamorphic grade, despite the wide range of prograde P-T histories represented by the five tectonometamorphic units (with the highest-grade rocks having experienced peak temperatures near 750 °C). Metamafic rocks similarly show no change in Li concentration, as a function of grade, that can be directly related to varying degrees of devolatilization. The Li concentrations and δ7Li of metasedimentary rocks appear to best correlate with a chemical alteration index that likely reflects differential extents of weathering in the sediment sources. The δ7Li of most metamafic rocks (including gabbroic metaconglomerate cobbles), veins and mélange matrix reflects overprinting of the Li isotope compositions derived (through transfer of Li in fluids) from sedimentary lithologies in the same tectonometamorphic units. Thus it appears that Li was mobilized in fluids released from metasedimentary rocks but that the loss was insufficient to affect the bulk δ7Li and Li content of the metasedimentary rocks. Veins and mélange matrix have the highest Li concentrations of all samples analyzed, ranging up to 70 ppm. Some veins and amphibolite-grade mafic rocks have low δ7Li suggestive of Li diffusion through an intergranular fluid. Taken together, these observations suggest mobilization of Li during high pressure/temperature (P/T) fluid-rock interactions that also resulted in enrichments in B, N, and some large ion lithophile elements (K, Rb, Cs, Ba) in the veins, mélange matrix, and cobbles. This study demonstrates that, although Li is mobile in high P/T metamorphic fluids, it does not partition into fluids to the extent demonstrated for N, B, and Cs, the latter elements showing large depletions in the higher-grade Catalina Schist metasedimentary rocks. Thus, even in relatively warm subduction zones, deeply subducted metasedimentary rocks are expected to largely retain their Li inventory to depths of at least 40 km, the maximum depth at which units of the Catalina Schist experienced peak metamorphism.

  19. 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.

  20. 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.

  1. 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.

  2. 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.

  3. 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

  4. 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

  5. Deep Postseismic Viscoelastic Relaxation Excited By an Intra-Slab Normal Faulting Earthquake in the Chile Subduction Zone

    NASA Astrophysics Data System (ADS)

    Bie, L.; Ryder, I. M. A.

    2014-12-01

    Geodetic observations of postseismic deformation have been widely modeled to constrain the rheology of lower crust and/or upper mantle in variable tectonic environments. In subduction zones, postseismic deformation following megathrust earthquakes is often explained by afterslip and/or viscoelastic relaxation (VER) processes, and explicitly modeled to infer the viscosity of the mantle wedge. Besides the megathrust events, large intermediate-depth normal faulting earthquakes also induce stresses in surrounding lithosphere. Presumably, these stresses will be gradually released by viscoelastic relaxation in any nearby weak layer(s) and hence provide opportunities to infer rheological properties. The 2005 Mw 7.8 Tarapaca earthquake is identified as a result of normal faulting on a west-dipping plane at depths between 90 and 115 km within the subducting slab of northern Chile (Delouis and Legrand, 2007). The Envisat-recorded short-term postseismic deformation shows an elliptical pattern of subsidence across the fault's surface projection. We construct a semi-analytical three-dimensional model, which takes into account the vertical and horizontal heterogeneities of viscosity. Our preliminary results of VER modelling show a first-order similarity in spatial pattern with the InSAR observed deformation, assuming a Maxwell viscosity of 5e18 Pa s for the continental upper mantle and 1e20 Pa s for the oceanic upper mantle. This study demonstrates the feasibility of using normal faulting earthquakes to constrain deep subduction zone rheology.

  6. 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.

  7. 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

  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. 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

  10. 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 landward and downward. At the toe of the deformation front, velocities are higher than 4.0 km/s, indicating extensive dewatering of deep, oceanic sediment. Further landward, considerable velocity variation is found, which indicates major breaks throughout the accretionary history.

  11. 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.

  12. 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 the sedimentary infill of the trench is almost nonexistent, short-term shallow coupling occurs and friction between the frontal sector of the overriding plate and the downgoing plate triggers upper-plate bending. In this sector, after the early short-term coupling, the overriding plate is hereafter decoupled from the subducting slab. Moving along trench-strike, where sediments amount increases, the upper plate couples with the subducting plate and is dragged coherently downwards. If a large amount of sediments is stored in the trench the overriding plate is scraped off and incorporated as fragments along the plate interface. Our results suggest that a) one main parameter controlling coupling at convergent plate margins is the occurrence and the amount of sediment at the trench; b) the upper plate margin is dragged to depth or destroyed only where sediments thickness at the trench is large enough to promote interplate coupling, suggesting that a variation of sediment amount along the trench-axis influences the amount and style of transport of upper-plate material in the mantle.

  13. The metamorphic evolution of the eclogitic rocks from the 100-2000m core of Chinese Continental Scientific Drilling, China

    NASA Astrophysics Data System (ADS)

    You, Z.; Su, S.; Liang, F.; Zhang, Z.

    2004-12-01

    On the basis of petrographic study of the metamorphic rocks from 100-2000m in the main hole of the Chinese Continental Scientific Drilling (CCSD) it is revealed that the major lithological types are: (1)Eclogites and garnet clinopyroxenites; (2)Eclogitic gneisses; (3)Garnet peridotites; (4)Biotite (hornblende) two-feldspar gneiss (granitic gneiss) and (5) Fault breccia and mylonites. Most of the eclogites and eclogitic gneisses had a long residence history in the crust before the deep subduction of the continental slab. However the garnet clinopyroxenites are closely related with garnet peridotites which occur within 607-783m of the drill hole. Based on their mineralogical and petrochemical characteristics they were ultramafic-mafic complexes incorporated into the subducted slab from the overlying mantle wedge during the subduction of the continental slab and had undergone ultrahigh"Cpressure metamorphism (UHPM). The eclogitic gneisses, equivalent partly to paragneiss as called by some geologists, are UHPM rocks of intermediate acidic protolith associated with crustal eclogites. The granitic gneisses are of diverse origin, most of them are products of decompressive melting of the retrogressive amphibolite and biotite gneiss derived from eclogitic rocks. The metamorphic evolution of the eclogitic rocks can be divided into 3 major stages. The first (M1) is the ultrahigh pressure metamorphism (UHPM) as evidenced by coesite inclusions within garnet and omphacite grains in eclogites. The second (M2) is a retrogressive stage sequence during which most of the UHPM rocks were turned to rocks of high-pressure eclogite facies, amphibolite facies and then epidote amphibolite facies. At this stage the eclogites are characterized by the growth of symplectites and coronas. The eclogitic gneiss is retrograded to be biotite (hornblende) plagioclase gneiss and epidote biotite (amphibole) gneiss. The granitic rocks are turned to biotite hornblende two feldspar gneiss (so-call orthogneiss) due to partial melting or K-metasomatism of the HP supercritical fluid. Some of them are allanite (Ce)"Cbearing, the La/Ce=0.42¡ª0.72. The allanite are zonal and are rimmed by epidote, representing the later overprint of epidote amphibolite facies retrometamorphism. The third stage (M3) is characterized by the formation of tectonic breccia and mylonites in response to the brittle and ductile-brittle deformation related with uplift. The matrix of these tectonites containing chlorite, actinolite and calcite indicates the greenschist facies of metamorphism (M3). All of the above-mentioned 3 stages of metamorphism are records of the tectonic processes surpassed by these eclogitic rocks. They delineate a clock-wise metamorphic PTt D path of the Donghai UHP terrane which is comparable in pattern with those of Dabieshan. The metamorphic evolution and the successive deformational events found in the 100¡ª2000m core from the CCSD main hole confirms that during the collision of the two (Yangtze and North China) plates voluminous crustal materials including granitic rocks can deeply be subducted to mantle depth and then rapidly returned back to the earth surface. Acknowledgement: This work is sponsored by the Major State Key Project to CCSD: 2003CB716501 and major project from NSFC: 40399142.

  14. Coseismic Slip Beneath Forearc Basins in Great Subduction Zone Earthquakes: Implications for the Size and Mode of Rupture on the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Wells, R. E.; Blakely, R.; Sugiyama, Y.; Scholl, D.

    2002-12-01

    We have examined the relationship between coseismic slip and forearc structure for 29 of the largest circum-Pacific megathrust earthquakes. Coseismic slip distributions were compiled from published seismic, geodetic, and tsunami waveform inversions, and we interpreted forearc structure from satellite gravity and bathymetry and marine geology. Seismogenic slip is generally focused beneath forearc deep sea terraces and basins, which are underlain by relatively high velocity arc or continental crust. Along the non-accretionary margins of NE Japan, Kuriles, and Kamchatka, the high slip areas roughly correlate with the width of the deep sea terrace, and the landward edge of the terrace approximately coincides with the landward limit of interplate thrust focal mechanisms. Along accretionary margins, coseismic slip is commonly focused beneath the offshore forearc basins. In the 1923, 1944, 1946, and 1968 earthquakes along the Nankai and Sagamai Trough of SW Japan, slip was focused beneath five forearc basins, and the presently locked Tokai source region is centered on a sixth. The steep gravity gradient marking the landward edge of the basins coincides with the landward decrease in coseismic slip and the 350°C isotherm on the plate boundary, approximately marking the down-dip limit to stick-slip behavior. Similar coseismic slip beneath basins is also observed along the Aleutian, Mexico, Peru, and S. Chile subduction zones. Transverse forearc gravity highs which separate the basins commonly overlie areas of lower coseismic slip, as at Cape Erimo separating the 1952 and 1968 Tokachi-oki earthquakes off Hokkaido, the Shumagin gap separating the 1938 and 1946 earthquakes in S. Alaska, and the Portlock anticline separating the Kodiak and Prince William Sound asperities in 1964. If the long-term slip budget is balanced along the margin, then the intervening gravity highs may be future sources of great slip not observed historically, or more likely are regions of smaller interseismic strain accumulation, as is observed in the Shumagin gap. The empirical relationship between high coseismic slip and forearc basins suggests that forearc basins may be useful indicators of long-term seismic moment release in some subduction zones. The inferred source zone of the 1700 AD Mw~9 Cascadia earthquake contains five very large basin-centered gravity lows, the largest of which is 350 km long off the mouth of the Columbia River. These lows, corresponding to the Eel, Coos Bay, Newport-Willapa, and Olympic basins, lie within the locked and transition zones inferred from geodetic data and may indicate at depth. The steep gravity gradient marking the inboard edge of the basins and presumably the downdip limit to large coseismic slip lies beneath Grays Harbor and the western Olympic Peninsula in Washington and just offshore Oregon, between the 350°C and 450° C isotherms on the megathrust. Transverse gravity highs between the basins suggest the margin is seismically segmented and may produce a variety of large earthquakes.

  15. Rare earth element budgets in subduction-zone fluids

    NASA Astrophysics Data System (ADS)

    Tsay, A.; Zajacz, Z.; Sanchez-Valle, C.

    2012-12-01

    Subduction zone fluids play a fundamental role in the geochemical cycle of the Earth. The nature and composition of these fluids are determined by complex processes and still poorly understood. As a result of a variety of metasomatic and partial melting events, arc-related magmas display a typical trace element abundance spectrum, in which the rare earth elements' (REE) signature is an important record of petrogenetic processes. Therefore, investigating the behavior of REE in fluids at high pressure (P) and temperature (T) conditions is crucial for constraining fluid composition, as well as understanding subduction-zone processes in general. However, up to date, the experimental studies on REE solubility and speciation are limited to quite low P-T conditions (300 °C, saturated water vapor pressure) [1]. The theoretical predictions of the stability of REE complexes have been performed up to 350 °C [2] and 1000 °C, 0.5 GPa [3] by the extrapolation of thermodynamic data obtained at ambient conditions. In this study we present new experimental data on REE silicate (REE2Si2O7) solubility in aqueous quartz saturated fluids, containing various ligands, at conditions relevant for subducting slabs (600, 700, 800 °C, 2.6 GPa). The aim of the experiments was to investigate the relative effect of temperature and ligands on the solubility of REE. The experiments were conducted in an end-loaded piston-cylinder apparatus and the fluids were in situ sampled at P-T in the form of primary fluid inclusions in quartz [4]. The gold capsule was typically loaded with a chip of synthetic REE silicate (La,Nd,Gd,Dy,Er,Yb)2Si2O7, an aqueous fluid (~20 wt.%) and a piece of natural quartz. During the experiment (24-48 h) a thermal gradient along the capsule promoted intensive dissolution of quartz at the hottest part and precipitation of new quartz at the cooler part of the capsule, allowing the primary fluid inclusions to be trapped (~30-50 μm). Rubidium and cesium were added to the fluid as the internal standards for LA-ICPMS analyses. The solubility of REE in quartz saturated water, free of additional ligands, increases more than an order of magnitude as temperature is increased from 600 °C to 800 °C. The effect of halogen ligands (Cl-, F-) on the solubility of REE was tested on experiments conducted at 800 °C. Addition of 1.5 m NaCl enhances the solubilities of all REE by a factor of 2 to 4 and induces moderate LREE/HREE fractionation; the La/Yb ratio increases by factor of 2. Unlike chlorine, the presence of fluorine ligands in the fluid (0.3 m NaF) promotes increase in HREE solubilities with almost no change in LREE solubilities compared to water, hence decreasing the La/Yb ratio by a factor of 2. The results of our experiments suggest that temperature plays an important role in mobilization of all REE by fluids. The presence of Cl- and F- ligands in the fluid shows opposing effects on the REE pattern: Cl- seems to be a more efficient ligand for LREE, while F- tends to form more stable complexes with HREE. [1] Migdisov A. A. et al., 2009, Geochim Cosmochim Acta, 73, 7087-7109 [2] Wood S. A. 1990b, Chem Geo., 82, 159-186 [3] Haas J. R. 1995, Geochim Cosmochim Acta, 59, 4329-4350 [4] Bali E. et al., 2011, Contrib Mineral Petrol, 161, 597-613

  16. 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.

  17. Rheology of magnesite and implications for subduction zone dynamics

    NASA Astrophysics Data System (ADS)

    Holyoke, C. W.; Kronenberg, A. K.; Newman, J.; Ulrich, C. A.

    2013-12-01

    We deformed two natural magnesite aggregates over a wide range of temperatures (400-1000oC) and strain rates (10-7 - 10-4/s) in order to determine the deformation mechanisms of magnesite and their respective rheologies. The two magnesite aggregates have similar compositions, but different grain sizes (1 vs. 100 μm). Experiments using fine-grained magnesite were performed in a Heard-type gas confining medium rock deformation apparatus at a constant effective pressure (= confining pressure - CO2 pressure) of 300 MPa. Experiments using coarse-grained magnesite were performed using molten salt or solid salt assemblies in a Griggs-type piston-cylinder rock deformation apparatus at a constant effective pressure of 900 MPa. At low temperatures (T≤600oC, strain rate = 10-5/s) both magnesite aggregates deform by crystal plastic mechanisms predominated by dislocation glide. However, at higher temperatures the coarse-grained magnesite aggregate deforms by dislocation creep and the fine-grained magnesite aggregate deforms by diffusion creep. The strain rate and temperature dependence of the low temperature plasticity, dislocation creep and diffusion creep rheologies can be described by power laws with stress exponents (n) of 19.7, 3.0 and 1.1 and activation enthalpies of 229, 410 and 209 kJ/mol, respectively. The rheology of the low temperature plasticity data can also be described using an exponential flow law with α = 0.022 MPa-1 with a best-fit activation enthalpy of 233 kJ/mol. Extrapolation of the experimentally determined rheological data to natural conditions indicates that magnesite is generally stronger than calcite and dolomite assuming similar grain sizes. However, its strength is orders of magnitude lower than olivine at all conditions in the Earth's mantle. Thus magnesite may act as a weak phase in altered lithosphere of subduction zones, and it may even promote deep-focus earthquakes through ductile instabilities.

  18. The earliest mantle fabrics formed during subduction zone infancy

    NASA Astrophysics Data System (ADS)

    Harigane, Y.; Michibayashi, K.; Morishita, T.; Tani, K.; Dick, H. J.; Ishizuka, O.

    2013-12-01

    Harzburgites obtained from the oldest crust-mantle section in the Philippine Sea plate along the landward slope of the southern Izu-Ogasawara Trench in Izu-Bonin-Mariana arc, that explored by Dive 7K417 of the ROV Kaiko 7000II during R/V Kairei cruise KR08-07, and Dredge 31 of R/V Hakuho-Maru cruise KH07-02, operated by the Japan Agency for Marine-Earth Science and Technology. Harzburgites preserve mantle fabrics formed during the infancy of the subduction zone; that is during the initial stages of Pacific plate subduction beneath the Philippine Sea plate. The main constituent minerals of harzburgites are olivine (15.6%), orthopyroxene (Opx; 13.1%) and spinel (0.5%), along with serpentine (70.8%) as a secondary mineral. Microstructure shows inequigranular interlobate (or protogranular) textures. There is no secondary deformation such as porphyroclastic or fine-grained textures. The secondary serpentine shows undeformed mesh texture in the harzburgites. Harzburgites have crystal preferred orientation patterns in olivine (001)[100] and Opx (100)[001]. The mineral chemistry in harzburgites have high olivine forsterite (90.6-92.1 mol.%) and NiO (~0.4 wt%) contents, low Opx Al2O3 (<~1.5 wt%) and Na2O (<0.03 wt%), and high spinel Cr# (65-67). This has the characteristics of residual peridotites, whereas the dunites, obtained from the same location as the harzburgites, provide evidence for the earliest stages of arc volcanism during the inception of subduction. Therefore, we propose that the (001)[100] olivine patterns began forming in immature fore-arc mantle with an increase in slab-derived hydrous fluids during the initial stages of subduction in in situ oceanic island arc.

  19. 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 accretionary prism.

  20. 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.

  1. 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 record the progressive thermal maturation of the juvenile Neotethyan subduction zone. This period of ~23 myr between subduction initiation and thermal "steady state" is significantly shorter than that obtained for the Rio San Juan Complex (~60 myr; Krebs et al. 2008, Lithos, 103, 106-137), but compares well with that for the Franciscan Complex (~22 myr; Anczkiewicz et al. 2004, EPSL, 225, 147-161) and falls in the range predicted in numerical simulations (e.g., Gerya et al. 2002, Tectonics, 21/6, 1056).

  2. Slab dehydration and fluid-producing metamorphic reactions in early subduction stages: the record of the metamorphic sole of the Mont Albert ophiolite (Quebec, Canada)

    NASA Astrophysics Data System (ADS)

    Jewison, Ella; Soret, Mathieu; Dubacq, Benot; Agard, Philippe; Labrousse, Loc

    2015-04-01

    Metamorphic soles found at the base of obducted ophiolites provide valuable information on the early history of the subduction / obduction system. Metamorphic soles are characterised by rocks originating from the ocean floor (basalts and sediments in variable proportions) metamorphosed up to granulite facies, where the intensity of metamorphism increases to the top of the unit, towards the contact with peridotite. Their mafic and less frequently pelitic lithologies make them sensitive recorders of their pressure-temperature conditions of crystallization and allow radiometric dating. In addition, metamorphic soles have directly witnessed slab dehydration as they underwent similar fluid-producing metamorphic reactions before being accreted to the mantle wedge peridotites (i.e. before "underplating"). The mechanisms of underplating remain uncertain, because of the somewhat obscure link between weakening through fluid production and hardening via garnet crystallization, with direct consequences on the rheology of the plate interface. In this study, we document fluid-producing reactions occurring during the prograde history of the metamorphic sole of the Taconian (ca. 460 Ma) ophiolite from Mont Albert (Quebec, Canada). This metamorphic sole shows variably metamorphosed mafic and pelitic rocks with metamorphic gradients over the scale of 10 metres, with clinopyroxene-garnet-amphibole granulite facies mafic rocks at the contact with the overlying peridotites. Evidences of melting of pelitic lithologies increase towards the contact, and no remains of metapelites have been found within about 20 m from the contact. Fluid channelization and melt migration is evidenced by decimetric dykes and veins. Away from the contact, metamorphism intensity gradually decreases to greenschist facies with abundant hydrated silicates. The aim of the study is to provide constraints (i) on the nature of the fluids produced (aqueous versus melt), (ii) on their composition and (iii) on the pressure-temperature conditions of their production. This will allow a better understanding of the rheological behaviour of subducting slabs in subduction zones and of amphibolites in the lower continental crust.

  3. 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.

  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 imaged in our previous study because we had to impose smoothing constraints. Our result implies that transdimensional imaging can be used to reveal detailed seismic structures from spatially non-uniform data.

  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 fluid flow into the existing thermo-mechanical finite element code and present first results of benchmarks (e.g. solitary wave) 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.

  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 result of very large gravitational instability from phase transitions. Prominent slow anomalies are also revealed in the mantle under the subducting slabs, which may represent either mantle plumes or upwelling flows associated with the deep subduction of the slabs.

  7. Seismogenic Coupling at Convergent Margins - Geophysical Observations from the South American Subduction Zone and the Alpine Rock Record

    NASA Astrophysics Data System (ADS)

    Oncken, O.

    2008-12-01

    Convergent continental margins are the Earth's principal locus of important earthquake hazards with nearly all interplate megathrust earthquakes (M>8) in the seismogenic coupling zone between the converging plates. Despite the key importance of this zone, the processes that shape it are poorly understood. This is underscored by a number of novel observations attributed to processes in the interface zone that are attracting increasing attention: silent slip events, non-volcanic tremors, afterslip, locked patches embedded in a creeping environment, etc. We here compare the rock record from a field study with recent results from two major geophysical experiments (ANCORP and TIPTEQ) that have imaged the South Chilean subduction zone at the site of the largest historically recorded earthquake (Valdivia, 1969; Mw = 9.5) and the plate boundary in Northern Chile, where a major seismic event is expected in the near future (Iquique segment). The reflection seismic data exhibit well defined changes of reflectivity and Vp/Vs ratio along the plate interface that can be correlated with different parts of the coupling zone as well as with changes during the seismic cycle. Observations suggest an important role of the hydraulic system. The rock record from the exhumed Early Tertiary seismogenic coupling zone of the European Alps provides indications for the mechanisms and processes responsible for the geophysical images. Fabric formation and metamorphism in a largely preserved subduction channel chiefly record the deformation conditions of the pre-collisional setting along the plate interface. We identify an unstable slip domain from pseudotachylytes occurring in the temperature range between 200-300°C. This zone coincides with a domain of intense veining in the subduction mélange with mineral growth into open cavities, indicating fast, possibly seismic, rupture. Evidence for transient near-lithostatic fluid pressure as well as brittle fractures competing with mylonitic shear zones continues into the region below the occurrence of pseudotachylytes, possibly reflecting a zone of conditionally stable slip. This latter zone is characterized by a pervasive fabric in the subduction mélange with elongated clasts that may well contribute to a layered reflection image. The zone above the unstable slip area has a chaotic appearance and is devoid of veins, but displays ample evidence of fluid-assisted processes similar to the deeper zone: solution-precipitation creep and dehydration reactions in the mélange matrix, hydration and sealing of the base of the upper plate. Seismic rupture here is possibly expressed by ubiquitous localized deformation zones. We hypothesize that trenchward sealing of parts of the plate interface as well as reaction-enhanced destruction of upper plate permeability is an important component localizing the unstable slip zone. Temporal variations in the hydraulic system are probably related to the seismic cycle and may be candidates for the geophysical observations identified.

  8. Serpent: Magnetic signatures of serpentinized mantle and mesoscale oceanic variability along the Alaska/Aleutian subduction zone

    NASA Astrophysics Data System (ADS)

    Purucker, Michael; Serpent Team

    2010-05-01

    NASA recently solicited suborbital missions as a part of its new Earth Venture program element. These missions are designed as complete PI-led investigations to conduct innovative, integrated, hypothesis or scientific question driven approaches to pressing questions in Earth System science. The missions should require sustained observations (<5 years) and significant resources (<30 million USD). The submitted mission proposals have been under evaluation since last November, and NASA is scheduled to make a decision in April. We, a team led by Raytheon's Photon Research Associates, propose to carry out a suborbital magnetic survey of the Aleutian subduction zone using NASA's Global Hawk to test the magnetic serpentinite hypothesis. This hypothesis states that dewatering of the descending slab within subduction zones produces an observable static magnetic signature through the formation of serpentinite in the overriding mantle. This signature may serve as a predictor of the location of large megathrust earthquakes and their associated tsunamis. Magnetic field measurements from 20 km (sub-orbital) altitude are essential to the testing of this hypothesis; analysis shows orbital and/or near-surface measurements are not likely to provide sufficient sensitivity and uniform calibration to confirm or reject the hypothesis, nor to consistently map its presence around the world. Static and dynamic magnetic signatures from the motion of seawater in the earth's magnetic field have the potential to confound an evaluation of the magnetic serpentinite hypothesis. Through a combination of modeling and exact repeat surveys over the subduction zone, spaced weeks to as much as six months apart, we can study the magnetic signature of the motion that characterizes the mesoscale oceanic circulation in order to develop the best possible corrections for lithospheric imaging, and elucidating the intrinsic and unique oceanic information content in the magnetic fields for the first time ever. The role of water in subduction zones, and in the overlying ocean, can be traced by sustained suborbital observations of the magnetic field. At critical depths of 40 to 50 km, subducting ocean crust goes through important metamorphic changes that release large amounts of water into overriding mantle rocks. Introduction of water into the mantle produces serpentinite, a highly magnetic, low-density rock. Thermal models indicate that, in many of the world's subduction zones, this part of the mantle is cooler than the Curie temperature of magnetite, the most important magnetic mineral in serpentinite, and thus large volumes of mantle in subduction-margin settings should be magnetic. Indeed, analysis of magnetic data from some subduction zones indicates that magnetic mantle can be detected in long-wavelength magnetic anomalies. The presence of serpentinite in subduction margins has two important links to large within-slab and giant megathrust earthquakes, and associated tsunamis. First, release of water from the subducting slab is thought to embrittle the slab, thereby promoting within-slab earthquakes (M 7-8). Thus, we expect to see a spatial association between this type of earthquake and mantle magnetic anomalies. Second, in cool subduction margins, the down-dip limit of megathrust earthquakes (M 8.0-9.6) is controlled by the slab's first encounter with serpentinized mantle. Again, we expect to see a spatial association between these devastating earthquakes and magnetic anomalies. The magnetic serpentinite hypothesis can be tested by comparison to free-air gravity, geologic, topographic, and bathymetric data of comparable resolution. Significant static and dynamic magnetic fields also originate as a consequence of oceanic flow in electrically conducting ocean water above the subduction zone. Although these signals are of much lower amplitude than the magnetic field associated with serpentinite, they can have significant power at short spatial scales, and thus have the potential to confound estimated magnetic source depths that rely on inferences from the horizontal magnetic gradient. Although a source of noise from the perspective of imaging the lithosphere, the motionally induced magnetic fields also present new opportunities for remote sensing in ocean and climate studies.

  9. 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.

  10. Temperature Distribution and Mineralogy of Deep Subduction Zones: Insights From Numerical Modelling Including Phase Transitions

    NASA Astrophysics Data System (ADS)

    Valera, J. L.; Negredo, A. M.; Carminati, E.

    A kinematic model to determine the thermal evolution of subducting slabs and sur- rounding mantle is carried out. We develop a finite difference algorithm to solve the energy equation using the alternating difference implicit method. As an innovation to commonly used thermal models, which only account for heat advection and conduc- tion, our model includes latent heat release due to phase transformation, shear and adiabatic heating and radiogenic heat production. This latter feature allows to model subduction of continental lithosphere. Moreover, the heat transfer between the slab and the surrounding mantle is accounted for in a self consistent way. We incorpo- rate in our modelling two different simplified disequilibrium phase diagrams of the olivine-spinel transition. As in previous models we find that for sufficiently cold slab geotherms, the olivine to spinel phase change is hindered and a metastable wedge of olivine (MTWO) protrudes into the transition zone. Olivine to spinel reactions within MTWO are suggested to generate deep focus earthquakes. We investigate the effect of subduction velocity and dip, and of the age of the subducting lithosphere on the formation of MTWO and, for several deep subduction zones occurring worldwide, we look for a correlation between conditions favourable for the formation of MTWO and the occurrence of deep earthquakes. Furthermore, the computed density distribution is an output relevant for future dynamic models of subduction to quantify the com- bined effects of negative thermal buoyancy due to low temperature within the slab and buoyancy forces arising from density contrasts between olivine and spinel. The pres- ence of MTWO in the transition zone is shown to create a negative density contrast with respect to the surrounding mantle in spinel phase, and therefore tends to oppose subduction.

  11. 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 the activity of the arc. The arc crust of the northern MANGO 3 becomes significantly thinner in the backarc region due to extension, whereas the data from MANGO 2 likely show thermal activity from the adjacent arc volcanism.

  12. 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 concentrated in the lower crust and in the mantle wedge, close to the island arc beneath the inner forearc domain. In comparison, little seismicity is observed beneath the outer forearc domain. We propose that the difference of the seismicity beneath the inner and the outer forearc is related to a difference of crustal structure between the inner forearc interpreted as a dense, thick and rigid crustal block and the lighter and more flexible outer forearc. Seismicity is enhanced beneath the inner forearc because it likely increases the vertical stress applied to the subducting plate. At depth, interplate earthquakes observed between 35 and 45 km depth, deeper than the Moho of the forearc (~30 km), possibly reveal the downdip limit of the seismogenic zone. The Thales Scientific Party is composed of: Bayrakci, G., Bécel, A., Charvis, P., Diaz, J., Evain, M., Flueh, E., Gallart, J., Gailler, A., Galve, A., Hello, Y., Hirn, A., Kopp, H., Krabbenhoeft, A., Laigle, M., Lebrun, J. F., Monfret, T., Papenberg, C., Planert, L., Ruiz, M., Sapin, M., Weinzierl, W.

  13. Mantle convection and crustal tectonics in the Tethyan subduction zone

    NASA Astrophysics Data System (ADS)

    Jolivet, L.; Sternai, P.; Menant, A.; Faccenna, C.; Becker, T. W.; Burov, E. B.

    2013-12-01

    Mantle convection drives plate tectonics and the size, number and thermotectonic age of plates codetermines the convection pattern. However, the degree of coupling of surface deformation and mantle flow is unclear. The use of SKS waves seismic anisotropy shows a coherence of mantle and surface deformation, but significant examples depart from this scenario. We review geological observations and present kinematic reconstructions of the Aegean and Middle East and 3D numerical models to discuss the role of asthenospheric flow in crustal deformation. At the scale of the Mediterranean backarcs, lithosphere-mantle coupling is effective below the most extended regions as shown by the alignment of SKS fast orientations and stretching directions in MCCs. In the Aegean, the directions of mantle flow, crustal stretching and GPS velocities are almost parallel, while, below the main part of the Anatolian plate, SKS fast orientations are oblique to GPS velocities. When considering the long-term geological history of the Tethyan convergent, one can conclude that asthenospheric flow has been an important player. The case of Himalaya and Tibet strongly supports a major contribution of a northward asthenospheric push, with no persistent slab that could drive India after collision, large thrust planes being then decoupling zones between deep convection and surface tectonics. The African plate repeatedly fragmented during its migration, with rifting of large pieces of continents that had then been moving northward faster than Africa (Apulia, Arabia). This also suggests a dominant role of an underlying flow at large scale, dragging and mechanically eroding plates and breaking them into fragments, then passively carried. Mantle flow thus seems to be able to carry plates toward subduction zones, break-away pieces of plates, and deform backarc upper crust where the lithosphere is the thinnest. Most numerical models of lithospheric deformation are designed such that strain is a consequence of kinematic boundary conditions (push or pull on lateral sides), and rarely account for basal stresses due to mantle flow. On the other hand, convection models often treat the lithosphere as a single-layer stagnant lid with vertically undeformable surface. There is thus a gap between convection models and lithospheric-scale geodynamic models. We test different degrees of coupling using 3-D lithospheric deformation models. Preliminary results suggest that lithosphere can be carried by asthenospheric flow, which may lead to plate fragmentation, especially if this flow is applied on a large surface and involves mantle upwelling. However, the presence of a ductile lower crust inhibits the upward transmission of stresses. A highly extended crust in a hot environment such as a backarc domain, with no lithospheric mantle and a ductile lower crust in direct contact with asthenosphere, is more prone to follow the mantle flow than a thick and stratified lithosphere.

  14. Serpentine rheology and its significance on subduction zone processes

    NASA Astrophysics Data System (ADS)

    Katayama, I.; Hirauchi, K.

    2011-12-01

    Serpentinite is expected to present at the subducting plate interface, where released water from the descending slab reacts to the mantle rocks. Serpentinites are characterized by low seismic velocity and high Poisson's ratio, and such anomalies are detected in various subduction systems (e.g. Kamiya and Kobayashi 2000; Hyndman and Peacock 2003). In order to understand rheological behavior of serpentine at the plate interface, we are performing deformation experiments of serpentine at high pressures and temperatures. In this study, we introduce several topics related to serpentine rheology in subduction systems, including (1) down-dip limit of interplate earthquake, (2) excess pore fluid pressure and low-frequency tremor and (3) coupling-decoupling at the subducting plate interface. (1) Down-dip limit of interplate earthquake is generally controlled by the brittle-ductile transition at temperatures around 350-400C. However, in some subduction zones where a cold plate is subducting, the down-dip limit coincides with the depth of crustal Moho at temperatures lower than the brittle-ductile transition (Oleskevich et al. 1999). Seno (2005) proposed this seaward shift of down-dip limit resulted from weak serpentine at the plate interface. To test this hypothesis, we performed deformation experiments of serpentine at conditions corresponding to the Moho of cold subduction systems (P=1GPa, T=200-300C). Experimental results show that deformation of serpentine is mostly controlled by plastic flow rather than brittle failure, suggesting that the presence of serpentine at plate interface inhibits the initiation of subduction interplate earthquake. (2) Low-frequency tremor is mostly located at depths of 35-40 km, where the subducting plate meets the island arc Moho (Shelley et al. 2006). Such regions are characterized by low velocity anomaly and high Poisson's ratio, suggesting the presence of serpentine with excess aqueous fluids. This can be resulted of back stopped fluid migration at the island arc Moho due to the permeability contrast between serpentinite and gabbro. The excess fluids could cause a stick-slip type unstable sliding of serpentinite, and may trigger the low-frequency earthquake at the tip of mantle wedge. (3) Coupling/decoupling between mantle wedge and subducting plate is a key for material circulation and thermal structure of subduction systems. Numerical modeling shows that the low-viscosity layer such as serpentinite at the plate interface causes decoupling, and the forearc mantle wedge becomes stagnant (Wada et al. 2008). We are investigating the viscosity contrast between serpentines and olivine by laboratory experiments. Preliminary results show that the high-temperature serpentine (antigorite) is slightly weaker than olivine by a factor of 2, whereas low-temperature serpentines (lizardite, chrysotile) are characterized by one order of magnitude lower viscosity than that of olivine. This indicates that the coupling-decoupling phenomena are largely influenced by the type of serpentine stable at the subducting plate interface.

  15. Contrasting styles of deformation along the Sumatra subduction zone

    NASA Astrophysics Data System (ADS)

    Franke, D.; Gaedicke, C.; Ladage, S.; Tappin, D.; Neben, S.; Ehrhardt, A.; Mueller, C.; Djajadihardja, Y.

    2006-12-01

    Along the Sumatra subduction zone we distinguish two styles of deformation and accretion. This study bases on some 9000 km multi-channel seismic profiles acquired by the Federal Institute for Geosciences and Natural Resources (BGR) of Germany during two expeditions in 2006 (cruises RV Sonne 186 and 189) between Aceh in the north and Enggano Island in the south. The first style of deformation is characterized by landward and seaward dipping thrusts. These occur above a gently dipping oceanic crust (about 0.5° over 250 km) from the eastern flank of the Ninety East Ridge to the deformation front off northern Sumatra. The sediments covering the oceanic crust at the slope toe are between 3.0 and 3.2 s (TWT) thick. The subducting oceanic crust dips at ~3° under the frontal part of the wedge for at least 60 km. A proto-thrust develops in most cases already some 5 to 10 km seaward of the slope toe. The basally detached trench sediments in most cases are sliding along a seaward dipping thrust plane onto older accreted material forming 3-5 thrust slices. The thrust plane forming the ramp is probably evolving from the set of reverse faults that develop already distal from the deformation front in the sediments covering the oceanic crust. The second deformation style we observe is a conventional imbricate stack of thrust slices. This occurs in the region offshore of Simeulue Island, south of approx 2.5° N, where the architecture of the fore arc changes significantly. Here, the oceanic crust dips at an angle of ~7° within the first 50 km beneath the lower slope. Mainly 2.5 s (TWT) sediments fill the deepest part of the trench, but in seaward direction, we observe a sharp decrease in the sedimentary cover to less than 1.0 s (TWT) thickness. Deformation starts at the slope toe with an anticline developing above a landward dipping thrust fault. The steep slope rises continually to the outer arc high in the prolongation of the Islands of Simeulue and Nias. The wedge is built up by a stack of thrust slices bounded by landward dipping reverse faults that rise from a common basal detachment some 0.2 s (TWT) above top oceanic crust reflection. The oceanic crust beneath the trench off the Mentawai Islands (south of 1° S) is intensively faulted with offsets up to 0.6 s (TWT). These faults are often listric, leading to half grabens or rotated crustal fault blocks. The oceanic crust morphology in the area depicts several ridges and other bathymetric highs. The sedimentary fill of the morphologically expressed trench is about 1.2 to 1.7 s (TWT) at the deformation front. As offshore northern Sumatra, both landward and seaward verging folds developed at the deformation front. The incoming sediment pile is completely involved in thrusting and accretion. The frontal wedge is build up of two to three larger thrust slices. Further upslope a major landward dipping thrust fault, presumably an out of sequence thrust, breaks through the seafloor. Upslope this thrust the slope is merely gently dipping and has a plateau- like morphology.

  16. Uncertainty in turbidite correlations along the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Atwater, B. F.

    2012-12-01

    An influential reconstruction of Cascadia earthquakes builds on tenous evidence for synchronous shaking. The synchroneity has been inferred by correlating deep-sea turbidites with one another (http://pubs.usgs.gov/pp/pp1661f/). The correlated deposits record turbidity currents that likely evolved from other kinds of subaqueous mass movements. The stratigraphic correlations reportedly distinguish between long ruptures and series of shorter ones; the greater the extent of turbidite correlation along the plate boundary, the longer the inferred fault rupture. The correlations have been interpreted as exact enough to show whether the initial mass movements began during the same few minutes in areas hundreds of kilometers apart. A recent review of these findings focuses on deep-sea channels that head offshore Washington (http://pubs.usgs.gov/of/2012/1043/). Two of the points reviewed: (1) Sensitivity of Cascadia Channel turbidites to the extent of fault rupture beneath its tributary canyons. The Juan de Fuca tributaries head mainly in submarine canyons offshore northern Washington, while the combined Quinault, Grays, and Willapa tributaries head offshore southern Washington. It has been assumed that flows from north and south, if initiated at the same time, were both large enough to merge at the head of Cascadia Channel and to continue together for hundreds of kilometers downstream. This so-called confluence test is often cited as evidence that 13 fault ruptures of the past 7,500-7,800 years extended along the full length of the Washington coast. But the test is confounded by uncertainties in proximal turbidite counts, flow paths, and relative sizes of flows. (2) Ability of individual sandy layers within a turbidite to mimic the series of strong-motion pulses during an earthquake. According to the reconstruction: (a) A series of ground-motion pulses during an earthquake produces a series of sediment pulses in a turbidity current. (b) The sediment pulses yield a sequence 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.

  17. An Integrated Geophysical View of the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Egbert, G. D.; Fouch, M. J.; Schultz, A.

    2009-04-01

    The goal of this study is to evaluate correlations between new models of conductivity and seismic structure of the Cascadia subduction zone in the northwestern United States. These models were obtained using data from EarthScope's USArray long period magnetotelluric (MT) and broadband seismic Transportable Array (TA) systems. Initial MT deployments in 2006 and 2007 acquired long period (10-10000s) data at 110 sites covering the US Pacific Northwest with the same nominal spacing as the USArray seismic transportable array (~70 km). Three-dimensional inversion of this dataset reveals extensive areas of high conductivity in the lower crust beneath the Northwest Basin and Range, and beneath the Cascade Mountains, contrasting with very resistive crust in Siletzia, the accreted thick ocean crust which forms the basement rocks in the Cascadia forearc and the Columbia Embayment). The conductive lower crust beneath the southeastern part of the array is inferred to result from fluids (including possibly partial melt at depth) associated with magmatic underplating. Beneath the Cascades high conductivities probably result from fluids released by the subducting Juan de Fuca slab. Resistive Siletzia represents a stronger crustal block, accommodating deformation in the surrounding crust by rigid rotation. Significant variations in upper mantle conductivity are also revealed by the inversions, with the most conductive mantle beneath the Washington backarc in the northeastern part of the array, and the most resistive corresponding to subducting oceanic lithosphere. The Juan de Fuca slab is well imaged in body wave seismic tomography models, extending to depths of ~500 km and perhaps deeper [Roth et al., 2008]. Significant zones of focused reduced seismic velocities are evident beneath both the Newberry region and the surface expression of the Columbia River basalts. We find no evidence for a zone of low velocities beneath the Juan de Fuca slab. Further, we demonstrate that the absence of a slab signature beneath central Oregon, interpreted by some groups as a "hole" in the slab, is an inversion artifact due to imperfect ray coverage and the presence of the reduced velocity zone coincident with Newberry. While the MT analyses have so far focused on the lower crustal structure which this data is most sensitive to, and the seismic results to date have emphasized upper mantle structure, notable relationships between the models are already evident. For instance, the Juan de Fuca slab signature is clear in both models, and in both cases exhibits lateral continuity. In addition, the low velocity feature beneath Newberry coincides with a a circular zone of enhanced conductivity in the upper mantle. Interestingly, some features in the MT image, such as the high conductivity zone at shallow depths in SE Oregon, correlate well with structure of similar geometry in the seismic images at mantle depths

  18. Investigation of the crustal structure of the Manilla subduction zone offshore southern Taiwan using multi-channel seismic reflection and wide-angle refraction data

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    In 2009 we conducted a seismic experiment with the R/V Marcus G. Langseth in conjunction with the TAIGER project in the deep water Luzon Strait offshore Taiwan. The program acquired 12 high-quality MCS and OBS profiles across the Manila subduction zone in an effort to gain a further understanding of the crustal evolution of pure subduction south of ~20.5° N to initial-stage arc-continent collision north of ~21.3° N along the Manilla trench. Two of the main MCS profiles are oriented E-W perpendicular to the strike of the Manila trench at approximately 21.3° N and 20.3° N. Here Eurasian crust subducts to the east beneath oceanic crust of the Philippine Sea plate. Both transects extend across the entirety of the subduction zone making it possible to track and correlate the structure and geometry of the subducting Eurasian slab from the surface at the trench to well below the accretionary prism as well as the structure and geometry of the encroaching Luzon volcanic arc. Pre-stack time migration imaging of the two MCS profiles show an east dipping Eurasian slab top of crust reflector beneath the deforming accretionary prism on the western margin of the overriding Philippine Sea plate. Imaging also shows highly irregular Eurasian crust consisting of faulted rotated blocks, uncharacteristic of oceanic crust, entering the subduction zone with intermittent moho reflections potentially supporting the hypothesis that this area of the South China Sea is composed of transitional oceanic-continental material. Both MCS profiles are complimented by wide-angle OBS data acquired over the same transects. Records from 40 OBS instruments on the northern transect and 25 instruments on the southern transect combined with the corresponding MCS data will be used in tomographic inversion to develop a preliminary, first arrival crustal scale velocity model across the Manilla subduction zone. The preliminary models will provide information that will shed light on the composition and dip of the subducting Eurasian slab, composition of accretionary wedge material, and structure of the forearc basement. Further work will include an analysis of the forearc region of the subduction zone in the MCS data in order to delineate the geometry of the forearc basement. Is the forearc basement being used as an abrupt backstop for accretionary prism development and deformation? How is the forearc basin being deformed from normal subduction to incipient collision?

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

    NASA Astrophysics Data System (ADS)

    Hardebeck, Jeanne L.

    2015-09-01

    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 an angle of 45° to 60° with respect 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.

  20. 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

  1. 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

  2. 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.

  3. 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.

  4. Triassic high-pressure metamorphism in the Huwan shear zone: Tracking the initial subduction of continental crust in the whole Dabie orogen

    NASA Astrophysics Data System (ADS)

    Liu, Xiaochi; Wu, Yuanbao; Gao, Shan; Peng, Min; Wang, Jing; Wang, Hao; Gong, Hujun; Yuan, Honglin

    2012-04-01

    LA-(MC)-ICPMS trace element, U-Pb and Hf isotope analyses were performed on zircons from continental-derived retrograde high-pressure metamorphic rocks, which were attested by the occurrence of HP mineral inclusions in metamorphic zircons, from the Huwan shear zone in the western Dabie orogen. Zircons in a schist sample are mostly detrital and gave U-Pb ages from 1212 to 2312 Ma, suggesting that the detritus were derived from multiple sources or suffered partial Pb-loss during subsequent metamorphism. Magmatic zircons in an amphibolite sample yielded an upper intercept U-Pb age of 1806 ± 63 Ma, which was interpreted as its protolith formation age. The zircons in the amphibolite show a weighted ɛHf (t) value of - 16.3, and a weighted Hf model age of 3455 Ma, suggesting that the protolith was derived from reworking of ancient continental materials. Metamorphic zircons in a quartz-rich leucosome sample and the amphibolite sample gave weighted mean 206Pb/238U ages of 241 ± 1 and 243 ± 4 Ma, respectively. These metamorphic zircons contain mineral inclusions of garnet and omphacite and are characterized by relatively flat HREE patterns with slight negative Eu anomalies. This indicates that they formed in the presence of garnet but the absence of plagioclase, and thus under eclogite facies conditions. The U-Pb age of ca. 243 Ma is consistent with eclogite-facies metamorphic ages of the juvenile oceanic crust in the western Dabie orogen, suggesting that the initial continental subduction for high-pressure metamorphism of the whole Dabie orogen can be traced back to ca. 243 Ma. In addition, the western Dabie HP-UHP orogenic belt might have suffered multi-slice subduction and exhumation during the Triassic.

  5. 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

  6. 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 700 C and 10 Pa s (both models) at 1200 C. The extremely low melt viscosity is caused primarily by the formation of liquids (release of OH-) from hydrous illite, and secondarily by small volume fraction (< 20%) of grains in the PT. Because illite is commonly present in accretionary prisms, generation of a low viscosity melt from illite would lead to fault lubrication and hence control the efficiency of stored strain energy release and earthquake magnitude in subduction zones.

  7. Coseismic and interseismic displacements at a subduction zone - a parameter study using finite-element modelling

    NASA Astrophysics Data System (ADS)

    Li, Tao; Hampel, Andrea

    2013-04-01

    Tide-gauge and geodetic measurements of coseismic and interseismic displacements in the forearc of subduction zones showed that the coastal region undergoes uplift during the interseismic phase and subsidence during the coseismic phase, while opposite vertical movements are observed in the neighbouring regions (e.g., Savage & Thatcher 1992; Hyndman & Wang 1995). Horizontal displacements during the interseismic phase are typically directed landward, whereas the forearc moves seaward during the earthquake (e.g., Klotz et al. 1999). Here we use two-dimensional finite-element modelling to evaluate how the friction coefficient along the plate interface, the length and the position of the downdip end of the locked zone affect the coseismic and interseismic displacements. Our model consists of a deformable, rheologically stratified upper plate and an undeformable oceanic plate, which rotates at a prescribed angular velocity (cf. Cailleau & Oncken, 2008). The frictional plate interface is divided - from the trench to the base of the continental lithosphere - into a seismogenic zone, a transition zone and a landward free slip zone. During an initial phase, the seismogenic zone is locked, which leads to the accumulation of elastic strain in the forearc. During the subsequent coseismic phase, the strain is released and causes sudden slip of several meters on the plate interface. During the next interseismic phase, the seismogenic zone is locked again. Our model results show patterns of vertical and horizontal displacements that are in general agreement with geodetically observed patterns. A sensitivity analysis reveals that the magnitude of the vertical displacements is strongly influenced by the friction coefficients of the seismogenic zone and the transition zone. The location of the zones of maximum interseismic uplift and coseismic subsidence in the coastal regions depends on the length and position of the locked zone. Preliminary results from three-dimensional models show that the lateral extent of the rupture zone also affects the direction of the coseismic surface displacements, as recorded by GPS stations during the 2011 Japan earthquake (Ozawa et al. 2011). References: Cailleau, B., O. Oncken (2008) Past forearc deformation in Nicaragua and coupling at the megathrust interface: Evidence for subduction retreat? Geochemistry, Geophysics, Geosystems, 9, Q03016, doi:10.1029/2007GC001754. Hyndman, R.D., K. Wang (1995) The rupture zone of Cascadia great earthquakes from current deformation and the thermal regime. Journal of Geophysical Research, 100, 22133-22154. Klotz, J., D. Angermann, G.W. Michel, R. Porth, C. Reigber, J. Reinking, J. Reinking, J. Viramonte, R. Perdomo, V.H. Rios, S. Barrientos, R. Barriga, O. Cifuentes (1999) GPS-derived Deformation of the Central Andes Including the 1995 Antofagasta Mw=8.0 Earthquake. Pure and Applied Geophysics, 154, 709-730. Ozawa, S., T. Nishimura, H. Suito, T. Kobayashi, M. Tobita, T. Imakiire (2011) Coseismic and postseismic slip of the 2011 magnitude-9 Tohoku-Oki earthquake. Nature, 475, 373-376. Savage, J.C., Thatcher, W. (1992) Interseismic deformation at the Nankai Trough, Japan, subduction zone. Journal of Geophysical Research, 97, 11117-11135.

  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 and 65 Ma, most likely as a thermal response of a geodynamic perturbation toward the end of the Cretaceous.

  9. The proportionality between relative plate velocity and seismicity in subduction zones

    NASA Astrophysics Data System (ADS)

    Ide, S.

    2013-12-01

    Seismic activity differs among subduction zones due to various factors such as relative plate velocity, temperature, stress, and subducting materials. Relative plate velocity has a direct control on tectonic deformation and an overall correlation with seismicity has been suggested, as a global average or for large regions. Here I show a positive correlation between relative plate velocity and seismicity by estimating the background seismicity rate for 117 sections of subduction zones worldwide using the epidemic type aftershock sequence (ETAS) model. The background rate is stably estimated even for the period following M9-class earthquakes in Chile and Japan. A prominent proportional relationship is evident in the southwestern Pacific Ocean. Given that M9-class earthquakes occur independently of one another, the lack of M9 earthquakes in the southwestern Pacific Ocean over the last century is difficult to explain by chance. On the other hand, some subduction zones have extremely low background seismicity, and have experienced very large earthquakes. Slow earthquakes have been discovered in many of these quiet zones. Thus, this proportionality relation may be useful in assessing the seismic risk in subduction zones worldwide between two apparently confusing end members: 'active and moderate' and 'quiet and extreme'.

  10. 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.

  11. Imaging of Upwelling Fluids and Partial Melt in the Subduction Zone with 3D Regional Electrical Resistivity Structure by the Network-MT Data

    NASA Astrophysics Data System (ADS)

    Hata, M.; Oshiman, N.; Yoshimura, R.; Tanaka, Y.; Uyeshima, M.

    2012-04-01

    Subduction zones are where oceanic plate and seawater (aqueous fluids) return to Earth's mantle and the fluids released into the mantle from the downgoing slab as a consequence of metamorphic reactions [e.g., Tatsumi, 1989]. Such the fluids trigger partial melt of upper mantle and crust. As a result, igneous activity forms volcanoes as typical surface expressions in subduction zones because the partial melt rises owing to its lower density. The island of Kyushu in the Southwestern Japan is a typical high angle subduction zone, at which the hot Shikoku basin (15-27 Ma) and the cold Philippine Sea plate (45-55 Ma) subduct beneath the Eurasian plate. Kyushu can be separated into three parts; northern Kyushu, central Kyushu and southern Kyushu, and there exist many quaternary active volcanoes, as the Aso and Kuju volcanoes in the northern part and Kirishima and Sakurajima volcanoes in the southern part, with defining a volcanic front. Moreover, the central Kyushu is devoid of active volcanoes, where it is considered that the buoyant Kyushu-Palau Ridge subducts. It is important to investigate structure beneath Kyushu for understanding the volcanic formation. In the Kyushu district, the Network-Magnetotelluric (MT) observations were carried out from 1993 to 1998 to cover the whole island of Kyushu. The Network-MT method employs metallic wires in a commercial telephone network to measure the electric potential difference with a dipole length of ten to several tens of kilometers. The Network-MT data provide valuable information on fluid and partial melt (magma) generation because the electromagnetic soundings are highly sensitive to the presence of a few percent of interconnected fluids (aqueous and/or melt). We analyzed the Network-MT data sets, which have geoelectromagnetic information from the crust to upper mantle, in order to determine regional scale electrical resistivity structure. We applied three-dimensional (3D) inversion analyses using the WSINV3DMT inversion code of the version for the Network-MT impedance responses [Uyeshima et al., 2008]. Two remarkable features are found that a conductive block exists beneath the volcano of which the bottom extends to the backarc side and the forearc side including the Philippine Sea plate is resistive. Moreover, the resistive region distributes along the hinge line of the subducting plate as imaged by seismicity. The former conductor is thought to represent fluids released from the slab or partial melt related to the fluid, constituting a magma source for subduction zone volcanoes. In this presentation, we would like to explain details on the 3D resistivity structure related to the subducting Philippine Sea plate and the active volcanoes.

  12. Flow Zone Isolation in Sedimentary Inputs to the Nankai Trough Subduction Zone, IODP Expedition 322 (Invited)

    NASA Astrophysics Data System (ADS)

    Dugan, B.; Torres, M. E.; Destrigneville, C.; Heuer, V.; Underwood, M. B.; Saito, S.; Iodp Expedition 322 Shipboard Scientific Party

    2010-12-01

    Based on porewater chemistry observations at Integrated Ocean Drilling Program (IODP) Sites C0011 (on the flank of Kashinosaki Knoll) and C0012 (near the crest of Kashinosaki Knoll), we interpret two independent flow systems within the input sediments to the Nankai subduction zone. We integrate lithology data and laboratory-constrained permeability results to develop a conceptual framework that describes these two flow systems: one sourced from sediment within the subduction zone and one through the upper oceanic crust seaward of the subduction zone. In porefluids from Site C0011, methane, ethane, propane, and iso-butane concentrations reach maxima at the base of the Lower Shikoku Basin turbidite facies (850 mbsf). Site C0012 has peak methane and ethane concentrations at the base of the same turbidite facies (418 mbsf); however neither ethane nor iso-butane is present. The presence of propane and iso-butane at Site C0011 may indicate a thermogenic source and migration of fluids from the subduction zone. Lower methane and ethane concentrations at Site C0012 may provide insights on flow rates. Hydrocarbon gas concentrations at Site C0011 decrease gradually up-section to near zero by 430 mbsf; the downhole trend decreases more rapidly. At Site C0012, hydrocarbon gas concentrations decrease symmetrically around the concentration maxima and reach near-zero concentrations by 290 mbsf and 530 mbsf. We also observe porewater freshening, up to 7%, at Site C0011. This freshening could indicate migration of freshened porefluids that originate from depth within the subduction zone, consistent with the origin of thermogenic gases. Site C0012 does not show porefluid freshening, potentially indicating that freshened porefluids have yet to migrate from the subduction zone to Site C0012. Site C0012, however, shows seawater-porefluid mixing (up to 20% seawater) deeper in the sedimentary section, above the igneous basement within volcaniclastic sandstones (500 mbsf). A seawater contribution is inferred from proportional reversals in all major cation concentrations (e.g., Na, K, Ca and Mg) and the presence of sulfate. This seawater source may be linked to a flow system within the upper basaltic basement and the overlying volcaniclastic sandstones. This deeper seawater flux must be separate and isolated from the sulfate-depleted, hydrocarbon-gas-bearing fluids migrating from the subduction zone. Permeability data and lithologic variability are used to define flow pathways and flow barriers that facilitate the existence of these flow systems and prevent their mixing.

  13. 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 slab related mantle flow, the coincidence of shallow plumes with changes in plate boundaries suggests an origin related to sites of weakness or extension in the oceanic crust.

  14. A comparison of seismicity in world's subduction zones: Implication by the difference of b-values

    NASA Astrophysics Data System (ADS)

    Nishikawa, T.; Ide, S.

    2013-12-01

    Since the pioneering study of Uyeda and Kanamori (1979), it has been thought that world's subduction zones can be classified into two types: Chile and Mariana types. Ruff and Kanamori (1980) suggested that the maximum earthquake size within each subduction zone correlates with convergence rate and age of subducting lithosphere. Subduction zones with younger lithosphere and larger convergence rates are associated with great earthquakes (Chile), while subduction zones with older lithosphere and smaller convergence rates have low seismicity (Mariana). However, these correlations are obscured after the 2004 Sumatra earthquake and the 2009 Tohoku earthquake. Furthermore, McCaffrey (2008) pointed out that the history of observation is much shorter than the recurrence times of very large earthquakes, suggesting a possibility that any subduction zone may produce earthquakes larger than magnitude 9. In the present study, we compare world's subduction zones in terms of b-values in the Gutenberg-Richer relation. We divided world's subduction zones into 146 regions, each of which is bordered by a trench section of about 500 km and extends for 200 km from the trench section in the direction of relative plate motion. In each region, earthquakes equal to or larger than M4.5 occurring during 1988-2009 were extracted from ISC catalog. We find a positive correlation between b-values and ages of subducting lithosphere, which is one of the two important variables discussed in Ruff and Kanamori (1980). Subduction zones with younger lithosphere are associated with high b-values and vice versa, while we cannot find a correlation between b-values and convergence rates. We used the ages determined by Müller et al. (2008) and convergence rate calculated using PB2002 (Bird, 2003) for convergence rate. We also found a negative correlation between b-values and the estimates of seismic coupling, which is defined as the ratio of the observed seismic moment release rate to the rate calculated from plate tectonic velocities (Scholz and Campos, 2012). Lithosphere age also has a weak negative correlation with the degree of seismic coupling. Based on differences in b-values for the types of faulting, Schorlemmer et al. (2005) suggested that b-value depends inversely on differential stress. This idea, taken together with correlations in the present study, suggests a model where the buoyancy of subducting slabs which depends on the lithosphere age determines stress state and the b-value in each sunbduction zone. The stress state also controls the seismic coupling. This model is basically consistent with the idea of Ruff and Kanamori (1980). Subduction zones with younger and lighter lithosphere are in a compressive stress state and associate with high coupling and small b-values (Chile), while those with older and heavier lithosphere are in a tensional stress state and correlate with low coupling and large b-values (Mariana). Subduction zones such as Nicaragua and El Salvador where b-values are much higher than the expectation from the above correlations may be explained by considering the fact that local tectonics affects the seismic coupling (LaFemina et al., 2009; Scholz and Campos, 2012).

  15. 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.

  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. 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.

  18. Slab1.0: A three-dimensional model of global subduction zone geometries

    USGS Publications Warehouse

    Hayes, G.P.; Wald, D.J.; Johnson, R.L.

    2012-01-01

    We describe and present a new model of global subduction zone geometries, called Slab1.0. An extension of previous efforts to constrain the two-dimensional non-planar geometry of subduction zones around the focus of large earthquakes, Slab1.0 describes the detailed, non-planar, three-dimensional geometry of approximately 85% of subduction zones worldwide. While the model focuses on the detailed form of each slab from their trenches through the seismogenic zone, where it combines data sets from active source and passive seismology, it also continues to the limits of their seismic extent in the upper-mid mantle, providing a uniform approach to the definition of the entire seismically active slab geometry. Examples are shown for two well-constrained global locations; models for many other regions are available and can be freely downloaded in several formats from our new Slab1.0 website, http://on.doi.gov/ d9ARbS. We describe improvements in our two-dimensional geometry constraint inversion, including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking, derived from the interpolation between other active source seismic data along-strike in the same subduction zone. We include several analyses of the uncertainty and robustness of our three-dimensional interpolation methods. In addition, we use the filtered, subduction-related earthquake data sets compiled to build Slab1.0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far, concluding that the width of these seismogenic zones is on average 30% larger than previous studies have suggested. Copyright 2011 by the American Geophysical Union.

  19. Evidence for Deep Tectonic Tremor in the Alaska-Aleutian Subduction Zone

    NASA Astrophysics Data System (ADS)

    Brown, J. R.; Prejean, S. G.; Beroza, G. C.; Gomberg, J. S.; Haeussler, P. J.

    2010-12-01

    We search for, characterize, and locate tremor not associated with volcanoes along the Alaska-Aleutian subduction zone using continuous seismic data recorded by the Alaska Volcano Observatory and Alaska Earthquake Information Center from 2005 to the present. Visual inspection of waveform spectra and time series reveal dozens of 10 to 20-minute bursts of tremor throughout the Alaska-Aleutian subduction zone (Peterson, 2009). Using autocorrelation methods, we show that these tremor signals are composed of hundreds of repeating low-frequency earthquakes (LFEs) as has been found in other circum-Pacific subduction zones. We infer deep sources based on phase arrival move-out times of less than 4 seconds across multiple monitoring networks (max. inter-station distances of 50 km), which are designed to monitor individual volcanoes. We find tremor activity is localized in 7 segments: Cook Inlet, Shelikof Strait, Alaska Peninsula, King Cove, Unalaska-Dutch Harbor, Andreanof Islands, and the Rat Islands. Locations along the Cook Inlet, Shelikof Straight and Alaska Peninsula are well constrained due to adequate station coverage. LFE hypocenters in these regions are located on the plate interface and form a sharp edge near the down-dip limit of the 1964 M 9.2 rupture area. Although the geometry, age, thermal structure, frictional and other relevant properties of the Alaska-Aleutian subduction are poorly known, it is likely these characteristics differ along its entire length, and also differ from other subduction zones where tremor has been found. LFE hypocenters in the remaining areas are also located down-dip of the most recent M 8+ megathrust earthquakes, between 60-75 km depth and almost directly under the volcanic arc. Although these locations are less well constrained, our preliminary results suggest LFE/tremor activity marks the down-dip rupture limit for megathrust earthquakes in this subduction zone. Also, we cannot rule out the possibility that our observations could be related deep magmatic processes.

  20. 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.

  1. Thermal models for the magmatic accretion and subsequent metamorphism of continental crust

    NASA Astrophysics Data System (ADS)

    Wells, P. R. A.

    1980-01-01

    I present one-dimensional conductive relaxation models for the thermal evolution of continental crust which has undergone magmatic thickening in a predominantly recumbent tectonic regime. The sustained addition of magma, and associated heat, profoundly influences the conductive relaxation of the affected sialic crust. The mechanism of accretion, successively beneath (under-accretion), or successively above (over-accretion), previously added material, strongly affects the type of P- T-time history sustained by the rocks in the middle and deep crust. The pressure and temperature conditions which might be recorded by mineral assemblages equilibrating in such tectonothermal regimes may bear very little relation to conductive heat supplies from the upper mantle. P- T values obtained by geobarometry and geothermometry in such complexes must be used with caution to constrain steady state heat flows and geothermal gradients. In a contemporary plate tectonic context I consider the tectonothermal models developed here to apply particularly to island arcs and sites of continental collision associated with extensive plutonic igneous activity. Precambrian orthogneiss complexes, such as the Archaean of southern West Greenland, probably represent exhumed deep crustal elements involved in the intense magmatic thickening processes considered here.

  2. 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 stratigraphic correlation of turbidites to determine whether turbidites deposited in separate channel systems are correlative - triggered by a common event. In most cases, these tests can separate earthquake-triggered turbidity currents from other possible sources. The 10,000-year turbidite record along the Cascadia margin passes several tests for synchronous triggering and correlates well with the shorter onshore paleoseismic record. The synchroneity of a 10,000-year turbidite-event record for 500 km along the northern half of the Cascadia subduction zone is best explained by paleoseismic triggering by great earthquakes. Similarly, we find a likely synchronous record in southern Cascadia, including correlated additional events along the southern margin. We examine the applicability of other regional triggers, such as storm waves, storm surges, hyperpycnal flows, and teletsunami, specifically for the Cascadia margin. The average age of the oldest turbidite emplacement event in the 10-0-ka series is 9,800±~210 cal yr B.P. and the youngest is 270±~120 cal yr B.P., indistinguishable from the A.D. 1700 (250 cal yr B.P.) Cascadia earthquake. The northern events define a great earthquake recurrence of ~500-530 years. The recurrence times and averages are supported by the thickness of hemipelagic sediment deposited between turbidite beds. The southern Oregon and northern California margins represent at least three segments that include all of the northern ruptures, as well as ~22 thinner turbidites of restricted latitude range that are correlated between multiple sites. At least two northern California sites, Trinidad and Eel Canyon/pools, record additional turbidites, which may be a mix of earthquake and sedimentologically or storm-triggered events, particularly during the early Holocene when a close connection existed between these canyons and associated river systems. The combined stratigraphic correlations, hemipelagic analysis, and 14C framework suggest that the Cascadia margin has three rupture modes: (1) 19-20 full-length or nearly full length ruptures; (2) three or four ruptures comprising the southern 50-70 percent of the margin; and (3) 18-20 smaller southern-margin ruptures during the past 10 k.y., with the possibility of additional southern-margin events that are presently uncorrelated. The shorter rupture extents and thinner turbidites of the southern margin correspond well with spatial extents interpreted from the limited onshore paleoseismic record, supporting margin segmentation of southern Cascadia. The sequence of 41 events defines an average recurrence period for the southern Cascadia margin of ~240 years during the past 10 k.y. Time-independent probabilities for segmented ruptures range from 7-12 percent in 50 years for full or nearly full margin ruptures to ~21 percent in 50 years for a southern-margin rupture. Time-dependent probabilities are similar for northern margin events at ~7-12 percent and 37-42 percent in 50 years for the southern margin. Failure analysis suggests that by the year 2060, Cascadia will have exceeded ~27 percent of Holocene recurrence intervals for the northern margin and 85 percent of recurrence intervals for the southern margin. The long earthquake record established in Cascadia allows tests of recurrence models rarely possible elsewhere. Turbidite mass per event along the Cascadia margin reveals a consistent record for many of the Cascadia turbidites. We infer that larger turbidites likely represent larger earthquakes. Mass per event and magnitude estimates also correlate modestly with following time intervals for each event, suggesting that Cascadia full or nearly full margin ruptures weakly support a time-predictable model of recurrence. The long paleoseismic record also suggests a pattern of clustered earthquakes that includes four or five cycles of two to five earthquakes during the past 10 k.y., separated by unusually long intervals. We suggest that the pattern of long time intervals and longer ruptures for the northern and central margins may be a function of high sediment supply on the incoming plate, smoothing asperities, and potential barriers. The smaller southern Cascadia segments correspond to thinner incoming sediment sections and potentially greater interaction between lower-plate and upper-plate heterogeneities. The Cascadia Basin turbidite record establishes new paleoseismic techniques utilizing marine turbidite-event stratigraphy during sea-level highstands. These techniques can be applied in other specific settings worldwide, where an extensive fault traverses a continental margin that has several active turbidite systems.

  3. 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.

  4. The influence of subduction zone thermal structure on arc magma chemistry: B and fluid-mobile elements

    NASA Astrophysics Data System (ADS)

    Leeman, W. P.

    2001-12-01

    It is postulated that contents of B and other highly fluid-mobile elements (HFMs) in primitive arc lavas are sensitive to thermal structures of subducting slabs (sensu lato), which are the principal sources for these elements in subduction zones. Initial slab HFM inventories may be variably depleted in proportion to progressive fluid release as a consequence of gradual warming and metamorphism of descending slabs. Also, some variation in the rate of slab heating is expected from arc to arc due to differences in subduction rate, slab age, Benioff Zone (BZ) geometry, etc. Thus, a greater fraction of the initial slab HFM inventory will be available to modify subarc mantle domains in relatively cool subduction zones, and vice versa. Nearly 100-fold variation in B-enrichment (e.g., as monitored by B/Zr or similar ratios normalized to constant B content) is observed for volcanic front (VF) basalts from arcs worldwide. B-enrichment is highly correlated with the above measurable subduction parameters and with other temperature-sensitive parameters such as `slab length' (= down-dip extent of BZ seismicity). B-enrichment is also well correlated with slab-surface temperatures (SSTs) below the VF, as determined from numerical models (Huang et al., this volume). These relations suggest that slab HFM fluxes are strongly influenced by slab thermal history, and probably controlled by stability of HFM host minerals as well as the availability of fluid transport media. Different behavior is expected for various elements depending on their fluid-solubility and/or P-T stability of relevant host phases - in which case geochemical fractionations are readily feasible and may explain some of the geochemical variability among arc magma suites. Moreover, B/Zr and similar ratios potentially can be used to infer aspects of slab thermal structure. However, care must be taken that the ratios are representative of the most primitive mafic magmas and not modified by shallow crust-level processes. For example, interaction with lower crust rocks can lower normalized B/Zr values for relatively evolved lavas.

  5. 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 Science Letters, 163, 361-379,1998 [5] Wang et al, Review for Scientific Instruments, 74(6), 3002-3011, 2003 [6] Chantel et al., Earth and Planetary Science Letters, 349-350, 116-125, 2012

  6. 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 50 years of improved acquisition techniques, analysis capabilities, and imaging that greatly improves resolution in observational data. The first observatories in boreholes that place instruments closer to earthquake rupture zones have been deployed. Technological improvement should be strongly pursued to meet future challenges. Advanced seismic imaging and the new riser drilling vessel Chikyu are tools to significantly advance understanding of earthquake mechanics but availability is restricted by current global science budgets. The present scientific knowledge leaves great earthquakes and tsunamis an unpredictable "stealth" natural hazard of great proportions.

  7. 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. A detailed comparison of images constructed for distinct backazimuthal illuminations reveals deviations in the geometry of the subducted slab. These deviations are attributed to structural and/or compositional changes taking place directly to the north of the MEDUSA array, and are consistent with the existence of a slab tear beneath the Central Hellenic Shear Zone.

  8. 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 offshore. The evolution of this LVZ from the up-dip locked region through the transition zone to the down-dip freely slipping region will aid in our understanding of the hydration state of the Cascadia thrust.

  9. Subduction zone geometry and stress transfer during megathrust earthquakes, upper Cook Inlet basin, Alaska

    NASA Astrophysics Data System (ADS)

    Willis, Julie Barrott

    Field observations and elastic and viscoelastic dislocation models of the 1964 great Alaska earthquake, Mw 9.2, demonstrate how complex subduction zone geometry affects static and time-dependent stress transfer from megathrust ruptures to upper plate faults. A three-dimensional elastic dislocation model of the 1964 earthquake uses triangular subfaults to capture sharp changes in trend and spacing of Benioff contours that define the top of the subducting slab as it transitions from steep subduction of the Pacific Plate (PP) to shallow subduction of the Yakutat terrane (YT), an allochthonous continental fragment. The model demonstrates that slab geometry strongly influences coseismic stress transfer into the upper plate, ascertaining that the anomalous upper Cook Inlet stress field (maximum horizontal stress oriented 45 counterclockwise from subducting plate motion), is partially due to slip on the complex subduction interface. The model further ascertains that slip on the complex interface causes marked variations in coseismic stresses transferred to the Castle Mountain fault (CMF) which shows marked along-strike variability in seismogenic behavior and straddles the underlying edge of the YT. The western CMF is the only upper plate fault with unequivocal Holocene surface rupture in the greater Anchorage area. Herein, a previously unrecognized offset margin of a postglacial outwash channel is identified and used to establish a right lateral slip rate of 3.0+/-0.6 mm yr-1 and a potential rupture magnitude of 6.9-7.3 for that fault segment. A time-dependent viscoelastic Maxwell rheological model of the 1964 earthquake suggests a time-delayed rotation of principal stresses from a reverse to a strike-slip faulting regime along the CMF. The delay may indicate a temporal link between megathrust earthquakes and CMF rupture. The time-dependent model further suggest that megathrust-induced changes in shear and normal stresses, and thus in rupture potential, on the Castle Mountain fault and on transpressional faults of upper Cook Inlet basin will peak 60 years post-1964. The transpressional faults core hydrocarbon-producing anticlines of the Cook Inlet petroleum province. These faults and the western CMF pose a significant seismic hazard to the greater Anchorage area and to petroleum infrastructure of Cook Inlet basin.

  10. Across Arc Variation in Basaltic fO2: Influence of a Subduction Component in the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Rowe, M. C.; Kent, A. J.; Nielsen, R. L.

    2007-12-01

    Oxidation of the subarc mantle in subduction zones can greatly affect mineral phase equilibria, the speciation of volatiles, and the transfer of multivalent elements in basaltic magmas. While peridotite xenoliths provide the most direct approach to measuring mantle oxidation states, such xenoliths in continental arcs are rare. In this investigation, we applied an alternative method, the determination of sulfur speciation in olivine-hosted melt inclusions and chromite-olivine oxygen barometry. We present a first attempt to spatially correlate oxygen fugacity relative to the subduction zone in a continental arc. The overall range in oxygen fugacity, based on sulfur speciation measurements, is from <-0.25 log units to +1.9 log units (ΔFMQ). Sulfur oxidation and the concentration of fluid-mobile trace elements both generally increase from backarc to forearc. This correlation is interpreted to reflect a progressively greater proportion of fluid-rich, oxidized subduction component closer to the trench. Estimates of the amount of subduction component (up to ~6 wt%) required to generate the geochemical diversity based on flux melt modeling correlate with oxygen fugacity, with the exception of calc-alkaline basalts, with high oxygen fugacity and greater proportion subduction component closer to the trench. Two other important observations to come from the flux melt modeling are 1) as basalt oxygen fugacity increases, calculated mantle temperature decreases, and 2) shoshonitic basalts require a depleted mantle source, distinct from the more enriched mantle source of the low-K tholeiite, calc-alkaline and ocean island-like magmas. The potential mantle source for shoshonitic basalts has a predicted oxygen fugacity from +0.3 to +2.4 log units (ΔFMQ) while the mantle source for low-K tholeiite, calc-alkaline and ocean island-like basalts may range from -1.1 to +0.7 log units (ΔFMQ), consistent with estimates of oxidation state for oceanic lithosphere. Therefore, despite the volatile and fluid-mobile trace element enrichment, the subarc mantle need not be significantly oxidized relative to unmodified oceanic lithosphere to generate the diversity in Cascade arc basaltic magmas.

  11. Lithium as a tracer of fluid and metasomatic processes in subduction zone mélanges: Evidence from the Catalina Schist (Invited)

    NASA Astrophysics Data System (ADS)

    Penniston-Dorland, S. C.; Bebout, G. E.; Pogge von Strandmann, P.; Elliott, T.; Sorensen, S. S.

    2010-12-01

    The release and transport of fluids in subduction zones affect many fundamental Earth processes. The Catalina Schist subduction zone mélange is thought to have been a zone of fluid flow and mechanical mixing between the downgoing subducting slab and overlying mantle wedge. Study of rocks from this mélange zone can help elucidate processes of fluid release and transport occurring within subduction zones. Lithium is a fluid-mobile element which shows large variations in concentration and isotopic composition in subduction zone metamorphic rocks, thus providing a potentially useful tool in deciphering fluid flow pathways and mechanisms, fluid sources, and material transport. The well-studied Catalina Schist provides an opportunity to examine the effects of high-P/T devolatilization and other fluid-rock interactions on the behavior of Li in subduction zones. Whole-rock Li concentrations (10 to 45 ppm) and δ7Li (-2.3 to +4.9‰, all values reported relative to L-SVEC standard) of Catalina Schist metasedimentary rocks fall within the range of seafloor sedimentary rocks, and correlate with the chemical index of alteration, indicating that variations in Li at least partly reflect weathering processes. There is no discernible change in Li concentration or δ7Li in metasedimentary rocks with increasing metamorphic grade, an observation contrasting with that for B, Cs, and N, which show dramatic loss from metasedimentary rocks at higher grades. SIMS study of the same metasedimentary samples indicates the residency of Li in chlorite and phengite at lower grades and gradual transfer, at higher grades, to being dominantly in phengite (with chlorite breakdown). Metabasaltic rocks in the Catalina Schist are enriched in Li (10 to 32 ppm) relative to their MORB protoliths, showing a wide range of concentrations and, in most cases, δ7Li similar to that of nearby metasedimentary rocks (-6.7 to +1.8‰). These observations suggest metasomatic additions of Li mobilized during metasedimentary fluid-rock interactions. Lithium concentrations and δ7Li of metasomatized metamafic conglomerate cobbles (18 to 35 ppm, +1.2 to +3.5‰), veins (16 to 64 ppm, -2.6 to +2.2‰), and pegmatites (3 to 9 ppm, +1.1 to +3‰) also appear to reflect high-P/T mobility of Li in H2O-rich fluids or, at the highest grade, silicate melts. The majority of the metabasalts are relatively uniform in δ7Li, perhaps reflecting the fluid mixing and homogenization inferred from O, C, and N isotope systematics in the same complex. Several metabasaltic blocks in the amphibolite-grade mélange, and their metasomatic rinds, have lower δ7Li values (down to -6.7‰) that could reflect kinetic fractionation during the infiltrative/diffusive production of metasomatic gradients at the margins of the blocks.

  12. Areas of slip of recent earthquakes in the Mexican subduction zone

    NASA Astrophysics Data System (ADS)

    Hjorleifsdottir, V.; Sánchez-Reyes, H. S.; Singh, S.; Ji, C.; Iglesias, A.; Perez-Campos, X.

    2012-12-01

    The Mexican subduction zone is unusual: the width of the seismogenic zone is relatively narrow and a large portion of the co-seismic slip generally occurs below the coast, ~ 45 to 80 km from the trench. The earthquake recurrence interval is relatively short and almost the entire length of the zone has experienced a large (Mw≥7.4) earthquake in the last 100 years (Singh et al., 1981). In this study we present detailed analysis of the areas of significant slip during several recent (last 20 years) large earthquakes in the Mexican subduction zone. The most recent earthquake of 20 March 2012 (Mw7.4) occurred near the Guerrero/Oaxaca border. The slip was concentrated on the plate interface below land and the epicentral PGAs ranged between 0.2 and 0.7g. The updip portion of the plate interface had previously broken during the 25 Feb 1996 earthquake (Mw7.1), which was a slow earthquake and produced anomalously low PGAs (Iglesias et al., 2003). This indicates that in this region the area close to the trench is at least partially locked, with some earthquakes breaking the down-dip portion of the interface and others rupturing the up-dip portion. The Jalisco/Colima segment of the subduction zone seems to behave in a similar fashion. The 9 October 1995 (Mw 8.0) earthquake generated small accelerations relative to its size. The energy to moment ratio, E0/M0, is 4.2e-6 (Pérez-Campos, Singh and Beroza, 2003), a value similar to the Feb, 1996 earthquake. This value is low compared to other thrust events in the region. The earthquake also had the largest (Ms-Mw) disparity along the Mexican subduction zone, 7.4 vs 8.0. The event produced relatively large tsunami. On the contrary, the 3 June 1932 earthquake (Ms8.2, Mw8.0), that is believed to have broken the same segment of the subduction zone, appears to be "normal." Based on the available evidence, it may be concluded that the 1932 event broke a deeper patch of the plate interface relative to the 1995 event. The mode of rupture in the subduction zone between the two areas mentioned above is not known. This part of the subduction zone includes the rupture area of the 1985 Michoacán earthquake (Mw8.0) and the "Guerrero Gap" which is a section of the subduction zone that has not had a large earthquake in the last 100 years. The downdip and updip patches on the plate interface, which, generally, rupture independently may slip during one great earthquake. This possibility must be accounted for in the estimation of maximum-magnitude earthquake along the subduction zone.

  13. 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 recorded. After completion, the models were saturated with water and frozen, allowing for sectioning and profile inspection. The experiments were invariably characterized by different along-strike patterns of deformation, so that three distinct structural domains could be distinguished in all cases. Model descriptions are subdivided accordingly, including domain CC, simulating a continent-continent collision, domain OC, characterized by continent-ocean-continent collision and domain T, representing the transition zone between domain CC and domain OC. The latter zone varied in width and complexity depending on the contrast in structural style developed in the two other domains; in cases where domain OC developed very differently from domain CC, the transition zone was generally wider and more complex. A typical experiment displayed the following features and strain history: In domain CC two principal thrust sheets are displayed, which obviously developed in an in-sequence foreland-directed fashion. The lowermost detachment nucleated at the base of the High Strength Lithospheric Mantle analogue, whereas the uppermost thrust was anchored within the "lower crust". The two thrusts operated in concert, the surface trace of the deepest dominating in the west, and the shallowest in the east. The kinematic development of domain CC could be subdivided into four stages, including initiation of a symmetrical anticline with a minute amplitude and situated directly above the velocity discontinuity defined by the plate contact (stage 1), contemporaneous development of the two thrusts (stage 2) and an associated asymmetrical anticline (stage 3) with a central collapse graben in the latest phase (stage 4). It is noted that the segment CC as seen in a clear majority of the experiments followed this pattern of development. In contrast, the configuration of domain OC displayed greater variation, and included north and south-directed subduction, folding, growth of pop-up-structures and triangle zones. In the "ocean crust" domain, stage 1 was characterized by the growth of a fault-propagation anticline with an E-W-oriented fold axis, ending with the surfacing of a north-vergent thrust. In stage 2, the contraction was concentrated to the south in the oceanic domain, again ending with the surfacing of a thrust, here with top-south transport. By continued movement (stage 3), the thrust fault propagated towards the east, crossing into the "continental" domain and linking with the fault systems of the segment CC. The structure of domain T is dominated by the interference of faults propagating westwards from the domain CC and eastwards from the domain OC, respectively. The zone of overlap in the experiment was significant, and its central part had the geometry of a double "crocodile structure" (sensuMeissner 1989), separating the two areas of northerly and southerly subduction. Hence, its development is less easily subdivided into stages. Reference: Meissner,R., 1989: Rupture, creep lamellae and crocodiles: happenings in the continental crust. Terra Nova, 1, 17-28.

  14. A Comparison of Sub-Slab Anisotropy Beneath the Scotia and Caribbean Subduction Zones

    NASA Astrophysics Data System (ADS)

    Lynner, C.; Long, M. D.

    2011-12-01

    Subduction systems are vitally important to plate tectonics and mantle convection but questions remain about many aspects of subduction dynamics. In particular, the mantle flow field beneath subducting slabs remains poorly understood. Seismic anisotropy can shed light on the pattern of mantle flow in subduction zones, but most studies that have examined subduction zone anisotropy focus primarily on the anisotropy of the system as a whole, or on the mantle wedge. Here we examine shear wave splitting due to anisotropy in the sub-slab mantle beneath the Caribbean and Scotia subduction zones. These subduction systems share many morphological similarities, with high arc curvature and short arc length, and yet are substantially different morphologically from other subduction zones. We examine and compare the anisotropy beneath the Scotia and Caribbean slabs to determine if similarities are also found in the flow fields beneath the slabs. We use the source-side splitting technique for direct teleseismic S phases for earthquakes originating in the down going slabs. We restrict our analysis to seismic stations where we have examined the receiver side splitting in detail; for stations with complex upper mantle anisotropy on the receiver side, it is difficult to make accurate corrections for anisotropy near the station. Since we are only using stations which we examined and have simple single layer anisotropy, we can be very confident in the corrections made to the wave forms to account for the anisotropy along the receiver-side portion of the ray path. Once the receiver-side anisotropy has been removed from the incoming wave, the only anisotropic signal left is that of the sub-slab mantle as the rest of the wave path is through the isotropic lower mantle. The Scotia subduction zone shows weak toroidal flow around the down going slab, possibly due to the curved nature of the subducting slab coupled with trench roll back. We are currently working on characterizing the Caribbean subduction zone sub-slab anisotropy, and while the seismicity of the Caribbean subduction system is substantially less than that of Scotia, we hope to see similar toroidal flow.

  15. 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-P times in a manner similar to double-difference tomography. Obtaining a reliable Vp/Vs model of the subduction zone is more helpful for understanding its mechanical and petrologic properties. Our applications of the original version of double-difference tomography to several subduction zones beneath northern Honshu, Japan, the Wellington region, New Zealand, and Alaska, United States, have shown evident velocity variations within and around the subducting slab, which likely is evidence of dehydration reactions of various hydrous minerals that are hypothesized to be responsible for intermediate depth earthquakes. We will show the new velocity models for these subduction zones by applying our advanced tomographic methods.

  16. Fluid-Mediated Redox Processes at Subduction Zones (Invited)

    NASA Astrophysics Data System (ADS)

    Malaspina, N.; Langenhorst, F.; Poli, S.

    2013-12-01

    The mechanism of slab-to-mantle volatile transfer is strongly related to the fluid speciation, which in turn is a function of oxygen fugacity, in a system buffered by equilibria involving redox-sensitive elements. However, the redox processes taking place in the portion of mantle wedge on top of the subducting slab are poorly investigated and the oxidising power of fluids is still unknown. Information on such fluid/melt related processes can be gained by the study of orogenic metasomatised ultramafic rocks associated with deeply subducted crust. We present two case studies of mantle-derived garnet peridotites from Sulu (China) and the Western Gneiss Region (Norway), unique examples of metasomatised mantle wedge that interacted with COH fluids subducted up to 200 km depth. Sulu peridotites record a multistage metasomatism by alkali-rich silicate melt, and a subsequent influx of a slab-derived incompatible element and silicate-rich fluid during the Triassic UHP metamorphism. We performed Fe3+/ΣFe flank method and electron energy loss spectroscopy measurements on garnet and pyroxenes, to quantify the Fe3+ distribution among the peridotite phases and estimate the bulk oxidation state of the peridotite. The results indicate that garnets are zoned, with Fe2O3 increasing from ~0.8 to ~2.5 wt.%, and clinopyroxenes contain high Fe3+/ΣFe ratios (0.48 to 0.51) and Na contents. Peridotites from Norway preserve remnants of crust-derived fluids which precipitated daughter Cr- spinel + phlogopite/K-amphibole + dolomite/magnesite + graphite/diamond in polyphase inclusions hosted by majoritic garnet. They witness COH fluid/mineral interaction responsible for diamond formation. We determined the fO2 of the peak mineral assemblage starting from Fe3+ analyses in majoritic garnet. The fO2 values are up FMQ-2 along a trend from arc lavas (FMQ+1.5 - FMQ+3) to mantle wedge garnet peridotites from Sulu (FMQ - FMQ+2). The fO2 determination together with Fe3+ distribution among the hydrate-carbonate-bearing mineral association of Sulu and Bardane peridotites suggest that slab-derived metasomatic COH fluids are H2O-CO2 mixtures, whereby the H2O/CO2 ratio increases with increasing pressure. The peculiar composition of majorite-hosted diamond-bearing polyphase inclusions from Bardane and the speciation of its COH component point to an 'oxidised' silicate-rich aqueous fluid contaminated by the subducted slab to the mantle wedge which could be regarded as carrier of dissolved oxidised components from the subducted slab to the mantle wedge. Also, the Fe3+ enrichment of clinopyroxene in the Sulu peridotite is apparently related to a progressive enrichment in Na, as aegerine component, and could be favoured by the influx of Fe2O3- and alkali-rich metasomatic fluid phases. Such mechanisms open new possibilities to unravel the redox processes occurring in arc mantle sources and their role in precipitating diamond.

  17. 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.

  18. Geothermal modelling of faulted metamorphic crystalline crust: a new model of the Continental Deep Drilling Site KTB (Germany)

    NASA Astrophysics Data System (ADS)

    Szalaiová, Eva; Rabbel, Wolfgang; Marquart, Gabriele; Vogt, Christian

    2015-11-01

    The area of the 9.1-km-deep Continental Deep Drillhole (KTB) in Germany is used as a case study for a geothermal reservoir situated in folded and faulted metamorphic crystalline crust. The presented approach is based on the analysis of 3-D seismic reflection data combined with borehole data and hydrothermal numerical modelling. The KTB location exemplarily contains all elements that make seismic prospecting in crystalline environment often more difficult than in sedimentary units, basically complicated tectonics and fracturing and low-coherent strata. In a first step major rock units including two known nearly parallel fault zones are identified down to a depth of 12 km. These units form the basis of a gridded 3-D numerical model for investigating temperature and fluid flow. Conductive and advective heat transport takes place mainly in a metamorphic block composed of gneisses and metabasites that show considerable differences in thermal conductivity and heat production. Therefore, in a second step, the structure of this unit is investigated by seismic waveform modelling. The third step of interpretation consists of applying wavenumber filtering and log-Gabor-filtering for locating fractures. Since fracture networks are the major fluid pathways in the crystalline, we associate the fracture density distribution with distributions of relative porosity and permeability that can be calibrated by logging data and forward modelling of the temperature field. The resulting permeability distribution shows values between 10-16 and 10-19 m2 and does not correlate with particular rock units. Once thermohydraulic rock properties are attributed to the numerical model, the differential equations for heat and fluid transport in porous media are solved numerically based on a finite difference approach. The hydraulic potential caused by topography and a heat flux of 54 mW m-2 were applied as boundary conditions at the top and bottom of the model. Fluid flow is generally slow and mainly occurring within the two fault zones. Thus, our model confirms the previous finding that diffusive heat transport is the dominant process at the KTB site. Fitting the observed temperature-depth profile requires a correction for palaeoclimate of about 4 K at 1 km depth. Modelled and observed temperature data fit well within 0.2 °C bounds. Whereas thermal conditions are suitable for geothermal energy production, hydraulic conditions are unfavourable without engineered stimulation.

  19. Seismic imaging of the subduction zone in Southern Central Chile

    NASA Astrophysics Data System (ADS)

    Buske, Stefan; Gross, Kolja; Shapiro, Serge; Wigger, Peter

    2010-05-01

    We present the results of a three-component reflection seismic survey across the seismogenic coupling zone in the area of the 1960 Valdivia earthquake in Southern Central Chile (38.2 deg S). This data set has been acquired within the framework of project TIPTEQ (from The Incoming Plate to megaThrust Earthquake Processes) which aimed at deriving the structural and petrophysical properties of the hypocentral area as well as the surrounding crust and mantle. Our main focus was on the application of advanced seismic imaging techniques in order to obtain a high-resolution structural image. We have applied Kirchhoff-Prestack-Depth-Migration (KPSDM) and Fresnel-Volume-Migration (FVM) to enhance the structural image as well as Reflection-Image-Spectroscopy (RIS) to characterize the subsurface in terms of its scattering properties. The KPSDM and FVM sections show varying reflectivity along the subducting Nazca plate. Below the coast the plate interface can be observed at 25 km depth as the sharp lower boundary of a 2-5 km thick, highly reflective region which we interpret as a subduction channel. The plate interface itself can be traced down to depths of 50-60 km where we observe strong reflectivity along the plate interface as well as in the continental mantle wedge above it. The sections show a segmented forearc crust and major features in the accretionary wedge like the Lanalhue fault zone can be identified. At the eastern end of the profile a bright west-dipping reflector appears almost perpendicular to the plate interface. The same processing sequence has been applied to the horizontal wavefield components of the seismic reflection data set. The S-wave image (SS) shows basically the same features as the P-wave image (PP) with only slightly more diffuse reflectivity. The subduction channel appears in both images at almost the same depth with a similar thickness along the plate interface. The application of RIS distinguishes between the frequency-selective seismic response of the different parts of this subduction environment. The variable reflectivity along the plate interface appears to be a high-frequency effect mainly caused by the heterogeneous overburden and is probably not a property of the interface itself. In the intermediate-frequency image the Lanalhue fault zone clearly separates the subsurface into an almost transparent western part and a highly reflective eastern part. Finally a combination of the different frequency-selective images shows additional structural details and demonstrates the main benefit of the RIS approach in terms of an improved subsurface characterization. Finally, we present a comparison of the obtained seismic images with other geophysical data sets (local earthquake tomography, magnetotelluric images, etc.).

  20. Seismological detection of low-velocity anomalies surrounding the mantle transition zone in Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Liu, Zhen; Park, Jeffrey; Karato, Shun-ichiro

    2016-03-01

    In the Japan subduction zone, a locally depressed 660 discontinuity has been observed beneath northeast Asia, suggesting downwelling of materials from the mantle transition zone (MTZ). Vertical transport of water-rich MTZ materials across the major mineral phase changes could lead to water release and to partial melting in surrounding mantle regions, causing seismic low-velocity anomalies. Melt layers implied by low-velocity zones (LVZs) above the 410 discontinuity have been detected in many regions, but seismic evidence for partial melting below the 660 discontinuity has been limited. High-frequency migrated Ps receiver functions indicate LVZs below the depressed 660 discontinuity and above the 410 discontinuity in the deep Japan subduction zone, suggesting dehydration melting induced by water transport out of the MTZ. Our results provide insights into water circulation associated with dynamic interactions between the subducted slab and surrounding mantle.

  1. A Holocene average seismic flux rate for the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Scholz, C. H.

    2013-12-01

    Paleoseismic studies of the Cascadia subduction zone provide an almost complete record of great subduction earthquakes over the Holocene. Here we describe a simple model of this behavior. Because the width of the fully coupled subduction interface decreases to the south, constant stress-drop in earthquakes requires that their slip also decrease to the south. This requires, in order to satisfy the plate convergence constraints, that shorter and increasingly frequent earthquakes finger in towards the south, just in the way observed in the paleoseismic record. The model quantitatively agrees with independent estimates of plate convergence along the length of the plate boundary and shows, more completely than before, that this subduction zone is fully coupled seismically and that the paleoseismic record is indeed nearly complete. The seismic flux release rate averaged over the Holocene is estimated as 2.7x109 m3/yr.

  2. Boron as a tracer for material transfer in subduction zones

    NASA Astrophysics Data System (ADS)

    Rosner, Martin Siegfried

    2003-10-01

    Late Miocene to Quaternary volcanic rocks from the frontal arc to the back-arc region of the Central Volcanic Zone in the Andes show a wide range of delta 11B values (+4 to -7 ‰) and boron concentrations (6 to 60 ppm). Positive delta 11B values of samples from the volcanic front indicate involvement of a 11B-enriched slab component, most likely derived from altered oceanic crust, despite the thick Andean continental lithosphere, and rule out a pure crust-mantle origin for these lavas. The delta 11B values and B concentrations in the lavas decrease systematically with increasing depth of the Wadati-Benioff Zone. This across-arc variation in delta 11B values and decreasing B/Nb ratios from the arc to the back-arc samples are attributed to the combined effects of B-isotope fractionation during progressive dehydration in the slab and a steady decrease in slab-fluid flux towards the back arc, coupled with a relatively constant degree of crustal contamination as indicated by similar Sr, Nd and Pb isotope ratios in all samples. Modelling of fluid-mineral B-isotope fractionation as a function of temperature fits the across-arc variation in delta 11B and we conclude that the B-isotope composition of arc volcanics is dominated by changing delta 11B composition of B-rich slab-fluids during progressive dehydration. Crustal contamination becomes more important towards the back-arc due to the decrease in slab-derived fluid flux. Because of this isotope fractionation effect, high delta 11B signatures in volcanic arcs need not necessarily reflect differences in the initial composition of the subducting slab. Three-component mixing calculations for slab-derived fluid, the mantle wedge and the continental crust based on B, Sr and Nd isotope data indicate that the slab-fluid component dominates the B composition of the fertile mantle and that the primary arc magmas were contaminated by an average addition of 15 to 30 % crustal material. Spät-miozäne bis quartäre Vulkanite der vulkanischen Front und der Back-arc Region der Zentralen Vulkanischen Zone in den Anden weisen eine weite Spannbreite von delta 11B Werten (+4 bis –7 ‰) and Borkonzentrationen (6 bis 60 ppm) auf. Die positiven delta 11B Werte der Vulkanite der vulkanischen Front zeigen eine Beteiligung einer 11B-reichen Komponente am Aufbau der andinen Vulkanite, die am wahrscheinlichsten aus Fluiden der alterierten ozeanischen Kruste der abtauchenden Nazca-Platte stammt. Diese Beobachtung macht einen alleinigen Ursprung der untersuchten Laven aus der kontinentalen Kruste und/oder dem Mantelkeil unwahrscheinlich. Der Trend zu systematisch negativeren delta 11B Werten und kleineren B/Nb Verhältnissen von der vulkanischen Front zum Back-arc wird als Resultat einer Borisotopenfraktionierung einhergehend mit einer stetigen Abnahme der Fluidkomponente und einer relativ konstanten krustalen Kontamination, die sich durch relativ gleichbleibende Sr, Nd und Pb Isotopenverhältnisse ausdrückt, interpretiert. Weil die delta 11B Variation über den andinen vulkanischen Bogen sehr gut mit einer modellierten, sich als Funktion der Temperatur dynamisch verändernden, Zusammensetzung des Subduktionszonenfluides übereinstimmt, folgern wir, dass die Borisotopenzusammensetzung von Arc-Vulkaniten durch die sich dynamisch ändernde delta 11B Signatur eines Bor-reichen Subduktionsfluides bestimmt ird. Durch die Abnahme dieses Subduktionsfluides während der Subduktion nimmt der Einfluss der krustalen Kontamination auf die Borisotopie der Arc-Vulkanite im Back-arc zu. In Anbetracht der Borisotopenfraktionierung müssen hohe delta 11B Werte von Arc-Vulkaniten nicht notwendigerweise Unterschiede in der initialen Zusammensetzung der subduzierten Platte reflektieren. Eine Dreikomponenten Mischungskalkulation zwischen Subduktionsfluid, dem Mantelkeil und der kontinentalen Kruste, die auf Bor-, Strontium- und Neodymiumisotopendaten beruht, zeigt, dass das Subduktionsfluid die Borisotopie des fertilen Mantels dominiert und, dass die primären Arc-Magmen durchschnittlich einen Anteil von 15 bis 30 % krustalem Materiales aufweisen.

  3. Transition from dehydration to hydration during exhumation of the Sanbagawa metamorphic belt, Japan, revealed by the continuous P- T path recorded in garnet and amphibole zoning

    NASA Astrophysics Data System (ADS)

    Uno, Masaoki; Iwamori, Hikaru; Toriumi, Mitsuhiro

    2015-09-01

    To investigate the water budget in deep accretionary prisms, we undertook a detailed petrologic analysis of garnet-bearing amphibolites in the subduction-origin high-pressure Sanbagawa metamorphic belt, Japan. We obtained a continuous P- T path of exhumation by the application of differential thermodynamics to compositional zoning in both garnet and amphibole. The P- T path is characterized by three stages of decompression: (1) heating decompression from 1.5 GPa and 550 °C to 1.1 GPa and 600 °C; (2) isothermal decompression toward 0.8 GPa and 580 °C; and (3) cooling decompression to 0.3 GPa and 400 °C. Comparison with the results of numerical modeling of the thermal structures of subduction zones suggests that the peak-pressure condition is warmer than the slab surface temperature in a typical subduction zone, but is consistent with subduction of a young slab during the Sanbagawa metamorphism. The heating decompression just after the peak-pressure condition is explained by heating from the mantle wedge during ascent of the metamorphic body in the accretionary prism; heating stops when the metamorphic body reaches crustal depths. Mass balance calculations for the mineral reactions in an amphibolite reveal that the dehydration associated with the growth of Ca-poor garnet takes place during heating decompression, whereas hydration takes place during cooling decompression by the breakdown of garnet and pargasite to form chlorite and actinolite. Such a transition from dehydration to hydration in the exhuming metabasite body in an accretionary prism generates fluids between the continental Moho and the slab surface and may explain the seismic activity above the subduction plate boundary, such as supra-slab earthquakes.

  4. Source Parameters of Large Magnitude Subduction Zone Earthquakes Along Oaxaca, Mexico

    NASA Astrophysics Data System (ADS)

    Fannon, M. L.; Bilek, S. L.

    2014-12-01

    Subduction zones are host to temporally and spatially varying seismogenic activity including, megathrust earthquakes, slow slip events (SSE), nonvolcanic tremor (NVT), and ultra-slow velocity layers (USL). We explore these variations by determining source parameters for large earthquakes (M > 5.5) along the Oaxaca segment of the Mexico subduction zone, an area encompasses the wide range of activity noted above. We use waveform data for 36 earthquakes that occurred between January 1, 1990 to June 1, 2014, obtained from the IRIS DMC, generate synthetic Green's functions for the available stations, and deconvolve these from the ­­­observed records to determine a source time function for each event. From these source time functions, we measured rupture durations and scaled these by the cube root to calculate the normalized duration for each event. Within our dataset, four events located updip from the SSE, USL, and NVT areas have longer rupture durations than the other events in this analysis. Two of these four events, along with one other event, are located within the SSE and NVT areas. The results in this study show that large earthquakes just updip from SSE and NVT have slower rupture characteristics than other events along the subduction zone not adjacent to SSE, USL, and NVT zones. Based on our results, we suggest a transitional zone for the seismic behavior rather than a distinct change at a particular depth. This study will help aid in understanding seismogenic behavior that occurs along subduction zones and the rupture characteristics of earthquakes near areas of slow slip processes.

  5. Imaging of the subducted Kyushu-Palau Ridge in the Hyuga-nada region, western Nankai Trough subduction zone

    NASA Astrophysics Data System (ADS)

    Yamamoto, Yojiro; Obana, Koichiro; Takahashi, Tsutomu; Nakanishi, Ayako; Kodaira, Shuichi; Kaneda, Yoshiyuki

    2013-03-01

    We performed 3D seismic tomography of the Hyuga-nada region, western Nankai subduction zone, to investigate the relationship of the subducted part of Kyushu-Palau Ridge (KPR) to coseismic rupture propagation, seismicity, and shallow very low frequency earthquakes. Combining active-source and passive-source data recorded both onshore and offshore, we imaged the deep slab from near the trough axis to the coastal area. Our results show the subducted KPR as a low-velocity belt oriented NW-SE extending down the plate boundary to around 30 km depth. At this depth, we suggest that the subducted KPR detaches from the slab and becomes underplated on the overriding continental plate. As the coseismic slip areas of past large earthquakes do not extend into the subducted KPR, we suggest that it may inhibit rupture propagation. The interior of the subducted KPR shows active intraslab seismicity with a wide depth distribution. Shallow very low frequency earthquakes are continuously active above the location of the subducted KPR, whereas they are intermittent to the northeast of the subducted KPR. Thus, the subducted KPR appears to be an important factor in coseismic rupture propagation and seismic phenomena in this region.

  6. Combining GPS and repeating earthquakes for a high resolution analysis of subduction zone coupling

    NASA Astrophysics Data System (ADS)

    Weston, J.; Shirzaei, M.

    2016-01-01

    Increasingly complex spatiotemporal patterns of subduction zone coupling are being revealed by geodetic and seismic observations. Understanding the mechanisms which control it is useful for improving seismic hazard assessments. GPS and characteristically repeating earthquakes (CREs) are complementary datasets for monitoring aseismic slip. Here, both of them are combined to estimate the rate and distribution of creep on the northeast Japan subduction zone between 21 March 1996 and 24 September 2003. We find that the majority of the upper part at 0-30 km depth remains locked. There are three regions creeping at 7-8 cm/yr distributed along-strike at 40-70 km depth. We observe that these creeping regions occur in areas of low effective pressure and reduced porosity, which are inferred from Vp and Vs velocities. Moreover, an area of high clay content and high effective pressure coincides with the rupture area of the Tohoku-oki earthquake. We discuss these results in the context of potential mechanisms governing creep in northeast Japan. Our results highlight the benefits of combining GPS and CREs for advancing our understanding of the seismic cycle in subduction zones.

  7. Pressure dependence of fluid transport properties of shallow fault systems in the Nankai subduction zone

    NASA Astrophysics Data System (ADS)

    Tanikawa, Wataru; Mukoyoshi, Hideki; Lin, Weiren; Hirose, Takehiro; Tsutsumi, Akito

    2014-12-01

    We measured fluid transport properties at an effective pressure of 40 MPa in core samples of sediments and fault rocks collected by the Integrated Ocean Drilling Program (IODP) NanTroSEIZE drilling project Expedition 316 from the megasplay fault system (site C0004) and the frontal thrust (site C0007) in the Nankai subduction zone. Permeability decreased with effective pressure as a power law function. Permeability values in the fault zones were 8 × 10-18 m2 at site C0004 and 9 × 10-18 m2 at site C0007. Stratigraphic variation in transport properties suggests that the megasplay fault zone may act as a barrier to fluid flow, but the frontal thrust fault zone might not. Depth variation in permeability at site C0007 is probably controlled by the mechanical compaction of sediment. Hydraulic diffusivity at shallow depths was approximately 1 × 10-6 m2 s-1 in both fault zones, which is small enough to lead to pore pressure generation that can cause dynamic fault weakening. However, absence of a very low permeable zone, which may have formed in the Japan Trench subduction zone, might prevent facilitation of huge shallow slips during Nankai subduction zone earthquakes. Porosity tests under dry conditions might have overestimated the porosity.

  8. Depth Limits and Time Variations of Slip in Subduction Zones: Lessons from Alaska and Elsewhere

    NASA Astrophysics Data System (ADS)

    Freymueller, J. T.

    2012-12-01

    The distribution of slip with depth on the subduction interface is a critical factor in determining seismic potential. The largest subduction earthquakes generally feature large slip over a broad range of depths, while earthquakes that rupture smaller rupture patches generally feature less total slip as well. The position of the updip limit of slip is of great importance for tsunami generation, while the position of the downdip limit of slip is often more relevant for ground motion predictions, because the downdip limit of slip is located closer to cities and towns. Precise Global Positioning System (GPS) measurements provide an effective way to determine whether parts of the plate interface are creeping steadily or are locked by friction, by measuring the deformation of the surface that results from variations in slip. The position of the downdip limit is variable in time at most subduction zones. Over a ~300 km segment of the Alaska subduction zone near Anchorage and other population centers, we have observed two very large slow slip events (SSEs) on the plate interface, and also a transient increase in the width of the locked region on the megathrust interface. Although the position of the downdip end of the locked region is subject to short-term variations, geodetic data show large-scale along strike variations that correlate strongly with the pattern of slip in great subduction earthquakes. Thus, measurements of deformation are a powerful tool for assessing the potential for future seismic slip at subduction zones.

  9. Link between plate fabric, hydration and subduction zone seismicity in Alaska

    NASA Astrophysics Data System (ADS)

    Shillington, Donna J.; Bécel, Anne; Nedimović, Mladen R.; Kuehn, Harold; Webb, Spahr C.; Abers, Geoffrey A.; Keranen, Katie M.; Li, Jiyao; Delescluse, Matthias; Mattei-Salicrup, Gabriel A.

    2015-12-01

    Subduction zones worldwide exhibit remarkable variation in seismic activity over short distances of about tens of kilometres along their length. The properties of the subducting oceanic plate are believed to influence this seismic behaviour. However, comparisons between seismicity and plate attributes such as thermal structure made over large scales of hundreds of kilometres typically yield poor correlations. Here we present results from controlled-source seismic data collected offshore of the Alaska Peninsula. We find that fabric in the subducting oceanic plate--the orientation and style of remnant faults originally created at the mid-ocean ridge--can contribute to abrupt changes in faulting and hydration of the plate during bending before subduction. Variations in fabric, bending faulting and hydration correlate with changes in seismicity throughout the subduction zone. More interplate and intermediate-depth intraplate earthquakes are observed where the pre-existing fabric is aligned with the trench and there is more bend faulting and hydration. This suggests that pre-existing structures in the subducting plate are an important control on abrupt variations in deformation and plate hydration and on globally observed short-wavelength variations in seismicity at subduction zones.

  10. Carbon mobilized at shallow depths in subduction zones by carbonatitic liquids

    NASA Astrophysics Data System (ADS)

    Poli, Stefano

    2015-08-01

    More than half a gigaton of CO2 is subducted into Earth’s interior each year. At least 40% of this CO2 is returned to the atmosphere by arc volcanism. Processes that are known to release carbon from subducting slabs--decarbonation or carbonate dissolution in fluids--can account for only a portion of the CO2 released at arc volcanoes. Carbonatitic liquids may form from the subducting crust, but are thought to form only at very high temperatures. Melting of carbonated rocks could restrict the subduction of carbon into the deeper Earth. However, the behaviour of such rock types in subduction zones is unclear. Here I use laboratory experiments to show that calcium-rich hydrous carbonatitic liquids can form at temperatures as low as 870 to 900 °C, which corresponds to shallow depths of just 120 km beneath subduction zone arcs, in warm thermal regimes. I find that water strongly depresses the solidus for hydrous carbonate gabbro and limestone rocks, creating carbonatitic liquids that efficiently scavenge volatile elements, calcium and silicon, from the slab. These extremely mobile and reactive liquids are expected to percolate into the mantle wedge, and create a CO2 source for subduction zone magmatism. Carbonatitic liquids thus provide a potentially significant pathway for carbon recycling at shallow depths beneath arcs.

  11. High-temperature metamorphism during extreme thinning of the continental crust: a reappraisal of the north Pyrenean paleo-passive margin

    NASA Astrophysics Data System (ADS)

    Clerc, C.; Lahfid, A.; Monié, P.; Lagabrielle, Y.; Chopin, C.; Poujol, M.; Boulvais, P.; Ringenbach, J.-C.; Masini, E.; de St Blanquat, M.

    2015-02-01

    An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a dataset of more than one hundred peak-temperature estimates obtained using Raman spectroscopy of the carbonaceous material (RSCM). This dataset is completed by previous PT estimates based on mineral assemblages, and new Ar-Ar (amphibole, micas) and U-Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomargins are discussed. Ages range mainly from 110 to 90 Ma and no westward or eastward propagation of the metamorphism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key-localities of the Mauléon Basin, Arguenos-Moncaup, Lherz, Boucheville and the Bas-Agly, we analyse the thermal conditions prevailing during the Cretaceous crustal thinning. The results are synthetized into a series of three regional thematic maps, and into two detailed maps of the Arguenos-Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamorphism are intimately associated with the areas where subcontinental mantle rocks have been unroofed or exhumed.

  12. Formation of metamorphic core complexes in non-over-thickened continental crust: A case study of Liaodong Peninsula (East Asia)

    NASA Astrophysics Data System (ADS)

    Wang, Kun; Burov, Evgueni; Gumiaux, Charles; Chen, Yan; Lu, Gang; Mezri, Leila; Zhao, Liang

    2015-12-01

    Pre-thickened hot orogenic crust is often considered a necessary condition for the formation of continental metamorphic core complexes (MCCs). However, the discovery of MCCs in the Liaodong Peninsula, where the crust has a normal thickness (~ 35 km), challenges the universality of this scenario. Therefore, we implement a series of 2-D numerical thermo-mechanical modeling experiments in which we investigate the conditions of MCC formation in normal crusts, as well as the relationships between the underlying mechanisms and the syn-rift basin evolution. In these experiments, we explore the impact of the lithostratigraphic and thermo-rheological structure of the crust. We also examine the lithosphere thickness, strain softening, extension rate, and surface erosion/ sedimentation processes. The experiments demonstrate that high thermal gradients and crustal heterogeneities result only in a symmetric spreading dome, which is geometrically incompatible with the observations of the MCCs in the Liaodong Peninsula. According to our further findings, the strain softening should play a key role in the development of asymmetric strain localization and domal topography uplift, while synchronous surface erosion controls the polarity of the syn-rift basin. The synthetic model data are compatible with the geological observations and cooling history based on the thermo-chronology for the eastern part of the East Asia during the late Mesozoic to the early Cenozoic. The model-predicted P-T-t paths are essentially different from those inferred for the other known MCCs, confirming the exceptional character of the MCC formation in the wide rift system of the East Asia.

  13. Crustal structure and continental dynamics of Central China: A receiver function study and implications for ultrahigh-pressure metamorphism

    NASA Astrophysics Data System (ADS)

    He, Chuansong; Dong, Shuwen; Chen, Xuanhua; Santosh, M.; Li, Qiusheng

    2014-01-01

    The Qinling-Tongbai-Hong'an-Dabie-Sulu orogenic belt records the tectonic history of Paleozoic convergence between the South China and North China Blocks. In this study, the distribution of crustal thickness and P- and S-wave velocity ratio (Vp/Vs) is obtained by using the H-k stacking technique from the Dabie-Sulu belt in central China. Our results show marked differences in the crustal structure between the Dabie and Sulu segments of the ultrahigh-pressure (UHP) orogen. The lower crust in the Dabie orogenic belt is dominantly of felsic-intermediate composition, whereas the crust beneath the Sulu segment is largely intermediate-mafic. The crust of the Dabie orogenic belt is thicker by ca. 3-5 km as compared to that of the surrounding region with the presence of an ‘orogenic root’. The crustal thickness is nearly uniform in the Dabie orogenic belt with a generally smooth crust-mantle boundary. A symmetrically thickened crust in the absence of any deep-structural features similar to that of the Yangtze block suggests no supportive evidence for the proposed northward subduction of the Yangtze continental block beneath the North China Block. We propose that the collision between the Yangtze and North China Blocks and extrusion caused crustal shortening and thickening, as well as delamination of the lower crust, resulting in asthenospheric upwelling and lower crustal UHP metamorphism along the Dabie Orogen. Our results also reveal the presence of a SE to NW dipping Moho in the North China Block (beneath the Tran-North China Orogen and Eastern Block), suggesting the fossil architecture of the northwestward subduction of the Kula plate.

  14. Experimental Study on Fluid Distribution at Ultra-High Metamorphic Conditions

    NASA Astrophysics Data System (ADS)

    Mönicke, K.; Burchard, M.; Duyster, J.; Maresch, W. V.; Röller, K.; Stöckhert, B.

    2001-12-01

    Ultra-high pressure (UHP) metamorphic rocks record deep subduction of continental crust. Insight into their rheological behavior at UHP metamorphic conditions is important for the understanding of the mechanical state and the kinematics within subduction zones. Amazingly, many exhumed UHP metamorphic rocks do not show evidence of significant deformation. Thus, it has been proposed that deformation is localized in low-strength zones controlled by partially wetting interstitial fluids [1]. Experimental results [2] show that at UHP metamorphic conditions only one homogenous fluid phase with variable composition exists, whose density and viscosity should be intermediate between those of conventional aqueous solutions and hydrous melts. Inclusions of such supercritical fluid have been recently described from a natural UHP metamorphic rock [3]. Motivated by these findings, experiments using a piston-cylinder apparatus were performed to study the fluid distribution in various rock types at pressures of 3.5 GPa and temperatures between 900 ° C and 600 ° C. Starting materials were natural UHP metamorphic specimens of (1) S-type granitic biotite-phengite-gneiss and (2) pyrope-quartzite, both from the Dora Maira Massif (Western Alps, Italy) and (3) a diamond-bearing garnet-mica-gneiss with granodioritic bulk composition from the Saxonian Erzgebirge (Germany), all with 2 wt.% water added. The supercritical fluids formed in these experiments can be quenched to form a silicic glass with demixing of an aqueous solution without changing the UHP fluid topology significantly. The shape of the fluid-filled interstices is irregular and complex, resulting in a low volume/interface area ratio and a potential of high stress concentration at the edges of wedge-shaped offshoots. We propose that the distribution of supercritical fluids has a pronounced effect on the strength of cool subducted crust, allowing deformation by grain boundary sliding and dissolution precipitation creep, or granular flow with grain shape accommodation by material transfer via an intergranular fluid [4], possibly localized in specific rock types. References: \

  15. Scattering beneath Western Pacific subduction zones: evidence for oceanic crust in the mid-mantle

    NASA Astrophysics Data System (ADS)

    Bentham, H. L. M.; Rost, S.

    2014-06-01

    Small-scale heterogeneities in the mantle can give important insight into the dynamics and composition of the Earth's interior. Here, we analyse seismic energy found as precursors to PP, which is scattered off small-scale heterogeneities related to subduction zones in the upper and mid-mantle. We use data from shallow earthquakes (less than 100 km depth) in the epicentral distance range of 90°-110° and use array methods to study a 100 s window prior to the PP arrival. Our analysis focuses on energy arriving off the great circle path between source and receiver. We select coherent arrivals automatically, based on a semblance weighted beampower spectrum, maximizing the selection of weak amplitude arrivals. Assuming single P-to-P scattering and using the directivity information from array processing, we locate the scattering origin by ray tracing through a 1-D velocity model. Using data from the small-aperture Eielson Array (ILAR) in Alaska, we are able to image structure related to heterogeneities in western Pacific subduction zones. We find evidence for ˜300 small-scale heterogeneities in the region around the present-day Japan, Izu-Bonin, Mariana and West Philippine subduction zones. Most of the detected heterogeneities are located in the crust and upper mantle, but 6 per cent of scatterers are located deeper than 600 km. Scatterers in the transition zone correlate well with edges of fast features in tomographic images and subducted slab contours derived from slab seismicity. We locate deeper scatterers beneath the Izu-Bonin/Mariana subduction zones, which outline a steeply dipping pseudo-planar feature to 1480 km depth, and beneath the ancient (84-144 Ma) Indonesian subduction trench down to 1880 km depth. We image the remnants of subducted crustal material, likely the underside reflection of the subducted Moho. The presence of deep scatterers related to past and present subduction provides evidence that the subducted crust does descend into the lower mantle at least for these steeply dipping subduction zones. Applying the same technique to other source-receiver paths will increase our knowledge of the small-scale structure of the mantle and will provide further constraints on geodynamic models.

  16. A non extensive statistical physics analysis of the Hellenic subduction zone seismicity

    NASA Astrophysics Data System (ADS)

    Vallianatos, F.; Papadakis, G.; Michas, G.; Sammonds, P.

    2012-04-01

    The Hellenic subduction zone is the most seismically active region in Europe [Becker & Meier, 2010]. The spatial and temporal distribution of seismicity as well as the analysis of the magnitude distribution of earthquakes concerning the Hellenic subduction zone, has been studied using the concept of Non-Extensive Statistical Physics (NESP) [Tsallis, 1988 ; Tsallis, 2009]. Non-Extensive Statistical Physics, which is a generalization of Boltzmann-Gibbs statistical physics, seems a suitable framework for studying complex systems (Vallianatos, 2011). Using this concept, Abe & Suzuki (2003;2005) investigated the spatial and temporal properties of the seismicity in California and Japan and recently Darooneh & Dadashinia (2008) in Iran. Furthermore, Telesca (2011) calculated the thermodynamic parameter q of the magnitude distribution of earthquakes of the southern California earthquake catalogue. Using the external seismic zones of 36 seismic sources of shallow earthquakes in the Aegean and the surrounding area [Papazachos, 1990], we formed a dataset concerning the seismicity of shallow earthquakes (focal depth ≤ 60km) of the subduction zone, which is based on the instrumental data of the Geodynamic Institute of the National Observatory of Athens (http://www.gein.noa.gr/, period 1990-2011). The catalogue consists of 12800 seismic events which correspond to 15 polygons of the aforementioned external seismic zones. These polygons define the subduction zone, as they are associated with the compressional stress field which characterizes a subducting regime. For each event, moment magnitude was calculated from ML according to the suggestions of Papazachos et al. (1997). The cumulative distribution functions of the inter-event times and the inter-event distances as well as the magnitude distribution for each seismic zone have been estimated, presenting a variation in the q-triplet along the Hellenic subduction zone. The models used, fit rather well to the observed distributions, implying the complexity of the spatiotemporal properties of seismicity and the usefulness of NESP in investigating such phenomena, exhibiting scale-free nature and long range memory effects. Acknowledgments. This work was supported in part by the THALES Program of the Ministry of Education of Greece and the European Union in the framework of the project entitled "Integrated understanding of Seismicity, using innovative Methodologies of Fracture mechanics along with Earthquake and non extensive statistical physics - Application to the geodynamic system of the Hellenic Arc. SEISMO FEAR HELLARC". GM and GP wish to acknowledge the partial support of the Greek State Scholarships Foundation (ΙΚΥ).

  17. Along strike variation of tremor activities and thermal structures in various subduction zones

    NASA Astrophysics Data System (ADS)

    Yabe, S.; Ide, S.; Yoshioka, S.

    2012-12-01

    A family of slow earthquakes, e.g., deep low frequency tremors, low frequency earthquakes (LFEs), very low frequency earthquakes (VLFs) and slow slip events (SSEs), are observed in various subduction zones. These phenomena represent shear slip on the plate interface, and they are thought to be related to brittle-ductile transition behavior on the plate interface because they are often located near the transition zones of interplate coupling estimated from GPS data. Such slip behavior along the plate interface would be controlled by temperature. Furthermore, tremors are considered to be related to fluid dehydrated from the subducting slab, through temperature dependent chemical reactions. Therefore, tremors occurrences are expected to be influenced by temperature, though some studies have questioned about the relationship between tremor activity and temperature. Here we investigate the source locations of deep tremor using an envelope correlation method and compare them with the temperature and shear strength profiles along the plate interface calculated using a numerical model (Yoshioka and Sanshadokoro, 2002). The study areas include New Zealand, southern Chile, and Mexico, where tremor behavior changes significantly along the strike of the plate interface. Investigating such along-strike variation in individual subduction zone may clarify the temperature dependence of tremor because environmental conditions affecting tremor occurrence are similar, unlike the comparison between different subduction zones. In the Hikurangi subduction zone beneath the North Island, New Zealand, the depth of SSE are quite different along the strike, e.g., deeper in the central region and shallower in the northern region (e.g. Wallace and Beavan, 2010). We reanalyze tremors detected by previous studies (Kim et al., 2011; Ide, 2012) to estimate their absolute depth and confirm that tremors in North Island are on the plate interface in both the central and the northern regions. Thermal modeling of the Hikurangi subduction zone suggests that change of seismic coupling depth is due to the change of the temperature profile, mainly controlled by shear heat on the plate interface, which is governed by fluid distribution, and plate convergence rate. The tremor sources are located near the brittle-ductile transition, which implies that tremor genesis is controlled by temperature. Along-strike variation is also observed in southern Chile, near the triple junction, and in the Colima-Jalisco region, Mexico. The automated analysis of Ide (2012) shows that the depth of tremors becomes continuously deeper as the subducting plate age increases, implying that temperature is a major factor controlling tremor genesis. We present the results of comparison between the accurate tremor location and thermal models.

  18. The influence of small stresses on the dynamics of glaciers and subduction zones

    NASA Astrophysics Data System (ADS)

    Walter, Jacob I.

    The fast flow of glaciers and rupture of earthquakes on subduction zones both occur at interfaces notoriously difficult to observe. When slip events occur on such interfaces, elastic energy is radiated in the form of seismic waves. Here, we analyze these and other signals, such as continuous GPS, for two glacier systems and a subduction zone. We find that the slip events at an Antarctic ice stream, a Greenland outlet glacier, and a subduction zone in Costa Rica are all modulated by the ocean tide, however, the manifestations of the modulations vary considerably. In Antarctica, we directly measure the bidaily rupture process of the Whillans Ice Plain using broadband seismometers. The average rupture speed of these events varies by a factor of 2, which is dependent upon the recurrence interval. Based on variations of rupture near the source region, we observe that rupture speed depends on loading conditions. In Greenland, the breakup of the sea ice and iceberg mixture that forms during the winter causes changes in the flow velocity of Store Glacier, West Greenland. We measure velocity using continuous GPS 16 km away from the terminus and time-lapse photography near the terminus. We observe a step-change in velocity near the terminus in response to the ice melange breakup, corresponding to a 30-60 kPa loss of buttressing stress. Further, we observe semi-diurnal periodicity in GPS speed perturbations 16 km from the terminus, likely due to ocean tides. At the subduction zone, we identify and locate tremor events, distinct from earthquakes in their low amplitude, which occur as shear failure on the plate interface. We locate the events on portions of the plate believed to be undergoing stable sliding, adjacent to locked portions of the plate. Furthermore, we provide multiple forms of geodetic and pressure evidence of an offshore event that occurred in 2008. Observations of various behaviors that include relatively small stresses may provide insight into the unique dynamics of glaciers and subduction zones. Small stresses that drive failure suggests either a weak basal interface or a system near its critical stress state.

  19. Volatiles in Tonga Arc magmas and their role in unraveling subduction zone processes

    NASA Astrophysics Data System (ADS)

    Cooper, Lauren Beth

    Subduction zones are locations where volatile species are recycled. Volatiles, especially H2O, contained within the subducted oceanic crust and overlying sediment are expelled into the mantle wedge via dehydration reactions, where they drive mantle melting. The resulting hydrous primary magmas are erupted, fueled by volatile degassing, through overlying volcanoes. To learn about subduction processes, undegassed primary H2O contents were estimated for seven submarine volcanoes within the Tonga Arc by measuring volatiles in melt inclusions (in addition to major and trace elements in melt inclusions, whole rocks, glasses, and phenocrysts). These are the first volatile measurements for Tonga. Tonga is an end-member subduction zone. Located in the southern Pacific, active spreading within the adjacent Lau Backarc Basin causes convergence rates at the trench to increase three-fold from south to north, where it is the fastest in the world. Mantle H2O contents are coupled indirectly to convergence rate, while the extents of mantle melting beneath the arc front are relatively constant over the length of the arc. The dominant signal is a strong variability in mantle source composition along the arc strike, due to prior melting in the Lau Basin. In support of this is the first documented occurrence of an active arc volcano erupting a rare rock-type called a boninite; this was discovered in the central portion of the arc, where extents of prior melting are the greatest. A backarc-arc evolutionary model is developed to explain the petrogenesis of this volcano. The model results, as well as geochemical arguments, suggest that many of the surrounding Tonga Arc volcanoes may also have a boninitic origin. Lastly, a new H2O/Ce fluid thermometer is used to estimate slab fluid temperatures within several global subduction zones to infer the temperature of the slab surface directly beneath the arc front. While temperatures range significantly between subductions zones, with Tonga being the coldest, all temperatures are greater than the wet solidus, implying that slab-derived fluids may be better described as hydrous melts than aqueous fluids. These estimates may be used to ground-truth geodynamic thermal models and provide constraints for the efficiency of volatile recycling within subduction zones.

  20. Two decades of spatiotemporal variations in subduction zone coupling offshore Japan

    NASA Astrophysics Data System (ADS)

    Loveless, John P.; Meade, Brendan J.

    2016-02-01

    Spatial patterns of interplate coupling on global subduction zones can be used to guide seismic hazard assessment, but estimates of coupling are often constrained using a limited temporal range of geodetic data. Here we analyze ∼19 years of geodetic observations from the GEONET network to assess time-dependent variations in the spatial distribution of coupling on the subduction zones offshore Japan. We divide the position time series into five, ∼3.75-year epochs each decomposed into best-fit velocity, annual periodic signals, coseismic offsets, and postseismic effects following seven major earthquakes. Nominally interseismic velocities are interpreted in terms of a combination of tectonic block motions and earthquake cycle activity. The duration of the inferred postseismic activity covaries with the linear velocity. To address this trade-off, we assume that the nominally interseismic velocity at each station varies minimally from epoch to epoch. This approach is distinct from prior time-series analysis across the earthquake cycle in that position data are not detrended using preseismic velocity, which inherently assumes that interseismic processes are spatially stable through time, but rather the best-fit velocity at each station may vary between epochs. These velocities reveal significant consistency since 1996 in the spatial distribution of coupling on the Nankai subduction zone, with variation limited primarily to the Tokai and Bungo Channel regions, where long-term slow slip events have occurred, and persistently coupled regions coincident with areas that slipped during historic great earthquakes. On the Sagami subduction zone south of Tokyo, we also estimate relatively stable coupling through time. On the Japan-Kuril Trench, we image significant coupling variations owing to effects of the 1994 MW = 7.7 Sanriku-oki, 2003 MW = 8.2 Tokachi-oki, and 2011 MW = 9.0 Tohoku-oki earthquakes. In particular, strong coupling becomes more spatially extensive following the 1994 event until 2011, coseismic-sense slip precedes the Tohoku-oki event, and coupling offshore northern Honshu is reduced after the 2011 earthquake. Despite the occurrence of the 2003 Tokachi-oki earthquake, persistent coupling offshore Hokkaido suggests ongoing seismic hazard, possibly similar to past MW ∼ 9-class earthquakes interpreted from coastal paleoseismic records. This time-dependent analysis of interseismic deformation illuminates rich diversity in the distribution of subduction zone coupling, including spatiotemporal stability in coupling, effective reduction in strongly coupled regions due to aseismic thrust-sense slip events, and broad changes in the distribution of coupling following major earthquakes.

  1. A Crustal Structure Study of the Southern Ryukyu Subduction Zone by Using the Aftershock Data

    NASA Astrophysics Data System (ADS)

    Cho, Y.; Lin, J.; Lee, C.

    2011-12-01

    The region along the Ryukyu subduction zone is known as a tsunami disaster zone. The biggest tsunami (85 m) of Japan history was recorded in the Ishigaki Island, Ryukyu, in 1771. The paleo-tsunami events show that it has a frequency of about 150 years. This thread makes the Ryukyu subduction zone as a concerned field for the earthquake studies. However, due to the long distance from the east coast of Taiwan, this is an area out of the effective earthquake detection zone from the Central Weather Bureau network. A main shock of M = 6.9 occurred near the Ishigaki Island in 2009 August 17. After this event, we quickly deployed the OBS and found many aftershocks with the magnitude greater than 5.0. The main shock was 240 km, NE direction from the Hualien city, Taiwan. If a tsunami occurred, it took only less than 15 minutes to arrive the coast. From the recorded data, we picked the P- and S-wave using the 1-D module (iasp91). There were 1500 recorded events during those time range, and most of the earthquakes were located around the Nanao Basin. Based on this, we study the southern Ryukyu subduction zone structure by using the results from focal mechanism solution. From the earthquake relocation it shows that two main groups of aftershocks. They tend in northwest - southeast with a left-lateral strike-slip fault. The left-lateral strike-slip fault is the main structures that link with the splay faults at the southern Ryukyu Trench. The stability and extension of the splay faults are one of the major concerns for the occurrence of mega earthquake. More than 500-km long of the splay fault, such as that in the Indonesia, Chile and Japan subduction zones, has attacked by mega earthquakes in the recent years. The second group of those aftershocks was located in the Gagua Ridge near the Ryukyu Trench. This group may represent the ridge structure relate to the Taitung canyon fault. The front of Ryukyu Trench was being as a locked subduction zone where it is easily to accumulate the earthquake stress. Because of these two earthquake groups are out of range of Taiwan Central Weather Bureau network and lack of information, it is worthwhile to focus our attentions on it.

  2. Rates of burial and exhumation of lawsonite blueschist/eclogite in subduction zones from in situ UV laser ablation 40Ar/39Ar phengite geochronology

    NASA Astrophysics Data System (ADS)

    Fornash, K.; Cosca, M. A.; Whitney, D. L.; Teyssier, C. P.

    2014-12-01

    Lawsonite eclogites and blueschists are accessible records of processes that occur at depth in subducting slabs and can therefore provide information about the chemical and physical evolution of subduction zones. In composite blueschist-eclogite terranes, blueschists may have formed (1) by prograde metamorphism (pre-eclogite), (2) at the same P-T conditions as eclogite-facies metamorphism as a result of differences in bulk composition, H2O content, or oxidation state, or (3) from retrogression of eclogite, e.g. during exhumation. Field and petrologic observations of lawsonite eclogite and blueschist in the Sivrihisar Massif, Turkey, suggest that some blueschist formed from eclogite during exhumation in the subduction channel, whereas results from thermobarometry suggest that some blueschist formed at the same P-T conditions as eclogite. To test the age, petrologic, and tectonic relationship of coexisting eclogite and blueschist, we applied in situ UV laser ablation 40Ar/39Ar phengite geochronology to eclogite- and blueschist-facies rocks representing different structural positions and displaying different phengite textures and coexisting mineral assemblages. Phengite from fresh lawsonite eclogite yield an age of 93 ± 2 Ma and have the narrowest spread in ages (<12 Ma) of any rock type analyzed. Retrogressed (epidote) eclogite yields a mean weighted age of 82 ± 2 Ma. In contrast to the tightly constrained ages obtained in eclogite pods, blueschists and blueschist-facies quartzite exhibit discrete age populations ranging from 82 Ma to 110 Ma. Deformed phengite clusters from lawsonite garnet blueschist record age populations at 82 Ma and 92 Ma. Phengite from lawsonite-garnet veins and glaucophane-rich margins of eclogite pods also record 92 Ma. Omphacite-bearing lawsonite blueschist and a blueschist-facies quartzite from the same structural position contain age populations at 97 Ma and 110 Ma. These results document a sequence of events from prograde blueschist-facies (> 93 Ma) to peak eclogite-facies metamorphism and vein formation (~93 Ma) to retrograde metamorphism (< 90 Ma). Combined with P-T estimates, these new results distinguish prograde from retrograde blueschist assemblages and allow calculation of burial/exhumation rates over the depth range ~80-45 km in a subduction zone.

  3. A Look Inside of Diamond-Forming Media in Deep Subduction Zones

    SciTech Connect

    Dobrzhinetskaya,L.; Wirth, R.; Green, II, H.

    2007-01-01

    Geologists have 'known' for many years that continental crust is buoyant and cannot be subducted very deep. Microdiamonds 10-80 {mu}m in size discovered in the 1980s within metamorphic rocks related to continental collisions clearly refute this statement, suggesting that material of continental crust has been subducted to a minimum depth of > 150 km and incorporated into mountain chains during tectonic exhumation. Over the past decade, the rapidly moving technological advancement has made it possible to examine these diamonds in detail, and to learn that they contain nanometric multiphase inclusions of crystalline and fluid phases and are characterized by a 'crustal' signature of carbon stable isotopes. Scanning and transmission electron microscopy, focused ion beam techniques, synchrotron infrared spectroscopy, and nano-secondary ion mass spectrometry studies of these diamonds provide evidence that they were crystallized from a supercritical carbon-oxygen-hydrogen fluid. These microdiamonds preserve evidence of the pathway by which carbon and water can be subducted to mantle depths and returned back to the earth's surface.

  4. A look inside of diamond-forming media in deep subduction zones

    PubMed Central

    Dobrzhinetskaya, Larissa F.; Wirth, Richard; Green, Harry W.

    2007-01-01

    Geologists have “known” for many years that continental crust is buoyant and cannot be subducted very deep. Microdiamonds 10–80 μm in size discovered in the 1980s within metamorphic rocks related to continental collisions clearly refute this statement, suggesting that material of continental crust has been subducted to a minimum depth of >150 km and incorporated into mountain chains during tectonic exhumation. Over the past decade, the rapidly moving technological advancement has made it possible to examine these diamonds in detail, and to learn that they contain nanometric multiphase inclusions of crystalline and fluid phases and are characterized by a “crustal” signature of carbon stable isotopes. Scanning and transmission electron microscopy, focused ion beam techniques, synchrotron infrared spectroscopy, and nano-secondary ion mass spectrometry studies of these diamonds provide evidence that they were crystallized from a supercritical carbon-oxygen-hydrogen fluid. These microdiamonds preserve evidence of the pathway by which carbon and water can be subducted to mantle depths and returned back to the earth's surface. PMID:17389388

  5. Three Dimensional Simulations of Strong Motions for Great Earthquakes on the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Delorey, A. A.; Frankel, A. D.; Stephenson, W. J.; Liu, P.

    2011-12-01

    Using a finite-fault rupture model, we ran a finite difference code to simulate a variety of Mw 8 and larger events on the Cascadia subduction zone using a 3D regional velocity model and two different 3D velocity models for the Seattle basin. Our results reveal the magnitude and duration of shaking that should be expected in the built environment for a megathrust event with a rupture length less than the entire length of the subduction zone. In the next step we will consider events that rupture the entire length of the subduction zone, similar in scope to the 1700 event, and compare our results to those considered for the national seismic hazard maps. In order to make predictions on the strength and duration of shaking in Cascadia due to a large megathrust event, we developed a kinematic fault rupture model based on a k-2 decay in final slip spectrum that has a scale-dependent rise time. This produces a ω-2 decay in the radiated displacement spectrum above the corner frequency, which is then modified by rupture directivity. In order to produce a k-2 decay in the final slip spectrum, we modeled the final slip as the sum of asperities with various wave numbers produced by calculating normal modes for a membrane, then shifting the phase of the standing waves to randomize the pattern. In this way, slip naturally decays towards the edges of the rupture without having to use a taper, we can produce a final slip model with any spectrum we choose, and each wave number can be assigned a unique rise time. The slip on each individual asperity initiates in time according to its closest distance to the hypocenter and the rupture velocity. The Cascadia subduction zone off the coast of northwestern United States and southwestern Canada is capable of producing megathrust earthquakes with magnitudes up to Mw 9.0 for margin-wide events and magnitudes greater than Mw 8.0 if only part of the subduction zone ruptures. The average recurrence interval for margin-wide megathrust earthquakes on the Cascadia subduction zone is about 500 years and the last margin-wide event occurred in 1700. The estimate of the recurrence interval for events that rupture only part of the subduction zone is more controversial, but the recurrence time is thought to be somewhere around 300 years. If a large event were to occur today, the cities of Portland, Oregon, Seattle, Washington, and Vancouver, British Columbia and other inhabited areas in the forearc basin and on the coast would be subjected to strong and long duration shaking and coastal areas would likely be subjected to a tsunami. Observations from the Maule, Chile and Tohoku-Oki, Japan earthquakes will be used to guide the specification of rupture time histories for the Cascadia 3D simulations. We are particularly interested in producing realistic ground motions at periods of 2 to 6 s that affect high-rise buildings. The amplitude and duration of these motions are strongly affected by the rise time of slip on the fault, the coherence of rupture propagation, and the presence of deep sedimentary basins such as the Seattle and Tacoma basins.

  6. Magnetic properties of Archean gneisses from the northeastern North China Craton: the relationship between magnetism and metamorphic grade in the deep continental crust

    NASA Astrophysics Data System (ADS)

    Wang, Hongcai; Liu, Qingsheng; Zhao, Weihua; Li, Zhiyong; Zheng, Jianping

    2015-04-01

    Magnetic mineralogy of crustal rocks has important implications for understanding continental crustal evolution and origin of regional magnetic anomalies. However, magnetic properties of the deep continental crust are still poorly understood. In this paper, measurements of density (ρ), mass-specific magnetic susceptibility (χ), natural remanent magnetization (NRM) and magnetic hysteresis loops, temperature-dependent magnetic susceptibility (χ-T), chemical and mineral analyses were conducted on Archean gneiss samples from the Jixian petrophysical section in the Precambrian terrain, northeastern North China Craton, with the aim of refining understanding of magnetic phase transformations in the deep crustal rocks. Results show that density and rock magnetic properties change distinctly with metamorphic facies. The dominant magnetic mineral is magnetite, while a little hematite is present in a few samples. Together with geochemical and mineralogical compositions, it is inferred that progressive increase in metamorphic grade from east to west is the major cause for magnetic enhancement of the lower crust in the studied section. Therefore, we conclude that study of magnetic phases of deep crustal rocks can offer important insights into the history of high metamorphic grade terranes.

  7. The Relationships of Upper Plate Ridge-Trench-Trench and Ridge-Trench-Transform Triple Junction Evolution to Arc Lengthening, Subduction Zone initiation and Ophiolitic Forearc Obduction

    NASA Astrophysics Data System (ADS)

    Casey, J.; Dewey, J. F.

    2013-12-01

    The principal enigma of large obducted ophiolite slabs is that they clearly must have been generated by some form of organized sea-floor spreading/plate-accretion, such as may be envisioned for the oceanic ridges, yet the volcanics commonly have arc affinity (Miyashiro) with boninites (high-temperature/low-pressure, high Mg and Si andesites), which are suggestive of a forearc origin. PT conditions under which boninites and metamorphic soles form and observations of modern forearc systems lead us to the conclusion that ophiolite formation is associated with overriding plate spreading centers that intersect the trench to form ridge-trench-trench of ridge-trench-tranform triple junctions. The spreading centers extend and lengthen the forearc parallel to the trench and by definition are in supra-subduction zone (SSZ) settings. Many ophiolites likewise have complexly-deformed associated mafic-ultramafic assemblages that suggest fracture zone/transform along their frontal edges, which in turn has led to models involving the nucleation of subduction zones on fracture zones or transpressional transforms. Hitherto, arc-related sea-floor-spreading has been considered to be either pre-arc (fore-arc boninites) or post-arc (classic Karig-style back arc basins that trench-parallel split arcs). Syn-arc boninites and forearc oceanic spreading centers that involve a stable ridge/trench/trench triple or a ridge-trench-transform triple junction, the ridge being between the two upper plates, are consistent with large slab ophiolite formation in an obduction-ready settting. The direction of subduction must be oblique with a different sense in the two subduction zones and the oblique subduction cannot be partitioned into trench orthogonal and parallel strike-slip components. As the ridge spreads, new oceanic lithosphere is created within the forearc, the arc and fore-arc lengthen significantly, and a syn-arc ophiolite forearc complex is generated by this mechanism. The ophiolite ages along arc-strike; a distinctive diachronous MORB-like to boninitic to arc volcanic stratigraphy develops vertically in the forearc and eruption centers progressively migrate from the forearc back to the main arc massif with time. Dikes in the ophiolite are commonly highly oblique to the trench (as are back-arc magnetic anomalies in modern environments). Boninites and high-mg andesites are generated in the fore-arc under the aqueous, low pressure/high temperature, regime at the ridge above the instantaneously developed subducting and dehydrating slab. We review both modern subduction environments and ancient obducted ophiolite analogues that illustrate this tectonic model for subduction initiation and the creation and rapid divergent-convergent plate tectonic transitions to ophiolitic forearcs.

  8. Mantle convection, tectonics and the evolution of the Tethyan subduction zone

    NASA Astrophysics Data System (ADS)

    Jolivet, Laurent; Sternai, Pietro; Menant, Armel; Faccenna, Claudio; Becker, Thorsten; Burov, Evguenii

    2014-05-01

    Mantle convection drives plate tectonics and the size, number and thermotectonic age of plates codetermines the convection pattern. However, the degree of coupling of surface deformation and mantle flow is unclear. Most numerical models of lithospheric deformation are designed such that strain is a consequence of kinematic boundary conditions, and rarely account for basal stresses due to mantle flow. On the other hand, convection models often treat the lithosphere as a single-layer stagnant lid with vertically undeformable surface. There is thus a gap between convection models and lithospheric-scale geodynamic models. The transmission of stresses from the flowing mantle to the crust is a complex process. The presence of a ductile lower crust inhibits the upward transmission of stresses but a highly extended crust in a hot environment such as a backarc domain, with no lithospheric mantle and a ductile lower crust in direct contact with asthenosphere, will be more prone to follow the mantle flow than a thick and stratified lithosphere. We review geological observations and present reconstructions of the Aegean and Middle East and discuss the possible role played by basal drag in governing lithospheric deformation. In Mediterranean backarc regions, lithosphere-mantle coupling is effective on geological time scale as shown by the consistency of SKS fast orientations in the mantle with stretching directions in the crust. The long-term geological history of the Tethyan convergent zone suggests that asthenospheric flow has been an important player. The case of Himalaya and Tibet strongly supports a major contribution of a northward asthenospheric push, with no persistent slab that could drive India after collision, large thrust planes being then decoupling zones between deep convection and surface tectonics. The African plate repeatedly fragmented during its northward migration with the separation of Apulia and Arabia. Indeed, extension has been active on the northern side of Africa from the Jurassic until the collision in the Oligocene, and even afterward when Arabia formed by opening of the Red Sea and the Gulf of Aden. This also suggests a dominant role of an underlying flow at large scale, dragging and mechanically eroding plates and breaking them into fragments, then passively carried. Only during a short period of the Late Cretaceous did the situation change drastically with the obduction event giving the large ophiolitic nappes observed from Oman to Turkey. This obduction event has never been really explained. It has been shown to be coeval with faster plate velocities and more active formation of oceanic crust globally, which in turn suggests a link with deep mantle convection. We discuss this succession of events and propose to relate them with the basal drag induced by convective mantle flow below the African continental lithosphere. We discuss the effects of convection on crustal deformation at different scales from deep convection related to plumes and subduction zones to more local mantle flow due to slab retreat and tearing.

  9. The Southern Chilean Subduction Zone: Local Earthquake Tomography and State of Stress

    NASA Astrophysics Data System (ADS)

    Lange, D.; Cembrano, J.; Rietbrock, A.; Haberland, C.; Dahm, T.; Bataille, K.

    2007-12-01

    While the northern and central part of the South American Subduction zone has been intensively studied the southern part has attracted less attention, which might be caused by its difficult accessibility and lower seismic activity. However, the southern part exhibits strong seismic and tsunamogenic potential with the prominent example of the Mw=9.5~May~22, 1960 Valdivia earthquake. Here we present data from an amphibious seismic array (Project TIPTEQ) located between 41.5--43.5°S reaching from the trench to the active magmatic arc incorporating the Island of Chiloé and the magmatic arc with the N-S trending Liquiñe-Ofqui fault zone~(LOFZ). 364 local events were observed in a 11-month period from November 2004 until October 2005 with magnitudes between 0.5 to 5.1~Ml. The observed seismicity allows us to constrain the current state of stress of the subducting plate and magmatic arc as well as the local seismic velocity structure. The downgoing Wadati-Benioff zone is readily identifiable as an eastward dipping plane with an inclination of about 29-33°. Besides events in in the Benioff Zone, 75~shallow crustal events with depths shallower than 25~km were observed mainly occurring in different clusters along the magmatic arc. These crustal clusters of seismicity are related to the LOFZ, to the volcanoes Chaitén, Michinmahuida and Corcovado, and to active faulting on secondary faults. Further activity along the LOFZ is indicated by individual events located in direct vicinity of the LOFZ. Focal mechanisms were calculated using moment tensor inversion of amplitude spectra for body waves which mostly yield strike slip mechanisms with SE-NW oriented direction of~σ1~for the LOFZ at this latitude. In contrast to the strike-slip mechanism of the events along the LOFZ, the focal mechanisms of the stronger events in the Benioff Zone yield mainly thrust mechanism down to 55~km depth. Focal Mechanism Stress Inversion (FMSI) was carried out for the crustal events and the events in the Benioff zone indicating a strike slip regime along the arc and thrust regime in the Benioff zone, respectively. We suggest that the observed deformation along the LOFZ combined with teleseismic observations is a confirmation for the proposed northward movement of the forearc sliver acting as a detached continental micro-plate as outlined by Forsythe~& Nelson (1985) and Beck et al.~(1993). A high quality subset of events was inverted for a 2-D velocity model using the SIMULPS inversion code. First results suggest a 10~km high mantle bulge (vp>7.8~km/s) below the longitudinal valley and low velocity structure below the coastal cordillera.

  10. Petrofabrics of high-pressure rocks exhumed at the slab-mantle interface from the "point of no return" in a subduction zone (Sivrihisar, Turkey)

    NASA Astrophysics Data System (ADS)

    Whitney, Donna L.; Teyssier, Christian; Seaton, Nicholas C. A.; Fornash, Katherine F.

    2014-12-01

    The highest pressure recorded by metamorphic rocks exhumed from oceanic subduction zones is ~2.5 GPa, corresponding to the maximum decoupling depth (MDD) (80 ± 10 km) identified in active subduction zones; beyond the MDD (the "point of no return") exhumation is unlikely. The Sivrihisar massif (Turkey) is a coherent terrane of lawsonite eclogite and blueschist facies rocks in which assemblages and fabrics record P-T-fluid-deformation conditions during exhumation from ~80 to 45 km. Crystallographic fabrics and other features of high-pressure metasedimentary and metabasaltic rocks record transitions during exhumation. In quartzite, microstructures and crystallographic fabrics record deformation in the dislocation creep regime, including dynamic recrystallization during decompression, and a transition from prism slip to activation of rhomb and basal slip that may be related to a decrease in water fugacity during decompression (~2.5 to ~1.5 GPa). Phengite, lawsonite, and omphacite or glaucophane in quartzite and metabasalt remained stable during deformation, and omphacite developed an L-type crystallographic fabric. In marble, aragonite developed columnar textures with strong crystallographic fabrics that persisted during partial to complete dynamic recrystallization that was likely achieved in the stability field of aragonite (P > ~1.2 GPa). Results of kinematic vorticity analysis based on lawsonite shape fabrics are consistent with shear criteria in quartzite and metabasalt and indicate a large component of coaxial deformation in the exhuming channel beneath a simple shear dominated interface. This large coaxial component may have multiplied the exhuming power of the subduction channel and forced deeply subducted rocks to flow back from the point of no return.

  11. In-situ formation of Indian Mantle in global subduction zones

    NASA Astrophysics Data System (ADS)

    Nebel, Oliver; Arculus, Richard; Davies, Rhodri

    2014-05-01

    The isotopic signatures of Sr-Nd-Pb-Hf-Os in mid-ocean ridge basalts (MORB) in the Indian Ocean are clearly distinct compared with their Atlantic/Pacific (A/P) counterparts. The origin of this isotopic distinction has been a matter of debate since its discovery by Dupré and Allègre (1983). Current models advocate: (i) delamination of ancient, negatively buoyant lower crust/lithosphere from a supercontinent; (ii) contamination of A/P-style mantle with plumes (the original association with the DUPAL anomaly); or (iii) long-term overprint by a subduction component (SC) surrounding a former supercontinent. The sum of various stable and radiogenic isotope proxies appears to support a delamination scenario, but alternatives, or the combination of the aforementioned scenarios, are possible. Irrespective of the origin of the Indian mantle domain, isotopic signatures similar to those of Indian MORB and hot-spots are observed in arc/back-arc systems associated with western Pacific subduction zones. These isotope signatures have been regarded as unequivocally derived from Indian-type mantle, and accordingly used to trace eastward flow of that type of mantle. Here we show the majority of igneous rocks associated with subduction zone systems mimic Indian-type mantle in Pb isotope space, but are distinct in Hf-Nd isotope co-variations. We suggest isotopic signatures believed to be derived from Indian mantle in subduction zones are the result of medium-term subduction overprint of evolving A/P-type mantle wedges. This feature results from the relative mobility of U-Pb>Sm-Nd>Lu-Hf in subducted slab-derived components and Th/U (k) fractionation in the mantle wedge. Elevation of k in the wedge from 2.6 (MORB) to about 6-12 can account for the shift in Pb isotope space over a duration of ca. 100-200 Myrs; "decoupling" of Hf-Nd isotopes reflect the subduction component vs mantle wedge contribution. More generally, "Pseudo-Indian mantle" is noted as common in subduction zones globally, and not limited to the western Pacific, supporting the in-situ generation of isotope signatures akin to Indian mantle. Radiogenic ingrowth in modified wedge mantle requires shallow storage of affected parts of the mantle wedge over tens to hundreds of millions of years. Convection models support the feasibility of this scenario, provided that wedge rheology is modified through hydration (as is required by Th addition) in, at least, a small region of the wedge center. We argue that most if not all Indian-mantle signatures in global subduction zones are not related to the actual Indian mantle domain and associated geotectonic models employing this proxy in subduction zones need revision

  12. Invited review paper: The control of subduction zone structural complexity and geometry on margin segmentation and seismicity

    NASA Astrophysics Data System (ADS)

    Kopp, H.

    2013-03-01

    Convergent plate boundaries around the globe show a high degree of structural complexity and variability in site-specific geometry and mass flux. The heterogeneity in the structural evolution, the interior regime as well as external architecture of individual margins is reflected in their seismic character, resulting in a segmentation along-strike as well as along-dip. Subduction zones generate more than 80% of global earthquakes above magnitude 8.0, but rupture characteristics are highly individual and linked to margin specific geometrical conditions. Major segments of subduction zones 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 proven crucial to improve our knowledge on the geological framework of the different types of subduction zones. The aim of this review paper is to unravel the structural diversity of convergent margins and between individual subduction zone segments. Field data from different margins around the globe deliver images of the seafloor and subsurface in unprecedented resolution, which show segmentation to be far more complex than previously inferred. Along-strike segmentation results in accretionary segments contiguous to erosive segments along a single margin. Modes of mass transfer must hence be viewed as transient processes dependent on sediment supply and lower plate structure. Along-strike segment boundaries commonly correlate with underthrusting lower plate relief that controls the deep deformation of a subduction zone and the spatial and temporal variations in slip behavior. Examples of underthrusting oceanic basement relief at different stages of subduction elucidate their impact on the inner geometry of the margin. Lower plate heterogeneities occur at subduction zones worldwide and thus pose a common phenomenon, whose role as barriers to seismic rupture constitute a central control on subduction zone seismicity and segmentation.

  13. Evolution of the northern Sierra Nevada metamorphic belt: Petrological, structural, and Ar/Ar constraints

    SciTech Connect

    Hacker, B.R.

    1993-05-01

    The Sierra Nevada metamorphic belt constitutes an important record of the growth of continental crust from essentially oceanic materials. In the northern Sierra, the central part of the belt is made up of volcanoplutonic arcs and sediment-dominated units inferred to be accretionary wedges or closed ocean basins. The latter are broken formation and melange composed of radiolarian chert, lava, and volcanogenic and continental turbidites. Sedimentary detritus in the largest of these units can be plausibly linked to sources farther east in the Sierra, suggesting that deposition occurred near the eastern Sierran arc. Isoclinal folds, steeply dipping foliations, and steeply plunging down-dip lineations are characteristics structures. The westernmost unit is only feebly recrystallized, and deformation was accomplished principally by stress solution and local redeposition in veins. More easterly, inboard units are compositionally similar, but they recrystallized at pumpellyite-actinolite-and blueschist-facies conditions and deformed via solution-transfer and dislocation creep. Phengite silica contents, the degree of quartz veining, and the locations of pseudo-isograds support an eastward increase in metamorphic pressure and temperature. Metamorphic conditions during the growth of pumpellyite and actinolite ranged from {approximately}150-350 {degrees}C and 200-400 MPa, compatible with recrystallization and deformation in subduction zones or the deeper levels of magmatic arcs. Ar/Ar ages of volcanisclastic rocks and crosscutting plutons constrain the age of deformation and metamorphism in the western part of the region to 174-165 Ma. Deformation and recrystallization in more easterly units may have been coeval or begun as early as Triassic time. 58 refs., 14 figs., 4 tabs.

  14. Absolute plate motions since 130 Ma constrained by subduction zone kinematics

    NASA Astrophysics Data System (ADS)

    Williams, Simon; Flament, Nicolas; Dietmar Müller, R.; Butterworth, Nathaniel

    2015-05-01

    The absolute motions of the lithospheric plates relative to the Earth's deep interior are commonly constrained using observations from paleomagnetism and age-progressive seamount trails. In contrast, an absolute plate motion (APM) model linking surface plate motions to subducted slab remnants mapped from seismic tomography has recently been proposed. Absolute plate motion models (or "reference frames") derived using different methodologies, different subsets of hotspots, or differing assumptions of hotspot motion, have contrasting implications for parameters that describe the long term state of the plate-mantle system, such as the balance between advance and retreat of subduction zones, plate velocities, and net lithospheric rotation. Previous studies of contemporary plate motions have used subduction zone kinematics as a constraint on the most likely APM model. Here we use a relative plate motion model to compute these values for the last 130 Myr for a range of alternative reference frames, and quantitatively compare the results. We find that hotspot and tomographic slab-remnant reference frames yield similar results for the last 70 Myr. For the 130-70 Ma period, where hotspot reference frames are less well constrained, these models yield a much more dispersed distribution of slab advance and retreat velocities. By contrast, plate motions calculated using the slab-remnant reference frame, or using a reference frame designed to minimise net rotation, yield more consistent subduction zone kinematics for times older than 70 Ma. Introducing the global optimisation of trench migration characteristics as a key criterion in the construction of APM models forms the foundation of a new method of constraining APMs (and in particular paleolongitude) in deep geological time.

  15. A Thick, Deformed Sedimentary Wedge in an Erosional Subduction Zone, Southern Costa Rica

    NASA Astrophysics Data System (ADS)

    Silver, E. A.; Kluesner, J. W.; Edwards, J. H.; Vannucchi, P.

    2014-12-01

    A paradigm of erosional subduction zones is that the lower part of the wedge is composed of strong, crystalline basement (Clift and Vannucchi, Rev. Geophys., 42, RG2001, 2004). The CRISP 3D seismic reflection study of the southern part of the Costa Rica subduction zone shows quite the opposite. Here the slope is underlain by a series of fault-cored anticlines, with faults dipping both landward and seaward that root into the plate boundary. Deformation intensity increases with depth, and young, near-surface deformation follows that of the deeper structures but with basin inversions indicating a dynamic evolution (Edwards et al., this meeting). Fold wavelength increases landward, consistent with the folding of a landward-thickening wedge. Offscraping in accretion is minimal because incoming sediments on the lower plate are very thin. Within the wedge, thrust faulting dominates at depth in the wedge, whereas normal faulting dominates close to the surface, possibly reflecting uplift of the deforming anticlines. Normal faults form a mesh of NNW and ENE-trending structures, whereas thrust faults are oriented approximately parallel to the dominant fold orientation, which in turn follows the direction of roughness on the subducting plate. Rapid subduction erosion just prior to 2 Ma is inferred from IODP Expedition 334 (Vannucchi et al., 2013, Geology, 49:995-998). Crystalline basement may have been largely removed from the slope region during this rapid erosional event, and the modern wedge may consist of rapidly redeposited material (Expedition 344 Scientists, 2013) that has been undergoing deformation since its inception, producing a structure quite different from that expected of an eroding subduction zone.

  16. Dynamics of the opposite-verging subduction zones in the Taiwan region: Insights from numerical models

    NASA Astrophysics Data System (ADS)

    Lin, Shu-Chuan; Kuo, Ban-Yuan

    2016-03-01

    The Taiwan mountain belt between the Eurasian and the Philippine Sea plate is a rare example for an orogen bracketed by two opposite-verging subduction zones. The influences of the double subduction zones on regional dynamics have long remained unknown. In this study lithospheric deformation and mantle circulation in the Taiwan region are calculated with double subduction-collision models. The results show that the limitedly deformed subducted Eurasian plate separates the highly deforming orogen above from the highly deforming mantle below. The edge flow driven by the rollback of the Philippine Sea slab dominates in the asthenosphere primarily as a result of the longer slab if the gap between the two slabs beneath Taiwan is sufficiently wide. The induced toroidal current gives rise to a pattern of seismic anisotropy compatible with that measured with teleseismic phases and coincidentally accordant with the strike of the orogen. The additional presence of a frequently hypothesized lithospheric fragment or slab tear disrupts the toroidal circulation and mars the model predictions for seismic anisotropy. We found that the rollback of the Eurasian slab deflects the plate downward and neutralizes the uplift, posing difficulty to models with Eurasian slab extending too far north. These results bolster the view that the Taiwan mountain belt is a subduction-dominated orogen and both subduction zones play a key role on regional dynamics. Conceptual models specifically developed for large-scale continent-continent collision zones that have been commonly applied to the Taiwan region are inadequate for this ocean-continent, opposite-verging subduction-collision system.

  17. Estimation of strong ground motions from hypothetical earthquakes on the Cascadia subduction zone, Pacific Northwest

    USGS Publications Warehouse

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

    1989-01-01

    Strong ground motions are estimated for the Pacific Northwest assuming that large shallow earthquakes, similar to those experienced in southern Chile, southwestern Japan, and Colombia, may also occur on the Cascadia subduction zone. Fifty-six strong motion recordings for twenty-five subduction earthquakes of Ms???7.0 are used to estimate the response spectra that may result from earthquakes Mw<81/4. Large variations in observed ground motion levels are noted for a given site distance and earthquake magnitude. When compared with motions that have been observed in the western United States, large subduction zone earthquakes produce relatively large ground motions at surprisingly large distances. An earthquake similar to the 22 May 1960 Chilean earthquake (Mw 9.5) is the largest event that is considered to be plausible for the Cascadia subduction zone. This event has a moment which is two orders of magnitude larger than the largest earthquake for which we have strong motion records. The empirical Green's function technique is used to synthesize strong ground motions for such giant earthquakes. Observed teleseismic P-waveforms from giant earthquakes are also modeled using the empirical Green's function technique in order to constrain model parameters. The teleseismic modeling in the period range of 1.0 to 50 sec strongly suggests that fewer Green's functions should be randomly summed than is required to match the long-period moments of giant earthquakes. It appears that a large portion of the moment associated with giant earthquakes occurs at very long periods that are outside the frequency band of interest for strong ground motions. Nevertheless, the occurrence of a giant earthquake in the Pacific Northwest may produce quite strong shaking over a very large region. ?? 1989 Birkha??user Verlag.

  18. Compaction Further Enhances Inelastic Wedge Failure in Shallow Subduction Zone Earthquakes

    NASA Astrophysics Data System (ADS)

    Ma, S.; Hirakawa, E. T.

    2014-12-01

    Accretionary wedges in the subduction zone are typically highly porous due to continuous influx of unconsolidated sediments on the seafloor. A well-known deformation mode for porous materials is compaction. When porous materials are compacted in the undrained condition (as in the rapid loading of earthquake rupture) the pore pressure increases significantly when pore space closes while pore fluids are largely incompressible, resulting in a much larger pore pressure increase than that by the elastic compression alone. In this work, we incorporate compaction in our poroplastic model of shallow subduction zone earthquakes (Ma, 2012; Ma and Hirakawa, 2013) by using an end-cap yield criterion (e.g., Wong, 1997). We show that for a wedge on the verge of compactant failure initially (due to large porosity) up-dip rupture causes significant wedge compaction, which induces large pore pressure increase, reduces effective stress, and causes more easily shear failure. The wedge needs not be on the verge of Coulomb failure initially. Dilatancy accompanying shear failure near the end of deformation process can reduce volume loss due to initial compaction, resulting in a small net volumetric plastic strain. Compaction significantly enhances failure in the wedge, which makes our wedge-failure mechanism more plausible in explaining well-documented anomalous observations for shallow subduction zone earthquakes, such as slow rupture velocity, efficient tsunami generation, deficiency in high-frequency radiation, and low moment-scaled radiated energy. The dynamically elevated pore pressure may also explain numerous observations on the hydrologic activities in the wedge (e.g., mud volcanoes).

  19. Full waveform modelling of aftershock seismicity in the Chilean subduction zone using the VERCE platform

    NASA Astrophysics Data System (ADS)

    Garth, T.; Hicks, S. P.; Fuenzalida Velasco, A. J.; Casarotti, E.; Spinuso, A.; Rietbrock, A.

    2014-12-01

    The VERCE platform allows high resolution waveforms to be simulated through an interactive web-based portal. The platform runs on a variety of HPC clusters, and waveforms are calculated using SPECFEM3D. We use the full waveform modelling techniques supported on the VERCE platform to test the validity of a number of subduction zone velocity models from the Chilean subduction zone. Waveforms are calculated for aftershocks of the 2010 Mw 8.8 Maule (central Chile) and the Mw 8.1 2014 Pisagua (Northern Chile) earthquakes. For the Maule region, we use a 2D tomographic model of the rupture area (Hicks et al., 2012), and the focal mechanisms of Agurto et al., (2012). For the Pisagua earthquake, we use a 2.5D composite velocity model based on tomographic studies of the region (e.g. Husen et al., 2000, Contreras-Reyes et al., 2012) and Slab1.0 (Hayes et al., 2012). Focal mechanisms for the Pisagua aftershock sequence are produced from waveforms recorded on the IPOC network using the program ISOLA (Sokos and Zahradnik, 2008). We also test a number of synthetic velocity models. The simulated waveforms are directly compared to waveforms recorded on the temporary deployment for the Maule earthquake aftershocks, and waveforms recorded on the IPOC network for the Pisagua earthquake aftershocks. The waveforms produced by the 3D full waveform simulations are also compared to the waveforms produced by the focal mechanism inversion, which assume a 1D velocity model. The VERCE platform allows waveforms from the full 3D model to be produced easily, and allows us to quantifiably assess the validity of both the velocity model and the source mechanisms. In particular the dependence of the dip of the focal mechanism on the velocity model used is explored, in order to assess the reliability of current models of the plate interface geometry in the Chilean subduction zone.

  20. Full waveform modelling using the VERCE platform - application to aftershock seismicity in the Chile subduction zone

    NASA Astrophysics Data System (ADS)

    Garth, Thomas; Rietbrock, Andreas; Hicks, Steve; Fuenzalida Velasco, Amaya; Casarotti, Emanuele; Spinuso, Alessandro

    2015-04-01

    The VERCE platform is an online portal that allows full waveform simulations to be run for any region where a suitable velocity model exists. We use this facility to simulate the waveforms from aftershock earthquakes from the 2014 Pisagua earthquake, and 2010 Maule earthquake that occurred at the subduction zone mega thrust in Northern and Central Chile respectively. Simulations are performed using focal mechanisms from both global earthquake catalogues, and regional earthquake catalogues. The VERCE platform supports specFEM Cartesian, and simulations are run using meshes produced by CUBIT. The full waveform modelling techniques supported on the VERCE platform are used to test the validity of a number of subduction zone velocity models from the Chilean subduction zone. For the Maule earthquake we use a 2D and 3D travel time tomography model of the rupture area (Hicks et al. 2011; 2014). For the Pisagua earthquake we test a 2D/3D composite velocity model based on tomographic studies of the region (e.g. Husen et al. 2000, Contreyes-Reyes et al. 2012) and slab1.0 (Hayes et al. 2012). Focal mechanisms from the cGMT catalogue and local focal mechanisms calculated using ISOLA (e.g. Agurto et al. 2012) are used in the simulations. The waveforms produced are directly compared to waveforms recorded on the temporary deployment for the Maule earthquake aftershocks, and waveforms recorded on the IPOC network for the Pisagua earthquake aftershocks. This work demonstrates how the VERCE platform allows waveforms from the full 3D simulations to be easily produced, allowing us to quantify the validity of both the velocity model and the source mechanisms. These simulations therefore provide an independent test of the velocity models produced synthetically and by travel time tomography studies. Initial results show that the waveform is reasonably well reproduced in the 0.05 - 0.25 frequency band using a refined 3D travel time tomography, and locally calculated focal mechanisms.

  1. Recovering the slip history of a scenario earthquake in the Mexican subduction zone

    NASA Astrophysics Data System (ADS)

    Hjorleifsdottir, V.; Perez-Campos, X.; Iglesias, A.; Cruz-Atienza, V.; Ji, C.; Legrand, D.; Husker, A. L.; Kostoglodov, V.; Valdes Gonzalez, C.

    2011-12-01

    The Guerrero segment of the Mexican subduction zone has not experienced a large earthquake for almost 100 years (Singh et al., 1981). Due to its proximity to Mexico City, which was devastated by an earthquake in the more distant Michoacan segment in 1985, it has been studied extensively in recent years. Silent slip events have been observed by a local GPS network (Kostoglodov et al. 2003) and seismic observations from a dense linear array of broadband seismometers (MASE) have provided detailed images of the crustal structure of this part of the subduction zone (see for example Prez-Campos et al., 2008, Iglesias et al., 2010). Interestingly the part of the fault zone that is locked during the inter-seismic period is thought to reach up to or inland from the coast line. In the event of a large megathrust earthquake, this geometry could allow recordings from above the fault interface. These types of recordings can be critical to resolve the history of slip as a function of time on the fault plane during the earthquake. A well constrained model of slip-time history, together with other observations as mentioned above, could provide very valuable insights into earthquake physics and the earthquake cycle. In order to prepare the scientific response for such an event we generate a scenario earthquake in the Guerrero segment of the subduction zone. We calculate synthetic strong motion records, seismograms for global stations and static offsets on the Earth's surface. To simulate the real data available we add real noise, recorded during times of no earthquake, to the synthetic data. We use a simulated annealing inversion algorithm (Ji et al., 1999) to invert the different datasets and combinations thereof for the time-history of slip on the fault plane. We present the recovery of the slip model using the different datasets, as well as idealized datasets, investigating the expected and best possible levels of recovery.

  2. Extending the global coverage of Slab1.0 3D subduction zone models

    NASA Astrophysics Data System (ADS)

    Seidman, L.; Hayes, G. P.

    2013-12-01

    Slab1.0 is a three-dimensional model of subduction zone geometries that covers approximately 85% of global slabs by area. It is built from an automated interpolation of a combined dataset made up from subduction-related earthquakes, moment tensors, interpretations of active source seismic data, and models of bathymetry and sediment thickness. Those subduction zones that are missing from the model are difficult to characterize with this automated approach because of sparse teleseismically located, interplate seismicity (e.g., Cascadia, Hikurangi), complex geometry (e.g., Halmahera, southern Philippine Sea), or some combination of these issues (e.g., Caribbean). Here we attempt to solve this problem with a straightforward modification of the Slab1.0 approach. Instead of constructing a series of automated spline fits to our geophysical data in two-dimensional cross sections, we produce hand-contoured two-dimensional fits; under the assumption that where seismicity is sparse or geometry complex, a human guided by tectonic knowledge can produce a better fit to geometry than can a computer algorithm. These manual 2D sections are then interpolated into a 3D surface in the same way automated 2D fits are processed for Slab1.0. Following this approach, we produce models for slabs in the Caribbean, the Makran, the Manila Trench, the Halmahera Plate, and the Hellenic Arc. We also address regions of current models (e.g., Peru) that were poorly characterized by the original automated approach. These new models thus provide valuable information on subduction zone structure from the trench and into the mantle in regions previously missing from Slab1.0, and help to make existing models more accurate, and thus more useful, than was previously possible. In turn, the models can be used to better characterize associated seismic hazards.

  3. Revisiting Parts of the Makran Subduction Zone in Search of its Paleoearthquakes

    NASA Astrophysics Data System (ADS)

    Rajendran, C.

    2013-05-01

    In contrast to the subduction zones of southern Chile, Cascadia, Southern Alaska, Sumatra-Andaman and the Japan Trench that have generated the largest tsunamigenic earthquakes, the Makran subduction zone has not generated any megathrust earthquake. However, the 2004 earthquake and the unprecedented Indian Ocean tsunami that followed is a reminder that the tsunamigenic potential of a subduction zone cannot be evaluated solely on the basis of its current seismicity. The Mw 8.1 earthquake of 1945 and the consequent tsunami that originated on the eastern part of Makran are the only historically known hazardous events in this region; the history of its past earthquakes remains unexplored. Here we use tectonic proxies along the Makran Coast, available constraints on historical seismicity and the tell-tale characteristics of sea floor morphology to track its seismotectonic history. We explored the accessible coastal sites for evidence of past earthquake-related deformation and secondary features such as liquefaction. In the near-shore regions of the central part of Makran, near Chabahar, we found evidence of seismically induced liquefaction, possibly from the 1945 earthquake. Westward sites are remarkably devoid of liquefaction features, at least within the shallow stratigraphy. Whether it is an indication that no recent earthquakes have impacted the western part of Makran Coast region is an issue that needs to be explored. Elevated marine terraces on the western Makran and their uplift rates are indicative of long-term tectonic activity, comparable to that of the central part of Makran. The offshore data suggest occurrences of recently active submarine slumps on the eastern part of Makran, reflective of shaking events, from the great 1945 earthquake. The western segment appears to be potentially seismogenic, and is possibly going through a long interseismic period.

  4. Heat Flow Surveys on the Washington Margin of the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    Understanding the temperature distribution along an active subducting plate interface can improve our understanding of subduction zone dynamics and our ability to estimate seismic hazards. The 'locking' mechanism on the fault appears temperature dependent, where the up-dip (shallow) limit of the seismic zone ranges from 100-150°and the down-dip (deep) limit is a transition zone between 350°C and 450°C. Heat flow measurements provide the most direct method for resolving the subduction zone thermal environment. The Cascadia Subduction Zone currently poses the single largest seismic hazard to population centers within the Northwest United States. In August 2013, heat flow data were collected offshore Grays Harbor, WA, along a profile perpendicular to the accretionary wedge. Measurements extend seaward of the Cascadia deformation front and landward over the accretionary wedge and are collocated along line 4 of the 2012 R/V Langseth multi-channel seismology (MCS) profiles. These data consist of 43 long probe (3 m) measurements, 204 ROV Jason II probe (0.6 m) and 27 thermal blanket heat flow measurements Preliminary results indicate a mean heat flow of 110 mW/m2 over the incoming plate, a decrease to 30 mW/m2 at the first deformation ridge, then heat flow varying between 90 to 120 mW/m2 over the lower accretionary wedge. BSR derived heat flow decreases from 90 mW/m2 at the deformation front to 60 mW/m2 60 km landward and up the accretionary wedge. Regionally the heat flow values are consistent with the subduction of a thickly sedimented and young oceanic plate and local heat flow variations likely reflect advective and conductive heat transfer within the shallow portion of the accretionary wedge.

  5. PTt path in metamorphic rocks of the Khoy region (northwest Iran) and their tectonic significance for Cretaceous Tertiary continental collision

    NASA Astrophysics Data System (ADS)

    Azizi, H.; Moinevaziri, H.; Mohajjel, M.; Yagobpoor, A.

    2006-06-01

    Metamorphic rocks in the Khoy region are exposed between obducted ophiolites to the southwest and sedimentary rocks of Precambrian-Paleozoic age to the northeast. The Qom formation (Oligocene-Miocene) with a basal conglomerate transgressively overlies all of these rocks. The metamorphic rocks consist of both metasediments and metabasites. The metasediments are micaschist, garnet-staurolite schist and garnet-staurolite sillimanite schist with some meta-arkose, marble and quartzite. The metabasites are metamorphosed to greenschist and amphibolite facies from a basaltic and gabbroic protolith of tholeiitic and calc-alkaline rocks. Geothermobarometry based on the equivalence of minerals stability and their paragenesis in these rocks and microprobe analyses by several different methods indicate that metamorphism occurred in a temperature range between 450 and 680 °C at 5.5 and 7.5 kb pressure. Rims of minerals reveal a considerable decrease of pressure (<2 kb) and insignificant decrease of temperature. The PTt path of this metamorphism is normal. The MFG line passes above the triple junction of Al 2SiO 5 polymorphs, and the average geothermal gradient during metamorphism was from 27 to 37 °C/km, which is more concordant with the temperature regime of collision zones. We infer that crustal thickening during post-Cretaceous (possibly Eocene) collision of the Arabian plate and the Azerbaijan-Albourz block was the main factor that caused the metamorphism in the studied area.

  6. Sweet Spot Tremor Triggered by Intraslab Earthquakes in the Nankai Subduction Zone

    NASA Astrophysics Data System (ADS)

    Aiken, C.; Obara, K.; Peng, Z.; Chao, K.; Maeda, T.

    2014-12-01

    Deep tectonic tremor has been observed at several major plate-bounding faults around the Pacific Rim. Tremor­ in these regions can be triggered by small stresses arising from solid earth tides as well as passing seismic waves of large, distant earthquakes. While large, distant earthquakes are capable of repeatedly triggering tremor in the same region (i.e., a sweet spot), it is less understood how intraslab earthquakes interact with sweet spot tremor areas. We conduct a systematic survey of tremor triggered in the Nankai subduction zone by intraslab earthquakes to better understand what governs fault slip along the Eurasian-Philippine Sea Plate boundary. We examine 3 tremor sweet spots in the Nankai subduction zone: Shikoku West, Kii North, and Tokai. In each region, we select earthquakes from the Japan Meteorological Agency (JMA) catalog that occur from mid-2009 to mid-2014 with magnitude (M) greater than 2, that occur within the down-going Philippine Sea Plate, and within a 300 km epicentral distance of the sweet spot region. Using these selection criteria, we obtain ~1,200 earthquakes in each region. We examine a tremor catalog immediately before and after these local events as well as visually inspect filtered waveforms from short-period Hi-net seismic stations surrounding the sweet spot areas to identify additional tremor signals. From our initial analysis, we have identified 18 clear cases of increased tremor activity immediately following intraslab earthquakes in Shikoku West, most of which occur down-dip of the Shikoku West sweet spot. In comparison, we have identified only 5 triggering earthquakes in Kii North, and our investigation at Tokai is still ongoing. Our results so far are in agreement with triggering susceptibility being dependent upon background activity rates, as has been suggested for remote triggering of microearthquakes in geothermal regions by large, distant earthquakes as well as for remotely triggered tremor in the Nankai subduction zone. Next, we plan to evaluate whether the observed triggering cases reflect a true causal relationship between intraslab earthquakes and interplate tremors or if the observations are due to coincidence. By doing so, we aim to better understand the physical mechanisms responsible for the triggering of the sweet spot tremor in the Nankai subduction zone.

  7. Active Crustal Faults in the Forearc Region, Guerrero Sector of the Mexican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Gaidzik, Krzysztof; Ramírez-Herrera, Maria Teresa; Kostoglodov, Vladimir

    2016-01-01

    This work explores the characteristics and the seismogenic potential of crustal faults on the overriding plate in an area of high seismic hazard associated with the occurrence of subduction earthquakes and shallow earthquakes of the overriding plate. We present the results of geomorphic, structural, and fault kinematic analyses conducted on the convergent margin between the Cocos plate and the forearc region of the overriding North American plate, within the Guerrero sector of the Mexican subduction zone. We aim to determine the active tectonic processes in the forearc region of the subduction zone, using the river network pattern, topography, and structural data. We suggest that in the studied forearc region, both strike-slip and normal crustal faults sub-parallel to the subduction zone show evidence of activity. The left-lateral offsets of the main stream courses of the largest river basins, GPS measurements, and obliquity of plate convergence along the Cocos subduction zone in the Guerrero sector suggest the activity of sub-latitudinal left-lateral strike-slip faults. Notably, the regional left-lateral strike-slip fault that offsets the Papagayo River near the town of La Venta named "La Venta Fault" shows evidence of recent activity, corroborated also by GPS measurements (4-5 mm/year of sinistral motion). Assuming that during a probable earthquake the whole mapped length of this fault would rupture, it would produce an event of maximum moment magnitude Mw = 7.7. Even though only a few focal mechanism solutions indicate a stress regime relevant for reactivation of these strike-slip structures, we hypothesize that these faults are active and suggest two probable explanations: (1) these faults are characterized by long recurrence period, i.e., beyond the instrumental record, or (2) they experience slow slip events and/or associated fault creep. The analysis of focal mechanism solutions of small magnitude earthquakes in the upper plate, for the period between 1995 and 2008, revealed that frequent normal faults, sub-parallel to the trench, could be reactivated in the current stress field related to the Cocos subduction. Moreover, these features could also be reactivated by subduction megathrust earthquakes.

  8. Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip.

    PubMed

    Rogers, Garry; Dragert, Herb

    2003-06-20

    We found that repeated slow slip events observed on the deeper interface of the northern Cascadia subduction zone, which were at first thought to be silent, have unique nonearthquake seismic signatures. Tremorlike seismic signals were found to correlate temporally and spatially with slip events identified from crustal motion data spanning the past 6 years. During the period between slips, tremor activity is minor or nonexistent. We call this associated tremor and slip phenomenon episodic tremor and slip (ETS) and propose that ETS activity can be used as a real-time indicator of stress loading of the Cascadia megathrust earthquake zone. PMID:12738870

  9. Shallow velocity structure of the Alaska Peninsula subduction zone and implications for controls on seismic behavior

    NASA Astrophysics Data System (ADS)

    Li, J.; Shillington, D. J.; Becel, A.; Nedimovic, M. R.; Kuehn, H.; Webb, S. C.; Abers, G. A.; Keranen, K. M.; Saffer, D. M.

    2014-12-01

    Downdip and along-strike variations in the seismic behavior of subduction zone megathrust faults are thought to be strongly controlled by changes in the material properties along the plate boundary. Roughness and hydration of the incoming plate, fluid pressure and lithology in the subducting sediment channel are likely to control the distribution of shallower rupture. Here, we focus on the subduction zone offshore of the Alaska Peninsula. In 2011, the ALEUT program acquired deep penetration multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data across the apparently freely sliding Shumagin Gap, the locked Semidi segment that last ruptured in 1938 M8.2 earthquake, and the locked western Kodiak asperity, which ruptured in the 1964 M9.2 earthquake. Seismic reflection data from the ALEUT cruise reveal significant variability in the thickness of sediment on the incoming plate and entering the trench, and the roughness and degree of hydration of the incoming plate. Oceanic crust entering the trench in the Shumagin gap is rugged with extensive faults and only a thin layer of sediment (<0.5 km thick). Farther east in the Semidi segment, the subducting plate has a smoother surface with thicker sediments (~1 km thick) and less faulting/hydration. To better constrain the properties of the accretionary prism and shallow part of the plate boundary, we are undertaking travel time tomography using reflection/refraction phases in OBS and MCS data, and constraints on the interface geometry from MCS images to estimate the detailed shallow velocity structure, with particular focus on properties within the shallow subduction channel. We observe refractions and reflections in OBS data from the shallow part of the subduction zone in both the Shumagin Gap and Semidi segment, including reflections off the top and base of what appears to be a layer of subducting sediment, which can be used for this work. We plan to present initial models of the shallow part of the subduction zone from both segments and discuss comparisons between the two.

  10. Transient uplift after a 17th-century earthquake along the kuril subduction zone

    USGS Publications Warehouse

    Sawai, Y.; Satake, K.; Kamataki, T.; Nasu, H.; Shishikura, M.; Atwater, B.F.; Horton, B.P.; Kelsey, H.M.; Nagumo, T.; Yamaguchi, M.

    2004-01-01

    In eastern Hokkaido, 60 to 80 kilometers above a subducting oceanic plate, tidal mudflats changed into freshwater forests during the first decades after a 17th-century tsunami. The mudflats gradually rose by a meter, as judged from fossil diatom assemblages. Both the tsunami and the ensuing uplift exceeded any in the region's 200 years of written history, and both resulted from a shallow plate-boundary earthquake of unusually large size along the Kuril subduction zone. This earthquake probably induced more creep farther down the plate boundary than did any of the region's historical events.

  11. Upper-mantle seismic discontinuities and the thermal structure of subduction zones

    USGS Publications Warehouse

    Vidale, J.E.; Benz, H.M.

    1992-01-01

    The precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western United States. The boundary near 410 km depth is locally elevated, that near 660 km depressed. The depths of these boundaries, which mark phase transitions, provide an in situ thermometer in subduction zones: the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth. In addition, a reflector near 210 km depth may mark the bottom of the aesthenosphere.

  12. Trace element behavior in hydrothermal experiments: Implications for fluid processes at shallow depths in subduction zones

    NASA Astrophysics Data System (ADS)

    You, C.-F.; Castillo, P. R.; Gieskes, J. M.; Chan, L. H.; Spivack, A. J.

    1996-05-01

    Chemical evaluation of fluids affected during progressive water-sediment interactions provides critical information regarding the role of slab dehydration and/or crustal recycling in subduction zones. To place some constraints on geochemical processes during sediment subduction, reactions between décollement sediments and synthetic NaCl-CaCl 2 solutions at 25-350°C and 800 bar were monitored in laboratory hydrothermal experiments using an autoclave apparatus. This is the first attempt in a single set of experiments to investigate the relative mobilities of many subduction zone volatiles and trace elements but, because of difficulties in conducting hydrothermal experiments on sediments at high P-T conditions, the experiments could only be designed for a shallow (˜ 10 km) depth. The experimental results demonstrate mobilization of volatiles (B and NH 4) and incompatible elements (As, Be, Cs, Li, Pb, Rb) in hydrothermal fluids at relatively low temperatures (˜ 300°C). In addition, a limited fractionation of light from heavy rare earth elements (REEs) occurs under hydrothermal conditions. On the other hand, the high field strength elements (HFSEs) Cr, Hf, Nb, Ta, Ti, and Zr are not mobile in the reacted fluids. The observed behavior of volatiles and trace elements in hydrothermal fluids is similar to the observed enrichment in As, B, Cs, Li, Pb, Rb, and light REEs and depletion in HFSEs in arc magmas relative to magmas derived directly from the upper mantle. Thus, our work suggests a link between relative mobilities of trace elements in hydrothermal fluids and deep arc magma generation in subduction zones. The experimental results are highly consistent with the proposal that the addition of subduction zone hydrous fluids to the subarc mantle, which has been depleted by previous melting events, can produce the unique characteristics of arc magmas. Moreover, the results suggest that deeply subducted sediments may no longer have the composition necessary to generate the other distinct characteristics, such as the B-δ 11 B and B- 10Be systematics, of arc lavas. Finally, the mobilization of B, Cs, Pb, and light REEs relative to heavy REEs in the hydrothermal fluids fractionate the ratios of B/Be, B/Nb, Cs/Rb, Pb/Ce, La/Ba and LREE/HREE, which behave conservatively during normal magmatic processes. These results demonstrate that the composition of slab-derived fluids has great implications for the recycling of elements; not only in arc magmas but also in mantle plumes.

  13. Insights from trace element geochemistry as to the roles of subduction zone geometry and subduction input on the chemistry of arc magmas

    NASA Astrophysics Data System (ADS)

    Wehrmann, Heidi; Hoernle, Kaj; Garbe-Schönberg, Dieter; Jacques, Guillaume; Mahlke, Julia; Schumann, Kai

    2014-10-01

    Subduction zones of continental, transitional, and oceanic settings, relative to the nature of the overriding plate, are compared in terms of trace element compositions of mafic to intermediate arc rocks, in order to evaluate the relationship between subduction parameters and the presence of subduction fluids. The continental Chilean Southern Volcanic Zone (SVZ) and the transitional to oceanic Central American Volcanic Arc (CAVA) show increasing degrees of melting with increasing involvement of slab fluids, as is typical for hydrous flux melting beneath arc volcanoes. At the SVZ, the central segment with the thinnest continental crust/lithosphere erupted the highest-degree melts from the most depleted sources, similar to the oceanic-like Nicaraguan segment of the CAVA. The northern part of the SVZ, located on the thickest continental crust/lithosphere, exhibits features more similar to Costa Rica situated on the Caribbean Large Igneous Province, with lower degrees of melting from more enriched source materials. The composition of the slab fluids is characteristic for each arc system, with a particularly pronounced enrichment in Pb at the SVZ and in Ba at the CAVA. A direct compositional relationship between the arc rocks and the corresponding marine sediments that are subducted at the trenches clearly shows that the compositional signature of the lavas erupted in the different arcs carries an inherited signal from the subducted sediments.

  14. Frictional properties of sediments entering the Costa Rica subduction zone offshore the Osa Peninsula: implications for fault slip in shallow subduction zones

    NASA Astrophysics Data System (ADS)

    Namiki, Yuka; Tsutsumi, Akito; Ujiie, Kohtaro; Kameda, Jun

    2014-12-01

    We examined the frictional properties of sediments on the Cocos plate offshore the Osa Peninsula, Costa Rica, and explored variations in the intrinsic frictional properties of the sediment inputs to the Costa Rica subduction zone. Sediment samples were collected at Site U1381A during the Integrated Ocean Drilling Program Expedition 334, and include hemipelagic clay to silty clay material (Unit I) and pelagic silicic to calcareous ooze (Unit II). The frictional properties of the samples were tested at a normal stress of 5 MPa under water-saturated conditions and with slip velocities ranging from 0.0028 to 2.8 mm/s for up to 340 mm of displacement. The experimental results reveal that the steady-state friction coefficient values of clay to silty clay samples are as low as ~0.2, whereas those of silicic to calcareous ooze samples are as high as 0.6 to 0.8. The clay to silty clay samples show a positive dependence of friction on velocity for all tested slip velocities. In contrast, the silicic to calcareous ooze samples show a negative dependence of friction on velocity at velocities of 0.0028 to 0.28 mm/s and either neutral or positive dependence at velocities higher than 0.28 mm/s. Given the low frictional coefficient values observed for the clay to silty clay samples of Unit I, the décollement at the Costa Rica Seismogenesis Project transect offshore the Osa Peninsula likely initiates in Unit I and is initially very weak. In addition, the velocity-strengthening behavior of the clay to silty clay suggests that faults in the very shallow portion of the Costa Rica subduction zone are stable and thus behave as creeping segments. In contrast, the velocity-weakening behavior of the silicic to calcareous ooze favors unstable slip along faults. The shallow seismicity occurred at a depth as shallow as ~9 km along the Costa Rica margin offshore the Osa Peninsula (Mw 6.4, June 2002), indicating that materials characterized by velocity-weakening behavior constitute the fault zone at the depth of the seismicity. Fault slip nucleating along a fault in Unit II would be a likely candidate for the source of the shallow earthquake event.

  15. Evidence for Constructing Continental Crust by Deep Accretion of Sedimentary Rocks

    NASA Astrophysics Data System (ADS)

    Hourigan, J. K.; Brandon, M. T.

    2009-12-01

    The ultimate fate of sediment in subduction zones is not completely understood. The mass balance of continental crust, and thus the growth history thereof, requires better quantification of the fraction of sediment that is subducted into the mantle vs. that which is ultimately underplated or otherwise reincorporated into the continents. Microanalytical zircon U-Th-Pb chronometry has been critical for revealing the age and origin of metasedimentary rocks. We highlight three recent examples from the literature and our own work in the Sredinniy Range of Kamchatka which demonstrate that ultimate fate substantial volumes of sediment that are deeply accreted and become otherwise indistinguishable from average continental crust. These examples are: (1) the Pelona-Orucopia-Rand-Sierra de Salinas schist (e.g., Grove et al, 2009); (2) the Swakane gneiss (Matzel et al, 2004); (3) Pamir xenoliths (Ducea et al.,2003; ); and our work in (4) the Sredinniy Range, Kamchatka (Hourigan et al.,2009); and (5) the Olympic Mtns of Washington. Each of these suites of rocks exhibit a range of detrital zircon ages with young peak that is similar to the age of sediment subduction. The range implies incorporation of isotopically evolved (continental) and relatively juvenile (volcanic arc) source regions. In these cases, subduction, attendant metamorphic transformation and deep accretion of these metasedimentary sequences resulted in reincorporation large volumes of sediment to the base of the crust. In this contribution we summarize the salient features of the literature examples and detail the results of our own work from Kamchatka. There within an Eocene-age arc-continent collision zone Cretaceous and Paleocene sedimentary rocks belonging to the margin of northeastern Russia were deeply buried and subjected to metamorphism at upper amphibolite to granulite facies. Statisitcal similarity of detrital zircon grain-age spectra from these high grade metasediments and Paleogene strata of the NE Russian margin establish the local derivation of the metasedimentary protoliths. The sediments achieved peak metamorphic grade within 10 m.y. of deposition (at ~52 Ma) and then were exhumed rapidly to mid-to-upper crustal levels. The P-T path of these rocks is a atypical of a subduction zone setting; we argue that Eocene mafic to ultramafic plutonic rocks observed in the southern part range were a critical ingredient in the metamorphic transformation and ascent of these high-grade metasediments. The seismic velocity structure throughout Kamchatka is that of average continental crust despite the widespread surface exposure of island arc rocks.

  16. Unraveling an antique subduction process from metamorphic basement around Medellín city, Central Cordillera of Colombian Andes

    NASA Astrophysics Data System (ADS)

    Bustamante, Andres; Juliani, Caetano

    2011-10-01

    In the surroundings of Caldas and El Retiro cities (Colombia) metamorphic rocks derived from basic and pelitic protoliths comprise the Caldas amphibole schist and the Ancón schist respectively. Subordinated metamorphosed granite bodies (La Miel gneiss) are associated to these units, and The El Retiro amphibolites, migmatites and granulites crops out eastwards of these units, separated by shear zones. The Caldas amphibole schist and the Ancón schist protoliths could have been formed in a distal marine reduced environment and amalgamated to the South American continent in an apparent Triassic subduction event. The El Retiro rocks are akin to a continental basement and possible include impure metasediments of continental margin, whose metamorphism originated granulite facies rocks and migmatites as a result of the anatexis of quartz-feldspathic rocks. The metamorphism was accompanied by intense deformation, which has juxtaposed both migmatites and granulite blocks. Afterward, heat and fluid circulation associated with the emplacement of minor igneous intrusions resulted in intense fluid-rock interaction, variations in the grain size of the minerals and, especially, intense retrograde metamorphic re-equilibrium. Thermobarometric estimations for the Caldas amphibole schist indicate metamorphism in the Barrovian amphibolite facies. The metamorphic path is counter-clockwise, but retrograde evolution could not be precisely defined. The pressures of the metamorphism in these rocks range from 6.3 to 13.5 kbar, with narrow temperature ranging from 550 to 630 °C. For the Ancón schist metapelites the P- T path is also counter-clockwise, with a temperature increase evidenced by the occurrence of sillimanite and the cooling by later kyanite. The progressive metamorphism event occurred at pressures of 7.6-7.2 kbar and temperatures of 645-635 °C for one sample and temperature between 500 and 600 °C under constant pressure of 6 kbar. The temperature estimated for these rocks varies between 400 and 555 °C at pressures of 5-6 kbar in the retrograde metamorphic path. The El Retiro rocks evidence strong decompression with narrow variation in temperature, showing pressure values between 8.7 and 2.7 kbar at temperatures of 740-633 °C. These metamorphic fragments of the basement in the Central Cordillera of the Colombian Andes could represent a close relationship with an antique subduction zone.

  17. Early to Middle Ordovician organized and disorganized supra-subduction zone spreading along the Laurentian margin of Iapetus, Newfoundland Appalachians

    NASA Astrophysics Data System (ADS)

    Zagorevski, Alexandre; van Staal, Cees R.; McNicoll, Vicki J.; Cutts, Jamie A.

    2013-04-01

    The Annieopsquotch accretionary tract (AAT) comprises a thrust stack of Lower to Middle Ordovician arc and backarc terranes that were accreted to the Laurentian margin of Iapetus during Middle to Upper Ordovician. Geological relationships suggest that the constituent terranes of the AAT initially formed outboard of composite Laurentian margin in an extensional arc that underwent multiple rifting episodes prior to its accretion. The initiation of AAT magmatism led to the development of Tremadocian to Floian supra-subduction zone ophiolites with organized ridges indicated by well developed sheeted dyke complexes. This spreading centre propagated through a fragment of Laurentian crust and separated it from the composite Laurentian margin. This crustal block formed the basement to subsequent Floian to Darriwilian AAT arc magmatism. The Floian arc rifted leading to organized spreading in the Lloyds River backarc basin which was floored by juvenile backarc ophiolitic crust. The latest Floian to earliest Dapingian arc magmatism occurred above thickened crust, locally leading to eruption of andesitic rocks. The establishment of the Darriwilian arc was in part coeval with and followed yet another stage of rifting. Darriwilian magmatism is characterised by great along-strike variability, ranging from continental to intraoceanic calc-alkaline arc to tholeiitic back-arc magmatism, and lack of sheeted dyke complexes. The diversity of the magmatism can be attributed to fragmentation and magmatic reworking of the Laurenia-derived basement along strike in the same arc that is undergoing disorganized spreading. The constituent terranes of AAT were underplated beneath the composite Laurentian margin within ~5 Ma following their formation, coeval with extensional arc magmatism. This and other constraints suggest that the AAT thrust stack formed following subduction initiation in the backarc. AAT magmatism was terminated by the closure of the main tract of Iapetus and arrival of the peri-Gondwanan Victoria arc at the Laurentian margin.

  18. Imaging the Chilean Subduction Zone at 38° S Using Prestack Kirchhoff Depth Migration Within Project TIPTEQ

    NASA Astrophysics Data System (ADS)

    Gross, K.; Buske, S.; Wigger, P.; Micksch, U.; Krawczyk, C. M.; Stiller, M.; Araneda, M.; Bataille, K.; Bribach, J.; Lüth, S.; Mechie, J.; Schulze, A.; Shapiro, S. A.; Ziegenhagen, T.

    2005-12-01

    The Chilean continental margin is one of the most seismically active subduction systems and serves as a natural laboratory to study mega-thrust earthquakes. Investigations with respect to the structural and petrophysical properties of the corresponding seismogenic coupling zone are the key to understand the triggering mechanisms and processes that generate those mega-thrust earthquakes. This is one of the main aims of the interdisciplinary project TIPTEQ (from The Incoming Plate to mega-Thrust EarthQuake processes). As part of this project an approximately 95 km long 3C near-vertical reflection seismic profile was shot in southern Central Chile at about 38° S in January 2005. The profile runs from the Pacific Ocean to the Central Valley along a west-east trending line crossing the relocated hypocentre of the historic 1960 Valdivia earthquake (Mw=9.5). An additional expanding spread (ESP) experiment focuses on the down-dip limit (30-50 km depth) of the seismogenic coupling zone. Furthermore, three-hole (Camouflet) shooting configurations were carried out as a pilot study to test SH-wave generation in a crustal regime. This paper focuses on the structural imaging results using prestack Kirchhoff depth migration. Close to the coast the oceanic crust is clearly visible and can be traced below a strongly heterogeneous accreted overburden further inland down to depths of about 50 km. At the eastern end of the seismic section, a strong reflector appears at about 60 km depth, which runs perpendicular to the extrapolated plate interface. Taking into account results from previous experiments and other components of project TIPTEQ, we will present an interpretation of the subduction zone image with special emphasis on its structural features.

  19. Important role for organic carbon in subduction-zone fluids in the deep carbon cycle

    NASA Astrophysics Data System (ADS)

    Sverjensky, Dimitri A.; Stagno, Vincenzo; Huang, Fang

    2014-12-01

    Supercritical aqueous fluids link subducting plates and the return of carbon to Earth's surface in the deep carbon cycle. The amount of carbon in the fluids and the identities of the dissolved carbon species are not known, which leaves the deep carbon budget poorly constrained. Traditional models, which assume that carbon exists in deep fluids as dissolved gas molecules, cannot predict the solubility and ionic speciation of carbon in its silicate rock environment. Recent advances enable these limitations to be overcome when evaluating the deep carbon cycle. Here we use the Deep Earth Water theoretical model to calculate carbon speciation and solubility in fluids under upper mantle conditions. We find that fluids in equilibrium with mantle peridotite minerals generally contain carbon in a dissolved gas molecule form. However, fluids in equilibrium with diamonds and eclogitic minerals in the subducting slab contain abundant dissolved organic and inorganic ionic carbon species. The high concentrations of dissolved carbon species provide a mechanism to transport large amounts of carbon out of the subduction zone, where the ionic carbon species may influence the oxidation state of the mantle wedge. Our results also identify novel mechanisms that can lead to diamond formation and the variability of carbon isotopic composition via precipitation of the dissolved organic carbon species in the subduction-zone fluids.

  20. Subducting Seamounts and the Rupturing Process of Great Subduction Zone Earthquakes

    NASA Astrophysics Data System (ADS)

    Das, S.

    2009-05-01

    It was suggested in the 1970's that subducting ocean floor features may delimit the along-strike rupture lengths of large subduction zone earthquakes. With the dramatic improvement in data quality, both for seismic and ocean floor bathmetry data, we can now see how the actual rupturing process of great earthquakes is also influenced by such subducting features. Here we present three great (Mw > 8) subduction zone earthquakes, in very different parts of the world, for which a relation between the ocean floor and the earthquake source process is seen. These include the 1986 Andreanof Islands, Alaska and the 1996 Biak, Indonesia earthquakes, in which the regions of large slip concentrate in patches, reminiscent of the "asperity model" of earthquakes, and appear to be related to subducted seamounts. For the 2001 Peru earthquake, a subducting fracture zone, with its associated bathymetric peak and trough, seems to have been the cause of the rupture being stalled for ~30s, before producing an earthquake of Mw 8.4, the third largest earthquake worldwide since 1965. Similarities and differences in the earthquake rupturing properties for these two different types of subducting features will be discussed. An outstanding question is what controls whether a seamount obducts or subducts.

  1. Thrust-type subduction-zone earthquakes and seamount asperites: A physical model for seismic rupture

    SciTech Connect

    Cloos, M. )

    1992-07-01

    A thrust-type subduction-zone earthquake of M{sub W} 7.6 ruptures an area of {approximately}6,000 km{sup 2}, has a seismic slip of {approximately}1 m, and is nucleated by the rupture of an asperity {approximately}25km across. A model for thrust-type subduction-zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick-slip faulting. A M{sub W} 7.6 event would correspond to the near-basal rupture of a {approximately}2-km-tall seamount. The base of the seamount is surrounded by a low shear-strength layer composed of subducting sediment that also deforms between seismic events by distributed strain (viscous flow). Planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometers per second. The faults in the shear zone are disrupted after the event by aseismic, slow viscous flow of the subducting sediment layer. Consequently, the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture.

  2. GPS Measurements of Interseismic,Coseismic, and Postseismic Deformation in Puysegur Subduction Zone, New Zealand

    NASA Astrophysics Data System (ADS)

    Benz, N.; Wallace, L. M.; Hreinsdottir, S.; Denys, P. H.; Pearson, C. F.

    2014-12-01

    A combination of continuous and campaign style geodetic measurements from sites on the South Island of New Zealand help us to constrain long-term and short-term deformation adjacent to the Puysegur subduction zone, offshore Fiordland, New Zealand. We update an elastic block model for the region that consists of an array of microplates bounded by active faults. We invert GPS velocities, and long-term fault slip rates and earthquake slip vectors for tectonic block rotations, fault coupling coefficients, and permanent block strain. Three large subduction interface earthquakes, 2003 Secretary Island (Mw 7.2), 2007 George Sound (Mw 6.7), and 2009 Dusky Sound (Mw 7.8) complicate interpretation of the GPS time series over the last decade in the Fiordland region. To deal with this, we also conduct time-dependent inversions for coseismic and postseismic slip in those earthquakes using TDefnode (McCaffrey, 2009), fitting the campaign and continuous GPS timeseries with the best-fitting coseismic and postseismic slip models for those events. We evaluate the interseismic coupling model for the Puysegur subduction zone in the context of the recent earthquakes, and in particular find that the source region of the Mw 7.8 Dusky Sound earthquake was interseismically coupled prior to 2009. Overall, the campaign and continuous GPS are well-fit by a partially locked subduction interface undergoing seismic and aseismic slip as well as distributed (likely permanent) deformation east of the main plate boundary fault.

  3. Exhumation of serpentinized peridotite in the northern Manila subduction zone inferred from forward gravity modeling

    NASA Astrophysics Data System (ADS)

    Doo, Wen-Bin; Lo, Chung-Liang; Kuo-Chen, Hao; Brown, Dennis; Hsu, Shu-Kun

    2015-10-01

    The Taiwan Integrated Geodynamic Research program (TAIGER) collected two wide-angle and reflection seismic transects across the northern Manila subduction zone that provide constraints on the seismic velocity structure of the crust. Two-dimensional gravity modeling along these two transects shows a significant, relatively high density (3.12 and 3.02 g/cm3) in the fore-arc region, at the interface between the subducting Eurasian Plate and the accretionary prism in front of the Luzon arc on the overriding Philippine Sea Plate. The anomalous density in this zone is higher than that in the fore-arc crust and the accretionary prism but lower than that in mantle. Numerous geophysical and geological data, together with numerical models, have indicated that serpentinization of the fore-arc mantle is both expected and observed. Serpentinization of mantle rocks can dramatically reduce their seismic velocity and therefore their seismic velocity in a density to velocity conversion. Therefore, the source of the high-density material could be serpentinized fore-arc mantle, with serpentinization caused by the dehydration of the subducting Eurasian Plate. We interpret that positive buoyancy combined with weak plate coupling forces in the northern Manila subduction zone is resulting in this serpentinized fore-arc mantle peridotite being exhumed.

  4. Ambient seafloor noise excited by earthquakes in the Nankai subduction zone.

    PubMed

    Tonegawa, Takashi; Fukao, Yoshio; Takahashi, Tsutomu; Obana, Koichiro; Kodaira, Shuichi; Kaneda, Yoshiyuki

    2015-01-01

    Excitations of seismic background noises are mostly related to fluid disturbances in the atmosphere, ocean and the solid Earth. Earthquakes have not been considered as a stationary excitation source because they occur intermittently. Here we report that acoustic-coupled Rayleigh waves (at 0.7-2.0 Hz) travelling in the ocean and marine sediments, retrieved by correlating ambient noise on a hydrophone array deployed through a shallow to deep seafloor (100-4,800 m) across the Nankai Trough, Japan, are incessantly excited by nearby small earthquakes. The observed cross-correlation functions and 2D numerical simulations for wave propagation through a laterally heterogeneous ocean-crust system show that, in a subduction zone, energetic wave sources are located primarily under the seafloor in directions consistent with nearby seismicity, and secondarily in the ocean. Short-period background noise in the ocean-crust system in the Nankai subduction zone is mainly attributed to ocean-acoustic Rayleigh waves of earthquake origin. PMID:25635384

  5. Effect of depth-dependent shear modulus on tsunami generation along subduction zones

    USGS Publications Warehouse

    Geist, E.L.; Bilek, S.L.

    2001-01-01

    Estimates of the initial size of tsunamis generated by subduction zone earthquakes are significantly affected by the choice of shear modulus at shallow depths. Analysis of over 360 circum-Pacific subduction zone earthquakes indicates that for a given seismic moment, source duration increases significantly with decreasing depth (Bilek and Lay, 1998; 1999). Under the assumption that stress drop is constant, the increase of source duration is explained by a 5-fold reduction of shear modulus from depths of 20 km to 5 km. This much lower value of shear modulus at shallow depths in comparison to standard earth models has the effect of increasing the amount of slip estimated from seismic moment determinations, thereby increasing tsunami amplitude. The effect of using depth dependent shear modulus values is tested by modeling the tsunami from the 1992 Nicaraguan tsunami earthquake using a previously determined moment distribution (lhmle??, 1996a). We find that the tide gauge record of this tsunami is well matched by synthetics created using the depth dependent shear modulus and moment distribution. Because excitation of seismic waves also depends on elastic heterogeneity, it is important, particularly for the inversion of short period waves, that a consistent seismic/tsunami shear modulus model be used for calculating slip distributions.

  6. Ambient seafloor noise excited by earthquakes in the Nankai subduction zone

    PubMed Central

    Tonegawa, Takashi; Fukao, Yoshio; Takahashi, Tsutomu; Obana, Koichiro; Kodaira, Shuichi; Kaneda, Yoshiyuki

    2015-01-01

    Excitations of seismic background noises are mostly related to fluid disturbances in the atmosphere, ocean and the solid Earth. Earthquakes have not been considered as a stationary excitation source because they occur intermittently. Here we report that acoustic-coupled Rayleigh waves (at 0.7–2.0 Hz) travelling in the ocean and marine sediments, retrieved by correlating ambient noise on a hydrophone array deployed through a shallow to deep seafloor (100–4,800 m) across the Nankai Trough, Japan, are incessantly excited by nearby small earthquakes. The observed cross-correlation functions and 2D numerical simulations for wave propagation through a laterally heterogeneous ocean–crust system show that, in a subduction zone, energetic wave sources are located primarily under the seafloor in directions consistent with nearby seismicity, and secondarily in the ocean. Short-period background noise in the ocean–crust system in the Nankai subduction zone is mainly attributed to ocean-acoustic Rayleigh waves of earthquake origin. PMID:25635384

  7. Lithosphere tearing at STEP faults: response to edges of subduction zones

    NASA Astrophysics Data System (ADS)

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

    2005-07-01

    Slab edges are a relatively common feature in plate tectonics. Two prominent examples are the northern end of the Tonga subduction zone and the southern end of the New Hebrides subduction zone. Near such horizontal terminations of subduction trenches, ongoing tearing of oceanic lithosphere is a geometric consequence. We refer to such kinks in the plate boundary as a Subduction-Transform Edge Propagator, or STEP. Other STEPs are the north and south ends of the Lesser Antilles trench, the north end of the South Sandwich trench, the south end of the Vrancea trench, and both ends of the Calabria trench. Volcanism near STEPs is distinct from typical arc volcanism. In some cases, slab edges appear to coincide with mantle plumes. Using 3D mechanical models, we establish that STEP faults are stable plate tectonic features in most circumstances. In the (probably rare) cases that the resistance to fault propagation is high, slab break-off will occur. Relative motion along the transform segment of the plate boundary often is non-uniform, and the STEP is not a transform plate boundary in the (rigid) plate tectonics sense of the phrase. STEP propagation may result in substantial deformation, rotation, topography and sedimentary basins, with a very specific time-space evolution. Surface velocities are substantially affected by nearby STEPs.

  8. Aftereffects of Subduction-Zone Earthquakes: Potential Tsunami Hazards along the Japan Sea Coast.

    PubMed

    Minoura, Koji; Sugawara, Daisuke; Yamanoi, Tohru; Yamada, Tsutomu

    2015-01-01

    The 2011 Tohoku-Oki Earthquake is a typical subduction-zone earthquake and is the 4th largest earthquake after the beginning of instrumental observation of earthquakes in the 19th century. In fact, the 2011 Tohoku-Oki Earthquake displaced the northeast Japan island arc horizontally and vertically. The displacement largely changed the tectonic situation of the arc from compressive to tensile. The 9th century in Japan was a period of natural hazards caused by frequent large-scale earthquakes. The aseismic tsunamis that inflicted damage on the Japan Sea coast in the 11th century were related to the occurrence of massive earthquakes that represented the final stage of a period of high seismic activity. Anti-compressive tectonics triggered by the subduction-zone earthquakes induced gravitational instability, which resulted in the generation of tsunamis caused by slope failing at the arc-back-arc boundary. The crustal displacement after the 2011 earthquake infers an increased risk of unexpected local tsunami flooding in the Japan Sea coastal areas. PMID:26399180

  9. The source scaling and depth-dependent stress drops for subduction zone events

    NASA Astrophysics Data System (ADS)

    Ko, Y.-T.; Kuo, B.-Y.; Hung, S.-H.

    2012-04-01

    The issue of source scaling between earthquake corner frequency (fc) and seismic moment (Mo) has caused incessant debate. Both self-similarity and non-similarity of earthquakes across a certain range of magnitudes and event depths are appealing to different investigators. The major difficulty in this topic is that the source characteristic is always concealed from several effects along the raypaths. The only chance to resolve this issue is to robustly extract the source spectra from the observed data. A few commonly used approaches, i.e. empirical Green's function (EGF) method, mitigate the problem; however the reference event must be chosen which may limit the applicability of the method. In this work, we apply the cluster-event method (CEM) to robustly determine the source corner frequency of the events in Japan subduction zone. We demonstrate that the source parameter of each event is statistically better determined with the CEM than with conventional methods, leading to a better constraint on path effect. We found that the corner frequencies satisfy a relationship with seismic moment of Mo ≡ f c-3, implying earthquake self-similarity for subduction zone events. The results of this study agree with those of previous studies, except with an upward deviation due to higher corner frequencies and stress drops. This leads to an interesting issue of the depth-dependency of stress drop, suggesting that there are some fundamental changes in strength of fault zone or in mechanism of earthquake source.

  10. The source scaling between corner frequency and seismic moment for intermediate-depth subduction zone events

    NASA Astrophysics Data System (ADS)

    Ko, Y.; Kuo, B.; Hung, S.

    2011-12-01

    The source scaling between earthquake corner frequency and seismic moment is a controversial issue. Both self-similarity and non-similarity of earthquakes across a certain range of magnitude are appealing to different investigators. The major difficulty in this topic is that the source characteristic is always concealed from several effects along the raypaths. The only chance to resolve this issue is to robustly extract the source spectra from the observed data. A few commonly used approaches, i.e. empirical Green's function (EGF) method, mitigate the problem; however the reference event must be chosen which may limit the applicability of the method. In this work, we apply the cluster-event method (CEM) to robustly determine the source corner frequency of the events in Japan subduction zone. We demonstrate that the source parameter of each event is statistically better determined with CEM than with conventional methods, leading to a better constraint on path effect. We found that the corner frequencies satisfy a relationship with seismic moment of Mo ∝ fc^(-3), implying earthquake self-similarity for subduction zone events. The CEM not only provides a new insight into the field of the source scaling, but also is a powerful tool in mapping the large-scale attenuation structure of the region.

  11. A mixed seismic-aseismic stress release episode in the Andean subduction zone

    NASA Astrophysics Data System (ADS)

    Villegas-Lanza, J. C.; Nocquet, J.-M.; Rolandone, F.; Vallée, M.; Tavera, H.; Bondoux, F.; Tran, T.; Martin, X.; Chlieh, M.

    2016-02-01

    In subduction zones, stress is released by earthquakes and transient aseismic slip. The latter falls into two categories: slow slip and afterslip. Slow-slip events emerge spontaneously during the interseismic phase, and show a progressive acceleration of slip with a negligible contribution of synchronous tremors or microseismicity to the energy, or moment release. In contrast, afterslip occurs immediately after large and moderate earthquakes, decelerates over time, and releases between 20 and 400% of the moment released by the preceding earthquake. Here we use seismic and GPS data to identify transient aseismic slip that does not fit into either of these categories. We document a seismic-aseismic slip sequence which occurred at shallow depths along a weakly coupled part of the Andean subduction zone in northern Peru and lasted seven months. The sequence generated several moderate earthquakes that together account for about 25% of the total moment released during the full sequence, equivalent to magnitude 6.7. Transient slip immediately followed two of the earthquakes, with slip slowing at a logarithmic rate. Considered separately, the moment released by transient slip following the second earthquake was more than 1,000% of the moment released during the earthquake itself, a value incompatible with classical models of afterslip. Synchronous seismic swarms and aseismic slip may therefore define a stress-release process that is distinct from slow-slip events and afterslip.

  12. Relationship between the location of chemosynthetic benthic communities and geologic structure on the Cascadia subduction zone

    SciTech Connect

    Lewis, B.T.R. ); Cochrane, G.C. )

    1990-06-10

    Chemosynthetic benthic communities, which live symbiotically with microbes capable of metabolizing nutrients dissolved in water seeping out of the seafloor, are widespread along the Cascadia subduction zone. These seeps and vents are therefore indicative of one mode of fluid migration out of the subduction zone sediments. The authors have used deep-towed seismic methods, including hydrophones mounted on Alvin, to examine the detailed geologic structure under two of these vent sites. At one of the sites, located on a seaward dipping thrust zone, the benthic communities are associated with a disruption of the subsurface acoustic layering in the thrust zone. It appears that at this site, dewatering is occurring along fractures in the disrupted sediments which connect to permeable layers in the undeformed sediments and not along the main thrust fault. The other site is located near the top of a ridge which has been uplifted by thrusting along a landward dipping thrust. Most of the benthic communities are found to exist at the outcrop of a steeply dipping unconformity between recent slope-basin sediments and the older uplifted sediments, with the unconformity serving as the fluid pathway. Underlying this unconformity at the most active dewatering location is a complexly deformed structure which appears to enhance the flow of fluid to the unconformity. The source of the fluids could be the older uplifted sediments or the recent slope-basin turbidites. In either case the source is shallow, less than about 1 km.

  13. Deep Magnetotelluric survey on Crete Island across the Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

    Kalisperi, D.; Smirnov, M.; Kokologiannakis, A.; Pentes, G.; Makris, J. P.

    2013-12-01

    Crete Island is located in a prominent position at the fore-arc of the Hellenic Subduction Zone (HSZ), thus enabling onshore study of the Earth's deep structure. The area is characterized by a complicated geological and geotectonic setting as well as by intense geodynamics that manifests itself in high seismicity. The aim of the ongoing research project 'MagnetoTellurics in studying Geodynamics of the hEllenic ARc (MT-GEAR)' is to contribute to the investigation of the geoelectric structure of Southern Aegean, and particularly to try to image the Hellenic Subduction Zone. In this context, onshore magnetotelluric (MT) measurements were carried out in July 2013 on Crete Island, comprising three parallel profiles aligned to the North-South (NS) direction, yielding a site spacing of about 5 to 10 km. In total, 21 broad-band MT soundings were conducted in the period range of 0.003-1000 s organized in the three 36Km, 30Km and 42Km long NS trending profiles. Data were collected using two different types of MT instruments (an EMI MT24LF and two Uppsala type MTU2000 systems) which were running simultaneously. We present the resulting model of the conductivity structure of the HSZ in the area of Crete.

  14. In-situ Nd isotope measurements on accessory minerals: Insights into isotope equilibration during metamorphism

    NASA Astrophysics Data System (ADS)

    Hammerli, Johannes; Spandler, Carl; Kemp, Tony; Pirard, Cassian

    2015-04-01

    Understanding isotope equilibration processes during metamorphism has huge implications for a range of geoscience applications, ranging from provenance studies of sedimentary units to the origin of magmas and ore bodies. Furthermore, recent claims of isotope disequilibrium situations during the melting of continental crust have questioned the reliability of using certain isotope systems to track magma sources. Our recent work investigated a prograde sequence of high-temperature, low-pressure (350-650 ?C, ~3-5 kbar) metasedimentary rocks from the Mt. Lofty Ranges, South Australia that underwent widespread pervasive fluid flow at peak metamorphism. In situ Nd-isotope analyses by LA-MC-ICP-MS found that the detrital signature of apatite survives temperatures of 500 C. However, the observed isotope equilibration of REE-bearing accessory minerals at ~600 C, before the onset of partial melting, suggests that isotope disequilibrium is unlikely during high-grade metamorphism of upper crustal rocks where fluid induced melting takes place. Here, we extend our research to metasedimentary rocks from (ultra)-high pressure metamorphic terrains from northern New Caledonia, and Dabieshan, China that represent pressure and temperature conditions found in subduction zones. Our study helps to understand isotope equilibration processes from heterogeneous protoliths as well as the impact of retrogression and the resetting of isotope systems over a pressure-temperature range from ~350 C to 700 C and ~15 kbar to 40 kbar. Nd isotope analyses of apatite, allanite, titanite, xenotime, monazite, lawsonite and epidote in pelitic and psammitic samples allow the investigation of isotope equilibration on a mineral and sub-mineral scale, as well as comparison with traditional bulk rock isotope analyses. Our preliminary results show that under high-pressure conditions (~20 to 30 kbar) and temperatures to ~650 C, REE-bearing phases show variable ?Nd values in some cases. These differences cannot be simply explained with retrogressive processes and the partial resetting of the isotope system. The results rather suggest that isotopic exchange between different REE-phases in such systems might be limited, and hence heterogeneous isotope signatures of protolithic sedimentary rocks can survive to great depths in subduction-zone environments. These results may have bearing on the reliability of isotopic tracking of the subducted slab component of arc magmas.

  15. Geophysical signature of hydration-dehydration processes in active subduction zones

    NASA Astrophysics Data System (ADS)

    Reynard, Bruno

    2013-04-01

    Seismological and magneto-telluric tomographies are potential tools for imaging fluid circulation when combined with petrophysical models. Recent measurements of the physical properties of serpentine allow refining hydration of the mantle and fluid circulation in the mantle wedge from geophysical data. In the slab lithospheric mantle, serpentinization caused by bending at the trench is limited to a few kilometers below the oceanic crust (<5 km). Double Wadati-Benioff zones, 20-30 km below the crust, are explained by deformation of dry peridotites, not by serpentine dehydration. It reduces the required amount of water stored in solid phases in the slab (Reynard et al., 2010). In the cold (<700°C) fore-arc mantle wedge above the subducting slab, serpentinization is caused by the release of large amounts of hydrous fluids in the cold mantle above the dehydrating subducted plate. Low seismic velocities in the wedge give a time-integrated estimate of hydration and serpentinization. Serpentinization reaches 50-100% in hot subduction, while it is below 10% in cold subduction (Bezacier et al., 2010; Reynard, 2012). Electromagnetic profiles of the mantle wedge reveal high electrical-conductivity bodies. In hot areas of the mantle wedge (> 700°C), water released by dehydration of the slab induces melting of the mantle under volcanic arcs, explaining the observed high conductivities. In the cold melt-free wedge (< 700°C), high conductivities in electromagnetic profiles provide "instantaneous" images of fluid circulation because the measured electrical conductivity of serpentine is below 0.1 mS/m (Reynard et al., 2011). A small fraction (ca. 1% in volume) of connective high-salinity fluids accounts for the highest observed conductivities. Low-salinity fluids (≤ 0.1 m) released by slab dehydration evolve towards high-salinity (≥ 1 m) fluids during progressive serpentinization in the wedge. These fluids can mix with arc magmas at depths and account for high-chlorine melt inclusions in arc lavas. High electrical conductivities up to 1 S/m in the hydrated wedge of the hot subductions (Ryukyu, Kyushu, Cascadia) reflect high fluid concentration, while low to moderate (<0.01 S/m) conductivities in the cold subductions (N-E Japan, Bolivia) reflect low fluid flow. This is consistent with the seismic observations of extensive shallow serpentinization in hot subduction zones, while serpentinization is sluggish in cold subduction zones. Bezacier, L., et al. 2010. Elasticity of antigorite, seismic detection of serpentinites, and anisotropy in subduction zones. Earth and Planetary Science Letters, 289, 198-208. Reynard, B., 2012. Serpentine in active subduction zones. Lithos, http://dx.doi.org/10.1016/j.lithos.2012.10.012. Reynard, B., Mibe, K. & Van de Moortele, B., 2011. Electrical conductivity of the serpentinised mantle and fluid flow in subduction zones. Earth and Planetary Science Letters, 307, 387-394. Reynard, B., Nakajima, J. & Kawakatsu, H., 2010. Earthquakes and plastic deformation of anhydrous slab mantle in double Wadati-Benioff zones. Geophysical Research Letters, 37, L24309.

  16. Implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary (non-collisional) and collisional (suturing) orogens

    USGS Publications Warehouse

    Scholl, D. W.; Von Huene, R.

    2009-01-01

    Arc magmatism at subduction zones (SZs) most voluminously supplies juvenile igneous material to build rafts of continental and intra-oceanic or island arc (CIA) crust. Return or recycling of accumulated CIA material to the mantle is also most vigorous at SZs. Recycling is effected by the processes of sediment subduction, subduction erosion, and detachment and sinking of deeply underthrust sectors of CIA crust. Long-term (>10-20 Ma) rates of additions and losses can be estimated from observational data gathered where oceanic crust underruns modern, long-running (Cenozoic to mid-Mesozoic) ocean-margin subduction zones (OMSZs, e.g. Aleutian and South America SZs). Long-term rates can also be observationally assessed at Mesozoic and older crust-suturing subduction zone (CSSZs) where thick bodies of CIA crust collided in tectonic contact (e.g. Wopmay and Appalachian orogens, India and SE Asia). At modern OMSZs arc magmatic additions at intra-oceanic arcs and at continental margins are globally estimated at c. 1.5 AU and c. 1.0 AU, respectively (1 AU, or Armstrong Unit,= 1 km3 a-1 of solid material). During collisional suturing at fossil CSSZs, global arc magmatic addition is estimated at 0.2 AU. This assessment presumes that in the past the global length of crustal collision zones averaged c. 6000 km, which is one-half that under way since the early Tertiary. The average long-term rate of arc magmatic additions extracted from modern OMSZs and older CSSZs is thus evaluated at 2.7 AU. Crustal recycling at Mesozoic and younger OMSZs is assessed at c. 60 km3 Ma-1 km-1 (c. 60% by subduction erosion). The corresponding global recycling rate is c. 2.5 AU. At CSSZs of Mesozoic, Palaeozoic and Proterozoic age, the combined upper and lower plate losses of CIA crust via subduction erosion, sediment subduction, and lower plate crustal detachment and sinking are assessed far less securely at c. 115 km3 Ma-1 km-1. At a global length of 6000 km, recycling at CSSZs is accordingly c. 0.7 AU. The collective loss of CIA crust estimated for modern OMSZs and for older CSSZs is thus estimated at c. 3.2 AU. SZ additions (2.7 AU) and subtractions (23.2 AU) are similar. Because many uncertainties and assumptions are involved in assessing and applying them to the deep past, the net growth of CIA crust during at least Phanerozoic time is viewed as effectively nil. With increasing uncertainty, the long-term balance can be applied to the Proterozoic, but not before the initiation of the present style of subduction at c. 3 Ga. Allowing that since this time a rounded-down rate of recycling of 3 AU is applicable, a startlingly high volume of CIA crust equal to that existing now has been recycled to the mantle. Although the recycled volume (c. 9 ?? 109 km3) is small (c. 1%) compared with that of the mantle, it is large enough to impart to the mantle the signature of recycled CIA crust. Because subduction zones are not spatially fixed, and their average global lengths have episodically been less or greater than at present, recycling must have contributed significantly to creating recognized heterogeneities in mantle geochemistry. ?? The Geological Society of London 2009.

  17. Crustal Structure of the Caribbean-South American Diffuse Plate Boundary: Subduction Zone Migration and Polarity Reversal Along BOLIVAR Profile 64W

    NASA Astrophysics Data System (ADS)

    Clark, S. A.; Levander, A.; Magnani, M.; Zelt, C. A.; Sawyer, D. S.; Ave Lallemant, H. G.

    2005-12-01

    The BOLIVAR (Broadband Ocean-Land Investigation of Venezuela and the Antilles arc Region) project is an NSF funded, collaborative seismic experiment in the southeast Caribbean region. The purpose of the project is to understand the diffuse plate boundary created by the oblique collision between the Caribbean and South American plates. Profile 64W of the BOLIVAR experiment, a 450 km-long, N-S transect onshore and offshore Venezuela located at ~64°W longitude, images the deep crustal structures formed by this collision. The active source components of profile 64W include 300 km of MCS reflection data, 33 coincident OBSs, and 344 land seismic stations which recorded 7500 offshore airgun shots and 2 explosive land shots. Results from the reflection and refraction seismic data along 64W show complex crustal structure across the entire span of the diffuse plate boundary. The onshore portion of 64W crosses the fold and thrust belt of the Serrania del Interior, which formed at ~16 Ma by collision of the Caribbean forearc with the northern South American passive margin. Underlying the Serrania del Interior is a south-vergent, remnant Lesser Antillean subduction zone. As this Lesser Antilles subduction impinged on continental crust, it caused a polarity reversal and jump offshore to the north. Convergence was initially localized in the closure and inversion of the Grenada Basin. However, subduction could not develop because of the ~20-km-thick crust of the Aves Ridge; instead, north-vergent subduction initiated further to the north, where ~12-km-thick Caribbean oceanic crust of the Venezuela Basin began to subduct beneath the Aves Ridge in the Pliocene (~4 Ma) and appears to continue subducting today. Between the remnant subduction zone and the modern one, the El Pilar and Coche dextral strike-slip faults accommodate most of the transform motion of the plate boundary. From the Serrania del Interior to the Aves Ridge, ~260 km of accreted orogenic float comprises the diffuse plate boundary.

  18. Constraining input and output fluxes of the southern-central Chile subduction zone: water, chlorine and sulfur

    NASA Astrophysics Data System (ADS)

    Völker, David; Wehrmann, Heidi; Kutterolf, Steffen; Iyer, Karthik; Rabbel, Wolfgang; Geersen, Jacob; Hoernle, Kaj

    2014-10-01

    In this paper, we constrain the input and output fluxes of H2O, Cl and S into the southern-central Chilean subduction zone (31°S-46°S). We determine the input flux by calculating the amounts of water, chlorine and sulfur that are carried into the subduction zone in subducted sediments, igneous crust and hydrated lithospheric mantle. The applied models take into account that latitudinal variations in the subducting Nazca plate impact the crustal porosity and the degree of upper mantle serpentinization and thus water storage in the crust and mantle. In another step, we constrain the output fluxes of the subduction zone both to the subcontinental lithospheric mantle and to the atmosphere-geosphere-ocean by the combined use of gas flux determinations at the volcanic arc, volume calculations of volcanic rocks and the combination of mineralogical and geothermal models of the subduction zone. The calculations indicate that about 68 Tg/m/Ma of water enters the subduction zone, as averaged over its total length of 1,480 km. The volcanic output on the other hand accounts for 2 Tg/m/Ma or 3 % of that input. We presume that a large fraction of the volatiles that are captured within the subducting sediments (which accounts for roughly one-third of the input) are cycled back into the ocean through the forearc. This assumption is however questioned by the present lack of evidence for major venting systems of the submarine forearc. The largest part of the water that is carried into the subduction zone in the crust and hydrated mantle (accounting for two-thirds of the input) appears to be transported beyond the volcanic arc.

  19. New estimates of subducted water from depths of extensional outer rise earthquakes at the Northwestern Pacific subduction zones

    NASA Astrophysics Data System (ADS)

    Emry, E. L.; Wiens, D. A.

    2012-12-01

    The presence of water within the subducting slab mantle may have important implications for subduction zone water budgets, intermediate depth earthquakes, and transport of water into Earth's deep mantle. However, the amount of water stored in hydrous slab mantle rocks prior to subduction is not well constrained. Large extensional faults formed as the plate bends at the subduction zone outer rise are thought to be the main pathway by which water can travel into and hydrate the slab mantle; yet for many subduction zones accurate depths of extensional outer rise faulting are also not well known. Therefore, we attempt to identify the maximum observed depth of extensional faulting, and thereby identify the possible depth extent of slab mantle hydration, by accurately locating and determining depths for outer rise and trench axis earthquakes at Northern and Western Pacific subduction zones. For each region, we relocate all earthquakes seaward of the trench axis as well as forearc earthquakes within 60 km landward of the trench axis using ISC arrival times and the hypocentroidal decomposition relative location algorithm. We then model P- and SH- waveforms and their associated depth phases for all earthquakes with Mw 5.0+ since 1990 that exhibit good signal-to-noise ratios and do not have shallow-dipping thrust focal mechanisms, which are indicative of subduction zone plate interface earthquakes. In total, we redetermined epicenters and depths for over 70 earthquakes at the Alaskan, Aleutian, Kamchatka, Kuril, Japan, and Izu-Bonin-Mariana trenches. We find that at most Pacific subduction zones there is evidence for extensional faulting down to 10-15 km within the top of the oceanic plate mantle, and in total, 95% of our analyzed extensional outer rise events occur within the crust or top 15 km of the mantle. However some regions, such as the Bonin and Aleutian Islands, show evidence for extensional faulting as deep as 20 km below the base of the crust. If the mantle of the subducting slab is hydrated down to ~15 km (with ~2-3.5 wt. % water), and assuming published values for the amount of water in the slab crust [1], then we expect that ~10^10 Tg/Myr of water are input into Northwestern Pacific subduction zones. This value for only the Northwestern Pacific subduction zones is then 10 times larger than previous global estimates [1] and indicates a need to reevaluate recent subduction water flux calculations. [1] Van Keken et al (2011), JGR, 116, B01401.

  20. Sharp Permeability Transitions due to Shallow Diagenesis of Subduction Zone Sediments

    NASA Astrophysics Data System (ADS)

    James, S.; Screaton, E.

    2013-12-01

    The permeability of hemipelagic sediments is an important factor in fluid flow in subduction zones and can be affected by porosity changes and cementation-dissolution processes acting during diagenesis. Anomalously high porosities have been observed in cores from the Shikoku Basin sediments approaching the Nankai Trough subduction zone. These high porosities have been attributed to the presence of minor amounts of amorphous silica cement that strengthen the sediment and inhibit consolidation. The porosity rapidly drops from 66-68% to 54-56% at a diagenetic boundary where the amorphous silica cement dissolves. Although the anomalous porosity profiles at Nankai have received attention, the magnitude of the corresponding permeability change has not been addressed. In this study, permeability profiles were constructed using permeability-porosity relationships from previous studies, to estimate the magnitude and rate of permeability changes with depth. The predicted permeability profiles for the Nankai Trough sediment cores indicate that permeability drops by almost one order of magnitude across the diagenetic boundary. This abrupt drop in permeability has the potential to facilitate significant changes in pore fluid pressures and thus to influence the deformation of the sediment onto the accretionary prism. At the Costa Rica subduction zone, results vary with location. Site U1414 offshore the Osa Peninsula shows porosities stable at 69% above 145 mbsf and then decrease to 54% over a 40 m interval. A porosity drop of that magnitude is predicted to correlate to an order of magnitude permeability decrease. In contrast, porosity profiles from Site 1039 offshore the Nicoya Peninsula and Site U1381 offshore the Osa Peninsula show anomalously high porosities but no sharp drop. It is likely that sediments do not cross the diagenetic boundary due to the extremely low (<10°C/km) thermal gradient at Site 1039 and the thin (<100 m) sediment cover at Site U1381. At these locations, the porosity loss and permeability reduction may occur after the sediment is subducted and contribute to high pore pressures at the plate boundary.

  1. Seismological investigations of the subduction zone plate interface: New advances and challenges

    NASA Astrophysics Data System (ADS)

    Rietbrock, Andreas; Garth, Tom; Hicks, Stephen

    2015-04-01

    In the last decade, huge advances have been made in analysing the slip distribution of large megathrust earthquakes and how slip relates to geodetic locking, shedding light on the character of the seismic cycle in subduction zones. Recently, a number of studies have suggested that at convergent plate boundaries, geodetic locking may be closely related to slip distribution of subsequent large earthquakes, as found recently for the Maule 2010 and Tohoku 2011 earthquakes. However, the physical (e.g. seismic) properties along the subduction zone interface are still poorly constrained, posing a major limitation to our physical understanding of both geodetic locking and earthquake rupture process. Here, we present high-resolution seismic tomography results (P- and S-wave velocity), as well as earthquake locations to make a detailed investigation of seismic properties along the portion of the plate interface that ruptured during the 2010 Maule earthquake. Additionally, to test the robustness of our models, we performed numerous numerical tests including changes to the parameterization, synthetic recovery tests and bootstrap analysis. We find P-wave velocities of about 5.7 km/s at 10 km depth and linearly increasing to 7.5 km/s at a depth of 30 km. Between 30 km and 43 km, P-wave velocities are relatively constant at around 7.5 km/s before a subsequent increase to 8.3 km/s at larger depths (>60 km) is observed. The Poisson's ratio is significantly elevated, at values of up to 0.35 at shallow depths of 10km to 15km, before reaching less elevated values of 0.28-0.29 in the depth range between 20km and 43km. Comparison of these velocities to petrological models shows good agreement below 30 - 50 km depth. At shallower depths though P-wave velocities are significantly lower, which together with the elevated poisons ratio indicates that this portion of the mega thrust is highly hydrated, suggesting that material properties may in part control the seismogenic character of subduction megathrusts Comparison of our findings to other regional tomographic models from other subduction zones worldwide (Japan, Sumatra) shows excellent agreements with our results.

  2. The Effects of the Brittle-Ductile Transition on the India-Asia Intracontinental Subduction Zone

    NASA Astrophysics Data System (ADS)

    Cannon, J. M.; Murphy, M. A.

    2014-12-01

    The intracontinental subduction zone between India and Asia is termed the Main Himalayan thrust. Beneath the range front and Lesser Himalaya the subduction interface is locked and only ruptures during great earthquakes. However beneath the High Himalaya and southern Tibet a rheologic transition from brittle to ductile deformation along the subduction zone has been defined based on microseismicity, geodesy, and thermal modelling. Geodetic data suggests that the crust above the transition zone strains to accommodate much of India-Asia interseismic convergence as southern Tibet creeps toward the locked portion of the Main Himalayan thrust. We combine regional geology with stream profile analysis, thermal modelling, geodesy, and microseismicity to understand spatial and temporal characteristics of the intracontinental subduction in this region of the Himalaya. This continuous flux of material against the locked portion of the subduction zone stores elastic strain which drives great earthquakes that rupture the plate boundary from southern Tibet to the range front. While the brittle-ductile transition zone stores a significant amount of interseismic strain it has also been a locus of permanent shortening and crustal thickening as shown by the presence of the Dolpo and Manaslu folds in Nepal and Kakhtang thrust in Bhutan, which were all active in the Middle Miocene. The forelimb of the Dolpo fold, a 400 km long 50 km half wavelength gentle fold, coincides with a 200 km x 50 km swath of oversteepened river channels indicative of rapid rock uplift suggesting the fold is currently being uplifted and shortened. The Dolpo fold experienced persistent shortening and crustal thickening despite being located adjacent to, and active concurrently with the Gurla Mandhata core complex indicating simultaneous N-S shortening and E-W extension implying a constrictional train field since the Middle Miocene. We interpret the Dolpo fold to reflect an active antiformal duplex along the Main Himalayan thrust in which shear zones stack ductile lower-middle crust and propagate into the seismogenic upper crust becoming nucleation sites for microseismicity. We predict the position of the duplex to be controlled not by a footwall ramp but rather by the transition from ductile to brittle rheology along the Main Himalayan thrust.

  3. Seismic characteristics of outer-rise earthquakes in the different seismic coupling subduction zones

    NASA Astrophysics Data System (ADS)

    Lee, Hsin-Hua; Lin, Jing-Yi

    2013-04-01

    Characterizing the seismogenic zone of major subduction plate boundaries provides us a possible to reduce large earthquakes hazard. In the past several decades, many scientists have analyzed various geophysical methods and datasets, such as seismic and geodetic ground motion data, historical tsunami deposits, aftershock distributions, and seafloor bathymetry, trying to understand the mechanisms behind great devastating earthquakes, and to estimate the probability of a major earthquake occurrence in the future. In this study, by using the global earthquake catalog (GCMT) from January 1, 1976 to December 31, 2011. We firstly re-examines the outer-rise earthquake model proposed by the Christensen (1988) at the subduction zones suggested to have different coupling levels. The compressive stress cumulated during the subducting processes are often reflected by the occurrence of compressional outer-rise earthquakes. Thus, in the region where the compressional outer-rise earthquakes take place without any corresponding large underthrusting earthquakes, the seismic potential is usually considered to be high. We re-examined the high seismic potential areas determined by this criteria in Christensen (1988) and confirm that the large underthrusting earthquakes did really occur in the 30 years following the appearance of compressional outer-rise events, such as in Tonga region in the vicinity of 20S, a Mw 8.3 large earthquake occurred in 2006. This result represents that the outer-rise earthquake model could be an indicator for the generation of large earthquakes along subduction zones. In addition, to have a more accurate estimation for the seismic potential, we discuss the relationship between the generation of earthquakes and the change of cumulative gravitational potential energy caused by earthquakes (ΔGPE) over time. Our result shows an acceleration of ΔGPE before large earthquakes. Our result also shows that the extensional outer-rise events for strong seismic coupling subduction zone only presented after the occurrence of earthquakes with magnitude larger than 8, for instance, after the 2012 March Mw 9.0 Tohoku, the 2010 February Mw 8.8 Chili and the 2006 November Mw8.3 Kamchatka earthquakes, which is consistent with the analysis performed by Christensen (1988). Based on our analysis, the outer rise earthquakes occur immediately after the main event which does not coincide with the result stating in Christensen (1988) that they occur in the 30 years after the earthquake. In addition, the duration of the extensional outer-rise earthquakes occurrence appears to be correlated with its magnitude. Meanwhile, for the earthquakes with magnitude smaller than 8, as well as in the weak coupling areas, this observation is not engaged.

  4. Searching for slow-slip events at the Sumatran subduction zone

    NASA Astrophysics Data System (ADS)

    Feng, L.; Hill, E.; Qiu, Q.; Elosegui, P.; Banerjee, P.; Hermawan, I.; Lubis, A.; Barbot, S.; Sieh, K.

    2013-12-01

    Slow-slip events were first discovered on the deeper Cascadia subduction interface about a decade ago. Since then, slow-slip events have been observed at various subduction environments including Cascadia, Nankai, Alaska, New Zealand, Mexico, and Costa Rica. Conspicuously absent from this list, however, is the Sumatran subduction zone, along which no slow-slip events have yet been discovered. As one of the most active subduction zones currently in the world, the Sumatran subduction zone has experienced a series of great earthquakes over the last decade, along with numerous moderate and smaller earthquakes. We conducted a systematic search for transient aseismic slip in time series from the Sumatran GPS Array (SuGAr) between 2002 and 2012. We reprocessed the time series using GIPSY 6.2, and included the most up-to-date atmosphere and ocean models in processing to reduce day-to-day scatter of the raw time series. The raw data are dominated signals from coseismic and postseismic deformation. In order to search for much smaller transient signals, on the order of a few millimeters, we estimated and removed long-term background rates simultaneously with a large number of earthquake parameters, thus generating daily residuals. After our careful inspection of the daily residuals, we concluded that no episodic slip events have been recorded. Regarding non-episodic slip events, one station (BSAT) recorded a rate change in a few months immediately after the 2004 Sumatra-Andaman earthquake. Some other nearby stations that were installed later recorded a portion of this change. However, there were no other independent observations to confirm if this signal was indeed slow-slip. Except this suspicious signal, no other positive evidence could provide support for the occurrence of slow-slip events in Sumatra. This may indicate that slow-slip events have not occurred on the Sunda megathrust over the last decade, but may also highlight the limitation of the SuGAr network to detect slow-slip events. In testing the capability of the SuGAr network to resolve slow-slip events along the Sumatran subduction interface similar to those observed at Cascadia (~Mw 6.7) and Nankai (~Mw 6.0), we show that the current SuGAr network cannot fully resolve slow-slip events in the deeper part. This information is useful for augmenting and strengthening the network in the future.

  5. Marine electromagnetics: A new tool for mapping fluids at subduction zones

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    The recent adoption of marine electromagnetic (EM) methods by the hydrocarbon exploration industry has driven technological innovations in acquisition hardware and modeling software that have created new opportunities for studying plate boundary structure at subduction zones. Because the bulk electrical resistivity measured by EM surveys is strongly dependent on crustal porosity and hence fluid content, EM data can provide valuable constraints on crustal hydration in the incoming oceanic plate, fluids released through sediment compaction and dehydration reactions occurring after the plate is subducted, and fluids escaping through the overlying forearc crust. Since water also plays an important role in regulating subduction earthquake processes and frictional behavior along the plate boundary, EM data have the potential to reveal new insights on the causes of large subduction zone earthquakes and their potential for generating tsunamis. As a demonstration of this novel technique, we present new results from the first controlled-source EM survey of a subduction zone, carried out at the Middle America Trench offshore Nicaragua in 2010. During this survey 50 seafloor EM receivers were deployed along a 280 km profile extending from the abyssal plain, across the trench and onto the forearc. Controlled-source EM signals were broadcast to the receivers by deep-towing a low-frequency electric dipole transmitter close to the seafloor along the entire survey profile, generating diffusive EM waves that traveled through the crust and uppermost mantle. Non-linear two-dimensional inversion of the data reveals a significant decrease in crustal resistivity with the onset of bending faults at the trench outer rise and images a continuous zone of low resistivity porous sediments being carried down with the subducting plate to at least 10 km down dip from the trench. Further landward at about 25 km from the trench, a sub-vertical low-resistivity zone extending from the plate boundary into the overlying forearc crust is consistent with the fluid release expected from the smectite-illite transformation and occurs directly beneath the location of known seafloor fluid seeps. Potential future surveys at other margins such as Cascadia, Alaska, New Zealand and Japan and integrated interpretation with other geophysical, geochemical and geological studies offers the chance for greatly enhancing our understanding of subduction processes.

  6. Phengite-hosted LILE enrichment in eclogite and related rocks: Implications for fluid-mediated mass transfer in subduction zones and arc magma genesis

    USGS Publications Warehouse

    Sorensen, Sorena S.; Grossman, J.N.; Perfit, M.R.

    1997-01-01

    Geochemical differences between island arc basalts (LAB) and ocean-floor basalts (mid-ocean ridge basalts; MORB) suggest that the large-ion lithophile elements (LILE) K, Ba, Rb and Cs are probably mobilized in subduction zone fluids and melts. This study documents LILE enrichment of eclogite, amphibolite, and epidote ?? garnet blueschist tectonic blocks and related rocks from melanges of two subduction complexes. The samples are from six localities of the Franciscan Complex, California, and related terranes of Oregon and Baja California, and from the Samana Metamorphic Complex, Samana Peninsula, Dominican Republic. Most Franciscan blocks are MORB-like in their contents of rare earth elements (REE) and high field strength elements (HFSE); in contrast, most Samana blocks show an LAB signature of these elements. The whole-rock K2O contents of both groups range from 1 to 3 wt %; K, Ba, Rb, and Cs are all strongly intercorrelated. Many blocks display K/Ba similar to melasomatized transition zones and rinds at their outer margins. Some transition zones and rinds are enriched in LILE compared with host blocks; others are relatively depleted in these elements. Some LILE-rich blocks contain 'early' coarse-grained muscovite that is aligned in the foliation defined by coarse-grained omphacite or amphibole grains. Others display 'late' muscovite in veins and as a partial replacement of garnet; many contain both textural types. The muscovite is phengite that contains ???3??25-3??55 Si per 11 oxygens, and ???0??25-0??50 Mgper 11 oxygens. Lower-Si phengite has a significant paragonite component: Na per 11 oxygens ranges to ???0??12. Ba contents of phengite range to over 1 wt % (0??027 per 11 oxygens). Ba in phengite does not covary strongly with either Na or K. Ba contents of phengite increase from some blocks to their transition zones or rinds, or from blocks to their veins. Averaged KlBa ratios for phengite and host samples define an array which describes other subsamples of the block and other analyzed blocks. Phengite carries essentially all of the LILE in otherwise mafic eclogite, amphibolite, and garnet blueschist blocks that are enriched in these elements compared with MORE. It evidently tracks a distinctive type of LILE metasomatism that attends both high-T and retrograde subduction zone metamorphism. An obvious source for the LILE is a fluid in equilibrium with metasedimentary rocks. High-grade semipelitic schists from subduction complexes and subductable sediment display LILE values that resemble those seen in the most LILE-rich blocks. Modeling of Ba and Ti suggests that 1-40 wt % of phengite added to MORB can produce their observed LILE enrichment. Thus, the release of LILE from such rocks to fluids or melts in very high-T and -P parts of subduction zones probably depends critically on the stability and solubility relations of phengite, which is thought to be stable at pressures as high as 95-110 kbar at T= 750-1050??C.

  7. New U-Pb Age and Trace Element Composition of Young Metamorphic Zircon Rims from the UHP Tso Morari Complex, NW Himalaya, Distinguishes Peak from Retrograde Metamorphism

    NASA Astrophysics Data System (ADS)

    Leech, M. L.; Coble, M. A.; Singh, S.; Guillot, S.; Jain, A. K.

    2014-12-01

    The ultrahigh-pressure (UHP) Tso Morari Complex (TMC) sits in the footwall of the Indus-Yarlung suture zone in the NW Himalaya. The timing of metamorphism during subduction and exhumation in the complex is critical to constraining the age of the India-Asia collision. de Sigoyer et al. (2000) and Leech et al. (2005) reported mean U-Pb ages for thin outer rims of sectioned zircon between 55 ± 6 Ma and 53.3 ± 0.7 Ma, respectively, for the age of peak UHP through retrograde metamorphism, and Leech et al. (2005) used these data to calculate the minimum age for the start of continental subduction at 57 ± 1 Ma. Recently published results for the TMC have reignited debate on the age of metamorphism and thus, the timing of India-Asia collision. We used the same TM38 sample analyzed for the results described in Leech et al. (2005) and performed new SIMS U-Pb depth-profiling analyses to target only the outermost ~1.5 micron rims of zircon. Our results yield a mean age of 44.9 ± 0.7 Ma; adjacent spots for REE analyses yielded positive, enriched HREE profiles with negative Eu anomalies and corresponding Ti-in-zircon temperatures of ~550° to 680° C. Sharp boundaries between zircon domains are clearly resolved with CL and BSE imaging of TM38 zircons, and there is a large age difference between rims and protolith core ages; any mixing during depth-profiling through rims is clear. The positive HREE profiles imply the period of zircon growth in the TMC at c. 45 Ma to be retrograde. We suggest that the 47-43 Ma peak ages and flat heavy REE profiles with no Eu anomaly recently reported by Donaldson et al. (2013) on sectioned zircons, and interpreted as the age of UHP metamorphism of the TMC, may actually represent mixing between zircon rims and cores. The Leech et al. (2005) collision age of 57 ± 1 Ma assumed the TMC represents the leading edge of India. However, numerical modeling of Warren et al. (2008) suggests all exhumed material is derived from the central part of the pro-continental margin 200 km inboard of the ocean-continent transition. Recalculating using the geometric model of Leech et al. (2005) and the 53.3 ± 0.7 Ma age of peak UHP metamorphism yields a revised age of 61.1 ± 1.0 Ma for the entry of northwestern Greater Indian continental crust into the subduction zone beneath the Kohistan-Ladakh arc.

  8. Slow slip event within a gap between tremor and locked zones in the Nankai subduction zone

    NASA Astrophysics Data System (ADS)

    Takagi, Ryota; Obara, Kazushige; Maeda, Takuto

    2016-02-01

    We report on two small long-term slow slip events (SSEs) within a gap between tremor and locked zones in the Nankai subduction zone, southwest Japan. The SSEs were detected by subtracting the steady state component and postseismic effects of large earthquakes from long-term and high-density Global Navigation Satellite System data. Both SSEs occurred in adjacent regions of the Bungo channel following long-term SSEs in the Bungo channel in 2003 and 2010. The estimated slip was 1-5 cm/year that lasted at least 1-2 years after 2004 and 2011, partly accommodating plate convergence. As the low-frequency tremor in the downdip region is activated at the same time as the Bungo channel long-term SSE, a spatiotemporal correlation was observed between the detected SSEs and long-term tremor activity in the downdip region. This correlation indicates along-dip interaction of the slips on the subducting plate interface.

  9. In situ Raman spectroscopic investigation of the structure of subduction-zone fluids

    USGS Publications Warehouse

    Mibe, Kenji; Chou, I.-Ming; Bassett, William A.

    2008-01-01

    In situ Raman spectra of synthetic subduction-zone fluids (KAlSi3O8-H2O system) were measured to 900?? and 2.3 GPa using a hydrothermal diamond-anvil cell. The structures of aqueous fluid and hydrous melt become closer when conditions approach the second critical endpoint. Almost no three-dimensional network was observed in the supercritical fluid above 2 GPa although a large amount of silicate component is dissolved, suggesting that the physical and chemical properties of these phases change drastically at around the second critical endpoint. Our experimental results indicate that the fluids released from a subducting slab change from aqueous fluid to supercritical fluid with increasing depth under the volcanic arcs. Copyright 2008 by the American Geophysical Union.

  10. High-resolution image of the North Chilean subduction zone: seismicity, reflectivity and fluids

    NASA Astrophysics Data System (ADS)

    Bloch, W.; Kummerow, J.; Salazar, P.; Wigger, P.; Shapiro, S. A.

    2014-06-01

    We obtained high-precision locations for 5250 earthquakes in the Iquique segment of the northern Chilean subduction zone from two temporary local seismic networks around 21°S. A double seismic zone in the downgoing Nazca slab can be clearly identified. One band of seismicity is located at the plate interface and a second one 20-25 km deeper in the oceanic mantle. It can be traced updip to uncommonly shallow levels of 50 km. A combined interpretation of seismicity and reflectivity along the seismic ANCORP'96 experiment suggests the prevalence of fluid processes in the subducted oceanic crust as well as in the uppermost 20 km of the mantle. Crustal seismicity is pervasive below the Coastal Cordillera. Beneath the Precordillera, the lower bound of crustal seismicity delineates a sharp west-dipping boundary down to 20 km depth, consistent with earlier findings indicating a rheological boundary.

  11. Long-Term Perspectives on Giant Earthquakes and Tsunamis at Subduction Zones

    NASA Astrophysics Data System (ADS)

    Satake, Kenji; Atwater, Brian F.

    2007-05-01

    Histories of earthquakes and tsunamis, inferred from geological evidence, aid in anticipating future catastrophes. This natural warning system now influences building codes and tsunami planning in the United States, Canada, and Japan, particularly where geology demonstrates the past occurrence of earthquakes and tsunamis larger than those known from written and instrumental records. Under favorable circumstances, paleoseismology can thus provide long-term advisories of unusually large tsunamis. The extraordinary Indian Ocean tsunami of 2004 resulted from a fault rupture more than 1000 km in length that included and dwarfed fault patches that had broken historically during lesser shocks. Such variation in rupture mode, known from written history at a few subduction zones, is also characteristic of earthquake histories inferred from geology on the Pacific Rim.

  12. Nonlinear study of seismicity in the Mexican subduction zone by means of visual recurrence analysis

    NASA Astrophysics Data System (ADS)

    Ramirez Rojas, A.; Moreno-Torres, R. L.

    2012-12-01

    The subduction in the Mexican South Pacific coast might be approximated as a subhorizontal slab bounded at the edge by the steep subduction geometry of the Cocos plate beneath the Caribbean plate to the east and of the Rivera plate beneath North America to the west. Singh et al. (1983), reported a study that takes into account the geometry of the subducted Rivera and Cocos plates beneath the North American lithosphere defining, according their geometry, four regions: Jalisco, Michoacán, Guerrero and Oaxaca. In this work we study the seismicity occurred in Mexico, for each region, by means of the visual recurrence analysis (VRA). Our analysis shows important differences between each region that could be associated with nonlinear dynamical properties of each region. Singh, S.K., M. Rodriguez, and L. Esteva (1983), Statistics of small earthquakes and frequency of occurrence of large earthquakes along the Mexican subduction zone, Bull. Seismol. Soc. Am. 73, 6A, 1779-1796.

  13. Long-term perspectives on giant earthquakes and tsunamis at subduction zones

    USGS Publications Warehouse

    Satake, K.; Atwater, B.F.

    2007-01-01

    Histories of earthquakes and tsunamis, inferred from geological evidence, aid in anticipating future catastrophes. This natural warning system now influences building codes and tsunami planning in the United States, Canada, and Japan, particularly where geology demonstrates the past occurrence of earthquakes and tsunamis larger than those known from written and instrumental records. Under favorable circumstances, paleoseismology can thus provide long-term advisories of unusually large tsunamis. The extraordinary Indian Ocean tsunami of 2004 resulted from a fault rupture more than 1000 km in length that included and dwarfed fault patches that had broken historically during lesser shocks. Such variation in rupture mode, known from written history at a few subduction zones, is also characteristic of earthquake histories inferred from geology on the Pacific Rim. Copyright ?? 2007 by Annual Reviews. All rights reserved.

  14. The impact of megasplay faulting and permeability contrasts on Nankai Trough subduction zone pore pressures

    NASA Astrophysics Data System (ADS)

    Screaton, Elizabeth J.; Ge, Shemin

    2012-11-01

    Results of a coupled fluid flow and deformation model indicate that megasplay fault slip may impact excess pore pressures within an accretionary prism. In a simulation with homogeneous low permeability representative of clay-rich sediment, excess pore pressures build where compression occurs in the hanging wall and directly beneath the splay fault. In another simulation with heterogeneous permeability, the impact of high permeability fault zones and turbidite horizons lessens the effect of the megasplay fault slip. Overall, excess pore pressures are lower for the heterogeneous simulation than for the simulation with uniform low permeability. Within the footwall of the megasplay fault zone, simulated pore pressures increase across a transition from more permeable uplifted turbidites to less permeable clay-rich sediments. Strong permeability contrasts might provide an alternate explanation for a previously observed decrease in seismic velocity beneath the megasplay fault of the Nankai Trough subduction zone.

  15. Seismotectonics of accretive versus erosive subduction zones - insights from analog seismic cycle simulation

    NASA Astrophysics Data System (ADS)

    Rosenau, M.; Bachmann, R.; Oncken, O.

    2007-12-01

    Accretive and erosive subduction zones differ both in their forearc structure and seismic release character. For instance the greatest historical megathrust earthquakes concentrated along accretive margins, tectonically characterized by forearc shortening (e.g. Sumatra, Southern Chile, Alaska), whereas erosive margins, tectonically characterized by forearc extension (e.g. Peru, Kuriles), have often been the locus of tsunami earthquakes (i.e. slow and shallow events). Here we investigate the implied link between internal forearc deformation and megathrust seismogenesis and its implications for seismic hazard in subduction zones. We interpret quasi two- dimensional plastoelastic (allowing deformation to localize, permanent shortening dominates) and elastoplastic (elastic deformation dominates, minor internal deformation) granular wedge models as analogs of accretive and erosive subduction forearcs, respectively, overlying a rate-state frictional plate interface which represents a seismogenic megathrust. Experimental observations support current hypotheses that internal forearc deformation is controlled by stress changes associated with the megathrust seismic cycle: Consistent with the theory of dynamic Coulomb wedges, coseismic compression at the updip limit of great earthquakes triggers shallow postseismic forearc deformation. Plastic shortening of the outer forearc wedge and shallow afterslip both are interpreted as transient postseismic relaxation mechanisms with the first being dominant in plastoelastic/accretive settings and the second being dominant in elastoplastic/erosive settings. Interseismically, permanent crustal shortening localizes in both settings above the downdip limit of great earthquakes and may lead to uplift of a coastal cordillera. Longterm coastal uplift rates at elastoplastic/erosive margins are about one order of magnitude lower than in plastoelastic/accretive settings, and associated with permanent crustal extension above the seismogenic zone. Extension here appears as an effect of postseismic relaxation coeval to interseismic reloading. Internal shortening of plastoelastic/accretive margins reduces the subduction velocity stretching the recurrence intervals. We found, however, no evidence for that longer recurrence intervals lead to larger earthquake slip (e.g. due to stronger healing effects). Consequently, the seismic moment release rate tends to be smaller at accretive than at erosive margins, at least in the 2D case presented here. Since megathrust earthquakes of magnitude 8 and higher gain their size not primarily by increasing coseismic slip but by unzipping larger parts of the plate interface along strike of the subduction zone, the historical concentration of great events along accretive margins, if statistically significant, is probably a 3D effect associated with lateral rupture propagation. These findings may have important implications for seismic hazard in subduction zones since (1) forearc anatomy is suggested to reflect the seismogenic structure at depth, for instance in accretive settings, (2) erosive settings tend to release elastic energy by shallow megathrust afterslip and thus have a generally higher risk of tsunami earthquakes than accretive margins and (3) erosive margins are characterized by more frequent megathrust earthquakes than accretive margins. Very great earthquakes along accretive margins may be triggered either by a longer wavelength along-strike-segmentation of the subduction interface or by more synchronized seismic cycles of neighboring segments compared to erosive margins thus boosting lateral rupture propagation during a single giant event.

  16. Geodetic Observations of Interseismic Strain Segmentation at the Sumatra Subduction Zone

    NASA Technical Reports Server (NTRS)

    Prawirodirdjo, L.; Bock, Y.; McCaffrey, R.; Genrich, J.; Calais, E.; Puntodewo, S. S. O.; Subarya, C.; Rais, J.; Zwick, P.; Fauzi

    1997-01-01

    Deformation above the Sumatra subduction zone, revealed by Global Positioning System (GPS) geodetic surveys, shows nearly complete coupling of the forearc to the subducting plate south of 0.5 deg S and half as much to north. The abrupt change in plate coupling coincides with the boundary between the rupture zones of the 1833 and 1861 (Mw greater than 8) thrust earthquakes. The rupture boundary appears as an abrupt change in strain accumulation well into the interseismic cycle, suggesting that seismic segmentation is controlled by properties of the plate interface that persist occupied through more than one earthquake cycle. Structural evidence indicates that differences in basal shear stress may be related to elevated pore pressure in the north.

  17. Deep low-frequency earthquakes in tremor localize to the plate interface in multiple subduction zones

    USGS Publications Warehouse

    Brown, J.R.; Beroza, G.C.; Ide, S.; Ohta, K.; Shelly, D.R.; Schwartz, S.Y.; Rabbel, W.; Thorwart, M.; Kao, H.

    2009-01-01

    Deep tremor under Shikoku, Japan, consists primarily, and perhaps entirely, of swarms of low-frequency earthquakes (LFEs) that occur as shear slip on the plate interface. Although tremor is observed at other plate boundaries, the lack of cataloged low-frequency earthquakes has precluded a similar conclusion about tremor in those locales. We use a network autocorrelation approach to detect and locate LFEs within tremor recorded at three subduction zones characterized by different thermal structures and levels of interplate seismicity: southwest Japan, northern Cascadia, and Costa Rica. In each case we find that LFEs are the primary constituent of tremor and that they locate on the deep continuation of the plate boundary. This suggests that tremor in these regions shares a common mechanism and that temperature is not the primary control on such activity. Copyright 2009 by the American Geophysical Union.

  18. A slow slip event in the south central Alaska Subduction Zone and related seismicity anomaly

    NASA Astrophysics Data System (ADS)

    Wei, Meng; McGuire, Jeffrey J.; Richardson, Eliza

    2012-08-01

    We detected a slow slip event in the south central Alaska Subduction Zone by analyzing continuous GPS data from the Plate Boundary Observatory (PBO) network. The slow slip event started in early 2010 at a depth of 35 km beneath the Cook Inlet, near the down-dip end of the locked zone, and is ongoing as of November 2011 with an accumulated magnitude of Mw 6.9. Analysis of the earthquake catalog in the same area using the stochastic Epidemic Type Aftershock Sequence model (ETAS) shows a small but detectable seismicity increase during the slow slip event. We also find a change in seismicity rate around 1998, that may suggest an earlier slow slip event in the same region. Slow slip events in Alaska appear more widespread than previously thought but have remained undetected due to their long durations, the time intervals between them, and the limited time records from the continuous GPS.

  19. Reaction Weakening of Dunite in Friction Experiments at Hydrothermal Conditions and Its Relevance to Subduction Zones

    NASA Astrophysics Data System (ADS)

    Moore, D. E.; Lockner, D. A.

    2014-12-01

    To improve our understanding of processes occurring in the mantle wedge near the downdip limit of seismicity in subduction zones, we conducted triaxial friction tests on dunite gouge at temperatures in the range 200-350°C, 50 MPa fluid pressure and 100 MPa effective normal stress. Dunite, quartzite, and granite forcing blocks were used respectively to approximate changing rock/fluid chemistry with decreasing distance above the subduction thrust. All experiments were characterized by an initial increase in frictional strength to a peak value, followed by a decrease associated with shearing-enhanced alteration of the dunite gouge. Reaction products and the extent of weakening varied with the chemical environment. In the dunite-block experiments, strength gradually declined from the peak value to a coefficient of friction, µ ~ 0.5-0.6, consistent with the frictional strength of serpentine that formed on the shear surfaces from alteration of the gouge. Interaction of dunite gouge with quartzite and granite driving blocks resulted in significantly greater weakening, to μ ~ 0.3, at temperatures of 250°C and higher. Talc and serpentine partly replaced dunite gouge sheared between quartzite blocks, and metastable saponitic smectite clays crystallized in dunite sheared between granite blocks, as a result of fluid-assisted chemical exchange with the minerals in the wall rocks. These results suggest that rapid and substantial weakening can occur in the mantle wedge immediately overlying the subducting slab. Whichever the chemical environment, attainment of peak strength typically was accompanied by oscillatory slip with small stress drops that gradually was replaced by stable slip with increasing displacement. This oscillatory behavior in some ways resembles the tremor events that have been reported near the forearc mantle corner in subduction zones, and it may indicate the possible involvement of mineral reactions in some instances of tremor.

  20. Helium solubility in mica and mechanisms for deep transport of noble gases in subduction zones

    NASA Astrophysics Data System (ADS)

    Jackson, C.; Parman, S. W.; Kelley, S. P.; Cooper, R. F.

    2012-12-01

    We have experimentally determined helium solubility in mica to explore possible transport mechanisms of noble gases in subduction zones. Helium solubility in single crystals of muscovite and F-phlogopite investigated is relatively high, ~1 [He]/PHe (ppm/kbar). This solubility is approximately two orders of magnitude greater than values recently measured for olivine [1], and similar to values recently measured for amphibole with a low density of unoccupied ring sites [2]. Helium was dissolved into the micas by subjecting them to a high pressure noble gas atmosphere (1.26-1.48 He-Ne-Ar kbar) at moderate temperatures (450-700 C), allowing the micas to diffusively equilibrate with the imposed helium fugacity. Diffusion of Ne and Ar is too slow in both micas at the explored conditions to quantify their solubility. Experiments were conducted using a TZM gas pressure medium apparatus (Brown University). Analysis was completed by noble gas LA-MS (Open University, UK). Muscovite (dioctehedral) and F-phlogopite (trioctehedral) represent the two basic structural groups of micas, suggesting micas stabilized at higher pressures and temperatures, such as phengite, can provide a relatively deep transport mechanism for noble gases delivered to subduction zones. Thus, phengite may play a role in explaining atmospheric signatures in mantle derived noble gases [e.g.3,4]. 1. Heber, V. S., Brooker, R. A., Kelley, S. P. & Wood, B. J., GCA, 71, 1041-1061 2. Jackson C.R.M, Kelley S.P., Parman S.W., Cooper R.F., Goldschmidt 2012 Abstract 3. Holland, G. & Ballentine, C. J., Nature 441, 186-191 4. Mukhopadhyay, S.. Nature 486, 101-104

  1. Jurassic Early Cretaceous intermediate virtual geomagnetic poles and Pangaean subduction zones

    NASA Astrophysics Data System (ADS)

    Vizán, Haroldo; Van Zele, María Andrea

    2008-02-01

    The objective of this paper is to show that the distribution of Jurassic-Early Cretaceous intermediate virtual geomagnetic poles (VGPs) seems to be conditioned by Pangaean subducted slabs. Palaeomagnetic data from between ˜ 200Ma and 125Ma were compiled from reliable studies and their VGPs repositioned in their Jurassic-Early Cretaceous geographic location considering a "zero-longitude" motion of Africa over the last 200m.y. and the corresponding palaeomagnetic poles from each sequence. Those repositioned VGPs lying between latitudes of ± 60° were considered to be intermediate. To avoid bias as a function of simple sampling numbers for those sequences with more data, each VGP was weighted by Love's methodology. A colour-scale map (shadow-scale map in printed issue) of density of the weighted intermediate VGPs was obtained and compared with the Pangaean subduction zones. There is a good visual correlation between the distribution of these VGPs and the location of the subduction zones during the Jurassic, suggesting that there is a relationship between the Jurassic-Early Cretaceous geomagnetic reversals and the plate tectonic setting at that time. Minima of intermediate VGPs correlate well with the absence of VGPs predicted with a tomographic model and the intermediate VGP distribution is also well correlated with zones of faster seismic wave propagation in the lower mantle (just above of the core-mantle boundary), which suggests that the Jurassic geomagnetic polarity transitions could have been controlled by a structure of the core-mantle boundary similar to that at the Present time. We suspect that the subducted lithospheric slabs refrigerated the deepest mantle causing more heat than average flowing out from the core and controlling the geometry of the Jurassic-Early Cretaceous polarity transitions. The Earth's lithospheric plate motion history could have played a controlling role in the geometry of the geomagnetic reversals.

  2. Offshore Structure of the Cascadia Subduction Zone from Full-wave Tomography

    NASA Astrophysics Data System (ADS)

    Gao, H.

    2014-12-01

    We construct a preliminary offshore model of the crust and uppermost mantle at the Cascadia subduction zone using a full-wave tomographic method. We include the ocean bottom seismometers deployed by the Cascadia Initiative community experiment and Neptune Canada from 2011-2013, and the available broadband stations on land. We have extracted the empirical Green's functions from continuous seismic records on the vertical components of the OBS and inland station pairs with a frequency-time normalization method, which provide useful Rayleigh-wave signals within the periods of 7-50 s. We have also selected ~50 regional earthquakes between 2011-2013 offshore of the Cascadia subduction zone, which generated useful surface-wave signals up to 75 s period. We simulate wave propagation within a 3D Earth structure using a finite-difference method to generate a station Strain Greens Tensor database and synthetic waveforms. Rayleigh wave phase delays are obtained by cross-correlating the observed and synthetic waveforms. The sensitivity kernels of Rayleigh waves on the perturbations of Vp and Vs are calculated based on the Strain Greens Tensor database. We then invert for the velocity perturbation from the reference model and progressively improve the model resolution. Our preliminary full-wave tomographic imaging using the EGFs and earthquake Rayleigh waves shows: (1) Segmented low-velocity anomalies along the forearc, which are spatially correlated with the patterns of offshore basins and high slip patches; (2) Low velocities beneath the Blanco fracture zone; (3) The distribution of pseudofaults defines the seismic velocity heterogeneities; and (4) A low-velocity zone beneath the oceanic Moho near the trench, which may indicate serpentinization of the mantle lithosphere.

  3. Crustal Input into the Mantle through Subduction Zones, an Isotopic Perspective

    NASA Astrophysics Data System (ADS)

    Chauvel, C.

    2014-12-01

    Subduction zones are key areas on Earth where material once exposed at the surface is processed: part of the material almost instantaneously finds its way back to the surface in the island arcs but most of it is recycled into the mantle. However, all elements are not transferred in the same way from the surface to the island arc or deep mantle. Most fluid-mobile elements such as B, Li or U escape the subducted slab in arc magmas while others such as Hf and the heavy REE are mainly recycled into the mantle. Here I revisit the information provided by elements such as HFSE, REE, Pb and Sr whose isotopic compositions constrain their partitioning between arc magmas and the slab. A compilation of data on arcs around the globe shows that two main processes dominate the arc signature: in some cases subducted sediments contribute significant amounts of both fluid-mobile and immobile elements to arc magmas; in other cases they contribute very little, not even for the fluid-mobile elements. While Sr and Pb are highly enriched in lavas produced in fluid-dominated environments, their average Sr isotopic composition is only about 0.7035 and their average 206Pb/204Pb is about 18.5, values that are similar to those of average depleted mantle. This indicates that the fluids come mainly from the basaltic portion of the subducted slab. It also indicates that fluid-mobile elements in the subducted sediments are quantitatively sent back into the mantle unless the sediment melts, an unlikely situation in present-day subduction zones.

  4. Source Variations of Small Magnitude Events in the Downdip Region of the Mexican Subduction Zone.

    NASA Astrophysics Data System (ADS)

    Wang, R.; Bilek, S. L.; Brudzinski, M. R.; Cabral-Cano, E.; Arciniega-Ceballos, A.

    2014-12-01

    Slip in subduction zones produce a range of events including typical seismicity, slow slip events, and non-volcanic tremor. Understanding the nature of the transition between these different types of slip is an important area of study in order to advance our understanding of the conditions required for the spectrum of slip. The portion of the Middle America subduction zone underneath Oaxaca, Mexico is an ideal place to study this relationship because a local land based seismic and geodetic network lies above a large portion of the seismogenic zone to capture large and small magnitude earthquakes. This is also an area with slow slip events, non-volcanic tremor, and evidence for an ultra slow velocity layer near the downdip transitional region. In order to study the earthquakes at the downdip transition, we use a catalog of small magnitude (M < 5.5) locally recorded events from 2006 to 2012, prior to the 2012 Mw 7.4 Ometepec, Mexico earthquake. For each downdip event in the catalog, we compute first motion focal mechanisms using a standard community algorithm, FocMec. We see significant heterogeneity in our focal mechanism solutions. Our results show a lack of thrust events in an area with a known megathrust seismic gamp updid and prominent slow slip events observed downdip. We also find an abundance of thrust events in an area with an ultra slow layer and east of the slow slip events. Further analysis of stress drop will be used to study temporal patterns relative to the slow slip events as well as the Ometepec event.

  5. Fault plane orientations of deep earthquakes in the Izu-Bonin-Marianas subduction zone system

    NASA Astrophysics Data System (ADS)

    Myhill, R.; Warren, L. M.

    2011-12-01

    We present the results of directivity analysis on 45 deep earthquakes within the Izu-Bonin-Marianas subduction zone between 1993 and 2011. The age of the subducting Pacific plate increases from north to south along the trench, from 120 Ma offshore Tokyo to over 150 Ma east of the Mariana Islands. The dip of the deep slab generally increases from north to south, and is steep to overturned beneath the southern Bonin Islands and Marianas. Between 34 and 26 degrees north, a peak in seismicity at 350-450 km depth marks a decrease in dip as the slab approaches the base of the upper mantle. We observe directivity for around 60 percent of the analysed earthquakes, and use the propagation characteristics to find the best fitting rupture vector. In 60-70 percent of cases with well constrained rupture directivity, the best fitting rupture vector allows discrimination of the fault plane and the auxiliary plane of the focal mechanism. The identified fault planes between 100 km and 500 km are predominantly near-horizontal or south-southwest dipping. Rotated into the plane of the slab, the fault plane poles form a single cluster, since the more steeply dipping fault planes are found within more steeply dipping sections of slab. The dominance of near-horizontal fault planes at intermediate depth agrees with results from previous studies of the Tonga and Middle-America subduction zones. However, the presence of a single preferred fault plane orientation for large deep-focus earthquakes has not been previously reported, and contrasts with the situation for deep-focus earthquakes in the Tonga-Kermadec subduction system. Ruptures tend to propagate away from the top surface of the slab. We discuss potential causes of preferred fault plane orientations within subducting slabs in the light of existing available data, and the implications for mechanisms of faulting at great depths within the Earth.

  6. Characterizing Seismic Anisotropy across the Peruvian Flat-Slab Subduction Zone: Shear Wave Splitting from PULSE

    NASA Astrophysics Data System (ADS)

    Eakin, C. M.; Long, M. D.; Beck, S. L.; Wagner, L. S.; Tavera, H.

    2013-12-01

    Although 10% of subduction zones worldwide today exhibit shallow or flat subduction, we are yet to fully understand how and why these slabs go flat. An excellent study location for such a problem is in Peru, where the largest region of flat-subduction currently exists, extending ~1500 km in length (from 3 °S to 15 °S) and ~300 km in width. Across this region we investigate the pattern of seismic anisotropy, an indicator for past and/or ongoing deformation in the upper mantle. To achieve this we conduct shear wave splitting analyzes at 40 broadband stations from the PULSE project (PerU Lithosphere and Slab Experiment). These stations were deployed for 2+ years across the southern half of the Peruvian flat-slab region. We present detailed shear wave splitting results for deep and teleseismic events, making use of a wide variety of available phases that sample the upper mantle directly beneath the stations (such as SKS, SKKS, PKS, sSKS, SKiKS, ScS and local/direct S). We analyze the variability of our results with respect to initial polarizations and ray paths, as well as spatial variability between stations as the underlying slab morphology changes. Preliminary results show predominately NW-SE fast polarizations (trench oblique to sub-parallel) over the flat-slab region east of Lima. These results are consistent with observations of more complex multi-layered anisotropy beneath a nearby permanent station (NNA). Further south, towards the transition to steeper subduction, the splitting pattern becomes increasingly dominated by null measurements. Over to the east however, beyond Cuzco, where the mantle wedge might begin to play a role, we record fast polarizations quasi-parallel to the local slab contours. We carefully evaluate the different possible source locations within the subduction zone for this seismic anisotropy and observe increasing evidence for distinct anisotropy within the slab as well as the sub-slab mantle.

  7. Deformation Mechanisms of Antigorite Serpentinite at Subduction Zone Conditions Determined from Experimentally and Naturally Deformed Rocks

    NASA Astrophysics Data System (ADS)

    Auzende, A. L.; Escartin, J.; Walte, N.; Guillot, S.; Hirth, G.; Frost, D. J.

    2014-12-01

    The rheology of serpentinite, and particularly that of antigorite-bearing rocks, is of prime importance for understanding subduction zone proceses, including decoupling between the downwelling slab and the overriding plate, exhumation of high-pressure rocks, fluids pathways and, more generally, mantle wedge dynamics. We present results from deformation-DIA experiments on antigorite serpentinite performed under conditions relevant of subduction zones (1-3.5 GPa ; 400-650°C). We complemented our study with a sample deformed in a Griggs-type apparatus at 1 GPa and 400°C (Chernak and Hirth, EPSL, 2010), and with natural samples from Cuba and the Alps deformed under blueschist/eclogitic conditions. Our observations on experimental samples of antigorite deformed within its stability field show that deformation is dominated by cataclastic flow; we can only document a minor contribution of plastic deformation. In naturally deformed samples, deformation-related plastic structures largely dominate strain accommodation, but we also document a minor contribution of brittle deformation. When dehydration occurs in experiments, plasticity increases, and is coupled to local embrittlement attributed to hydraulic fracturating due to the migration of dehydration fluids. Our results thus show that semibrittle deformation operates within and above the stability field of antigorite. We also document that the corrugated structure of antigorite has a control on the strain accommodation mechanisms under subduction conditions, with preferred inter and intra-cracking along (001) and gliding along both a and b. Deformation dominated by brittle processes, as observed in experiments, may occur during deformation at elevated (seismic?) strain rates, while plastic deformation, as observed in naturally deformed rocks, may correspond instead to low strain rates instead (aseismic creep?). We also discuss the role of antigorite rheology and mode of deformation on fluid transport.

  8. Deformation mechanisms of antigorite serpentinite at subduction zone conditions determined from experimentally and naturally deformed rocks

    NASA Astrophysics Data System (ADS)

    Auzende, Anne-Line; Escartin, Javier; Walte, Nicolas P.; Guillot, Stéphane; Hirth, Greg; Frost, Daniel J.

    2015-02-01

    We performed deformation-DIA experiments on antigorite serpentinite at pressures of 1-3.5 GPa and temperatures of between 400 and 650 °C, bracketing the stability of antigorite under subduction zone conditions. For each set of pressure-temperature (P-T) conditions, we conducted two runs at strain rates of 5 ×10-5 and 1 ×10-4 s-1. We complemented our study with a sample deformed in a Griggs-type apparatus at 1 GPa and 400 °C (Chernak and Hirth, 2010), and with natural samples from Cuba and the Alps deformed under blueschist/eclogitic conditions. Optical and transmission electron microscopies were used for microstructural characterization and determination of deformation mechanisms. Our observations on experimentally deformed antigorite prior to breakdown show that deformation is dominated by cataclastic flow with observable but minor contribution of plastic deformation (microkinking and (001) gliding mainly expressed by stacking disorder mainly). In contrast, in naturally deformed samples, plastic deformation structures are dominant (stacking disorder, kinking, pressure solution), with minor but also perceptible contribution of brittle deformation. When dehydration occurs in experiments, plasticity increases and is coupled to local embrittlement that we attribute to antigorite dehydration. In dehydrating samples collected in the Alps, embrittlement is also observed suggesting that dehydration may contribute to intermediate-depth seismicity. Our results thus show that semibrittle deformation operates within and above the stability field of antigorite. However, the plastic deformation recorded by naturally deformed samples was likely acquired at low strain rates. We also document that the corrugated structure of antigorite controls the strain accommodation mechanisms under subduction conditions, with preferred inter- and intra-grain cracking along (001) and gliding along both a and b. We also show that antigorite rheology in subduction zones is partly controlled by the presence of fluids, which can percolate within the exhumation channel via deformation-induced interconnected porosity.

  9. Paleoseismic Records of Multiple Great Earthquakes from the Subduction Zones of Sumatra, Chile, and Alaska

    NASA Astrophysics Data System (ADS)

    Dura, T.; Horton, B.; Briggs, R. W.; Cisternas, M.; Ely, L. L.; Kelsey, H. M.; Nelson, A. R.; Rubin, C. M.

    2014-12-01

    Instrumental and historical records have proved too short to estimate the potential magnitudes and recurrence intervals of rare events such as the 2004 Indian Ocean and 2011 Tohoku-Oki great earthquakes and tsunamis. Paleoseismology improves our understanding of subduction zone hazards by extending earthquake histories thousands of years into the past. Through paleoseismic investigations at subduction zones, we scrutinize coastal sediments in low-energy depositional environments to reconstruct relative sea-level (RSL) changes related to upper-plate deformation from past earthquakes and tsunami inundation. Microfossils—long recognized as valuable RSL indicators—in coastal sediment provide an independent test of earthquake related RSL change and tsunami deposition inferred from coastal stratigraphy. Here, we illustrate the value and potential of paleoseismic methods at megathrust sites in Sumatra, Chile, and the eastern Aleutian Islands to identify both coseismic subsidence and uplift. In western Sumatra, coastal sediments beneath a lowland near the city of Padang contain stratigraphic evidence for two mid Holocene earthquakes, each resulting in > 1 m of coseismic subsidence. A site along the most populated portion of the central Chilean coast near Valparaíso records stratigraphic and diatom evidence of six instances of early to mid-Holocene coseismic uplift (< 1 m) accompanied by tsunamis. Coastal marsh sediment on the eastern Aleutian Island of Sitkinak contains stratigraphic and diatom evidence for three instances of uplift and two of subsidence during great earthquakes. Such biostratigraphic evidence of past great earthquakes and tsunamis shows that the absence of great megathrust events near our sites in the last 200-300 years is not representative of the potential for great events on the megathrusts. In addition, we discuss how our earthquake and tsunami records fit within the context of past regional RSL histories and emphasize the importance of RSL on the preservation and interpretation of evidence of past events.

  10. Could a Sumatra-like megathrust earthquake occur in the south Ryukyu subduction zone?

    NASA Astrophysics Data System (ADS)

    Lin, Jing-Yi; Sibuet, Jean-Claude; Hsu, Shu-Kun; Wu, Wen-Nan

    2014-12-01

    A comparison of the geological and geophysical environments between the Himalaya-Sumatra and Taiwan-Ryukyu collision-subduction systems revealed close tectonic similarities. Both regions are characterized by strongly oblique convergent processes and dominated by similar tectonic stress regimes. In the two areas, the intersections of the oceanic fracture zones with the subduction systems are characterized by trench-parallel high free-air gravity anomaly features in the fore-arcs and the epicenters of large earthquakes were located at the boundary between the positive and negative gravity anomalies. These event distributions and high-gravity anomalies indicate a strong coupling degree of the intersection area, which was probably induced by a strong resistance of the fracture features during the subduction. Moreover, the seismicity distribution in the Ryukyu area was very similar to the pre-seismic activity pattern of the 2004 Sumatra event. That is, thrust-type earthquakes with a trench-normal P-axis occurred frequently along the oceanward side of the mainshock, whereas only a few thrust earthquakes occurred along the continentward side. Therefore, the aseismic area located west of 128°E in the western Ryukyu subduction zone could have resulted from the strong plate locking effect beneath the high gravity anomaly zone. By analogy with the tectonic environment of the Sumatra subduction zone, the occurrence of a potential Sumatra-like earthquake in the south Ryukyu arc is highly likely and the rupture will mainly propagate continentward to fulfill the region of low seismicity (approximately 125° E to 129° E; 23° N to 26.5° N), which may generate a hazardous tsunami.

  11. Interseismic deformation along the Philippine Fault system and Manila subduction zone

    NASA Astrophysics Data System (ADS)

    Hsu, Y.; Yu, S.; Loveless, J. P.; Bacolcol, T.

    2013-12-01

    Southern Taiwan and Luzon is bounded between the Sunda plate and the Philippine Sea plate. The Philippine Sea plate converges obliquely with the Sunda plate with a rate of 50-90 mm/yr and results in a major sinistral strike-slip fault, the Philippine fault, extending 1300 km from Luzon to Mindanao. Using GPS data collected between 2000 and 2013 as well as a block modeling approach, we decompose the crustal motion into multiple rotating blocks, homogeneous intrablock strain, and the elastic deformation due to fault slip at block boundaries. Our preferred model composed of 9 blocks, produced a mean residual velocity of 3.1 mm/yr at 92 GPS stations. The estimated slip rates on the Philippine fault increase toward the south, from ~10 mm/yr at latitude 17°N to ~40 mm/yr at latitude 15°N. Estimated slip rates on the Philippine fault are higher than the long-term geological slip rates of 9-17 mm/yr, partly due to the postseismic deformation of the 1990 Ms 7.9 Luzon earthquake. Along the Manila subduction zone, estimated slip rates are ~70 mm/yr on the northern segment (offshore SW Taiwan, 21°N~22.5°N), ~10 mm/yr on the central segment (17°N~20°N), and 20 mm/yr on the southern segment (14°N~17N). High plate coupling ratios is inferred at shallow depths on the northern Manila subduction zone, although the resolution is poor due to sparse GPS data. The central segment is likely to be creeping and the southern segment is possibly partially locked.

  12. Evaluation of strain accumulation in global subduction zones from seismicity data

    NASA Astrophysics Data System (ADS)

    Ikuta, Ryoya; Mitsui, Yuta; Kurokawa, Yuri; Ando, Masataka

    2015-12-01

    We compared the cumulative seismic slip of interplate earthquakes (≥M5.5) with relative plate motion at subduction zones. By assuming that each interplate earthquake occurred on a stick-slip patch, we used the slip history of each patch to calculate the interplate slip in the surrounding area. We considered that areas in which interplate earthquakes occurred but that had small cumulative slips compared with relative plate motion were accumulating strain, and we calculated the size of these areas. We first used this method to test the rupture areas of six M9-class interplate earthquakes that have occurred during the past 100 years. The cumulative seismic slip preceding and following the six earthquakes was smaller than the relative plate motion in the rupture areas of the earthquakes. We interpret the areas of slip-deficient stick-slip patches to be the rupture areas of future huge earthquakes. We applied the same procedure to global subduction zones and found that slip-deficient stick-slip patches with large spatial extents (equivalent to the rupture area of M9-class earthquakes) occur in an additional 25 locations. Considering that six M9-class earthquakes have occurred in the past 110 years and that the recurrence interval in each case is probably between a few hundred and a thousand years, it is not surprising that 25 regions globally are capable of producing M9-class earthquakes. These regions may be the most likely candidates for the rupture areas of future M9-class interplate earthquakes.

  13. Forearc Basin Structure in the Andaman-Nicobar Segment of the Sumatra-Andaman Subduction Zone: Insight from High-Resolution Seismic Reflection Data

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    The Andaman-Nicobar subduction is the northernmost segment of the Sumatra-Andaman subduction zone and marks the western boundary of the Andaman Sea, which is a complex backarc extensional basin. We present the interpretation of a new set of deep seismic reflection data acquired across the Andaman-Nicobar forearc basin, from 8°N to 11°N, to understand the structure and evolution of the forearc basin, focusing on how the obliquity of convergence affects deformation in the forearc, as well as on the Diligent (DF) and Eastern Margin Faults (EMF). Constraining the evolution of this basin, which is strongly related to the collision of India and Eurasia, can help shed light onto present-day deformation processes along this segment of the subduction zone, where convergence is highly oblique and little data is available. We find that he DF is a backthrust and corresponds to the Mentawai (MFZ) and West Andaman Fault (WAF) systems further south, offshore Sumatra. The DF is expressed as a series of mostly landward verging folds and faults, deforming the early to late Miocene sediments. The DF seems to root from the boundary between the accretionary complex and the continental backstop, where it meets the EMF. The EMF marks the western boundary of the forearc basin; it is associated with subsidence and is expressed as a deep piggyback basin, associated with recent Pliocene to Pleistocene subsidence at the western edge of the forearc basin. The eastern edge of the forearc basin is marked by the Invisible Bank (IB), which is thought to be tilted and uplifted continental crustal block. Subsidence along the EMF and uplift and tilting of the IB seem to be related to different opening phases in the Andaman Sea. The sliver Andaman-Nicobar Fault (ANF), which is the active northward extension of the Great Sumatra sliver Fault (GSF), lies to the east of the IB, and marks the boundary between continental crust underlying the forearc basin and crust accreted at the Andaman Sea Spreading Center.

  14. Cretaceous high-pressure metamorphic belts of the Central Pontides (northern Turkey): pre-collisional Pacific-type accretionary continental growth of Laurasian Margin

    NASA Astrophysics Data System (ADS)

    Aygul, Mesut; Okay, Aral I.; Oberhaensli, Roland; Sudo, Masafumi

    2014-05-01

    Cretaceous blueschist-facies metamorphic rocks crop out widely in the central part of the Pontides, an east-west trending mountain belt in northern Turkey. They comprise an accretionary wedge along to the southern Laurasian active continental margin and predate the opening of Black Sea basin. From North to South, the wedge consists of a low grade metaflysch unit with marble, Na-amphibole-bearing metabasite and serpentinite blocks. An extensional shear zone separates the accreted distal terrigenous sediments from HP/LT micaschists and metabasites of oceanic origin, known as Domuzdaǧ Complex. The shear zone reaches up to one km in thickness and consists of tectonic slices of serpentinite, metabasite, marble, phyllite and micaschist with top to the NW sense of shear. The Domuzdaǧ Complex predominantly consists of carbonaceous micaschist and metabasite with serpentinite, and minor metachert, marble and metagabbro. Metabasites consist mainly of epidote-blueschists sometimes with garnet. Fresh lawsonite-blueschists are found as blocks within the shear zone. Peak metamorphic assemblages in the micaschists are chloritoid-glaucophane and garnet-chloritoid-glaucophane-lawsonite in addition to phengite, paragonite, quartz, chlorite and rutile (P: 17 ± 1 Kbar, T: 390-450 °C). To the south, lithologies change slightly, with metabasite and thick, pale marble with few metachert and metapelitic horizons. The degree of metamorphism also changes. The metabasites range from high-pressure upper-greenschist facies with growth of sodic-amphibole to lower greenschist without any HP index mineral, suggesting a general decrease in pressure toward south within the prism. While Domuzdaǧ Complex represents deep-seated underplated oceanic sediments and basalts, the carbonate-rich southern parts can be interpreted as seamounts integrated into the accretionary prism. Ar/Ar dating on phengite separates both from terrigenous and oceanic metasediments give consistent plateau ages of 100 ± 2 Ma. One of the Cld-micaschist, exposed to the South, gives a 92 ± 2 Ma age. This documents a southward younging of metamorphism within the accretionary prism. A mid-Jurassic (160 Ma) age, previously reported from a micaschist in the southern part of Domuzdaǧ Complex, is also supported in this study. These rocks however differ from the Cretaceous HP unit both in lithology and degree of metamorphism (P: 10 ± 2 Kbar, T: 620 ± 30°C; Okay et al. 2013). It is not clear whether these rocks indicate episodic subduction process or represent tectonically emplaced slivers of the overriding plate which has widespread Mid-Jurassic high-grade metamorphic rocks and intrusions. The Cretaceous accretionary complex structurally overlies an arc-related low-grade metavolcanic unit, which is thrusted over the ophiolitic rocks of the main Tethyan İzmir-Ankara-Erzincan Suture zone that separates the Pontides from the Gondwana-derived terranes. In the tectonic framework discussed above, the study area represents subduction and accretion related units, which are sandwiched between the southern Laurasian active margin and the Gondwana-derived Kırşehir Block without any continental fragments. This indicates that Pacific-type pre-collisional accretion has a major role in the Tethyan geology of the Central Pontides during Cretaceous. Okay et al. (2013) Tectonics 32: 1247-1271.

  15. The energy release in earthquakes, and subduction zone seismicity and stress in slabs. Ph.D. Thesis

    NASA Technical Reports Server (NTRS)

    Vassiliou, M. S.

    1983-01-01

    Energy release in earthquakes is discussed. Dynamic energy from source time function, a simplified procedure for modeling deep focus events, static energy estimates, near source energy studies, and energy and magnitude are addressed. Subduction zone seismicity and stress in slabs are also discussed.

  16. Thermal modeling of the southern Alaska subduction zone: Insight into the petrology of the subducting slab and overlying mantle wedge

    SciTech Connect

    Ponko, S.C.; Peacock, S.M.

    1995-11-10

    This report discusses a two-dimensional thermal model of the southern Alaska subduction zone. This model allows specfic predictions to be made about the pressure-temperature conditions and mineralogy of the subducting oceanic crust and the mantle wedge and assess different petrologic models for the generation of Alaskan arc magmas.

  17. Formulation and Application of a Physically-Based Rupture Probability Model for Large Earthquakes on Subduction Zones: A Case Study of Earthquakes on Nazca Plate

    NASA Astrophysics Data System (ADS)

    Mahdyiar, M.; Galgana, G.; Shen-Tu, B.; Klein, E.; Pontbriand, C. W.

    2014-12-01

    Most time dependent rupture probability (TDRP) models are basically designed for a single-mode rupture, i.e. a single characteristic earthquake on a fault. However, most subduction zones rupture in complex patterns that create overlapping earthquakes of different magnitudes. Additionally, the limited historic earthquake data does not provide sufficient information to estimate reliable mean recurrence intervals for earthquakes. This makes it difficult to identify a single characteristic earthquake for TDRP analysis. Physical models based on geodetic data have been successfully used to obtain information on the state of coupling and slip deficit rates for subduction zones. Coupling information provides valuable insight into the complexity of subduction zone rupture processes. In this study we present a TDRP model that is formulated based on subduction zone slip deficit rate distribution. A subduction zone is represented by an integrated network of cells. Each cell ruptures multiple times from numerous earthquakes that have overlapping rupture areas. The rate of rupture for each cell is calculated using a moment balance concept that is calibrated based on historic earthquake data. The information in conjunction with estimates of coseismic slip from past earthquakes is used to formulate time dependent rupture probability models for cells. Earthquakes on the subduction zone and their rupture probabilities are calculated by integrating different combinations of cells. The resulting rupture probability estimates are fully consistent with the state of coupling of the subduction zone and the regional and local earthquake history as the model takes into account the impact of all large (M>7.5) earthquakes on the subduction zone. The granular rupture model as developed in this study allows estimating rupture probabilities for large earthquakes other than just a single characteristic magnitude earthquake. This provides a general framework for formulating physically-based rupture probability models for large earthquakes on subduction zones that is consistent with their true locking state and earthquake history. We will present the formulation of the proposed model and its application to the Nazca plate subduction zone.

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

    The Etirol-Levaz Slice in the Penninic Alps (Valtournenche, Italy) is a piece of eclogite-facies continental basement sandwiched between two oceanic units, the blueschist-facies Combin Zone in the hanging wall and the eclogite-facies Zermatt-Saas Zone in the footwall. It has been interpreted as an extensional allochthon from the continental margin of Adria, emplaced onto ultramafic and mafic basement of the future Zermatt-Saas Zone by Jurassic, rifting-related detachment faulting, and later subducted together with the future Zermatt-Saas Zone. Alternatively, the Etirol-Levaz Slice could be derived from a different paleogeographic domain and be separated from the Zermatt-Saas Zone by an Alpine shear zone. We present Lu-Hf whole rock-garnet ages of two eclogite samples, one from the center of the unit and one from the border to the Zermatt-Saas Zone below. These data are accompanied by a new geological map of the Etirol-Levaz Slice and the surrounding area, as well as detailed petrology of these two samples. Assemblages, mineral compositions and garnet zoning in both samples indicate a clockwise PT-path and peak-metamorphic conditions of about 550-600 °C/20-25 kbar, similar to conditions proposed for the underlying Zermatt-Saas Zone. Prograde garnet ages of the two samples are 61.8 ± 1.8 Ma and 52.4 ± 2.1 Ma and reflect different timing of subduction. One of these is significantly older than published ages of eclogite-facies metamorphism in the Zermatt-Saas Zone and thus contradicts the hypothesis of Mesozoic emplacement. The occurrence of serpentinite and metagabbro bodies possibly derived from the Zermatt-Saas Zone inside the Etirol-Levaz Slice suggests that the latter is a tectonic composite. The basement slivers forming the Etirol-Levaz Slice and other continental fragments were subducted earlier than the Zermatt-Saas Zone, but nonetheless experienced similar pressure-temperature histories. Our results support the hypothesis that the Zermatt-Saas Zone and the overlying continental slivers do not represent a coherent paleogeographic unit but rather formed by successive, in-sequence subduction and accretion of different fragments.

  19. Potential links between porphyry copper deposits and exhumed metamorphic basement complexes in northern Chile

    NASA Astrophysics Data System (ADS)

    Cooper, Frances; Docherty, Alistair; Perkins, Rebecca

    2014-05-01

    Porphyry copper deposits (PCDs) are typically associated with magmatic arcs in compressional subduction zone settings where thickened crust and fractionated calc-alkaline magmas produce favourable conditions for copper mineralisation. A classic example is the Eocene-Oligocene PCD belt of Chile, the world's leading copper producing country. In other parts of the world, older late Cretaceous to early Tertiary PCDs are found in regions of former subduction-related magmatism that have undergone subsequent post-orogenic crustal extension, such as the Basin and Range province of western North America, and the Eurasian Balkan-Carpathian-Dinaride belt. In the Basin and Range there is a striking correlation between the location of many PCDs and exhumed metamorphic core complexes (isolated remnants of the middle to lower crust exhumed during extensional normal faulting). This close spatial relationship raises questions about the links between the two. For example, are their exhumation histories related? Could the presence of impermeable metamorphic rocks at depth affect and localise mineralising fluids? In Chile there appears to be a similar spatial relationship between PCDs and isolated outcrops of exhumed metamorphic basement. In northern Chile, isolated exposures of high-grade metamorphic gneisses and amphibolites are thought to be exhumed remnants of the pre-subduction Proterozoic-Paleozoic continental margin of Gondwana [2], although little is known about when they were exhumed and by what mechanism. For example, the Limón Verde metamorphic complex, exhumed from a depth of ca. 50 km, is situated adjacent to Chuquicamata, the largest open pit copper mine in the world. In northernmost Chile, another metamorphic exposure, the Belén complex, sits close to the Dos Hermanos PCD, a small deposit that is not actively mined. Comprising garnet-bearing gneisses and amphibolites, the Belén is thought to have been exhumed from a depth of ca. 25 km, but when and how is unclear [3]. We present new mapping, structural analysis, and geochronologic data from both the Limón Verde and the Belén metamorphic complexes and explore the relationship between these isolated outcrops and PCD formation in northern Chile. References: [1] Seedorff et al., 2008, Root Zones of Porphyry Systems: Extending the Porphyry Model to Depth, Ec. Geol., 103, 939-956. [2] Hervé et al., 2007, Metamorphic and Plutonic basement complexes, in: The Geology of Chile, Geol. Soc. Lond., 5-19. [3] Wörner et al., 2000, Precambrian and Early Paleozoic evolution of the Andean basement at Belen (northern Chile) and Cerro Uyarani (western Bolivia Altiplano), 13, 717-737.

  20. Complex flow resulted from along-strike variations in slab length in the southern Chilean subduction zone

    NASA Astrophysics Data System (ADS)

    Lin, S.; Chung, S.

    2013-12-01

    Complicated patterns and trench-parallel components of seismic anisotropy in the central portion of the subduction zones have not been link to three-dimensional flow in general, as toroidal circulations may mostly occur in the vicinity of a plate edge. Nevertheless, recent dynamical models demonstrate that complex flow forms when lateral heterogeneities in the subduction zone are considered. Seismic studies often show gradual variations in slab length along strike. The southern Chilean subduction zone is a notable example. The Peru-Chile Trench roughly extending 5900 km is the longest trench worldwide. Most of the regions are distant from the subduction zone edges. In this study the effects of slab-length variations are examined with numerical experiments. The results are used to characterize the regional mantle circulations and the potential heat sources for the recent Patagonian plateau basalts in the southern South America. The model results show that rollback subduction induces both significant poloidal components and trench-parallel flow, extending over 1500 kilometers along the strike of the subduction zone. The trajectories of a set of passive tracers show intricate patterns such as helical streamlines, revealing complex mantle circulations in systems with the along-arc slab-length variations. The upwelling may lead to decompression melting of the heterogeneous asthenosphere to cause extensive lavas in the back arc. Mantle upwelling of the sub-slab mantle moving towards supra-slab regions is observed in the southernmost portion of the subduction system, but only for models with slab segments roughly deeper than 100 km depth. The results indicate that certain segments of the Antarctic slab may reach at least 100 km depth.