Sample records for continental subduction-zone metamorphism

  1. Fluids from the Deeply Continental Subduction Zone and the Metamorphic Chemical Geodynamics

    NASA Astrophysics Data System (ADS)

    Zhang, Z.; Wang, J.; Shen, K.

    2009-05-01

    The complex vein associations hosted in southern Sulu ultrahigh-pressure (UHP) eclogites contain quartz À omphacite (or jadeite) À kyanite À allanite À zoisite À rutile À garnet. These minerals have chemical compositions similar to those of host eclogites. Inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet, and coesite inclusions were found in vein zircon. These facts suggest that the veins together with host eclogites have been subjected to synchronous UHP metamorphism. The vein minerals contain relatively high concentrations of rare earth elements (REE), high field strength elements (HFSE) and transition metal elements (TME). A kyanite-quartz vein has a whole-rock composition similar to adjacent UHP metamorphic granitic gneisses. Abundant primary multi-solid fluid inclusions trapped within UHP vein minerals contain complex daughter minerals of muscovite, calcite, anhydrite, magnetite, pyrite, apatite, celestite and liquid and gas phase of H2O with solids up to 30 to 70% of the inclusion volume. Presence of daughter mineral anhydrite and magnetite indicates the high oxygen fugacity in subduction released fluids, and provides a feasible interpretation to the high oxygen fugacity in convergent margins. These characteristics imply that the UHP vein minerals were crystallized from supercritical silicate-rich aqueous fluids that were in equilibrium with peak-UHP minerals, and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as HREE, HFSE and TME. Such fluids might have resulted in major fractionation between Nb and Ta, i.e. the UHP fluids have subchondritic Nb/Ta values, whereas the host eclogites after extraction of the fluids have suprachondritic Nb/Ta values. Therefore, voluminous residual eclogites with high Nb/Ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs.

  2. 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.

  3. 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 ?15N 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 ?15NAir (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.

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

    NASA Astrophysics Data System (ADS)

    Ernst, W. G.; Tsujimori, T.; Zhang, R.; Liou, J. G.

    2006-12-01

    Convergent plate motion over portions of the interval from 320 to 210 Ma generated the Tongbai-Dabie-Sulu (east-central China)-Imjingang-Gyeonggi (central Korea)-Renge-Suo (Southwestern Japan)-Sikhote-Alin contractional orogen along the paleo-Pacific edge of cratonal Asia. This amalgamated belt reflects collision between the Sino-Korean and Yangtze cratons along the southwestern part, and accretion of outboard oceanic arcs and minor sialic fragments against the northeastern 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 mid-crustal levels from depths exceeding 90 to 200 km after continental collision in east-central China and Korea, and decoupled from depths of about 30 to 50 km after arrival of far-travelled oceanic terranes in Southwestern Japan and the Russian Far East. On achieving neutral buoyancy and stalling out at 10 to 20 km depth, later doming, gravitational collapse, and erosion exposed parts of the high- and ultrahigh-pressure complexes. Since its accretion, this curvilinear orogen has been segmented and offset by major and minor transverse faults. Also, regional backarc spreading has opened marginal basins behind the Permo-Triassic convergent suture zone, further displacing portions of the orogenic belt oceanward.

  5. 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.

  6. Metamorphic chemical geodynamics of subduction zones Gray E. Bebout

    E-print Network

    Bebout, Gray E.

    Frontiers Metamorphic chemical geodynamics of subduction zones Gray E. Bebout Lehigh University.N. Halliday Available online 12 June 2007 Abstract Study of metamorphic suites directly representing the deep-pressure (UHP) metamorphic suites incorporating knowledge of mineral chemistry and reactions, kinetics

  7. Ultramafic cumulates of oceanic mantle affinity in a continental subduction zone: UHP garnet

    E-print Network

    Ultramafic cumulates of oceanic mantle affinity in a continental subduction zone: UHP garnet of Ljubljana, Slovenia 4 Dept. of Earth Sciences, University of Ottawa, Canada Rare UHP garnet peridotites have,2]. Peak P-T conditions of the garnet peridotites reached up to 4 GPa and 900°C [1]. The UHP metamorphic

  8. Metamorphic chemical geodynamics of subduction zones

    Microsoft Academic Search

    Gray E. Bebout

    2007-01-01

    Study of metamorphic suites directly representing the deep subduction of altered oceanic crust and sediments can help elucidate the geochemical evolution of the forearc-to-subarc slab mantle interface, the nature of slab-derived fluids added to arc lava source regions, and the chemical changes in subducting rocks potentially contributing to the geochemical heterogeneity of the deeper mantle. The stage is set for

  9. 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.

  10. Dating Subduction Zone Metamorphism with Garnet and Lawsonite Geochronology

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Lawsonite [CaAl2Si2O7(OH)2 H2O] is a critical index mineral for high- to ultrahigh-pressure metamorphism associated with subduction. Lawsonite is an important carrier of water into the mantle, a likely contributor to subduction zone seismicity, and a bearer of trace elements that link metamorphism to arc magmatism. Due to its limited pressure-temperature stability, lawsonite can serve as a powerful petrogenetic indicator of specific metamorphic events. Lu-Hf dating of lawsonite, therefore provides a potentially powerful new tool for constraining subduction zone processes in a pressure-temperature window where few successful geochronometers exist. Broad application of lawsonite Lu-Hf geochronology requires constraining the role of pressure-temperature path, lawsonite forming reactions, and the Lu and Hf systematics within lawsonite and other blueschist facies minerals. We are working to address the role of the metamorphic path on the applicability of lawsonite Lu-Hf geochronology within the Franciscan Complex of California. The Franciscan Complex preserves mafic high-grade exotic blocks in melange that underwent a counterclockwise pressure-temperature path wherein garnet, which strongly partitions heavy rare-earth elements, formed prior to lawsonite. Coherent mafic rocks within the Franciscan Complex, however, underwent a clockwise pressure-temperature path and lawsonite growth occurred prior to garnet. We sampled exotic blocks of garnet-hornblendite, garnet-epidote amphibolite, garnet-epidote blueschist, and lawsonite blueschist from the Berkeley Hills and Tiburon Peninsula of California. We collected four samples from coherent lawsonite blueschist across the lawsonite-pumpellyite-epidote isograds in Ward Creek, near Cazadero California. High-grade blocks give ages similar to existing Franciscan geochronology: multi-stage garnet in hornblendite gives the following ages: 171×1.3 Ma (MSWD 2.8) for the core and 159.4×0.9 Ma (MSWD 2.0) for the corresponding rim; 166×0.9 Ma (MSWD 1.0) for garnet-epidote amphibolite; and 156.2×1.0 Ma (MSWD 0.35) for garnet-epidote blueschist. Samples from retrograde exotic blocks contain lawsonite formed by garnet breakdown reactions and exhibit elevated Lu concentrations (?0.5-1.3 ppm) and 176Lu/177 Hf ratios (?2.2). Two samples we dated from the Berkeley Hills and Tiburon, respectively, gave lawsonite-whole rock ages of 129.2×1.2 Ma (MSWD 1.0) and 144.9×1.2 Ma (MSWD 0.64) . In the younger sample, garnet replaced by lawsonite is dated at 149.2×1.6 Ma. Lawsonite from Ward Creek that formed by prograde reactions involving plagioclase minerals, in contrast, have low Lu concentrations (?0.3 ppm) and low 176Lu/177 Hf ratios (?0.45). A garnet age of 151.6×1.3 Ma (MSWD 12.7) was obtained from garnet-epidote blueschist. While the ages from retrograde lawsonite within exotic blocks are similar to existing ages from the Franciscan Complex, no meaningful lawsonite ages were obtained from prograde coherent blocks of Ward Creek. The results suggest that lawsonite formed from the breakdown of garnet along a retrograde path provides meaningful Lu-Hf ages, while the very-low temperature lawsonite formed along a prograde path is not well suited for geochronology. This may be due to factors such as non-equilibrium at low metamorphic grades, low bulk rock Lu content, and the prevalence of micro-zircon within these samples.

  11. 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.

  12. Seismic investigation of the transition from continental to oceanic subduction along the western Hellenic Subduction Zone

    E-print Network

    Pearce, Frederick Douglas

    The western Hellenic subduction zone (WHSZ) exhibits well-documented along-strike variations in lithosphere density (i.e., oceanic versus continental), subduction rates, and overriding plate extension. Differences in slab ...

  13. Experimental modeling of subduction zone metamorphism and devolatilization

    NASA Astrophysics Data System (ADS)

    Perchuk, Alexei L.; Korepanova, Olga S.

    2010-05-01

    Subduction zones are characterized by valuable thermal gradients owing to descend of cold oceanic plate into the hot mantle. We propose for the first time to apply high-gradient zones of the piston-cylinder apparatus to study the interaction between subducting plate and hanging wall mantle. In this contribution, we present peculiar features of the glaucophane schist-olivine interaction as analogs of crust and mantle, respectively. The glaucophane schist is from the Atbashi complex, Tien Shan, the olivine is collected from the quarry Ahaim, Norway. ?-? conditions of the runs correspond to so-called "warm" geotherm. We present results of the two runs carried out at pressure of 2.4 and 2.6 GPa and both at the temperature of 1050 grad ? at the upper range of the capsule. Temperature of ~700 grad C the lower edge of the 7 mm capsule was inferred from the numerical modeling based on the real sizes of materials after the runs. The experiments reveal that transformation of the glaucophane schist is strongly controlled by the pressure. The transformation was resulted in growth of barroisitic amphibole rims around the primary glaucophane at 2.4 GPa and patchy omphacite+quartz aggregate at 2.6 GPa. Thus we didn't observe the typical eclogite assemblage (omphacite+garnet) in the metabasic rock well within the eclogite facies P-T conditions. The devolatalization owing to the breakdown of hydrous mineral of the glaucophane schist lead to the formation of thin (30 micron) orthopyroxene layer at the contact of olive with glaucophane schist. In addition, first portions of melt (dacite composition) appear in between the orthopyroxene layer and the glaucophane schist. The melt (fluid) was migrated above the opx layer but owing to the low Si-content didn't react with the host olivine. The study demonstrated that high-gradient zones of the "piston-cylinder" apparatus are very efficient for study mantle-crust interaction in the subduction zones. Financial support by RFBR grants N 09-05-01217 and 09-05-00991.

  14. 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

  15. 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.

  16. High-pressure metamorphism in the Aegean, eastern Mediterranean: Underplating and exhumation from the Late Cretaceous until the Miocene to Recent above the retreating Hellenic subduction zone

    NASA Astrophysics Data System (ADS)

    Ring, Uwe; Layer, Paul W.

    2003-06-01

    We report 40Ar/39Ar ages from various tectonic units in the Aegean and westernmost Turkey. On the basis of published geochronologic data and our 40Ar/39Ar ages we propose that the Aegean is made up of several high-pressure units, which were successively underplated from the Late Cretaceous until the Miocene. Ages for high-pressure metamorphism range from 80-83 Ma in parts of the Vardar-Izmir-Ankara suture zone in the north to 21-24 Ma for the Basal unit in the Cyclades and the external high-pressure belt on Crete in the south. Published seismic data suggest that high-pressure metamorphism is currently occurring underneath Crete. Younging of high-pressure metamorphism in a southerly direction mimics the southward retreat of the Hellenic subduction zone. We propose that distinct stages of high-pressure metamorphism were controlled by the underthrusting of fragments of mainly thinned continental crust and that these punctuated events were superposed on progressive slab retreat. By far most of the exhumation of the high-pressure units occurred early during the orogenic history in a forearc position.

  17. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Depending on the magnitude of the poorly constrained C flux in ultramafic rocks, on a global basis, sediments and altered oceanic crust (AOC) together deliver 70-95% of the C currently entering subduction zones. We are investigating extents of retention and metamorphic release of C in deeply subducted AOC and carbonate-rich sediment represented by HP/UHP meta-ophiolitic and metasedimentary rocks in the Italian Alps and in the Tianshan. Study of metapelite devolatilization in the same W. Alps suite (Bebout et al., 2013, Chem. Geol.) provides a geochemical framework for study of C behavior along prograde P-T paths similar to those experienced in forearcs of most modern subduction margins. Study of veins in the Tianshan affords examination of C mobility in UHP fluids, in later stages as metabasaltic rocks were fragmented in the subduction channel. Our results for sediments and AOC indicate impressive retention of oxidized C (carbonate) and reduced C (variably metamorphosed organic matter) to depths approaching those beneath arc volcanic fronts. In metasedimentary rocks, extensive isotopic exchange between the oxidized and reduced C resulted in shifts in both reservoirs toward upper mantle compositions. Much of the carbonate in metabasalts has C and O isotopic compositions overlapping with those for carbonate in AOC, with some HP/UHP metamorphic veins showing greater influence of organic C signatures from metasedimentary rocks. Calculations of prograde devolatilization histories using Perple-X demonstrate that, in most forearcs, very little decarbonation occurs in the more carbonate-rich rocks unless they are flushed by H2O-rich fluids from an external source, for example, from the hydrated ultramafic section of subducting slabs (cf. Gorman et al., 2006; G3) or from more nearby rocks experiencing dehydration (e.g., metapelites). A comparison of the most recently published thermal models for modern subduction zones (van Keken et al., 2011, JGR) with calculated and experimentally determined phase relations indicates that significant C loss during devolatilization (and partial melting) should occur as subducting sections traverse depths beneath arcs. The extent of C mobility due to carbonate dissolution remains uncertain. On a global basis, imbalance between subducted C input and C return flux by magmatism (excluding ultramafic inputs, ~40×20% of subducted C return via arcs and ~80×20% by all magmatism; Bebout, 2013, Treat. Geochem.) indicates net modern C return to the mantle, perhaps a reversal of Archean net outgassing (despite more rapid subduction). Global C cycle models predict that relatively small (and geologically plausible) change in the subduction/volcanic C flux could significantly affect atmospheric CO2 levels and thus global climate.

  19. Low-grade metamorphism around the down-dip limit of seismogenic subduction zones: Example from an ancient

    E-print Network

    Paris-Sud XI, Université de

    a subduction zone are progressively modified by dewatering, lithification, and deformation (Cowan, 1982; Byrne­aseismic transitions occurs at temperatures of ~150 °C and is believed to be related to dewatering of subducted

  20. Modification of subcontinental lithospheric mantle above continental subduction zone: Constraints from geochemistry of Mesozoic gabbroic rocks in southeastern North China

    NASA Astrophysics Data System (ADS)

    Yang, Qing-Liang; Zhao, Zi-Fu; Zheng, Yong-Fei

    2012-08-01

    The subcontinental lithospheric mantle (SCLM) in the southern margin of the North China Block was modified by the Triassic subduction of the South China Block, but less is known about subsequent events. This study presents a geochemical study of its anatectic products, Mesozoic gabbroic rocks from the southeastern edge of the North China Block. Zircon U-Pb dating gave ages of 119 ± 4 to 131 ± 1 Ma for magma crystallization. The gabbroic rocks have high contents of MgO, Cr and Ni as well as high Mg# values, but low contents of SiO2. They exhibit arc-like trace element patterns, enriched Sr-Nd-Hf isotope compositions, and high ?18O values. These geochemical features indicate their derivation from partial melting of a fertile mantle source that is enriched not only in large ion lithophile elements and light rare earth elements but also in radiogenic isotopes. In particular, they have two-stage whole-rock Nd and zircon Hf model ages of Mesoproterozoic to Paleoproterozoic, which are similar to those of ultrahigh-pressure metaigneous rocks in the Dabie-Sulu orogenic belt. This suggests a genetic relationship in precursor origin between them. Therefore, the mantle source for the gabbroic rocks would be generated by crustal metasomatism through underplate reaction of the overlying SCLM-wedge peridotite with felsic melts derived from subducting continental crustal rocks of the South China Block during the Triassic continental collision. The fertile and enriched signatures in the mantle source were imparted by the crustally derived melts in the continental subduction channel, giving rise to the metasomatised SCLM in the Triassic. Because of its fertile and enriched properties, the metasomatised SCLM was susceptible to partial melting in an extensional tectonic setting, resulting in gabbroic magmatism in the Early Cretaceous. Therefore, these gabbroic rocks provide petrological and geochemical records of the crust-mantle interaction above a continental subduction zone. The spatial effect of continental deep subduction on the overlying SCLM wedge may extend to 200-300 km distant from the collisional orogen. The melt metasomatism would have modified the chemical and physical properties of the SCLM in the southern North China Block, facilitating the SCLM thinning in the Mesozoic.

  1. 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 (2?) 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,?)38Cl(?)38Ar). The high temperature release of 38ArCl may result from smaller fluid inclusions (<1-2 ?m). 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.

  2. An explanation for the age independence of oceanic elastic thickness estimates from flexural profiles at subduction zones, and implications for continental rheology

    NASA Astrophysics Data System (ADS)

    Craig, Timothy J.; Copley, Alex

    2014-04-01

    Most properties of oceanic lithosphere are widely observed to be dependent on the age of the plate, such as water depth, heat flow, and seismogenic thickness. However, estimates of the ‘effective elastic thickness' of oceanic lithosphere based on the deflection of the plate as it enters a subduction zone show little correlation with the age of the incoming lithosphere. This paradox requires reconciliation if we are to gain a full understanding of the structure, rheology, and behaviour of oceanic lithosphere. Here, we show that the permanent deformation of the plate due to outer-rise faulting, combined with uncertainties in the yield stress of the lithosphere, the in-plane forces transmitted through subduction zones, and the levels of noise in bathymetric and gravity data, prevents simple elastic plate modelling from accurately capturing the underlying rheological structure of the incoming plate. The age-independent estimates of effective elastic thickness obtained by purely elastic plate modelling are therefore not likely to represent the true rheology of the plate, and hence are not expected to correspond to the plate age. Similar effects may apply to estimates of elastic thickness from continental forelands, with implications for our understanding of continental rheology.

  3. 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.

  4. The 2006 slow slip event and nonvolcanic tremor in the Mexican subduction zone

    E-print Network

    Clayton, Robert W.

    The 2006 slow slip event and nonvolcanic tremor in the Mexican subduction zone Vladimir Kostoglodov) and nonvolcanic tremor (NVT) in different subduction zones and continental faults. Many observations show that SSE- nic tremor in the Mexican subduction zone, Geophys. Res. Lett., 37, L24301, doi:10.1029/2010GL045424

  5. 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

  6. Are subduction zones inherently weak?

    NASA Astrophysics Data System (ADS)

    Duarte, Joao; Schellart, Wouter; Cruden, Alexander; Gutscher, Marc-André

    2014-05-01

    An outstanding question in geodynamics is how weak are plate boundaries when compared to their interiors? Particularly, how weak are subduction zone interfaces? Because subduction is believed to be the major driver of plate tectonics, this question is of fundamental importance for geodynamics. Several lines of evidence suggest that subduction zones are weak and that the unique availability of water on Earth plays a key role. We have evaluated the strength of subduction zone interfaces using two approaches: i) an empirical relationship between shear stress at the interface and subduction velocity, deduced from laboratory experiments of subduction; and ii) a parametric study of natural subduction zones that provides new insights on subduction zone interface strength. Our results suggest that subduction is only mechanically feasible when shear stresses along the plate interface are relatively low (< 33 MPa). To account for this global requirement, we propose that there is a feedback mechanism between subduction velocity, water release rate from the subducting plate and serpentinization and weakening of the forearc mantle that may explain how relatively low shear stresses are maintained at subduction interfaces globally.

  7. Rare earth element redistribution during high-pressure low-temperature metamorphism in ophiolitic Fe-gabbros (Liguria, northwestern Italy): Implications for light REE mobility in subduction zones

    Microsoft Academic Search

    Riccardo Tribuzio; Bruno Messiga; Riccardo Vannucci; Piero Bottazzi

    1996-01-01

    To unravel the rare earth element (REE) redistribution during high-pressure low-temperature metamorphism, we have analyzed by ion microprobe all the minerals from representative Fe-gabbros from Ligurian metaophiolites (northwestern Italy). Contrary to what is observed for fresh Fe-gabbros, the clinopyroxene contribution to the whole-rock REE inventory of blueschists and eclogitized Fe-gabbros is minor or negligible. In both blueschists and eclogites, REE

  8. 2006 Slow Slip Event and Nonvolcanic Tremor in the Mexican Subduction Zone Vladimir Kostoglodov1

    E-print Network

    1 2006 Slow Slip Event and Nonvolcanic Tremor in the Mexican Subduction Zone Vladimir Kostoglodov1) and Nonvolcanic Tremor (NVT) in different subduction zones and continental faults. Many observations show that SSE] and Nonvolcanic Tremor (NVT) [Obara, 2002; Rubinstein et al., 2010]. SSE and NVT observed now in different

  9. SUBDUCTION ZONES Robert J. Stern

    E-print Network

    Stern, Robert J.

    by the sinking slab interacts with water and incompatible elements rising from the sinking plate-ocean ridges to spread, with the result that plate tectonics and subduc- tion zones are surficial and interior, only Earth appears to have subduction zones and plate tectonics. Mercury and Earth's moon

  10. Absolute gravity and GPS deformation rates in the regions of mid-continental North America and the northern Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Henton, J. A.; Lambert, A.; Mazzotti, S.; James, T. S.; Courtier, N.; Dragert, H.

    2011-12-01

    High-precision absolute gravity (AG) observations are sensitive to vertical motion of the observation site as well as mass redistribution within (and below) the underlying deforming crust. The deformation gravity gradient (DGG) may be defined as the ratio of the time rate of change of surface gravity (g-dot) to vertical crustal velocity (h-dot) and its value provides insight into the deformation process. In the mid-continent region of North America, we have examined yearly AG and continuous vertical GPS time series between 1995 and 2010 at eight collocated (or nearby) sites. GPS data have been re-processed with NRCan's Precise Point Positioning (PPP) software using IGS repro1 products. For this region the comparison of AG and GPS trends shows (1) DGG = -0.16 ± 0.01 ?Gal mm-1, and (2) an offset of -0.05 ± 0.08 ?Gal yr-1 (-0.31 ± 0.48 mm yr-1). This gravity/uplift ratio is consistent with postglacial rebound (PGR) model predictions and confirms that this ratio is similar in regions of fast and slow uplift. On southern Vancouver Island, situated in the forearc of the northern Cascadia Subduction Zone, AG measurements have been made typically three to four times per year for over a decade at four sites. For these four stations, earlier comparisons between the observed gravity trends and vertical GPS rates indicated a linear DGG that is appropriate for a subduction zone. However there appears to be an offset bias between the gravity trends and the GPS rates in these earlier analyses. We investigate this bias further by systematically reprocessing continuous GPS data from stations of the Western Canada Deformation Array using the PPP software in a manner identical to the mid-continent data. Key aspects of the reprocessing are more consistent and accurate including absolute antenna calibrations of both station and satellite antennas, the IGS05 realization of the ITRF2005 reference frame, and the latest versions of GPS products, processing software and procedures.

  11. Deep crustal metamorphism during continental extension: modern and ancient examples

    Microsoft Academic Search

    Michael Sandiford; Roger Powell

    1986-01-01

    Granulite facies metamorphism in the lower levels of continental crust which is undergoing extension is indicated by unusually high heat flow in modern-day extensional regimes. For certain geometries of extension, particularly those involving crustal-penetrative detachment zones, this metamorphism may occur on a regional scale. The predicted pressure-temperature-time (P-T-t) paths for such metamorphism involve heating into the granulite facies at constant

  12. Yin of birthing and the Yang of destroying continental crust at ocean-margin and crust-suturing subduction zones—exploring evidence about processes, amounts, and the Phanerozoic balance

    NASA Astrophysics Data System (ADS)

    Scholl, D. W.; Stern, R. J.

    2009-12-01

    INTRODUCTION: Field evidence implies that the creation of juvenile continental crust by mantle melting is paired with the destruction of older crust and its recycling to the mantle. The tectonic notion of birthing and annihilation is philosophically expressed by the Chinese concept of the twained and inseparable processes of yin-yang. Since at least the Phanerozoic these opposing processes appear to have been most active at the subduction zones (SZs) of ocean margins (i.e., Peru-Chile margin) and suture-building collision zones (i.e. closing of Tethys). Crust is also created and destroyed by non-plate tectonic processes, in particular additions by hotspot melting and losses by foundering of lower crust. BASIC OBSERVATIONS: Evidence exists that the creation of juvenile crust is impressively voluminous at newly formed SZs, in particular those that build the massifs of offshore arcs. Modern intra-oceanic arcs (e.g., the Aleutian and IBM arcs) have since the early Tertiary grown at a rate of 100-150 km3/Myr/km of SZ, thus adding to the inventory of continental crust at a global rate of about 1.5 km3/yr (= 1.5 Armstrong units or AU). The arc massifs forming along continental margin (e.g., Andean arc) compile at a much slower rate (30 km3/Myr/km of SZ) but globally account for about +1.0 AU of juvenile growth. An additional +0.7 AU is estimated supplied by continental rift and hotspot volcanism. The opposite or destructive Yang processes of sediment subduction and subduction erosion remove and transport crustal material toward and into the mantle at modern ocean-margin SZs. The global rate is evidentially estimated at about -2.5 AU. Similar observations of missing material estimate that a large volume, at least -0.7 AU or greater, of continental crust is loss at the SZs of colliding or suturing crustal blocks. The greater volume of the loss is effected by the detachment and en-mass sinking of deeply underthrust continental edges. THE BALANCE: During the Phanerozoic, creation and destruction of continental crust has either struck a long-term balance at about 3.2 AU, or that more crust has being destroyed than created. This seems possible or even likely to us because the uncertainty comes from limited observations to assess the en-mass loss of deeply underthrust crust and the foundering (sinking) of densified lower crustal rock to which we have presently assigned no estimate.

  13. Sumatran segment of the Indonesian subduction zone: morphology of the Wadati-Benioff zone and seismotectonic pattern of the continental wedge

    Microsoft Academic Search

    V. Hanuš; A. Špi?ák; J. Van?k

    1996-01-01

    The results of a detailed study on the geometrical distribution of earthquake foci in the Sumatran region allowed the authors to distinguish the foci belonging to the recent Wadati-Benioff zone from those occurring in the continental wedge. The morphology of the Wadati-Benioff zone, its main geometrical parameters and the variable depth of its penetration into the upper mantle were established.

  14. 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.

  15. Serpentine in active subduction zones

    NASA Astrophysics Data System (ADS)

    Reynard, Bruno

    2013-09-01

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

  16. Divergence in subduction zones and exhumation of high pressure rocks (Eocene Western Alps)

    NASA Astrophysics Data System (ADS)

    Malusà, Marco G.; Faccenna, Claudio; Garzanti, Eduardo; Polino, Riccardo

    2011-10-01

    Exhumation of high-pressure rocks has long remained a controversial issue in the Earth sciences. In this article, we analyze the tectono-metamorphic, stratigraphic and plate-motion constraints from the Western Alps region, providing new insights on exhumation mechanisms and tectonic evolution during the earliest orogenic stages. Eocene eclogites of the Western Alps form a 20-25 km wide belt on the upper-plate side of the orogen (Eclogite belt), exposed beneath extensional shear zones at the rear of a lower-pressure accretionary wedge. Units of the Eclogite belt show the youngest peak-pressure assemblages within the subduction zone, and experienced superfast tectonic exhumation since 45-40 Ma. The role of erosion was negligible during the whole of this stage. Eocene foreland basins remained starved, and the massive arrival of axial-belt detritus began well after exhumation was completed. Tectonic reconstructions based on fixed-boundaries exhumation models (e.g. channel flow), and/or implying fast erosion at the surface (e.g. slab breakoff), are thus not consistent with geological evidence. In the lack of erosion, exhumation through the overburden requires divergence within the subduction zone. We demonstrate that this was not attained by rollback of the lower plate (Europe), but was instead attained by NNEward motion of the upper plate (Adria-Africa) away from the Western Alps trench. Such motion induced localized extension within the weak portion of the upper plate, at the rear of the accretionary wedge, and allowed tectonic emplacement of the Eclogite belt in the upper crust at rates much faster than subduction rates. Tectonic exhumation ceased in the Oligocene, when oblique-divergence along the Western Alps traverse changed into oblique-convergence. The onset of slow erosional unroofing was synchronously recorded by pressure-temperature paths in all major tectonic units of the Western Alps, and by arrival of orogenic detritus in sedimentary basins. This work demonstrates that divergence between upper plate and trench is a viable mechanism to exhume large and coherent eclogite units in continental subduction zones. Our exhumation model can be applied to other eclogite belts showing a similar exhumational record, including the Western Gneiss Region, the Dabie-Sulu, and eastern Papua New Guinea.

  17. Sediment-derived fluids in subduction zones: Isotopic evidence from veins in blueschist and eclogite of the Franciscan Complex, California

    SciTech Connect

    Nelson, B.K. (Univ. of Washington, Seattle (United States))

    1991-10-01

    Isotopic analyses of minerals from veins that cut high-grade blueschist and eclogite blocks in the central belt of the Franciscan Complex provide constraints on the chronology of metamorphic events and on the origin and movement of fluids within the subduction zone. A Rb-Sr age of 153 {plus minus}1 Ma obtained for minerals from veins and open cavities that formed contemporaneously with retrograde blueschist facies metamorphism is a minimum age for the prograde metamorphism. The veining precedes the last episode of sedimentary-matrix melange formation by a minimum 15 to 20 Ma, during which time the blocks must have been stored within the subduction complex at low temperatures and without undergoing penetrative deformation. Initial Nd-isotope compositions ({epsilon}{sub Nd}) of the vein minerals range from +10.8 to {minus}2.4, indicating that some fluids were derived predominantly from dehydration of subducted mid-ocean ridge basalt, but that other fluids had a component derived from subducted sediment. The provenance of the subducted sediment was within old continental crust, thus associating the Franciscan paleo-subduction complex with a continental craton by the time of vein formation.

  18. 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.

  19. Investigation of upper mantle discontinuities near northwestern Pacific subduction zones using precursors to sSH

    Microsoft Academic Search

    Zhi Zhang; Thorne Lay

    1993-01-01

    Tangential component recordings of deep and intermediate depth earthquakes are analyzed for the presence of sS precursors. It is established that these precursors can be used to determine mantle stratification above subduction zones. For the northwestern Pacific area under study, oceanic crust beneath Izu Japan, continental crust under North Korea with 36 km crustal thickness and 18 percent shear wave

  20. Ancient subduction zone in Sakhalin Island

    NASA Astrophysics Data System (ADS)

    Rodnikov, A. G.; Sergeyeva, N. A.; Zabarinskaya, L. P.

    2013-07-01

    The northern part of Sakhalin Island is an area of recent intensive tectonic movements and hydrothermal processes, as well as a place of accumulation of useful minerals. The deep structure of the lithosphere beneath the region of the Neftegorsk earthquake of May 27, 1995 in North Sakhalin, which killed residents and caused significant destruction, is examined in this paper. Our geodynamic model shows that North Sakhalin consists of the North Sakhalin Basin, Deryugin Basin and an ophiolite complex located between them. The Deryugin Basin was formed in place of an ancient deep trench after subducting the Okhotsk Sea Plate under Sakhalin in the Late Cretaceous-Paleogene. The North Sakhalin Basin was formed on the side of the back-arc basin at that time. The ophiolite complex is fixed in the position of ancient subduction zone that was active in the Late Cretaceous-Paleogene. Approximately in the Miocene, the subduction of the Okhotsk lithosphere apparently ceased. The remains of the subduction zone in the form of an ophiolite complex have been identified from geological and geophysical data. On the surface, the subduction zone is manifested as deep faults stretched along Sakhalin. It is probable that the Neftegorsk earthquake was a result of activation of this ancient subduction zone.

  1. Serpentine and the subduction zone water cycle

    Microsoft Academic Search

    J. P. Morgan; M. Hort; J. A. D. Connolly

    2004-01-01

    Abstract This study explores a chemo-thermo-dynamic subduction zone model that solves for slab dehydration during subduction. We investigate how,changes in the incoming,plate’s hydration and thermal structure may,effect the efficiency of sub-arc water release from sediments, crust, and serpentinized mantle. We find that serpentinized lithospheric mantle may not only be an important fluid source to trigger arc melting but is also

  2. Serpentine and the subduction zone water cycle

    Microsoft Academic Search

    Lars H Rüpke; Jason Phipps Morgan; Matthias Hort; James A. D Connolly

    2004-01-01

    This study explores a chemo-thermo-dynamic subduction zone model that solves for slab dehydration during subduction. We investigate how changes in the incoming plate's hydration and thermal structure may effect the efficiency of sub-arc water release from sediments, crust, and serpentinized mantle. We find that serpentinized lithospheric mantle may not only be an important fluid source to trigger arc melting but

  3. 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.

  4. On the initiation of subduction zones

    NASA Astrophysics Data System (ADS)

    Cloetingh, Sierd; Wortel, Rinus; Vlaar, N. J.

    1989-03-01

    Analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere. Numerical model studies show that if after a short evolution of a passive margin (time span a few tens of million years) subduction has not yet started, continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin. Although much geological evidence is available in supporting the key role small ocean basins play in orogeny and ophiolite emplacement, evolutionary frameworks of the Wilson cycle usually are cast in terms of opening and closing of wide ocean basins. We propose a more limited role for large oceans in the Wilson cycle concept. In general, initiation of subduction at passive margins requires the action of external plate-tectonic forces, which will be most effective for young passive margins prestressed by thick sedimentary loads. It is not clear how major subduction zones (such as those presently ringing the Pacific Basin) form but it is unlikely they form merely by aging of oceanic lithosphere. Conditions likely to exist in very young oceanic regions are quite favorable for the development of subduction zones, which might explain the lack of preservation of back-arc basins and marginal seas. Plate reorganizations probably occur predominantly by the formation of new spreading ridges, because stress relaxation in the lithosphere takes place much more efficiently through this process than through the formation of new subduction zones.

  5. 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.

  6. 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

  7. Two types of peridotite in North Qaidam UHPM belt and their tectonic implications for oceanic and continental subduction: A review

    Microsoft Academic Search

    Shuguang Song; Li Su; Yaoling Niu; Guibin Zhang; Lifei Zhang

    2009-01-01

    Two types of peridotites are recognized in the North Qaidam continental-type UHP metamorphic belt. (1) Garnet peridotite, which includes garnet lherzolite, garnet-bearing dunite, garnet-free dunite and garnet pyroxenite, is one of the most informative lithologies in a continental-type subduction zone. Observations such as diamond inclusion in a zircon crystal and decompression exsolutions in garnet and olivine, plus thermobarometric calculations, argue

  8. A detailed receiver function image of the upper mantle discontinuities in the Japan subduction zone

    Microsoft Academic Search

    X. Li; S. V. Sobolev; R. Kind; X. Yuan; Ch. Estabrook

    2000-01-01

    We have imaged the upper mantle discontinuities in a 30×20° large region at the active continental margin of the Japan subduction zone and neighboring areas, using P-to-S converted phases from teleseismic records of permanent broadband stations. The 410 km discontinuity is detected within ±10 km of its global average position. An interesting exception in its observation is a gap near

  9. Fluids in deeply subducted continental crust: Petrology, mineral chemistry and fluid inclusion of UHP metamorphic veins from the Sulu orogen, eastern China

    NASA Astrophysics Data System (ADS)

    Zhang, Ze-Ming; Shen, Kun; Sun, Wei-Dong; Liu, Yong-Sheng; Liou, J. G.; Shi, Cao; Wang, Jin-Li

    2008-07-01

    The complex vein associations hosted in southern Sulu ultrahigh-pressure (UHP) eclogites contain quartz ± omphacite (or jadeite) ± kyanite ± allanite ± zoisite ± rutile ± garnet. These minerals have chemical compositions similar to those of host eclogites. Inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet, and coesite inclusions were found in vein zircon. These facts suggest that the veins together with host eclogites have been subjected to synchronous UHP metamorphism. The vein minerals contain relatively high concentrations of rare earth elements (REE), high-field-strength elements (HFSE) and transition metal elements (TME). A kyanite-quartz vein has a whole-rock composition similar to adjacent UHP metamorphic granitic gneisses. Abundant primary multi-solid fluid inclusions trapped within UHP vein minerals contain complex daughter minerals of muscovite, calcite, anhydrite, magnetite, pyrite, apatite, celestite and liquid and gas phase of H 2O with solids up to 30-70% of the inclusion volume. The presence of daughter minerals anhydrite and magnetite indicates the subduction fluids were oxidizing, and provides a possible interpretation for the high oxygen fugacity of subduction zone magmas. These characteristics imply that the UHP vein minerals were crystallized from supercritical silicate-rich aqueous fluids that were in equilibrium with peak-UHP minerals, and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as HREE, HFSE and TME. Such fluids might have resulted in major fractionation between Nb and Ta, i.e. the UHP fluids have subchondritic Nb/Ta values, whereas the host eclogites after extraction of the fluids have suprachondritic Nb/Ta values. Therefore, voluminous residual eclogites with high Nb/Ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs.

  10. Improving Seismic Constraints on Subduction Zone Geometry

    NASA Astrophysics Data System (ADS)

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

    2007-12-01

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

  11. Gravity modelling of the Hellenic subduction zone — a regional study

    NASA Astrophysics Data System (ADS)

    Casten, U.; Snopek, K.

    2006-05-01

    The Hellenic subduction zone is clearly expressed in the arc-shaped distribution of earthquake epicenters and gravity anomalies, which connect the Peloponnesos with Crete and Anatolia. In this region, oceanic crust of the African plate collides northward with continental crust of the Aegean microplate, which itself is pushed apart to the south-west by the Anatolian plate and, at the same time, is characterised by crustal extension. The result is an overall collision rate of up to 4 cm/year and a retreating subduction process. Recent passive and active seismic studies on and around Crete gave first, but not in all details consistent, structural results useful for supporting gravity modelling. This was undertaken with the aim of presenting the first 3D density structure of the entire subduction zone. Gravity interpretation was based on a Bouguer map, newly compiled using data from land, marine and satellite sources. The anomalies range from + 170 mGal (Cretan Sea) to - 10 mGal (Mediterranean Ridge). 3D gravity modelling was done applying the modelling software IGMAS. The computed Bouguer map fits the low frequency part of the observed one, which is controlled by variations in Moho depth (less than 20 km below the Cretan Sea and extending 30 km below Crete) and the extremely thick sedimentary cover (partly up to 18 km) of the Mediterranean Ridge. The southernmost edge of the Eurasian plate, with its more triangular-shaped backstop area, was traced south off Crete. Only 50 to 100 km further to the south, the edge of the African continent was traced as well. In between these boundaries there is African oceanic crust, which has a clear arc-shaped detachment line situated at the Eurasian continental edge. The subduction arc is open towards the north, its slab separates hotter mantle material (lower density) below the updoming Moho of the Cretan Sea from colder one (higher density) in the south. Subjacent to the upper continental crust of Crete is a thickened layer of lower crust followed by the subducted oceanic crust with some mantle material as intermediate layer. The depth of the oceanic Moho below Crete is 50 km. The presence and structure of subducted or underplated sediments remains uncertain.

  12. The earthquake cycle in subduction zones

    NASA Technical Reports Server (NTRS)

    Melosh, H. J.; Fleitout, L.

    1982-01-01

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

  13. The dynamics of reactive fluid escape in subduction zones

    NASA Astrophysics Data System (ADS)

    Plümper, Oliver; John, Timm; Podladchikov, Yuri; Scambelluri, Marco

    2014-05-01

    At subduction zones seawater-altered oceanic lithosphere is returned to the Earth's mantle, where increasing pressures and temperatures cause the progressive destabilization of hydrous minerals to liberate immense quantities of aqueous fluids. Understanding the mechanism and non-lithostatic fluid (thermo)dynamics of how fluids are liberated and escape from the subducting oceanic plate is key to develop a quantitative understanding of geochemical cycles and geodynamical processes associated with subduction zones. Fluids released from the subducting slab induce sub-arc mantle melting causing volcanism and induce petrophysical changes during dehydration that can lead to intermediate-depth seismicity. In all these cases large-scale transport systems need to form, where fluids are able to escape from the subducting slab to either migrate up-dip along the subduction channel or into the overlying mantle wedge. Nevertheless, permeability is minimal at the depths and confining pressures relevant to subduction settings, thus insufficient to allow for pervasive fluid flow with high enough fluxes to efficiently drain the subducting oceanic plate. Evidence from the volatile cycle indicates that a fluid extraction mechanism must exist that can keep pace with the slab descent velocity of cm/year to avoid the fluid being lost to the mantle. The tendency of fluid flow to occur channelized in space and time, demonstrated in almost all high-pressure terrains as vein networks, points to a possible mechanism. Channelized fluid flow would enable efficient fluid release rates with high local fluid fluxes over long distances. However, the unresolved questions is; how does a dehydrating system with an intially low, pervasive fluid production develop into a channelized fluid extraction network, allowing effective large-scale fluid transport? By using a combined approach of field observations, reaction microstructures down to the nanoscale and state-of-the-art numerical modelling we investigate one of the most prominent dehydration reaction for the deep volatile cycle, the breakdown of antigorite to form anhydrous olivine. For the first time we are able to link micro- to nanoscale breakdown reaction microstructures characteristic of initial fluid pooling to the development of large-scale, channelizing dehydration vein networks. On the basis of our findings we formulate a consistent mechanistic model of metamorphic fluid escape during mineral dehydration.

  14. 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.

  15. Detachment and/or exhumation depth clusters in subduction zones? Highlights from W. Turkey and comparison with Oman, Corsica and New Caledonia

    NASA Astrophysics Data System (ADS)

    Plunder, Alexis; Agard, Philippe; Chopin, Christian

    2014-05-01

    Recent studies have shown that exhumation of rocks is a fundamentally discontinuous process acting over short-lived time periods (~10 My) during the 'life' of a subduction zone. Recent advances in analytical techniques and in estimating P-T conditions now allow petrologists to attempt characterizing the subduction interface itself and better understanding the mechanisms that enable rocks to detach from the downgoing slab. Important clues would be provided by answering such questions as: (i) Is it the exhumation and/or the detachment preluding to exhumation that is rather a continuous process? (ii) Do the rocks primarily originate from specific depths (thereby pointing to particular conditions of mechanical coupling there) or from all along the subduction interface? Obduction (i.e., emplacement of oceanic lithosphere atop continents) and associated subduction processes provide insight into mechanical coupling at the plate interface, the rheology of the lithosphere and fossilize the different steps of an evolving subduction zone. Field-based data and petrological study in western Turkey are here used to highlight processes acting in a cooling subduction zone during both oceanic and continental subduction and are then compared with other similar geodynamic settings. In western Turkey, the Tav?anl? zone is made of oceanic lithosphere and of a thinned continental margin sequentially subducted below an oceanic plate during the Late Cretaceous. It represents an exceptionally well-preserved subduction interface thanks to later mild collision between the Anatolide-Tauride block and Eurasia. The Tav?anl? zone is divided into three major tectonic units from top to bottom: the obducted ophiolite, an accretionary complex and the continental margin. Among these three main tectonic units, two related either to oceanic or continental subduction consist of HP-LT metamorphic units that are: (i) the oceanic accretionary complex, subdivided in three tectonic units (namely complex 1, 2 and 3 from top to bottom) with different PT conditions (200°C and < 8kbar; 300°C and 12 kbar; 450°C and 17 kbar, respectively); (ii) the cover of the continental margin, which yielded eclogite-facies conditions of 500°C and 24 kbar. Comparisons with similar geodynamic settings (i.e. oceanic then continental subduction without collision: Oman, New Caledonia and Corsica) allow us to point out very similar maximum burial depths for each of those units sharing an equivalent structural position. In each setting up to three clusters of HP-LT conditions might be recognizable, chiefly at 300°C-12 kbar and 500°C-23kbar, and possibly at 450°C-17 kbar too. Those PT conditions show that slicing of kilometre-scale units occurs at fairly specific depths along the subduction interface. We finally tentatively relate these observations to the different seismic events documented in present-day subduction zones along the plate interface.

  16. 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.

  17. 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.

  18. 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.

  19. Controls on earthquake rupture and triggering mechanisms in subduction zones

    E-print Network

    Llenos, Andrea Lesley

    2010-01-01

    Large earthquake rupture and triggering mechanisms that drive seismicity in subduction zones are investigated in this thesis using a combination of earthquake observations, statistical and physical modeling. A comparison ...

  20. Seismic Cycle at the Alaska-Aleutian Subduction Zone

    Microsoft Academic Search

    J. T. Freymueller

    2008-01-01

    Over the last century the Alaska-Aleutian subduction zone has generated an M8+ earthquake about once every 15 years, and it has generated three of the ten largest earthquakes in instrumental history. But the Alaska-Aleutian subduction zone does not display uniform properties along strike. Instead, the shallow plate interface features several distinct segments; individual segments generally have either a very wide

  1. Earthquake size distribution in subduction zones linked to slab buoyancy

    NASA Astrophysics Data System (ADS)

    Nishikawa, Tomoaki; Ide, Satoshi

    2014-12-01

    The occurrence of subduction zone earthquakes is primarily controlled by the state of stress on the interface between the subducting and overriding plates. This stress state is influenced by tectonic properties, such as the age of the subducting plate and the rate of plate motion. It is difficult to directly measure stress on a plate interface. However, the stress state can be inferred using the Gutenberg-Richter relationship's b-value, which characterizes the relative number of small compared to large earthquakes and correlates negatively with differential stress. That is, a subduction zone characterized by relatively frequent large earthquakes has a low b-value and a high stress state. The b-value for subduction zones worldwide varies significantly, but the source of this variance is unclear. Here we use the Advanced National Seismic System earthquake catalogue to estimate b-values for 88 sections in different subduction zones globally and compare the b-values with the age of the subducting plate and plate motions. The b-value correlates positively with subducting plate age, so that large earthquakes occur more frequently in subduction zones with younger slabs, but there is no correlation between b-value and plate motion. Given that younger slabs are warmer and more buoyant, we suggest that slab buoyancy is the primary control on the stress state and earthquake size distribution in subduction zones.

  2. 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.

  3. 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.

  4. 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 ?QFM ~+ 1.5) and at the East African Rift (reduced conditions ?QFM ~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 ?34S values are attributed to recycling of subducted oxidized sulfur (sulfate ~ + 20 ‰) through the CA arc. The ?34S 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 ?34S 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 ?34S value similar to that of Masaya gas (+5 × 0.5 ‰). Considering ?34S 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

  5. Initiation of Subduction Zones: A Consequence of Lateral Compositional Buoyancy Contrast Within the Lithosphere

    NASA Astrophysics Data System (ADS)

    Niu, Y.; O'Hara, M. J.; Pearce, J. A.

    2001-12-01

    Subduction of oceanic lithosphere into deep mantle is one of the key aspects of plate tectonics. Pull by the subducting-slab due to its negative buoyancy is widely accepted as the major driving force for plate motion and plate tectonics. Hence, there would be no plate tectonics if there were no subduction zones. Yet how a subduction zone initiates remains poorly known. Here we show that lateral compositional (vs. thermal) buoyancy contrast within the lithosphere creates the favored and necessary condition for the initiation of a subduction zone by (1) comparing the compositional and density differences between normal oceanic lithosphere (NOL) represented by abyssal peridotites (AP) and subarc lithosphere (SAL) represented by forearc peridotites (FP), and (2) simple physical analysis. As the gravitational attraction is the principal driving force of the subducting slab, it would be optimal if one part of the lithosphere experiences a greater gravitational attraction than its adjacent neighbor prior to or during the initiation of a subduction. This requires the pre-existence of a density contrast within the lithosphere. If the lithosphere is thermally uniform as is often the case, then the density contrast must result from a compositional contrast. This hypothesis can be tested by examining the lithospheric materials on both sides of a subduction zone. Subduction of a dense NOL beneath a buoyant continental lithosphere is straightforward, but intra-oceanic subduction such as in the western Pacific requires a scrutiny. Our data show that FP of Mariana and Tonga - two of the most important intra-oceanic subduction zones on Earth - are compositionally more depleted than AP: Cr#-sp (mean+/- 1? ) = 0.584+/-0.084(FP) vs. 0.307+/-0.134(AP); Mg#-ol = 0.915+/-0.006(FP) vs. 0.898+/-0.082(AP); Mg#-opx = 0.917+/-0.006(FP) vs. 0.908+/-0.006(AP); Mg#-cpx = 0.929+/-0.021(FP) vs. 0.917+/-0.011(AP). As a result, SAL is > 0.7% less dense than NOL. This density contrast due to compositional difference is equivalent to ? T = ~230° C, which is similar to or greater than the postulated thermal buoyancy contrast between a hot mantle plume and its surroundings. While the depleted nature of FP has been interpreted to result from subducting-slab dehydration induced high extents of mantle wedge melting, evidence indicates that the depletion of these FP predates the inception of the subduction, thus these FP are not residues of present-day arc magmatism. Hence, the compositional buoyancy contrast already existed within the lithosphere before the inception of the subduction in the western Pacific. Much of the Mariana SAL may be fragments of old continental lithosphere, whereas the Tonga/Fiji plateau and Kamchatka lithosphere may be remnants of buoyant, hence unsubductable oceanic plateaus (mantle plume head materials) for the Louisville and Hawaiian hotspots respectively. Passive continental margins, where the largest compositional buoyancy contrast exists within the lithosphere, are the loci of future subduction zones. Geometrical analysis shows that the compositional buoyancy contrast within the lithosphere under compression (e.g., ridge push) induces transtensional planes. The weakest plane in the vicinity of the compositional buoyancy contrast develops into a reverse fault. The dense NOL (the foot-wall) tends to sink into the hot and less dense asthenosphere. Calculations show that this tendency to sink reduces both the normal stress to, and shear resistance along, the fault plane, thus easing the sinking and favoring the initiation of a subduction zone. This concept also explains other observations and makes testable predictions on important geodynamic problems.

  6. 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.

  7. 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 (?ST), subduction thrust curvature (CST), and trench curvature angle (?T). 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 (?ST ? 30°), low subduction thrust curvature (CST ? 2.0 × 10-13 m-2) and low trench curvature angles (-6.3° ? ?T ? 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?, ?ST, CST and ?T). 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.

  8. Animation of Volcanism at a Subduction Zone

    NSDL National Science Digital Library

    Jennifer Loomis

    This animation illustrates subduction of an oceanic plate beneath continental crust and the processes that can result in volcanic eruptions. Users can stop, play, fast forward and rewind the animation at any time to stress important points.

  9. Chemical Fluxes in Subduction Zones: Implications for Forearc and Ocean Chemistry

    NASA Astrophysics Data System (ADS)

    Kastner, M.; Martin, J.; Deyhle, A.

    2001-12-01

    Subduction zone pore fluid chemical and isotopic profiles and recent modeling of the fluid flow regimes and solute fluxes indicate rapid and intense altration of the accreted and subducted sediments and mostly episodic expulsion of considerably altered seawater (pore water) into the ocean. Fluid flow is along higher permeability conduits. The extent of altration and the fluid fluxes involved vary, they depend on the type of sediments involved, the ratio of accreted to subducted sediments, the subduction rate, the thermal regime, and the geophysical properties of the subduction zone. The important diagenetic and low-grade metamorphic reactions which modify the fluid compositions, and concurrently the physical and thermal properties of the solids through which they flow are: bacterial and thermal degradation of organic matter; carbonate precipitation and recrystallization; formation and dissociation of gas hydrates; dehydration and transformation of hydrous minerals, especially of clay minerals and opal-A; volcanic ash hydration and alteration, principally zeolitization and smectite formation; and higher temperatures hydrous silicates formation. These fluid-sediment diagenetic and low-grade methemorphic reactions alter the sediment properties of the subduction system. The degree to which these fluid regimes influence the global chemical and isotopic systems, for example the seawater and mantle Cl isotopic compositions, are significant for some components and insignificant for others. An evaluation of the fluid fluxes and associated Li, Cl, Sr, Ca, and Mg chemical and or isotopic budgets will be considered, assuming that the ocean is circulating through the global subduction zones once in 200 to 300 million years. The fluid-sediment reactions and fluid and solutes expelled may alter the bulk chemical composition of the underthrust sediments. If so, it would alter the original concentrations of some typical sediment signatures in volcanic arcs.

  10. Extreme Nb/Ta fractionation in metamorphic titanite from ultrahigh-pressure metagranite

    NASA Astrophysics Data System (ADS)

    Chen, Yi-Xiang; Zheng, Yong-Fei

    2015-02-01

    Extremely high Nb/Ta ratios (up to 239) occur in metamorphic titanite from ultrahigh-pressure metagranite in the Sulu orogen. This indicates significant Nb/Ta fractionation in subduction-zone fluids. By means of U-Pb dating and trace element analysis of titanite, we distinguish the metamorphic domains from the anatectic domains. Titanite U-Pb dating yields lower intercept ages of 215 ± 12 Ma to 222 ± 27 Ma for the metagranite samples, with regardless of the compositional differences between the two types of titanite domains. This indicates the two generations of titanite growth during exhumation of deeply subducted continental crust. The metamorphic titanite shows significantly elevated Nb but decreased Ta and thus higher Nb/Ta ratios than the anatectic titanite. The increase of Nb/Ta ratios for the metamorphic titanite is associated more with a decrease of Ta than an increase of Nb, suggesting the control of fluid composition on the titanite Nb/Ta ratios. Because the metamorphic titanite grew during the exhumation of deeply subducted continental crust, its unusually high Nb/Ta ratios are ascribed to the breakdown of hydrous minerals such as phengite and biotite that host much more Nb than Ta. This implies that the composition of subduction-zone fluids is primarily dictated by the geochemical property of hydrous minerals that break down during dehydration reaction at high-pressure to ultrahigh-pressure conditions. Therefore, significant Nb/Ta fractionation in Ti-rich accessory minerals such as titanite and rutile, at least on the mineral scale, during subduction-zone processes is possibly much more common than previously thought.

  11. Seismic Imaging of the Middle America Subduction Zone Beneath Mexico

    NASA Astrophysics Data System (ADS)

    Miller, M. S.; Kim, Y.; Pearce, F. D.; Clayton, R. W.

    2011-12-01

    P-wave coda from teleseismic events were used to compute receiver functions followed by formal inversions for discontinuous variations in elastic properties beneath a dense seismic array that crosses Mexico from Acapulco on the Pacific coast, through Mexico City, almost to Tempico on the Gulf of Mexico. Broadband data from the Meso-America Subduction Experiment (MASE) line were used to image the subducted Cocos plate and the overriding continental lithosphere beneath central Mexico using a generalized radon transform based migration. Our images provide insight into the process of subducting relatively young oceanic lithosphere. We observe nearly horizontal tectonic underplating of the Cocos oceanic lithosphere beneath the North American continent for a distance of approximately 300 km from the Middle America Trench, with a clear image of a very thin low-velocity oceanic crust (7-8 km) which dips at 15-20 degrees then flattens and slightly thickens (~10 km). At approximately 250 km inland the inferred subducting crust undergoes a change in seismic character, specifically a disruption in the crustal velocity signature, which may reflect the initiation of partial eclogitization of the subducting crust or release of fluids via dehydration that would result in a reduced velocity contrast at the Moho. Farther inland the slab then appears to abruptly change from nearly horizontal to a steeply dipping geometry of approximately 75 degrees underneath the Trans-Mexican Volcanic Belt (TMVB). The image of the steeply subducted Cocos slab underneath the TMVB is enhanced by using the P-to-S converted phases, following the method used in southern Central America to image a steeply dipping subducted slab (> 60 degrees) for the TUCAN experiment (MacKenzie et al, 2010), however is complicated by the wide active volcanic arc and deep sedimentary basins in the middle of the array. The continental Moho is clearly imaged at ~40 km deep beneath the TMVB and shallows (~25 km) towards the Gulf of Mexico. The deeper seismic structure underneath the TMVB shows a prominent negative discontinuity (fast-to-slow) at ~70-80 km within the upper mantle. This feature, which spans horizontally beneath the entire arc (~150 km), may delineate the top of a layer of ponded partial melt, which is consistent with previous geodynamic modeling of melt migration, and also evidenced from P-wave velocity tomographic images of this and other subduction zones although images of this layer was not sharply resolved.

  12. Mobility of HSEs in subduction zone fluids: Evidence from the Franciscan Complex, CA

    NASA Astrophysics Data System (ADS)

    Penniston-Dorland, S. C.; Walker, R. J.; Pitcher, L.; Sorensen, S. S.

    2009-12-01

    The substantial differences in 187Os/188Os and abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Pd) in mantle peridotites compared to subduction zone oceanic crust indicates this suite of elements may prove useful in assessing mantle-slab material transport. There is currently, however, limited information regarding the mobility of the highly siderophile elements (HSE) in aqueous fluids in subduction zone environments. To explore this issue further, we measured HSE abundances and Os isotopic compositions of high-grade meter-scale blocks of metabasite and related rocks from subduction-related mélange within the Franciscan Complex, CA. Different parts of the blocks underwent variable degrees of fluid-rock interaction during subduction-zone metamorphism. Rinds of actinolite-chlorite-phengite rinds observed in high grade blocks have been interpreted by some to be products of fluid-rock interaction. As such they would be metasomatic features that formed when fluids that serpentinized the surrounding ultramafic matrix interacted with the outer portions of the basaltic blocks. The cores of the blocks are characterized by low abundances (<0.05 ppb) of Os, Ir and Ru, and modest concentrations (<1 ppb) of Pt and Pd. Cores are variably radiogenic, with 187Os/188Os ranging from 0.200 to 2.94. These are characteristics consistent with little-modified basaltic presursors. In surprising contrast, HSE concentrations (except for Re) in all of the rinds are elevated relative to typical basalts. Absolute and relative abundances approach values typical of peridotites, and 187Os/188Os are typical of upper mantle peridotites, ranging from 0.126 to 0.132. Both HSE concentrations and isotopic ratios suggest efficient transference of mantle HSE to the block rinds. These results suggest that HSE can be highly mobile in the fluids that serpentinized the ultramafic matrix and interacted with basaltic blocks within the subduction zone. Such mobility is also possible in deeper, UHP-type settings as well, likely constrained only by the depths at which devolatilization occurs.

  13. 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.

  14. Contrasting styles of deformation along the Sumatra subduction zone

    Microsoft Academic Search

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

    2006-01-01

    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

  15. Evidence for large earthquakes at the Cascadia Subduction Zone

    Microsoft Academic Search

    John J. Clague

    1997-01-01

    Large, historically unprecedented earthquakes at the Cascadia subduction zone in western North America have left signs of sudden land level change, tsunamis, and strong shaking in coastal sediments. The coastal geological evidence suggests that many of the earthquakes occurred at the boundary between the overriding North American plate and the subducting Juan de Fuca plate. This hypothesis is consistent with

  16. Evidence for large earthquakes at the Cascadia subduction zone

    Microsoft Academic Search

    John J. Clague

    1997-01-01

    Large, historically unprecedented earthquakes at the Cascadia subduction zone in western North America have left signs of sudden land level change, tsunamis, and strong shaking in coastal sediments. The coastal geological evidence suggests that many of the earthquakes occurred at the boundary between the overriding North American plate and the subducting Juan de Fuca plate. This hypothesis is consistent with

  17. Geodetic Constraints on Fault Coupling on the Cascadia Subduction Zone

    Microsoft Academic Search

    R. McCaffrey; A. Qamar; C. Williams; Z. Ning; P. Wallenberger; R. W. King

    2002-01-01

    The degree of stick-slip coupling on the Cascadia subduction zone and the motions of the surrounding plates, large and small, are inferred by simultaneous inversion of GPS velocities, surface tilt rates, surface uplift rates, surface horizontal strain rates, spreading rates, slip vectors, and transform fault azimuths. Motion of the Pacific plate relative to North America (NA) is fixed and we

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Tracing the cycling of fluids and volatiles through subduction zones continues to be a challenging task with budgets still having large error bars attached to them. In this contribution we show how numerical models can help to integrate various geological, geophysical, and geochemical datasets and how they can be used to put better bounds on the likely amounts of water being subducted, released into the arc and back-arc melting regions, and recycled to the deeper mantle. To achieve this task we use a suite of numerical models. Bending related faulting and hydration of the incoming lithosphere is resolved using a reactive flow model that solves for crustal scale fluid flow and mantle serpentinization using reaction kinetics. Seismic tomography studies from offshore Chile and Central America are used to evaluate and constrain the effective reaction rate. These rates are then used to assess the contribution of serpentinization to the water budget at subduction zones. The pattern of hydration is controlled by the reaction kinetics and serpentinization is most intense around the 270°C isotherm. The depth of this isotherm correlates well with the dominant spacing of double seismic zones observed globally. Comparison of the results with heat flow data suggests that observed seafloor temperature gradients in the bend-fault region are too low to be caused by ';one-pass' downward water flow into the serpentinizing lithosphere, but rather suggest that bend-faults are areas of active hydrothermal circulation. This implies that serpentine-sourced vents and chemosynthetic vent communities should be found in this deep-sea environment as well. Dehydration reactions are resolved with a 2D kinematic subduction zone model that computes the temperature field and the likely locations and volumes of slab fluid release due to metamorphic dehydration reactions. Here we find that up to 1/3 of the subducted water may be transported into the deeper mantle for the coldest subduction zones. Slab fluids that do flux the mantle wedge are commonly believed to trigger arc melting. Finally, the fate of these fluids and the likely mantle flow field within the mantle wedge are resolved in 3D. We find that the classical 2D corner-flow solution is only a small subset of all possible mantle wedge flow fields. In fact, a more 'natural' flow field involves 3D diapirs fuelled by low-density slab fluids rising from the slab surface. These diapirs provide a potential mechanism for decompression melting in the mantle wedge, break the classic corner flow solution, and illustrate the need for high-resolution three-dimensional subduction zones models. In summary we find that numerical models are capable to resolve the key geological processes that control the subduction zone water cycle and help us to better relate subduction input to arc output.

  19. Links Between Geologic Conditions and Lateral Seismicity Variations in Circum-Pacific Subduction Zones (Invited)

    Microsoft Academic Search

    S. L. Bilek; J. Stankova-Pursley

    2009-01-01

    In general, the vast majority of global seismicity occurs within the upper 60 km of subduction zones. However, focused examination of earthquake locations within any given subduction zone shows a complex distribution, with high concentrations of seismicity adjacent to relatively quiet zones. In addition, there is a wide spectrum of earthquake rupture characteristics found in subduction zones, including complex slip

  20. From oceanic subduction to continental collision: An overview of HP-UHP metamorphic rocks in the North Qaidam UHP belt, NW China

    NASA Astrophysics Data System (ADS)

    Zhang, Guibin; Zhang, Lifei; Christy, Andrew G.

    2013-02-01

    The North Qaidam UHPM belt is unique among "continental type" subduction zones in showing well-preserved subducted oceanic relics that predate continental subduction and collision. We review petrologic, thermobarometric, geochemical and geochronological studies for the Yuka, Luliang Shan, Xitieshan and Dulan terranes in this belt. UHP conditions are demonstrated by coesite inclusions in eclogite and country gneiss, and diamond inclusions from garnet peridotite. The relict subducted oceanic lithology crops out in the Shaliuhe cross-section in Dulan terrane; it originated as the floor of a Paleo-Qilian Ocean that existed between Qaidam and Qilian blocks before the early Ordovician. Whereas most eclogites of this belt are derived from mafic rocks of either a Neoproterozoic continental rift or incipient oceanic basin setting, which were subducted along with continental rocks in the early Paleozoic. The data, especially the two protolith sources for eclogites, demonstrate tectonic evolution of the North Qaidam UHPM belt from oceanic subduction to continental collision.

  1. 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.

    2015-02-01

    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.

  2. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  4. 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.

  5. 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.

  6. Electromagnetic Images of the South and Central American Subduction Zones

    Microsoft Academic Search

    Heinrich Brasse

    \\u000a Current and fossil plate margins offer some of the most rewarding targets for geophysical studies. Particularly, the fluid\\/melt\\u000a cycle in subduction zones continues to be of major interest for seismological as well as deep electromagnetic (EM), specifically\\u000a magnetotelluric investigations. In this contribution we describe a number of experiments which have been conducted in several\\u000a ocean-continent convergence zones around the world,

  7. 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

  8. 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

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

    NASA Astrophysics Data System (ADS)

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

    2014-07-01

    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.

  10. Numerical modelling of seismic wave propagation along the plate contact of the Hellenic Subduction Zone-the influence of a deep subduction channel

    Microsoft Academic Search

    Katja Essen; Mandy Braatz; Lars Ceranna; Wolfgang Friederich; Thomas Meier

    2009-01-01

    We model seismic wave propagation from intermediate depth earthquakes in a subduction zone using a 2-D Chebyshev pseudospectral method. Particular attention is directed to the influence of a deep, low-viscosity subduction channel on top of the plate contact where metamorphic rocks may be exhumed by forced return flow. The study is motivated by observations of complicated dispersive and high-amplitude P-

  11. Lithospheric Scale Deformation in Mega-thrust Subduction Zones

    NASA Astrophysics Data System (ADS)

    Furlong, K. P.; Ammon, C.; Lay, T.

    2008-12-01

    Although the general plate tectonic model of subduction zone deformation and its relationship to the earthquake cycle for mega-thrust earthquakes is well known, there is neither consistency in such descriptions nor compatibility among seismological, geodetic, and geologic frameworks for such events. In particular in most seismologic studies of mega-thrust earthquakes there is an implicit assumption that the co-seismic slip is essentially symmetric across the fault surface - that is both the upper and lower plates moved equal amounts (but in opposite directions) during the rupture. Implicit in many geologic studies along convergent margins is the assumption that most permanent deformation is within the upper plate and the subducting slab basically transits the seismogenic zone with little permanent deformation. This perspective serves as the framework for many animations of subduction zone tectonics. Two subduction zone locales, the Kuriles and Solomon Islands, that have hosted recent Mw 8+ earthquakes demonstrate two end-member styles of subduction zone processes neither consistent with the conventional view. The November 2006 (thrust) and January 2007 (normal) earthquake pair in the Kuriles provide an opportunity to quantify the deformation within the subducting Pacific slab during the interseismic period. Based on the correspondence in slip during these events, we are able to both estimate the deformation (dominantly in the subducting slab and not in the overriding plate) and place a constraint on the static frictional strength of the megathrust interface of approximately 2-5 MPa. The 2007 Solomon Island Mw 8+ earthquake shows a distinctly different pattern of interseismic deformation. During this event, the propagating rupture traversed an active transform plate boundary between the separately subducting Australia and Solomon Sea plates. We interpret this to represent a situation in which interseismic deformation is primarily in the upper (Pacific) plate allowing the rupture to jump the fundamental barrier of a plate boundary. This is also compatible with limited GPS data available for the Australia plate near the trench indicating unimpeded subduction of Australia and thus little internal deformation of the subducting slab. These two subduction regimes indicate that there is likely a full continuum in how deformation is accommodated during subduction, and implies that attempts to determine the megathrust (and associated tsunami) potential of subduction zones using observations of upper-plate deformation is problematic.

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

    NASA Astrophysics Data System (ADS)

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

    2015-02-01

    New combined P receiver functions and seismicity data obtained from the EGELADOS network employing 65 seismological stations within the Aegean constrained new information on the geometry of the Hellenic subduction zone. The dense network and large data set enabled us to estimate the Moho depth of the continental Aegean plate across the whole area. Presence of a negative contrast at the Moho boundary indicating the serpentinized mantle wedge above the subducting African plate was seen along the entire forearc. Furthermore, low seismicity was observed within the serpentinized mantle wedge. We found a relatively thick continental crust (30-43 km) with a maximum thickness of about 48 km beneath the Peloponnese Peninsula, whereas a thinner crust of about 27-30 km was observed beneath western Turkey. The crust of the overriding plate is thinning beneath the southern and central Aegean and reaches 23-27 km. Unusual low Vp / Vs ratios were estimated beneath the central Aegean, which most likely represent indications on the pronounced felsic character of the extended continental Aegean crust. Moreover, P receiver functions imaged the subducted African Moho as a strong converted phase down to a depth of about 100 km. However, the converted Moho phase appears to be weak for the deeper parts of the African plate suggesting nearly complete phase transitions of crustal material into denser phases. We show the subducting African crust along eight profiles covering the whole southern and central Aegean. Seismicity of the western Hellenic subduction zone was taken from the relocated EHB-ISC catalogue, whereas for the eastern Hellenic subduction zone, we used the catalogues of manually picked hypocentre locations of temporary networks within the Aegean. Accurate hypocentre locations reveal a significant change in the dip angle of the Wadati-Benioff zone (WBZ) from west (~ 25°) to the eastern part (~ 35°) of the Hellenic subduction zone. Furthermore, a zone of high deformation can be characterized by a vertical offset of about 40 km of the WBZ beneath the eastern Cretan Sea. This deformation zone may separate a shallower N-ward dipping slab in the west from a steeper NW-ward dipping slab in the east. In contrast to hypocentre locations, we found very weak evidence for the presence of the slab at larger depths in the P receiver functions, which may result from the strong appearance of the Moho multiples as well as eclogitization of the oceanic crust. The presence of the top of a strong low-velocity zone at about 60 km depth in the central Aegean may be related to the asthenosphere below the Aegean continental lithosphere and above the subducting slab. Thus, the Aegean mantle lithosphere seems to be 30-40 km thick, which means that its thickness increased again since the removal of the mantle lithosphere about 15 to 35 Ma ago.

  14. Subduction Zone Processes and Implications for Changing Composition of the Upper and Lower Mantle

    NASA Astrophysics Data System (ADS)

    Morris, J. D.; Ryan, J. G.

    2003-12-01

    With ca. forty thousand kilometers of subduction zones and convergence rates from 30 km Ma-1 to 180 km Ma-1, subduction carries massive amounts of material into seafloor trenches, and beyond. Most of the subducting plate is made of mantle material returning to the depths from which it originated. The hydrated and altered upper oceanic section and the overlying sediments, however, carry a record of low-temperature interaction with the ocean, atmosphere, and continents. Subduction and recycling of these components into the mantle has the potential to change mantle composition in terms of volatile contents, heat-producing elements, radiogenic isotope systematics, and trace element abundances. Enrichments in volatile and potassium, uranium, and thorium contents could change the rheological, thermal, and geodynamical behavior of portions of the mantle. Changing isotope and trace-element systematics provide a means for tracking mantle mixing and the possible subduction modification of the deep mantle. A large number of studies point to possible contributions of subducted sediments and altered oceanic crust (AOC) to the mantle-source region for enriched mantle II (EMII) and high mu (HiMU) enriched oceanic island basalts. Transit through the subduction zone, however, changes the composition of the subducting sediment and AOC from that measured outboard of trenches.This chapter focuses on subduction zone processes and their implications for mantle composition. It examines subduction contributions to the shallow mantle that may be left behind in the wedge following arc magma genesis, as well as the changing composition of the slab as it is processed beneath the fore-arc, volcanic front and rear arc on its way to the deep mantle. Much of this chapter uses boron and the beryllium isotopes as index tracers: boron, because it appears to be completely recycled in volcanic arcs with little to none subducted into the deep mantle, and cosmogenic 10Be, with a 1.5 Ma half-life, because it uniquely tracks the contribution from the subducted sediments.The focus here is on subduction processes from trench to rear arc. This chapter starts with a brief discussion of recent thermal models for the downgoing plate and the prograde metamorphic mineralogy of the oceanic crust and sedimentary veneer; the reader is referred to Schmidt and Poli (Chapter 3.17), for an extensive discussion. In the next step it uses 10Be to estimate the absolute mass of sediments subducted to the volcanic arc, in comparison to that supplied to the subduction trenches. Flux balances for 10Be subducted in the sediments versus that erupted in the volcanic arc provide estimates of the fraction of 10Be extracted from the downgoing plate, which can be extrapolated to other elements (cf. Plank and Langmuir, 1993). It subsequently looks at chemical changes for selected elements across the subduction zone, using data from fore-arc serpentinite mud volcanoes, subduction-assemblage metamorphic rocks, high-pressure eclogites, and volcanic lavas from Kurile cross-arc transects, and examines boron-isotope systematics across the convergent margin. Lithium-isotope systematics and comparison of 10Be with uranium-series systematics sometimes delineate multiple stages of subduction modification of the mantle and pinpoint the compositional effects of prior subduction modification on the upper mantle. This contribution ends with estimates of the efficiency of arsenic, antimony, potassium, caesium, rubidium, barium, strontium, uranium, thorium, lead, cerium, samarium, neodymium, lutetium, and hafnium recycling from trench to rear arc, relative to that of boron and beryllium.2.11.2. Thermal Structure and Mineralogy of The Subducting PlateCentral to understanding the recycling of subducted elements in the arc or their subduction to the deep mantle is the temperature variation in the subducting slab, and the prograde mineral assemblages in the sediment, oceanic crust, and lithospheric mantle. Together, they determine where dehydration of the sediments, crust, and deeper subducting mantle occurs, a

  15. 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.

  16. Large earthquake processes in the northern Vanuatu subduction zone

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

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

    PubMed

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

    2007-03-15

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

  18. Variable Rupture Mode at Subduction Zones Around the Pacific

    NASA Astrophysics Data System (ADS)

    Satake, K.

    2005-12-01

    The enormity of the 2004 Sumatra-Andaman earthquake, in comparison with 19th- and 20th-century earthquakes in its rupture area, serves as a reminder that a subduction zone may produce earthquakes larger than those in recorded in the past. Historical record and paleoseismological data show that variability in rupture mode is characteristic of some subduction zones. Infrequent, gigantic earthquakes predominate in geologic records, while historic data tell of more frequent, smaller earthquakes. This implies that along the Cascadia subduction zone, great (M > 8) earthquake can occur more frequently than estimated from paleoseismological record. Like the 2004 Sumatra-Andaman earthquake, the giant 1960 Chilean earthquake (Mw 9.5) was unusually large. Historical predecessors of the 1960 earthquake occurred in 1837, 1737, and 1575. However, midway along the 1960 rupture, only the 1575 event produced geologic records of subsidence and tsunami as obvious as those of 1960. The 1837 and 1737 ruptures were probably small, at least at this latitude (Cisternas et al., 2005). Along the Nankai trough of southwest Japan, recurrence of semi-regular earthquakes has been documented in the 1300 years' written history, with an indication of some variability. The easternmost Suruga trough was ruptured in 1854 but not in 1944, leaving a seismic gap for the anticipated Tokai earthquake. The 1707 earthquake ruptured both Nankai and Tokai sources that ruptured separately in 1854 and in 1944 and 1946. The 1605 earthquake seems to be an unusual tsunami earthquake. Near Tokyo, along the Sagami trough, historical records and marine terraces show two types of large earthquakes (1923 type and 1703 type; Shishikura, 2003); their average recurrence intervals are estimated geologically as several hundred years and a few thousand years, respectively. Earthquakes larger than Mw 8.2 can happen along the southern Kuril trench even though they are unknown from the 200-year written history of Hokkaido. Plate-boundary earthquakes close to M 8, at intervals of 100 years or less, had been considered characteristic in this subduction zone. The 2003 Tokachi-oki earthquake (M 8.0), for instance, was preceded by similar earthquakes, from slightly different source areas, in 1952 and 1843. However, tsunami deposits show that unusually large tsunamis repeated at intervals averaging about 500 yr, with the most recent event in the 17th century (Hirakawa et al., 2000; Nanayama et al., 2003). The inferred inundation area is much wider than those typical earthquakes, and is best explained by earthquakes that broke more than one of the historical segments. Only these multi-segment earthquakes triggered deep postseismic creep that produced decimeters of coastal uplift (Sawai et al., 2004).

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

    NASA Astrophysics Data System (ADS)

    Suarez, G.; Mortera, C.

    2013-05-01

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

  20. Evaluation of seismic reflection characteristics from non-asperities along the subduction zone to actively monitor subduction zone behavior

    NASA Astrophysics Data System (ADS)

    Murase, K.; Tsuruga, K.; Kasahara, J.

    2008-12-01

    It is widely accepted that shallow-intermediate depth major earthquakes along subdution zones occur in asperities surrounding by non-asperities. Non-asperities might comprise liquid-like layer which can generate strong seismic reflections. Rapid change of physical state in the non-asperities might trigger seismic slip along asperities because of high strain accumulation there. Strong seismic reflections were observed along the Japan Trench (Fujie et al., 2002) and the Nankai subduction zones (Iidaka et al, 2003). The observation of strong reflections along the subduction zone suggests the presence of liquid-like layer which comprises non-asperities. We showed the possibility to detect change of physical state in the strong seismic reflection zone in the Tokai subduction zone using ACROSS seismic system (Tsuruga et al., 2006). To perform a better observation, it is necessary to know seismic characteristics assuming presence of liquid-like phase along the subduction zone. For this purpose, we performed waveform simulation. We examined the seismic refraction and wide-angle reflection phases in the subduction zone. In a structural model, the trench axis is located at x =100km at the center of the model, and an oceanic plate subducts beneath the forearc basin between 0 and 100 km. The zone between 100 and 200 km is pure oceanic region. The oceanic crust has 7 km in thick. The thickness of forearc basin is thinning toward the trench axis. Above the subducting plate, 500-m thick decollement with Vp=1.6-2.2 km/s is placed. Vp at just top of the oceanic mantle is 8.0 km/s. The seismograms and travel times were calculated by 2D-FDM (Larsen, 2000) and graph method (Kubota et al., 2005), respectively. Assuming appropriate Vp, Vs, density and Q-values structural models, we computed shot-gather records using 4-Hz Ricker wavelet explosive sources placed at the ocean bottom and receivers aligned at 30-m below the sea surface. Grid space in space is 30 m, and time step is 2 ms. At particular locations, we can recognize the strong reflection from the decollement with negative polarity due to the negative impedance contrast as follows: 1) At 100 km from the trench axis, clear reflection from the decollement at 6-km below the ocean bottom and PmP from the subducting oceanic Moho are identified. Reflection from the decollement has large amplitude between offset distance of 0 and 30"@km, but PmP does large amplitude between 30-50 km. Pn traveling in the oceanic mantle has clear appearance. 2) At 50 km, characteristics of shot-gather records have similar characteristics of ones at 100km case. 3) At 100 km (trench axis), synthetic waveforms at the offset distance > 100km are similar to the seismic records at typical oceanic crust. Large Pg and PmP are identified. Pn is seen for both sides of trench axis. Reflection from PmP around 0-offset is weak, but it has very large amplitude at 20-40 km by wide-angle reflection. The thickness of decollement used is a little thick (0.5km) in our numerical test. If 16-Hz wavelet is used as a source similar to real observation, such layer has about 125 m in thick.. Although we assume a pure liquid layer, we can expect strong seismic reflection from the target layer even for not-pure liquid layer. Through the above simulation, we can choose appropriate observation method for active monitoring of the subduction zone dynamics.

  1. 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.

  2. 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.

  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. Dense GPS Array Observations Across the Nankai Subduction Zone, Southwest Japan

    Microsoft Academic Search

    T. Tabei; S. Miyazaki; M. Hashimoto; T. Matsushima; T. Kato; S. Kato

    2004-01-01

    Interseismic deformation in an oblique subduction zone is a mixture of short-term crustal shortening in the direction of plate convergence and permanent margin-parallel movement of a forearc block. We have deployed two dense GPS traverse arrays across the southwest Japan arc to better illustrate strain partitioning in the Nankai subduction zone. In 1998 we constructed the first array that composed

  5. Distribution of slip from 11 Mw > 6 earthquakes in the northern Chile subduction zone

    E-print Network

    Simons, Mark

    Distribution of slip from 11 Mw > 6 earthquakes in the northern Chile subduction zone M. E in northern Chile (23°­25°S) between the years 1993 and 2000. We invert body wave waveforms and geodetic data. Simons (2006), Distribution of slip from 11 Mw > 6 earthquakes in the northern Chile subduction zone, J

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

    USGS Publications Warehouse

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

    2000-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    Boron Isotope Evidence for Shallow Fluid Transfer Across Subduction Zones by Serpentinized Mantle M. Scambelluri (1), S. Tonarini (2), S. Agostini (2), E. Cannaò (1) (1) Dipartimento di Scienze della Terra, Ambiente e vita, University of Genova, Italy (2) Istituto di Geoscienze e Georisorse-CNR, Pisa, Italy In subduction zones, fluid-mediated chemical exchange between slabs and mantle dictates volatile and incompatible element cycles and influences arc magmatism. Outstanding issues concern the sources of water for arc magmas and its slab-to-mantle wedge transport. Does it occur by slab dehydration beneath arc fronts, or by hydration of fore-arc mantle and subsequent subduction of the hydrated mantle? So far, the deep slab dehydration hypothesis had strong support, but the hydrated mantle wedge idea is advancing supported by studies of fluid-mobile elements in serpentinized wedge peridotites and their subducted high-pressure (HP) equivalents. Serpentinites are volatile and fluid-mobile element reservoirs for subduction: their dehydration causes large fluid and element flux to the mantle.However, direct evidence for their key role in arc magmatism and identification of dehydration environments has been elusive and boron isotopes can trace the process. Until recently, the altered oceanic crust (AOC) was considered the 11B reservoir for arcs, which largely display positive ?11B. However, shallow slab dehydration transfers 11B to the fore-arc mantle and leaves the residual AOC very depleted in 11B below arcs. Here we present high positive ?11B of HP serpentinized peridotites from Erro Tobbio (Ligurian Alps), recording subduction metamorphism from hydration at low-grade to eclogite-facies dehydration. We show a connection among serpentinite dehydration, release of 11B-rich fluids and arc magmatism. The dataset is completed by B isotope data on other HP Alpine serpentinites from Liguria and Lanzo Massif. In general, the ?11B of these rocks is heavy (16 to + 30 permil). No significant B loss and 11B fractionation occurs with burial. Their B and 11B abundance shows that high budgets acquired during shallow hydration are transferred to HP fluids, providing the heavy-boron component requested for arcs. The B compositions of Erro-Tobbio are unexpected for slabs, deputed to loose B and 11B during dehydration: its isotopic composition can be achieved diluting in the mantle shallow subduction-fluids (30 km). The serpentinizing fluids and the fluid-transfer mechanism in Erro-Tobbio are clarified integrating B with O-H and Sr isotopes. Low ?D (-102permil), high ?18O (8permil) of early serpentinites suggest low-temperature hydration by metamorphic fluids. 87Sr/86Sr (0.7044 to 0.7065) is lower than oceanic serpentinites formed from seawater. We conclude that alteration was distant from mid-ocean ridges and occurred at the slab-mantle interface or in forearc environments. We thus provide evidence for delivery of water and 11B at sub-arcs by serpentinized mantle altered by subduction-fluid infiltration atop of the slab since the early stages of burial, witnessing shallow fluid transfer across the subduction zone. Similarity of the B composition of Erro Tobbio with other Alpine serpentinized peridotites suggests that these materials might have spent much of their subduction lifetime at the plate interface, fed by B and 11Bich fluids uprising from the slab.

  8. Resolution experiments for NW Pacific subduction zone tomography

    NASA Technical Reports Server (NTRS)

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

    1989-01-01

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

  9. 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

  10. Deformation across the Alaska-Aleutian Subduction Zone near Kodiak

    USGS Publications Warehouse

    Savage, J.C.; Svarc, J.L.; Prescott, W.H.

    1999-01-01

    The Kodiak-Katmai geodetic array, nine monuments distributed along a profile trending north-northwestward across Kodiak Island and the Alaska Peninsula, was surveyed in 1993, 1995 and 1997 to determine the deformation at the Alaska-Aleutian subduction zone. Velocities on Kodiak Island measured relative to the stable North American plate decrease with distance from the Alaska-Aleutian trench (distance range 106 to 250 km), whereas no appreciable deformation was measured on the Alaska Peninsula (distances 250 to 370 km from the trench). The measured deformation is reasonably well predicted by the conventional dislocation representation of subduction with the model parameters determined independently (i.e., not simply by fitting the observations). The deformation of Kodiak Island is in striking contrast to the very minor deformation measured in the similarly situated Shumagin Islands, 450 km southwest of Kodiak along the Alaska-Aleutian trench.

  11. 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.

  12. Fluid Influence on the Trace Element Compositions of Subduction Zone Magmas

    NASA Astrophysics Data System (ADS)

    Saunders, A. D.; Norry, M. J.; Tarney, J.

    1991-05-01

    Subduction zones represent major sites of chemical fractionation within the Earth. Element pairs which behave coherently during normal mantle melting may become strongly decoupled from one another during the slab dehydration processes and during hydrous melting conditions in the slab and in the mantle wedge. This results in the large ion lithophile elements (e.g. K, Rb, Th, U, Ba) and the light rare earth elements being transferred from the slab to the mantle wedge, and being concentrated within the mantle wedge by hydrous fluids, stabilized in hydrous phases such as hornblende, and phlogopite, from where they are eventually extracted as magmas and contribute to growth of the continental crust. High-field strength elements (e.g. Nb, Ta, Ti, P, Zr) are insoluble in hydrous fluids and relatively insoluble in hydrous melts, and remain in the subducted slab and the adjacent parts of the mantle which are dragged down and contribute to the source for ocean island basalts. The required element fractionations result from interaction between specific mineral phases (hornblende, phlogopite, rutile, sphene, etc.) and hydrous fluids. In present day subduction magmatism the mantle wedge contributes dominantly to the chemical budget, and there is a requirement for significant convection to maintain the element flux. In the Precambrain, melting of subducted ocean crust may have been easier, providing an enhanced slab contribution to continental growth.

  13. Carbon dioxide released from subduction zones by fluid-mediated reactions

    NASA Astrophysics Data System (ADS)

    Ague, Jay J.; Nicolescu, Stefan

    2014-05-01

    The balance between the subduction of carbonate mineral-bearing rocks into Earth's mantle and the return of CO2 to the atmosphere by volcanic and metamorphic degassing is critical to the carbon cycle. Carbon is thought to be released from subducted rocks mostly by simple devolatilization reactions. However, these reactions will also retain large amounts of carbon within the subducting slab and have difficulty in accounting for the mass of CO2 emitted from volcanic arcs. Carbon release may therefore occur via fluid-induced dissolution of calcium carbonate. Here we use carbonate ?18O and ?13C systematics, combined with analyses of rock and fluid inclusion mineralogy and geochemistry, to investigate the alteration of the exhumed Eocene Cycladic subduction complex on the Syros and Tinos islands, Greece. We find that in marble rocks adjacent to two fluid conduits that were active during subduction, the abundance of calcium carbonate drastically decreases approaching the conduits, whereas silicate minerals increase. Up to 60-90% of the CO2 was released from the rocks--far greater than expected via simple devolatilization reactions. The ?18O of the carbonate minerals is 5-10 lighter than is typical for metamorphosed carbonate rocks, implying that isotopically light oxygen was transported by fluid infiltration from the surroundings. We suggest that fluid-mediated carbonate mineral removal, accompanied by silicate mineral precipitation, provides a mechanism for the release of enormous amounts of CO2 from subduction zones.

  14. 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.

  15. 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

  16. 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

  17. 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; (

  18. 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.

  19. 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

  20. 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.

  1. Isotopic Characteristics of Thermal Fluids from Mexican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Taran, Y.; Inguaggiato, S.

    2007-05-01

    Chemical (major and trace elements) and isotopic (H,O,N,C,He) composition of waters and gases from thermal springs and geothermal wells of Mexican subduction zone have been measured. Three main geochemical profiles have been realized: (1) along the frontal Trans-Mexican Volcanic Belt (TMVB) zone through high- temperature Acoculco, Los Humeros, Los Azufres and La Primavera hydrothermal systems, Colima and Ceboruco volcanoes; (2) along the for-arc region of Pacific coast (12 groups of hot springs); (3) across the zone, from Pacific coast to TMVB, through the Jalisco Block. Fluids from El Chichon volcano in Chiapanecan arc system and Tacana volcano from the Central America Volcanic Arc have also been sampled. The frontal zone of TMVB is characterized by high 3He/4He ratios, from 7.2Ra in Ceboruco fumaroles to 7.6Ra in gases from Acoculco and Los Humeros calderas (Ra is atmospheric value of 1.4x10-6). These values are significantly higher than those published earlier in 80-s (up to 6.8Ra). Gases from coastal springs are low in 3He, usually < 1Ra with a minimum value of 0.2Ra in the northernmost submarine Punta Mita hot springs and a maximum value of 2.4Ra in La Tuna springs at the southern board of the Colima graben. An important feature of the TMVB thermal fluids is the absence of excess nitrogen in gases and, as a consequence, close to zero d15N values. In contrast, some coastal for-arc gases and gases from the Jalisco Block have high N2/Ar ratios and d15N up to +5 permil. Isotopic composition of carbon of CO2 along TMVB is close to typical "magmatic" values from -3 permil to -5 permil, but d13C of methane varies significantly indicating multiple sources of CH4 in geothermal fluids and a partial temperature control. High 3He/4He ratios and pure atmospheric nitrogen may indicate a low contribution of subducted sediments into the TMVB magmas and magmatic fluids. In contrast, El Chichon and Tacana fluids show some excess nitrogen (N2/Ar up to 500) and variable d15N, but quite different 3He/4He (up to 8.1Ra at El Chichon and <6.5Ra at Tacana). The data obtained are discussed in terms of the volatile budget of Mexican subduction zone and the local water-rock interaction.

  2. 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.

  3. 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.

  4. 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, N.; 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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Métois, Marianne; Vigny, Christophe; Socquet, Anne

    2014-05-01

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

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

    SciTech Connect

    Scholz, C.H.; Campos, J. [Institut de Physique du Globe de Paris (France)] [Institut de Physique du Globe de Paris (France)

    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.

  8. 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.

  9. A strong-motion database from the Peru–Chile subduction zone

    Microsoft Academic Search

    Maria C. Arango; Fleur O. Strasser; Julian J. Bommer; Ruben Boroschek; Diana Comte; Hernando Tavera

    2011-01-01

    Earthquake hazard along the Peru–Chile subduction zone is amongst the highest in the world. The development of a database\\u000a of subduction-zone strong-motion recordings is, therefore, of great importance for ground-motion prediction in this region.\\u000a Accelerograms recorded by the different networks operators in Peru and Chile have been compiled and processed in a uniform\\u000a manner, and information on the source parameters

  10. H2O release in cold subduction zones: eclogitization vs. lawsonite stability

    NASA Astrophysics Data System (ADS)

    Vitale Brovarone, A.; Groppo, C.; Hetényi, G.; Compagnoni, R.

    2012-04-01

    Transition from blueschist to eclogite facies is considered as a major step of dehydration during subduction of oceanic crust. In cold subduction zones, this critical transitional field is characterized by the stability of lawsonite, which represents the major H2O carrier in HP basaltic rocks. Lawsonite-bearing eclogites are commonly associated with lawsonite-blueschist [1]. This association is commonly referred to prograde (i.e. from blueschist- to eclogite-facies conditions) or retrograde (i.e. from eclogite- to blueschist-facies conditions) incomplete re-equilibration. However, field, microstructural and petrological data indicate that the two assemblages can coexist over a wide PT field. In Alpine Corsica (France), deeply subducted metabasalts are well preserved as lawsonite-bearing eclogite (Law-Ecl) and lawsonite-bearing blueschist (Law-Bs), providing a unique access to these rocks rarely preserved elsewhere. The Corsican Law-Ecl, consisting of omphacite + lawsonite + garnet + phengite + titanite, commonly occur as single undeformed metabasaltic pillows surrounded by Law-Bs. Law-Bs are found as variably deformed metabasaltic pillows locally cross-cut by eclogitic veins and consist of glaucophane + actinolite + lawsonite + garnet + phengite + titanite. Field evidence and microstructures reveal that both Law-Ecl and Law-Bs are stable at the metamorphic peak in the lawsonite-eclogite stability field. Isochemical phase diagrams (pseudosections) calculated in the system MnNKCFMASH for representative Law-Ecl and Law-Bs samples indicate that both lithologies equilibrated at the same conditions of 520 ± 20 °C and 2.3 ± 0.1 GPa, in response of primary differences in the bulk rock compositions, probably acquired during igneous or seafloor metasomatic processes [2]. These PT estimates are comparable with and therefore representative of common PT values registered and preserved by exhumed rocks in HP orogenic belts. Despite the two rocks are omphacite-free (i.e. Law-Bs) and amphibole-free (i.e. Law-Ecl), respectively, PT pseudosections indicate that the water content of the two coexisting rocks is very similar (difference ~ 1% wt). On the contrary, a much more significant water release (~ 3-4%) is observed by crossing the lawsonite-epidote boundary independently from the occurrence of eclogite, blueschist or both coexisting assemblages. This feature indicates that significant water release in cold subduction zones occurs i) at greater depth with respect to eclogitization in rocks that will be incorporated into the mantle, or ii) at lower depth, during the retrograde path in rocks detached from the subducting slab and exhumed. [1] Tsujimori, T., Sisson, V.B., Liou, J.G., Harlow, G.E. & Sorensen, S.S., 2006. Very-low temperature record of the subduction process: a review of worldwide lawsonite eclogites. Lithos, 92, 609-624. [2] Vitale Brovarone, A., Groppo, C., Hetenyi, G., Compagnoni, R. & Malavieille, J., 2011b. Coexistence of lawsonite-eclogite and blueschist: phase diagram calculations from Alpine Corsica metabasalts. J. Metamorph. Geol., doi:10.1111/j.1525-1314.2011.00931.x.

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

    NASA Astrophysics Data System (ADS)

    Shimizu, N.; Mandeville, C. W.

    2011-12-01

    While there is a broad consensus that mantle melting in subduction zones occurs as a result of transport of aqueous fluid (or H2O-rich components) from the subducting slab to the mantle wedge, how and where the transport occurs is still one of the outstanding questions. We report recent SIMS-based sulfur isotope data of input to (pyrites in eclogites) and output from (un-degassed olivine-hosted primitive melt inclusions from arcs) subduction zones, and argue, on the basis of sulfur isotope mass balance, that our results do not support a widely held view of deep fluid transfer from slab to wedge. We suggest, instead, that hydration of the mantle wedge occurs at shallow levels with subsequent subduction and dehydration as the likely source of H2O-rich components for magma generation. Our data from olivine-hosted un-degassed primitive melt inclusions from Galunggung (?34S ranging from -3 to +10 %, average = +2.9% with 1000 - 2000 ppm S), Krakatau (+1.6 - +8.7 %, av = +4.2%, 1200 - 2400 ppm S), and Augustine (+11 - +17%, 2500 - 5200 ppm S) clearly show that mantle wedge (?34S ~0%, ~250 ppm S) has been significantly modified by slab-derived fluid (e.g., seawater with +21%, ~900 ppm S). On the other hand, eclogitic pyrites from the Western Gneiss Region, Norway (2 - 2.5 GPa, 700 - 850°C: Kylander-Clark et al., 2007) range in ?34S from -3.4 to +2.8%, similar to that for altered oceanic crust (e.g., Alt, 1995). Fluid in equilibrium with the eclogitic pyrites could have ?34S up to +10% (Ohmoto and Rye, 1979) and could contain up to ~1000 ppm S, based on the solubility data of Newton and Manning (2005). Mass balance calculations show that more than 10 wt.% of this fluid would be needed for modifying ?34S of the mantle wedge with ~250 ppm S from 0% to +5%, at least an order of magnitude greater than predicted by trace element-based arguments. For fluids with more seawater-like salinity, much more would be necessary for modifying the sulfur isotopic composition of the mantle wedge to that observed in arc magmas, suggesting that deep fluid transfer from slab to wedge is inconsistent with our observations. The results are consistent with a hypothesis for subduction and dehydration of hydrated mantle wedge as a source of H2O-rich components for magma generation (e.g., Hattori and Guillot, 2003; Grove et al., 2009). Alt, J. C. (1995) Geology, 23, 585-588. Grove, T. L. et al. (2009) Nature, 459, 694-697. Hattori, K. and Guillot, S. (2003) Geology, 31, 525-528. Kylander-Clark, A. et al. (2007) Chem. Geol., 242, 137-154. Newton, R. C. and Manning, C. E. (2005) J. Petrol., 46, 701-716. Ohmoto, H. and Rye, R. O. (1979) in Geochemistry of Hydrothermal Ore Deposits, pp 509-567

  12. 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 ?m). 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

  13. Controls on the Migration of Fluids in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Wilson, C. R.; Spiegelman, M. W.; Van Keken, P. E.; Kelemen, P. B.; Hacker, B. R.

    2013-12-01

    Arc volcanism associated with subduction is generally considered to be caused by the transport in the slab of hydrated minerals to sub-arc depths. In a qualitative sense it appears clear that progressive dehydration reactions in the down-going slab release fluids to the hot overlying mantle wedge, causing flux melting and the migration of melts to the volcanic front. However, the quantitative details of fluid release, migration, melt generation and transport in the wedge remain poorly understood. In particular, there are two fundamental observations that defy quantitative modeling. The first is the location of the volcanic front with respect to intermediate depth earthquakes (e.g. 100+/-40 km; England et al., 2004, Syracuse and Abers, 2006) which is remarkably robust yet insensitive to subduction parameters. This is particularly surprising given new estimates on the variability of fluid release in global subduction zones (e.g. van Keken et al. 2011) which show great sensitivity of fluid release to slab thermal conditions. Reconciling these results implies some robust mechanism for focusing fluids and/or melts toward the wedge corner. The second observation is the global existence of thermally hot erupted basalts and andesites that, if derived from flux melting of the mantle requires sub-arc mantle temperatures of 1300 degrees C over shallow pressures of 1-2 GPa which are not that different from mid-ocean ridge conditions. These observations impose significant challenges for geodynamic models of subduction zones, and in particular for those that do not include the explicit transport of fluids and melts. We present a range of high-resolution models that include a more complete description of coupled fluid and solid mechanics (allowing the fluid to interact with solid rheological variations) together with rheologically consistent solution for temperature and solid flow. Focusing on end-members of a global suite of arc geometries and thermal histories we discuss how successful these interactions are at focusing both fluids and hot solids to sub-arc regions worldwide. We will also evaluate the efficacy of current wet melting parameterizations in these models. When driven by buoyancy alone, fluid migrates through the mantle wedge along a near vertical trajectory. Only interactions with the solid flow at very low values of permeability or high values of fluid viscosity can cause deviations from this path. However, in a viscous, permeable medium, additional pressure gradients are generated by volumetric deformation due to variations in fluid flux. These pressure gradients can significantly modify the fluid flow paths. At shallow depths, compaction channels form along the rheological contrast with the overriding plate while in the mantle wedge itself porosity waves concentrate the fluid. When considering multiple, distributed sources of fluid, interaction between layers in the slab itself can also cause significant focusing. As well as permeability, rheological controls and numerical regularizations place upper and lower bounds on the length-scales over which such interactions occur further modifying the degree of focusing seen. The wide range of behaviors described here is modeled using TerraFERMA (the Transparent Finite Element Rapid Model Assembler), which harnesses the advanced computational libraries FEniCS, PETSc and SPuD to provide the required numerical flexibility.

  14. Heat-flow density across the Central Andean subduction zone

    NASA Astrophysics Data System (ADS)

    Springer, Michael; Förster, Andrea

    1998-06-01

    New and revised heat-flow density determinations for the Central Andes in Chile and Bolivia between 60° and 75°W and between 15° and 30°S are presented. Heat flow was determined from temperature logs obtained from fourteen boreholes in northern Chile and from a large Bottom-Hole Temperature (BHT) data set in Bolivia. Thermal data were corrected for perturbations resulting from drilling, topography, and fluid circulation, and effects of porosity and temperature were considered in adjusting thermal conductivity for in-situ conditions. Thus, the 74 reliable heat-flow values available were used to interpret heat flow in the context of lithospheric conditions. The heat flow in the Andean orogen shows a strong dependence on the different tectono-morphologic units. Clusters of heat-flow values projected onto a W-E profile reflect significant changes from the Nazca Plate in the west to the Chaco Basin in the east. Heat-flow density is low (20 mW/m 2) in the area of the Coastal Cordillera and high (50-180 mW/m 2) in the active magmatic arc and the Altiplano. The back-arc region is characterized by a comparably high heat flow (80 mW/m 2) whereas the Andean foreland has a heat flow of 40 mW/m 2, which is slightly lower than the heat flow in the adjacent Brazilian Shield farther east. The Central Andean subduction zone shows a heat-flow pattern similar to the active margin of the North American continent, but differs in the subduction parameters. Consequently, this results in different lithospheric thermal conditions.

  15. 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.

  16. Evidence from ophiolites, blueschists, and ultrahigh-pressure metamorphic terranes that the modern episode of subduction tectonics began in Neoproterozoic time

    NASA Astrophysics Data System (ADS)

    Stern, Robert J.

    2005-07-01

    Earth is the only known planet with subduction zones and plate tectonics, and this fact demonstrates that special conditions are required for this mode of planetary heat loss. Sinking of cold, dense lithosphere in subduction zones is the principal plate-driving force, so plate tectonics could not have begun until Earth cooled sufficiently to allow lithosphere to collapse into the underlying asthenosphere. Direct geologic evidence for when the modern episode of subduction tectonics began focuses on the first appearance of ophiolitic graveyards, blueschist facies metamorphic rocks, and ultrahigh-pressure metamorphic terranes. Ophiolites manifest two modes of lithospheric motion expected from subduction tectonics: seafloor spreading and obduction. High-pressure, low-temperature metamorphic blueschists and ultrahigh-pressure terranes indicate subduction and exhumation of oceanic and continental crust, respectively. These lines of evidence indicate that the modern style of subduction tectonics began in Neoproterozoic time. This revolution in the functioning of the solid Earth may have driven wild fluctuations in Earth's climate, described under the “snowball Earth” hypothesis. These conclusions may be controversial, but suggest fruitful avenues for research in geodynamics and paleoclimate.

  17. Thermo-mechanical model of the mantle wedge in Central Mexican subduction zone and a blob tracing approach for the magma transport

    NASA Astrophysics Data System (ADS)

    Manea, Vlad Constantin; Manea, Marina; Kostoglodov, Vladimir; Sewell, Granville

    2005-03-01

    The origin of the Central Mexican Volcanic Belt (CMVB) and the influence of the subducting Cocos plate on the CMVB volcanism are still controversial. In this study, the temperature and mantle wedge flow models for the Mexican subduction zone are developed using the finite element method to investigate the thermal structure below CMVB. The numerical scheme solves a system of 2D Stokes equations and 2D steady-state heat transfer equation. Two models are considered for the mantle wedge: the first one with an isoviscous mantle wedge and the second one with strong temperature-dependent viscosity. The first model reveals a maximum temperature of ˜830 °C in the mantle wedge, which is not sufficient for melting of wet peridotite. Also, the geotherm of the subducting plate upper surface does not intersect the dehydration-melting solidus for mafic minerals. The second model predicts temperatures of more than 1200 °C beneath the CMVB for a wide range of rheological parameters (reference viscosity and activation energy). Up to 0.6 wt.% H 2O can be released down to 60 km depth through metamorphic changes in the oceanic crust of the subducting slab. The melting of this oceanic crust apparently occurs in a narrow depth range of 50-60 km and also melting of the mantle wedge hydrated peridotite is now expected to take place beneath CMVB. Considering that the melting processes on and in the vicinity of the subducting plate surface generate the most of the volcanic material, a dynamic model for the blob tracers is developed using Stokes flow at infinite Prandtl number. The blobs of 0.2-10.0 km in diameter migrate along very different trajectories only at low wrapping viscosities ( ?w = 10 14-5 × 10 17 Pa s). The modeling results show that the "fast" trajectories terminate at the same focus location at the base of the continental crust, while the arrival points of "slow" trajectories, which are common for the blobs of smaller size (˜0.4-0.5 km), are scattered away from the average focus location. This observation may give us a hint on a possible mechanism of strato and mono volcanoes genesis. The rise time, which the blob detached from the subducted plate, needs to reach the bottom of the continental crust, is from 0.001 up to 14 million years depending on the blob diameter and surrounding viscosity.

  18. 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.

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

    USGS Publications Warehouse

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

    1993-01-01

    The ability of a subducted slab or subducted sediment to contribute many incompatible trace elements to arc source regions may depend on the stabilities of accessory minerals within these rocks, which can only be studied indirectly. In contrast, the role of accessory minerals in lower-T and -P metasomatic processes within paleo-subduction zones can be studied directly in subduction-zone metamorphic terranes. The Gee Point-Iron Mountain locality of the Shuksan Metamorphic Suite, North Cascades, Washington State, is a high-T me??lange of metamafic blocks in a matrix of meta-ultramafic rocks. This me??lange is similar in geologic setting and petrology to the upper part of an unnamed amphibolite unit of the Catalina Schist, Santa Catalina Island, southern California. Both are interpreted as shear zones between mantle and slab rocks that formed during the early stages of subduction. Some garnet amphibolite blocks from the Gee Point-Iron Mountain locality display trace-element enrichments similar to those in counterparts from the Catalina Schist. Some Catalina blocks are highly enriched in Th, rare-earth elements (REE), the high-field-strength elements Ti, Nb, Ta, Zr and Hf (HFSE), U and Sr compared to mid-ocean ridge basalt (MORB), and to other garnet amphibolite blocks in the same unit. Textural and geochemical data indicate that accessory minerals of metamorphic origin control the enrichment of Th, REE and HFSE in blocks from both areas. The Mg-rich rinds around blocks and the meta-ultramafic matrix from both me??langes are highly enriched in a large number of trace elements compared to harzburgites, dunites and serpentinites. Evidence for recrystallization or formation of accessory minerals in the former rocks suggests that these minerals control some of the trace-element enrichments. Data from the Gee Point and Catalina me??langes 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. ?? 1993.

  20. Thermal structure and megathrust seismogenic potential of the Makran subduction zone

    NASA Astrophysics Data System (ADS)

    Smith, Gemma L.; McNeill, Lisa C.; Wang, Kelin; He, Jiangheng; Henstock, Timothy J.

    2013-04-01

    The Makran subduction zone experienced a tsunamigenic Mw 8.1 earthquake in 1945 and recent, smaller earthquakes also suggest seismicity on the megathrust; however, its historical record is limited and hazard potential enigmatic. We have developed a 2-D thermal model of the subduction zone. The results are twofold: (1) The thick sediment cover on the incoming plate leads to high (~150°) plate boundary temperatures at the deformation front making the megathrust potentially seismogenic to a shallow depth, and (2) the shallow dip of the subducting plate leads to a wide potential seismogenic zone (up to ~350 km). Combining these results with along strike rupture scenarios indicates that Mw8.7-9.2 earthquakes are possible in the seaward Makran subduction zone. These results have important earthquake and tsunami hazard implications, particularly for the adjacent coastlines of Pakistan, Iran, Oman, and India, as the Makran has not been previously considered a likely candidate for a Mw > 9 earthquake.

  1. Dependence of earthquake size distributions on convergence rates at subduction zones

    SciTech Connect

    Mccaffrey, R. [Rensselaer Polytechnic Inst., Troy, NY (United States)] [Rensselaer Polytechnic Inst., Troy, NY (United States)

    1994-10-01

    The correlation of numbers of thrust earthquakes of moment magnitude 7 or greater in this century at subduction zones with convergence rate results from a combination of lower recurrence intervals for earthquakes of a given size where slip rates are high and peak in the global distribution of subduction zone convergence rates at high values (55 to 90 mm/yr). Hence, physical mechanisms related to convergence rate, such as plate interface force, slab pull, or thermal effects, are not required to explain the distribution of large earthquakes with convergence rate. The seismic coupling coefficient ranges from 10% to 100% at subduction zone segments where convergence is faster than 45 mm/yr but does not correlate with rate. The coefficient is generally orders of magnitude lower at rates below 40 mm/yr which may be due to long recurrence intervals and a short sampling period (94 years).

  2. Dehydrated fluid and seismic deformation in deep subduction zone

    NASA Astrophysics Data System (ADS)

    Okamoto, K.

    2013-12-01

    It has been considered that there is a correlation between the double seismic zone and metamorphic dehydration reaction in deep slab. The location of the upper limits of the upper seismic plane correspond to metamorphic facies boundary where H2O contents change in subducting crust; numerous earthquakes from 60 to 110 km depths in the lawsonite-blueschist facies, many earthquakes in the lower crust of the slab from 110 to 150 km depths in the lawsonite-amphibole eclogite facies and few earthquakes in the lawsonite eclogite facies. There is still minor amount of H2O present in the lawsonite eclogite facies. The dehydrated fluid is not the only trigger to cause slab earthquake. Recent petrological researches have revealed that both blueschist and lawsonite eclogite are stable in the same pressure and temperature condition because chemical variation including water content creates both lawsonite-amphibole eclogite and lawsonite eclogite in different portion of subducted crust. It would cause stress localization and hydro-fracturing in the slab in the shallower part (depths ranging from 60 to 110 km) and lawsonite amphibole eclogite in the lower crust in the deeper part (from 110 to 150 km depth) in the upper seismic plane. The lower plane of the double seismic zone, is considered to be related to dehydration reaction in the slab. Metamorphic olivine has been described in vein from serpentinite mylonite. The vein was created by dehydration reaction to decompose antigorite under shear deformation. In the cold slab beneath Tohoku arc, the reaction has a negative slope in P-T space and forms olivine+orthopyroxene+fluid. In the warm slab beneath SW Japan, the reaction has a positive slope in P-T space and forms olivine+talc+fluid. The above these dehydration reactions are well-described in the serpentinite from high P/T metamorphic belt from Spain, and Italy, respectively.

  3. Thermobarometric and fluid expulsion history of subduction zones

    SciTech Connect

    Ernst, W.G. (Univ. of California, Los Angeles (United States))

    1990-06-10

    Phanerozoic, unmetamorphosed, weathered, and altered lithotectonic complexes subjected to subduction exhibit the prograde metamorphic facies sequence: zeolite {r arrow} prehnite-pumpellyite {r arrow} glaucophane schist {r arrow} 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 eclogites at depth. Inclusion studies suggest that two-phase immiscible volatiles 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 (e.g., Western Alps) have been partially overprinted by later greenschist and/or epidote-amphibolite facies assemblages. Less common blueschist terranes (e.g., 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. 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 P{sub 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).

  4. 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.

  5. 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.

  6. 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.

  7. Incoming plate faulting in the Northern and Western Pacific and implications for subduction zone water budgets

    NASA Astrophysics Data System (ADS)

    Emry, Erica L.; Wiens, Douglas A.

    2015-03-01

    The greatest uncertainty in the amount of water input into the Earth at subduction zones results from poor constraints on the degree of mantle serpentinization in the incoming plate. Recent studies suggest that the depth of serpentinization within the incoming plate mantle is likely controlled by the depth of extensional faulting caused by lithospheric bending at the outer rise and trench. We explore the maximum depth of extension within the incoming plate at Northwestern Pacific subduction zones in order to estimate the depth limit of serpentinization and to identify any significant variation between subduction zone segments. We relocate trench earthquakes to identify which events occurred within the incoming plate and determine accurate depths for 63 incoming plate earthquakes occurring during 1988-2011 by inverting teleseismic broadband P and SH waveforms. We observe that the top 10-15 km of the incoming plate mantle experiences extensional faulting at all of the subduction zones with a reasonable sample of earthquakes; 60% of the total number of extensional earthquakes occur at crustal depths or within the top 5 km of the incoming plate mantle, 80% occur above 10 km within the mantle, and 95% occur above 15 km. There is evidence for variation throughout the different regions of study, for example extensional earthquakes occur down to 20 km below the crust in the western Aleutians and Izu-Bonin. We propose that the incoming plate mantle is most strongly hydrated in the upper 5 km, and that partial serpentinization exists regionally within the incoming plate mantle to ?15 km. Making reasonable assumptions about the degree of serpentinization and incorporating previous estimates of crustal water, we calculate that the total water carried into the Northern and Western Pacific subduction zones is generally higher than previous estimates, and is approximately 4- 6 ×108 Tg /Myr, or ? 45- 70 ×103 Tg /Myr per kilometer of subduction zone.

  8. Evidence for Seafloor Deformation During Great Subduction Zone Earthquakes of the Sumatran Subduction zone: Results From the First Seafloor Survey Onboard the HMS Scott, 2005

    Microsoft Academic Search

    L. McNeill; T. Henstock; D. Tappin

    2005-01-01

    The 26 December 2004 Mw 9.3 Sumatra-Andaman Islands subduction zone earthquake was the second largest earthquake recorded with fault slip up to 10-25 m and provided the first opportunity to investigate evidence of seafloor deformation immediately after an event of this size. The Royal Navy's HMS Scott conducted a bathymetric survey over the rupture zone during Jan-Feb, 2005. The data

  9. 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 growt

  10. 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.

  11. 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.

  12. Temporal variations of non-volcanic tremor (NVT) locations in the Mexican subduction zone: finding the NVT sweet spot

    E-print Network

    Paris-Sud XI, Université de

    Temporal variations of non-volcanic tremor (NVT) locations in the Mexican subduction zone: finding, México Abstract Epicentral locations of non-volcanic tremors (NVT) in the Mexican subduction zone and are therefore triggered by the SSE. 1. Introduction Non-volcanic tremor (NVT) in the Japan, Cascadia, and Costa

  13. Shallow Thermal Conditions in the Central and Southern Cascadia Subduction Zone J.R. McKenna1

    E-print Network

    Southern Methodist University

    > 9) subduction zone thrust earthquakes. Thermal cross- sections based on measured heat flow. The anomalous character of the Cascadia Subduction Zone (CSZ) is immediately obvious because, based on the instrumental record, almost no shallow thrust (interslab) earthquakes have occurred in the forearc region. Most

  14. 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.

  15. INTRODUCTION The subduction zones to the north and east of Australia

    E-print Network

    INTRODUCTION The subduction zones to the north and east of Australia provide frequent events over in southeastern Australia and dif- ferent parts of Western Australia (Kennett 2003). Fundamental and higher mode (1996) analysed the data from the SKIPPY stations in eastern Australia using the Partitioned Waveform

  16. Subduction zone coupling and tectonic block rotations in the North Island, New Zealand

    E-print Network

    McCaffrey, Robert

    Subduction zone coupling and tectonic block rotations in the North Island, New Zealand Laura M. Wallace and John Beavan Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand Robert Mc Darby Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand Received 17 June 2004

  17. 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.

  18. Fluid Influence on the Trace Element Compositions of Subduction Zone Magmas

    Microsoft Academic Search

    A. D. Saunders; M. J. Norry; J. Tarney

    1991-01-01

    Subduction zones represent major sites of chemical fractionation within the Earth. Element pairs which behave coherently during normal mantle melting may become strongly decoupled from one another during the slab dehydration processes and during hydrous melting conditions in the slab and in the mantle wedge. This results in the large ion lithophile elements (e.g. K, Rb, Th, U, Ba) and

  19. Episodic Tremor and Slip on the Cascadia Subduction Zone: The Chatter of Silent Slip

    Microsoft Academic Search

    Garry Rogers; Herb Dragert

    2003-01-01

    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

  20. Nonvolcanic tremor observed in the Mexican subduction zone Juan S. Payero,1,2

    E-print Network

    Shapiro, Nikolai

    Nonvolcanic tremor observed in the Mexican subduction zone Juan S. Payero,1,2 Vladimir Kostoglodov] Nonvolcanic tremor (NVT) activity is revealed as episodes of higher spectral amplitude at 1­8 Hz in daily, and then a new large SSE occurred in 2006. The tremor burst is dominated by S-waves. More than 100 strong NVT

  1. Inferred pore pressures at the Costa Rica subduction zone: implications for dewatering processes

    E-print Network

    Fisher, Andrew

    Inferred pore pressures at the Costa Rica subduction zone: implications for dewatering processes a direct measure of the rate and magnitude of sediment dewatering. Laboratory consolidation tests indicate use a simple model of fluid flow to demonstrate that dewatering of the underthrust sediments can occur

  2. The Potential of Abiotic Organic Synthesis in Alkaline Environments of Subduction Zones

    NASA Astrophysics Data System (ADS)

    Holm, N. G.

    2010-04-01

    The high pH in parts of subduction zones may promote abiotic formation of carbohydrates like pentose sugars as well as amino acids, nucleosides and even nucleotides, since pyrophosphate is stable under alkaline conditions at low water-rocks ratios.

  3. Tomography of the westernmost Ryukyu subduction zone and the serpentinization of the fore-arc mantle

    E-print Network

    Lin, Andrew Tien-Shun

    Tomography of the westernmost Ryukyu subduction zone and the serpentinization of the fore is not reproduced in this study. We convert VS and VP/VS to perturbations of temperature and serpentinization theoretical modeling of subduction. The serpentinization reaches $15%, or 2% water content, at 50 km depth

  4. Topography of the Calabria subduction zone (southern Italy): Clues for the origin of Mt. Etna

    Microsoft Academic Search

    C. Faccenna; P. Molin; B. Orecchio; V. Olivetti; O. Bellier; F. Funiciello; L. Minelli; C. Piromallo; A. Billi

    2011-01-01

    Calabria represents an ideal site to analyze the topography of a subduction zone as it is located on top of a narrow active Wadati-Benioff zone and shows evidence of rapid uplift. We analyzed a pattern of surface deformation using elevation data with different filters and showed the existence of a long wavelength (>100 km) relatively positive topographic signal at the

  5. Crustal structure and seismic activity at subduction zones Struktura i seizmicka aktivnost subdukcione zone zemljine kore

    E-print Network

    Nedimoviæ, Mladen R.

    , subdukcione i Wadati-Benioff. Ovaj rad ima za cilj da citaocu ukratko izlozi savremena saznanja o subdukcionim i Wadati-Benioff zemljotresima, cija je pojava iskljucivo vezana za zone podvlacenja. Posebna paznja and Wadati-Benioff or intraslab earthquakes, both of which are specific to subduction zones. Much of what we

  6. Geoelectric investigation of the Hellenic subduction zone using long period magnetotelluric data

    Microsoft Academic Search

    D. Galanopoulos; V. Sakkas; D. Kosmatos; E. Lagios

    2005-01-01

    The strongest evidence up to date for a subduction zone in the Hellenic region is a clearly identified Wadati-Benioff zone below the central Aegean Sea, to a maximum depth of 180 km. Alternative seismic tomography models suggest that subduction process continues deeper than the Wadati-Benioff zone to a maximum depth of at least 600 km. So far the lack of

  7. A satellite magnetic perspective of subduction zones, large igneous provinces, rifts, and diffuse plate boundary zones

    Microsoft Academic Search

    M. E. Purucker; K. A. Whaler

    2008-01-01

    Large and intermediate-scale tectonic features such as subduction zones, large igneous provinces, rifts, and diffuse plate boundary zones are often seen to have a magnetic signature visible from the perspective of near-Earth magnetic field satellites such as CHAMP and Orsted. Why do these tectonic features have a magnetic signature, while others do not? A new model of the lithospheric field

  8. Regional P wave velocity structure of the Northern Cascadia Subduction Zone

    Microsoft Academic Search

    K. Ramachandran; R. D. Hyndman; T. M. Brocher

    2006-01-01

    This paper presents the first regional three-dimensional P wave velocity model for the Northern Cascadia Subduction Zone (SW British Columbia and NW Washington State) constructed through tomographic inversion of first-arrival traveltime data from active source experiments together with earthquake traveltime data recorded at permanent stations. The velocity model images the structure of the subducting Juan de Fuca plate, megathrust, and

  9. A low viscosity wedge in subduction zones Magali I. Billen *, Michael Gurnis

    E-print Network

    Billen, Magali I.

    A low viscosity wedge in subduction zones Magali I. Billen *, Michael Gurnis Seismological mantle wedge), which could decrease the viscosity of the mantle locally by several orders of magnitude. Using numerical models we demonstrate that a low viscosity wedge has a dramatic influence on the force

  10. Variability of Near-Term Probability for the Next Great Earthquake on the Cascadia Subduction Zone

    Microsoft Academic Search

    Stephane Mazzotti; John Adams

    2004-01-01

    The threat of a great (M 9) earthquake along the Cascadia subduction zone is evidenced by both paleoseismology data and current strain accumulation along the fault. On the basis of recent information on the characteristics of this subduction system, we estimate the conditional probabilities of a great earthquake occurring within the next 50 years and their variabilities. The most important

  11. Lesser Antilles Subduction Zone Investigation by a Cluster of Large Seismic Experiments in the Forearc Region

    Microsoft Academic Search

    T. Last

    2007-01-01

    Thales LAST stands for Lesser Antilles Subduction zone Team which gathers the scientific teams of a cluster of surveys and cruises that have been carried out in 2007 and coordinated under the European Union THALES WAS RIGHT project (Coord. A. Hirn). This cluster is composed by the German cruise TRAIL with the vessel F\\/S Merian (PI E. Flueh and H.

  12. CAFE: a seismic investigation of water percolation in the Cascadia subduction zone

    Microsoft Academic Search

    S. Rondenay; G. A. Abers; K. C. Creager; S. D. Malone; L. MacKenzie; Z. Zhang; P. E. van Keken; A. G. Wech; J. R. Sweet; T. I. Melbourne; B. R. Hacker

    2008-01-01

    Subduction zones transport water into the Earth's interior. The subsequent release of this water through dehydration reactions may trigger intraslab earthquakes and arc volcanism, regulate slip on the plate interface, control plate buoyancy, and regulate the long-term budget of water on the planet's surface. As part of Earthscope, we have undertaken an experiment named CAFE (Cascadia Arrays for Earthscope) seeking

  13. 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).

  14. 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.

  15. 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.

  16. Comment on "The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle" by W.P. Schellart

    E-print Network

    Doglioni, Carlo

    Discussion Comment on "The potential influence of subduction zone polarity on overriding plate online xxxx The Schellart's [Schellart, W.P., 2007, The potential influence of subduction zone polarity are misleading for the study of the net rotation of the lithosphere since the first 125 km of subduction zones

  17. 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.

  18. 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~?1~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.

  19. 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 Mohorovi?i? 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.

  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. Metamorphic P-T-t evolution of mafic HP granulites in the northeastern segment of the Tarim Craton (Dunhuang block): Evidence for early Paleozoic continental subduction

    NASA Astrophysics Data System (ADS)

    He, Zhenyu; Zhang, Zeming; Zong, Keqing; Xiang, Hua; Klemd, R.

    2014-05-01

    The Dunhuang block, which constitutes the northeastern segment of the Tarim Craton, is located between the Central Asian Orogenic Belt (CAOB) to the north and the Altyn-Qilian Orogenic Belt to the south. The present study reveals that the early Paleozoic HP mafic granulites from the Dunhuang block underwent four stages of metamorphism: prograde amphibolite-facies (M1), peak high-pressure (HP) granulite-facies (M2) and two late stages of amphibolite-facies retrograde metamorphism. Based on phase equilibrium modeling, P-T conditions of the four stages of metamorphism are estimated at 720-750 °C and 11-13 kbar, 760-800 °C and 14-16 kbar, 690-720 °C and 8-8.5 kbar, and at < 6 kbar and < 640 °C respectively. Peak granulite-facies metamorphism is characterized by a low geothermal gradient of ca. 16 °C/km. Zircon U-Pb dating shows that the HP granulite-facies metamorphism occurred at ca. 431 Ma and the early retrograde amphibolite-facies overprint at ca. 403 Ma. Thus the investigated mafic rocks here reveal a clockwise P-T-t path involving burial heating before peak granulite-facies metamorphism and subsequent decompression-cooling with an uplift rate of ca. 0.8 km/Ma. This, together with a continental affinity of the HP metamorphic rocks, indicates that the Dunhuang block experienced a collisional orogenesis during the early Paleozoic.

  2. 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.

  3. 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. (Washington (Paul A.), Southern Pines, NC (United States))

    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.

  4. Record of high-pressure overprint in metamorphic soles of the Tav?anli zone, Western Anatolia

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

    Large obducted ophiolites correspond to the emplacement of dense oceanic lithosphere on top of a continent and thereby provide insights into rheological and thermal coupling between plates or fluid budgets. Obducted ophiolites thrust onto the continental margin of the Anatolide-Tauride block (Western Anatolia, south of the Izmir-Ankara suture zone) are dated through their metamorphic sole at ca. 90-95Ma and derive from the same intra-oceanic Neotethyan subduction. We herein focus on the metamorphic soles of the Tav?anl? zone, which show a variable high-pressure low-temperature (HP-LT) overprint of the initial amphibolitic metamorphic conditions (Önen & Hall, 1993; Dilek & Whitney, 1997; Okay et al, 1998). Systematic sampling was done in both the already studied areas as well as new locations. PT conditions were estimated at 8 kbar and 700°C for the amphibolitic stage with the assemblage hornblende + plagioclase ± garnet ± epidote. The HP-LT metamorphic overprint reached incipient blueschist to blueschist facies PT conditions. Development of the characteristic assemblage glaucophane + lawsonite yields PT estimates of >6-7 kbar and 300°C. The high-pressure stage is similar to the one observed for the underlying accretionary-complex unit of the Tav?anl? zone (Plunder et al, this meeting). This HP overprint was not observed in other obduction contexts such as Oman or New Caledonia but was documented in Fransciscan Complex amphibolites (Wakayabashi, 1990). The record of two metamorphic events can be understood as: (1) rapid cooling of the subduction zone after initiation and the exhumation of the metamorphic sole; (2) reburial after or during exhumation of the amphibolite initially welded at the base of the ophiolite. Several observations (i.e., lack of tectonic contact between the ophiolitic body and the metamorphic sole, PT estimates,...) point to cooling as the most likely hypothesis. Metamorphic soles allow to highlight: (1) the dynamics of obducted material and the evolution of the interplate coupling during subduction and obduction and, based on the available geochronological data, (2) the timing of hanging-wall thermal reequilibration of a young and hot subduction zone to <10 My. Dilek, Y., Whitney, D.L., 1997. Counterclockwise P-T-t trajectory from the metamorphic sole of a Neo-Tethyan ophiolite (Turkey). Tectonophysics 280, 295-310. Okay, A.I., Harris, N.B.W., Kelley, S., 1998. Exhumation of blueschists along a Tethyan suture in northwest Turkey. Tectonophysics 285, 275-299. Önen, A.P., Hall, R., 1993. Ophiolites and related metamorphic rocks from the Kutahya region, north-west Turkey. Geological Journal 28, 399-412. Plunder, A., Agard, P., Chopin, C., Okay, A.I., 2013. Tectono-metamorphic evolution of the Tav?anl? zone, (Western Anatolia): implications for mechanical coupling during subduction/obduction processes. EGU General Assembly 2013, Abstract 8404. Wakabayashi, J., 1990. Counterclockwise P-T-t paths from amphibolites, Franciscan complex, California: relics from the early stages of subduction zone metamorphism. The Journal of Geology, v. 98, p. 657-680.

  5. Geophysical assessment of migration and storage conditions of fluids in subduction zones

    NASA Astrophysics Data System (ADS)

    Pommier, Anne

    2014-12-01

    By enhancing mass transfer and energy release, the cycle of volatiles and melt is a major component of subduction. Investigating this fluid cycle is therefore critical to understand the past and current activity of subduction zones. Fluids can significantly affect rock electrical conductivity and elastic parameters that are measured using electromagnetic and seismic methods, respectively. This letter emphasizes how these geophysical methods complement each other to provide information about the storage of fluids in subduction systems. By compiling electromagnetic and seismic results from various subduction zones, a possible correlation between electrical conductivity and seismic wave attenuation anomalies in the mantle wedge is observed, consistent with fluid accumulation. A possible relationship between geophysical properties and the slab age is also suggested, whereas no significant trend is observed between electrical conductivity or seismic wave attenuation and estimates of water flux in the mantle wedge. These field-based relationships require further constrains, emphasizing the need for new measurements in the laboratory.

  6. Nitrogen systematics and gas fluxes of subduction zones: Insights from Costa Rica arc volatiles

    Microsoft Academic Search

    Mindy M. Zimmer; Tobias P. Fischer; David R. Hilton; Guillermo E. Alvarado; Zachary D. Sharp; James A. Walker

    2004-01-01

    Volcanic gases are a powerful tool for assessing magmatic processes in subduction zones. We report gas chemistry and nitrogen isotope compositions of fumaroles, bubbling springs, and geothermal wells from the Costa Rican segment of the Central American volcanic segment (CAVS), and new correlation spectroscopy (COSPEC) SO2 flux measurements of Poás and Arenal volcanoes. N2\\/He ratios (100-8,250) and nitrogen isotope compositions

  7. Nitrogen systematics and gas fluxes of subduction zones: Insights from Costa Rica arc volatiles

    Microsoft Academic Search

    Mindy M. Zimmer; Tobias P. Fischer; David R. Hilton; Guillermo E. Alvarado; Zachary D. Sharp; James A. Walker

    2004-01-01

    Volcanic gases are a powerful tool for assessing magmatic processes in subduction zones. We report gas chemistry and nitrogen isotope compositions of fumaroles, bubbling springs, and geothermal wells from the Costa Rican segment of the Central American volcanic segment (CAVS), and new correlation spectroscopy (COSPEC) SO2 flux measurements of Poás and Arenal volcanoes. N2\\/He ratios (100–8,250) and nitrogen isotope compositions

  8. The redox state of subduction zones: insights from arc-peridotites

    Microsoft Academic Search

    Ian J. Parkinson; Richard J. Arculus

    1999-01-01

    Spinel peridotites from a variety of island arcs have been utilised to calculate the redox state of the mantle wedge above subduction zones. Oxygen fugacities (fO2 values) calculated from the ferric iron content of spinels, measured by Electron Microprobe (EMP) using secondary standards [Wood, B.J., Virgo, D., 1989. Upper mantle oxidation state: ferric iron contents of lherzolite spinels by 57Fe

  9. Adaptive finite element methods in geodynamics : Convection dominated mid-ocean ridge and subduction zone simulations

    Microsoft Academic Search

    D. R. Davies; J. H. Davies; O. Hassan; K. Morgan; P. Nithiarasu

    2008-01-01

    Purpose – The purpose of this paper is to present an adaptive finite element procedure that improves the quality of convection dominated mid-ocean ridge (MOR) and subduction zone (SZ) simulations in geodynamics. Design\\/methodology\\/approach – The method adapts the mesh automatically around regions of high-solution gradient, yielding enhanced resolution of the associated flow features. The approach utilizes an automatic, unstructured mesh

  10. Pronounced zonation of seismic anisotropy in the Western Hellenic subduction zone and its geodynamic significance

    NASA Astrophysics Data System (ADS)

    Olive, Jean-Arthur; Pearce, Frederick; Rondenay, Stéphane; Behn, Mark

    2014-05-01

    Many subduction zones exhibit significant retrograde motion of their arc and trench. The observation of fast shear-wave velocities parallel to the trench in such settings has been inferred to represent trench-parallel mantle flow beneath a retreating slab. Here, we investigate this process by measuring seismic anisotropy in the shallow Aegean mantle. We carry out shear-wave splitting analysis on a dense array of seismometers across the Western Hellenic Subduction Zone, and find a pronounced zonation of anisotropy at the scale of the subduction zone. Fast SKS splitting directions subparallel to the trench-retreat direction dominate the region nearest to the trench. Fast splitting directions abruptly transition to trench-parallel above the corner of the mantle wedge, and rotate back to trench-normal over the back-arc. We argue that the trench-normal anisotropy near the trench is explained by entrainment of an asthenospheric layer beneath the shallow-dipping portion of the slab. Toward the volcanic arc this signature is overprinted by trench-parallel anisotropy in the mantle wedge, likely caused by a layer of strained serpentine immediately above the slab. Arcward steepening of the slab and horizontal divergence of mantle flow due to rollback may generate an additional component of sub-slab trench-parallel anisotropy in this region. Poloidal flow above the retreating slab is likely the dominant source of back-arc trench-normal anisotropy. We hypothesize that trench-normal anisotropy associated with significant entrainment of the asthenospheric mantle near the trench may be widespread, but only observable at shallow-dipping subduction zones where stations nearest the trench do not overlie the mantle wedge.

  11. A probabilistic tsunami hazard assessment for the Makran subduction zone at the northwestern Indian Ocean

    Microsoft Academic Search

    Mohammad Heidarzadeh; Andrzej Kijko

    2011-01-01

    A probabilistic tsunami hazard assessment is performed for the Makran subduction zone (MSZ) at the northwestern Indian Ocean\\u000a employing a combination of probability evaluation of offshore earthquake occurrence and numerical modeling of resulting tsunamis.\\u000a In our method, we extend the Kijko and Sellevoll’s (1992) probabilistic analysis from earthquakes to tsunamis. The results suggest that the southern coasts of Iran and

  12. H2O transport and release in subduction zones: Experimental constraints on basaltic and andesitic systems

    Microsoft Academic Search

    Stefano Poli; Max W. Schmidt

    1995-01-01

    Phase relationships in natural andesitic and synthetic basaltic systems were experimentally investigated from 2.2 to 7.7 GPa, and 550°C to 950°C, in the presence of an aqueous fluid, in order to determine the stability of hydrous phases in natural subducted crustal material and to constrain reactions resulting in the release of water from subduction zones to the mantle wedge. Water

  13. A revised dislocation model of interseismic deformation of the Cascadia subduction zone

    Microsoft Academic Search

    Kelin Wang; Ray Wells; Stephane Mazzotti; Roy D. Hyndman; Takeshi Sagiya

    2003-01-01

    CAS3D-2, a new three-dimensional (3-D) dislocation model, is developed to model interseismic deformation rates at the Cascadia subduction zone. The model is considered a snapshot description of the deformation field that changes with time. The effect of northward secular motion of the central and southern Cascadia forearc sliver is subtracted to obtain the effective convergence between the subducting plate and

  14. High-Resolution Subduction Zone Seismicity and Velocity Structure in Ibaraki, Japan

    Microsoft Academic Search

    D. R. Shelly; G. C. Beroza; H. Zhang; C. H. Thurber; S. Ide

    2004-01-01

    We use double-difference tomography (tomoDD) [Zhang and Thurber, 2003] and waveform-derived cross-correlation differential arrival times to invert for the earthquake locations and P- and S-wave velocity distributions in the subduction zone under Ibaraki Prefecture of north-central Honshu, Japan. The Ibaraki region is attractive for its high rate of slab seismicity and for the presence of an intermediate-depth double seismic zone.

  15. The 1945 Balochistan earthquake and probabilistic tsunami hazard assessment for the Makran subduction zone

    NASA Astrophysics Data System (ADS)

    Höchner, Andreas; Babeyko, Andrey; Zamora, Natalia

    2014-05-01

    Iran and Pakistan are countries quite frequently affected by destructive earthquakes. For instance, the magnitude 6.6 Bam earthquake in 2003 in Iran with about 30'000 casualties, or the magnitude 7.6 Kashmir earthquake 2005 in Pakistan with about 80'000 casualties. Both events took place inland, but in terms of magnitude, even significantly larger events can be expected to happen offshore, at the Makran subduction zone. This small subduction zone is seismically rather quiescent, but a tsunami caused by a thrust event in 1945 (Balochistan earthquake) led to about 4000 casualties. Nowadays, the coastal regions are more densely populated and vulnerable to similar events. Additionally, some recent publications raise the question of the possiblity of rare but huge magnitude 9 events at the Makran subduction zone. We first model the historic Balochistan event and its effect in terms of coastal wave heights, and then generate various synthetic earthquake and tsunami catalogs including the possibility of large events in order to asses the tsunami hazard at the affected coastal regions. Finally, we show how an effective tsunami early warning could be achieved by the use of an array of high-precision real-time GNSS (Global Navigation Satellite System) receivers along the coast.

  16. 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.

  17. B-type olivine fabric in the mantle wedge: Insights from high-resolution non-Newtonian subduction zone models

    Microsoft Academic Search

    Erik A. Kneller; Peter E. van Keken; Shun-ichiro Karato; Jeffrey Park

    2005-01-01

    Several hypotheses have been proposed to explain trench-parallel shear wave splitting in the mantle wedge of subduction zones. These include 3-D flow effects, parallel melt filled cracks, and B-type olivine fabric. We predict the distribution of B-type and other fabrics with high-resolution thermal and stress models of subduction zones. A composite viscous rheology is used that incorporates wet diffusion creep,

  18. 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 (???).

  19. 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.

  20. 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 ?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. PMID:17389388

  1. 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: \

  2. The subducted oceanic crust within continental-type UHP metamorphic belt in the North Qaidam, NW China: Evidence from petrology, geochemistry and geochronology

    Microsoft Academic Search

    Guibin Zhang; Shuguang Song; Lifei Zhang; Yaoling Niu

    2008-01-01

    Three types of eclogite, together with a serpentinized harzburgite, coexist as blocks within granitic and pelitic gneisses along the Shaliuhe cross section, the eastern part of the North Qaidam continental-type ultrahigh-pressure (UHP) metamorphic belt, NW China. The olivine (Ol1) and orthopyroxene in the harzburgite are compositionally similar to present-day abyssal peridotites. The kyanite–eclogite is derived from a troctolitic protolith, whereas

  3. Multistage metamorphism of orogenic garnet–lherzolite from Zhimafang, Sulu UHP terrane, E. China: Implications for mantle wedge convection during progressive oceanic and continental subduction

    Microsoft Academic Search

    Kai Ye; Yan Ru Song; Yi Chen; Hai Jin Xu; Jing Bo Liu; Min Sun

    2009-01-01

    Seven stages of mineral assemblage were recognized in the Zhimafang orogenic garnet lherzolite from the Sulu ultrahigh-pressure (UHP) metamorphic terrane of eastern China, which suggest that it experienced progressive mantle wedge convection during subduction of previous oceanic and subsequent continental slabs. M1 is recorded by inclusions of high-Mg olivine, high-Mg–Al–Cr–Ca orthopyroxene and high-Mg clinopyroxene in the high Ca–Cr cores of

  4. Frictional Behaviour of Sediment Entering The Nankai Subduction Zone and Its Implications For Wedge Taper and Onset of Seismicity

    NASA Astrophysics Data System (ADS)

    Kopf, A. J.; Brown, K. M.; Weinberger, J. L.

    Multi-component sediment recovered from numerous drilling legs off SW Japan allow to characterize the input into the Nankai subduction zone. The succession approach- ing the large accretionary wedge consists of fine-grained turbidites interspersed with tephra, the first of which are dominated by smectite- (S) or illite-rich (I) clayey in- tervals within silts and fine sands. Combined ring shear and direct shear testing of saturated sediment up to effective normal stresses of 40MPa show friction coefficients (?) of 0.36>?<0.65 for the coarser, sand- and tephra-rich intervals and lower strength (0.1>?<0.26) for clay-rich specimens. Along plate boundary subduction thrusts, one of the crucial mineralogical processes thought to affect frictional response and seis- mic faulting is the sm-il transition at 70-150rC. Within the range of effective normal stresses tested (similar to those along the subduction thrust in vivo), however, both sm and il show a weak response (?=0.1-0.22) and velocity strengthening behaviour. Monitored pore pressures across the fault plane rarely rise above 2The increase in effective shear stress as well as ? along the décollement with distance from the de- formation front, as calculated from our experiments and pore fluid data, agrees well with a steepening of the prism and abundant out-of-sequence thrusting. Given the low intrinsic friction of illite, however, the S-to-I reaction seems an unlikely candidate to be responsible for the aseismic-seismic transition along subduction thrusts. An overall increase in ? along the plate boundary fault (in conjunction with thermal structure and overall fluid supply) may well constrain the position of the up-dip limit of the seis- mogenic zone at depth, most likely as a function of quartz cementation, zeolite and low-grade metamorphic processes, or incipient lithification.

  5. Mantle peridotites from continental rifts to ocean basins to subduction zones

    Microsoft Academic Search

    Enrico Bonatti; Peter J. Michael

    1989-01-01

    Some key parameters for mantle-derived spinel peridotites from the North Atlantic, such as reconstructed primary modal and bulk composition, Al2O3 content of orthopyroxene (opx), Fo content of olivine, and 100 Cr\\/(Cr + Al) of spinel, have been compared with the same parameters for peridotite bodies from preoceanic rifts (Zabargad Island in the Red Sea), passive ocean margins (Iberian and Spitsbergen

  6. Evidence for Seafloor Deformation During Great Subduction Zone Earthquakes of the Sumatran Subduction zone: Results From the First Seafloor Survey Onboard the HMS Scott, 2005

    NASA Astrophysics Data System (ADS)

    McNeill, L.; Henstock, T.; Tappin, D.

    2005-12-01

    The 26 December 2004 Mw 9.3 Sumatra-Andaman Islands subduction zone earthquake was the second largest earthquake recorded with fault slip up to 10-25 m and provided the first opportunity to investigate evidence of seafloor deformation immediately after an event of this size. The Royal Navy's HMS Scott conducted a bathymetric survey over the rupture zone during Jan-Feb, 2005. The data reveal the geomorphology of the deformation front, accretionary wedge, outer-arc high fault system and forearc basin of the southern 2004 rupture zone. Primary surface rupture during large thrust/reverse fault earthquakes is rare and complex and the nature of shallow slip during submarine subduction zone events is particularly poorly known. Data from the southern part of the 26/12/04 rupture zone reveal numerous small neotectonic features at the toe of the accretionary prism. These take the form of scarps, folds and depressions 5-100 m in height on the seaward limb of the frontal thrust ridges. If these features are formed during coseismic slip and slip is focused on a few faults, they may represent slip during a few or possibly a single great earthquake. The morphology of the frontal thrusts strongly suggests landward-vergent (seaward-dipping) faulting. A model of backthrust fault slip or bending moment folding during coseismic plate boundary slip can explain the position of these features on the seaward fold limb of the frontal thrust. We suggest that rupture may propagate to the seafloor close to the deformation front in major plate boundary earthquakes.

  7. 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.

  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. 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.

  10. Seismic versus aseismic slip: Probing mechanical properties of the northeast Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Shirzaei, M.; Bürgmann, R.; Uchida, N.; Hu, Y.; Pollitz, F.; Matsuzawa, T.

    2014-11-01

    Fault slip may involve slow aseismic creep and fast seismic rupture, radiating seismic waves manifested as earthquakes. These two complementary behaviors accommodate the long-term plate convergence of major subduction zones and are attributed to fault frictional properties. It is conventionally assumed that zones capable of seismic rupture on the subduction megathrust are confined to between about 10 to 50 km depth; however, the actual spatiotemporal distribution of fault mechanical parameters remains elusive for most subduction zones. The 2011 Tohoku Mw 9.0 earthquake ruptured with >50 m slip up to the trench, thus challenging this conventional assumption, and provides a unique opportunity to probe the mechanical properties of the Japan subduction zone. Drawing on the inferred distribution of coseismic and postseismic slip, it has recently been suggested that portions of the megathrust are capable of switching between seismic and aseismic behavior. Kinematic models of the coseismic rupture and 15-month postseismic afterslip of this event suggest that the coseismic rupture triggered widespread frictional afterslip with equivalent moment magnitude of 8.17-8.53, in addition to viscoelastic relaxation in the underlying mantle. The identified linear relation between modeled afterslip, slip inferred from repeating earthquakes on the plate interface, and the cumulative number of aftershocks within 15 km distance of the subduction thrust suggests that most aftershocks are a direct result of afterslip. We constrain heterogeneous rate-state friction parameters of the subduction thrust from the computed coseismic stress changes and afterslip response. Our results indicate a variable pattern along dip and strike, characterizing areas down-dip and south of the main rupture zone as having velocity-strengthening properties. In agreement with seismic tomographic models of plate boundary elastic properties and geologic evidence for previous M >8.5 megathrust earthquakes on this section of the plate boundary, we suggest that the obtained pattern of the frictional properties is characteristic of subducted material and thus persistent in time and space.

  11. 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.

  12. Interseismic locking on the Hikurangi subduction zone: Uncertainties from slow-slip events

    NASA Astrophysics Data System (ADS)

    McCaffrey, Robert

    2014-10-01

    Interseismic locking on the Hikurangi subduction zone in New Zealand is examined in light of alternative assumed locking distributions and the impact of transients (slow-slip and volcanic sources) on temporal and spatial resolution. The modern pattern of locking in the north is poorly resolved and, based on simulations of possible transient behavior, may be an ephemeral feature of the subduction cycle. While there appears to be some contemporary locking in the northern half of the Hikurangi subduction zone (HSZ), its location is model dependent, and hence, its relationship to structure, slow-slip, or any transition zone there is unclear. Simulations of site velocities using the 14 year history of transient events reveal that in the timescale of the interseismic period the northern half of the HSZ could be either locked or unlocked, and this may not be resolvable for decades. In the southern half, there is strong contemporary locking in the 15 to 40 km depth range, but again, the slow-slip history leads to uncertainty in the long-term pattern. Slow-slip events not only reduce the long-term locking by aseismic slip but also greatly hinder our ability to see it. It is within the range of possible models that the slip deficit rate at the HSZ is more uniform along strike, and the modern appearance is controlled by the particular pattern of transients over the past 10 to 20 years when the GPS data were collected. Similarly, uncertainties in surface velocities will be large at any subduction zone with large transients.

  13. 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.

  14. 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.

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

    SciTech Connect

    Hacker, B.R. [Stanford Univ., CA (United States)] [Stanford Univ., CA (United States)

    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.

  16. Molecular modelling of rare earth element complexation in subduction zone fluids

    NASA Astrophysics Data System (ADS)

    van Sijl, Jelle; Allan, Neil L.; Davies, Gareth R.; Westrenen, Wim van

    2009-07-01

    Complexation of (trace) elements in fluids plays a critical role in determining element mobility in subduction zones, but to date, the atomic-scale processes controlling elemental solubilities are poorly understood. As a first step towards computer simulation of element complexation in subduction zone fluids, a thermodynamic cycle was developed to investigate the hydration environment and energetics of lanthanide complexes using density functional theory. The first solvation shell is explicitly defined and the remaining part of the aqueous fluid is modelled using a polarisable continuum model, which allows extrapolation to a broad pressure and temperature range. We illustrate our method by comparing solvation of lanthanide series elements in H 2O in the presence of fluoride or chloride for conditions relevant to subduction zones. The energetics of lanthanide- and lanthanide-fluoride/chloride hydration complexes were determined computationally. Calculated hydration free energies for trivalent lanthanides with explicit eight- and nine-fold coordinated first hydration shells show good agreement with literature data at room pressure and temperature. The hydration free energy is more negative for smaller complexes (heavy lanthanides) relative to larger complexes (light lanthanides), with the difference between La and Lu in water amounting to 361 kJ mol -1. The hydration free energy of all lanthanide ions becomes less negative with increasing pressure ( p) and temperature ( T) up to 2.5 GPa and 1000 K (typical conditions in the upper part of subducting slabs). The free energy difference between light- and heavy-lanthanides remains essentially unchanged at elevated ( p, T) conditions. There are minor geometrical differences in local hydration environment between light lanthanide-chloride (La-Nd) and heavy lanthanide-chloride (Pm-Lu) hydrated complexes, without a distinguishable energy difference. Complexation with fluoride is energetically more favourable than with chloride by 206 ± 4 kJ mol -1 across the entire lanthanide series. The association of fluoride-water and chloride-water fragments with lanthanide-water complexes is energetically more favourable for aqueous lanthanide complexes surrounded by fewer first hydration shell water molecules. The methods developed in this study, in conjunction with simulation of the energetics of trace element incorporation into minerals, open the possibility to use molecular modelling to constrain elemental behaviour in subduction zones.

  17. Seismic structure around the 660 in subduction zones and its implications

    NASA Astrophysics Data System (ADS)

    Wang, Tao

    Velocity structure around the 660-km seismic discontinuity in subduction zones is crucial for understanding the thermal state, chemical composition, and dynamics of the mantle. Because of limited spatial coverage of data and resolution, the complicated structure, especially in sub-slab regions, remains ambiguous. Topographic variation of the oceanward 660-km discontinuity in the Western Pacific is investigated by stacking S-to-P converted phases using large-array data. Beneath the Philippine Sea and Southern Kuril Islands, the mean depth of this discontinuity is elevated ~10 km and ~6.5 km with respect to the global average depth. Meanwhile, it is normal beneath the Sea of Japan where the subducted slab appears to be near-horizontally flattened above the 660-km discontinuity. Elsewhere, the 660-km discontinuity is ~11.5 km and ~6.4 km shallower beneath the Marianas and Tonga Islands subduction zones, respectively. This topographic variation suggests that the 660-km discontinuity beneath subducting slabs shallows as the dip of the slab increases. Using sources in the western Pacific and stations in China, 3D triplication waveform modeling of structure beneath the Southern Kuril subduction zone suggests that the high-velocity slab (+5% anomaly and ~100km thick) subducts to at least 560 km. No obvious high-velocity anomaly of the slab is observed around the 660 km depth. To match late AB and BC branch arrives at large distances, one of two possible low-velocity anomalies (-3% and ~30km thick) is necessary. The first is located beneath the subducted slab and might be due to a hot sinking anomaly emplaced at shallower depths away from the subduction zone. The second location is on the landward side of the subducted slab and above the 660-km discontinuity. We suggest this low-velocity anomaly could be generated by hot upwelling from lower mantle or small scale convection above the deep slab. Numerical modeling of mantle convection demonstrates that slab entrainment of hot sinking anomalies can explain both the topographic variation of the 660-km discontinuity oceanward of slabs and the possible low-velocity anomaly beneath the subducted slab in the Southern Kuriles.

  18. 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.

  19. 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-? 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.

  20. Lithospheric structure of Pampean flat slab (latitude 30-33degreesS) and Northern Costa Rica (latitude 9-11degreesN) subduction zones

    NASA Astrophysics Data System (ADS)

    Linkimer Abarca, Lepolt

    The Pampean flat slab subduction in west-central Argentina (latitude 30-33S) and the steeply dipping Northern Costa Rica subduction zone (latitude 9-11N) show significant along-trench variations in both the subducting and overriding plates. This dissertation contains the results of three seismological studies using broadband instruments conducted in these subduction zones, with the aim of understanding the structure of the lithosphere and the correlation between the variability observed in the downgoing and the overriding plates. In the Costa Rica region, by analyzing teleseismic receiver functions we investigate the variability in the hydration state of the subducting Cocos Plate and the nature of three distinct crustal terranes within the overriding Caribbean Plate: the Nicoya and Chorotega terranes that display an oceanic character, and the Mesquito Terrane, which is more compatible with continental crust. In the Pampean region of Argentina, we apply a regional-scale double-difference tomography algorithm to earthquake data recorded by the SIEMBRA (2007-2009) and ESP (2008-2010) broadband seismic networks to obtain high-resolution images of the South America lithosphere. We find that most of the upper mantle has seismic properties consistent with a depleted lherzolite or harzburgite, with two anomalous regions above the flat slab: a higher Vp/Vs ratio anomaly consistent with up to 10% hydration of mantle peridotite and a localized lower Vp/Vs ratio anomaly consistent with orthopyroxene enrichment. In addition, we study the geometry and brittle deformation of the subducting Nazca Plate by determining high-quality earthquake locations, slab contours, and focal mechanisms. Our results suggest that the subduction of the incoming Juan Fernandez Ridge controls the slab geometry and that ridge buoyancy and slab pull are key factors in the deformation of the slab. The spatial distribution of the slab seismicity suggests variability in the hydration state of the subducting Nazca Plate and/or in strain due to slab bending. These observations support the hypothesis that the along-trench variability in bathymetric features and hydration state of the incoming plate has profound effects in the subducting slab geometry and the upper plate structure in both flat and steeply dipping subduction zones.

  1. Serpentinization and infiltration metasomatism in the Trinity peridotite, Klamath province, northern California: implications for subduction zones

    Microsoft Academic Search

    Simon M. Peacock

    1987-01-01

    The Trinity peridotite was emplaced over metabasalts and metasedimentary rocks of the central metamorphic belt along the Devonian Trinity thrust zone. Three metamorphic events can be recognized in the Trinity peridotite: (1) antigorite (dD= -63 to -65%.) formation related to regional underthrusting of the central metamorphic belt; (2) contact metamorphism associated with Mesozoic dioritic plutons; and (3) late-stage formation of

  2. 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.

  3. 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.

  4. 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.

  5. 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 (?), mass-specific magnetic susceptibility (?), natural remanent magnetization (NRM) and magnetic hysteresis loops, temperature-dependent magnetic susceptibility (?-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.

  6. 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.

  7. 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.

  8. 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.

  9. 3D Finite-Difference Modeling of Scattered Teleseismic Wavefields in a Subduction Zone

    NASA Astrophysics Data System (ADS)

    Morozov, I. B.; Zheng, H.

    2005-12-01

    For a teleseismic array targeting subducting crust in a zone of active subduction, scattering from the zone underlying the trench result in subhorizontally-propagating waves that could be difficult to distinguish from converted P- and S- wave backscattered from the surface. Because back-scattered modes often provide the most spectacular images of subducting slabs, it is important to understand their differences from the arrivals scattered from the trench zone. To investigate the detailed teleseismic wavefield in a subduction zone environment, we performed a full-waveform, 3-D visco-elastic finite-difference modeling of teleseismic wave propagation using a Beowulf cluster. The synthetics show strong scattering from the trench zone, dominated by the mantle and crustal P-waves propagating at 6.2-8.1.km/s and slower. These scattered waves occupy the same time and moveout intervals as the backscattered modes, and also have similar amplitudes. Although their amplitude decay characters are different, with the uncertainties in the velocity and density structure of the subduction zone, unambiguous distinguishing of these modes appears difficult. However, under minimal assumptions (in particular, without invoking slab dehydration), recent observations of receiver function amplitudes decreasing away from the trench favor the interpretation of trench-zone scattering.

  10. Seismicity and structural heterogeneities around the western Nankai Trough subduction zone, southwestern Japan

    NASA Astrophysics Data System (ADS)

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

    2014-06-01

    The Nankai and Hyuga-nada seismogenic segments, in the western part of the Nankai subduction zone off southwestern Japan, have sometimes ruptured separately and sometimes simultaneously. To investigate the relationships among heterogeneities of seismic structure, spatial variation of the incoming plate, and the seismogenic segments, we carried out seismic observations in the western Nankai subduction zone and modeled the area with 3D seismic tomography using both onshore and offshore seismic data. Our seismic observations suggested that the pattern of seismicity is related to heterogeneities within the subducted plate rather than the seismogenic segments. The up-dip depth limit of seismicity along the plate boundary and in the oceanic crust is typically around 15 km, corresponding to the depth of dehydration of the oceanic crust. In addition, the seaward-extended seismicity observed where the subducted plate was considered to have rough internal structures. In the resulting velocity model, the up-dip limit of the area where the P-wave velocity just above the plate boundary exceeds 6 km/s corresponds to the up-dip limit of coseismic slip in the 1968 Hyuga-nada and 1946 Nankai earthquakes. Between the two coseismic rupture zones is an area of lower P-wave velocity about 40 km wide that is evidence of lateral heterogeneities in the upper plate along the trough-parallel direction. Structural heterogeneities in the upper plate may explain the variety of coseismic slip patterns in this region.

  11. 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.

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

    SciTech Connect

    Cloos, M. (Univ. of Texas, Austin (United States))

    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.

  13. A strong-motion database from the Peru-Chile subduction zone

    NASA Astrophysics Data System (ADS)

    Arango, Maria C.; Strasser, Fleur O.; Bommer, Julian J.; Boroschek, Ruben; Comte, Diana; Tavera, Hernando

    2011-01-01

    Earthquake hazard along the Peru-Chile subduction zone is amongst the highest in the world. The development of a database of subduction-zone strong-motion recordings is, therefore, of great importance for ground-motion prediction in this region. Accelerograms recorded by the different networks operators in Peru and Chile have been compiled and processed in a uniform manner, and information on the source parameters of the causative earthquakes, fault-plane geometries and local site conditions at the recording stations has been collected and reviewed to obtain high-quality metadata. The compiled database consists of 98 triaxial ground-motion recordings from 15 subduction-type events with moment magnitudes ranging from 6.3 to 8.4, recorded at 59 different sites in Peru and Chile, between 1966 and 2007. While the database presented in this study is not sufficient for the derivation of a new predictive equation for ground motions from subduction events in the Peru-Chile region, it significantly expands the global database of strong-motion data and associated metadata that can be used in the derivation of predictive equations for subduction environments. Additionally, the compiled database will allow the assessment of existing predictive models for subduction-type events in terms of their suitability for the Peru-Chile region, which directly influences seismic hazard assessment in this region.

  14. 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.

  15. 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

  16. A satellite magnetic perspective of subduction zones, large igneous provinces, rifts, and diffuse plate boundary zones

    NASA Astrophysics Data System (ADS)

    Purucker, M. E.; Whaler, K. A.

    2008-12-01

    Large and intermediate-scale tectonic features such as subduction zones, large igneous provinces, rifts, and diffuse plate boundary zones are often seen to have a magnetic signature visible from the perspective of near-Earth magnetic field satellites such as CHAMP and Orsted. Why do these tectonic features have a magnetic signature, while others do not? A new model of the lithospheric field (MF-6, Maus et al., 2008) extending to spherical harmonic degree 120 (333 km wavelength) has been used to evaluate the magnetic state of the lithosphere under the assumption that the magnetization is either induced (with a seismic starting model), or remanent (with a minimum norm approach). Some of the features identified from these images include the Tethyan and NE Siberian diffuse plate boundary zones, the Red Sea rift, and Cretaceous rift basins developed on the West African shield. Almost without exception, subduction zones exhibit a magnetic signature, as do many large igneous provinces. In this talk we discuss some of the new insights this magnetic perspective provides, and speculate on the controls which determine whether tectonic features will be expressed magnetically.

  17. 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

  18. Locking of the Chile subduction zone controlled by fluid pressure before the 2010 earthquake

    NASA Astrophysics Data System (ADS)

    Moreno, Marcos; Haberland, Christian; Oncken, Onno; Rietbrock, Andreas; Angiboust, Samuel; Heidbach, Oliver

    2014-04-01

    Constraints on the potential size and recurrence time of strong subduction-zone earthquakes come from the degree of locking between the down-going and overriding plates, in the period between large earthquakes. In many cases, this interseismic locking degree correlates with slip during large earthquakes or is attributed to variations in fluid content at the plate interface. Here we use geodetic and seismological data to explore the links between pore-fluid pressure and locking patterns at the subduction interface ruptured during the magnitude 8.8 Chile earthquake in 2010. High-resolution three-dimensional seismic tomography reveals variations in the ratio of seismic P- to S-wave velocities (Vp/Vs) along the length of the subduction-zone interface. High Vp/Vs domains, interpreted as zones of elevated pore-fluid pressure, correlate spatially with parts of the plate interface that are poorly locked and slip aseismically. In contrast, low Vp/Vs domains, interpreted as zones of lower pore-fluid pressure, correlate with locked parts of the plate interface, where unstable slip and earthquakes occur. Variations in pore-fluid pressure are caused by the subduction and dehydration of a hydrothermally altered oceanic fracture zone. We conclude that variations in pore-fluid pressure at the plate interface control the degree of interseismic locking and therefore the slip distribution of large earthquake ruptures.

  19. Magmatic and metamorphic development of an early to mid-Paleozoic continental margin arc in the southernmost Central Asian Orogenic Belt, Inner Mongolia, China

    NASA Astrophysics Data System (ADS)

    Zhang, Wei; Jian, Ping; Kröner, Alfred; Shi, Yuruo

    2013-08-01

    The Bainaimiao arc in Inner Mongolia, China, comprises a weakly metamorphosed volcani-sedimentary sequence and a low-P/T metamorphic complex. In this study we present SHRIMP zircon ages and geochemical data to document the temporal and genetic relationships between these two tectonic units. Zircons from a rhyolite and two dacites (high-K calc-alkaline) of the volcani-sedimentary sequence yielded 206Pb/238U ages of 474 ± 7 Ma, 453 ± 7 Ma and 436 ± 9 Ma respectively, that we interpret as recording the timing of three volcanic arc episodes. Rocks from the low-P/T metamorphic complex yielded zircon ages of 462 ± 11 Ma for a sillimanite gneiss and 437 ± 5 Ma for a plagioclase-hornblende gneiss, reflecting two distinct anatectic events. Also from the low-P/T complex, the protolith age of a metadiorite is 438 ± 2 Ma. A diorite from a weakly deformed diorite-granodiorite pluton in the low-P/T complex has a zircon age of 419 ± 10 Ma that correlates in time with regional collisional magmatism. An undeformed pegmatite dike cutting the low-P/T metamorphic complex has an age of 411 ± 8 Ma, which postdates collision and provides an upper limit for the termination of orogeny. Accordingly, we conclude that the volcani-sedimentary sequence and the low-P/T metamorphic complex evolved coherently in the early to mid-Paleozoic and formed an integral continental margin arc in the southernmost Central Asian Orogenic Belt.

  20. Slab Geometry Control on Mantle Flow Regime: A case study from Central South America Subduction Zone

    NASA Astrophysics Data System (ADS)

    Biryol, C. B.; Beck, S. L.; Zandt, G.; Wagner, L. S.

    2013-12-01

    The subduction of oceanic lithosphere along convergent plate margins plays an important role in the dynamics of the upper mantle beneath convergent margins and major orogenic belts. Many studies of mantle dynamics show that the flow pattern of the mantle varies greatly between different subduction zones as well as within the same subduction zone. The factors that control such variations are poorly understood and need to be investigated further in order to develop a better understanding of various subduction zone processes such as the deformation of mantle beneath convergent plate margins and transport of melts and volatiles in the mantle wedge above subducting slabs. Earlier studies of mantle flow inferred from seismic anisotropy via shear-wave splitting analysis indicated that the dynamics and deformation of subducting and overriding plates as well as the slab geometry have important roles on mantle flow regime. In an effort to test the significance of these factors in constraining the mantle dynamics along the central South America subduction zone, we carried out a shear-wave splitting analysis. Our study area covers southern Peru and northwestern Bolivia encompassing the northernmost Altiplano plateau where subduction of the Nazca plate begins to gradually flatten towards the north. The major part of the data for our analysis comes from the CAUGHT temporary seismic deployment (2010 - 2012) with 49 three-component broadband seismometers. In our study we used SKS, SKKS and PKS arrivals from over 80 teleseismic earthquakes, located between the distance-range of 60 to 140 degrees. We determined polarization direction and delay-time of shear-wave arrivals that are polarized into fast and slow components and split in time. The resultant fast polarization directions indicate the direction of mantle flow beneath the study area and the delay-times show the strength and depth extend of the associated seismic anisotropy. The results of our analysis revealed a geographically varying pattern of fast polarization directions with delay times ranging between 0.3 and 1.2 seconds. With the exception of few outlier measurements, larger delay times (over 1.0 second) are observed in the southwestern parts of the study area, closer to the trench. The fast polarization directions associated with these measurements are NE-SW and are nearly perpendicular to the strike of the trench and parallel to the absolute plate motion direction. Based on these observations and the shallow location of the slab, we infer that the location of the anisotropy in the southwest is below the slab and most probably due to the mantle flow entrained by the subducting Nazca plate. In the northwest the delay times reduce dramatically towards the Peruvian flat slab and we cannot detect significant azimuthal anisotropy. We attribute this to the change in flow pattern in the mantle related to the flattening slab. Based on these results we conclude that the geometry of the slab has a significant control on the mantle flow pattern in the central South America subduction zone.

  1. 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.

  2. 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.

  3. A tremor and slip event on the CocosCaribbean subduction zone as measured by a global positioning system (GPS) and seismic

    E-print Network

    Biggs, Juliet

    ; published 12 October 2010. [1] In May 2007 a network of global positioning systems (GPS) and seismic), A tremor and slip event on the CocosCaribbean subduction zone as measured by a global positioning systemA tremor and slip event on the CocosCaribbean subduction zone as measured by a global positioning

  4. Talc and "talc"-bearing dehydrating serpentinite rheology within subduction zones

    NASA Astrophysics Data System (ADS)

    Hilairet, N.; Wang, Y.; Reynard, B.

    2008-12-01

    Fluids released by subducting slabs hydrate peridotites within the mantle wedge and produce weak phyllosilicates in significant quantities (1). Depending on the original chemistry of peridotites and on the silica content of the fluids, either talc or the high-pressure (HP) variety of serpentine antigorite, or both, can form down to 180 km depth. A talc-like phase is also produced transiently during serpentine dehydration (2). The extent to which such weak materials at the slab-mantle wedge interface can influence the dynamics of subduction zones from human (seismicity - post-seismic ground deformations) to geological (convection) timescales is still unknown. Promising deformation experiments on antigorite have shown that its low strength makes it a potential actor for governing silent earthquakes that release elastic energy in subduction zones and for localizing deformation at the slab interface, thereby modifying the mantle wedge convection, heat fluxes and seismic anisotropy (3). The strength of talc at low pressure is also exceptionally low compared to other silicates (4) and at sub-surface conditions the weakness of talc is thought to govern deformation in major faults such as San Andreas (5). At low pressures (< 500 MPa), talc behaves cataclastically and hardly achieves distributed deformation (4). However, higher pressures should promote crystal plasticity i.e. different deformation mechanisms in talc. Investigating the rheology of talc, and talc-bearing assemblage during serpentine dehydration, at P-T conditions corresponding to subducting slabs, is necessary in order to better understand localization of deformation in the slab-mantle wedge interface as well as the nature of this interface. We conducted deformation experiments using the D-DIA (6) apparatus at GSE-CARS (APS sector 13), with in situ strain and stress measurements using synchrotron X-ray imaging and diffraction, respectively. Stress-strain data were obtained on talc at P-T conditions between 2 and 5 GPa, and T between 350 and 700C, at strain rates between 10-4 and 10-5 s-1. Dehydration of serpentine was conducted around 4 GPa and 10-5 s-1. The results will be presented together with their implications, in particular the influence of the talc-like phase within a dehydrating serpentine, and more generally for subduction zones dynamics. 1 Ulmer and Trommsdorff GSA spec. publ. (1999). 2 Perrillat et al EPSL (2005) 3 Hilairet et al Science (2007) 4 Escartin et al EPSL (2008) 5 Moore and Rymer Nature (2007) 6 Wang et al RSI (2003)

  5. Ambient Tremor, But No Triggered Tremor at the Northern Costa Rica Subduction Zone

    NASA Astrophysics Data System (ADS)

    Swiecki, Z.; Schwartz, S. Y.

    2010-12-01

    Non-volcanic tremor (NVT) has been found to be triggered during the passage of surface waves from various teleseismic events in locations around the world including Cascadia, Southwest Japan, Taiwan, and California. In this study we examine the northern Costa Rica subduction zone for evidence of triggered tremor. The Nicoya Peninsula segment of the northern Costa Rica margin experiences both slow-slip and tremor and is thus a prime candidate for triggered tremor observations. Eleven teleseismic events with magnitudes (Mw) greater than 8 occurring between 2006 and 2010 were examined using data from both broadband and short period sensors deployed on the Nicoya Peninsula, Costa Rica. Waveforms from several large regional events were also considered. The largest teleseismic and regional events (27 February 2010 Chile, Mw 8.8 and 28 May 2009 Honduras, Mw 7.3) induced peak ground velocities (PGV) at the NIcoya stations of ~2 and 6 mm/s, respectively; larger than PGVs in other locations that have triggered tremor. Many of the earthquakes examined occurred during small episodes of background ambient tremor. In spite of this, no triggered tremor was observed during the passage of seismic waves from any event. This is significant because other studies have demonstrated that NVT is not triggered everywhere by all events above some threshold magnitude, indicating that unique conditions are required for its occurrence. The lack of triggered tremor at the Costa Rica margin can help to better quantify the requisite conditions and triggering mechanisms. An inherent difference between the Costa Rica margin and the other subduction zones where triggered tremor exists is its erosional rather than accretionary nature. Its relatively low sediment supply likely results in a drier, lower pore fluid pressure, stronger and less compliant thrust interface that is less receptive to triggering tremor from external stresses generated by teleseismic or strong local earthquakes. Another important factor is Costa Rica’s relatively cool subduction zone structure where temperatures required for the fluid generating basalt/ecloginte reaction are not reached until far below tremor producing depths.

  6. GPS constraints on 35+ slow slip events within the Cascadia subduction zone, 1997- February, 2007

    NASA Astrophysics Data System (ADS)

    Melbourne, T.; Santillan, M.; Szeliga, W.; Miller, M.

    2007-05-01

    Refinements to GPS analyses in which we factor geodetic time series to better estimate both reference frames and transient deformation resolve 35 slow slip events (SSE) located throughout the Cascadia subduction zone from 1997 through early 2007. Timing of transient onset is determined with wavelet-transformation of the geodetic time series. 30 continuous GPS stations are included in this study up through 2005, and over 70 stations for the 2007 event. Events are analyzed that range from northern California to southwestern British Columbia, with station density generally increasing towards the north. The improved analyses better resolves the largest creep and also identifies many smaller events. At 48.5N latitude, the 14-month average recurrence interval still applies, four events after first recognition. Elsewhere, such periodicity is not observed. Along central Vancouver Island to the north (49N), a host of smaller events distinct from the 14-month periodicity occur with no obvious periodicity. Sporadic smaller events also appear throughout the subduction zone to the south, including some within the region of the 14-month periodicity of larger events. In southern Washington State, some of the largest transient displacements are observed, but lack any obvious periodicity in their recurrence. Along central Oregon, an 18-month recurrence is evident, while in northern California (Yreka) the 11-month periodicity continues through 2005. To invert GPS offsets of the 12 best-recorded events for slip, we use a cross-validation scheme to derive optimal smoothing of non-negative thrust faulting along a plate interface divided into 40 along strike and 24 down-dip subfaults. Those events have equivalent moment magnitudes ranging from 6.3 (smallest resolvable with GPS) to 6.8, and typically 2-3 cm of slip. The largest spatial extent of all events resolved to date is just under 350 km along strike, with a maximum observed duration of seven weeks across the network; the majority last less than one month and show typically half the spatial extent. Unlike other subduction zones like the Middle America Trench, no longer-duration Cascadia events are observed, nor cumulative deformation greater than 0.6 cm. The frequency, size and style of the many newly resolved smaller transient deformation events show they occur frequently here, that GPS captures only the largest events, and that smaller SSE's routinely occur at much greater occurrence rate at levels not detectable with GPS. Moreover, the location of the slow slip events, and the general pattern of deformation, suggest that the characteristics of seismogenic locking beneath Cascadia remains enigmatic.

  7. Geophysical Properties of Serpentine and Imaging of Fluid Flow in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Reynard, B.

    2011-12-01

    Water is recycled to the Earth's interior at subduction zones, and a large portion of the subducting fluid is mobilised at depths shallower than 150 km. Seimological and magneto-telluric methods are potential tools for imaging fluid circulation when combined with petrophysical models. Recent measurements of the physical properties of serpentine are presented, and allow refining fluid circulation in the mantle wedge from geophysical data. In the cold (<700°C) melt-free fore-arc mantle wedge above the subducting slab, serpentinisation is caused by the release of large amounts of hydrous fluids in the cold mantle above the dehydrating subducting plate. Low seismic velocities in the wedge give a time-integrated image of extensive hydration and serpentinisation within the stability of serpentine below 700°C. Using experimentally measured elastic properties of serpentine (Bezacier et al., Earth Planet Sci Letters, 2010), the amount of serpentinization is calculated to reach 50-100% in hot subductions, while it is below 10% in cold subduction. This amount corresponds to a time-integrated reaction of water-rich fluids originated from the dehydrating slab with the overlying mantle wedge. 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 wedge (< 700°C), high conductivities in electromagnetic profiles provide "instantaneous" images of fluid circulation because the measured electrical conductivity of serpentine is below 10-4 S.m-1 (Reynard et al., Earth Planet Sci Letters, 2011). A small fraction (ca. 1% in volume) of connective high-salinity fluids accounts for the highest observed conductivities. Modelling shows that low-salinity fluids (? 0.1 m) released by slab dehydration can evolve towards high-salinity (? 1 m) fluids during progressive serpentinisation in the wedge. These fluids can mix with arc magmas at depths and account for high-chlorine melt inclusions in arc lavas. Electrical conductivities up to 1 S.m-1 have been observed in the hydrated wedge of the hot subductions (Ryukyu, Kyushu, Cascadia), while moderate conductivities are observed in the cold subductions (N-E Japan, Bolivia), reflecting low fluid flow in the cold wedge of the latter. This is consistent with the seismic observations of extensive shallow serpentinisation in hot subduction zones, while serpentinisation is sluggish in cold subduction zones.

  8. Metamorphism and Continental Collision

    NSDL National Science Digital Library

    Kenneth Howard

    Physical Geology students are required to understand the processes involved in plate tectonics. They are expected to know the geologic differences between continents and ocean basins and should be able to recall and use simple geologic terms to describe geologic processes and events. This activity is designed to improve student comprehension of the varied Earth materials and complex processes involved in plate collisions. The activity synthesizes material covered during the first eight weeks of Physical Geology on plate tectonics, rock types, volcanoes, and Earth's composition. The instructor introduces the exercise to the students as a component of the college's Critical Thinking Initiative. The "hook" for the students is that the exercise represents a chance for self-appraisal of course content and understanding prior to the next semester test. The grading rubric for the lab is discussed with the students in terms of the Bloom Pyramid so that they can assess their level of progress in the course.

  9. 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.

  10. Plate Tectonics and Sea-Floor Spreading, Subduction Zones, "Hot Spots", and the "Ring of Fire"

    NSDL National Science Digital Library

    This site is part of the United States Geological Survey, Cascade Volcano Observatory web site. It provides general information about the theory of plate tectonics. It correlates specific landform types and physical processes with the types of plate boundaries where they occur. The explanation of each boundary type includes real world examples and links to United States Geological Survey web pages about each example. The links between volcanism, earthquakes, and plate boundaries is also discussed. There is a section of the site that explores the types of volcanism that occur at spreading ridges, subduction zones, and hot spots (intraplate volcanism). Links are also provided to information on specific areas. These areas include: Cascade Range Volcanoes, Gorda Ridge, Juan de Fuca Plate, Juan de Fuca Ridge, North Cascades, Olympic Mountains, and the Yellowstone Caldera.

  11. Unusually large shear wave anisotropy for chlorite in subduction zone settings

    NASA Astrophysics Data System (ADS)

    Mookherjee, Mainak; Mainprice, David

    2014-03-01

    Using first principle simulations we calculated the elasticity of chlorite. At a density ?~ 2.60 g cm-3, the elastic constant tensor reveals significant elastic anisotropy: VP ~27%, VS1 ~56%, and VS2 ~43%. The shear anisotropy is exceptionally large for chlorite and enhances upon compression. Upon compression, the shear elastic constant component C44 and C55 decreases, whereas C66 shear component stiffens. The softening in C44 and C55 is reflected in shear modulus, G, and the shear wave velocity, VS. Our results on elastic anisotropy at conditions relevant to the mantle wedge indicates that a 10-20 km layer of hydrated peridotite with serpentine and chlorite could account for the observed shear polarization anisotropy and associated large delay times of 1-2 s observed in some subduction zone settings. In addition, chlorite could also explain the low VP/VS ratios that have been observed in recent high-resolution seismological studies.

  12. Modeling seismically induced deformation and fluid flow in the Nankai subduction zone

    NASA Astrophysics Data System (ADS)

    Ge, S.; Screaton, E.

    2005-09-01

    Fluid pressure changes induced by seismic strains in the Nankai subduction zone were investigated through numerical modeling. Seismic strains resulting from dislocations along fault surfaces were coupled to pore pressure generation, and subsequent transient fluid flow was simulated. This study is distinct from previous efforts that modeled homogeneous systems. Effects of variable mechanical and hydrologic properties were investigated by assigning different mechanical and hydrological properties to marine sediments, the decollement zone, and the upper oceanic crust. Model sensitivity studies suggest that for a reasonable range of parameter scenarios, transient pressure head signals caused by discrete dislocations of a few meters in the updip region of the seismogenic zone can be observed over large areas of the margin from within shallow depths of the sediment wedge to the oceanic crust below the decollement.

  13. Nonextensivity at the Circum-Pacific subduction zones-Preliminary studies

    NASA Astrophysics Data System (ADS)

    Scherrer, T. M.; França, G. S.; Silva, R.; de Freitas, D. B.; Vilar, C. S.

    2015-05-01

    Following the fragment-asperity interaction model introduced by Sotolongo-Costa and Posadas (2004) and revised by Silva et al. (2006), we try to explain the nonextensive effect in the context of the asperity model designed by Lay and Kanamori (1981). To address this issue, we used data from the NEIC catalog in the decade between 2001 and 2010, in order to investigate the so-called Circum-Pacific subduction zones. We propose a geophysical explanation to nonextensive parameter q. The results need further investigation however evidence of correlation between the nonextensive parameter and the asperity model is shown, i.e., we show that q-value is higher for areas with larger asperities and stronger coupling.

  14. Hot 'nough for ya?: Controls and Constraints on modeling flux melting in subduction zones

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    The qualitative concept of flux-melting in subduction zones is well established. Progressive dehydration reactions in the down-going slab release fluids to the hot overlying mantle wedge, causing flux melting and the migration of melts to the volcanic front. However, the quantitative details of fluid release, migration, melt generation and transport in the wedge remain poorly understood. In particular, there are two fundamental observations that defy quantitative modeling. The first is the location of the volcanic front with respect to intermediate depth earthquake (e.g. ˜ 100±40 km; England et al., 2004, Syracuse and Abers, 2006) which is remarkably robust yet insensitive to subduction parameters. This is particularly surprising given new estimates on the variability of fluid release in global subduction zones (e.g. van Keken et al. 2011) which show great sensitivity of fluid release to slab thermal conditions. Reconciling these results implies some robust mechanism for focusing fluids/melts toward the wedge corner. The second observation is the global existence of thermally hot erupted basalts and andesites that, if derived from flux melting of the mantle requires sub-arc mantle temperatures of ˜ 1300° C over shallow pressures of 1-2 GPa which are not that different from mid-ocean ridge conditions. These thermodynamic constraints are also implicit in recent parameterizations of wet melting (e.g. Kelley et al, 2010) which tend to produce significant amounts of melt only near the dry solidus. These observations impose significant challenges for geodynamic models of subduction zones, and in particular for those that don't include the explicit transport of fluids and melts. We present new high-resolution model results that suggest that a more complete description of coupled fluid/solid mechanics (allowing the fluid to interact with solid rheological variations) together with rheologically consistent solutions for temperature and solid flow, may provide the required ingredients that allow for robust focusing of both fluids and hot solids to the sub-arc regions. We demonstrate coupled fluid/solid flow models for simplified geometries to understand the basic processes, as well as for more geologically relevant models from a range of observed arc geometries. We will also evaluate the efficacy of current wet melting parameterizations in these models. All of these models have been built using new modeling software we have been developing that allows unprecedented flexibility in the composition and solution of coupled multi-physics problems. Dubbed TerraFERMA (the transparent Finite Element Rapid Model Assembler...no relation to the convection code TERRA), this new software leverages several advanced computational libraries (FEniCS/PETSc/Spud) to make it significantly easier to construct and explore a wide range of models of varying complexity. Subduction zones provide an ideal application area for understanding the role of different degrees of coupling of fluid and solid dynamics and their relation to observations.

  15. 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.

  16. From an active continental plate margin to continental collision: New constraints from the petrological, structural and geochronological record of the (ultra) high-P metamorphic Rhodope domain (N-Greece)

    NASA Astrophysics Data System (ADS)

    Mposkos, E.; Krohe, A.; Wawrzenitz, N.; Romer, R. L.

    2012-04-01

    The Rhodope domain occupies a key area along the suture between the European and the Apulian/Adriatic plate (Schmid et al., 2008), which collided in the early Tertiary (closure of the Vardar/Axios ocean, cf. Mposkos & Krohe, 2006). An integrated study of the geochronological, tectonic and petrological data of the Rhodope domain provides the unique opportunity resolving a 160 my lasting metamorphic evolution (Jurassic to Miocene) of an active plate margin to a high degree. The Greek Rhodope consists of several composite metamorphic complexes bounded by the Nestos thrust and several normal detachment systems. The PT- and structural records of the complexes constrain metamorphic, magmatic and tectonic processes, associated with subduction along a convergent plate margin including UHP metamorphism, MP to HP metamorphism associated with continental collision, and core complex formation linked to Aegean back arc extension. We focus on the Sidironero Complex that shows a polymetamorphic history. This is documented by SHRIMP and LA-ICP-MS U-Pb zircon ages of ca. 150 Ma from garnet-kyanite gneisses that are interpreted to record the HP/UHP metamorphism (Liati, 2005; Krenn et al., 2010). SHRIMP zircon ages of ca. 51 Ma from an amphibolitized eclogite is interpreted by Liati (2005) to record a second Eocene HP metamorphic event. We present new data from an integrated petrological, geochronological and tectonic study. Granulite facies and upper amphibolite facies metamorphic conditions are recorded by the mineral assemblage Grt-Ky-Bt-Pl-Kfs-Qtz-Rt and Grt-Ky-Bt-Ms-Pl-Qtz-Rt, respectively, in deformed migmatitic metapelites. Deformation occurred under granulite facies conditions. Monazites from the matrix, that formed during the granulite facies deformation, lack core/rim structures and are only locally patchy zoned. Monazite chemical compositions are related to varying reaction partners. Single grains and fractions of few grains yield ID-TIMS U-Pb ages that plot along the concordia between 64 to 60 Ma. One date of 55 Ma might represent Pb-loss during later fluid-induced dissolution-reprecipitation. We discuss the following questions: What is the history of the high-P metamorphic rocks in the Sidironero Complex? Were high-P rocks that have been already exhumed again dragged into the subduction channel? Which rocks from the upper plate are affected by high-P metamorphism evincing that subduction erosion is an important mechanism? We reconsider the significance of the P-T-t evolution in the light of the tectonic processes that took place along the depth extension of a convergent plate interface and during subsequent continental collision along the European/Apulian Suture zone. Krenn et al., 2010. Tectonics 29, TC4001. Liati, A., 2005. Contribution to Mineralogy and Petrology 150, 608-630. Mposkos, E. & Krohe, A. 2006. Canadian Journal of Earth Sciences 43, 1755-1776. Schmid S.M., et al. 2008. Swiss Journal of Geoscience 101, 139-183.

  17. 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. [Arizona State Univ., Tempe, AZ (United States)] [Arizona State Univ., Tempe, AZ (United States)

    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.

  18. Interseismic uplift rates for western Oregon and along-strike variation in locking on the Cascadia subduction zone

    Microsoft Academic Search

    Reed J. Burgette; Ray J. Weldon II; David A. Schmidt

    2009-01-01

    We quantify the spatial pattern of uplift rate in western Oregon and northernmost California using tidal and leveling records to better understand the pattern of interseismic locking on the Cascadia subduction zone. We extend relative sea level time series of the six primary NOAA tide gauges to include all observations from 1925 to 2006. Previously unidentified bench mark instability biases

  19. The West Philippine Basin: An Eocene to early Oligocene back arc basin opened between two opposed subduction zones

    E-print Network

    Demouchy, Sylvie

    The West Philippine Basin: An Eocene to early Oligocene back arc basin opened between two opposed Basin and its boundaries, we propose a comprehensive Cenozoic history of the basin. Our model shows that it is a back arc basin that developed between two opposed subduction zones. Rifting started around 55 Ma

  20. Magnitude Limits of Subduction Zone Earthquakes by Yufang Rong, David D. Jackson, Harold Magistrale, and Chris Goldfinger

    E-print Network

    Goldfinger, Chris

    Magnitude Limits of Subduction Zone Earthquakes by Yufang Rong, David D. Jackson, Harold Magistrale, and Chris Goldfinger Abstract Maximum earthquake magnitude (mx) is a critical parameter in seismic hazard. In this study, we use probable maximum earthquake magnitude within a time period of interest, mpT, to replace mx

  1. Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays

    Microsoft Academic Search

    Rigobert Tibi; Douglas A. Wiens

    2005-01-01

    Recordings of deep Tonga earthquakes from two arrays of 12 broadband seismographs each in the Fiji and Tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the Tonga subduction zone. The arrays operated as part of the Seismic Arrays in Fiji and Tonga (SAFT) experiment from July 2001 to August 2002. In comparison with

  2. Receiver functions in northeast China – implications for slab penetration into the lower mantle in northwest Pacific subduction zone

    Microsoft Academic Search

    Xueqing Li; Xiaohui Yuan

    2003-01-01

    Seismic studies of the subducting lithosphere and the upper mantle discontinuities in the northwest Pacific subduction zone beneath Japan and northeast China have suggested contrary subduction scenarios. There was little consensus on the issue whether the subducting slab penetrates the upper mantle discontinuities into the lower mantle or it is deflected atop of the 660-km discontinuity over several hundred kilometers.

  3. Seismic Properties of Rocks at High PT Conditions. Laboratory Measurements on Mafic and Ultramafic Rocks Representative of Subduction Zones

    Microsoft Academic Search

    R. M. Prelicz; L. Burlini; K. Kunze; J. Burg

    2003-01-01

    The geological interpretation of geophysical data requires the knowledge of the physical properties of rocks and in particular of the effects of confining pressure and temperature. Of particular interest for the interpretation of the structures in subduction zones is the relationship between seismic anisotropy and rock fabric. Eclogites, peridotes and pyroxenites as well as some amphibolitic mylonites were collected in

  4. B-type olivine fabric in the mantle wedge: Insights from high-resolution non-Newtonian subduction zone models

    E-print Network

    van Keken, Peter

    B-type olivine fabric in the mantle wedge: Insights from high-resolution non-Newtonian subduction of subduction zones. These include 3-D flow effects, parallel melt filled cracks, and B-type olivine fabric. We predict the distribution of B- type and other fabrics with high-resolution thermal and stress models

  5. 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.

  6. Investigating Along-Strike Variations of Source Parameters for Relocated Thrust Earthquakes Along the Sumatra-Java Subduction Zone

    Microsoft Academic Search

    M. El Hariri; S. L. Bilek; H. R. Deshon; E. R. Engdahl

    2009-01-01

    Some earthquakes generate anomalously large tsunami waves relative to their surface wave magnitudes (Ms). This class of events, known as tsunami earthquakes, is characterized by having a long rupture duration and low radiated energy at long periods. These earthquakes are relatively rare. There have been only 9 documented cases, including 2 in the Java subduction zone (1994 Mw=7.8 and the

  7. Control of high oceanic features and subduction channel on earthquake ruptures along the Chile–Peru subduction zone

    Microsoft Academic Search

    Eduardo Contreras-Reyes; Daniel Carrizo

    2011-01-01

    We discuss the earthquake rupture behavior along the Chile–Peru subduction zone in terms of the buoyancy of the subducting high oceanic features (HOF's), and the effect of the interplay between HOF and subduction channel thickness on the degree of interplate coupling. We show a strong relation between subduction of HOF's and earthquake rupture segments along the Chile–Peru margin, elucidating how

  8. Short Notes Variability of Near-Term Probability for the Next Great Earthquake on the Cascadia Subduction Zone

    Microsoft Academic Search

    Stephane Mazzotti; John Adams

    2004-01-01

    The threat of a great (M 9) earthquake along the Cascadia subduction zone is evidenced by both paleoseismology data and current strain accumulation along the fault. On the basis of recent information on the characteristics of this subduction system, we estimate the conditional probabilities of a great earthquake occurring within the next 50 years and their variabilities. The most important

  9. The Distribution of Interseismic Locking on the Central Cascadia Subduction Zone Inferred From Coastal Uplift Rates in Oregon

    Microsoft Academic Search

    D. Schmidt; R. Burgette; R. Weldon

    2007-01-01

    We invert for the distribution of locking along the Oregon portion of the Cascadia subduction zone using an updated dataset of the interseismic vertical displacement rates. Uplift rates are inferred from spirit leveling that is tied into an absolute vertical reference frame using tide gauge records. With absolute uncertainties less than 1 mm\\/yr, the data provide good constraints on the

  10. The Distribution of Interseismic Locking on the Central Cascadia Subduction Zone Inferred From Coastal Uplift Rates in Oregon

    Microsoft Academic Search

    D. Schmidt; R. Burgette; R. Weldon

    2004-01-01

    We invert for the distribution of locking along the Oregon portion of the Cascadia subduction zone using an updated dataset of the interseismic vertical displacement rates. Uplift rates are inferred from spirit leveling that is tied into an absolute vertical reference frame using tide gauge records. With absolute uncertainties less than 1 mm\\/yr, the data provide good constraints on the

  11. Interseismic deformation of the Nankai subduction zone, southwest Japan, inferred from three-dimensional crustal velocity fields

    Microsoft Academic Search

    T. Tabei; M. Adachi; S. Miyazaki; T. Watanabe; S. Kato

    2007-01-01

    We have studied crustal deformation in the Nankai subduction zone, southwest Japan, based on three-dimensional GPS velocity fields. Oblique subduction of the Philippine Sea plate has caused two different modes of deformation of the overriding plate: interseismic crustal shortening in the direction of plate convergence, and permanent lateral movement of the forearc. The block boundary dividing the forearc is the

  12. Inverse Modeling of Surface Deformation Data to Determine the Depth of Coupling at the Cascadia Subduction Zone

    Microsoft Academic Search

    D. Verdonck

    2003-01-01

    Understanding the extent of seismic coupling along the Cascadia subduction zone is important in assessing earthquake hazards in the Pacific Northwest of the United States and southwestern Canada. Recently developed models suggest that surface deformation can be the result of seismic coupling deeper on the subduction interface than previously accepted models indicate. Models also indicate that the depth of coupling

  13. 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.

  14. 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.

  15. Rayleigh Wave Tomography in the Nicaragua-Costa Rica Subduction Zone

    NASA Astrophysics Data System (ADS)

    Salas-de La Cruz, M.; Fischer, K. M.; Forsyth, D. W.; Abers, G. A.; Strauch, W.; Protti, M.; Gonzalez, V.

    2006-12-01

    The goal of this study is to image crust and mantle structure in the Nicaragua-Costa Rica subduction zone by applying Rayleigh wave tomography to waveforms recorded by the TUCAN Broadband Seismometer Experiment. The 48-station TUCAN array included two dense station lines normal to the arc, one in Nicaragua and the other in Costa Rica, and two sparser lines along the fore-arc and in the back-arc. Stations were in the field from July, 2004, until March, 2006. The method we employ inverts teleseismic event phase and amplitude measurements at different periods for 1) phase velocity in a grid surrounding the array and 2) six parameters that describe the incoming wavefield (the phase, amplitude and propagation direction of two interfering plane waves). We are analyzing events at epicentral distances of 35° to 120° at 19 periods ranging from 15 s to 167 s. Phase velocity maps will then be inverted for regional shear-wave structure. Ample sources with a good azimuthal distribution were recorded over the duration of the array, and, given the station distribution, we anticipate that good resolution of the overriding plate beneath the region will be possible. Geochemical data in this region contain strong along-arc variations; these are consistent with a mantle wedge beneath Nicaragua that contains a greater depth and extent of melting and larger input of slab fluids than are present in the mantle wedge beneath Costa Rica. Constraints on the thickness of the upper plate lithosphere and its velocity contrast with the mantle wedge should provide useful constraints on subduction zone thermal structure and may help to explain the source of the along-arc geochemical variation.

  16. Rayleigh-Wave Tomography in the Nicaragua-Costa Rica Subduction Zone

    NASA Astrophysics Data System (ADS)

    Salas de La Cruz, M.; Fischer, K. M.; Forsyth, D. W.; Abers, G. A.; Strauch, W.; Protti, J. M.; Gonzalez, V.

    2008-12-01

    The goal of this study is to image crust and mantle structure in the Nicaragua-Costa Rica subduction zone by applying Rayleigh wave tomography to waveforms recorded by the TUCAN Broadband Seismometer Experiment. The 48-station TUCAN array included two dense station lines normal to the arc, one in Nicaragua and the other in Costa Rica, and two sparser lines along the fore-arc and in the back-arc. Stations were in the field from July 2004 until March 2006. Two-dimensional phase velocity maps of the fundamental mode from 15 to 167 s were inverted for three-dimensional shear-wave structure, using a starting model with an average crustal thickness based on receiver functions at TUCAN stations. The primary features in the shear wave models are fast velocities associated with the subducting slab and upper plate, and a slow mantle wedge. The subducting slab can be best seen at depths of 90-200 km with fast velocity anomalies of 4-7% compared to the regional average. The slow wedge region beneath Nicaragua occupies a larger volume and has lower velocities than are found in Costa Rica. Geochemical data contain strong along-arc variations between Nicaragua and Costa Rica that are consistent with a mantle wedge beneath Nicaragua that is more hydrated and may contain a greater extent and depth of melting. The shear velocities obtained from the Rayleigh wave inversions therefore correlate with the geochemical trends. They also match observations of slower velocities and greater shear attenuation in the mantle wedge obtained in local body wave tomography with TUCAN data. The average thickness of the upper plate in the back-arc in the Rayleigh wave inversions (~55 km) roughly corresponds to its thickness seen in shear attenuation models, a result that can be used to constrain thermal models for the subduction zone.

  17. 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.

  18. 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.

  19. 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.

  20. An Ongoing Large Slow Slip Event in the Southern Central Alaska Subduction Zone

    NASA Astrophysics Data System (ADS)

    Freymueller, J. T.; Fu, Y.

    2012-12-01

    We analyze time series of GPS measurement in southern central Alaska, and identify an ongoing long-term slow slip event (SSE) occurring since the end of 2008 around Anchorage Alaska area, starting 7-8 years later after the end of the previous 1998-2001 SSE. The slow slip event is counter-acting the background linear trend of plate convergence between Pacific and North America plates. Significant velocity changes are very obvious in north and height directions, and detectable in the east. Detailed analysis of continuous GPS time series indicates this slow slip event started at ~2008.96, and is still continuing since then. This slow slip event appears to have a very slow rise time, unlike the 1998-2001 event. Through mid-2012, GPS time series during this long-term slow slip can be well fitted with a linear term plus an exponential (or logarithmic) term. The velocity changes are located around and north of Anchorage, which indicates that the SSE is occurring at the downdip of the locked seismogenic zone of the southern central Alaska subduction zone, like the 1998-2001 event. We will use both continuous and campaign (including the campaign measurements of later summer of 2012) GPS measurements to better constrain the location and amplitude of the ongoing SSE in southern central Alaska, and compare with the 1964 earthquake rupture zone to discuss the relationship between locked zone, SSE zone and free slip zone in terms of their contributions to large earthquake in Alaska subduction zone.

  1. Paleomagnetic Evidence for Pliocene Initiation of Clockwise Block Rotation in Central Greece and Implications for the Evolution of the Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

    Bradley, K. E.; Hosa, A. M.; Vassilakis, E.; Weiss, B. P.

    2012-12-01

    The Aegean region presents one of the best examples of distributed continental extension in the upper plate of a rapidly retreating oceanic slab. We present new paleomagnetic data from Early Miocene to Pliocene terrestrial sediments and volcanics in Central Greece that constrain the history of vertical-axis rotation along the central part of the western limb of the Aegean arc. The present-day pattern of rapid block rotation within a broad zone of distributed deformation linking the right-lateral North Anatolian and Kephalonia continental transform faults initiated after Early Pliocene time, resulting in a uniform clockwise rotation of 24.3 ± 6.5° over a region >250 km long and >150 km wide encompassing Central Greece and the western Cycladic archipelago. Pliocene initiation of rapid clockwise rotation in Central and Western Greece reflects the propagation of the North Anatolia-Kephalionia Fault system through the previously extending Aegean Sea region in response to entry of dense oceanic lithosphere of the Ionian Sea into the Hellenic subduction zone and consequent accelerated slab rollback. Comparison of our results with the published paleomagnetic dataset indicates that the Aegean geometric arc formed in two short-duration pulses characterized by rapid vertical-axis rotation and strong upper-plate deformation.

  2. Fluid-metapelite interaction in an ultramafic mélange: implications for mass transfer along the slab-mantle interface in subduction zones

    NASA Astrophysics Data System (ADS)

    Mori, Yasushi; Shigeno, Miki; Nishiyama, Tadao

    2014-12-01

    The slab-mantle interface in subduction zones is a site of tectonic mixing of crustal and mantle rocks. It is the interface for fluid flow of slab-derived components into the mantle wedge. To assess the fluid-rock interaction along the slab-mantle interface, we studied the bleaching of pelitic schist in an ultramafic mélange. The Nishisonogi metamorphic rocks in Kyushu, Japan, comprise ultramafic mélanges intercalated with epidote-blueschist facies schists. The ultramafic mélange consists of tectonic blocks of various lithologies and a matrix of chlorite-actinolite schist and serpentinite. Along the contact with the mélange matrix, pelitic schist blocks are bleached mainly due to the modal increase of albite and the consumption of carbonaceous material. The bleaching is probably attributed to infiltration of Na-rich external fluid from the mélange matrix. Mass balance analysis indicates losses of C, Rb, K2O, Ba, Pb, and SiO2 from the bleached pelitic schist, although Al2O3, TiO2, Sc, Y, Zr, Nb, La, Ce, and Nd remain immobile. This suggests fractionation of large-ion lithophile elements (LILE) and Pb from the high-field-strength elements and rare earth elements during the bleaching. If this ultramafic mélange is analogous to the slab-mantle interface, similar infiltration metasomatism will promote liberation of C, Si, LILE, and Pb from subducting metapelites and enhance metasomatism of the mantle wedge.

  3. Metamorphic evolution of the high-pressure metamorphic rocks from the Kemer area (Biga Peninsula, NW Turkey): Implications for the accretionary continental growth and closure of the Intra-Pontide Ocean

    NASA Astrophysics Data System (ADS)

    Aygül, M.; Topuz, G.; Sat?r, M.

    2009-04-01

    The northwestern part of the Biga Peninsula, NW Turkey, corresponds to the so-called Intra-Pontide suture, separating the Rhodope-Strandja zone to the north and Sakarya zone to the south. This suture zone is marked by the exposures of high-pressure metamorphics, oceanic accretionary complexes and ophiolites, which are widely covered by Eocene to Miocene volcanics and volcano-clastics, and crosscut by Eocene to Miocene granites. The Kemer area is one of the key-areas where high-pressure metamorphics and an ophiolitic mélange were widely exposed. This contribution deals with the metamorphic evolution of the Kemer high-pressure rocks, and their implications for the geodynamic evolution. The Kemer high-pressure rocks comprise predominantly micaschist, calcschist, marble and minor metabasite and serpentinite. The micaschists contain mineral assemblages involving garnet, phengite (3,30-3,44 c.p.f.u.), paragonite, epidote, chlorite, albite and titanite, and the metabasites consists of garnet, barrosite, albite, chlorite, epidote, albite and titanite. The equilibrium conditions are poorly constrained as 550 ± 50 °C temperature and >8-10 kbar pressure by Fe-Mg partitioning between garnet and phengite, and phengite-barometry, respectively. Timing of the high-pressure metamorphism is constrained as 84-64 Ma by Rb-Sr phengite-whole rock dating on four samples. Although the obtained age values display a wide scatter, they are consistent with geochronological data from the neighboring high-pressure areas: 86 ± 2 Ma from the ?arköy blueschists (Topuz et al. 2008) and 65-69 Ma from the Çaml?ca garnet-micaschists (Okay and Sat?r, 2000). These data in conjunction with those from the literature suggest that Late Cretaceous represent a time of substantional accretionary continental growth related to the northward subduction of the Intra-Pontide Ocean. The closure of the Intra-Pontide Ocean is constrained between Late Cretaceous and Eocene, because Eocene volcanics and volcano-clastics cover the suture zone, and Eocene granitoids (~52 Ma, Beccaletto et al. (2007)) crosscut the high-pressure rocks. References: Beccaletto et al. (2007) Geological Magazine 144: 393-400. Okay AI & Sat?r M (2000) Turkish Journal of Earth Sciences 9: 47-56. Topuz et al. (2008) Journal of Metamorphic Geology 26: 895-913.

  4. Lead transport in intra-oceanic subduction zones: 2D geochemical-thermo-mechanical modeling of isotopic signatures

    NASA Astrophysics Data System (ADS)

    Baitsch-Ghirardello, Bettina; Stracke, Andreas; Connolly, James A. D.; Nikolaeva, Ksenia M.; Gerya, Taras V.

    2014-11-01

    Understanding the physical-chemical mechanisms and pathways of geochemical transport in subduction zones remains a long-standing goal of subduction-related research. In this study, we perform fully coupled geochemical-thermo-mechanical (GcTM) numerical simulations to investigate Pb isotopic signatures of the two key "outputs" of subduction zones: (A) serpentinite mélanges and (B) arc basalts. With this approach we analyze three different geodynamic regimes of intra-oceanic subduction systems: (1) retreating subduction with backarc spreading, (2) stable subduction with high fluid-related weakening, and (3) stable subduction with low fluid-related weakening. Numerical results suggest a three-stage Pb geochemical transport in subduction zones: (I) from subducting sediments and oceanic crust to serpentinite mélanges, (II) from subducting serpentinite mélanges to subarc asthenospheric wedge and (III) from the mantle wedge to arc volcanics. Mechanical mixing and fluid-assisted geochemical transport above slabs result in spatially and temporarily variable Pb concentrations in the serpentinized forearc mantle as well as in arc basalts. The Pb isotopic ratios are strongly heterogeneous and show five types of geochemical mixing trends: (i) binary mantle-MORB, (i) binary MORB-sediments, (iii) double binary MORB-mantle and MORB-sediments, (iv) double binary MORB-mantle and mantle-sediments and (v) triple MORB-sediment-mantle. Double binary and triple mixing trends are transient and characterize relatively early stages of subduction. In contrast, steady-state binary mantle-MORB and MORB-sediments trends are typical for mature subduction zones with respectively low and high intensity of sedimentary melange subduction. Predictions from our GcTM models are in agreement with Pb isotopic data from some natural subduction zones.

  5. A case of obduction-related high-pressure, low-temperature metamorphism in upper crustal nappes, Arabian continental margin, Oman: P-T paths and kinematic interpretation

    NASA Astrophysics Data System (ADS)

    Goffé, Bruno; Michard, André; Kienast, Jean Robert; Le Mer, Olivier

    1988-09-01

    In the southeastern Oman Mountains, the Saih Hatat tectonic window reveals Arabian continental material beneath the oceanic (sensu. lato) nappes i.e., the Hawasina sedimentary units and the huge Sumail ophiolite. The northeastern (internal) half of the window suffered an intense deformation associated with a high-pressure, low-temperature ( HP- LT) metamorphism, approximately dated at 80-75 Ma according to the available stratigraphie data. After the deformation, due to the absence of collision, the obduction-inherited structural and metamorphic pattern was virtually unmodified. A metamorphic map is established based on mineral assemblages in metapelites and metabasites. Low-grade assemblages occur in the highest units (Muscat nappes, Quryat unit). Characteristic minerals are {Fe}/{Mg} carpholite (± lawsonite) associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites. High-grade rocks outcrop in the deepest and easternmost units (As Sifah antiform) and are typified by an epidote-glaucophane-bearing eclogite assemblage. Intermediate units (Hulw) show assemblages of intermediate grade (chloritoid schists and glaucophane-bearing metabasites). Higher grade assemblages such as talc-chloritoid were not found. Inverted gradients are observed locally in the Muscat nappes (pyrophyllitebearing units above kaolinite-quartz-bearing units) and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures. A complex P- T evolution is fairly well documented by the mineral assemblages. Prograde evolution in the subophiolitic (continental rocks) began with a rather "hot" gradient of about 30°-35°C/km (with chloritoid growth in the higher units). Thereafter pressure increased with the temperature remaining constant or even decreasing (chloritoid replaced by kaolinite + oxides and carpholite growth) in synkinematic conditions. The peak P- T conditions are close to 8 kbar, 270°C for the Muscat nappes and 11 kbar, 400°C for the As Sifah unit. The corresponding surface-related gradient is low (10°-15°C/km). During the retrograde evolution, pressure and temperature generally decreased concomitantly, but in some of the upper units temperature remained constant or increased slightly for some time, at which point sudoite growth from carpholite took place. The metamorphic pattern and mineral evolution are interpreted in the framework of the Late Cretaceous obduction. The already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin (with coeval basaltic volcanism). Pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite, the maximum thickness of which was about 25 km in the As Sifah area. In this area, the overburden was also attributable to the accumulation of continental material (up to 5 km thick) by means of a complex set of synmetamorphic, often conjugated folds and thrusts. The basal peridotites of the obducted slab were probably not hotter than 300° C during its thrusting upon the continental margin. The temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas. Late thrusting in the same southerly direction controlled the retrograde evolution, together with erosional and gravity-driven tectonic unloading.

  6. Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental?crust

    PubMed Central

    Ernst, W. G.; Maruyama, S.; Wallis, S.

    1997-01-01

    Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90–125 km require unusual conditions. Our subduction model involves underflow of a salient (250 ± 150 km wide, 90–125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a thin (2–15 km thick), low-density slice; finally, uplift, backfolding, normal faulting, and exposure of the UHP terrane. Sustained over ?20 million years, rapid (?5 mm/year) exhumation of the thin-aspect ratio UHP sialic sheet caught between cooler hanging-wall plate and refrigerating, downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces. The intracratonal position of most UHP complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone. UHP metamorphic terranes consist chiefly of transformed, yet relatively low-density continental crust compared with displaced mantle material—otherwise such complexes could not return to shallow depths. Relatively rare metabasaltic, metagabbroic, and metacherty lithologies retain traces of phases characteristic of UHP conditions because they are massive, virtually impervious to fluids, and nearly anhydrous. In contrast, H2O-rich quartzofeldspathic, gneissose/schistose, more permeable metasedimentary and metagranitic units have backreacted thoroughly, so coesite and other UHP silicates are exceedingly rare. Because of the initial presence of biogenic carbon, and its especially sluggish transformation rate, UHP paragneisses contain the most abundantly preserved crustal diamonds. PMID:11038569

  7. Buoyancy-Driven, Rapid Exhumation of Ultrahigh-Pressure Metamorphosed Continental Crust

    NASA Astrophysics Data System (ADS)

    Ernst, W. G.; Maruyama, S.; Wallis, S.

    1997-09-01

    Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90-125 km require unusual conditions. Our subduction model involves underflow of a salient (250 ± 150 km wide, 90-125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a thin (2-15 km thick), low-density slice; finally, uplift, backfolding, normal faulting, and exposure of the UHP terrane. Sustained over ? 20 million years, rapid (? 5 mm/year) exhumation of the thin-aspect ratio UHP sialic sheet caught between cooler hanging-wall plate and refrigerating, downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces. The intracratonal position of most UHP complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone. UHP metamorphic terranes consist chiefly of transformed, yet relatively low-density continental crust compared with displaced mantle material--otherwise such complexes could not return to shallow depths. Relatively rare metabasaltic, metagabbroic, and metacherty lithologies retain traces of phases characteristic of UHP conditions because they are massive, virtually impervious to fluids, and nearly anhydrous. In contrast, H2O-rich quartzofeldspathic, gneissose/schistose, more permeable metasedimentary and metagranitic units have backreacted thoroughly, so coesite and other UHP silicates are exceedingly rare. Because of the initial presence of biogenic carbon, and its especially sluggish transformation rate, UHP paragneisses contain the most abundantly preserved crustal diamonds.

  8. Magmatic peridotites and pyroxenites, Andong Ultramafic Complex, Korea: Geochemical evidence for supra-subduction zone formation and extensive melt-rock interaction

    NASA Astrophysics Data System (ADS)

    Whattam, Scott A.; Cho, Moonsup; Smith, Ian E. M.

    2011-12-01

    The Andong Ultramafic Complex (AUC) mainly comprises peridotites (wehrlites ± plagioclase or spinel; or plagioclase + spinel) and related serpentinites with subordinate low-Al pyroxenites (clinopyroxenites, orthopyroxenites, and websterites). These rocks are compositionally similar to sub-continental lithospheric mantle peridotites and pyroxenites. Wehrlites formed predominantly by fractional crystallization processes within supra-subduction zone magmas and the pyroxenites are generally consistent with segregation and accumulation in similar magmas. Bulk rock ratios of Al2O3/SiO2 (0.01-0.03) and MgO/SiO2 (up to > 1) exhibited by the wehrlites and serpentinites indicate crystallization from a refractory source that underwent high degrees of melt extraction. Spinel chemistry confirms this and demonstrates that wehrlite and clinoproxenite protoliths underwent approximately 20-23% and 12-15% partial melting, respectively. Wehrlites and serpentinites also preserve evidence of extensive melt-peridotite interaction manifest as bulk rock SiO2-depletions and FeOt-enrichments relative to mantle residua as well as low Mg# (0.39-0.45) spinels with variable Ti contents but constant Cr# (0.42-0.47). These features are identical to those of ‘impregnated' plagioclase-peridotites of abyssal and sub-continental environments and compositional trends in spinel space imply reaction between secondary, MORB-like melts saturated in olivine + clinopyroxene or olivine and a harzburgitic protolith. High olivine:pyroxene (~ 3:1) and clinopyroxene:orthopyroxene ratios of the wehrlites coupled with chemical data dictate that reactions entailed orthopyroxene dissolution and olivine recrystallization. All AUC rock types exhibit primitive mantle-normalized incompatible element signatures characterized by LILE-enrichments, high fluid-mobile/immobile element ratios (Sr/Nd, Ba/La and Pb/Ce ? 1) and prominent HFSE (Nb, Zr, and Ti) depletions indicative of generation in a sub-arc environment within a supra-subduction zone system. A candidate for the associated arc-system is the one responsible for nearby arc-related Jurassic granitoids. Southeast-directed thrusting along the Andong Fault System may account for subsequent emplacement of the AUC into the Gyeongsang Basin.

  9. 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.

  10. 3D double difference velocity tomography of the Middle America subduction zone beneath Nicaragua and Costa Rica

    NASA Astrophysics Data System (ADS)

    Driskell, Melissa M.

    Waveform and arrival onset data collected on five amphibious arrays deployed along the Costa Rica-Nicaragua portion of the Middle America subduction zone are integrated to conduct high resolution velocity and location studies. Pick quality is evaluated using an automated arrival detection algorithm based on the wavelet transform and Akaike information criterion, resulting in revised pick weights for inversion studies. I explore the effect of new weighting and removal of poor data by relocating hypocenters through a minimum one dimensional velocity model and conducting double-difference local earthquake tomography (LET). Analysis of the hypocenter relocation and seismic velocity tomography results suggest that using the improved quality determinations improve sharpness in the velocity images. Double-difference LET, utilizing catalog derived absolute and differential times and waveform cross-correlation derived differential times, is conducted with the quality-controlled dataset. Results show improved hypocentral locations of seismogenic zone earthquakes and compressional and shear velocity structure of the seismogenic zone. There is high variability in seismic structure along the length of the margin. I find that the up-dip limit of seismogenic zone microseismicity is variable but approximately corresponds to the 150°C isotherm. The downdip limit of interseismic microseismicity occurs near the continental Moho intersection with the subducting plate interface. Seismicity is sparser in Nicaragua and low velocities dominate the margin. Low Vp and high Vp/Vs in the oceanic mantle suggest serpentinization. Seismogenic zone seismicity resides in a low Vp and low Vp/Vs band that parallels the top of the high-velocity subducting slab. This low velocity band thin and weakens from north to south. This may reflect decreasing hydration or changes in overpressure along the plate interface. The addition of data from the TUCAN experiment provides additional raypath coverage along margin due to the broader aperture of the array. The results are compared to the coarse model and an ancillary method of calculating high-resolution Vp/Vs measurements. Using the highest resolution data along the Nicoya peninsula, we find that regions with subduction tremor and slow slip are associated with high Vp zones. These regions may be a better proxy signal for defining the limit of rupture during major earthquakes.

  11. Shear-wave Velocity Structure and Inter-Seismic Strain Accumulation in the Up-Dip Region of the Cascadia Subduction Zone: Similarities to Tohoku?

    NASA Astrophysics Data System (ADS)

    Collins, J. A.; McGuire, J. J.; Wei, M.

    2013-12-01

    The up-dip region of subduction zone thrusts is difficult to study using land-based seismic and geodetic networks, yet documenting its ability to store and release elastic strain is critical for understanding the mechanics of great subduction earthquakes and tsunami generation. The 2011 Tohoku earthquake produced extremely large slip in the shallowest portion of the subduction zone beneath a region of the fore-arc that is comprised of extremely low-velocity, unconsolidated sediments [Tsuru et al. JGR 2012]. The influence of the sediment material properties on the co-seismic slip distribution and tsunami generation can be considerable through both the effects on the dynamic wavefield during the rupture [Kozdon and Dunham, BSSA 2012] and potentially the build up of strain during the inter-seismic period. As part of the 2010-2011 SeaJade experiment [Scherwath et al, EOS 2011], we deployed 10 ocean bottom seismographs (OBS) on the continental slope offshore of Vancouver Island in the region of the NEPTUNE Canada observatory. One goal of the experiment is to measure the shear modulus of the sediments lying above the subducting plate using the seafloor compliance technique. Using seafloor acceleration measured by broadband seismometer and seafloor pressure measured by Differential Pressure Gauge (DPG), we estimate the compliance spectrum in the infra-gravity wave band (~0.002-0.04 Hz) at 9 sites following the methodology of Crawford et al. [JGR, 1991]. We calibrated DPG sensitivities using laboratory measurements and by comparing teleseismic Rayleigh arrivals recorded on the seismometer and DPG channels [Webb, pers. comm]. We correct the vertical-component seismometer data for tilt using the procedure of Crawford and Webb [BSSA, 2000], Corrections for the gravitational attraction of the surface gravity waves [Crawford et al., JGR, 1998] are important at frequencies of 0.003-0.006 Hz only. Typically, the coherences are high (>0.7) in the 0.006 to 0.03 Hz range. We invert the measured compliances in this frequency band using a genetic algorithm that solves for the S-wave velocity, P-wave velocity, and density in a layered structure. By including constraints on the Vp distribution from active-source studies, these parameters appear well constrained down to about 4 km depth from our dataset. There is a clear difference in observed compliance values between stations close to the deformation front (~10 km) and those further up the continental slope (~30-40 km) indicating a region of unconsolidated, high-porosity sediment similar to the off-Tohoku region. The low S-wave velocities and high Vp/Vs ratios in the up-dip region correspond to unconsolidated high-porosity sediments. We calculated the effect of this material property contrast on the inter-seismic strain accumulation in the up-dip region of the subduction zone using a finite element model and find that the sediments can increase the amount of inter-seismic strain accumulated in the up-dip region by >100% relative to a homogenous elastic model.

  12. The impact of splay faults on the geochemistry and fluid budget of a subduction zone

    NASA Astrophysics Data System (ADS)

    Lauer, Rachel M.; Saffer, Dm

    2010-05-01

    Fluid samples obtained from convergent margins indicate that pore fluids are geochemically distinct from seawater, and characterized by reaction products derived from deeper within the subduction zone. The geochemical anomalies indicate a hydraulic connection from the site of these reactions to the seafloor, although it is unclear how the observed geochemical signature is related to the underlying permeability architecture, and overall fluid budget. In erosive margins like Costa Rica, the majority of sediments on the incoming oceanic plate are subducted, and therefore represent significant fluid sources derived through processes of compaction and mineral dehydration. In regions characterized by thin sediment cover and low heat flow (eg. Costa Rica) temperatures sufficient to drive mineral dehydration are not reached prior to subduction. In these settings, the geochemical anomalies observed at seafloor seeps most likely represent fluids flowing from deep within the subduction zone to the seafloor via the décollement or major subsidiary faults that connect the plate boundary to the seafloor. Observations of geochemical anomalies and seeps far upslope highlight the role of the latter features in both chemical transport and overall fluid budgets. Although previous modeling studies designed to estimate pore pressures, flow rates, and fluid budgets for convergent margins have successfully predicted pore water geochemical signatures and flow rates at seafloor seeps, they have not investigated the role of major splay faults. Here, we address this problem using a 2-D numerical model of coupled fluid flow and transport in order to quantitatively evaluate the physical and chemical hydrogeology of the forearc using the well studied Costa Rica margin as an example. Costa Rica provides an ideal setting for the proposed modeling study, as it has been well constrained through ocean drilling and borehole observatories that continuously monitor pressure, temperature, and geochemistry along a transect perpendicular to the trench. Specifically, we investigate the effect of realistic permeability architecture on the fluid budget, distribution of fluid pressures, and the spatial pattern of geochemical signals in the fluids expelled at the seafloor. Our model consists of a cross section perpendicular to the Middle American trench, extending from 10 km seaward of the deformation front to 50 km landward. In our models, we assign fluid sources within the subducted sediment section to represent both compaction and clay dehydration in order to evaluate the distribution and fate of freshened (dehydration derived) water from deep in the subduction zone to the seafloor. We assign sediment porosity following an exponential decrease with depth, and define permeability using a relation to porosity derived from laboratory data. Because it is not composed of accreted sediment, we assign a uniform permeability to the overriding margin wedge. We evaluate the effect of clay dehydration by first establishing a baseline model with wedge and décollement permeabilities of 10-19 m2 and 10-15 m2respectively, without including faults. In subsequent model runs, we vary the splay fault and décollement permeability from 10-17 m2 to 10-13 m2 in order to examine the effect of these changes on the distribution of fluid pressures, the fluid budget, and distribution of fresh pore water through comparison with the baseline model. Comparison of modeling results with direct measurements from seafloor seeps and borehole observatories will clarify the link between geochemical data and the underlying physical processes that determine fluid pressures and the distribution of fluids in the forearc. Preliminary results show that flow rates at the seafloor are consistent with those estimated from gravity coring (0.3-1.5 cm/yr) and the distribution of fluids (ie. fluid budget) is sensitive to changes in the permeability of the faults and décollement, especially in cases where fault permeability is greater than décollement permeability.

  13. Fractal analysis of the spatial distribution of earthquakes along the Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

    Papadakis, Giorgos; Vallianatos, Filippos; Sammonds, Peter

    2014-05-01

    The Hellenic Subduction Zone (HSZ) is the most seismically active region in Europe. Many destructive earthquakes have taken place along the HSZ in the past. The evolution of such active regions is expressed through seismicity and is characterized by complex phenomenology. The understanding of the tectonic evolution process and the physical state of subducting regimes is crucial in earthquake prediction. In recent years, there is a growing interest concerning an approach to seismicity based on the science of complex systems (Papadakis et al., 2013; Vallianatos et al., 2012). In this study we calculate the fractal dimension of the spatial distribution of earthquakes along the HSZ and we aim to understand the significance of the obtained values to the tectonic and geodynamic evolution of this area. We use the external seismic sources provided by Papaioannou and Papazachos (2000) to create a dataset regarding the subduction zone. According to the aforementioned authors, we define five seismic zones. Then, we structure an earthquake dataset which is based on the updated and extended earthquake catalogue for Greece and the adjacent areas by Makropoulos et al. (2012), covering the period 1976-2009. The fractal dimension of the spatial distribution of earthquakes is calculated for each seismic zone and for the HSZ as a unified system using the box-counting method (Turcotte, 1997; Robertson et al., 1995; Caneva and Smirnov, 2004). Moreover, the variation of the fractal dimension is demonstrated in different time windows. These spatiotemporal variations could be used as an additional index to inform us about the physical state of each seismic zone. As a precursor in earthquake forecasting, the use of the fractal dimension appears to be a very interesting future work. Acknowledgements Giorgos Papadakis wish to acknowledge the Greek State Scholarships Foundation (IKY). References Caneva, A., Smirnov, V., 2004. Using the fractal dimension of earthquake distributions and the slope of the recurrence curve to forecast earthquakes in Colombia. Earth Sci. Res. J., 8, 3-9. Makropoulos, K., Kaviris, G., Kouskouna, V., 2012. An updated and extended earthquake catalogue for Greece and adjacent areas since 1900. Nat. Hazards Earth Syst. Sci., 12, 1425-1430. Papadakis, G., Vallianatos, F., Sammonds, P., 2013. Evidence of non extensive statistical physics behavior of the Hellenic Subduction Zone seismicity. Tectonophysics, 608, 1037-1048. Papaioannou, C.A., Papazachos, B.C., 2000. Time-independent and time-dependent seismic hazard in Greece based on seismogenic sources. Bull. Seismol. Soc. Am., 90, 22-33. Robertson, M.C., Sammis, C.G., Sahimi, M., Martin, A.J., 1995. Fractal analysis of three-dimensional spatial distributions of earthquakes with a percolation interpretation. J. Geophys. Res., 100, 609-620. Turcotte, D.L., 1997. Fractals and chaos in geology and geophysics. Second Edition, Cambridge University Press. Vallianatos, F., Michas, G., Papadakis, G., Sammonds, P., 2012. A non-extensive statistical physics view to the spatiotemporal properties of the June 1995, Aigion earthquake (M6.2) aftershock sequence (West Corinth rift, Greece). Acta Geophys., 60, 758-768.

  14. Along-strike variations in temperature and tectonic tremor activity along the Hikurangi subduction zone, New Zealand

    NASA Astrophysics Data System (ADS)

    Yabe, Suguru; Ide, Satoshi; Yoshioka, Shoichi

    2014-12-01

    In the Hikurangi subduction zone, situated along the east coast of the North Island, New Zealand, where the old oceanic Pacific Plate is subducting beneath the Australian Plate, several slow slip events and tectonic tremors have recently been documented. These observations are somewhat surprising because such slow seismic phenomena tend to be common in subduction zones where relatively young oceanic plate is subducting. The locations of tectonic tremors, down-dip limit of slow slip events and seismic coupling transition change along strike from greater depths in the south to shallower depths in the north, suggesting significant along-strike variations in the characteristics of the plate interface. Similar along-strike variations have been observed for other characteristic features of the Hikurangi subduction zone. Here, we demonstrate that along-strike variations observed for tectonic tremors, slow slip events, and seismic coupling can be explained by lateral differences in the thermal structure of the subduction zone, which are controlled mainly by variations in convergence rate and friction along the plate interface. To demonstrate this, we first confirm that tectonic tremors occur around the plate interface. Then, we calculate the thermal structure of the Hikurangi subduction zone using a two-dimensional finite difference code. To explain the along-strike variation in the heat flow observed in the forearc region, temperatures along the plate interface should be systematically higher in the northern region than in the southern region, which we interpret as a consequence of higher convergence rates and greater frictional heating in the northern region. We compare the along-strike variation of seismic characteristics with calculated thermal structure and highlight that this along-strike variation in temperature controls the depth of the brittle-ductile transition, which is consistent with the observed spatial variations in tectonic tremors, down-dip limit of slow slip events and seismic coupling. Our results suggest that tectonic tremors recorded within subduction zones reflect the transient rheology of the materials being subducted, which is controlled by variations in temperature along the plate interface.

  15. Metamorphic core complexes vs. synkinematic plutons in continental extension setting: insights from key structures (Shandong Province, eastern China)

    E-print Network

    Paris-Sud XI, Université de

    ) the exhumation of the Late Jurassic-Early Cretaceous Linglong MCC below the SE-dipping Linglong detachment fault by partially-melted lower to middle crust upward into the Linglong MCC should be revised to Late Jurassic-Early Cretaceous period. Key-words: Mesozoic extension, eastern Asia, Metamorphic Core Complex, synkinematic pluton

  16. Seismic imaging along a 600 km transect of the Alaska Subduction zone (Invited)

    NASA Astrophysics Data System (ADS)

    Calkins, J. A.; Abers, G. A.; Freymueller, J. T.; Rondenay, S.; Christensen, D. H.

    2010-12-01

    We present earthquake locations, scattered wavefield migration images, and phase velocity maps from preliminary analysis of combined seismic data from the Broadband Experiment Across the Alaska Range (BEAAR) and Multidisciplinary Observations of Onshore Subduction (MOOS) projects. Together, these PASSCAL broadband arrays sampled a 500+ km transect across a portion of the subduction zone characterized by the Yakutat terrane/Pacific plate boundary in the downgoing plate, and the Denali volcanic gap in the overriding plate. These are the first results from the MOOS experiment, a 34-station array that was deployed from 2006-2008 to fill in the gap between the TACT offshore refraction profile (south and east of the coastline of the Kenai Peninsula), and the BEAAR array (spanning the Alaska Range between Talkeetna and Fairbanks). 2-D images of the upper 150 km of the subduction zone were produced by migrating forward- and back-scattered arrivals in the coda of P waves from large teleseismic earthquakes, highlighting S-velocity perturbations from a smoothly-varying background model. The migration images reveal a shallowly north-dipping low velocity zone that is contiguous near 20 km depth on its updip end with previously obtained images of the subducting plate offshore. The low velocity zone steepens further to the north, and terminates near 120 km beneath the Alaska Range. We interpret this low velocity zone to be the crust of the downgoing plate, and the reduced seismic velocities to be indicative of hydrated gabbroic compositions. Earthquakes located using the temporary arrays and nearby stations of the Alaska Regional Seismic Network correlate spatially with the inferred subducting crust. Cross-sections taken along nearly orthogonal strike lines through the MOOS array reveal that both the dip angle and the thickness of the subducting low velocity zone change abruptly across a roughly NNW-SSE striking line drawn through the eastern Kenai Peninsula, coincident with a distinct change in locking at the subduction interface as revealed by previous geodetic studies. On the west end of the Kenai Peninsula, where seismically imaged downgoing crust appears oceanic, the geodetic signal mainly reflects postseismic deformation from the 1964 earthquake as evinced by southeast trending displacement vectors (with respect to fixed North America). While postseismic relaxation continues east of the boundary, NNW-directed elastic deformation due to locking at the plate boundary dominates the geodetic signal, and imaging reveals thickened Yakutat crust is subducting. The collocation of sharp changes in both deep structure and surface deformation suggest that the nature of the plate interface changes drastically across the western edge of the Yakutat block and that variations in downgoing plate structure control the strain field in the overriding plate.

  17. Interseismic coupling and seismic potential along the Central Andes subduction zone

    NASA Astrophysics Data System (ADS)

    Chlieh, Mohamed; Perfettini, Hugo; Tavera, Hernando; Avouac, Jean-Philippe; Remy, Dominique; Nocquet, Jean-Mathieu; Rolandone, FréDéRique; Bondoux, Francis; Gabalda, Germinal; Bonvalot, Sylvain

    2011-12-01

    We use about two decades of geodetic measurements to characterize interseismic strain build up along the Central Andes subduction zone from Lima, Peru, to Antofagasta, Chile. These measurements are modeled assuming a 3-plate model (Nazca, Andean sliver and South America Craton) and spatially varying interseismic coupling (ISC) on the Nazca megathrust interface. We also determine slip models of the 1996 Mw = 7.7 Nazca, the 2001 Mw = 8.4 Arequipa, the 2007 Mw = 8.0 Pisco and the Mw = 7.7 Tocopilla earthquakes. We find that the data require a highly heterogeneous ISC pattern and that, overall, areas with large seismic slip coincide with areas which remain locked in the interseismic period (with high ISC). Offshore Lima where the ISC is high, a Mw˜8.6-8.8 earthquake occurred in 1746. This area ruptured again in a sequence of four Mw˜8.0 earthquakes in 1940, 1966, 1974 and 2007 but these events released only a small fraction of the elastic strain which has built up since 1746 so that enough elastic strain might be available there to generate a Mw > 8.5 earthquake. The region where the Nazca ridge subducts appears to be mostly creeping aseismically in the interseismic period (low ISC) and seems to act as a permanent barrier as no large earthquake ruptured through it in the last 500 years. In southern Peru, ISC is relatively high and the deficit of moment accumulated since the Mw˜8.8 earthquake of 1868 is equivalent to a magnitude Mw˜8.4 earthquake. Two asperities separated by a subtle aseismic creeping patch are revealed there. This aseismic patch may arrest some rupture as happened during the 2001 Arequipa earthquake, but the larger earthquakes of 1604 and 1868 were able to rupture through it. In northern Chile, ISC is very high and the rupture of the 2007 Tocopilla earthquake has released only 4% of the elastic strain that has accumulated since 1877. The deficit of moment which has accumulated there is equivalent to a magnitude Mw˜8.7 earthquake. This study thus provides elements to assess the location, size and magnitude of future large megathurst earthquakes in the Central Andes subduction zone. Caveats of this study are that interseismic strain of the forearc is assumed time invariant and entirely elastic. Also a major source of uncertainty is due to fact that the available data place very little constraints on interseismic coupling at shallow depth near the trench, except offshore Lima where sea bottom geodetic measurements have been collected suggesting strong coupling.

  18. Imaging melt and thermal structure in subduction zones: what does seismic attenuation tell us?

    NASA Astrophysics Data System (ADS)

    Abers, G. A.; Fischer, K. M.; Hirth, G.; Holtzman, B. K.; McCarthy, C.; Plank, T. A.; Wiens, D. A.

    2013-12-01

    Subduction zones provide opportunities for observation of the mantle melting region not easily available elsewhere. Earthquakes within subducting plates can be recorded in the overlying plate. These paths sample the volumes where melting occurs with high resolution and short ray paths, and produce simple signals with much higher frequency content than available elsewhere. Also, arc volcanoes provide a direct sample of mantle melting products, and magmas record H2O concentrations, temperature, and pressure in their geochemical compositions. Beneath both volcanic arcs and back-arc basins, seismic waves exhibit very high attenuation (1/Q) for both P and S waves. Several recent field experiments have shown that the region of high 1/Q is localized and more than an order of magnitude more attenuating than adjacent regions in the forearc or slab. We have systematically re-analyzed data from two sets of these experiments, from Central America and the Marianas, where 1/Q anomalies are well defined and where arc or backarc lavas provide independent constraints on mantle properties. These analyses show strong attenuation anomalies, with Qs at 1 Hz no lower than 60-80 beneath Costa Rica but lower beneath other arcs and back-arc basins, to Qs<40. The systematic decrease in Qs (increase in attenuation) correlates well with temperature from geothermometers based on major-element chemistry. However, these Qs values are a factor of 2-4 lower than predicted from temperature by current laboratory-based calibrations in olivine-dominated rocks, at relevant conditions. We refine the Qs predictions using a grain size evolution model and estimates of mantle water content from olivine-hosted melt inclusions, effects which decrease but do not eliminate the discrepancy. We conclude that melt must have a significant impact on Q, bigger than predicted by models of grain-boundary dissipation with equilibrium grain geometries. One possibility is that in these very high 1/Q regions additional attenuation mechanisms such as melt squirt may be important at 1 Hz frequencies, as predicted for very high aspect ratio pore geometries. The high Poisson's ratios inferred for the sub-arc mantle also indicate such pore geometries. Thus, subduction zones are providing good calibration of models for predicting seismic attenuation, and may be providing insight into the mechanical behavior of melt-bearing rocks.

  19. Back-arc strain in subduction zones: Statistical observations versus numerical modeling

    NASA Astrophysics Data System (ADS)

    Arcay, D.; Lallemand, S.; Doin, M.-P.

    2008-05-01

    Recent statistical analysis by Lallemand et al. (2008) of subduction zone parameters revealed that the back-arc deformation mode depends on the combination between the subducting (vsub) and upper (vup) plate velocities. No significant strain is recorded in the arc area if plate kinematics verifies vup = 0.5 vsub - 2.3 (cm/a) in the HS3 reference frame. Arc spreading (shortening) occurs if vup is greater (lower) than the preceding relationship. We test this statistical law with numerical models of subduction, by applying constant plate velocities far away from the subduction zone. The subducting lithosphere is free to deform at all depths. We quantify the force applied on the two converging plates to sustain constant surface velocities. The simulated rheology combined viscous (non-Newtonian) and brittle behaviors, and depends on water content. The influence of subduction rate vs is first studied for a fixed upper plate. After 950 km of convergence (steady state slab pull), the transition from extensional to compressive stresses in the upper plate occurs for vs ˜ 1.4 cm/a. The effect of upper plate velocity is then tested at constant subduction rate. Upper plate retreat (advance) with respect to the trench increases extension (compression) in the arc lithosphere and increases (decreases) the subducting plate dip. Our modeling confirms the statistical kinematic relationship between vsub and vup that describes the transition from extensional to compressive stresses in the arc lithosphere, even if the modeled law is shifted toward higher rates of upper plate retreat, using our set of physical parameters (e.g., 100 km thick subducting oceanic plate) and short-term simulations. Our results make valid the choice of the HS3 reference frame for assessing plate velocity influence on arc tectonic regime. The subduction model suggests that friction along the interplate contact and the mantle Stokes reaction could be the two main forces competing against slab pull for upper mantle subductions. Besides, our simulations show that the arc deformation mode is strongly time dependent.

  20. Apparent stress, fault maturity and seismic hazard for normal-fault earthquakes at subduction zones

    USGS Publications Warehouse

    Choy, G.L.; Kirby, S.H.

    2004-01-01

    The behavior of apparent stress for normal-fault earthquakes at subduction zones is derived by examining the apparent stress (?? a = ??Es/Mo, where E s is radiated energy and Mo is seismic moment) of all globally distributed shallow (depth, ?? 1 MPa) are also generally intraslab, but occur where the lithosphere has just begun subduction beneath the overriding plate. They usually occur in cold slabs near trenches where the direction of plate motion across the trench is oblique to the trench axis, or where there are local contortions or geometrical complexities of the plate boundary. Lower ??a (< 1 MPa) is associated with events occurring at the outer rise (OR) complex (between the OR and the trench axis), as well as with intracrustal events occurring just landward of the trench. The average apparent stress of intraslab-normal-fault earthquakes is considerably higher than the average apparent stress of interplate-thrust-fault earthquakes. In turn, the average ?? a of strike-slip earthquakes in intraoceanic environments is considerably higher than that of intraslab-normal-fault earthquakes. The variation of average ??a with focal mechanism and tectonic regime suggests that the level of ?? a is related to fault maturity. Lower stress drops are needed to rupture mature faults such as those found at plate interfaces that have been smoothed by large cumulative displacements (from hundreds to thousands of kilometres). In contrast, immature faults, such as those on which intraslab-normal-fault earthquakes generally occur, are found in cold and intact lithosphere in which total fault displacement has been much less (from hundreds of metres to a few kilometres). Also, faults on which high ??a oceanic strike-slip earthquakes occur are predominantly intraplate or at evolving ends of transforms. At subduction zones, earthquakes occurring on immature faults are likely to be more hazardous as they tend to generate higher amounts of radiated energy per unit of moment than earthquakes occurring on mature faults. We have identified earthquake pairs in which an interplate-thrust and an intraslab-normal earthquake occurred remarkably close in space and time. The intraslab-normal member of each pair radiated anomalously high amounts of energy compared to its thrust-fault counterpart. These intraslab earthquakes probably ruptured intact slab mantle and are dramatic examples in which Mc (an energy magnitude) is shown to be a far better estimate of the potential for earthquake damage than Mw. This discovery may help explain why loss of life as a result of intraslab earthquakes was greater in the 20th century in Latin America than the fatalities associated with interplate-thrust events that represented much higher total moment release. ?? 2004 RAS.

  1. THE MISSING EARTHQUAKES OF HUMBOLDT COUNTY: RECONCILING RECURRENCE INTERVAL ESTIMATES, SOUTHERN CASCADIA SUBDUCTION ZONE

    NASA Astrophysics Data System (ADS)

    Patton, J. R.; Leroy, T. H.

    2009-12-01

    Earthquake and tsunami hazard for northwestern California and southern Oregon is predominately based on estimates of recurrence for earthquakes on the Cascadia subduction zone and upper plate thrust faults, each with unique deformation and recurrence histories. Coastal northern California is uniquely located to enable us to distinguish these different sources of seismic hazard as the accretionary prism extends on land in this region. This region experiences ground deformation from rupture of upper plate thrust faults like the Little Salmon fault. Most of this region is thought to be above the locked zone of the megathrust, so is subject to vertical deformation during the earthquake cycle. Secondary evidence of earthquake history is found here in the form of marsh soils that coseismically subside and commonly are overlain by estuarine mud and rarely tsunami sand. It is not currently known what the source of the subsidence is for this region; it may be due to upper plate rupture, megathrust rupture, or a combination of the two. Given that many earlier investigations utilized bulk peat for 14C age determinations and that these early studies were largely reconnaissance work, these studies need to be reevaluated. Recurrence Interval estimates are inconsistent when comparing terrestrial (~500 years) and marine (~220 years) data sets. This inconsistency may be due to 1) different sources of archival bias in marine and terrestrial data sets and/or 2) different sources of deformation. Factors controlling successful archiving of paleoseismic data are considered as this relates to geologic setting and how that might change through time. We compile, evaluate, and rank existing paleoseismic data in order to prioritize future paleoseismic investigations. 14C ages are recalibrated and quality assessments are made for each age determination. We then evaluate geologic setting and prioritize important research locations and goals based on these existing data. Terrestrial core transects are located in each of eight archival domains in order to evaluate archival bias and potential deformation sources for the southern Cascadia subduction zone. These domains are located in the Eel River, Humboldt Bay, Humboldt Lagoons, and Crescent City regions. In any given domain, evidence of earthquakes can be regional, local, or both. Core transects are designed to capture archival bias due to 1) interseismic deformation in the upper plate or the megathrust, 2) rupture on upper plate thrust faults, 3) rupture on the megathrust, or 4) rupture on both. Modern biogeochemical transects are used to calibrate paleontologic estimates. Based on our assessment, we determine which sites need better age control, which sites need supplemental coring, and key new research areas that need to be investigated.

  2. Finite element modeling on stress field of subduction zones and island arcs during megathrust earthquake cycles

    NASA Astrophysics Data System (ADS)

    Muto, J.; Shibazaki, B.; Iidaka, T.; Ohzono, M.

    2013-12-01

    A subduction zone earthquake cycle includes a great earthquake and subsequent strain accumulation in to the next earthquake. Such cycles in viscoelastic earth perturbs crustal stresses. The observations of shear-wave splitting during crustal earthquakes in the forearc of the NE Japan have revealed the presence of almost NS polarization azimuths, while the volcanic front to backarc show the EW polarization azimuths. This indicates that the stress field in the forearc crust is not horizontal EW compression during the interseismic period. In order to clarify how crustal stress fields are perturbed during earthquake cycles, we have conducted a finite element model on subduction zones earthquake cycles in the NE Japan. We developed a two-dimensional finite element model oriented perpendicular to the Japan Trench extending 1000 km to the west and 600 km to the east of the Trench and 800 km depth. The model also transects an area of large coseismic slip of the 2011 Tohoku Oki earthquake with the slip magnitude exceeding 60 m. The subsurface crustal and mantle wedge structures, and subducting slab geometry were developed based on an offshore seismic reflection survey and high-precision seismic tomography of the crust, mantle wedge structures, and subducting slab in this region. Deformation along plate boundary is the kinematically assigned using the split node method. For a subduction plate boundary, a shallow portion is assumed to be locked and from a certain depth downdip, the boundary is assumed to slip at the full plate convergence rate of 80 mm/yr during interseismic period. At the coseismic step, the amount of slip corresponding to slip deficit during the interseismic period is achieved along the shallow portion. From preliminary results for cycles up to 10 earthquakes, the horizontal stress was oscillated through the cycles: horizontal EW compression during interseismic periods and sudden extension by coseismic deformations. The horizontal stress in the shallower portion of the forearc side just prior to an earthquake gradually becomes extension regime with cycles. The portion of this extension regime roughly corresponds to the region with NS polarization azimuths of the shear wave splitting of crustal earthquakes in the NE Japan. This indicates that the formation of extensional stress regime in the forearc during intersesimic period might be originated from the buckling of the island arc lithosphere and relaxation of compressive stress during the intersesimic period.

  3. Global Patterns of Radiated Energy for Thrust Earthquakes in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Choy, G. L.; Kirby, S. H.

    2008-12-01

    Teleseismic studies have found that, for a given seismic moment M0, the radiated energy ES of an earthquake can range over two orders of magnitude. Intraplate and intraslab earthquakes having unusually elevated radiated energy are largely confined to specific high-deformation tectonic settings, an observation which can lead to improving estimates of seismic hazard potential (Choy et al., GJI, 2002, 2004). In contrast, the population of interplate thrust subduction earthquakes has the lowest average ES/M0 ratio among plate-boundary earthquakes. Nevertheless, among subduction plate-boundary earthquakes, there is considerable variability in this ratio. For example, some large earthquakes of this type have anomalously low energy (i.e., low ES/M0 ratio), a characteristic associated with tsunamigenic events involving slow rupture. In a global reconnaissance of the radiated energies of more than 1300 large shallow thrust earthquakes (magnitude greater than about 5.5 and depth < 70 km) that occurred from 1987 to 2006 in subduction zones, we found 270 earthquakes with anomalously low energy radiation comparable to that of tsunamigenic slow earthquakes. The enervated (low energy) earthquakes have a magnitude differential ? M > 0.5 (where ? M is the difference between energy magnitude Me and moment magnitude Mw). This reconnaissance also found 152 earthquakes with anomalously high energy radiation. These energetic thrust events have ? M values less than about -0.2. The global distributions of energetic and enervated events are not random and typically do not overlap. Enervated events are nearly always located at the top surface of a Wadati-Benioff zone defining a narrow zone that can be interpreted as the slab interface. Energetic events occur in high-deformation tectonic settings, such as some marine collision zones involving seamount chains, submerged continent-continent collisions, colliding slabs, regions of complex plate interactions, and slab distortions. Some of these may be intraslab based on their greater depths compared to shallower events that are presumed to be on subduction boundaries. There is no obvious spatial correlation of these enervated events with locations of notable pre-digital tsunami earthquakes, such as the 1896 Japan and 1946 Aleutian events. The majority of enervated thrust events occur beneath forearc basins and not beneath frontal prisms (events south of Java in 1994 and 2006 being notable exceptions). The above trends in the locations of enervated and energetic thrust earthquakes may provide insight into variation of deformation within subduction zones.

  4. CAFE: a seismic investigation of water percolation in the Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Rondenay, S.; Abers, G. A.; Creager, K. C.; Malone, S. D.; MacKenzie, L.; Zhang, Z.; van Keken, P. E.; Wech, A. G.; Sweet, J. R.; Melbourne, T. I.; Hacker, B. R.

    2008-12-01

    Subduction zones transport water into the Earth's interior. The subsequent release of this water through dehydration reactions may trigger intraslab earthquakes and arc volcanism, regulate slip on the plate interface, control plate buoyancy, and regulate the long-term budget of water on the planet's surface. As part of Earthscope, we have undertaken an experiment named CAFE (Cascadia Arrays for Earthscope) seeking to better constrain these effects in the Cascadia subduction zone. The basic experiment has four components: (1) a 47-element broadband imaging array of Flexible Array instruments integrated with Bigfoot; (2) three small-aperture seismic arrays with 15 additional short-period instruments near known sources of Episodic Tremor and Slip (ETS) events; (3) analysis of the PBO and PANGA GPS data sets to define the details of episodic slip events; and (4) integrative modeling with complementary constraints from petrology and geodynamics. Here, we present a summary of the results that have been obtained to date by CAFE, with a focus on high-resolution seismic imaging. A 250 km-long by 120 km-deep seismic profile extending eastward from the Washington coast was generated by 2-D Generalized Radon Transform Inversion of the broadband data. It images the subducted crust as a shallow-dipping, low-velocity layer from 20km depth beneath the coast to 40km depth beneath the forearc. The termination of the low-velocity layer is consistent with the depth at which hydrated metabasalts of the subducted crust are expected to undergo eclogitization, a reaction that is accompanied by the release of water and an increase in seismic velocities. Slab earthquakes are located in both the oceanic crust and mantle at depths <40 km, and exclusively in the oceanic mantle at greater depth, as would be expected if they are related to slab dehydration. Two ETS events have occurred during the course of the deployment. They were precisely located and are confined to the region above which the crust exhibits low-velocities and is believed to undergo progressive dehydration, further supporting the proposition that water plays a role in ETS.

  5. Low to negligible BrO/SO2 ratios at two subduction-zone volcanoes

    NASA Astrophysics Data System (ADS)

    Bobrowski, Nicole; Hörmann, Christoph; Mori, Toshiya; Platt, Ulrich

    2014-05-01

    In July 2013 a measurement campaign took place on Kyushu, Japan, investigating the BrO/SO2 ratio in the plume of Sakurajima and Aso. Multi-Axis-Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were carried out at four sides on Sakurajima Island, with a maximum distance of about 5 km downwind, and assuming a wind speed of 5 m/s (corresponding to a plume age of about 15 minutes). At Aso measurements took place on the western slope of the active crater and at the crater rim. The MAX-DOAS data of both sites were evaluated for BrO and SO2 slant column densities (SCDs). In the following, BrO/SO2 ratios were calculated to overcome dilution effects and to investigate the BrO formation processes in the ash-laden plume of Sakurajima and the volcanic plume of Aso which is characterized by emissions from a fumarolic area and a mud pool. The BrO/SO2 ratios of the measurement have been below the detection limit for Aso as well as during most of the measurement days at Sakurajima with the only exception on 15th July 2013, when a BrO/SO2 ratio of ~ 1 x 10-5 could be determined. After very high BrO/SO2 ratios at Sakurajima that were reported by C. Lee et al. (2005) our results seem to be unexpected but nevertheless match the general geological settings at both volcanoes. In a recent paper, Shinohara (2013) summarized and compared chlorine emissions from the Japanese volcanic arc with global chlorine emissions from arc volcanoes and pointed out that the volcanic gas emissions in Japan are quite Cl-poor compared to those at other subduction zones. In the recent past it has been found that low chlorine emissions can occur together with nevertheless high bromine emissions (Nyiragongo, Bobrowski et al., 2013). However, looking up Br/Cl ratios (of condensate measurements at fumaroles) of the Japanese arc volcanism summarized in Gerlach, 2004 a comparatively low Br/Cl ratio is added with 6-7 x 10-4 (global arc mean 2 x 10-3) to the already poor chlorine emissions. We will present upper limits of BrO/SO2 ratios and give an estimate on bromine emissions of Aso and Sakurajima for July 2013. To our knowledge these are the so far lowest bromine emission from an arc volcano. Possible reasons will be discussed in the light of today's available literature - pointing out the geological particularities on Eurasian-Philippine plate subduction zone.

  6. Possible control of subduction zone slow-earthquake periodicity by silica enrichment

    NASA Astrophysics Data System (ADS)

    Audet, Pascal; Bürgmann, Roland

    2014-06-01

    Seismic and geodetic observations in subduction zone forearcs indicate that slow earthquakes, including episodic tremor and slip, recur at intervals of less than six months to more than two years. In Cascadia, slow slip is segmented along strike and tremor data show a gradation from large, infrequent slip episodes to small, frequent slip events with increasing depth of the plate interface. Observations and models of slow slip and tremor require the presence of near-lithostatic pore-fluid pressures in slow-earthquake source regions; however, direct evidence of factors controlling the variability in recurrence times is elusive. Here we compile seismic data from subduction zone forearcs exhibiting recurring slow earthquakes and show that the average ratio of compressional (P)-wave velocity to shear (S)-wave velocity (vP/vS) of the overlying forearc crust ranges between 1.6 and 2.0 and is linearly related to the average recurrence time of slow earthquakes. In northern Cascadia, forearc vP/vS values decrease with increasing depth of the plate interface and with decreasing tremor-episode recurrence intervals. Low vP/vS values require a large addition of quartz in a mostly mafic forearc environment. We propose that silica enrichment varying from 5 per cent to 15 per cent by volume from slab-derived fluids and upward mineralization in quartz veins can explain the range of observed vP/vS values as well as the downdip decrease in vP/vS. The solubility of silica depends on temperature, and deposition prevails near the base of the forearc crust. We further propose that the strong temperature dependence of healing and permeability reduction in silica-rich fault gouge via dissolution-precipitation creep can explain the reduction in tremor recurrence time with progressive silica enrichment. Lower gouge permeability at higher temperatures leads to faster fluid overpressure development and low effective fault-normal stress, and therefore shorter recurrence times. Our results also agree with numerical models of slip stabilization under fault zone dilatancy strengthening caused by decreasing fluid pressure as pore space increases. This implies that temperature-dependent silica deposition, permeability reduction and fluid overpressure development control dilatancy and slow-earthquake behaviour.

  7. High-Resolution Subduction Zone Seismicity and Velocity Structure in Ibaraki, Japan

    NASA Astrophysics Data System (ADS)

    Shelly, D. R.; Beroza, G. C.; Zhang, H.; Thurber, C. H.; Ide, S.

    2004-12-01

    We use double-difference tomography (tomoDD) [Zhang and Thurber, 2003] and waveform-derived cross-correlation differential arrival times to invert for the earthquake locations and P- and S-wave velocity distributions in the subduction zone under Ibaraki Prefecture of north-central Honshu, Japan. The Ibaraki region is attractive for its high rate of slab seismicity and for the presence of an intermediate-depth double seismic zone. We relocate ~8000 events occurring in this region between June 2002 and June 2004. We use a combination of ~200,000 absolute travel times, ~5 million catalog-derived differential times, and ~5 million cross-correlation differential times derived from more than 150,000 waveforms, with roughly equal numbers of P- and S-wave data. Many of the waveforms are from HiNet borehole stations that provide particularly high-quality data. We also use data from JMA, the University of Tokyo, and Tohoku University. Since it is natural to expect sharp velocity contrasts in a subduction zone, we regularize the inversion using the total variation (TV) approach implemented through iteratively reweighted least squares. Because TV is an L1-norm regularization, sharp changes in velocity are penalized no more than gradual ones, but undulations in the velocity model remain damped. We will compare the TV results with those determined by standard least-squares, L2-norm regularization. Our results show increasingly organized seismicity including narrowing by up to 50% of the upper and lower limbs of the double seismic zone as viewed in cross-section. We find a zone of interplate events extending as deep as 60 km, forming a very distinct lineation in cross-section. Focal mechanisms support the interpretation that these are low angle, subduction interface events. These earthquakes are accompanied by a zone of very high Vp/Vs ratio within the downgoing plate, just beneath the seismicity, suggesting that high pore-pressures may enable seismic slip on the subduction interface at depths where aseismic slip would otherwise predominate. These events represent significantly deeper seismic coupling than the 37-43 km maximum depth observed in this area previously by Tichelaar and Ruff [1993], but are consistent with the maximum depth of 50-70 km for low-angle thrust events found by Igarashi et al. [2001] farther to the north.

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

    Microsoft Academic Search

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

    2001-01-01

    Recent diffusion modeling of eclogitic garnets from the Great Caucasus, Russia, and Yukon, Canada, have shown that the preservation of garnet growth zoning in rocks that have equilibrated at high temperature (680–700 °C) is possible only if rates of pressure and temperature change on the burial and\\/or exhumation paths are in the order of several cm\\/year and several hundreds of

  9. Beach ridges as paleoseismic indicators of abrupt coastal subsidence during subduction zone earthquakes, and implications for Alaska-Aleutian subduction zone paleoseismology, southeast coast of the Kenai Peninsula, Alaska

    USGS Publications Warehouse

    Kelsey, Harvey M.; Witter, Robert C.; Engelhart, Simon E.; Briggs, Richard; Nelson, Alan R.; Haeussler, Peter J.; Corbett, D. Reide

    2015-01-01

    The Kenai section of the eastern Alaska-Aleutian subduction zone straddles two areas of high slip in the 1964 great Alaska earthquake and is the least studied of the three megathrust segments (Kodiak, Kenai, Prince William Sound) that ruptured in 1964. Investigation of two coastal sites in the eastern part of the Kenai segment, on the southeast coast of the Kenai Peninsula, identified evidence for two subduction zone earthquakes that predate the 1964 earthquake. Both coastal sites provide paleoseismic data through inferred coseismic subsidence of wetlands and associated subsidence-induced erosion of beach ridges. At Verdant Cove, paleo-beach ridges record the paleoseismic history; whereas at Quicksand Cove, buried soils in drowned coastal wetlands are the primary indicators of paleoearthquake occurrence and age. The timing of submergence and death of trees mark the oldest earthquake at Verdant Cove that is consistent with the age of a well documented ?900-year-ago subduction zone earthquake that ruptured the Prince William Sound segment of the megathrust to the east and the Kodiak segment to the west. Soils buried within the last 400–450 years mark the penultimate earthquake on the southeast coast of the Kenai Peninsula. The penultimate earthquake probably occurred before AD 1840 from its absence in Russian historical accounts. The penultimate subduction zone earthquake on the Kenai segment did not rupture in conjunction with the Prince William Sound to the northeast. Therefore the Kenai segment, which is presently creeping, can rupture independently of the adjacent Prince William Sound segment that is presently locked.

  10. 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 Domuzda? 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 Domuzda? 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 Domuzda? 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 Domuzda? 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 ?zmir-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 K?r?ehir 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.

  11. Crustal earthquake triggering by pre-historic great earthquakes on subduction zone thrusts

    USGS Publications Warehouse

    Sherrod, Brian; Gomberg, Joan

    2014-01-01

    Triggering of earthquakes on upper plate faults during and shortly after recent great (M>8.0) subduction thrust earthquakes raises concerns about earthquake triggering following Cascadia subduction zone earthquakes. Of particular regard to Cascadia was the previously noted, but only qualitatively identified, clustering of M>~6.5 crustal earthquakes in the Puget Sound region between about 1200–900?cal?yr?B.P. and the possibility that this was triggered by a great Cascadia thrust subduction thrust earthquake, and therefore portends future such clusters. We confirm quantitatively the extraordinary nature of the Puget Sound region crustal earthquake clustering between 1200–900?cal?yr?B.P., at least over the last 16,000. We conclude that this cluster was not triggered by the penultimate, and possibly full-margin, great Cascadia subduction thrust earthquake. However, we also show that the paleoseismic record for Cascadia is consistent with conclusions of our companion study of the global modern record outside Cascadia, that M>8.6 subduction thrust events have a high probability of triggering at least one or more M>~6.5 crustal earthquakes.

  12. Structural control on the nucleation of megathrust earthquakes in the Nankai subduction zone

    NASA Astrophysics Data System (ADS)

    Liu, Xin; Zhao, Dapeng

    2014-12-01

    To clarify the causal mechanisms of megathrust earthquakes, we studied the detailed three-dimensional P and S wave velocities (V), attenuation (Q), and Poisson's ratio (?) structures of the Nankai subduction zone in southwest Japan, using a large number of high-quality arrival time and t* data measured precisely from seismograms of local earthquakes. The suboceanic earthquakes used are relocated precisely using sP depth phase and ocean bottom seismometer data. Our results show the existence of two prominent high-V, high-Q, and low-? patches separated by low-V, low-Q, and high-? anomalies in the Nankai megathrust zone. Megathrust earthquakes during 1900 to 2013 nucleated in or around the high-V, high-Q, and low-? patches, which may represent strongly coupled areas (i.e., asperities) in the megathrust zone. Our results indicate that structural heterogeneities in the megathrust zone, such as the subducting seafloor topography and compositional variations, control the nucleation of the Nankai megathrust earthquakes.

  13. Spatial distribution of seismic energy rate of tectonic tremors in subduction zones

    NASA Astrophysics Data System (ADS)

    Yabe, Suguru; Ide, Satoshi

    2014-11-01

    The sizes of deep tectonic tremors have never been accurately evaluated as a physical quantity. Here we estimate tremor size as the band-limited seismic energy rate at 2-8 Hz, with accurate evaluation of the path attenuation and site amplification of seismic waves in four subduction zones: Nankai, Cascadia, Jalisco, and South Chile. The size-frequency statistics of seismic energy rate, which are characterized by the median measure for each subregion, are spatially variable. The spatial variations are categorized into three types, with each type corresponding to a different tremor migration behavior. In type A regions where tremor zone is wider, seismic energy rates are highly variable in the dip direction, and tremor activities are usually initiated in the less energetic tremor zone. Some of them further penetrate into the energetic tremor zone and subsequently migrate for long distances in the strike direction. Type B regions are characterized by relatively narrow tremor zones, minor variations in energy rates in the dip direction, and long-distance migration in the strike direction. Type C regions are characterized by isolated clusters of tremor activities without migration and by independent failure of each small tremor cluster. Given that the spatial distributions of tremor energy rates reflect heterogeneities in the strength of the plate interface, such distributions, which would be controlled by the width of tremor zone, may determine the regional style of slow-earthquake behavior. Some energetic tremor regions act as switches that trigger large slow slip events, especially in type A regions.

  14. Effect of Murray ridge on the tsunami propagation from Makran subduction zone

    NASA Astrophysics Data System (ADS)

    Swapna, M.; Srivastava, Kirti

    2014-12-01

    The aseismic Murray ridge (MR) is a continuation of the Owen fracture zone which marks the boundary between the Indian and Arabian plates. Due to large variation in morphology and structure within this NE-SW trending ridge in the Arabian Sea a large variation of the bathymetry from few hundred metres to about 4000 m is seen. Observed seismicity on the ridge system is predominantly strike-slip. Tsunamis generated in the Makran subduction zone (MSZ) will propagate through the MR system due to its proximity. As the tsunami speed depends on the depth of the ocean, bathymetry plays a vital role on tsunami propagation. In this paper, the effect of tsunami propagation through the MR system is carried out with the existing bathymetry and comparing the results by removing the bathymetry. To study this phenomenon the 1945 Makran tsunami and worst possible tsunamigenic earthquakes form eastern and western MSZ are considered. The directivity of tsunami propagation with the ridge system is seen to change after crossing the MR towards the southeast direction for tsunamis generated in the eastern MSZ. For tsunami generated in the western MSZ there is no change in its directivity and is almost same as without the ridge with propagation being towards the open sea. Hence the MR not only affects the amplitude of the tsunamis but also the directionality and the arrival times.

  15. Origin of magmas in subduction zones: a review of experimental studies.

    PubMed

    Kushiro, Ikuo

    2007-02-01

    Studies of the origin of magmas in subduction zones, particularly in the Japanese island arc, have been significantly advanced by petrological, geochemical, geophysical and experimental studies during last 50 years. Kuno's original model(1)) for magma generation in the Japanese island arc, that tholeiite magmas are formed at relatively shallow levels in the mantle on the Pacific Ocean side whereas alkali basalt magmas are formed in deeper levels on the Japan Sea side, stimulated subsequent studies, particularly high-pressure experimental studies in which the author participated. Recent seismic tomographic studies of regions beneath the Japanese island arc demonstrate that seismic low-velocity zones where primary magmas are formed have finger-like shapes and rise obliquely from the Japan Sea side toward the Pacific Ocean side. Based on experimental studies, it is suggested that the compositions of primary magmas depend mainly on the H2O content and degree of melting in the melting zones, and that primary tholeiite magmas are formed by 10-25% melting of the source mantle containing less than 0.2 wt.% H2O. High-alumina basalt and alkali basalt magmas are formed by smaller degrees of melting of similar mantle, whereas primary boninite magmas are formed by more than 20% melting of the source mantle with more than 0.2 wt.% H2O, and finally, high-magnesia andesite magmas are formed by smaller degrees of melting of similar mantle. PMID:24019580

  16. Origin of magmas in subduction zones: a review of experimental studies

    PubMed Central

    Kushiro, Ikuo

    2007-01-01

    Studies of the origin of magmas in subduction zones, particularly in the Japanese island arc, have been significantly advanced by petrological, geochemical, geophysical and experimental studies during last 50 years. Kuno’s original model1) for magma generation in the Japanese island arc, that tholeiite magmas are formed at relatively shallow levels in the mantle on the Pacific Ocean side whereas alkali basalt magmas are formed in deeper levels on the Japan Sea side, stimulated subsequent studies, particularly high-pressure experimental studies in which the author participated. Recent seismic tomographic studies of regions beneath the Japanese island arc demonstrate that seismic low-velocity zones where primary magmas are formed have finger-like shapes and rise obliquely from the Japan Sea side toward the Pacific Ocean side. Based on experimental studies, it is suggested that the compositions of primary magmas depend mainly on the H2O content and degree of melting in the melting zones, and that primary tholeiite magmas are formed by 10–25% melting of the source mantle containing less than 0.2 wt.% H2O. High-alumina basalt and alkali basalt magmas are formed by smaller degrees of melting of similar mantle, whereas primary boninite magmas are formed by more than 20% melting of the source mantle with more than 0.2 wt.% H2O, and finally, high-magnesia andesite magmas are formed by smaller degrees of melting of similar mantle. PMID:24019580

  17. The low velocity layer in subduction zone: Structure and elasticity of glaucophane at high pressures

    NASA Astrophysics Data System (ADS)

    Mookherjee, Mainak; Bezacier, Lucile

    2012-10-01

    We investigated the structure, equation of state, and elasticity of glaucophane [Na2Mg3Al2Si8O22(OH)2], up to 9 GPa, which encompasses its experimentally observed stability field. We find that the pressure-volume results for glaucophane are well represented by a third order Birch-Murnaghan formulation, with K0 = 81 GPa, K0' = 4.5 and V0 = 899.4 Å3. The full elastic constant tensor reveals significantly larger stiffness along the (1 0 0) plane. The [1 0 0] direction is the relatively softer. This could be rationalized in terms of the stacking of the stiffer tetrahedral units along [0 1 0] and [0 0 1] directions within the crystal structure. Glaucophane is a dominant mineral constituent of blueschist facies rock, and has significantly lower velocities compared to garnet bearing eclogites. In addition, glaucophane is anisotropic and could account for the observed low velocity layer in the subducting slabs at depth range within the thermodynamic stability of glaucophane. At high-pressures, beyond stability of glaucophane, hydrous phase such as lawsonite could account for the observed low velocity layers in certain subduction zones.

  18. Repeated large Slow Slip Events at the southcentral Alaska subduction zone

    NASA Astrophysics Data System (ADS)

    Fu, Yuning; Freymueller, Jeffrey T.

    2013-08-01

    We identify and study an ongoing Slow Slip Event (SSE) in the southcentral Alaska subduction zone using GPS measurements. This is the second large SSE in this region since modern geodetic measurements became available in 1993. We divide the ongoing SSE into two phases according to their transient displacement time evolution; their slip distributions are similar to each other but slip rates are slightly different. This ongoing SSE occurs downdip of the main asperity that ruptured in the 1964 Alaska earthquake, on the same part of the subduction interface as the earlier 1998-2001 SSE. The average slip rate of this SSE is ?4-5 cm/yr, with a cumulative moment magnitude of Mw 7.5 (Mw 7.3 and Mw 7.1 For Phases I and II, respectively) through the end of 2012. The time and space dependence of the GPS displacements suggest that the slip area remained nearly the same during Phase I, while the slip rate increased with time. The SSEs occur on a transitional section of the subduction plate interface between the fully locked updip part and the freely slipping deeper part. During the 1964 earthquake, slip on the region of the SSE was much lower than slip in the updip region. Based on this observation and the repeated SSEs, we conclude that this part of the interface slips repeatedly in SSEs throughout the interseismic period and does not build up a large slip deficit to be released through large slip in earthquakes.

  19. High-resolution subduction zone seismicity and velocity structure beneath Ibaraki Prefecture, Japan

    NASA Astrophysics Data System (ADS)

    Shelly, David R.; Beroza, Gregory C.; Zhang, Haijiang; Thurber, Clifford H.; Ide, Satoshi

    2006-06-01

    We use double-difference tomography and waveform-derived cross-correlation differential times to estimate earthquake locations and P and S wave velocity structure in the subduction zone under Ibaraki and neighboring prefectures of north central Honshu. We find evidence in both earthquake hypocenters and the velocity structure that the Philippine Sea plate, or perhaps a microplate fragment, may be caught between the subducting Pacific plate and overriding Okhotsk plate in this region. In the southern part of the study area, we find a zone of interplate events extending as deep as 60 km, forming a distinct lineation in cross section. Focal mechanisms support the interpretation that these are low-angle, subduction interface events. We infer that these events probably occur on the interface between the Pacific and Philippine Sea plates rather than between the Pacific and Okhotsk plates. In the upper part of the downgoing Pacific plate, we find a zone of high Vp/Vs ratio (˜1.95), which may be explained by high pore fluid pressures within the subducting crust. At a depth of 60-80 km, this region of high Vp/Vs appears to diffuse into the overlying mantle wedge, possibly indicating the upward release of fluids from the slab.

  20. Slab top dips resolved by teleseismic converted waves in the Hellenic subduction zone

    NASA Astrophysics Data System (ADS)

    Gesret, A.; Laigle, M.; Diaz, J.; Sachpazi, M.; Charalampakis, M.; Hirn, A.

    2011-10-01

    The variations of the arrival times and polarities with backazimuth and distance of teleseismic P-to-S converted waves at interfaces bounding the slab crust under the upper plate mantle are used to constrain the depth, dip angle and azimuth of the slab of the Hellenic subduction zone. A grid search is designed to estimate the model parameters. Dip values of 16-18°, with an azimuth of 20° to 40°, are thus derived at 3 sites aligned over 50 km along the eastern coast of Peloponnesus. They are consistent with the variation from 54 to 61 km of the slab top depths constrained below each receiver. North of the Gulfs of Corinth and Evvia, a similar depth for the top of the slab is found at a distance from the subduction at least 100 km larger. This suggests flatter subduction of a different slab segment. Such a variation in slab attitude at depth across the region from south of the eastern Gulf of Corinth to north of Evvia is a candidate for the control of the recent or active localized crustal thinning of the upper plate we documented in earlier work, and of the surface deformation.

  1. Flat versus normal subduction zones: a comparison based on 3-D regional traveltime tomography and petrological modelling of central Chile and western Argentina (29°-35°S)

    NASA Astrophysics Data System (ADS)

    Marot, M.; Monfret, T.; Gerbault, M.; Nolet, G.; Ranalli, G.; Pardo, M.

    2014-12-01

    Our study compares the seismic properties between the flat and normal subduction regions in central Chile, to better understand the links between the slab geometry, surface deformation and the deeper structures. In comparison with previous studies, we show the most complete 3-D regional seismic tomography images for this region, in which we use (1) a larger seismic data set compiled from several short-term seismic catalogues, (2) a denser seismic array allowing a better resolution of the subduction zone from the trench to the backarc and into the upper ˜30 km of the slab and (3) a starting 1-D background velocity model specifically calculated for this region and refined over the years. We assess and discuss our tomography results using regional seismic attenuation models and estimating rock types on the basis of pressure and temperature conditions computed from thermomechanical models. Our results show significant seismic differences between the flat and normal subduction zones. As expected, the faster seismic velocities and increased seismicity within the flat slab and overriding lithosphere are generally consistent with a cooler thermal state. Our results are also consistent with dehydration of the mantle above the subducted Juan Fernandez Ridge at the eastern tip of the flat slab segment, indicating that the latter retains some fluids during subduction. However, fluids in the upper portion of the flat slab segment are not seismically detected, since we report instead fast slab seismic velocities which contradict the argument of its buoyancy being the cause of horizontal subduction. The forearc region, above the flat slab, exhibits high Vs and very low Vp/Vs ratios, uncorrelated with typical rock compositions, increased density or reduced temperature; this feature is possibly linked with the aftershock effects of the Mw7.1 1997 Punitaqui earthquake, the flat slab geometry and/or seismic anisotropy. At the surface, the seismic variations correlate with the geological terranes. The Andean crust is strongly reduced in seismic velocities along the La Ramada-Aconcagua deformation belt, suggesting structural damage. Slow seismic velocities along the Andean Moho match non-eclogitized hydrated rocks, consistent with a previous delamination event or a felsic composition, which in turn supports the extent of the Chilenia terrane at these depths. We confirm previous studies that suggest that the Cuyania terrane in the backarc region is mafic and contains an eclogitized lower crust below 50-km depth. We also hypothesize major Andean basement detachment faults (or shear zones) to extend towards the plate interface and canalize slab-derived fluids into the continental crust.

  2. Subduction zone tectonic studies to develop concepts for the occurrence of sediment subduction (Phase I). Final technical report

    SciTech Connect

    Hilde, T.W.C.

    1984-08-01

    The objective was to determine the fate of sediments at convergent lithospheric plate boundaries. The study focuses on the structures of the Circum-Pacific trenches and shallow portions of the associated subduction zones. Sediment distribution and the nature of sediment deformation was defined through the various stages of plate convergence to determine if the sediments are subducted or accreted. The controlling factors for sediment subduction and/or accretion were determined. 50 figs. (ACR)

  3. Seismic Structures Beneath the Deformation Front of the Northern Cascadia Subduction Zone: Preliminary Results from the SeaJade Project

    NASA Astrophysics Data System (ADS)

    Hutchinson, J. A.; Kao, H.; Obana, K.; Spence, G.

    2013-12-01

    The Cascadia subduction zone is a region capable of generating M~9 megathrust earthquakes, the most recent of which ruptured in 1700 AD. The Cascadia Initiative was established to monitor and analyze the Cascadia subduction zone. Sister to the Cascadia Initiative, the Seafloor Earthquake Array - Japan Canada Cascadia Experiment (SeaJade) is an international research collaboration among the Geological Survey of Canada (GSC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), the University of Victoria, Canada, and the Woods Hole Oceanographic Institution (WHOI) that was initiated in 2010 to increase our understanding of the northern extent of the Cascadia subduction zone, off the west coast of Vancouver island. For the first phase of SeaJade, thirty-five ocean bottom seismometers were deployed over a three-month period from June to September 2010. More than 1,200 events were recognized and located, with the highest seismicity along the strike-slip Nootka fault. Out of nearly 27,000 phase arrivals, over 300 hundred converted phases (S-to-P and P-to-S) were examined at seismic stations within a close proximity to earthquake epicenters. The timing of these converted phases is used to image the seismic structure beneath the deformation front of the northern Cascadia subduction zone, in particular the approximate depths and velocity contrasts of seismic interfaces within the lithosphere. Preliminary results indicate two interfaces at an average of 12 and 4 km below the SeaJade ocean bottom seismometers with a possible third interface at approximately 1 km depth. Upon completion of the second phase of SeaJade, which is scheduled in 2013-2014, the additional data will contribute to improving the resolution of our model and constraining the detailed structure of the lithosphere in the Cascadia deformation front.

  4. Motion on upper-plate faults during subduction zone earthquakes: Case of the Atacama Fault System, northern Chile

    Microsoft Academic Search

    J. P. Loveless; M. E. Pritchard

    2008-01-01

    Motion on the Atacama Fault System (AFS) in northern Chile is driven by Andean subduction zone processes. We use two approaches, observational and theoretical, to evaluate how the AFS and other forearc faults responded to coseismic stress induced by one well-studied megathrust earthquake, the 1995 Mw = 8.1 Antofagasta event. We use synthetic aperture radar interferometry (InSAR) to search for

  5. A real-time observation network of ocean-bottom-seismometers deployed at the Sagami trough subduction zone, central Japan

    Microsoft Academic Search

    Takao Eguchi; Yukio Fujinawa; Eisuke Fujita; Sin-Iti Iwasaki; Isao Watabe; Hiroyuki Fujiwara

    1998-01-01

    We installed a real-time operating regional observation network of Ocean-Bottom-Seismometers, connected to an electro-optical fiber communication cable, at the Sagami trough subduction zone, just south of the Tokyo metropolitan area, central Japan. The network, called ETMC, has six seismic observation sites at approximately 20 km spacing. In addition, there are three tsunami observation sites along the ETMC network to monitor the

  6. Along-Strike and DownDip Variations in Subduction Zone Slip Deficit: Persistent or Transient? (Invited)

    Microsoft Academic Search

    J. T. Freymueller

    2010-01-01

    The pattern of elastic deformation at subduction zones depends on the along-strike and down-dip variation in the slip deficit on the plate interface. The location and magnitude of contractional strain is controlled mainly by the down-dip transition from locked (high slip rate deficit) to creeping (low slip rate deficit) behavior of the interface. In southern Alaska, there are dramatic along-strike

  7. Mapping the b-value anomaly at 100 km depth in the Alaska and New Zealand subduction zones

    Microsoft Academic Search

    Stefan Wiemer; John P. Benoit

    1996-01-01

    A positive anomaly in the frequency-magnitude distribution (b-value) is detected at approximately 90-100 km depth in two subduction zones. The b-value in this anomalous zone is 40% higher than in adjacent volumes. We use regional catalogs with a magnitude of completeness of 2.6 (Central Alaska and Cook Inlet), 2.0 (Shumagin Islands), and 3.5 (New Zealand). To resolve the b-value as

  8. Reflection seismology: The continental crust (1986). Volume 14

    SciTech Connect

    Barazangi, M.; Brown, L.

    1986-01-01

    This volume explores the major problems of deep geology and continental tectonics, the issues of deep crustal fluids, and the geological significance of deep reflections. It reports on seismic results from Precambrian crust to Paleozoic structures, including seismic results from the Appalachian-Hercynian system and new results from the Cordillera of western North America, the young subduction zone of western Canada, and Antarctica.

  9. Growth and differentiation of the Earth`s continental crust

    E-print Network

    Siebel, Wolfgang

    ) Felsic TTG`s Bimodal compositions also in continental flood basalt provinces but: at subduction zones Rocks from many Archean terranes are bimodal in their silica distribution 1) High-magnesium basalts 2 on Geochemistry, Vol 3: The Crust #12;Petrogenetic model for Archean TTG's Derived from (meta)basaltic source

  10. Coupling of oceanic and continental crust during Eocene eclogite-facies metamorphism: evidence from the Monte Rosa nappe, western Alps

    Microsoft Academic Search

    Thomas J. Lapen; Clark M. Johnson; Lukas P. Baumgartner; Giorgio V. Dal Piaz; Susanne Skora; Brian L. Beard

    2007-01-01

    High precision U–Pb geochronology of rutile from quartz–carbonate–white mica–rutile veins that are hosted within eclogite\\u000a and schist of the Monte Rosa nappe, western Alps, Italy, indicate that the Monte Rosa nappe was at eclogite-facies metamorphic\\u000a conditions at 42.6 ± 0.6 Ma. The sample area [Indren glacier, Furgg zone; Dal Piaz (2001) Geology of the Monte Rosa massif: historical review and personal comments. SMPM

  11. Continental rifting and metamorphic core complex formation ahead of the Woodlark spreading ridge, D'Entrecasteaux Islands, Papua New Guinea

    Microsoft Academic Search

    Timothy A. Little; S. L. Baldwin; P. G. Fitzgerald; B. Monteleone

    2007-01-01

    We evaluate the role of a metamorphic core complex (MCC) on Normanby Island in the Woodlark rift. Located 1 km thickness of blueschist-derived mylonites formed in a midcrustal shear zone during the Pliocene at ?400–500°C. This top-to-the-north zone appears to have reactivated the gently dipping base of the Papuan ophiolite (Papuan Ultramafic Body, PUB), and its continued activity appears to

  12. Constraints from seismic reflection signature on the seismogenic region in the Alaska/Aleutian subduction zone from the 1938 Alaska rupture zone to the Shumagin gap

    NASA Astrophysics Data System (ADS)

    Li, J.; Becel, A.; Shillington, D. J.; Nedimovic, M. R.; Kuehn, H.; Webb, S. C.

    2013-12-01

    Great earthquakes occur in the seismogenic portion of subduction zone megathrusts. Downdip, the megathrust changes from stick-slip behavior to stable sliding. Competing models suggest that the transition is controlled by temperature or the intersection of the megathrust with the serpentinized forearc mantle wedge. In some subduction zones, changes in behavior appear to be accompanied by changes in seismic reflection signature. In 2011, ALEUT program acquired 3700 km of deep penetration multichannel seismic (MCS) reflection and 800 km of ocean bottom seismometer (OBS) data along a part of the Aleutian-Alaska subduction zone that exhibits lateral and downdip variability in present-day locking and earthquake history. One goal of this program is to use reflection signature of the megathrust to map out downdip and along-strike changes in plate boundary properties and correlate them with constraints on coupling and earthquake rupture history. Our study area encompassed the freely sliding Shumagin Gap, the locked Semidi segment and the locked western Kodiak asperity. The Semidi and the Western Kodiak segments have last ruptured in 1938 M8.2 Earthquake and 1964 M9.2 Earthquake, respectively, while no large earthquake ruptured the entire Shumagin Gap. Here we present seismic reflection profiles from two MCS lines that image the plate boundary at the transition between the Semidi segment and Shumagin Gap. ALEUT Line 4 is a ~300-km-long dip profile across the western edge of the 1938 M8.2 rupture zone. Reflections from the plate interface can be traced on this profile from the trench, at ~8 s twtt (~5.5-6.0 km depth), to 140 km landward of the trench, at 10-12 s twtt (~30-40 km depth). However, large variations in the reflection response are observed with depth. The plate interface is marked by a single, simple reflection within the 1938 Mw 8.2 earthquake rupture zone. This area is characterized by relatively sparse small-to-moderate megathrust seismicity. Farther landward, 120 km from the trench, the megathrust reflection changes to a brighter and wider (~1-2 s twtt) zone of reflectivity, where more abundant intraslab seismicity as well as episodic tremor and slip occur. The change in the megathrust reflection response appears to occur where it intersects a shallower band of reflectivity, which we tentatively interpret as the continental Moho. Thermal model predicts that the temperature on the thrust reaches 350 degrees Celsius, the temperature at which a change in rheology can occur, significantly deeper and farther landward. We will also show results from the along-strike ALEUT Line 7 that crosses the Semidi segment and Shumagin gap. The thickening of reflection band in the transition zone could be related to rheological change from brittle deformation to plastic shearing or a fluid-rich layer formed by dehydration process. We test this hypothesis by producing synthetic seismograms based on simple end member scenarios for the origin of the imaged megathrust reflection band (e.g., mylonites, a low velocity layer(s) with overpressured fluid) and comparing them with the data.

  13. Ophiolites and Continental Margins of the Mesozoic Western U.S. Cordillera

    NASA Astrophysics Data System (ADS)

    Dilek, Y.

    2001-12-01

    The Mesozoic tectonic history of the western U.S. Cordillera records evidence for multiple episodes of accretionary and collisional orogenic events and orogen-parallel strike-slip faulting. Paleozoic-Jurassic volcanic arc complexes and subduction zone assemblages extending from Mexico to Canada represent an East-Pacific magmatic arc system and an accretionary-type orogen evolved along the North American continental margin. Discontinuous exposures of Paleozoic upper mantle rocks and ophiolitic units structurally beneath this magmatic arc system are remnants of the Panthalassan oceanic lithosphere, which was consumed beneath the North American continent. Pieces of this subducted Panthalassan oceanic lithosphere that underwent high-P metamorphism are locally exposed in the Sierra Nevada foothills (e.g. Feather River Peridotite) indicating that they were subsequently (during the Jurassic) educted in an oblique convergent zone along the continental margin. This west-facing continental margin arc evolved in a broad graben system during much of the Jurassic as a result of extension in the upper plate, keeping pace with slab rollback of the east-dipping subduction zone. Lower to Middle Jurassic volcanoplutonic complexes underlain by an Upper Paleozoic-Lower Mesozoic polygenetic ophiolitic basement currently extend from Baja California-western Mexico through the Sierra-Klamath terranes to Stikinia-Intermontane Superterranes in Canada and represent an archipelago of an east-facing ensimatic arc terrane that developed west and outboard of the North American continental margin arc. The Smartville, Great Valley, and Coast Range ophiolites (S-GV-CR) in northern California are part of this ensimatic terrane and represent the island arc, arc basement, and back-arc tectonic settings, respectively. The oceanic Josephine-Rogue-Chetco-Rattlesnake-Hayfork tectonostratigraphic units in the Klamath Mountains constitute a west-facing island arc system in this ensimatic terrane as a counterpart of the east-facing S-GV-CR system to the south. The Guerrero intra-oceanic island arc system in Mexico was also part of the ensimatic arc terrane. Incorporation of this super arc terrane into the North American continent occurred diachronously along the irregular continental margin in the Middle Jurassic (in the north) through Early Cretaceous (in the south) during an arc-continent collision, marking a collisional orogenic episode in the North American Cordilleran history. Rifting of this accreted arc in the Late Jurassic (155-148 Ma) might have resulted from a sinistral transtensional deformation associated with the rapid NW motion of North America. Magmas generated during this rifting event probably migrated through the accreted arc crust and the continental margin units in the tectonic lower plate. The Franciscan subduction zone dipping eastwards beneath the continent was established in the latest Jurassic, following the collisional event and restoring the North American Cordillera back into an accretionary-type, Andean-style orogen. Different episodes of orogen-parallel intra-continental strike-slip faulting facilitated lateral dispersion of accreted terranes and continental margin units during the Early Cretaceous and transpressional deformation and batholithic magmatism in the Sierra Nevada magmatic arc in the Late Cretaceous. A Jurassic-Cretaceous island arc system (Wrangellia-Insular Superterrane) that had developed west of the Jurassic archipelago collapsed into the edge of North America during Late Cretaceous-Tertiary time and underwent northward lateral translation along the continental margin. These observations and interpretations have strong implications for the tectonic evolution of Central America and the Caribbean region.

  14. 3D absolute hypocentral determination - 13 years of seismicity in Ecuadorian subduction zone

    NASA Astrophysics Data System (ADS)

    Font, Yvonne; Segovia, Monica; Theunissen, Thomas

    2010-05-01

    In Ecuador, the Nazca plate is subducting beneath the North Andean Block. This subduction triggered, during the last century, 4 major earthquakes of magnitude greater than 7.7. Between 1994 and 2007, the Geophysical Institute (Escuela National Politecnica, Quito) recorded about 40 000 events in whole Ecuador ranging from Mb 1.5 to 6.9. Unfortunately, the local network shows great density discrepancy between the Coastal and Andean regions where numerous stations were installed to survey volcanic activity. Consequently, seismicity in and around the interplate seismogenic zone - producer of the most destructive earthquakes and tsunamis - is not well constrained. This study aims to improve the location of 13 years seismicity occurred during an interseismic period in order to better localize the seismic deformation and gaps. The first step consists in the construction of a 3D "georealistic" velocity model. Because local tomography cannot provide satisfactory model, we combined all local crustal/lithospheric information on the geometry and velocity properties of different geological units. Those information cover the oceanic Nazca plate and sedimentary coverture the subducting plate dip angle; the North Andean Block margin composed of accreted oceanic plateaus (the Moho depth is approximated using gravity modeling); the metamorphic volcanic chain (oceanic nature for the occidental cordillera and inter-andean valley, continental one for the oriental cordillera); The continental Guyana shield and sedimentary basins. The resulting 3D velocity model extends from 2°N to 6.5°S and 277°E to 283°E and reaches a depth of 300 km. It is discretized in constant velocity blocks of 12 x 12 x 3 km in x, y and z, respectively. The second step consists in selecting an adequate sub-set of seismic stations in order to correct the effect of station density disequilibrium between coastal and volcanic regions. Consequently, we only keep the most representative volcanic stations in terms of azimuthal coverage, record frequency and signal quality. Then, we define 5 domains: Offshore/coast, North-Andean margin, Volcanic chain, Southern Ecuador, and a domain deeper than 50 km. We process earthquake location only if at least 3 proximal stations exist in the event's domain. This data selection allows providing consistent quality location. The third step consists in improving the 3D MAXI technique that is well adapted to perform absolute earthquake location in velocity model presenting strong lateral Vp heterogeneities. The resulting catalogue allows specifying the deformation in the subduction system. All seismicity previously detected before trench occurs indeed between the trench and the coastal range. South of 0°, facing the subducting Carnegie Ridge, the seismicity aligns along the interplate seismogenic zone between an updip limit shallower than ~8 km and a downdip limit that reaches up to 50 km depth. The active seismogenic zone is interrupted by a gap that extends right beneath the coastal range. At these latitudes, a diffuse intraplate deformation also affects the subducting plate, probably induced by the locally thickened lithosphere flexure. Between the trench and the coast, earthquake distribution clearly defines a gap, which size is comparable to the 1942 M7.9 asperity (ellipse of axes ~55/35 km). A slab is clearly defines and dips around 25 to 30°. The slab seismicity is systematically interrupted between 100-170 km, approximately beneath the volcanic chain. North of 0°, i.e. in the megathrust earthquake domain, the interseismic activity is clearly reduced. The interplate distribution seems to gather along alignments perpendicular to the trench attesting probably of the margin segmentation. The North Andean overriding margin is undergoing active deformation, especially at the location where the Andean Chain strike changes of direction. At these latitudes, no earthquake occurs deeper than 100 km depth.

  15. Timing of eclogite-facies metamorphism of the Chuacús complex, Central Guatemala: Record of Late Cretaceous continental subduction of North America's sialic basement

    NASA Astrophysics Data System (ADS)

    Martens, Uwe C.; Brueckner, Hannes K.; Mattinson, Christopher G.; Liou, Juhn G.; Wooden, Joseph L.

    2012-08-01

    A Late Cretaceous collision of the southernmost portion of the North American continental margin with an undetermined southern block was first established based on the sedimentation history of the plate's supracrustal cover, which is overthrust by harzburgite-dominated nappes of the Guatemala Suture Complex. The collision is also well registered in the metamorphic evolution of continental eclogites of the Chuacús complex, a geologic unit that represents Mesoproterozoic-Triassic sialic basement of North America at the boundary with the Caribbean plate. Garnet-clinopyroxene-phengite thermobarometry of eclogites hosted in Chuacús gneisses indicates near ultra-high-pressure conditions to ~ 700 °C and ~ 2.1-2.4 GPa. SHRIMP-RG U-Pb dating of eclogite metamorphic zircon yielded a 75.5 ± 2 Ma age (95% confidence level). Chondrite-normalized rare-earth element patterns of zircon lack Eu anomalies and show depletions in heavy rare earths, consistent with zircon growing in a plagioclase-free, garnet-rich, eclogite-facies assemblage. Additionally, a Sm-Nd clinopyroxene-two garnet-whole rock isochron from an eclogite band yielded a less precise but consistent age of 77 ± 13 Ma. The above features imply subduction to > 60 km depth of at least a portion of the North American sialic basement during Late Cretaceous collision. The Chuacús complex was overprinted by an amphibolite-facies event. For instance, mafic high-pressure paragneiss contains symplectite, resorbed garnet, and amphibole + plagioclase poikiloblasts. Zircon rims from the paragneiss sample show rare-earth patterns consistent with plagioclase growth and garnet breakdown. Their 74.5 ± 3.5 Ma SHRIMP-RG U-Pb age is therefore interpreted as the time of retrogression, which is consistent with previously published results. Within error, the ages of the eclogite-facies event and the amphibolite-facies retrogression are equivalent. Thus exhumation of the Chuacús slab from mantle to mid-crustal depth was quick, taking few million years. During exhumation, partial melting of Chuacús gneisses generated ubiquitous pegmatites. One of the pegmatites intruded the North Motagua mélange, which is a serpentinite-rich subduction complex of the Guatemala Suture Complex containing Early Cretaceous oceanic eclogites. U-Pb, Rb-Sr, and K-Ar ages of the pegmatite range ~ 76-66 Ma. Thus initial juxtaposition of continental and oceanic high-pressure belts of the Guatemala Suture Complex predates Tertiary-present strike-slip faulting between the North-American and Caribbean plates.

  16. Physical properties and Consolidation behavior of sediments from the N. Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Valdez, R. D., II; Lauer, R. M.; Ikari, M.; Kitajima, H.; Saffer, D. M.

    2013-12-01

    Sediment hydraulic properties, consolidation state, and ambient pore pressure development are key parameters that affect fluid migration, deformation, and the slip behavior and mechanical strength of subduction zone megathrusts. In order to better understand the dynamics and mechanisms of large subduction earthquakes, Integrated Oceanic Drilling Program (IODP) Expedition 343, drilled into the toe of the Japan Trench subduction zone in a region of large shallow slip in the M 9.0 Tohoku earthquake, as part of the Japan Trench Fast Drilling Project (J-FAST). Here, we report on two constant rate of strain (CRS) uniaxial consolidation experiments and two triaxial deformation experiments on bedded claystone and clayey mudstone core samples collected from the frontal prism and subducted sediment section cored at Site C0019, 2.5 km landward of the Japan Trench, from depths of 697.18 and 831.45 mbsf. The goals of our experiments were: (1) to define the hydraulic and acoustic properties of sediments that host the subduction megathrust fault that slipped in the M 9.0 Tohoku earthquake; and (2) to constrain in-situ consolidation state and its implications for in-situ stress. The permeability-porosity trends are similar for the two samples, and both exhibit permeability that decreases systematically with increasing effective stress and decreasing porosity, and which varies log-linearly with porosity. Permeabilities of material from the frontal prism decrease from 5×10-18 m2 at 5 MPa effective stress, to 3.0×10-19 m2 at 70 MPa, and porosities decrease from 51% to 29%, while permeabilities of the subducted sediment sample decrease from 5×10-18 m2 at 5 MPa to 3.6×10-19 m2 at 90 MPa, and porosities decrease from 49% to 36%. In-situ permeabilities for the prism and underthrust sediment samples, estimated using laboratory defined permeability-porosity relationships, are 4.9×10-18 m2 and 3.7×10-18 m2, respectively. Elastic wavespeeds increase systematically with increasing effective stress. P-wave velocities (Vp) in the frontal prism sample increase from 2.1 km/s at 8 MPa to 2.7 km/s at 55 MPa effective stress, and velocities in the underthrust sediment sample increase from 2.3 km/s at 6 MPa to 3.0 km/s at 76.5 MPa. Estimated in-situ Vp for the frontal prism and underthrust sediment sample are 2.1 km/s and 2.4 km/s, respectively. This is slightly higher than both the logging while drilling (LWD) measurements and shipboard velocity measurements on discrete samples. We also estimated pre-consolidation pressures (Pc) for each sample using the work-stress method. Comparing Pc with the present day in-situ vertical stress calculated from shipboard bulk density data, we find that both samples are severely overconsolidated. We report this in terms of overconsolidation ratio (OCR), defined as the ratio of Pc to the in-situ stress expected for the case of normal consolidation. Values of OCR for the prism and underthrust samples are 3.95 and 4.28, respectively. This overconsolidation is broadly consistent with fully drained (non-overpressured) conditions, and may reflect uplift and unroofing of the sediments following peak burial greater than their current depth, a significant contribution from lateral tectonic stresses leading to an effective stress far greater than expected for the case of uniaxial burial, or cementation that leads to apparent overconsolidation.

  17. Supercritical aqueous fluids in subduction zones carrying carbon and sulfur: oxidants for the mantle wedge?

    NASA Astrophysics Data System (ADS)

    Sverjensky, Dimitri; Manning, Craig

    2014-05-01

    Much speculation surrounds the nature of aqueous fluids in subduction zones. Aqueous fluids likely trigger partial melting in the mantle wedge, influencing the chemistry of the magmas that erupt in island arcs. They also may play a role in transporting elements that could metasomatize and oxidize the overlying mantle wedge, most importantly C, S and Fe. However, full coupling of aqueous fluid chemistry with the silicate, carbonate, C, sulfide and sulfate minerals has remained limited to pressures of 0.5 GPa because of limitations on the HKF aqueous ion equation of state. Recent progress in developing a Deep Earth Water model (Sverjensky et al., 2014), calibrated with new experimental data, now enables a detailed evaluation of the evolution of aqueous fluid chemistry to a pressure of 6 GPa, well into subduction zone conditions. We report aqueous speciation models for eclogitic aqueous fluids constrained by model mineral assemblages that give preliminary indications of the solubilities of elements that could contribute to mass transfer and redox changes in the mantle wedge. For example, at 600 °C and 2.5 GPa, an aqueous fluid in equilibrium with jadeite, paragonite, muscovite, quartz, lawsonite, almandine, talc, magnesite and pyrite at QFM oxidation state with 0.1 molal total Cl, contains 5.5 molal C, 0.04 molal S, and 9 micromolal Fe. The fluid has a pH of 4.7, much greater than the neutral pH of 3.3; the predominant species and molalities are CO2 (5.0), Na+ (0.44), Si(OH)4 (0.36), HCO3- (0.26), H3SiO4- (0.23), CaHCO3+ (0.18), silica dimer (0.10), Cl- (0.09), K+ (0.08), HCOO- (0.06), H2S (0.03). Calculations for model eclogitic fluids at the higher pressures and temperatures of subarc conditions also show that the solubility of C is much greater than either S or Fe at QFM. However, in subarc eclogitic fluids of higher oxidation state (QFM +3 to +4) in equilibrium with hematite, anhydrite, jadeite, kyanite, phlogopite, coesite, lawsonite, almandine-pyrope, and magnesite, the C/S ratio can vary from 0.2 to 3.5 when temperature varies from 650 to 750 °C at 4 GPa. Fe concentrations remain negligible. These results strongly suggest that aqueous subarc eclogitic fluids that evolve to QFM +3 to +4, perhaps by reaction with metamorphosed Fe-oxide-bearing sediments, could transport significant amounts of C and/or S into the mantle wedge environment depending on the temperature. Hotter subduction should favor high C/S fluids, whereas colder subduction should favor low C/S fluids. Aqueous Fe transport is unlikely to be playing a significant role in oxidizing the mantle wedge. Sverjensky, D. A., Harrison, B., and Azzolini, D., 2014. Water in the deep Earth: the dielectric constant and the solubilities of quartz and corundum to 60 kb and 1,200°C. Geochim. et Cosmochim. Acta (in press).

  18. Local structural controls on outer-rise faulting, hydration, and seismicity in the Alaska subduction zone

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    We present evidence from marine geophysical data that pre-existing structures in the incoming oceanic plate off the Alaska Peninsula control bending faulting and hydration at the outer rise, which in turn correlate to changes in the abundance of interplate and intermediate-depth earthquakes within the subduction zone. Thus, pre-existing heterogeneities in the downgoing plate can result in significant variations in plate hydration over relatively small distances and may in part explain the observed global diversity of seismicity in subduction zones. ALEUT MCS and bathymetry data reveal large changes in the style and amount of bending in the incoming plate. To the west, outboard of the Shumagin Gap, there is significant bending faulting, with fault offsets up to ~250 m at the seafloor and larger offsets at depth. Faults create rugged topography at the seafloor, and sediment cover is thin (~0.5 km). Most faults have strikes within ~25 degrees of the trench. In contrast, the downgoing plate outboard of the Semidi segment to the east exhibits less dramatic bending faulting, with maximum offsets at the seafloor of 30 m, and the sediment cover is thicker (>1 km). These along-strike changes in faulting correlate with changes in the expected orientation of pre-existing structures in the incoming oceanic crust, which is nearly parallel to the trench near the Shumagin Gap, but highly oblique to the trench near the Semidi segment. This implies that more favorably-oriented pre-existing structures may facilitate bending faulting. P-wave velocity models from wide-angle seismic data reveal that along-strike changes in faulting are accompanied by variations in the velocity structure of the incoming plate. Mantle velocities are reduced by ~0.5 km/s at the outer rise off the Shumagin Gap, where significant bending faulting is observed. We interpret decreased velocities to represent serpentinization of the upper mantle. In contrast, the velocity structure is more variable off the Semidi segment, where the profile obliquely crosses seafloor fabric, but no comparable reduction is observed, implying less hydration at the outer rise. Variations in outer rise faulting and hydration correlate to changes in seismicity at depth. The Shumagin area is characterized by abundant interplate and intermediate depth microseismicity, while the Semidi segment exhibits relatively fewer events in both depth ranges. We suggest that local controls on bending faulting and hydration control the occurrence of both types of microseismicity. The greater deformation and hydration of the plate in the Shumagin Gap may enable intermediate depth earthquakes through dehydration embrittlement and/or the reactivation of bending faults. The paucity of both in the Semidi segment may limit the amount of intermediate depth seismicity. Likewise, the rougher plate surface in the Shumagin may form many small asperities when subducted and thus generate abundant small interplate earthquakes, while the smooth, sedimented surface of the Semidi segment would be associated with more distributed, even coupling and less seismicity when subducted, with fewer small events but capable of rupturing more easily in great (M>8) earthquakes.

  19. The Impact of Frictional Anisotropy of Serpentine on Thrust Fault Slip at Subduction Zones

    NASA Astrophysics Data System (ADS)

    Campione, M.; Capitani, G. C.

    2013-12-01

    Models describing the slip dynamics of a fault assume generally an isotropic interface giving rise to shear forces which are parallel to the slip direction. However, structured surfaces may be associated with an anisotropic frictional behavior, which is characterized by a dependence of the magnitude of friction force on slip direction and the presence of friction force components transverse to the slip direction. The arrangement of atoms in the crystal structure of the minerals composing the rock slab, the foliation and lineations originating from the rock deformation, and the presence of gouges formed as a consequence of rock erosion, contribute to the structuring of the surface on different length scales. Among these aspects, only the former is related to intrinsic properties of the fault, and its influence on the frictional properties is therefore independent on the rock history. In order to study the relation between frictional properties and crystal structure, we performed a nanotribological characterization of the surface of prominent mineral species with a scanning force microscope, where a micrometric tip mounted on an elastic cantilever is scanned by piezoelectric actuation along all directions of the sample surface on a nanoscopic area, while vertical deflection and lateral torsion of the cantilever are monitored in real-time, allowing for the quantitative determination of the friction force vectors. Antigorite, the high-temperature, high pressure polymorph of serpentine, is a mineral with a prominent role in defining the mechanical behavior of faulted regions. Indeed, there is general consensus that it forms in the forearc mantle wedge by hydration of olivine and pyroxenes through aqueous fluids released in the downgoing slab, upon increasing pressure and temperature. We asses frictional anisotropies as high as 100% of individual crystal domains of antigorite [1]. This anisotropy is related to the peculiar wavy arrangement of the TO-layer and the presence of reversals occurring along the a-axis of the tetrahedral sheets of antigorite. If we associate the known strain-induced lattice preferred orientation (LPO) of serpentine rocks to our nanoscopic observation, a strong anisotropic frictional behavior of sliding surfaces of serpentinized rocks in subduction zones can be inferred. The results obtained with other phyllosilicates such as mica muscovite and phlogopite will be also presented for comparison. Impressive phenomena can be envisaged if frictional anisotropy is introduced in the dynamical description of faults such as mechanical instabilities giving rise to a strong declination of the slip vector with respect to the plate convergence direction, explaining seismic degree of partitioning otherwise to be considered anomalous. On the other hand, for specific LPO mechanisms, a strain softening behavior of the serpentinized mantle wedge can be inferred, providing a physical explanation to its postulated aseismic character. [1] Campione M., and Capitani G. C., Subduction-zone earthquake complexity related to frictional anisotropy in antigorite, Nature Geoscience, in press, 2013.

  20. Seismic structure of the North Pacific oceanic crust prior plate bending at the Alaska subduction zone

    NASA Astrophysics Data System (ADS)

    Becel, A.; Shillington, D. J.; Nedimovic, M. R.; Kuehn, H.; Webb, S. C.; Holtzman, B. K.

    2012-12-01

    Seismic reflection profiles across North Pacific oceanic Plate reveal the internal structure of a mature oceanic crust (42-56Ma) formed at fast (70mm/yr, half rate) to intermediate (28mm/yr, half rate) spreading rates. Data used in this study were collected with the R/V Langseth in summer 2011 as part of the ALEUT (Alaska Langseth Experiment to Understand the megathrust) program. MCS data were acquired with two 8-km streamers and a 6600 cu. in. air gun array. We collected a series of profiles across the subduction zone system but also across the preexisting structures of the oceanic crust before being affected by subduction zone processes. Additionally, two 400-km OBS refraction lines were shot coincident with MCS profiles. The multi-channel seismic (MCS) data across oceanic crust formed at fast spreading rates contain abundant bright reflectors mostly confined in the lower crust above the Moho discontinuity and dipping predominantly toward the paleo-ridge. Along these profiles, the Moho discontinuity is observed as a bright event with remarkable lateral continuity. The lengths of the dipping reflectors are on the order of 5-km, with apparent dips between 10 and 30°. These reflectors represent discrete events, with spacing between 0.3 to 5 km without any obvious regularity. These dipping events appear to sole out within the middle crust (1 to 1.5 s beneath basement) and most of them terminate at the Moho. The Moho is much weaker or absent on the northern profiles acquired across the North Pacific oceanic crust formed at intermediate spreading rates. Basement topography is rougher and no clear dipping events have been imaged suggesting that the spreading rate may be an important factor that controls the strength and abundance of such dipping reflectors and the lateral change in the Moho reflection characters. Lower crustal dipping reflections (LCDR) have been only imaged at very few places across the Pacific oceanic crust: (Eittreim et al., 1988, Reston et al. 1999, Ranero et al., 1997, Hallenborg et al., 2003). These LCDR have been interpreted as being either formed near the spreading center during accretion or off-axis post-accretion structures. Interpretations proposed included lithologic banding from passive accretion, shear structures from active mantle upwelling, off-axis magmatism, or enhanced reflectivity of latent structures by crustal aging (e.g., hydrothermal circulation, etc). Another alternative that we propose is that the dipping reflectors are shear zones that form in the lower crustal mush zone due to active upwelling that contain frozen melts segregated into the shear zones during deformation. Characteristics of the wide-angle reflection data are also different between the two lines. Lateral variations in the Moho reflections and crustal refractions are clearly observed and will be discussed with respect to the structures imaged on the coincident reflection images. The transition between fast to intermediate spreading rates occurs near a triple junction that separated the Pacific, Kula and Farallon plates that ceased spreading in the Late Eocene. MCS profiles across and around the fossil triple junction reveal deep reflections into the crust and below the Moho down to 15-km depth. These reflections could possibly be caused by gabbroic melts that froze in the mantle lithosphere when the triple junction was abandoned.

  1. Quantifying potential earthquake and tsunami hazard in the Lesser Antilles subduction zone of the Caribbean region

    NASA Astrophysics Data System (ADS)

    Hayes, Gavin P.; McNamara, Daniel E.; Seidman, Lily; Roger, Jean

    2014-01-01

    In this study, we quantify the seismic and tsunami hazard in the Lesser Antilles subduction zone, focusing on the plate interface offshore of Guadeloupe. We compare potential strain accumulated via GPS-derived plate motions to strain release due to earthquakes that have occurred over the past 110 yr, and compute the resulting moment deficit. Our results suggest that enough strain is currently stored in the seismogenic zone of the Lesser Antilles subduction arc in the region of Guadeloupe to cause a large and damaging earthquake of magnitude Mw ˜ 8.2 ± 0.4. We model several scenario earthquakes over this magnitude range, using a variety of earthquake magnitudes and rupture areas, and utilizing the USGS ShakeMap and PAGER software packages. Strong ground shaking during the earthquake will likely cause loss of life and damage estimated to be in the range of several tens to several hundreds of fatalities and hundreds of millions to potentially billions of U.S. dollars of damage. In addition, such an event could produce a significant tsunami. Modelled tsunamis resulting from these scenario earthquakes predict meter-scale wave amplitudes even for events at the lower end of our magnitude range (M 7.8), and heights of over 3 m in several locations with our favoured scenario (M 8.0, partially locked interface from 15-45 km depth). In all scenarios, only short lead-times (on the order of tens of minutes) would be possible in the Caribbean before the arrival of damaging waves.

  2. Constraints on upper plate deformation in the Nicaraguan subduction zone from earthquake relocation and directivity analysis

    NASA Astrophysics Data System (ADS)

    French, S. W.; Warren, L. M.; Fischer, K. M.; Abers, G. A.; Strauch, W.; Protti, J. M.; Gonzalez, V.

    2010-03-01

    In the Nicaraguan segment of the Central American subduction zone, bookshelf faulting has been proposed as the dominant style of Caribbean plate deformation in response to oblique subduction of the Cocos plate. A key element of this model is left-lateral motion on arc-normal strike-slip faults. On 3 August 2005, a Mw 6.3 earthquake and its extensive foreshock and aftershock sequence occurred near Ometepe Island in Lake Nicaragua. To determine the fault plane that ruptured in the main shock, we relocated main shock, foreshock, and aftershock hypocenters and analyzed main shock source directivity using waveforms from the TUCAN Broadband Seismic Experiment. The relocation analysis was carried out by applying the hypoDD double-difference method to P and S onset times and differential traveltimes for event pairs determined by waveform cross correlation. The relocated hypocenters define a roughly vertical plane of seismicity with an N60°E strike. This plane aligns with one of the two nodal planes of the main shock source mechanism. The directivity analysis was based on waveforms from 16 TUCAN stations and indicates that rupture on the N60°E striking main shock nodal plane provides the best fit to the data. The relocation and directivity analyses identify the N60°E vertical nodal plane as the main shock fault plane, consistent with the style of faulting required by the bookshelf model. Relocated hypocenters also define a second fault plane that lies to the south of the main shock fault plane with a strike of N350°E-N355°E. This fault plane became seismically active 5 h after the main shock, suggesting the influence of stresses transferred from the main shock fault plane. The August 2005 earthquake sequence was preceded by a small eruption of a nearby volcano, Concepción, on 28 July 2005. However, the local seismicity does not provide evidence for earthquake triggering of the eruption or eruption triggering of the main shock through crustal stress transfer.

  3. Constraining Interseismic Deformation in the Cascadia Subduction Zone Using a Viscoelastic Earthquake Cycle Model

    NASA Astrophysics Data System (ADS)

    Kim, M. J.; Segall, P.; Johnson, K. M.

    2012-12-01

    Most recent models of interseismic deformation in Cascadia have been restricted to elastic half-spaces. In this study, we investigate the interseismic deformation in the Cascadia subduction zone using a viscoelastic earthquake cycle model in order to constrain the extent of plate coupling, elastic plate thickness, and the viscoelastic relaxation time. Our model of the plate interface consists of an elastic layer overlying a Maxwell viscoelastic half-space. The fault in the elastic layer is composed of a fully locked zone that slips during megathrust events at cycle time T= 500 years, and a transition zone where the interseismic slip rate changes from zero (fully coupled) to the plate velocity (zero coupling). Slip deficit within the transition zone is accommodated by either coseismic or rapid post-seismic slip. We model the slip rate in the transition zone using the analytic solution for slip at a constant resistive stress in an elastic full space. We explore ranges for the 4 model parameters: the elastic plate thickness, the relaxation time, and the upper and the lower bounds of the transition zone - that minimize the residual between the predicted surface velocities and the observed GPS data. GPS position solutions were provided by PANGA and our data consists of 29 GPS station velocities from 2002 to 2010 in the Olympic Peninsula - southern Vancouver Island region, since this region is least affected by forearc rotation. Our preliminary result suggests a shallow fully locked zone (< 15 km depth) with a short relaxation time (< 100 years) compared to the recurrence interval (~ 500 years). For a given degree of misfit to the data, accounting for the viscoelastic effect allows deeper locking depth compared to the fully elastic model.

  4. Seismic?wave attenuation determined from tectonic tremor in multiple subduction zones

    USGS Publications Warehouse

    Yabe, Suguru; Baltay, Annemarie S.; Ide, Satoshi; Beroza, Gregory C.

    2014-01-01

    Tectonic tremor provides a new source of observations that can be used to constrain the seismic attenuation parameter for ground?motion prediction and hazard mapping. Traditionally, recorded earthquakes of magnitude ?3–8 are used to develop ground?motion prediction equations; however, typical earthquake records may be sparse in areas of high hazard. In this study, we constrain the distance decay of seismic waves using measurements of the amplitude decay of tectonic tremor, which is plentiful in some regions. Tectonic tremor occurs in the frequency band of interest for ground?motion prediction (i.e., ?2–8??Hz) and is located on the subducting plate interface, at the lower boundary of where future large earthquakes are expected. We empirically fit the distance decay of peak ground velocity from tremor to determine the attenuation parameter in four subduction zones: Nankai, Japan; Cascadia, United States–Canada; Jalisco, Mexico; and southern Chile. With the large amount of data available from tremor, we show that in the upper plate, the lower crust is less attenuating than the upper crust. We apply the same analysis to intraslab events in Nankai and show the possibility that waves traveling from deeper intraslab events experience more attenuation than those from the shallower tremor due to ray paths that pass through the subducting and highly attenuating oceanic crust. This suggests that high pore?fluid pressure is present in the tremor source region. These differences imply that the attenuation parameter determined from intraslab earthquakes may underestimate ground motion for future large earthquakes on the plate interface.

  5. Mantle wedge anisotropy in Southern Tyrrhenian Subduction Zone (Italy), from receiver function analysis

    NASA Astrophysics Data System (ADS)

    Piana Agostinetti, Nicola; Park, Jeffrey; Lucente, Francesco Pio

    2008-12-01

    We constrain mantle wedge seismic structure in the Southern Tyrrhenian Subduction Zone (Italy) using teleseismic receiver functions (RF) recorded at station CUC of the Mednet seismographic network. Station CUC lies above the northern portion of the Calabrian slab segment, which is recognized from deep seismicity and tomographic imaging as a narrow, laterally high-arched slab fragment, extending from the surface below Calabria down to the transition zone. To better define the descending slab interface and possible shear-coupled flow in the mantle wedge above the slab, we computed receiver functions from the P-coda of 147 teleseismic events to analyze the back-azimuth dependence of Ps converted phases from interfaces beneath CUC. We stack the RF data-set with back azimuth to compute its harmonic expansion, which relates to the effects of interface dip and anisotropy at layer boundaries. The seismic structure constrained through the RF analysis is characterized in its upper part by a sub-horizontal Moho at about 25 km depth, overlying a thin isotropic layer at top of mantle. For the deeper part, back-azimuth variation suggests two alternative models, each with an anisotropic layer between two dipping interfaces near 70- and 90-km depth, with fast- and slow-symmetry axes, respectively, above the Apennines slab. Although independent evidence suggests a north-south strike for the slab beneath CUC, the trend of the inferred anisotropy is 45° clockwise from north, inconsistent with a simple downdip shear-coupled flow model in the supra-slab mantle wedge. However complexities of flow and induced rock fabric in the Tyrrhenian back arc may arise due to several concurring factors such as the arcuate shape of the Apennines slab, its retreating kinematics, or slab edge effects.

  6. The 2011 Northern Kermadec earthquake doublet and subduction zone faulting interactions

    NASA Astrophysics Data System (ADS)

    Todd, Erin K.; Lay, Thorne

    2013-01-01

    AbstractA large intraslab earthquake doublet (6 July 2011, Mw 7.6; 21 October 2011, Mw 7.4) occurred in the Pacific plate beneath the outer trench slope of the northern Kermadec <span class="hlt">subduction</span> <span class="hlt">zone</span>, seaward of an interseismically-coupled region of the megathrust. The first large event, a shallow (~24 km centroid depth) normal-faulting rupture, was followed by intraslab aftershocks within minutes and by aftershocks near the megathrust within hours. The second large event, a deeper (~45 km centroid depth) thrust-faulting rupture below the northern portion of the first rupture plane, was also followed by interplate and intraslab aftershocks. The last large interplate activity in northern Kermadec involved an underthrusting doublet on 14 January 1976 (Mw 7.8, 7.9). Previous studies interpret westward movement of the northern Kermadec Arc relative to the Australian Plate (from GPS measurements) to be a result of interseismic coupling on the northern Kermadec megathrust. A locked plate interface in this region should tend to reduce trench-slope extensional stresses. Coulomb stress change calculations using a finite-fault model determined from teleseismic body wave inversions for the normal-faulting event in the 2011 doublet favor activation of megathrust faulting. Coulomb stress changes for the October compressional event appear to reduce interplate thrust activity, and for a short time reversed the westward motion of the upper plate. The net effect of the doublet is a few bar increase of interplate thrust-fault driving stress, which may have advanced a future large megathrust event. Intraslab normal faulting may serve as a harbinger, not only a response, to large megathrust ruptures.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=USGSPUBS&redirectUrl=http://pubs.er.usgs.gov/publication/70073849"><span id="translatedtitle">Tsunami impact to Washington and northern Oregon from segment ruptures on the southern Cascadia <span class="hlt">subduction</span> <span class="hlt">zone</span></span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Priest, George R.; Zhang, Yinglong; Witter, Robert C.; Wang, Kelin; Goldfinger, Chris; Stimely, Laura</p> <p>2014-01-01</p> <p>This paper explores the size and arrival of tsunamis in Oregon and Washington from the most likely partial ruptures of the Cascadia <span class="hlt">subduction</span> <span class="hlt">zone</span> (CSZ) in order to determine (1) how quickly tsunami height declines away from sources, (2) evacuation time before significant inundation, and (3) extent of felt shaking that would trigger evacuation. According to interpretations of offshore turbidite deposits, the most frequent partial ruptures are of the southern CSZ. Combined recurrence of ruptures extending ~490 km from Cape Mendocino, California, to Waldport, Oregon (segment C) and ~320 km from Cape Mendocino to Cape Blanco, Oregon (segment D), is ~530 years. This recurrence is similar to frequency of full-margin ruptures on the CSZ inferred from paleoseismic data and to frequency of the largest distant tsunami sources threatening Washington and Oregon, ~Mw 9.2 earthquakes from the Gulf of Alaska. Simulated segment C and D ruptures produce relatively low-amplitude tsunamis north of source areas, even for extreme (20 m) peak slip on segment C. More than ~70 km north of segments C and D, the first tsunami arrival at the 10-m water depth has an amplitude of <1.9 m. The largest waves are trapped edge waves with amplitude ?4.2 m that arrive ?2 h after the earthquake. MM V–VI shaking could trigger evacuation of educated populaces as far north as Newport, Oregon for segment D events and Grays Harbor, Washington for segment C events. The NOAA and local warning systems will be the only warning at greater distances from sources.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2014JGRB..119.8478P"><span id="translatedtitle">Decoupling of Pacific <span class="hlt">subduction</span> <span class="hlt">zone</span> guided waves beneath central Japan: Evidence for thin slab</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Padhy, Simanchal; Furumura, Takashi; Maeda, Takuto</p> <p>2014-11-01</p> <p>The fine-scale seismic structure of the northeast Japan <span class="hlt">subduction</span> <span class="hlt">zone</span> is studied based on waveform analyses of moderate-sized (M4.5-6), deep-focus earthquakes (h >350 km) and the finite difference method (FDM) simulation of high-frequency (up to 8 Hz) wave propagation. Strong regional S wave attenuation anomalies for specific source-receiver paths connecting the cluster of events occurring in central part of the Sea of Japan recorded at fore arc stations in northern and central Japanese Islands (Honshu) are used to model the deeper structure of the subducting Pacific Plate, where recent teleseismic tomography has shown evidence for a possible slab tear westward beneath the Sea of Japan. The character of the observed anomalous S wave attenuation and the following high-frequency coda can be captured with the two-dimensional (2-D) FDM simulation of seismic waves in heterogeneous plate model, incorporating the thinning of the plate at depth, which is also compared with other possible causes of dramatic attenuation of high-frequency S wave due to low-Q or much weaker heterogeneities in the slab. The results of simulation clearly demonstrate that the dramatic loss of high-frequency S wavefield from the plate into the surrounding mantle occurred due to the variation in the plate geometry (i.e., thinning of the plate) at depth near the source rather than due to variation in physical properties, such as due to the lowered-Q and weaker heterogeneities in the plate. The presence of such a thin zone defocuses the high-frequency slab-guided S wave energy from the subducting plate into the surrounding mantle and acts as a geometric antiwaveguide. Based on the sequence of simulation results obtained, we propose thinning of Pacific Plate at depth subducting beneath northeastern Japan, localized to central part of Honshu, in agreement with the observations.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2014JGRB..119.2068S"><span id="translatedtitle">Antigorite-induced seismic anisotropy and implications for deformation in <span class="hlt">subduction</span> <span class="hlt">zones</span> and the Tibetan Plateau</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Shao, Tongbin; Ji, Shaocheng; Kondo, Yosuke; Michibayashi, Katsuyoshi; Wang, Qian; Xu, Zhiqin; Sun, Shengsi; Marcotte, Denis; Salisbury, Matthew H.</p> <p>2014-03-01</p> <p>The present study, which is a follow-up of the Journal of Geophysical Research paper by Ji et al. (2013a), provides a new calibration for both seismic and fabric properties of antigorite serpentinites. Comparisons of the laboratory velocities of antigorite serpentinites measured at high pressures with crystallographic-preferred orientation data measured using electron backscatter diffraction techniques demonstrate that seismic anisotropy in high T serpentinite, which is essentially controlled by the antigorite c axis fabric, is independent on the operating slip system but strongly dependent on the regime and magnitude of finite strain experienced by the rock. Extrapolation of the experimental data with both pressure and temperature suggests that Vp anisotropy decreases but shear wave splitting (?Vs) and Vp/Vs increase with increasing pressure in either cold or hot <span class="hlt">subduction</span> <span class="hlt">zones</span>. For a cold, steeply subducting slab, antigorite is most likely deformed by nearly coaxial flattening or trench-parallel movements, forming trench-parallel seismic anisotropy. For a hot, shallowly subducting slab, however, antigorite is most likely deformed by simple shear or transpression. Trench-normal seismic anisotropy can be observed when the subducting dip angle is smaller than 30°. The geophysical characteristics of the Tibetan Plateau such as strong heterogeneity in Vp, Vs and attenuation, shear wave splitting and electric conductivity may be explained by the presence of strongly deformed serpentinites in lithospheric shear zones reactivated along former suture zones between amalgamated blocks, hydrated zones of subducting lithospheric mantle, and the crust-mantle boundary if the temperature is below 700°C in the region of interest.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2013JAESc..78..291R"><span id="translatedtitle">Elastic thickness structure of the Andaman <span class="hlt">subduction</span> <span class="hlt">zone</span>: Implications for convergence of the Ninetyeast Ridge</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Ratheesh Kumar, R. T.; Windley, B. F.; Rajesh, V. J.; Santosh, M.</p> <p>2013-12-01</p> <p>We use the Bouguer coherence (Morlet isostatic response function) technique to compute the spatial variation of effective elastic thickness (Te) of the Andaman <span class="hlt">subduction</span> <span class="hlt">zone</span>. The recovered Te map resolves regional-scale features that correlate well with known surface structures of the subducting Indian plate and the overriding Burma plate. The major structure on the India plate, the Ninetyeast Ridge (NER), exhibits a weak mechanical strength, which is consistent with the expected signature of an oceanic ridge of hotspot origin. However, a markedly low strength (0 < Te < 3 km) in that region, where the NER is close to the Andaman trench (north of 10°N), receives our main attention in this study. The subduction geometry derived from the Bouguer gravity forward modeling suggests that the NER has indented beneath the Andaman arc. We infer that the bending stresses of the viscous plate, which were reinforced within the subducting oceanic plate as a result of the partial subduction of the NER buoyant load, have reduced the lithospheric strength. The correlation, Te < Ts (seismogenic thickness) reveals that the upper crust is actively deforming beneath the frontal arc Andaman region. The occurrence of normal-fault earthquakes in the frontal arc, low Te zone, is indicative of structural heterogeneities within the subducting plate. The fact that the NER along with its buoyant root is subducting under the Andaman region is inhibiting the subduction processes, as suggested by the changes in trench line, interrupted back-arc volcanism, variation in seismicity mechanism, slow subduction, etc. The low Te and thinned crustal structure of the Andaman back-arc basin are attributed to a thermomechanically weakened lithosphere. The present study reveals that the ongoing back-arc spreading and strike-slip motion along the West Andaman Fault coupled with the ridge subduction exerts an important control on the frequency and magnitude of seismicity in the Andaman region.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=USGSPUBS&redirectUrl=http://pubs.er.usgs.gov/publication/70057380"><span id="translatedtitle">Quantifying potential earthquake and tsunami hazard in the Lesser Antilles <span class="hlt">subduction</span> <span class="hlt">zone</span> of the Caribbean region</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Hayes, Gavin P.; McNamara, Daniel E.; Seidman, Lily; Roger, Jean</p> <p>2013-01-01</p> <p>In this study, we quantify the seismic and tsunami hazard in the Lesser Antilles <span class="hlt">subduction</span> <span class="hlt">zone</span>, focusing on the plate interface offshore of Guadeloupe. We compare potential strain accumulated via GPS-derived plate motions to strain release due to earthquakes that have occurred over the past 110 yr, and compute the resulting moment deficit. Our results suggest that enough strain is currently stored in the seismogenic zone of the Lesser Antilles subduction arc in the region of Guadeloupe to cause a large and damaging earthquake of magnitude Mw ? 8.2 ± 0.4. We model several scenario earthquakes over this magnitude range, using a variety of earthquake magnitudes and rupture areas, and utilizing the USGS ShakeMap and PAGER software packages. Strong ground shaking during the earthquake will likely cause loss of life and damage estimated to be in the range of several tens to several hundreds of fatalities and hundreds of millions to potentially billions of U.S. dollars of damage. In addition, such an event could produce a significant tsunami. Modelled tsunamis resulting from these scenario earthquakes predict meter-scale wave amplitudes even for events at the lower end of our magnitude range (M 7.8), and heights of over 3?m in several locations with our favoured scenario (M 8.0, partially locked interface from 15–45?km depth). In all scenarios, only short lead-times (on the order of tens of minutes) would be possible in the Caribbean before the arrival of damaging waves.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2014JGRB..119.8845W"><span id="translatedtitle">Seismic velocity structure and anisotropy of the Alaska <span class="hlt">subduction</span> <span class="hlt">zone</span> based on surface wave tomography</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Wang, Yun; Tape, Carl</p> <p>2014-12-01</p> <p>Southcentral Alaska is a complex tectonic region that transitions from subduction of Pacific crust to flat slab subduction—and collision—of overthickened Yakutat crust. Because much of the Yakutat crust has been subducted, seismic imaging is needed in order to understand the crustal and upper mantle structural framework for this active tectonic setting. Here we use teleseismic Rayleigh waves to image large-scale variations in shear wave structure. Our imaging technique employs a two-plane wave representation with finite frequency sensitivity kernels. Our 3-D isotropic model reveals several features: the subducting Pacific/Yakutat slab, slow wave speeds characterizing the onshore Yakutat collision zone, slow wave speeds of the Wrangell <span class="hlt">subduction</span> <span class="hlt">zone</span>, and a deep tomographic contrast at the eastern edge of the Pacific/Yakutat slab. We produce anisotropic phase velocity maps that exhibit variations in the fast direction of azimuthal anisotropy. These maps show the dominance of the Yakutat slab on the observed pattern of anisotropy. West of the Yakutat slab the fast directions are approximately aligned with the plate convergence direction. In the region of the Yakutat slab the pattern is more complicated. Along the margins of the slab the fast directions are roughly parallel to the margins. We identify notable differences and similarities with published SKS splitting measurements. Integrative modeling using 3-D anisotropy models and different seismic measurements will be needed in order to establish a detailed 3-D anisotropic velocity model for Alaska. This study provides a large-scale starting point for such an effort.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2012EGUGA..14.2976M"><span id="translatedtitle">Interseismic coupling, segmentation and mechanical behavior of the Central Chile <span class="hlt">subduction</span> <span class="hlt">zone</span>.</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Métois, M.; Socquet, A.; Vigny, C.</p> <p>2012-04-01</p> <p>Global Positioning System (GPS) measurements carried out in Chile over the last two decades showed that an entire portion of the Nazca-South America <span class="hlt">subduction</span> <span class="hlt">zone</span> (38°S-24°S) was locked over this period of time. The induced accumulation of elastic deformation in the upper-plate was not released until the recent Maule earthquake of 27 February 2010 (Mw 8.8) that ruptured the southern part of this section. Locking or coupling between the two plates varies both with depth and along strike. Here we use our own GPS data (an updated solution of our extended network in central Chile), combined with other published data sets, to quantify the spatial variations of the coupling that prevailed before the Maule earthquake. Using a simple elastic model based on the back-slip assumption, we show that coupling variations on the subduction plane are sufficient to explain the observed surface deformation, with no need of a sliver in central Chile. We identify four segments characterized by higher coupling and separated by narrow areas of lower coupling. This segmentation is in good agreement with historical and recent seismicity in Chile. In particular, the La Serena Bay (30°S-28°S) where the locked zone vanishes is a stable boundary where historical events stopped. The Maule region that ruptured in feb. 27th of 2010 (Mw 8.8) from 38 S to 34 S, was characterized by a well developped fully locked zone that extents far indepth and narrows where the earthquake stopped propagating (San Antonio at 34°S and south of Arauco peninsula at 38°S). These narrow zones of lower coupling are often associated with irregular bathymetric or coastal features (fracture zones or peninsulas). Finally, coseismic and early post-seismic slip distribution of the Maule earthquake, occurring either in previously highly or weakly coupled zones, map a complex distribution of velocity-weakening and velocity-strengthening patches on the subduction interface.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2014EP%26S...66...13K"><span id="translatedtitle">Quartz deposition and its influence on the deformation process of megathrusts in <span class="hlt">subduction</span> <span class="hlt">zones</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Kameda, Jun; Kawabata, Kuniyo; Hamada, Yohei; Yamaguchi, Asuka; Kimura, Gaku</p> <p>2014-12-01</p> <p>We present a quantitative examination of the liberation and subsequent deposition of silica at the <span class="hlt">subduction</span> <span class="hlt">zone</span> plate interface in the Mugi mélange, an exhumed accretionary complex in the Shimanto Belt of southwest Japan. Frequency and thickness measurements indicate that mineralized veins hosted in deformed shales make up approximately 0.4% of the volume of this exposure. In addition, whole-rock geochemical evidence suggests that the net volume of SiO2 liberated from the mélange at temperatures of < 200°C was as much as 35%, with up to 40% of the SiO2 loss related to the smectite-illite (S-I) conversion reaction, and the rest attributable to the pressure solution of detrital quartz and feldspar. Kinetic modeling of the S-I reaction indicates active liberation of SiO2 at approximately 70°C to 200°C, with peak SiO2 loss at around 100°C, although these estimates should be slightly shifted toward lower temperature conditions based on X-ray diffraction (XRD) analyses of mixed-layer S-I in the Mugi mélange. The onset of pressure solution was not fully constrained, but has been documented to occur at around 150°C in the study area. The deposition in deformed shales of quartz liberated by pressure solution and the S-I reaction is probably linked to seismogenic behavior along the plate interface by (1) progressively enhanced velocity-weakening properties, which are favorable for unstable seismogenic faulting, including very-low-frequency earthquakes and (2) increasing intrinsic frictional strength, which leads to a step-down of the plate boundary décollement into oceanic basalt.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2013AGUFM.S11B2380M"><span id="translatedtitle">Automatic picking and earthquake relocation for the Antilles <span class="hlt">subduction</span> <span class="hlt">zone</span> (1972-2013)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Massin, F.; Amorèse, D.; Bengoubou-Valerius, M.; Bernard, M.</p> <p>2013-12-01</p> <p>Locations for earthquake recorded in the Antilles <span class="hlt">subduction</span> <span class="hlt">zone</span> are processed separately by regional observatories and ISC. There is no earthquake location catalog available compiling all available first arrival data. We aim to produce a best complete earthquake catalog by merging all available first arrival data for better constrains on earthquake locations. ISC provides the first arrival data of 29243 earthquakes (magnitude range from 1.4 to 6.4) recorded by PRSN (Porto Rico), SRC (British West Indies), and form FUNVISIS (Venezuela). IPGP provided the first arrival data of 68718 earthquakes (magnitude from 0.1 to 7.5) recorded by OVSG (Guadeloupe, 53226 earthquakes since 1981) and by OVSM (Martinique, 29931 earthquakes since 1972). IPGP also provides the accelerometer waveform data of the GIS-RAP network in the Antilles. The final catalog contains 84979 earthquakes between 1972 and 2013, 24528 of which we compiled additional data. We achieved automatic picking using the Component Energy Correlation Method. The CECM provide high precision phase detection, a realistic estimation of picking error and realistic weights that can be used with manual pick weights. The CECM add an average of 3 P-waves and 2 S-waves arrivals to 3846 earthquakes recoded by the GIS-RAP network since 2002. Cluster analysis, earthquake local tomography and relative locations are to be applied in order to image active faulting and migration of seismicity. This will help to understand seismic coupling in the seismogenic zone as well as triggering mechanisms of intermediate depth seismicity like fluid migration beneath the volcanic arc.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=USGSPUBS&redirectUrl=http://pubs.er.usgs.gov/publication/ofr20071348"><span id="translatedtitle">Velocity and Density Models Incorporating the Cascadia <span class="hlt">Subduction</span> <span class="hlt">Zone</span> for 3D Earthquake Ground Motion Simulations</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Stephenson, William J.</p> <p>2007-01-01</p> <p>INTRODUCTION In support of earthquake hazards and ground motion studies in the Pacific Northwest, three-dimensional P- and S-wave velocity (3D Vp and Vs) and density (3D rho) models incorporating the Cascadia <span class="hlt">subduction</span> <span class="hlt">zone</span> have been developed for the region encompassed from about 40.2?N to 50?N latitude, and from about -122?W to -129?W longitude. The model volume includes elevations from 0 km to 60 km (elevation is opposite of depth in model coordinates). Stephenson and Frankel (2003) presented preliminary ground motion simulations valid up to 0.1 Hz using an earlier version of these models. The version of the model volume described here includes more structural and geophysical detail, particularly in the Puget Lowland as required for scenario earthquake simulations in the development of the Seattle Urban Hazards Maps (Frankel and others, 2007). Olsen and others (in press) used the model volume discussed here to perform a Cascadia simulation up to 0.5 Hz using a Sumatra-Andaman Islands rupture history. As research from the EarthScope Program (http://www.earthscope.org) is published, a wealth of important detail can be added to these model volumes, particularly to depths of the upper-mantle. However, at the time of development for this model version, no EarthScope-specific results were incorporated. This report is intended to be a reference for colleagues and associates who have used or are planning to use this preliminary model in their research. To this end, it is intended that these models will be considered a beginning template for a community velocity model of the Cascadia region as more data and results become available.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2015EP%26S...67...39H"><span id="translatedtitle">Estimation of slip rate and fault displacement during shallow earthquake rupture in the Nankai <span class="hlt">subduction</span> <span class="hlt">zone</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Hamada, Yohei; Sakaguchi, Arito; Tanikawa, Wataru; Yamaguchi, Asuka; Kameda, Jun; Kimura, Gaku</p> <p>2015-12-01</p> <p>Enormous earthquakes repeatedly occur in <span class="hlt">subduction</span> <span class="hlt">zones</span>, and the slips along megathrusts, in particular those propagating to the toe of the forearc wedge, generate ruinous tsunamis. Quantitative evaluation of slip parameters (i.e., slip velocity, rise time and slip distance) of past slip events at shallow, tsunamigenic part of the fault is critical to characterize such earthquakes. Here, we attempt to quantify these parameters of slips that may have occurred along the shallow megasplay fault and the plate boundary décollement in the Nankai Trough, off southwest Japan. We apply a kinetic modeling to vitrinite reflectance profiles on the two fault rock samples obtained from Integrated Ocean Drilling Program (IODP). This approach constitutes two calculation procedures: heat generation and numerical profile fitting of vitrinite reflectance data. For the purpose of obtaining optimal slip parameters, residue calculation is implemented to estimate fitting accuracy. As the result, the measured distribution of vitrinite reflectance is reasonably fitted with heat generation rate and slip duration ( t r ) of 16,600 J/s/m2 and 6,250 s, respectively, for the megasplay and 23,200 J/s/m2 and 2,350 s, respectively, for the frontal décollement, implying slow and long-term slips. The estimated slip parameters are then compared with previous reports. The maximum temperature, Tmax, for the Nankai megasplay fault is consistent with the temperature constraint suggested by a previous work. Slow slip velocity, long-term rise time, and large displacement are recognized in these fault zones (both of the megasplay, the frontal décollement). These parameters are longer and slower than typical coseismic slip, but are rather consistent with rapid afterslip.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2013AGUFMDI33A2215C"><span id="translatedtitle">Depth variation of upper mantle seismic discontinuities in the region of the Tonga <span class="hlt">subduction</span> <span class="hlt">zone</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Chen, J.; Wiens, D. A.; Emry, E.; Wei, S. S.; Cai, C.; Webb, S. C.; Menke, W. H.; Zha, Y.; Chen, Y. J.</p> <p>2013-12-01</p> <p>In order to study the mantle transition zone structure near the Tonga <span class="hlt">subduction</span> <span class="hlt">zone</span> in the southwestern Pacific, we analyzed receiver functions from teleseismic P waves recorded by both island broadband seismic stations and ocean bottom seismographs deployed as part of the RIDGE2000 Lau Basin Imaging Project. First, we used an iterative deconvolution in the time domain to obtain the receiver functions [Ammon, 1999]. The orientations of the OBS's were derived from a combination of the results of ambient noise correlation and Rayleigh-Wave polarization methods. Then a 3-D stacking approach [T. J. Owens, 2000] is adopted to stack those receiver functions for all station-event pairs. We binned the study area and stacked the traces within a certain radius from each bin at depths with a 10-km increment. The Tonga slab subducts with the fastest known convergence velocity, and the tectonics of this region are very complex. A flat slab with scattered seismicity is found to the west of the main Tonga slab in tomographic models. An elevation of the 410-km and a depression of the 660-km discontinuities are expected at and near the subducting slab since the olivine transitions are perturbed by the cold slab at these depths. From the on-going receiver function study, we are expecting to present detailed structure of the mantle transition zone at this complex plate boundary. Thomas J. Owens, Andrew A. Nyblade, Harold Gurrola, Charles A. Langston. Mantle transition zone structure beneath Tanzania, East Africa. Geophysical research letters, Vol. 27, No.6, Pages 827-830, 2000.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2009EGUGA..11.5251O"><span id="translatedtitle">The South Sandwich "Forgotten" <span class="hlt">Subduction</span> <span class="hlt">Zone</span> and Tsunami Hazard in the South Atlantic</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Okal, E. A.; Hartnady, C. J. H.; Synolakis, C. E.</p> <p>2009-04-01</p> <p>While no large interplate thrust earthquakes are know at the "forgotten" South Sandwich <span class="hlt">subduction</span> <span class="hlt">zone</span>, historical catalogues include a number of events with reported magnitudes 7 or more. A detailed seismological study of the largest event (27 June 1929; M (G&R) = 8.3) is presented. The earthquake relocates 80 km North of the Northwestern corner of the arc and its mechanism, inverted using the PDFM method, features normal faulting on a steeply dipping fault plane (phi, delta, lambda = 71, 70, 272 deg. respectively). The seismic moment of 1.7*10**28 dyn*cm supports Gutenberg and Richter's estimate, and is 28 times the largest shallow CMT in the region. This event is interpreted as representing a lateral tear in the South Atlantic plate, comparable to similar earthquakes in Samoa and Loyalty, deemed "STEP faults" by Gover and Wortel [2005]. Hydrodynamic simulations were performed using the MOST method [Titov and Synolakis, 1997]. Computed deep-water tsunami amplitudes of 30cm and 20cm were found off the coast of Brazil and along the Gulf of Guinea (Ivory Coast, Ghana) respectively. The 1929 moment was assigned to the geometries of other know earthquakes in the region, namely outer-rise normal faulting events at the center of the arc and its southern extremity, and an interplate thrust fault at the Southern corner, where the youngest lithosphere is subducted. Tsunami hydrodynamic simulation of these scenarios revealed strong focusing of tsunami wave energy by the SAR, the SWIOR and the Agulhas Rise, in Ghana, Southern Mozambique and certain parts of the coast of South Africa. This study documents the potential tsunami hazard to South Atlantic shorelines from earthquakes in this region, principally normal faulting events.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2008AGUFMOS51E..08O"><span id="translatedtitle">South Sandwich: The Forgotten <span class="hlt">Subduction</span> <span class="hlt">Zone</span> and Tsunami Hazard in the South Atlantic</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Okal, E. A.; Hartnady, C. J.</p> <p>2008-12-01</p> <p>While no large interplate thrust earthquakes are known at the South Sandwich <span class="hlt">subduction</span> <span class="hlt">zone</span>, historical catalogues include a number of earthquakes with reported magnitudes of 7 or more. We present a detailed seismological study of the largest one (27 June 1929; M (G&R) = 8.3). The earthquake relocates 80 km North of the Northwestern corner of the arc. Its mechanism, inverted using the PDFM method, features normal faulting on a steeply dipping fault plane (phi, delta, lambda = 71, 70, 272 deg.). The seismic moment, 1.7 10**28 dyn*cm, supports Gutenberg and Richter's estimate, and is 28 times the largest shallow CMT in the region. The 1929 event is interpreted as representing a lateral tear in the South Atlantic plate, comparable to similar earthquakes in Samoa and Loyalty, deemed "STEP faults" by Gover and Wortel [2005]. Hydrodynamic simulations using the MOST method [Titov and Synolakis, 1997] suggest deep-water tsunami amplitudes reaching 30 cm off the coast of Brazil, where it should have had observable run-up, and 20 cm along the Gulf of Guinea (Ivory Coast, Ghana). We also simulate a number of potential sources obtained by assigning the 1929 moment to the geometries of other known earthquakes in the region, namely outer-rise normal faulting events at the center of the arc and its southern extremity, and an interplate thrust fault at the Southern corner, where the youngest lithosphere is subducted. A common feature of these models is the strong focusing of tsunami waves by the SAR, the SWIOR, and the Agulhas Rise, resulting in amplitudes always enhanced in Ghana, Southern Mozambique and certain parts of the coast of South Africa. This study documents the potential tsunami hazard to South Atlantic shorelines from earthquakes in this region, principally normal faulting events.</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_16");'>16</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li class="active"><span>18</span></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_18 --> <div id="page_19" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li class="active"><span>19</span></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="361"> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=USGSPUBS&redirectUrl=http://pubs.er.usgs.gov/publication/70046869"><span id="translatedtitle">Coseismic and postseismic stress rotations due to great <span class="hlt">subduction</span> <span class="hlt">zone</span> earthquakes</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Hardebeck, Jeanne L.</p> <p>2012-01-01</p> <p>The three largest recent great <span class="hlt">subduction</span> <span class="hlt">zone</span> earthquakes (2011 M9.0 Tohoku, Japan; 2010 M8.8 Maule, Chile; and 2004 M9.2 Sumatra-Andaman) exhibit similar coseismic rotations of the principal stress axes. Prior to each mainshock, the maximum compressive stress axis was shallowly plunging, while immediately after the mainshock, both the maximum and minimum compressive stress axes plunge at ~45°. Dipping faults can be oriented for either reverse or normal faulting in this post-mainshock stress field, depending on their dip, explaining the observed normal-faulting aftershocks without requiring a complete reversal of the stress field. The significant stress rotations imply near-complete stress drop in the mainshocks, with >80% of the pre-mainshock stress relieved in the Tohoku and Maule earthquakes and in the northern part of the Sumatra-Andaman rupture. The southern part of the Sumatra-Andaman rupture relieved ~60% of the pre-mainshock stress. The stress axes rotated back rapidly in the months following the Tohoku and Maule mainshocks, and similarly in the southern part of the Sumatra-Andaman rupture. A rapid postseismic rotation is possible because the near-complete stress drop leaves very little “background” stress at the beginning of the postseismic reloading. In contrast, there has been little or no postseismic rotation in the northern part of the Sumatra-Andaman rupture over the 7 years since the mainshock. All M ?8.0 subduction earthquakes since 1990 with an adequate number of pre- and post-mainshock events were evaluated, and not all show similar coseismic stress rotations. Deeper earthquakes exhibit smaller coseismic stress rotations, likely due to increasing deviatoric stress with depth.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/1995Tecto..14..933J"><span id="translatedtitle">Structural and <span class="hlt">metamorphic</span> evolution of the Orocopia Schist and related rocks, southern California: Evidence for late movement on the Orocopia fault</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Jacobson, Carl E.; Dawson, M. Robert</p> <p>1995-08-01</p> <p>The Pelona, Orocopia, and Rand Schists (POR schists) of southern California and southwesternmost Arizona are late Mesozoic or early Tertiary subduction complexes that underlie Precambrian to Mesozoic <span class="hlt">continental</span> basement along the low-angle Vincent-Chocolate Mountains (VCM) fault system. The VCM faults are often considered to be remnants of the original <span class="hlt">subduction</span> <span class="hlt">zone</span>, but recent work indicates that many have undergone substantial postsubduction reactivation. In the Orocopia Mountains, for example, the Orocopia Schist exhibits an exceptionally complex structural and <span class="hlt">metamorphic</span> history due to multiple periods of movement along the Orocopia fault. Structures in the schist include isoclinal folds with axial-planar schistosity, open-to-tight folds that fold schistosity, penetrative stretching lineations, and crenulation lineations, all of which show a nearly 360° range in trend. Folds and lineations that trend approximately NE-SW occur throughout the schist and are thought to be part of an early phase of deformation related to subduction. Folds of this orientation show no consistent vergence. Folds and lineations that trend approximately NW-SE are concentrated near the Orocopia fault and are interpreted to have formed during exhumation of the schist. The NW-SE trending folds, and shear indicators in late-stage mylonite at the top of the schist, consistently verge NE. The exhumation event culminated in emplacement of the schist against brittlely deformed upper plate. Exhumation of the Orocopia Schist was accompanied by retrograde replacement of garnet, biotite, epidote, and calcic amphibole by chlorite, calcite, and sericite. Matrix amphibole has a lower Na/Al ratio than amphibole inclusions in albite, consistent with a late-stage decrease in pressure. As NE vergence in the Orocopia Mountains is associated with exhumation of the schist, the NE movement along other segments of the VCM fault may also be late and therefore have no bearing on the facing direction of the VCM <span class="hlt">subduction</span> <span class="hlt">zone</span>, contrary to past interpretations.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2007PhDT........59L"><span id="translatedtitle">An investigation of deformation and fluid flow at <span class="hlt">subduction</span> <span class="hlt">zones</span> using newly developed instrumentation and finite element modeling</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Labonte, Alison Louise</p> <p></p> <p>Detecting seafloor deformation events in the offshore convergent margin environment is of particular importance considering the significant seismic hazard at <span class="hlt">subduction</span> <span class="hlt">zones</span>. Efforts to gain insight into the earthquake cycle have been made at the Cascadia and Costa Rica subduction margins through recent expansions of onshore GPS and seismic networks. While these studies have given scientists the ability to quantify and locate slip events in the seismogenic zone, there is little technology available for adequately measuring offshore aseismic slip. This dissertation introduces an improved flow meter for detecting seismic and aseismic deformation in submarine environments. The value of such hydrologic measurements for quantifying the geodetics at offshore margins is verified through a finite element modeling (FEM) study in which the character of deformation in the shallow <span class="hlt">subduction</span> <span class="hlt">zone</span> is determined from previously recorded hydrologic events at the Costa Rica Pacific margin. Accurately sensing aseismic events is one key to determining the stress state in <span class="hlt">subduction</span> <span class="hlt">zones</span> as these slow-slip events act to load or unload the seismogenic zone during the interseismic period. One method for detecting seismic and aseismic strain events is to monitor the hydrogeologic response to strain events using fluid flow meters. Previous instrumentation, the Chemical Aqueous Transport (CAT) meter which measures flow rates through the sediment-water interface, can detect transient events at very low flowrates, down to 0.0001 m/yr. The CAT meter performs well in low flow rate environments and can capture gradual changes in flow rate, as might be expected during ultra slow slip events. However, it cannot accurately quantify high flow rates through fractures and conduits, nor does it have the temporal resolution and accuracy required for detecting transient flow events associated with rapid deformation. The Optical Tracer Injection System (OTIS) developed for this purpose is an electronic flow meter that can measure flow rates of 0.1 to >500 m/yr at a temporal resolution of 30 minutes to 0.5 minutes, respectively. Test deployments of the OTIS at cold seeps in the transpressional Monterey Bay demonstrated the OTIS functionality over this range of flow environments. Although no deformation events were detected during these test deployments, the OTIS's temporally accurate measurements at the vigorously flowing Monterey Bay cold seep rendered valuable insight into the plumbing of the seep system. In addition to the capability to detect transient flow events, a primary functional requirement of the OTIS was the ability to communicate and transfer data for long-term real-time monitoring deployments. Real-time data transfer from the OTIS to the desktop was successful during a test deployment of the Nootka Observatory, an acoustically-linked moored-buoy system. A small array of CAT meters was also deployed at the Nootka transform-Cascadia <span class="hlt">subduction</span> <span class="hlt">zone</span> triple junction. Four anomalous flow rate events were observed across all four meters during the yearlong deployment. Although the records have low temporal accuracy, a preliminary explanation for the regional changes in flow rate is made through comparison between flow rate records and seismic records. The flow events are thought to be a result of a tectonic deformation event, possibly with an aseismic component. Further constraints are not feasible given the unknown structure of faulting near the triple junction. In a final proof of concept study, I find that use these hydrologic instruments, which capture unique aseismic flow rate patterns, is a valuable method for extracting information about deformation events on the decollement in the offshore <span class="hlt">subduction</span> <span class="hlt">zone</span> margin. Transient flow events observed in the frontal prism during a 1999--2000 deployment of CAT meters on the Costa Rica Pacific margin suggest episodic slow-slip deformation events may be occurring in the shallow <span class="hlt">subduction</span> <span class="hlt">zone</span>. The FEM study to infer the character of the hypothetical deformation event driving flow transients verify</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=SCIGOV-MAS&redirectUrl=http://academic.research.microsoft.com/Publication/48932167"><span id="translatedtitle">Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to <span class="hlt">metamorphic</span> dehydration reactions?</span></a></p> <p><a target="_blank" href="http://academic.research.microsoft.com/">Microsoft Academic Search </a></p> <p>Bradley R. Hacker; Simon M. Peacock; Geoffrey A. Abers; Stephen D. Holloway</p> <p>2003-01-01</p> <p>New thermal-petrologic models of <span class="hlt">subduction</span> <span class="hlt">zones</span> are used to test the hypothesis that intermediate-depth intraslab earthquakes are linked to <span class="hlt">metamorphic</span> dehydration reactions in the subducting oceanic crust and mantle. We show that there is a correlation between the patterns of intermediate-depth seismicity and the locations of predicted hydrous minerals: Earthquakes occur in subducting slabs where dehydration is expected, and they</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=SCIGOV-MAS&redirectUrl=http://academic.research.microsoft.com/Publication/51758220"><span id="translatedtitle">Carbon Cycle in the Archean Plate Tectonics: Hydrothermal Carbonation, <span class="hlt">Metamorphic</span> Decarbonation and Storage in the Mantle</span></a></p> <p><a target="_blank" href="http://academic.research.microsoft.com/">Microsoft Academic Search </a></p> <p>S. Omori; K. Kitajima</p> <p>2005-01-01</p> <p>Geological studies in the Archean greenstone belt have shown that hydrothermal alteration at mid-oceanic ridge (MOR) had caused extensive carbonation in the oceanic crust. Hence, the MOR hydrothermal carbonation and subsequent <span class="hlt">subduction-zone</span> <span class="hlt">metamorphism</span> had an important role in the Archean carbon cycle. The aim of this study is phase-petrological characterization of carbonation-decarbonation processes in the Archean ocean floor and subduction</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2013Tectp.608....1W"><span id="translatedtitle">Rheology of the plate interface — Dissolution precipitation creep in high pressure <span class="hlt">metamorphic</span> rocks</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Wassmann, Sara; Stöckhert, Bernhard</p> <p>2013-11-01</p> <p><span class="hlt">Subduction</span> <span class="hlt">zone</span> models invoke deformation to be concentrated along the plate interface, in a region of particularly low temperature. Geophysical observations do not provide constraints on temperature, stress and deformation patterns with desired resolution. In contrast, the record of high pressure <span class="hlt">metamorphic</span> rocks exhumed from <span class="hlt">subduction</span> <span class="hlt">zones</span> provides details on P-T-history, deformation mechanisms, and stress state, albeit not readily correlated with the former dynamic situation on larger scale. Here we review available information on dissolution precipitation creep (DPC) in high pressure <span class="hlt">metamorphic</span> rocks, which - if representative for <span class="hlt">subduction</span> <span class="hlt">zones</span> in general - can pose constraints on conditions, rheology, and flow patterns along the plate interface. The key observations and conclusions are that: (1) Deformation is typically highly inhomogeneous and localized into shear zones; (2) stresses are generally too low to drive crystal plastic deformation; (3) microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism; (4) an aqueous fluid at quasi-lithostatic pressure is available throughout, allowing for tensile fracturing and crack healing or sealing; (5) low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity; and (6) contribution of shear heating to the thermal budget of <span class="hlt">subduction</span> <span class="hlt">zones</span> should be moderate. The dominant deformation mechanism DPC is reviewed in some detail, including experimental and theoretical approaches. Various examples of DPC in high pressure <span class="hlt">metamorphic</span> rocks are illustrated, emphasizing the role of interphase boundaries as sites of dissolution. Rheology governed by DPC is proposed to control interplate coupling and development of a subduction channel with return flow, being a likely candidate for rapid exhumation of high pressure <span class="hlt">metamorphic</span> rocks.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=PMC&redirectUrl=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=3503209"><span id="translatedtitle">Separation of supercritical slab-fluids to form aqueous fluid and melt components in <span class="hlt">subduction</span> <span class="hlt">zone</span> magmatism</span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pmc">PubMed Central</a></p> <p>Kawamoto, Tatsuhiko; Kanzaki, Masami; Mibe, Kenji; Ono, Shigeaki</p> <p>2012-01-01</p> <p><span class="hlt">Subduction-zone</span> magmatism is triggered by the addition of H2O-rich slab-derived components: aqueous fluid, hydrous partial melts, or supercritical fluids from the subducting slab. Geochemical analyses of island arc basalts suggest two slab-derived signatures of a melt and a fluid. These two liquids unite to a supercritical fluid under pressure and temperature conditions beyond a critical endpoint. We ascertain critical endpoints between aqueous fluids and sediment or high-Mg andesite (HMA) melts located, respectively, at 83-km and 92-km depths by using an in situ observation technique. These depths are within the mantle wedge underlying volcanic fronts, which are formed 90 to 200 km above subducting slabs. These data suggest that sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, react with mantle peridotite to form HMA supercritical fluids. Such HMA supercritical fluids separate into aqueous fluids and HMA melts at 92 km depth during ascent. The aqueous fluids are fluxed into the asthenospheric mantle to form arc basalts, which are locally associated with HMAs in hot <span class="hlt">subduction</span> <span class="hlt">zones</span>. The separated HMA melts retain their composition in limited equilibrium with the surrounding mantle. Alternatively, they equilibrate with the surrounding mantle and change the major element chemistry to basaltic composition. However, trace element signatures of sediment-derived supercritical fluids remain more in the melt-derived magma than in the fluid-induced magma, which inherits only fluid-mobile elements from the sediment-derived supercritical fluids. Separation of slab-derived supercritical fluids into melts and aqueous fluids can elucidate the two slab-derived components observed in <span class="hlt">subduction</span> <span class="hlt">zone</span> magma chemistry. PMID:23112158</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=PUBMED&redirectUrl=http://www.ncbi.nlm.nih.gov/pubmed/23112158"><span id="translatedtitle">Separation of supercritical slab-fluids to form aqueous fluid and melt components in <span class="hlt">subduction</span> <span class="hlt">zone</span> magmatism.</span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Kawamoto, Tatsuhiko; Kanzaki, Masami; Mibe, Kenji; Matsukage, Kyoko N; Ono, Shigeaki</p> <p>2012-11-13</p> <p><span class="hlt">Subduction-zone</span> magmatism is triggered by the addition of H(2)O-rich slab-derived components: aqueous fluid, hydrous partial melts, or supercritical fluids from the subducting slab. Geochemical analyses of island arc basalts suggest two slab-derived signatures of a melt and a fluid. These two liquids unite to a supercritical fluid under pressure and temperature conditions beyond a critical endpoint. We ascertain critical endpoints between aqueous fluids and sediment or high-Mg andesite (HMA) melts located, respectively, at 83-km and 92-km depths by using an in situ observation technique. These depths are within the mantle wedge underlying volcanic fronts, which are formed 90 to 200 km above subducting slabs. These data suggest that sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, react with mantle peridotite to form HMA supercritical fluids. Such HMA supercritical fluids separate into aqueous fluids and HMA melts at 92 km depth during ascent. The aqueous fluids are fluxed into the asthenospheric mantle to form arc basalts, which are locally associated with HMAs in hot <span class="hlt">subduction</span> <span class="hlt">zones</span>. The separated HMA melts retain their composition in limited equilibrium with the surrounding mantle. Alternatively, they equilibrate with the surrounding mantle and change the major element chemistry to basaltic composition. However, trace element signatures of sediment-derived supercritical fluids remain more in the melt-derived magma than in the fluid-induced magma, which inherits only fluid-mobile elements from the sediment-derived supercritical fluids. Separation of slab-derived supercritical fluids into melts and aqueous fluids can elucidate the two slab-derived components observed in <span class="hlt">subduction</span> <span class="hlt">zone</span> magma chemistry. PMID:23112158</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2012AGUFMNH11B1556W"><span id="translatedtitle">Estimated damage from the Cascadia <span class="hlt">Subduction</span> <span class="hlt">Zone</span> tsunami: A model comparisons using fragility curves</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Wiebe, D. M.; Cox, D. T.; Chen, Y.; Weber, B. A.; Chen, Y.</p> <p>2012-12-01</p> <p>Building damage from a hypothetical Cascadia <span class="hlt">Subduction</span> <span class="hlt">Zone</span> tsunami was estimated using two methods and applied at the community scale. The first method applies proposed guidelines for a new ASCE 7 standard to calculate the flow depth, flow velocity, and momentum flux from a known runup limit and estimate of the total tsunami energy at the shoreline. This procedure is based on a potential energy budget, uses the energy grade line, and accounts for frictional losses. The second method utilized numerical model results from previous studies to determine maximum flow depth, velocity, and momentum flux throughout the inundation zone. The towns of Seaside and Canon Beach, Oregon, were selected for analysis due to the availability of existing data from previously published works. Fragility curves, based on the hydrodynamic features of the tsunami flow (inundation depth, flow velocity, and momentum flux) and proposed design standards from ASCE 7 were used to estimate the probability of damage to structures located within the inundations zone. The analysis proceeded at the parcel level, using tax-lot data to identify construction type (wood, steel, and reinforced-concrete) and age, which was used as a performance measure when applying the fragility curves and design standards. The overall probability of damage to civil buildings was integrated for comparison between the two methods, and also analyzed spatially for damage patterns, which could be controlled by local bathymetric features. The two methods were compared to assess the sensitivity of the results to the uncertainty in the input hydrodynamic conditions and fragility curves, and the potential advantages of each method discussed. On-going work includes coupling the results of building damage and vulnerability to an economic input output model. This model assesses trade between business sectors located inside and outside the induction zone, and is used to measure the impact to the regional economy. Results highlight critical businesses sectors and infrastructure critical to the economic recovery effort, which could be retrofitted or relocated to survive the event. The results of this study improve community understanding of the tsunami hazard for civil buildings.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2004GGG.....5.5J11Z"><span id="translatedtitle">Nitrogen systematics and gas fluxes of <span class="hlt">subduction</span> <span class="hlt">zones</span>: Insights from Costa Rica arc volatiles</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Zimmer, Mindy M.; Fischer, Tobias P.; Hilton, David R.; Alvarado, Guillermo E.; Sharp, Zachary D.; Walker, James A.</p> <p>2004-05-01</p> <p>Volcanic gases are a powerful tool for assessing magmatic processes in <span class="hlt">subduction</span> <span class="hlt">zones</span>. We report gas chemistry and nitrogen isotope compositions of fumaroles, bubbling springs, and geothermal wells from the Costa Rican segment of the Central American volcanic segment (CAVS), and new correlation spectroscopy (COSPEC) SO2 flux measurements of Poás and Arenal volcanoes. N2/He ratios (100-8,250) and nitrogen isotope compositions (?15N = -3.02 to +1.69‰) of Costa Rica volatiles are consistent with sources ranging from typical arc-type end-members, with nitrogen addition from the subducting slab, to MORB end-member, having experienced no slab modification. Overall, nitrogen-helium chemistry of Costa Rican material indicates a diminished slab contribution versus other locations along the arc (e.g., Nicaragua and Guatemala). We use SO2 flux measurements of Poás and Arenal (1.80*105 ± 4.00*104 and 8.30*103 ± 4.00*103 kg/day, respectively, or 1.30*105 ± 6.25*104 and 2.81*106 ± 6.25*105 mol/day, respectively) to extrapolate a SO2 flux for the Costa Rica segment of 1.09*109 mol/day. Using CO2/St (St = total sulfur) of 2.7 and 5.9, we calculate CO2 fluxes of 1.88*108 and 4.11*108 kg/yr, respectively (2.94*109 and 6.42*109 mol/yr, respectively). Other volatile fluxes (N2, He, H2, Ar, HCl, and H2O) are calculated using CO2/St and regional gas chemistry. For Costa Rica, the output/input ratios of nitrogen are less than unity (0.03 to 0.06 for CO2/St of 2.7 and 5.9, respectively), suggesting more N is subducted than released in the subarc, possibly resulting from sediment offscraping, forearc devolatilization, limited fluid availability in the subarc, or subduction past the subarc.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2013AGUFM.T34C..03K"><span id="translatedtitle">He isotope ratios in the Nankai Trough and Costa Rica <span class="hlt">subduction</span> <span class="hlt">zones</span> - implications for volatile cycling</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Kastner, M.; Hilton, D. R.; Jenkins, W. J.; Solomon, E. A.; Spivack, A. J.</p> <p>2013-12-01</p> <p>The noble gas 3He is a clear indicator of primordial volatile flux from the mantle, thus providing important insights on the interaction between Earth's interior and exterior reservoirs. Volatile cycling at ridge-crests and its impact on the evolution of seawater chemistry is rather well known as constrained by the 3He flux, whereas the impact of volatile cycling at <span class="hlt">subduction</span> <span class="hlt">zones</span> (SZs) on seawater chemistry is as yet poorly known. Constraining chemical and isotopic cycling at SZs is important for understanding the evolution of the mantle-crust and ocean-atmosphere systems. To gain insights on volatile cycling in SZs, pore fluids were sampled for He concentration and isotopic analyses at two tectonically contrasting SZs, Nankai Trough (offshore Japan, Muroto and Kumano transects), an accretionary SZ, and Costa Rica (Offshore Osa Peninsula), an erosional SZ. Sampling for He was achieved by rapidly subsampling core sediments, cleaning and transferring these samples into Ti squeezers in a glove bag, and storing the squeezed pore fluids in crimped Cu tubes for shore-based He concentration and isotope ratio analyses. At the Nankai Trough SZ there is a remarkable range of He isotopic values. The 3He/4He ratios relative to atmospheric ratio (RA) range from mostly crustal 0.47 RA to 4.30 RA which is ~55% of the MORB value of 8 RA. Whereas at the Costa Rica SZ, offshore Osa Peninsula, the ratios range from 0.86 to 1.14 RA, indicating the dominance of crustal radiogenic 4He that is from U and Th decay. The distribution of the He isotope values at Nankai Trough is most interesting, fluids that contain significant mantle 3He components (3He/4He >1) were sampled along and adjacent to fluid conduits that were identified by several chemical and isotopic data (i.e. Cl, B, and Li), including the presence of thermogenic hydrocarbons. Whereas the fluids dominated by 4He (3He/4He ?1) were obtained from sediment sections that were between the fluid conduits. At Costa Rica, however, even along conduits, the fluids were not greatly enriched in 3He, hence there is no evidence for fluid advection from the subducting Cocos Ridge and numerous seamounts into the sediments, suggesting greatly diminished hydrothermal activity. Focused flow along faults, the décollement, splay and out of sequence faults, and fractured and permeable horizons at SZs play a key role in controlling fluid and heat transport, including mantle He, whereas diffuse flow plays a minor role; mud volcanoes and seeps as well play some role in volatile cycling.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2008AGUFM.U52A..02F"><span id="translatedtitle">Along-arc Variations in Volatile Cycling in the Nicaragua-Costa Rica <span class="hlt">Subduction</span> <span class="hlt">Zone</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Fischer, K. M.; Rychert, C. A.; Syracuse, E. M.; Abers, G. A.; Plank, T.</p> <p>2008-12-01</p> <p>Recent seismic imaging in the Nicaragua-Costa Rica <span class="hlt">subduction</span> <span class="hlt">zone</span>, in combination with geochemical data, has revealed systematic along-arc variations in volatile cycling from the subducting slab, through the mantle wedge, to the melts generated at arc volcanoes. Velocity and attenuation tomography based on P and S phases from local events recorded by the 2004-2006 TUCAN Broadband Seismic Experiment resolve a high velocity, low attenuation subducting slab, a shallow wedge corner with intermediate attenuation, and a slower, more highly attenuating mantle wedge beneath the arc and back-arc. However, velocity and attenuation structures also manifest strong along-arc variations. The subducting slab beneath Nicaragua contains a shallow layer of slow velocities and is more attenuating than the slab beneath Costa Rica, consistent with greater slab hydration (for example, 10-20% mantle serpentinization). Continuing the along-arc trend, attenuation in the Nicaraguan mantle wedge at depths of 60-100 km is significantly higher than in Costa Rica, suggesting that enhanced slab hydration beneath Nicaragua results in a more hydrated wedge. Moving to the arc, olivine melt inclusions indicate that Nicaraguan magmas are more water-rich on average than those erupted in the Costa Rican arc within the region best-imaged by the TUCAN array. Assuming that these wet melts equilibrated with olivine in the mantle wedge, then the inferred decrease in wedge hydration from Nicaragua to Costa Rica can explain the along-arc variation in wedge attenuation. In addition, although bulk magma compositions in the vicinity of the Nicaragua and Costa Rica attenuation profiles are significantly different, they imply a similar temperature of 1265 ± 25°C at a depth comparable to the middle of the wedge (75 km) when primary melt water content is taken into account in calculating olivine-liquid temperatures. A roughly vertical column of high Vp/Vs extends from the slab surface to the arc in Nicaragua, but this feature dies away along the arc to Costa Rica. This anomaly could reflect larger melt fractions in the Nicaraguan wedge, possibly produced by the apparently greater wedge hydration.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2012AGUFM.T22A..08C"><span id="translatedtitle">Outer forearc structure and fault property variations along and across the Sumatran <span class="hlt">Subduction</span> <span class="hlt">Zone</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Cook, B. J.; Henstock, T.; McNeill, L. C.; Bull, J.</p> <p>2012-12-01</p> <p>The Sumatran <span class="hlt">Subduction</span> <span class="hlt">Zone</span> exhibits along and across strike variation in forearc morphology, fault structure and reflectivity. Offshore North Sumatra, north of Simeulue Island, the outer forearc is characterised by a relatively narrow and steep outermost slope which transitions to a broad relatively flat plateau farther arcward; landward-verging (seaward-dipping) prism fault structures are relatively common at the prism toe and a strong negative polarity reflector, possibly acting as a décollement surface is present. Offshore Central Sumatra, near Nias Island, the outermost slope widens and shallows; landward-vergent faults are apparently less common than further north and the oceanic basement appears to act as the décollement surface. We present a detailed interpretation of multi-channel streamer (MCS) seismic reflection data collected offshore North-Central Sumatra to better characterise structural variation across and along the forearc and provide insight into the development of fault structures. MCS data were collected in 2008 following the 2004-2005 Sumatra earthquake sequence using a 5420 cu. in. gun array and recorded with a 192-channel 2.4 km long streamer. The data were pre-stack processed to optimally minimise strong swell and multiple noise before pre-stack time-migration. Interpretation was conducted incorporating observations from unmigrated and migrated, pre- and post-stack data. Just south of Nias Island, the outermost wedge has a steep slope of 5.5 deg and is very rugose reflecting prism thrust folding; landward the slope decreases to 3.0 deg and is much smoother due to increased draping by forearc high-derived sediment and presumed reduced activity of prism thrusts. The change in slope is coincident with a major, possibly out-of-sequence, fault structure and the locally steepest slope (23 deg) on the seaward limb of its fold. This transition may mark the inner-outer wedge boundary and/or a backstop within the forearc. This major structure is associated with a negative polarity fault plane reflection that appears to switch polarity landward. Fault planes imaged near the major structure and at the deformation front are also negative polarity; other fault planes are either non-reflective or not imaged well. Other profiles will be incorporated in order to map the subsurface variation in forearc structure and fault plane properties in detail and to compare with variations in forearc morphology and earthquake rupture behaviour.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2012EGUGA..1410487M"><span id="translatedtitle">Hydrodynamics of tsunamis in <span class="hlt">subduction</span> <span class="hlt">zones</span>. The differences between the Chile 2010 and Japan 2011 tsunamis</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Monardez, P.; Salinas, R.; Comte, D.</p> <p>2012-04-01</p> <p>Tsunamis due to large earthquakes in <span class="hlt">subduction</span> <span class="hlt">zones</span> have different hydrodynamic behaviors, depending on the location, the bathymetry and the geometry of the rupture associated to the large earthquake. When the width of the rupture (related to the length of the tsunami) is larger than its distance to the shore, the hydrodynamics in the near zone is completely different than the alternate case. In the first case, the earthquake triggers a tsunami composed by one or a group of a few waves with a few minutes in between propagating from the rupture, which reach the coast a few minutes after the earthquake. In the second case, the earthquake triggers a deformation in the water surface which cannot create a complete tsunami wave; there is not enough distance to complete it. Then, a succession of secondary effects are triggered, which are composed by several floods, up to seven or eight, separated several minutes (up to forty or more) and propagate parallel to the coast, which can be even perpendicular to the coast. This case is still poorly understood, even it has been observed and described in the literature over the past three centuries. The difference in hydrodynamic behavior was evidenced in the tsunamis of February of 2010 in Chile and March of 2011 in Japan. In this work we show a theory, which has been validated by field observations and numerical simulations based only on the hydrodynamics of the area, that explains the phenomena and it has been extended to other historical tsunamis in Chile. The effects of the Chile 2010 tsunami in the near field zone were complex. The small township of Cobquecura, located at 20 km from the epicenter, did not suffer major damage from the tsunami. The major port zone of Talcahuano at 100 km from the epicenter, received four destructive waves every forty minutes approximately, and lasted three hours after the occurrence of the earthquake, while the bay of San Vicente, adjacent to the above, only suffered a minor, but abrupt, rise in the sea level about 20 minutes after the end of the earthquake. Flux in general was reported to be parallel to the coast, from the north. In the case of Japan 2012 tsunami, the first wave arrived to shore from 1 to 50 min after the earthquake, depending on the distance to the rupture. This first wave was in the order of a few centimeters. The maximum wave arrived from 30 minutes to two hours after the earthquake, with high waves larger than 3 m, with flux perpendicular/diagonal to the coast.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2013EGUGA..1512818K"><span id="translatedtitle">Superconducting Gravity Effects of Earthquake at Cascadia <span class="hlt">Subduction</span> <span class="hlt">Zone</span> on Vancouver Island, Canada</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Kim, Jeong Woo; Neumeyer, Juergen; Kao, Ricky; Kabirzadeh, Hojjat; Henton, Joseph; Dragert, Herb; Lambert, Anthony</p> <p>2013-04-01</p> <p>Superconducting gravimeter (SG) iGrav #01 was deployed at NRCan's Pacific Geoscience Centre (PGC) on Vancouver Island near Sidney in British Columbia, Canada, in July 2012. The PGC is situated in the forearc of the northern Cascadia <span class="hlt">Subduction</span> <span class="hlt">Zone</span> (CSZ) and is equipped with FG-5 and A-10 absolute gravimeters, a borehole strainmeter, and a GPS network. In this area, a transient surface deformation accompanied by tremor-like seismic signals has been documented with a recurrence interval of 13 to 16 months. This phenomenon, named Episodic Tremor and Slip (ETS), has been interpreted to be associated with slow slip events (silent earthquakes) in the deeper (25-45 km) part of the CSZ. These slip events have been detected by transient horizontal displacements. The SG is not sensitive to horizontal displacements but it has the largest sensitivity in vertical direction. For detecting of ETS, the continuous SG recordings at the PGC site were reduced for the Earth and ocean tides, polar motion, atmospheric pressure and soil moisture, and, then were band-pass filtered and analyzed in the time and frequency domains and compared with the GPS-detected ETS. Furthermore, we present the gravity effect of the Haida Gwaii earthquake, which occurred near the plate boundary between the Pacific and North America plates (52.788N, 132.101W, 136 km south of Masset, Canada, on October 28th 2012 at 03:04:09 GMT with a magnitude 7.8 at a depth of 14 km). During the observation, a large co-seismic gravity change of -2.6 microGal was recorded at the onset of the Haida Gwaii earthquake. In addition, a significant decrease of gravity was observed from the 15 days prior to the earthquake, and the decrease lasted for 11 days after the earthquake. The analysis of other earthquakes, e.g. the southwestern Alaska earthquake (55.28N, 134.87W, January 5th 2013 with a magnitude 7.5) is also presented.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2013AGUFM.S21C2428M"><span id="translatedtitle">Intertidal land-level changes during the most recent megathrust earthquake at the Cascadia <span class="hlt">subduction</span> <span class="hlt">zone</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Milker, Y.; Horton, B.; Engelhart, S. E.; Nelson, A. R.; Witter, R. C.</p> <p>2013-12-01</p> <p>Estuarine marshes along the US-Pacific coast host unique stratigraphic sequences that record coseismic land-level changes and tsunamis during the most recent magnitude 8-9 earthquakes at the Cascadia <span class="hlt">subduction</span> <span class="hlt">zone</span>. Earlier studies have shown the great potential of microfossil reconstructions of land-level changes during past earthquakes, but these estimates are available only at a few sites and the precision of many is quite low with errors of >×0.3 m. To make more accurate reconstructions of earthquake-induced land-level changes along the Oregon coast of Cascadia, we sampled a surface transect (18 stations) in the Sough Slough estuary in order to identify live and dead benthic foraminiferal assemblages and their relations to their environment. We collected 0.5-m-long Russian cores at seven selected stations in the Sough Slough estuary in order to study the influence of infaunal living species on the reliability and precision of relative sea-level estimates. We also collected 3-4-m-long vibracores in South Slough and a second estuary 12 km to the south to examine changes in late Holocene fossil foraminiferal assemblages resulting from land-level changes and sea-level rise. New local and regional transfer functions using modern benthic foraminiferal assemblages will be developed and their performance will be evaluated before applying them to the fossil assemblages. The modern foraminiferal assemblages in the South Slough estuary show a strong vertical distribution with higher numbers of Miliammina fusca restricted to the mud flat and low marsh stations, and higher numbers of Jadammina macresences, Balticammina pseudomacrescens, Trochammina inflata and Haplophragmoides manilaensis in the middle marsh stations. Higher numbers of Trochamminita irregularis and Trochamminita sp. were observed in the high marsh and upland stations. Our initial results of the infaunal distribution study shows that the majority of living foraminifera (51 - 118 specimen per 10 cm3 sediment volume) are restricted to 0-2 cm depths, but lower numbers with 1 to 3 living specimen per 10 cm3 sediment volume have been found down to 50 cm depth. A preliminary local transfer function, based on the modern assemblages in the study area, shows a good potential predictive accuracy of × 0.25 m for earthquake-induced land-level estimates. A first application of this transfer function to the fossil assemblages indicates coseismic coastal subsidence of 0.42 m × 0.30 m during the most recent megathrust earthquake (AD 1700) in South Slough.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2014EGUGA..16.9105M"><span id="translatedtitle">Earthquake precise locations catalog for the Lesser Antilles <span class="hlt">subduction</span> <span class="hlt">zone</span> (1972-2013)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Massin, Frederick; Amorese, Daniel; Beauducel, Francois; Bengoubou-Valérius, Mendy; Bernard, Marie-Lise; Bertil, Didier</p> <p>2014-05-01</p> <p>Locations for earthquake recorded in the Lesser Antilles <span class="hlt">subduction</span> <span class="hlt">zone</span> are processed separately by regional observatories, NEIC and ISC. There is no earthquake location catalog available compiling all available phase arrival data. We propose a new best complete earthquake catalog by merging all available phase arrival data for better constrains on earthquake locations. ISC provides the phase arrival data of 29243 earthquakes (magnitude range from 1.4 to 6.4) recorded by PRSN (Porto Rico), SRC (British West Indies), and from FUNVISIS (Venezuela). We add phases data from IPGP observatories for 68718 earthquakes from magnitudes 0.1 to 7.5 (OVSG, Guadeloupe, recorded 53226 earthquakes since 1981, and OVSM, Martinique, recorded 29931 earthquakes since 1972). IPGP also provides the accelerometer waveform data of the GIS-RAP network. We achieved automatic picking on the GIS-RAP data using the Component Energy Correlation Method. The CECM provides high precision phase detection, a realistic estimation of picking error and realistic weights that can be used with manual pick weights. The CECM add an average of 3 P-waves and 2 S-waves arrivals to 3846 earthquakes recorded by the GIS-RAP network since 2002. The final catalog contains 84979 earthquakes between 1972 and 2013, 24528 of which we compiled additional data. We achieve earthquake location using NonLinLoc, regional P and S waves data and a set of one dimensional velocity models. We produce improved locations for 22974 earthquakes (better residuals, on equal or larger arrival dataset) and improved duration magnitudes for 6258 earthquakes (using duration data and improved locations). A subset of best constrained 15626 hypocenters (with more than 8 phases and an average RMS of 0.48±0.77s) could be used for structural analysis and earthquake local tomography. Relative locations are to be applied in order to image active faulting. We aim to understand coupling in the seismogenic zone as well as triggering mechanisms of intermediate depth seismicity like fluid migration beneath the volcanic arc.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2011AGUFM.U51D..06A"><span id="translatedtitle">The Fate of Eclogitized Oceanic Crust During Subduction: Implications for <span class="hlt">Subduction</span> <span class="hlt">Zone</span> Dynamics</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Angiboust, S.; Agard, P.; Langdon, R.; Waters, D.; Raimbourg, H.; Yamato, P.; Chopin, C.</p> <p>2011-12-01</p> <p>The Monviso ophiolite is composed of two main tectonic slices: the Monviso Unit (MU) to the west, which overlies the Lago Superiore Unit (LSU). Our PT estimates show that the MU has been subducted down to 480°C-23kbar (~70km) during Alpine subduction while the LSU reached slightly deeper conditions (540°C-26kbar, i.e. ~80km). This ophiolite, which comprises large (10-20km long) ophiolite fragments therefore does not correspond to a serpentinite mélange, and may be the southern extension of the Zermatt-Saas ophiolite (Angiboust et al., 2009; 2011). The well-preserved LSU constitutes an almost continuous upper fragment of oceanic lithosphere subducted between 50 and 40 Ma and later exhumed along the subduction interface. It therefore provides a unique opportunity to study strain partitioning and deep mechanical behaviour of the subducting lithosphere. The LSU comprises (i) several hundred meters of eclogitized basaltic crust (+ minor calcschist lenses) overlying a 100-400m thick metagabbroic body and (ii) a serpentinite sole (c. 1km thick). We herein focus on eclogite-facies shear zones, which are found at the boundary between basalts and gabbros, and between gabbros and serpentinites, i.e. between material with marked rheological contrasts. Eclogite facies blocks within the shear zones display intense fracturation, fragment rotation and dispersion in the serpentinite schists which line up the shear zones. We also report the first finding of eclogite-facies breccias, constituted of rotated eclogite mylonitic clasts cemented within an eclogite-facies matrix. Local fracturation of garnet within these breccias is attested by the presence of numerous fracture networks within garnet, generally healed by a Mg-enriched composition. The shear zones also preserve clear evidence of pervasive and channelized fluid flow (of variable duration) leading to alteration of bulk rock composition, weakening of the rock and widespread crystallization of lawsonite. Our results provide new constraints for deep mechanical coupling processes and meter-scale fluid-rock interaction occurring at depth in present-day <span class="hlt">subduction</span> <span class="hlt">zones</span>. In particular, we emphasize that (i) rheological contrasts in the field qualitatively support those inferred from experimental flow laws, (ii) fluid flow is channelized along deep eclogite facies shear zones (iii) brittle fracturing recorded by eclogite breccias and garnet fractures could be associated with deep interplate seismicity.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2007AGUFMDI51A0285R"><span id="translatedtitle">Nanoscale Properties of Rocks and <span class="hlt">Subduction</span> <span class="hlt">Zone</span> Rheology: Inferences for the Mechanisms of Deep Earthquakes</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Riedel, M. R.</p> <p>2007-12-01</p> <p>Grain boundaries are the key for the understanding of mineral reaction kinetics. More generally, nanometer scale processes involved in breaking and establishing bonds at reaction sites determine how and at which rate bulk rock properties change in response to external tectonic forcing and possibly feed back into various geodynamic processes. A particular problem is the effects of grain-boundary energy on the kinetics of the olivine-spinel phase transformation in subducting slabs. Slab rheology is affected in many ways by this (metastable) mineral phase change. Sluggish kinetics due to metastable hindrance is likely to cause particular difficulties, because of possible strong non-linear feedback loops between strain-rate and change of creep properties during transformation. In order to get these nanoscale properties included into thermo-mechanical models, reliable kinetic data is required. The measurement of grain-boundary energies is, however, a rather difficult problem. Conventional methods of grain boundary surface tension measurement include (a) equilibrium angles at triple junction (b) rotating ball method (c) thermal groove method, and others (Gottstein & Shvindlerman, 1999). Here I suggest a new method that allows for the derivation of grain-boundary energies for an isochemical phase transformation based on experimental (in-situ) kinetic data in combination with a corresponding dynamic scaling law (Riedel and Karato, 1997). The application of this method to the olivine-spinel phase transformation in subducting slabs provides a solution to the extrapolation problem of measured kinetic data: Any kinetic phase boundary measured at the laboratory time scale can be "scaled" to the correct critical isotherm at <span class="hlt">subduction</span> <span class="hlt">zones</span>, under experimentelly "forbidden" conditions (Liou et al., 2000). Consequences for the metastability hypothesis that relates deep seismicity with olivine metastability are derived and discussed. References: Gottstein G, Shvindlerman LS (1999) Grain Boundary Migration in Metals, CRC Press, 385 pp., New York. Riedel MR, Karato S (1997) Grain-Size Evolution in Subducted Oceanic Lithosphere Associated with the Olivine- Spinel Transformation and Its Effects on Rheology. EPSL 148: 27-43. Liou JG, Hacker BR, Zhang RY (2000) Into the forbidden zone. Science 287, 1215-1216.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2012AGUFMNH33A1655A"><span id="translatedtitle">Tsunami recurrence inferred from soil deposits on Ishigaki island along the Ryukyu <span class="hlt">subduction</span> <span class="hlt">zone</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Ando, M.; Shishikura, M.; Tu, Y.; Nakamura, M.; Arashiro, Y.</p> <p>2012-12-01</p> <p>The Ryukyu trench at the northwestern boundary of the Philippine Sea plate had no known thrust earthquakes Mw>8.0 in approximately the last 250 years. Due to this lack of associated large thrust earthquakes, an accepted common idea is that Ryukyu trench is aseismic. However, in 1771 a large tsunami struck Ishigaki islands and Miyako islands with the run-up height of up to 30-35 m. The 1771 source is suggested to be a tsunami (slow) earthquake with Mw=8.0 that occurred near the trench axis. Furthermore, very-low frequency earthquakes at shallow depths near the trench axis and slow-slip events at depths of 30km have been identified in the western Ryukyu trench. These findings suggest that the Ryukyu <span class="hlt">subduction</span> <span class="hlt">zone</span> has the potential to generate large thrust earthquakes. If the slip deficit has accumulated at the interface, the accumulated slip of more than 30 m would cause a large earthquake and a huge tsunami. Considering the abovementioned findings, estimate for the size and recurrence intervals of past tsunamis along the western Ryukyu trench was undertaken through excavation surveys of the Holocene deposits in Ishigaki and Miyako islands. The excavated sites are located on the lower Holocene marine terraces and implemented using a geoslicer or backhoes at 10 sites on November 2011 and June 2012. Stratigraphic and foraminiferal assemblages of tsunami sediment were compared with shallow beach sand to gain information on sediment source and depositional style. Based on the excavations, two tsunami layers were identified at 5 sites and provided estimates of sedimentation ages. The results obtained from stratigaraphic and foraminiferal analyses together with C14 dates of tsunami sediment indicated an event between 9-11th C. on Ishigaki and another or the same event occurred between 11th C. and 1771 on Miyako island. Consequently, if the 1771 earthquake is the only event that had occurred in the last 300 years over the 120 km, large earthquakes would potentially occur in the future along the rest of segments along the Western Ryukyu boundary.</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_17");'>17</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li class="active"><span>19</span></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_19 --> <div id="page_20" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li class="active"><span>20</span></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li><a href="#" onclick='return showDiv("page_22");'>22</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="381"> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2012AGUFM.T14A..03K"><span id="translatedtitle">Imaging megathrust zone and Yakutat/Pacific plate interface in Alaska <span class="hlt">subduction</span> <span class="hlt">zone</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Kim, Y.; Abers, G. A.; Li, J.; Christensen, D. H.; Calkins, J. A.</p> <p>2012-12-01</p> <p>We image the subducted slab underneath a 450 km long transect of the Alaska <span class="hlt">subduction</span> <span class="hlt">zone</span>. Dense stations in southern Alaska are set up to investigate (1) the geometry and velocity structure of the downgoing plate and their relation to slab seismicity, and (2) the interplate coupled zone where the great 1964 (magnitude 9.3) had greatest rupture. The joint teleseismic migration of two array datasets (MOOS, Multidisciplinary Observations of Onshore Subduction, and BEAAR, Broadband Experiment Across the Alaska Range) based on teleseismic receiver functions (RFs) using the MOOS data reveal a shallow-dipping prominent low-velocity layer at ~25-30 km depth in southern Alaska. Modeling of these RF amplitudes shows a thin (3-6.5 km) low-velocity layer (shear wave velocity less than 3 km/s), which is ~20-30% slower than normal oceanic crustal velocities, between the subducted slab and the overriding North America plate. The observed low-velocity megathrust layer (with Vp/Vs ratio exceeding 2.0) may be due to a thick sediment input from the trench in combination of elevated pore fluid pressure in the channel. The subducted crust below the low-velocity channel has gabbroic velocities with a thickness of 11-15 km. Both velocities and thickness of the low-velocity channel abruptly increase as the slab bends in central Alaska, which agrees with previously published RF results. Our image also includes an unusually thick low-velocity crust subducting with a ~20 degree dip down to 130 km depth at approximately 200 km inland beneath central Alaska. The unusual nature of this subducted segment has been suggested to be due to the subduction of the Yakutat terrane. Subduction of this buoyant crust could explain the shallow dip of the thrust zone beneath southern Alaska. We also show a clear image of the Yakutat and Pacific plate subduction beneath the Kenai Peninsula, and the along-strike boundary between them at megathrust depths. Our imaged western edge of the Yakutat terrane, at ~30-42 km depth in the central Kenai along the megathrust, aligns with the western end of the geodetically locked patch with high slip deficit, and coincides with the boundary of aftershock events from the 1964 earthquake. It seems plausible that this sharp change in the nature of the downgoing plate controls the slip distribution of great earthquakes on this plate interface.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2013AGUSM.T41A..08K"><span id="translatedtitle">Imaging megathrust zone and Yakutat/Pacific plate interface in Alaska <span class="hlt">subduction</span> <span class="hlt">zone</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Kim, Y.; Abers, G. A.; Li, J.; Christensen, D. H.; Calkins, J. A.</p> <p>2013-05-01</p> <p>We image the subducted slab underneath a 450 km long transect of the Alaska <span class="hlt">subduction</span> <span class="hlt">zone</span>. Dense stations in southern Alaska are set up to investigate (1) the geometry and velocity structure of the downgoing plate and their relation to slab seismicity, and (2) the interplate coupled zone where the great 1964 (magnitude 9.3) had greatest rupture. The joint teleseismic migration of two array datasets (MOOS, Multidisciplinary Observations of Onshore Subduction, and BEAAR, Broadband Experiment Across the Alaska Range) based on teleseismic receiver functions (RFs) using the MOOS data reveal a shallow-dipping prominent low-velocity layer at ~25-30 km depth in southern Alaska. Modeling of these RF amplitudes shows a thin (<6.5 km) low-velocity layer (shear wave velocity of ~3 km/s), which is ~20-30% slower than normal oceanic crustal velocities, between the subducted slab and the overriding North American plate. The observed low-velocity megathrust layer (with P-to-S velocity ratio (Vp/Vs) exceeding 2.0) may be due to a thick sediment input from the trench in combination of elevated pore fluid pressure in the channel. The subducted crust below the low-velocity channel has gabbroic velocities with a thickness of 11-12 km. Both velocities and thickness of the low-velocity channel abruptly increase as the slab bends in central Alaska, which agrees with previously published RF results. Our image also includes an unusually thick low-velocity crust subducting with a ~20 degree dip down to 130 km depth at approximately 200 km inland beneath central Alaska. The unusual nature of this subducted segment has been suggested to be due to the subduction of the Yakutat terrane. We also show a clear image of the Yakutat and Pacific plate subduction beneath the Kenai Peninsula, and the along-strike boundary between them at megathrust depths. Our imaged western edge of the Yakutat terrane, at 25-30 km depth in the central Kenai along the megathrust, aligns with the western end of the geodetically locked patch with high slip deficit, and coincides with the boundary of aftershock events from the 1964 earthquake. It seems plausible that this sharp change in the nature of the downgoing plate controls the slip distribution of great earthquakes on this plate interface.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2014GeCoA.140...20C"><span id="translatedtitle">Rare earth elements as indicators of hydrothermal processes within the East Scotia <span class="hlt">subduction</span> <span class="hlt">zone</span> system</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Cole, Catherine S.; James, Rachael H.; Connelly, Douglas P.; Hathorne, Ed C.</p> <p>2014-09-01</p> <p>The East Scotia <span class="hlt">subduction</span> <span class="hlt">zone</span>, located in the Atlantic sector of the Southern Ocean, hosts a number of hydrothermal sites in both back-arc and island-arc settings. High temperature (>348 °C) ‘black smoker’ vents have been sampled at three locations along segments E2 and E9 of the East Scotia back-arc spreading ridge, as well as ‘white smoker’ (<212 °C) and diffuse (<28 °C) hydrothermal fluids from within the caldera of the Kemp submarine volcano. The composition of the endmember fluids (Mg = 0 mmol/kg) is markedly different, with pH ranging from <1 to 3.4, [Cl-] from ?90 to 536 mM, [H2S] from 6.7 to ?200 mM and [F-] from 35 to ?1000 ?M. All of the vent sites are basalt- to basaltic andesite-hosted, providing an ideal opportunity for investigating the geochemical controls on rare earth element (REE) behaviour. Endmember hydrothermal fluids from E2 and E9 have total REE concentrations ranging from 7.3 to 123 nmol/kg, and chondrite-normalised distribution patterns are either light REE-enriched (LaCN/YbCN = 12.8-30.0) with a positive europium anomaly (EuCN/Eu?CN = 3.45-59.5), or mid REE-enriched (LaCN/NdCN = 0.61) with a negative Eu anomaly (EuCN/Eu?CN = 0.59). By contrast, fluids from the Kemp Caldera have almost flat REE patterns (LaCN/YbCN = 2.1-2.2; EuCN/Eu?CN = 1.2-2.2). We demonstrate that the REE geochemistry of fluids from the East Scotia back-arc spreading ridge is variably influenced by ion exchange with host minerals, phase separation, competitive complexation with ligands, and anhydrite deposition, whereas fluids from the Kemp submarine volcano are also affected by the injection of magmatic volatiles which enhances the solubility of all the REEs. We also show that the REE patterns of anhydrite deposits from Kemp differ from those of the present-day fluids, potentially providing critical information about the nature of hydrothermal activity in the past, where access to hydrothermal fluids is precluded.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2013AGUFM.T53F..05K"><span id="translatedtitle">Imaging the megathrust zone and Yakutat/Pacific plate interface in the Alaska <span class="hlt">subduction</span> <span class="hlt">zone</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Kim, Y.; Abers, G. A.; Li, J.; Christensen, D. H.; Rondenay, S.</p> <p>2013-12-01</p> <p>We image the subducted slab underneath a 450 km long transect of the Alaska <span class="hlt">subduction</span> <span class="hlt">zone</span>. Dense stations in southern Alaska are set up to investigate (1) the geometry and velocity structure of the downgoing plate and their relationship to slab seismicity, and (2) the interplate coupled zone where the great 1964 earthquake (magnitude 9.3) exhibited the largest amount of rupture. The joint teleseismic migration of two array datasets based on teleseismic receiver functions (RFs) reveals a prominent, shallow-dipping low-velocity layer at ~25-30 km depth in southern Alaska. Modeling of RF amplitudes suggests the existence of a thin (3-5 km) low-velocity layer (shear wave velocity of ~2.0-2.5 km/s) that is ~20-40% slower than underlying oceanic crustal velocities, and is sandwiched between the subducted slab and the overriding North America plate. The observed low-velocity megathrust layer (with P-to-S velocity ratio of 1.9-2.3) may be due to a thick sediment input from the trench in combination with elevated pore fluid pressure in the channel. The subducted crust below the low-velocity channel has gabbroic velocities with a thickness of ~15 km. Both velocities and thickness of the low-velocity channel abruptly increase downdip in central Alaska, which agrees with previously published results. Our image also includes an unusually thick low-velocity crust subducting with a ~20 degree dip down to 130 km depth at approximately 200 km inland beneath central Alaska. The unusual nature of this subducted segment has been suggested to be due to the subduction of the Yakutat terrane. We also show a clear image of the Yakutat and Pacific plate subduction beneath the Kenai Peninsula, and the along-strike boundary between them at megathrust depths. Our imaged western edge of the Yakutat terrane, at 25-30 km depth in the central Kenai along the megathrust, aligns with the western end of a geodetically locked patch with high slip deficit, and coincides with the boundary of aftershock events from the 1964 earthquake. It appears that this sharp change in the nature of the downgoing plate could control the slip distribution of great earthquakes on this plate interface.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=EPRINT&redirectUrl=http://www.geo.uu.nl/~forth/publications/Hinsbergen_2009a.pdf"><span id="translatedtitle">Geodynamics of collision and collapse at the AfricaArabiaEurasia <span class="hlt">subduction</span> <span class="hlt">zone</span> an introduction</span></a></p> <p><a target="_blank" href="http://www.osti.gov/eprints/">E-print Network</a></p> <p>Utrecht, Universiteit</p> <p></p> <p>; (2) <span class="hlt">continental</span> drift, sea-floor spreading and formation of ocean basins; (3) subduction initiation and drifting in the Western Mediterranean (Dercourt et al. 1986), and with initiation of the Tyrrhenian oceanic subduction stage closes the oceanic basin, even- tually resulting in the arrival of a <span class="hlt">continental</span></p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=USGSPUBS&redirectUrl=http://pubs.er.usgs.gov/publication/70010348"><span id="translatedtitle">Evolving <span class="hlt">subduction</span> <span class="hlt">zones</span> in the Western United States, as interpreted from igneous rocks</span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Lipman, P.W.; Prostka, H.J.; Christiansen, R.L.</p> <p>1971-01-01</p> <p>Variations in the ratio of K2O to SiO4 in andesitic rocks suggest early and middle Cenozoic subduction beneath the western United States along two subparallel imbricate zones dipping about 20 degrees eastward. The western zone emerged at the <span class="hlt">continental</span> margin, but the eastern zone was entirely beneath the <span class="hlt">continental</span> plate. Mesozoic subduction apparently occurred along a single steeper zone.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2014Tectp.637...20D"><span id="translatedtitle">Relative impact of mantle densification and eclogitization of slabs on subduction dynamics: A numerical thermodynamic/thermokinematic investigation of <span class="hlt">metamorphic</span> density evolution</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Duesterhoeft, Erik; Quinteros, Javier; Oberhänsli, Roland; Bousquet, Romain; de Capitani, Christian</p> <p>2014-12-01</p> <p>Understanding the relationships between density and spatio-thermal variations at convergent plate boundaries is important for deciphering the present-day dynamics and evolution of <span class="hlt">subduction</span> <span class="hlt">zones</span>. In particular, the interaction between densification due to mineralogical phase transitions and slab pull forces is subject to ongoing investigations. We have developed a two-dimensional <span class="hlt">subduction</span> <span class="hlt">zone</span> model that is based on thermodynamic equilibrium assemblage calculations and includes the effects of melting processes on the density distribution in the lithosphere. Our model calculates the '<span class="hlt">metamorphic</span> density' of rocks as a function of pressure, temperature and chemical composition in a <span class="hlt">subduction</span> <span class="hlt">zone</span> down to 250 km. We have used this model to show how the hydration, dehydration, partial melting and fractionation processes of rocks all influence the <span class="hlt">metamorphic</span> density and greatly depend on the temperature field within the subduction system. These processes are largely neglected by other approaches that reproduce the density distribution within this complex tectonic setting. Our model demonstrates that the initiation of eclogitization (i.e., when crustal rocks reach higher densities than the ambient mantle) of the slab is not the only significant process that makes the descending slab denser and generates the slab pull force. Instead, the densification of the lithospheric mantle of the sinking slab starts earlier than eclogitization and contributes significantly to slab pull in the early stages of subduction. Accordingly, the complex <span class="hlt">metamorphic</span> structure of the slab and the mantle wedge has an important impact on the development of <span class="hlt">subduction</span> <span class="hlt">zones</span>.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=EPRINT&redirectUrl=http://www.science.uottawa.ca/~khattori/Abst-Pohorje.pdf"><span id="translatedtitle">Ultramafic cumulates of oceanic affinity in an intracontinental <span class="hlt">subduction</span> <span class="hlt">zone</span>: UHP garnet peridotites from Pohorje (Eastern</span></a></p> <p><a target="_blank" href="http://www.osti.gov/eprints/">E-print Network</a></p> <p></p> <p></p> <p>peridotites from Pohorje (Eastern Alps, Slovenia) Jan C.M. De Hoog 1,* Marian Janák 2 Mirijam Vrabec 3 Keiko H Garnet peridotites from the Slovenska Bistrica Ultramafic Complex (SBUC) in the Pohorje Mts., Eastern depleted mantle of oceanic affinity. The <span class="hlt">metamorphic</span> assemblage of the garnet peridotites consists</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NSDL&redirectUrl=http://csmres.jmu.edu/geollab/Fichter/MetaRx/index.html"><span id="translatedtitle"><span class="hlt">Metamorphic</span> Rocks</span></a></p> <p><a target="_blank" href="http://nsdl.org/nsdl_dds/services/ddsws1-1/service_explorer.jsp">NSDL National Science Digital Library</a></p> <p></p> <p>2007-12-12</p> <p>Here is an in-depth description of <span class="hlt">metamorphic</span> rocks, from their classification to formation and identification. It covers types of <span class="hlt">metamorphism</span> (including Barrovian, or regional rock changes), classification by foliation, and <span class="hlt">metamorphic</span> processes (facies and zones). An alphabetical list of rocks with picture, composition, description, tectonic association, and type of <span class="hlt">metamorphism</span> is given. Common <span class="hlt">metamorphic</span> minerals are covered as well.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2011GeCoA..75.7502B"><span id="translatedtitle">Nitrogen isotopes in ophiolitic metagabbros: A re-evaluation of modern nitrogen fluxes in <span class="hlt">subduction</span> <span class="hlt">zones</span> and implication for the early Earth atmosphere</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Busigny, Vincent; Cartigny, Pierre; Philippot, Pascal</p> <p>2011-12-01</p> <p>Nitrogen contents and isotope compositions together with major and trace element concentrations were determined in a sequence of metagabbros from the western Alps (Europe) in order to constrain the evolution and behavior of N during hydrothermal alteration on the seafloor and progressive dehydration during subduction in a cold slab environment (8 °C/km). The rocks investigated include: (i) low-strain metagabbros that equilibrated under greenschist to amphibolite facies (Chenaillet Massif), blueschist facies (Queyras region) and eclogite facies (Monviso massif) conditions and (ii) highly-strained mylonites and associated eclogitic veins from the Monviso Massif. In all samples, nitrogen (2.6-55 ppm) occurs as bound ammonium ( NH4+) substituting for K or Na-Ca in minerals. Cu concentrations show a large variation, from 73.2 to 6.4 ppm, and are used as an index of hydrothermal alteration on the seafloor because of Cu fluid-mobility at relatively high temperature (>300 °C). In low-strain metagabbros, ?15N values of +0.8‰ to +8.1‰ are negatively correlated with Cu concentrations. Eclogitic mylonites and veins display Cu concentrations lower than 11 ppm and show a ?15N-Cu relationship that does not match the ?15N-Cu correlation found in low-strain rocks. This ?15N-Cu correlation preserved in low-strain rocks is best interpreted by leaching of Cu-N compounds, possibly of the form Cu(NH 3) 22+, during hydrothermal alteration. Recognition that the different types of low-strain metagabbros show the same ?15N-Cu correlation indicates that fluid release during <span class="hlt">subduction</span> <span class="hlt">zone</span> <span class="hlt">metamorphism</span> did not modify the original N and Cu contents of the parent hydrothermally-altered metagabbros. In contrast, the low Cu content present in eclogitic veins and mylonites implies that ductile deformation and veining were accompanied either by a loss of copper or that externally-derived nitrogen was added to the system. We estimate the global annual flux of N subducted by metagabbros as 4.2 (±2.0) × 10 11 g/yr. This value is about half that of sedimentary rocks, which suggests that gabbros carry a significant portion of the subducted nitrogen. The net budget between subducted N and that outgassed at volcanic arcs indicates that ˜80% of the subducted N is not recycled to the surface. On a global scale, the total amount of N buried to the mantle via <span class="hlt">subduction</span> <span class="hlt">zones</span> is estimated to be three times higher than that released from the mantle via mid-ocean ridges, arc and intraplate volcanoes and back-arc basins. This implies that N contained in Earth surface reservoirs, mainly in the atmosphere, is progressively transferred and sequestered into the mantle, with a net flux of ˜9.6 × 10 11 g/yr. Assuming a constant flux of subducted N over the Earth's history indicates that an amount equivalent to the present atmospheric N may have been sequestered into the silicate Earth over a period of 4 billion years.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=USGSPUBS&redirectUrl=http://pubs.er.usgs.gov/publication/70018866"><span id="translatedtitle">Radiocarbon evidence for extensive plate-boundary rupture about 300 years ago at the Cascadia <span class="hlt">subduction</span> <span class="hlt">zone</span></span></a></p> <p><a target="_blank" href="http://pubs.er.usgs.gov/pubs/index.jsp?view=adv">USGS Publications Warehouse</a></p> <p>Nelson, A.R.; Atwater, B.F.; Bobrowsky, P.T.; Bradley, L.-A.; Clague, J.J.; Carver, G.A.; Darienzo, M.E.; Grant, W.C.; Krueger, H.W.; Sparks, R.; Stafford, T.W., Jr.; Stuiver, M.</p> <p>1995-01-01</p> <p>THE Cascadia <span class="hlt">subduction</span> <span class="hlt">zone</span>, a region of converging tectonic plates along the Pacific coast of North America, has a geological history of very large plate-boundary earthquakes1,2, but no such earthquakes have struck this region since Euro-American settlement about 150 years ago. Geophysical estimates of the moment magnitudes (Mw) of the largest such earthquakes range from 8 (ref. 3).to 91/2 (ref. 4). Radiocarbon dating of earthquake-killed vegetation can set upper bounds on earthquake size by constraining the length of plate boundary that ruptured in individual earthquakes. Such dating has shown that the most recent rupture, or series of ruptures, extended at least 55 km along the Washington coast within a period of a few decades about 300 years ago5. Here we report 85 new 14C ages, which suggest that this most recent rupture (or series) extended at least 900 km between southern British Columbia and northern California. By comparing the 14C ages with written records of the past 150 years, we conclude that a single magnitude 9 earthquake, or a series of lesser earthquakes, ruptured most of the length of the Cascadia <span class="hlt">subduction</span> <span class="hlt">zone</span> between the late 1600s and early 1800s, and probably in the early 1700s.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2013AGUFM.V11A..04J"><span id="translatedtitle">A new method to investigate the dynamic properties of hydrous minerals and melts pertaining to <span class="hlt">subduction</span> <span class="hlt">zones</span> (Invited)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Jackson, J. M.</p> <p>2013-12-01</p> <p>Hydrous phases and melts play an important role in the geochemical and geodynamical evolution of the Earth's interior, and in particular <span class="hlt">subduction</span> <span class="hlt">zones</span>. Melts form an important basis for understanding the current state of our planet, as they are responsible for forming the crust of our planet and may likely play a role in understanding low-velocity regions deep within our planet. In an effort to obtain a more detailed understanding of minerals under high-pressure temperature conditions, we have developed a novel metric for detecting the solid-liquid phase boundary of iron-bearing materials at high-pressures using synchrotron Mössbauer spectroscopy (SMS), also known as nuclear forward scattering. Focused synchrotron radiation with 1 meV bandwidth passes through a laser-heated Fe-bearing sample inside a diamond anvil cell. The characteristic SMS time signature vanishes when melting occurs. This process is described by the Lamb-Mössbauer factor, a quantity that is directly related to the mean-square displacement of the iron atoms. Therefore, we measure the dynamics of the atoms in the material, in contrast to a static diffraction measurement. As this method monitors the dynamics of the atoms, the SMS technique provides a new and independent means of melting point determination for materials under high-pressure, as well as access to vibrational properties of the solid near its melting point (Jackson et al. EPSL 2013). In this presentation, we will discuss the applications to phases relevant to <span class="hlt">subduction</span> <span class="hlt">zones</span>.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2010EP%26S...62..665M"><span id="translatedtitle">Construction of semi-dynamic model of <span class="hlt">subduction</span> <span class="hlt">zone</span> with given plate kinematics in 3D sphere</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Morishige, M.; Honda, S.; Tackley, P. J.</p> <p>2010-09-01</p> <p>We present a semi-dynamic <span class="hlt">subduction</span> <span class="hlt">zone</span> model in a three-dimensional spherical shell. In this model, velocity is imposed on the top surface and in a small three-dimensional region around the shallow plate boundary while below this region, the slab is able to subduct under its own weight. Surface plate velocities are given by Euler's theorem of rigid plate rotation on a sphere. The velocity imposed in the region around the plate boundary is determined so that mass conservation inside the region is satisfied. A kinematic trench migration can be easily incorporated in this model. As an application of this model, mantle flow around slab edges is considered, and we find that the effect of Earth curvature is small by comparing our model with a similar one in a rectangular box, at least for the parameters used in this study. As a second application of the model, mantle flow around a plate junction is studied, and we find the existence of mantle return flow perpendicular to the plate boundary. Since this model can naturally incorporate the spherical geometry and plate movement on the sphere, it is useful for studying a specific <span class="hlt">subduction</span> <span class="hlt">zone</span> where the plate kinematics is well constrained.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/1988GeoRL..15.1219T"><span id="translatedtitle">Rupture extent of the 1978 Miyagi-Oki, Japan, earthquake and seismic coupling in the northern Honshu <span class="hlt">Subduction</span> <span class="hlt">Zone</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Tichelaar, Bart W.; Ruff, Larry J.</p> <p>1988-10-01</p> <p>Underthrusting at <span class="hlt">subduction</span> <span class="hlt">zones</span> can cause large earthquakes at shallow depths, but is always accommodated by aseismic creep below a certain depth. This transition depth is referred to as the depth of seismic coupling and can be directly observed in a <span class="hlt">subduction</span> <span class="hlt">zone</span> as the lower depth extent of significant moment release of the deepest large underthrusting earthquakes. In 1978, a large (Ms=7.5) earthquake occurred off the coast of Miyagi Prefecture in northern Honshu. Its focal mechanism represents underthrusting of the Pacific plate beneath Honshu. Since the hypocenter is located 150 km landward from the trench and there are no other large interplate earthquakes further landward from the trench axis, this event defines the maximum depth of the coupled zone. The lower limit of significant moment release of the Miyagi-Oki earthquake is obtained by analysis of the long-period P waves. The deconvolved source time function consists of a dominant single pulse with peak moment release at 12 s and a total duration of 18 s. The rupture extent of this dominant pulse does not extend deeper than 40 km, thus the transition from coupled to uncoupled in northern Honshu occurs at or above 40 km depth.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=SCIGOV-MAS&redirectUrl=http://www.geociencias.unal.edu.co/publicaciones/art/250/9-N1/06-The%20Romeral%20Fault.pdf"><span id="translatedtitle">THE ROMERAL FAULT SYSTEM: A SHEAR AND DEFORMED EXTINCT <span class="hlt">SUBDUCTION</span> <span class="hlt">ZONE</span> BETWEEN OCEANIC AND <span class="hlt">CONTINENTAL</span> LITHOSPHERES IN NORTHWESTERN SOUTH AMERICA</span></a></p> <p><a target="_blank" href="http://academic.research.microsoft.com/">Microsoft Academic Search </a></p> <p>GERMAN CHICANGANA</p> <p></p> <p>The Romeral Fault System (RFS) extends 1600 km from Barranquilla-Colombia to Talara city-Peru and before the Pliocene. In the Middle Eocene RFS defi ned the northwestern border of the South America plate, being originated by a triple junction rift - rift - rift occurred from lower to middle Jurassic, when the South American sector separated from Chortis, Oaxaca and Yucatan</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2014GeCoA.143..115S"><span id="translatedtitle">Common Pb isotope mapping of UHP <span class="hlt">metamorphic</span> zones in Dabie orogen, Central China: Implication for Pb isotopic structure of subducted <span class="hlt">continental</span> crust</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Shen, Ji; Wang, Ying; Li, Shu-Guang</p> <p>2014-10-01</p> <p>We report Pb isotopic compositions for feldspars separated from 57 orthogneisses and 2 paragneisses from three exhumed UHPM slices representing the North Dabie zone, the Central Dabie zone and the South Dabie zone of the Dabie orogen, central-east China. The feldspars from the gneisses were recrystallized during Triassic <span class="hlt">continental</span> subduction and UHP <span class="hlt">metamorphism</span>. Precursors of the orthogneisses are products of Neoproterozoic bimodal magmatic events, those in north Dabie zone emplaced into the lower crust and those in central and south Dabie zones into middle or upper crust, respectively. On a 207Pb/204Pb vs. 206Pb/204Pb diagram, almost all orthogneisses data lie to the left of the 0.23 Ga paleogeochron and plot along the model mantle evolution curve with the major portion of the data plotting below it. On a 208Pb/204Pb vs. 206Pb/204Pb diagram the most of data of north Dabie zone extend in elongate arrays along the lower crustal curve and others extend between the lower crustal curve to near the mantle evolution curve for the plumbotectonics model. This pattern demonstrates that the Pb isotopic evolution of the feldspars essentially ended at 0.23 Ga and the orthogneiss protoliths were principally dominated by reworking of ancient lower crust with some addition of juvenile mantle in the Neoproterozoic rifting tectonic zone. According to geological evolution history of the locally Dabie orogen, a four-stage Pb isotope evolution model including a long time evolution between 2.0 and 0.8 Ga with a lower crust type U/Pb ratio (? = 5-6) suggests that magmatic emplacement levels of the protoliths of the orthogneisses in the Dabie orogen at 0.8 Ga also play an important role in the Pb evolution of the exhumed UHPM slices, corresponding to their respective Pb characters at ca. 0.8-0.23 Ga. For example, north Dabie zone requires low ? values (3.4-9.6), while central and south Dabie zones require high ? values (10.9-17.2). On the other hand, Pb isotopic mixing between north and central or south Dabie zones during retro-grade <span class="hlt">metamorphism</span> enhanced by the extensive magmatism in the Cretaceous has also been observed in the 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pb vs. 206Pb/204Pb diagrams. A combined study of common Pb isotopic compositions of Dabie orthogneisses and Sulu UHPM rocks from the Chinese <span class="hlt">Continental</span> Scientific Drilling project demonstrates that a slab marked by extremely unradiogenic Pb observed in the main hole was absent in the Dabie orogen. However, occurrence of some Mesozoic granitoids with such unradiogenic character in the Dabie orogen suggests that their source may be a buried unradiogenic unit underlying below north Dabie zone. This case study clearly shows that whether the position of the Dabie data relative to the orogen curve of the plumbotectonic model is helpful in understanding the Pb isotopic structure and evolution of subducted <span class="hlt">continental</span> crust.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2005AGUFM.T42A..08S"><span id="translatedtitle">Strain Accumulation and Strain Partitioning in the Western Aleutian <span class="hlt">Subduction</span> <span class="hlt">Zone</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Steblov, G. M.; Kogan, M. G.</p> <p>2005-12-01</p> <p>The 2,200-km Aleutian megathrust demonstrates a rapid change in the sense of relative motion of the Pacific and North American plates, from nearly trench normal at Alaska to nearly trench parallel at Kamchatka, where the Aleutian and Kamchatka megathrusts connect making a cusp. This change is accompanied by an increase in the relative plate velocity from 61 to 76 mm/yr (an estimate based on our global GPS solution). Earthquake slip vector azimuths for thrust earthquakes along the arc support an idea of strain partitioning, i.e., the motion in seismic ruptures tends to be less oblique than the plate motion [MacCaffrey, 1992]. The strain partitioning predicts a steady westward motion of slivers of the hanging wall along strike-slip faults, resulting in an active collision of the far western arc with Kamchatka [Geist and Scholl, 1994]. GPS velocities measured on Aleutian islands progressively increase from 12 mm/yr at the eastern end of the arc (Kodiak) to as much as 49 mm/yr at the western end (Bering) with respect to the North American plate, which is 2/3 of the relative plate velocity. We show, using the constrained nonlinear inversion, that the high GPS velocity of Bering Is. can be alternatively explained by elastic strain accumulation resulting from locking at the subduction interface. In this scenario, there is no steady westward drift of arc slivers since the elastic strain is periodically released in earthquakes, with the islands returning to their original positions. Geodetic observations lasting for about a decade (continuous GPS was installed on Bering Is. in 1996) do not allow us to discriminate the periodic elastic strain accumulation from the along arc steady strike-slip motion. Yet there is a good argument in favor of the elastic strain at the interface beneath Bering Is.: small GPS velocities in Kamchatka at the Aleutian-Kamchatka cusp (<14 mm/yr) are easily explained by superposed elastic strains at Kamchatka and Aleutian subduction interfaces. There is simply no evidence in GPS velocities of intense compression near Cape Kamchatka as predicted by the collision scenario. Moreover, specific azimuths and values of GPS velocities in Kamchatka at the cusp can be easily explained by superposed elastic strains. Some amount of slivering in the westernmost Aleutians is clearly evidenced by abundant strike-slip seismic focal mechanisms, yet the rate of motion along fracture zones to the north and to the south of the arc currently is unknown; it can be small with respect to the elastic strain accumulation in an azimuth parallel to the arc. Unless longer time series at larger number of islands are observed, a unique interpretation of GPS velocities on the Aleutians is not possible. Recently, the first epoch of GPS at Medny (Copper) Is. was carried out, in 100 km from Bering and at greater distance from the Aleutian Trench. Comparison of GPS velocities at Bering and Medny should provide evidence on how significant is the strain partitioning. Bürgmann, R., M.G. Kogan, G.M. Steblov, G. Hilley, V.E. Levin, and T. Apel, Interseismic Coupling and Asperity Distribution Along the Kamchatka <span class="hlt">Subduction</span> <span class="hlt">Zone</span>, J. Geophys. Res., 110, B07405, doi:10.1029/2005JB003648, 2005.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2014IJEaS.tmp...12F"><span id="translatedtitle">Volatile (H2O, CO2, Cl, S) budget of the Central American <span class="hlt">subduction</span> <span class="hlt">zone</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Freundt, A.; Grevemeyer, I.; Rabbel, W.; Hansteen, T. H.; Hensen, C.; Wehrmann, H.; Kutterolf, S.; Halama, R.; Frische, M.</p> <p>2014-02-01</p> <p>After more than a decade of multidisciplinary studies of the Central American <span class="hlt">subduction</span> <span class="hlt">zone</span> mainly in the framework of two large research programmes, the US MARGINS program and the German Collaborative Research Center SFB 574, we here review and interpret the data pertinent to quantify the cycling of mineral-bound volatiles (H2O, CO2, Cl, S) through this subduction system. For input-flux calculations, we divide the Middle America Trench into four segments differing in convergence rate and slab lithological profiles, use the latest evidence for mantle serpentinization of the Cocos slab approaching the trench, and for the first time explicitly include subduction erosion of forearc basement. Resulting input fluxes are 40-62 (53) Tg/Ma/m H2O, 7.8-11.4 (9.3) Tg/Ma/m CO2, 1.3-1.9 (1.6) Tg/Ma/m Cl, and 1.3-2.1 (1.6) Tg/Ma/m S (bracketed are mean values for entire trench length). Output by cold seeps on the forearc amounts to 0.625-1.25 Tg/Ma/m H2O partly derived from the slab sediments as determined by geochemical analyses of fluids and carbonates. The major volatile output occurs at the Central American volcanic arc that is divided into ten arc segments by dextral strike-slip tectonics. Based on volcanic edifice and widespread tephra volumes as well as calculated parental magma masses needed to form observed evolved compositions, we determine long-term (105 years) average magma and K2O fluxes for each of the ten segments as 32-242 (106) Tg/Ma/m magma and 0.28-2.91 (1.38) Tg/Ma/m K2O (bracketed are mean values for entire Central American volcanic arc length). Volatile/K2O concentration ratios derived from melt inclusion analyses and petrologic modelling then allow to calculate volatile fluxes as 1.02-14.3 (6.2) Tg/Ma/m H2O, 0.02-0.45 (0.17) Tg/Ma/m CO2, and 0.07-0.34 (0.22) Tg/Ma/m Cl. The same approach yields long-term sulfur fluxes of 0.12-1.08 (0.54) Tg/Ma/m while present-day open-vent SO2-flux monitoring yields 0.06-2.37 (0.83) Tg/Ma/m S. Input-output comparisons show that the arc water fluxes only account for up to 40 % of the input even if we include an "invisible" plutonic component constrained by crustal growth. With 20-30 % of the H2O input transferred into the deeper mantle as suggested by petrologic modeling, there remains a deficiency of, say, 30-40 % in the water budget. At least some of this water is transferred into two upper-plate regions of low seismic velocity and electrical resistivity whose sizes vary along arc: one region widely envelopes the melt ascent paths from slab top to arc and the other extends obliquely from the slab below the forearc to below the arc. Whether these reservoirs are transient or steady remains unknown.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2004AGUFM.S51B0149Y"><span id="translatedtitle">Thrusting-related Fluid Flow in <span class="hlt">Subduction</span> <span class="hlt">Zone</span>: Geologic and Isotopic Evidence From Ancient Subduction Complex</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Yamaguchi, A.; Ujiie, K.; Kimura, G.</p> <p>2004-12-01</p> <p>Role of fluid in a seismogenic plate boundary of <span class="hlt">subduction</span> <span class="hlt">zone</span> remains poorly understood. To reveal the origin and behavior of syn-tectonic fluid in the ancient subduction complex, we examined occurrences of veins and carbon and oxygen isotope composition of vein calcite of the Mugi Melange in the Shimanto Belt. The melange preserves a relationship between rock deformation and fluid flow near the up-dip limit of seismogenic zone. The Mugi Melange is a late Cretaceous to Paleocene tectonic melange, and is characterized by blocks of sandstone and basalt in shale matrix. The Mugi Melange is divided into five units based on the repetition of basaltic layer of N-MORB, representing duplex-underplated assemblage. Paleo-temperature determined by vitrinite reflectance is 120-150 \\deg C in the studied section, and fluid inclusion thermo-barometry indicates 125-245 \\deg C and 92-149 MPa (Matsumura et al., 2003). This P-T condition is in the vicinity of up-dip limit of the seismogenic zone. Major deformational stage of the Mugi Melange is underthrusting (D1), underplating (D2), and uplifting (D3). There are various occurrences of quartz and calcite veins in melange formed during various stages. We classified three occurrences of veins named intra-boudin veins (IBV), network veins (NWV), and fault-parallel veins (FPV). IBV are observed only around the neck of boudined sandstone block in the black shale matrix, so they were precipitated when block-in-matrix structure was formed due to layer-parallel extension during D1. NWV are distributed in the damage zone of duplex ramp thrust, and are mainly perpendicular to shear surface of the thrust. Such occurrences suggest that NWV formed inter-faulting period of D2. FPV show implosion breccia-like texture (Sibson, 1986), and they are observed just below the basalt / shale boundary. Vein minerals are dominantly calcite. These features suggest that FPV would be precipitated as a result of co-slip fluid migration during oceanic crust underplating. We analyzed carbon and oxygen isotopic compositions of calcite obtained from these three occurrences of veins. Oxygen isotopic compositions are in the narrow range of +15.8 to +19.2 \\permil (vs. V-SMOW) for all veins. \\delta 18O values increase from IBV to FPV. Precipitation temperatures determined by fluid inclusion (Matsumura et al., 2003) are higher in NWV than in IBV. These results show that delta-18O values of fluids increase from IBV ( ˜ +4 \\permil) to FPV ( ˜ +10 \\permil). Assuming the same dehydration reaction of clay minerals of host rock and isotopic equilibrium, the dehydration temperature is lowest for IBV, moderate for NWV, and highest for FPV. In contrast to \\delta 18O values, carbon isotopic compositions of vein calcite are broad, in -17.2 to -3.7 \\permil (vs. V-PDB) for all veins. IBV and NWV include light carbon, while FPV take heavy one. This result suggests that organic carbon from shale and igneous carbon from basalt were mixed in the fault core. In conclusion, we suggest that the fluid dehydrated from both shale and basalt was accumulated in cracks in damage zone of plate boundary fault, and migrated to shallower portion in association with seismic event.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2012AGUFM.T13C2629K"><span id="translatedtitle">Structure of the <span class="hlt">subduction</span> <span class="hlt">zone</span> beneath the Wellington region, New Zealand , from passive seismic recordings</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Karalliyadda, S.; Savage, M. K.; Hall, C.; Stern, T. A.; Henrys, S. A.; Wech, A.; Townend, J.; Carrizales, A.</p> <p>2012-12-01</p> <p>The Seismic Array on the HiKurangi Experiment I consisted of 50 2-Hz seismometers deployed in a two-dimensional array and ten broadband seismometers deployed in a line above the Hikuangi <span class="hlt">Subduction</span> <span class="hlt">Zone</span> throughout the Wellington/Wairarapa region of New Zealand. Wellington is the capital and second largest city in New Zealand. Continuous signals were recorded between November 2009 and March 2010 on the short period sensors and up to 18 months on the broadband sensor. These stations densified the GeoNet network of two broadband and 11 1-Hz seismometers. Airgun shots and earthquakes were extracted for analysis. The E-W line was also occupied at several times with a high-density array of geophones deployed to record airgun shots and explosives. Here we summarize the results of preliminary analysis of earthquakes and seismic noise. Receiver function images of the plate boundary reveal similar structures to the results of active source analysis, suggesting that at long wavelength the S velocity and P velocity change at the same boundaries. A low velocity layer at the top of the plate and the within-slab Moho is well imaged. Deeper features are less clearly imaged but, like the controlled source reflectors, suggest that some converters are dipping in the same direction as the slab and some in the opposite direction. We use SKS phases recorded on the broadband array and permanent stations in the eastern part of our study area to investigate the deep anisotropic structure. Preliminary SKS splitting measurements display NE/SW fast polarization azimuths sub-parallel to Hikurangi trench and the predominant upper plate fault strike. Delay times of these splitting measurements range from 1.30 - 4.9 s (+/- 0.46 s) and SKS phases with large periods ( > 12 s) tend to show higher delay times ( > 2 s). Shear wave splitting on local earthquakes with magnitude greater than 4 yield mostly NE-SW polarization azimuths, consistent with previously determined local and SKS anisotropy at nearby stations. Smaller earthquakes yield more scattered fast directions. Delay times average 0.18 s and range from 0.02 to 0.7 seconds from earthquakes that extend from 5 to 180 km depth. However, delay times do not increase with depth, suggesting that most waveforms are re-split in the upper crust. Stacked seismic noise cross correlation functions obtained from broadband and short period stations alike exhibit coherent signals to distances of 80 km. Packets of coherent energy travel with speeds of between 5.2 km/s and 1.3 km/s, with the fastest group coherent to 60 km on the 2-stations. This analysis suggests that such high-frequency seismometers can usefully record surface waves with periods much longer than the natural period of the seismometers. We therefore expect to be able to use these data to determine surface wave velocity models of the uppermost 5 km of the crust, which will complement the controlled source profiles in the region.</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_18");'>18</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li class="active"><span>20</span></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li><a href="#" onclick='return showDiv("page_22");'>22</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_20 --> <div id="page_21" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li class="active"><span>21</span></li> <li><a href="#" onclick='return showDiv("page_22");'>22</a></li> <li><a href="#" onclick='return showDiv("page_23");'>23</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="401"> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2012AGUFM.V53D2872F"><span id="translatedtitle">In situ Raman study of dissolved CaCO3 minerals under <span class="hlt">subduction</span> <span class="hlt">zone</span> conditions</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Facq, S.; Daniel, I.; Sverjensky, D. A.</p> <p>2012-12-01</p> <p>The fate and the characteristics of the Earth's deep carbon reservoirs are still not well understood [1]. The connection between the surficial and the deep-Earth carbon cycles occurs at <span class="hlt">subduction</span> <span class="hlt">zones</span> where carbon is transported into the mantle and where hydrous silicates and carbonate minerals break down releasing H2O and C-species in fluids associated with mantle metasomatism and the generation of arc volcanism [2]. In order to obtain mass balance between recycling and burial in the deep mantle, the study of the dissolution of CaCO3 minerals in equilibrium with aqueous fluids under mantle conditions is crucial. We report a novel integrated experimental and theoretical study of the equilibration of CaCO3 minerals with aqueous solutions (pure water or NaCl solutions) at high pressures and temperatures (0.5 to 8 GPa and 250 to 500 °C). The fluid speciation was studied using in situ Raman spectroscopy coupled to an externally heated membrane type DAC equipped with 500 ?m pure synthetic diamond anvils. In a typical experiment, the aqueous fluid and a calcite crystal [3] were loaded in a rhenium gasket. The pressure was determined from the calibrated shift of the carbonate ?1 symmetric stretching mode of aragonite and the temperature measured with a K-type thermocouple. Raman spectra were recorded using a Labram HR800 Raman spectrometer (Horiba Jobin-Yvon) coupled to a Spectra Physics Ar+ laser. At equilibrium with an aragonite crystal, the Raman data show that bicarbonate is the most abundant species in low-pressure fluids (below 4 GPa) whereas carbonate becomes progressively dominant at higher pressure. After correction from their Raman cross-sections [4], the relative amounts of dissolved carbonate and bicarbonate were estimated from the areas of the Raman bands of the carbonate and bicarbonate ions (?1 and ?5 symmetric stretching modes, respectively). The presence of sodium chloride influences the speciation by extending the pressure field where the bicarbonate species is dominant in the fluid. The Raman data were also used to constrain a theoretical thermodynamic model of the fluid speciation in equilibrium with CaCO3. Building on published calcite solubility data from 400 - 700 °C and 2 - 16 kb [3, 5], revised thermodynamic properties of aqueous CO2 and HCO3-, data for the aqueous CaHCO3+ complex from 4 to 90 °C [6], and estimated dielectric constants of water, enabled an equation of state characterization of the standard Gibbs free energy of CaHCO3+ at the conditions of the Raman study. At 300 - 400 °C, the Raman speciation results were used to constrain equilibrium constants involving the carbonate ion. The results indicate that CO2 is a minor species in fluids in equilibrium with aragonite at 300 - 500 °C and P > 1.0 GPa. Instead, the CaHCO3+ species becomes important at high pressures until carbonate ion becomes the dominant C-species. [1] Marty, B. and Tolstikhin, I.N., Chemical Geology 145, 233 (1998) [2] Schmidt, M.W. and Poli, S., Earth and Planetary Science Letters 163, 163 (1998) [3] Caciagli, N.C. and Manning, C.E., Contributions to Mineralogy and Petrology 146, 275 (2003) [4] Frantz, J., Chemical Geology 152, 211 (1998) [5] Fein, J.B. and Walther, J.V., Contributions to Mineralogy and Petrology 103, 317 (1989) [6] Plummer, L.N. and Busenberg, E., Geochimica et Cosmochimica Acta 46, 1011 (1982)</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2011AGUFM.T21B2331C"><span id="translatedtitle">Structure of the Lesser Antilles <span class="hlt">subduction</span> <span class="hlt">zone</span> from seismic refraction tomography</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Charvis, P.; Evain, M.; Galve, A.; Laigle, M.; Ruiz Fernandez, M.; Kopp, H.; Hirn, A.; Flueh, E. R.; Party, T.</p> <p>2011-12-01</p> <p>In 2007, two wide-angle seismic experiments were conducted to constrain the structure of the central Lesser Antilles <span class="hlt">subduction</span> <span class="hlt">zone</span>. During the Sismantilles II experiment, seismic refraction data recorded by a network of 27 OBSs over an area of more than 6000 km2 provide new insights on the crustal structure of the forearc offshore Martinique and Dominica islands. The tomographic inversion of first arrival travel times provides a 3D P-wave velocity model down to 15 km. Basement velocity gradient shows the forearc made of two distinct units: A high velocity gradient domain named the inner forearc in comparison to a lower velocity gradient domain located further trenchward named the outer forearc. The inner forearc is likely the extension at depth of the Mesozoic magmatic crust outcropping to the north in La Désirade Island and along the scarp of the Karukera spur and then represent the eastern limit of the Caribbean Plateau. The outer forearc probably consists of magmatic rocks of a similar origin. It could be either part of the margin of the Caribbean Plateau, like the inner forearc, but the crust was thinned and fractured during the past tectonic history of the area or by recent subduction processes, or an oceanic terrane more recently accreted to the island arc. Whereas the inner forearc appears as a rigid block uplifted and tilted as a whole to the south, short wavelength deformations of the outer forearc basement are observed, beneath a 3 to 6 km thick sedimentary pile, in relation with the subduction of the Tiburon ridge and associated seamounts. North, offshore Dominica Island, the outer forearc is 70 km wide. It extends as far as 180 km to the east of the volcanic front where it acts as a backstop on which the accretionary wedge developed. Its width decreases strongly to the south to terminate offshore Martinique where the inner forearc acts as the backstop. Two dense wide-angle seismic refraction lines, shot during the Trail experiment (2007) provides a 2D velocity model across the whole arc north and south of Dominica Island. Wide-angle reflection travel times were inverted to constrain the deepest interfaces. It appears that the overriding plate is characterized by a thick crust, the Moho interface being at 30 km depth along both profile. To the south, an intra-crustal interface 20 km deep that strikes only beneath the inner forearc region is also imaged separating the upper crust from the lower crust. The Lesser Antilles 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.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2003AGUFM.S42I..05N"><span id="translatedtitle">Holocene History of Great Earthquakes in the Cascadia <span class="hlt">Subduction</span> <span class="hlt">Zone</span> Based on the Turbidite Event Stratigraphy</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Nelson, C. H.; Goldfinger, C.; Johnson, J. E.; Gutierrez-Pastor, J.</p> <p>2003-12-01</p> <p>Submarine channels along the Cascadia convergent margin have recorded a Holocene history of turbidity currents, in the form of turbidites, most likely triggered by great earthquakes. Turbidite systems from four regional sites, the Rogue, Astoria, Juan de Fuca, and Cascadia Channels, contain 13 correlative post Mazama turbidites (T1-13) based on the first occurrence of Mazama Ash (MA) at 7344 +/- 130 cal. yr BP below T 13 and another T18 datum of 9744 +/- 70 cal. yr BP. Based on these datums and tests of synchronicity, turbidity currents appear to have been triggered synchronously by great earthquakes along 660 km of the Cascadia <span class="hlt">Subduction</span> <span class="hlt">Zone</span> on average every 587 yr from T1-13 and every 480 yr from T13-18. Based on semi-independent methods of AMS radiocarbon ages and hemipelagic sediment thickness plus sedimentation rate beneath each turbidite, the average recurrence intervals of great earthquakes in Cascadia Basin are 530 years and 524 years respectively for the past 4000 yr, compared to coastal paleoseismic events of 533 yr at Willapa Bay, WA and 529 yr at the Sixes River,OR. The most complete and reliable hemipelagic record of recurrence intervals and ages for the 18 great earthquakes during the Holocene (past 9,744 years) has been determined at the Cascadia Channel site. Hemipelagic thicknesses and time intervals based on sedimentation rates have been calculated for every turbidite event from 8 replicate cores. When there has been some erosion of the hemipelagic sediment interval in one core, it is evident because the thicknesses in the other cores at the site are greater and usually equal. In general, there is a trend towards thicker hemipelagic sediment and longer recurrence times (hemipelagic average interval = 324 yr and radiocarbon average interval = 400 yr) toward the later Holocene. The maximum recurrence time, based on radiocarbon ages (1477 yr), occurs between T10 and T11 and coincides with the maximum interval (922 yr) and thickness of hemipelagic sediment. From T1 to T9 events, the variance of recurrence times is least, based on hemipelagic sediment thickness in Cascadia Channel, while radiocarbon results indicate a high variability for the same interval, consistently showing the intervals prior to T7 and T4 as long intervals > 1000 years. The T1 to T9 events for the past 4620 cal yr BP have a minimum recurrence time of 409 yr and maximum of 644 yr in Cascadia Channel using hemipelagic thickness, but a range of 321 to 1077 years (+ 120) with radiocarbon alone. The differences in the two methods appear to be the result of uniform hemipelagic thickness data following T8 and T5, whereas multiple radiocarbon ages at two sites suggest very long, > 1000 year intervals. These results also point out the need for further refinement of both methods, since the events can be correlated well without age information. When complete, the corrected age table should reveal an earthquake history ~ 10,000 years in length. This record can then be compared to the land record to identify differences that may reveal upper-plate, slab, or other events that did not trigger turbidity currents.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2007AGUFM.T53A1109L"><span id="translatedtitle">Lesser Antilles <span class="hlt">Subduction</span> <span class="hlt">Zone</span> Investigation by a Cluster of Large Seismic Experiments in the Forearc Region</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Last, T.</p> <p>2007-12-01</p> <p>Thales LAST stands for Lesser Antilles <span class="hlt">Subduction</span> <span class="hlt">zone</span> Team which gathers the scientific teams of a cluster of surveys and cruises that have been carried out in 2007 and coordinated under the European Union THALES WAS RIGHT project (Coord. A. Hirn). This cluster is composed by the German cruise TRAIL with the vessel F/S Merian (PI E. Flueh and H. Kopp, IFM-GEOMAR), the French cruise SISMANTILLES 2 with the IFREMER vessel N/O Atalante (PI M. Laigle, IPG Paris and JF. Lebrun, Univ. Antilles Guyane), and French cruise OBSANTILLES with the IRD vessel N/O Antea (PI P. Charvis, Geoazur, Nice, France). During these cruises and surveys, 84 Ocean Bottom 3-components Seismometers (OBS) and 20 Hydrophones (OBHs) have been brought together from several pools (Geoazur, INSU, IPGP, IFM-GEOMAR, AWI,), with up to 30 land stations (CSIC Barcelone, IPG Paris, INSU-RLBM and -LITHOSCOPE) in addition to the permanent onshore arrays of IPGP and SRU. The deployment of all these instruments has been supported principally by ANR Catastrophes Telluriques et Tsunamis (SUBSISMANTI), by the EU SALVADOR Programme of IFM-GEOMAR, as well as by the EU project THALES WAS RIGHT on the Antilles and Hellenic active subductions to which contribute IPGP, Geoazur, IFM-GEOMAR (Germany), ETH Zurich (Switzerland), CSIC Barcelona (Spain), Univ. Trieste (Italy) and NOA Athens (Greece). The main goal of this large seismic investigation effort is the understanding of the behaviour of the seismogenic zone and location of potential source regions of mega-thrust earthquakes. Specific goals are the mapping of the subduction interplate in the range where it may be seismogenic along the Lesser Antilles Arc from Antigua to southern Martinique Islands, as a contribution to identification and localisation in advance of main rupture zones of possible future major earthquakes, and to the search for transient signals of the activity. The forearc region, commonly considered as a proxy to the seismogenic portion of the subduction mega-thrust fault plane, and which is here the main target has been localized along 3 transects to the Arc thanks to a preliminary survey in 2001, the French SISMANTILLES cuise. We will present the first results obtained during these experiments dedicated specifically to image at depth the seismic structure and activity of this region. To image faults at depth and the detailed upper-crustal structure, 3700 km of multi-beam bathymetry and multi-channel reflection seismic profiles have been collected along a grid comprising 7 strike-lines of up to 300 km long and spaced by 15 km and 12 transects of up to 150 km long and spaced by 25 km (SISMANTILLES 2). All these airgun shots dedicated to deep penetration have been recorded by the 84 OBSs and 20 OBHs deployed by the F/S Merian and N/O Atalante on the nodes of this grid of profiles. It will permit to get Vp constraints on the deep forearc region and mantle wedge by wide-angle refraction studies, as well as constraints on the updip and downdip limits of the seismogenic part of the mega-thrust fault plane. Two of these transects have been extended across the whole arc during the TRAIL survey, with up to 50 OBSs deployed along both 240 km long profiles. All these OBSs remained several months beyond the shot experiments for local earthquakes Vp and Vp/Vs tomography. They have been recovered and partly redeployed by N/O Antea during the OBSANTILLES survey. A significant number of those instruments had broadband seismometers, a notable originality in the case of the OBSs to detect low-frequency transient signals.</p> </li> <li> <p><a target="resultTitleLink" href="http://www.science.gov/scigov/desktop/en/ostiblue/service/link/track?type=RESULT&searchId=topic-pages&collectionCode=NASAADS&redirectUrl=http://adsabs.harvard.edu/abs/2012AGUFM.P14A..08H"><span id="translatedtitle">A Model of <span class="hlt">Continental</span> Growth and Mantle Degassing Comparing Biotic and Abiotic Worlds</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Höning, D.; Hansen-Goos, H.; Spohn, T.</p> <p>2012-12-01</p> <p>While examples for interaction of the biosphere with the atmosphere can be easily cited (e.g., production and consumption of O2), interaction between the biosphere and the solid planet and its interior is much less established. It has been argued (e.g., Rosing et al. 2006; Sleep et al, 2012) that the formation of continents could be a consequence of bioactivity harvesting solar energy through photosynthesis to help build the continents and that the mantle should carry a chemical biosignature. We present an interaction model that includes mantle convection, mantle water vapor degassing at mid-oceanic ridges and regassing through <span class="hlt">subduction</span> <span class="hlt">zones</span>, <span class="hlt">continental</span> crust formation and erosion and water storage and transport in a porous oceanic crust that includes hydrous mineral phases. The mantle viscosity in this model depends on the water concentration in the mantle. We use boundary layer theory of mantle convection to parameterize the mantle convection flow rate and assume that the plate speed equals the mantle flow rate. The biosphere enters the calculation through the assumption that the <span class="hlt">continental</span> erosion rate is enhanced by a factor of several through bioactivity and through an assumed reduction of the kinetic barrier to diagenetic and <span class="hlt">metamorphic</span> reactions (e.g., Kim et al. 2004) in the sedimentary basins in <span class="hlt">subduction</span> <span class="hlt">zones</span> that would lead to increased water storage capacities. We further include a stochastic model of continent-to-continent interactions that limits the effective total length of <span class="hlt">subduction</span> <span class="hlt">zones</span>. We use present day parameters of the Earth and explore a phase plane spanned by the percentage of surface coverage of the Earth by continents and the total water content of the mantle. We vary the ratio of the erosion rate in a postulated abiotic Earth to the present Earth, as well as the activation barrier to diagenetic and <span class="hlt">metamorphic</span> reactions that affect the water storage capacity of the subducting crust. We find stable and unstable fixed points in the phase area where the net degassing and <span class="hlt">continental</span> growth rates are zero. Many of the parameter combinations result in one stable fixed point with a completely dry mantle that lacks continents altogether and a second stable fixed point with a continent coverage and mantle water concentration close to that of the present Earth. In addition, there is an unstable fixed point situated between the two. In general, the abiotic world has a larger zone of attraction for the fixed point with a dry mantle and no continents than the biotic world. Thus a biotic world is found to be more likely to develop continents and a have wet mantle. Furthermore, the biotic model is generally found to have a wetter mantle than an abiotic model with the same continent coverage. Through the effect of water on the mantle rheology, the biotic world would thus tend to be tectonically more active and have a more rapid long-term carbon silicate cycle. References: J. Kim, H. Dong, J. Seabaugh, S. W. Newell, D. D. Eberl, Science 303, 830-832, 2004 N. H. Sleep, D. K. Bird, E. Pope, Annu. Rev. Earth Planet. Sci. 40, 277-300, 2012 M. T. Rosing, D. K. Bird, N. H. Sleep, W. Glassley, F. Albarede, Paleo3 232, 90-113, 2006</p>