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1

Chemical geodynamics of continental subduction-zone metamorphism: Insights from studies of the Chinese Continental Scientific Drilling (CCSD) core samples  

Microsoft Academic Search

The Dabie–Sulu orogenic belt of east-central China has long been a type location for the study of geodynamic processes associated with ultrahigh-pressure (UHP) tectonics. Much of our understanding of the world's most enigmatic processes in continental deep-subduction zones has been deduced from various records in this belt. By taking advantage of having depth profiles from core samples of the Chinese

Yong-Fei Zheng; Ren-Xu Chen; Zi-Fu Zhao

2009-01-01

2

Chemical geodynamics of continental subduction-zone metamorphism: Insights from studies of the Chinese Continental Scientific Drilling (CCSD) core samples  

NASA Astrophysics Data System (ADS)

The Dabie-Sulu orogenic belt of east-central China has long been a type location for the study of geodynamic processes associated with ultrahigh-pressure (UHP) tectonics. Much of our understanding of the world's most enigmatic processes in continental deep-subduction zones has been deduced from various records in this belt. By taking advantage of having depth profiles from core samples of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen, a series of combined studies were carried out for UHP metamorphic rocks from the main hole (MH) at continuous depths of 100 to 5000 m. The results provide new insights into the chemical geodynamics of continental subduction-zone metamorphism, especially on the issues that are not able to be resolved from the surface outcrops. Available results from our geochemical studies of CCSD-MH core samples can be outlined as follows. (1) An O isotope profile of 100 to 5000 m is established for the UHP metamorphic minerals, with finding of 18O depletion as deep as 3300 m. Along with areal 18O depletion of over 30,000 km 2 along the Dabie-Sulu orogenic belt, three-dimensional 18O depletion of over 100,000 km 3 occurs along the northern margin of the South China Block. (2) Changes in mineral O isotope, H isotope and water content occur in eclogite-gneiss transitions, concordant with petrographic changes. The contact between different lithologies is thus the most favorable place for fluid action; fluid for retrogression of the eclogites away from the eclogite-gneiss boundary was derived from the decompression exsolution. For the eclogites adjacent to gneiss, in contrast, the retrograde metamorphism was principally caused by aqueous fluid from the gneiss that is relatively rich in water. Inspection of the relationship between the distance, petrography and ?18O values of adjacent samples shows O isotope heterogeneities between the different and same lithologies on scales of 20 to 50 cm, corresponding to the maximum scales of fluid mobility during the continental collision. (3) Studies of major and trace elements in the two continuous core segments indicate high mobility of LILE and LREE but immobility of HFSE and HREE. Some eclogites have andesitic compositions with high SiO 2, alkalis, LREE and LILE but low CaO, MgO and FeO contents. These features likely result from chemical exchange with gneisses, possibly due to the metasomatism of felsic melt produced by partial melting of the associated gneisses during the exhumation. On the other hand, some eclogites appear to have geochemical affinity to refractory rocks formed by melt extraction as evidence by strong LREE and LILE depletion and the absence of hydrous minerals. These results provide evidence for melt-induced element mobility in the UHP metamorphic rocks, and thus the possible presence of supercritical fluid during exhumation. In particular, large variations in the abundance of such elements as SiO 2, LREE and LILE occur at the contact between eclogite and gneiss. This indicates their mobility between different slab components, although it only occurs on small scales and is thus limited in local open-systems. (4) Despite the widespread retrogression, retrograde fluid was internally buffered in stable isotope compositions, and the retrograde fluid was of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl and molecular water in nominally anhydrous minerals. (5) A combined study of petrography and geochronology reveals the episode of HP eclogite-facies recrystallization at 216 ± 3 Ma, with timescale of 1.9 to 9.3 Myr or less. Collectively, the Dabie-Sulu UHP terrenes underwent the protracted exhumation (2-3 mm/yr) in the HP-UHP regime. (6) Zircon U-Pb ages and Hf isotopes indicate that mid-Neoproterozoic protoliths of bimodal UHP metaigneous rocks formed during supercontinental rifting along preexisting arc-continent collision orogen, corresponding to dual bimodal magmatism in response to the attempted breakup of the supercontinent Rodinia at about 780 Ma. The first type of bim

Zheng, Yong-Fei; Chen, Ren-Xu; Zhao, Zi-Fu

2009-09-01

3

Metamorphic chemical geodynamics of subduction zones Gray E. Bebout  

E-print Network

, and the chemical changes in subducting rocks potentially contributing to the geochemical heterogeneityFrontiers Metamorphic chemical geodynamics of subduction zones Gray E. Bebout Lehigh University, and experimental evidence for the physical and chemical properties of fluids at the pressures and temperatures

Bebout, Gray E.

4

Metamorphic chemical geodynamics of subduction zones  

Microsoft Academic Search

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

Gray E. Bebout

2007-01-01

5

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

E-print Network

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 ...

Pearce, Frederick Douglas

6

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

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.

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

2005-01-01

7

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

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

8

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

NASA Astrophysics Data System (ADS)

The Makbal Complex in the western Tianshan Mountains of Kazakhstan and Kyrgyzstan consists of HP/UHP metasedimentary host rocks which enclose various HP mafic blocks or boudins. These mafic rocks comprise rare eclogites (sensu stricto and sensu lato), garnet amphibolites (retrograded eclogites) and a newly discovered glaucophanite (glaucophane-garnet-omphacite bearing rock). So far the Makbal Complex has been interpreted to predominantly consist of continental lithologies and the mafic rocks were considered as dismembered dikes intruding continental metasediments. This interpretation is mainly based on the geological relationship and bulk rock chemistry of the different rock types. It was further suggested that the continental lithologies of the Makbal Complex underwent eclogite-facies metamorphism in a former subduction zone. In the present study we combined conventional geothermometry, P-T pseudosection modeling and major and trace element whole rock geochemistry for different mafic samples (glaucophanite and eclogites (sensu lato)) in order to shed light on both the metamorphic evolution and the protoliths of the mafic HP rocks in the Makbal Complex. Prograde to peak-pressure clockwise P-T paths of glaucophanite and eclogites (sensu lato) were modeled using garnet isopleth thermobarometry. The results show that the glaucophanite and eclogite (sensu lato) samples experienced similar prograde P-T paths and slightly different peak metamorphic conditions at ~ 560 °C at 2.4 GPa for the former and between ~ 520 °C at 2.2 GPa and ~ 555 °C at ~ 2.5 GPa for the latter, corresponding to burial depths between 70 and 85 km. Whole rock major and trace element analyses and petrological evidence imply that the various rock types at the Makbal Complex most likely originated from different precursor rocks. Eclogites (sensu lato) are believed to represent strongly retrogressed former eclogite-facies rocks that had never been eclogites (sensu stricto, i.e. > 70 vol.% garnet and omphacite) due to an unfavorable alkali-poor bulk composition (Na2O < 1 wt.%). The four high-pressure mafic samples investigated in this study originated from oceanic crust (Zr/Hf ratio of 33 to 35) which contradicts all previous studies suggesting a continental protolith for all mafic HP/UHP rocks at Makbal. The present study indicates that the mafic high-pressure rocks represent incoherent segments of exhumed oceanic crust. Juxtaposition of different mafic oceanic (this study) and continental rocks is suggested to be due to buoyancy-driven exhumation of the metasedimentary host rock in the subduction channel where dismembered fragments of the subducted oceanic crust were captured in different depths.

Meyer, Melanie; Klemd, Reiner; Konopelko, Dmitry

2013-09-01

9

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

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

10

Petrotectonic Significance of High and Ultrahigh-Pressure Metamorphic Belts: Inferences for Subduction-Zone Histories  

Microsoft Academic Search

The 900-km-long Franciscan accretionary prism of the California Coast Ranges consists chiefly of greywackes and dark shales of late Mesozoic age. The complex is characterized by monotonic growth toward the edge of the continent and by the systematic, ocean ward decrease in metamorphic intensity. High-pressure (HP) mineral assemblages include lawsonite, aragonite, glaucophane, and\\/or jadeitic pyroxene. Systematic seaward vergence and younging

W. Gary Ernst; Juhn G. Liou; Bradley R. Hacker

1994-01-01

11

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

E-print Network

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

Paris-Sud XI, Université de

12

Simon, N. S. C., Davies, G. R., Pearson, D. G. & Carlson, R. W. (2004). The southern African Kaapvaal craton: formation and modification of continental lithospheric mantle in Archaean subduction zones?  

E-print Network

Kaapvaal craton: formation and modification of continental lithospheric mantle in Archaean subduction zones and modification of continental lithospheric mantle in Archaean subduction zones? N.S.C. SIMON 1 , G.R. DAVIES 1 that garnet and cpx are not co-genetic. Calculated equilibrium liquids and Hf-Nd isotopic compositions for cpx

Simon, Nina

13

Seismic anisotropy, lithospheric deformation, and mantle flow in subduction zones, continental keels, and the core-mantle boundary  

NASA Astrophysics Data System (ADS)

This thesis consists of four chapters that evaluate the location, orientation, and strength of seismic anisotropy in the lithosphere and mantle in several tectonic settings, including two western Pacific subduction zones, the tectonically stable region of eastern North America, and two regions of the core-mantle boundary beneath the Pacific Ocean. The analyses in these chapters not only utilize existing methods, but also develop new, innovative techniques to determine and investigate patterns of shear wave splitting. Beneath northwest Pacific subduction zones, we found evidence for seismic anisotropy to depths as great as 410 km in some regions. In addition, we determined that fast directions are roughly parallel to the direction of absolute Pacific plate motion beneath Izu-Bonin, roughly parallel to the strike of the trench near Japan, and roughly parallel to the direction of transpressional shear in the southern Kurils near Sakhalin Island. In the Marianas subduction zone, we found strong evidence of frequency dependence in fast directions from phases that sample the northwestern portion of the subducting slab, but did not find evidence for frequency dependence in splitting times. All of the data can be explained by models containing anisotropy in the subducting slab and mantle wedge, and possibly anisotropy in the overriding Philippine Sea plate. Beneath eastern North America, we performed measurements of shear wave splitting and combined them with the results of a simple finite-difference model to examine mantle flow around a realistic continental keel. Using this model, we calculated predicted shear wave splitting produced in the mantle resulting from flow around and beneath the keel. We found that splitting produced by modified mantle flow can explain most, but not all, of the shear wave splitting observations in this region. Beneath the central and northern Pacific Oceans, we found evidence for seismic anisotropy within the lowermost mantle (D? ). Beneath the northern Pacific, this anisotropy may be due to lateral flow along the core-mantle boundary induced by lateral spreading of downwelling paleoslab material. Beneath the central Pacific, anisotropy may be due to lateral flow of lower mantle material toward the Hawai'ian plume source.

Fouch, Matthew James

14

Blueschist preservation in a retrograded, high-pressure, low-temperature metamorphic terrane, Tinos, Greece: Implications for fluid flow paths in subduction zones  

NASA Astrophysics Data System (ADS)

The preservation of high-pressure, low-temperature (HP-LT) mineral assemblages adjacent to marble unit contacts on the Cycladic island of Tinos in Greece was investigated using a new type of digital outcrop mapping and numerical modeling of metamorphic fluid infiltration. Mineral assemblage distributions in a large blueschist outcrop, adjacent to the basal contact of a 150-meter thick marble horizon, were mapped at centimeter-scale resolution onto digital photographs using a belt-worn computer and graphics editing software. Digital mapping reveals that while most HP-LT rocks in the outcrop were pervasively retrograded to greenschist facies, the marble-blueschist contact zone underwent an even more intense retrogression. Preservation of HP-LT mineral assemblages was mainly restricted to a 10-15 meter zone (or enclave) adjacent to the intensely retrograded lithologic contact. The degree and distribution of the retrograde overprint suggests that pervasively infiltrating fluids were channelized into the marble-blueschist contact and associated veins and flowed around the preserved HP-LT enclave. Numerical modeling of Darcian flow, based on the field observations, suggests that near the marble horizon, deflections in fluid flow paths caused by flow channelization along the high-permeability marble-blueschist contact zone likely resulted in very large fluid fluxes along the lithologic contact and significantly smaller fluxes (as much as 8 times smaller than the input flux) within the narrow, low-flux regions where HP-LT minerals were preserved adjacent to the contact. Our results indicate that lithologic contacts are important conduits for metamorphic fluid flow in subduction zones. Channelization of retrograde fluids into these discrete flow conduits played a critical role in the preservation of HP-LT assemblages.

Breeding, Christopher M.; Ague, Jay J.; BröCker, Michael; Bolton, Edward W.

2003-01-01

15

Evolution of a Subduction Zone  

NASA Astrophysics Data System (ADS)

The purpose of this study is to understand how Earth's surface might have evolved with time and to examine in a more general way the initiation and continuance of subduction zones and the possible formation of continents on an Earth-like planet. Plate tectonics and continents seem to influence the likelihood of a planet to harbour life, and both are strongly influenced by the planetary interior (e.g. mantle temperature and rheology) and surface conditions (e.g. stabilizing effect of continents, atmospheric temperature), but may also depend on the biosphere. Employing the Fortran convection code CHIC (developed at the Royal Observatory of Belgium), we simulate a subduction zone with a pre-defined weak zone (between oceanic and continental crust) and a fixed plate velocity for the subducting oceanic plate (Quinquis et al. in preparation). In our study we first investigate the main factors that influence the subduction process. We simulate the subduction of an oceanic plate beneath a continental plate (Noack et al., 2013). The crust is separated into an upper crust and a lower crust. We apply mixed Newtonian/non-Newtonian rheology and vary the parameters that are most likely to influence the subduction of the ocanic plate, as for example density of the crust/mantle, surface temperature, plate velocity and subduction angle. The second part of our study concentrates on the long-term evolution of a subduction zone. Even though we model only the upper mantle (until a depth of 670km), the subducted crust is allowed to flow into the lower mantle, where it is no longer subject to our investigation. This way we can model the subduction zone over long time spans, for which we assume a continuous inflow of the oceanic plate into the investigated domain. We include variations in mantle temperatures (via secular cooling and decay of radioactive heat sources) and dehydration of silicates (leading to stiffening of the material). We investigate how the mantle environment influences the subduction of the oceanic crust in terms of subduction velocity and subduction angle over time. We develop scaling laws combining the subduction velocity and angle depending on the mantle environment (and thus time). These laws can then be applied to continental growth simulations with 1D parameterized models (Höning et al., in press) or 2D/3D subduction zone simulations at specific geological times (using the correct subduction zone setting). References: Quinquis, M. et al. (in preparation). A comparison of thermo-mechanical subduction models. In preparation for G3. Noack, L., Van Hoolst, T., Dehant, V., and Breuer, D. (2013). Relevance of continents for habitability and self-consistent formation of continents on early Earth. XIII International Workshop on Modelling of Mantle and Lithosphere Dynamics, Hønefoss, Norway, 31. Aug. - 5. Sept. 2013. Höning, D., Hansen-Goos, H., Airo, A., and Spohn, T. (in press). Biotic vs. abiotic Earth: A model for mantle hydration and continental coverage. Planetary and Space Science.

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

2014-05-01

16

Metamorphic PT conditions and thermal structure of Chinese Continental Scientific Drilling main hole eclogites: Fe-Mg partitioning thermometer vs. Zr-in-rutile thermometer  

Microsoft Academic Search

Core rocks recovered from the main hole (5158 m deep) of the Chinese Continental Scientific Drilling (CCSD-MH) project, southern Sulu UHP terrane, east-central China, consist of eclogites, various gneisses and minor metaperidotite cumulates; this lithological section underwent subduction-zone UHP metamorphism. Coesite-bearing eclogites are mainly present between the depths of 100-2000 m, but below 2000 m, mafic eclogites are rare. Selected

R. Y. ZHANG; Y. IIZUKA; W. G. ERNST; J. G. LIOU; Z.-Q. XU; T. TSUJIMORI; C.-H. LO; B.-M. JAHN

2009-01-01

17

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

E-print Network

and shortterm SSE in the subduction zones of Japan and Cascadia supported mainly the hypothesisThe 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

Clayton, Robert W.

18

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

E-print Network

and short term SSE in the subduction zones of Japan and Cascadia supported mainly the hypothesisThe 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

Shapiro, Nikolai

19

Cascadia Subduction Zone  

USGS Publications Warehouse

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

Frankel, Arthur D.; Petersen, Mark D.

2008-01-01

20

Pore fluid pressures, porosity, and permeability of the Cascadia subduction zone plate boundary  

Microsoft Academic Search

The strength and seismogenic behavior of subduction zone plate boundaries depend critically on pore fluid pressure. Integrated over time, large amounts of H2O are released from subducting plates by tectonic compaction at shallow depths and by metamorphic dehydration reactions at deeper depths. In the Cascadia subduction zone, beneath southern Vancouver Island, converted teleseismic waves reveal anomalously high Poisson's ratios (average

S. M. Peacock; N. I. Christensen; M. G. Bostock; P. Audet

2009-01-01

21

Blueschist preservation in a retrograded, high-pressure, low-temperature metamorphic terrane, Tinos, Greece: Implications for fluid flow paths in subduction zones  

Microsoft Academic Search

The preservation of high-pressure, low-temperature (HP-LT) mineral assemblages adjacent to marble unit contacts on the Cycladic island of Tinos in Greece was investigated using a new type of digital outcrop mapping and numerical modeling of metamorphic fluid infiltration. Mineral assemblage distributions in a large blueschist outcrop, adjacent to the basal contact of a 150-meter thick marble horizon, were mapped at

Christopher M. Breeding; Jay J. Ague; Michael Bröcker; Edward W. Bolton

2003-01-01

22

Tectonic history of subduction zones inferred from retrograde blueschist P-T paths  

SciTech Connect

Many Phanerozoic convergent plate junctions are marked by discontinuous blueschist belts, reflecting relatively high-pressure (P) prograde trajectories. Common blueschist paragneisses, such as those of the western Alps, exhibit widespread overprinting by greenschist and/or epidote-amphibolite facies assemblages. For this type of high-P belt, retrograde metamorphism involved fairly rapid, nearly isothermal decompression; some terranes underwent continued heating during early stages of pressure release. Uplift probably occurred as a consequence of the entrance of an island arc, oceanic plateau, or segment of continental crust into the subduction zone (collision), resulting in marked deceleration or cessation of underflow and buoyant, approximately adiabatic rise of the stranded, recrystallized subduction complex. Other high-P belts, such as the Franciscan of western California, preserve metamorphic aragonite and lack a low-P overprint; retrogression approximately retraced the prograde P-T (temperature) path, or for early formed high-grade blocks, occurred at even higher P/T ratios. Parts of this type of metamorphic belt evidently migrated slowly back up the subduction zone in response to isostatic forces during continued plate descent and refrigeration. Upward motion took place as tectonically imbricated slices, as laminar return flow in melange zones, and perhaps partly a lateral spreading/extension of the underplated accretionary prism. Retrograde P-T trajectories of high-P belts therefore provide important constraints on the tectonic evolution of convergent plate junctions.

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

1988-12-01

23

Ultramafic cumulates of oceanic affinity in an intracontinental subduction zone: UHP garnet peridotites from Pohorje (Eastern  

E-print Network

Ultramafic cumulates of oceanic affinity in an intracontinental subduction zone: UHP garnet depleted mantle of oceanic affinity. The metamorphic assemblage of the garnet peridotites consists similar to those of associated Pohorje eclogites of predominantly MORB affinity. The shallow level

24

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

E-print Network

Possible emplacement of crustal rocks into the forearc mantle of the Cascadia Subduction Zone as a lower crustal shear zone above the subducting Juan de Fuca plate, extends into the upper mantle of the subduction zone, or (3) rocks from the lower continental crust that have been transported into the uppermost

Ramachandran, Kumar

25

Subduction zones Melt formation and crustal growth  

E-print Network

Subduction zones Melt formation and crustal growth #12;Ocean-ocean Island Arc Ocean Accretion -collision tectonics Lateral accretion at subduction zones has played a major role in the growth transfer between different geochemical reservoirs · Volcanic hazards Subduction Zone Magmatism #12;Factors

Siebel, Wolfgang

26

Earthquake hazards on the cascadia subduction zone  

Microsoft Academic Search

Large subduction earthquakes on the Cascadia subduction zone pose a potential seismic hazard. Very young oceanic lithosphere (10 million years old) is being subducted beneath North America at a rate of approximately 4 centimeters per year. The Cascadia subduction zone shares many characteristics with subduction zones in southern Chile, southwestern Japan, and Colombia, where comparably young oceanic lithosphere is also

T. H. Heaton; S. H. Hartzell

1987-01-01

27

Comparison of seismicity parameters in different subduction zones and its implications for the Cascadia subduction zone  

Microsoft Academic Search

The seismic potential associated with the Cascadia subduction zone is investigated by examining and comparing the number of shallow earthquakes with magnitude greater than 6 which have occurred in different subduction zones since 1910. It is shown that all the subduction zones exhibit a higher level of activity than Cascadia. Two of these zones with similar rates of plate motion

Hemendra Acharya

1992-01-01

28

Scientific teams analyze earthquake hazards of the Cascadia Subduction Zone  

Microsoft Academic Search

Scientists from GEOMAR, the U.S. Geological Survey (USGS), and Oregon State University recently collected seismic reflection and wide-angle seismic data for the continental margin off the Oregon and Washington coasts---a subduction zone that poses a significant earthquake hazard to populated areas of the Pacific Northwest. Geologic findings indicate that great earthquakes (magnitude 8 to 9) have occurred within the Cascadia

Ernst Flueh; Michael Fisher; David Scholl; Tom Parsons; Uri Ten Brink; Dirk Klaeschen; Nina Kukowski; Anne Trehu; Jonathan Childs; Joerg Bialas; Neus Vidal

1997-01-01

29

Earthquake hazards on the Cascadia subduction zone  

Microsoft Academic Search

Large subjection earthquakes on the Cascadia subduction zone pose a potential seismic hazard. Very young oceanic lithosphere (10 million years old) is being subducted beneath North America at a rate of approximately 4 cm per year. Very large subduction earthquakes, ranging in energy magnitude M(w) between 8 and 9.5 have occurred along these other subduction zones. If the Cascadia subjection

Thomas H. Heaton; Stephen H. Hartzell

1987-01-01

30

Density model of the Cascadia subduction zone  

USGS Publications Warehouse

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.

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

2001-01-01

31

Central Cascadia subduction zone creep  

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

32

Observe an animation of volcanism at a subduction zone  

NSDL National Science Digital Library

This Earth science resource features an animation of the volcanic activity that occurs when an oceanic plate subducts under a continental plate. An introductory paragraph provides background information about the events depicted in the animation. These events include the creation and eruption of magma and the formation of volcanic mountains at a subduction zone. Key features such as the asthenosphere are labeled at the beginning of the animation, and arrows indicate the direction of the oceanic plate's movement. Students are able to use movie control buttons to play, pause, and scroll frame by frame through the animation. Copyright 2005 Eisenhower National Clearinghouse

Education, Terc. C.; Littell, Mcdougal

2003-01-01

33

Earthquake hazards on the cascadia subduction zone.  

PubMed

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

Heaton, T H; Hartzell, S H

1987-04-10

34

Seismicity, metamorphism and rheology of the lower continental crust  

NASA Astrophysics Data System (ADS)

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.

Austrheim, Håkon

2014-05-01

35

Origin and metamorphism of ultrabasic rocks associated with a subducted continental margin, Naxos (Cyclades, Greece)  

Microsoft Academic Search

Meta-peridotites outcropping at diVerent structural levels within the Alpine metamorphic complex of the Cycladic island of Naxos were studied to re-examine their metamorphic evolution and possible tectonic mechanisms for emplacement of mantle material into the continental crust. The continental margin section exposed on Naxos, consisting of pre-Alpine basement and c. 7 km thick Mesozoic platform cover, has undergone intense metamorphism

Y. K ATZIR; D. AV; A. M ATTHEWS; Z. G ARFUNKEL; B. W. E VANS

36

Ancient subduction zone in Sakhalin Island  

NASA Astrophysics Data System (ADS)

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.

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

2013-07-01

37

Nonvolcanic tremors in the Mexican subduction zone  

Microsoft Academic Search

Nonvolcanic low frequency tremors (NVT) have been discovered and studied recently in Japan and Cascadia subduction zones and deep beneath the San Andreas Fault. The tremors activity is increasing during so-called silent earthquakes (SQ) in Japan and Cascadia. NVT clusters also migrate following the propagation of the SQ. The origin of the NVT is still unclear. The studies of NVT

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

2007-01-01

38

Earthquake Dynamics in the Greater Antilles Subduction Zone  

Microsoft Academic Search

The Greater Antilles subduction zone lies north of the densely populated islands of Hispaniola, Puerto Rico, and the Virgin Islands. A large earthquake on this subduction zone could cause significant structural damage and human suffering, and could furthermore trigger a large tsunami with equally devastating effects. The near-shore hanging wall of the subduction zone is cut by at least two

D. D. Oglesby; E. L. Geist

2009-01-01

39

Fluid\\/rock interaction and mass transfer in continental subduction zones: constraints from trace elements and isotopes (Li, B, O, Sr, Nd, Pb) in UHP rocks from the Chinese Continental Scientific Drilling Program, Sulu, East China  

Microsoft Academic Search

In order to better understand the role of fluids during subduction and subsequent exhumation, we have investigated whole-rock\\u000a and mineral chemistry (major and trace elements) and Li, B as well as O, Sr, Nd, Pb isotopes on selected continuous drill-core\\u000a profiles through contrasting lithological boundaries from the Chinese Continental Scientific Drilling Program (CCSD) in Sulu,\\u000a China. Four carefully selected sample

Yilin Xiao; Jochen Hoefs; Zhenhui Hou; Klaus Simon; Zeming Zhang

2011-01-01

40

Subduction-zone cycling of nitrogen in serpentinized mantle rocks  

NASA Astrophysics Data System (ADS)

Nitrogen (N) has shown great potential as a geochemical tracer of volatiles recycling, in part because of large differences in the N isotope composition of the various Earth reservoirs. The subduction flux of N in serpentinized oceanic mantle could be as important as N input flux in oceanic crust and even sediment because, although its N concentrations are lower, its volume is potentially far greater than that of the crust/sediment. However, recycling of oceanic mantle rocks is still poorly constrained for the N cycle, and N isotope data for subduction-related ultramafic rocks are scarce [1]. The primary goal of this study is to characterize the subduction flux of N in subducting altered oceanic mantle by documenting concentrations and isotopic compositions of N in mantle rocks that reflect different stages of the metamorphic subduction zone cycle. The results are crucial to assess the composition of N recycled into the mantle, to determine the extent to which N can be retained in subducted mantle rocks to depths approaching those beneath arcs, and to balance N subduction-zone inputs with outputs in arc volcanic gases. Moreover, information has been gained regarding the redistribution and isotope fractionation of N via ultramafic dehydration and metamorphic fluid-rock interaction. The samples analyzed in this study are ultramafic rocks from shallow oceanic environments to increasing P-T conditions up to depths of ~70 km. Three distinct metamorphic grades, reflecting seafloor fluid uptake, water release due to brucite breakdown and the final antigorite breakdown, were investigated: 1. Pre-subduction serpentinized mantle peridotite from non-subducted ophiolite sequences from the Northern Apennines, Italy (Monte Nero). 2. Eclogite-facies antigorite serpentinites from the Ligurian Alps, Italy (Erro Tobbio). 3. Eclogite-facies chlorite harzburgites derived from dehydration of serpentinites from the Betic Cordillera, Spain (Cerro de Almirez). The pre-subduction peridotites have the lowest N concentrations (1.3-2.1 ppm) and highly variable ?15N values (-4 to +3). High-pressure peridotites have higher N contents (up to 20 ppm) and mostly positive ?15N values. Data for veins in the peridotites are suggestive that N isotopes were not significantly fractionated by dehydration. Hence, N recycled through subduction zones via peridotites should show a range in ?15N similar to that of oceanic serpentinized peridotites. The range in ?15N for serpentinized peridotites, from pristine mantle values towards those typical of modern marine sediments, suggests derivation of N from organic-sedimentary sources, incorporated during bending-related faulting of the subducting slab and/or via metasomatic additions during subduction. N is enriched in serpentinized peridotites compared to pristine mantle rocks, and it is retained down to depths of at least 70 km and possibly deep into the mantle. Hence, serpentinized peridotites appear to represent an important reservoir for deep subduction zone N cycling. [1] Philippot et al. (2007), Min. Pet. 91:11-24

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

2010-12-01

41

Strain accumulation along the Cascadia subduction zone  

Microsoft Academic Search

We combine triangulation, trilateration, and GPS observations to determine horizontal strain rates along the Cascadia subduction zone from Cape Mendocino to the Strait of Juan de Fuca. Shear-strain rates are significantly greater than zero (95% confidence) in all forearc regions (26-167 nanoradians\\/yr), and are not significant in the arc and backarc regions. The deformation is primarily uniaxial contraction nearly parallel

Mark H. Murray; Michael Lisowski

2000-01-01

42

The Seismic Coupling of Subduction Zones Revisited  

NASA Astrophysics Data System (ADS)

The nature of seismic coupling for many of the world's subduction zones has been reevaluated. Geodetic estimates of seismic coupling obtained from GPS measurements of upper plate deformation during the interseismic period are summarized. We compared those with new estimates of seismic coupling obtained from seismological data. The results show that with a few notable exceptions the results using the two methods agree to within about 10%. The seismological estimates have been greatly improved over those made 20-30 years ago because of an abundance of paleoseismological data that greatly extend the temporal record of great subduction earthquakes and by the occurrence, in the intervening years, of an unusual number of great and giant earthquakes that have filled in some of the most critical holes in the seismic record. The data also, again with a few notable exceptions, support the frictional instability theory of seismic coupling, and in particular, the test of that theory made by Scholz and Campos [1995]. Overall, the results support their prediction that high coupling occurs for subduction zones subjected to high normal forces with a switch to low coupling occurring fairly abruptly as the normal force decreases below a critical value. There is also considerable variation of coupling within individual subduction zones. Earthquake asperities correlate with areas of high coupling and hence have a semblance of permanence, but the rupture zones and asperity distributions of great earthquakes may differ greatly between seismic cycles because of differences in the phase of seismic flux accumulation.

Scholz, C.; Campos, J.

2012-04-01

43

The seismic coupling of subduction zones revisited  

NASA Astrophysics Data System (ADS)

The nature of seismic coupling for many of the world's subduction zones has been reevaluated. Geodetic estimates of seismic coupling obtained from GPS measurements of upper plate deformation during the interseismic period are summarized. We compared those with new estimates of seismic coupling obtained from seismological data. The results show that with a few notable exceptions the two methods agree to within about 10%. The seismological estimates have been greatly improved over those made 20-30 years ago because of an abundance of paleoseismological data that greatly extend the temporal record of great subduction earthquakes and by the occurrence, in the intervening years, of an unusual number of great and giant earthquakes that have filled in some of the most critical holes in the seismic record. The data also, again with a few notable exceptions, support the frictional instability theory of seismic coupling, and in particular, the test of that theory made by Scholz and Campos (1995). Overall, the results support their prediction that high coupling occurs for subduction zones subjected to high normal forces with a switch to low coupling occurring fairly abruptly as the normal force decreases below a critical value. There is also considerable variation of coupling within individual subduction zones. Earthquake asperities correlate with areas of high coupling and hence have a semblance of permanence, but the rupture zones and asperity distributions of great earthquakes may differ greatly between seismic cycles because of differences in the phase of seismic flux accumulation.

Scholz, Christopher H.; Campos, Jaime

2012-05-01

44

Fluid flow and metasomatism in a subduction zone hydrothermal system: Catalina schist terrane, California  

Microsoft Academic Search

On Santa Catalina Island, southern California, bluechist to amphibolite facies metasedimentary, metamafic, and meta-ultramafic rocks show veining and alteration that reflect fluid flow and mass transfer at 25-45 km depths in an Early Cretaceous subduction zone. Synkinematic and postkinematic veins record fluid transport and metasomatism during prograde metamorphism and uplift. Vein and host-rock mineralogy and whole-rock compositions demonstrate large-scale chemical

M. D. Barton; G. E. Bebout

1989-01-01

45

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

NASA Astrophysics Data System (ADS)

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 (Ol 1) and orthopyroxene in the harzburgite are compositionally similar to present-day abyssal peridotites. The kyanite-eclogite is derived from a troctolitic protolith, whereas the epidote-eclogite from a gabbroic protolith, both having distinct positive Eu anomalies, low TiO 2, and high Al 2O 3 and MgO. The kyanite-eclogite shows inherited cumulate layering. The phengite-eclogite has high TiO 2, low Al 2O 3 and MgO with incompatible trace elements resembling enriched-type MORB. Sr-Nd isotope data indicate that the protoliths of both kyanite-eclogite and epidote-eclogite ([ 87Sr/ 86Sr] i ~ 0.703-0.704; ?Nd( T) ~ 5.9-8.0) are of mantle origin (e.g., ocean crust signatures). On the other hand, while the lower ?Nd( T) value (1.4-4.1) of phengite-eclogite is more or less consistent with an enriched MORB protolith, their high [ 87Sr/ 86Sr] i ratio (0.705-0.716) points to an additional enrichment in their history, probably in an subduction-zone environment. Field relations and geochemical analyses suggest that the serpentinized harzburgite and the three types of eclogite constitute the oceanic lithological section of an ophiolitic sequence from mantle peridotite, to cumulate, and to upper basaltic rocks. The presence of coesite pseudomorphs and quartz exsolution in omphacite plus thermobarometric calculations suggests that the eclogites have undergone ultrahigh pressure metamorphism (i.e., peak P ? 2.7 GPa). The harzburgite may also have experienced the same metamorphism, but the lack of garnet suggests that the pressure conditions of ? 3.0 GPa. Zircon U-Pb SHRIMP dating shows that the eclogites have a protolith age of 516 ± 8 Ma and a metamorphic age of 445 ± 7 Ma. These data indicate the presence of a Paleo-Qilian Ocean between Qaidam and Qilian blocks before the early Ordovician. The ophiolitic assemblage may be the relics of subducted oceanic crust prior to the subduction of continental materials during Ordovician-Silurian times and ultimate continent collision. These rocks, altogether, record a complete history of ocean crust subduction, to continental subduction, and to continental collision.

Zhang, Guibin; Song, Shuguang; Zhang, Lifei; Niu, Yaoling

2008-08-01

46

Satellite magnetic anomalies over subduction zones - The Aleutian Arc anomaly  

NASA Technical Reports Server (NTRS)

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

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

1985-01-01

47

Opening and closing slab windows in congested subduction zones  

NASA Astrophysics Data System (ADS)

Subduction zones often try to swallow buoyant material which is embedded in the oceanic lithosphere: plume material or hotspot residues, oceanic plateaux, and fragments of continental material. This often results in the formation of a slab window and it has been shown (Mason et al, 2010; Betts et al, 2012) that this window strongly influences the subsequent evolution of the slab and the advance/retreat rate of the trench. The buoyant material typically pushes the trench into a local state of advance, and the creation of the slab window allows the rest of the trench to retreat as the mantle behind the slab flows in through the window. This situation is inherently unstable: if the buoyancy anomaly is finite in size, then the retreating trench will soon move behind the anomaly and juxtapose negatively buoyant oceanic lithosphere with active subduction. This creates the potential to close the slab window and, in doing so, transfer the buoyant material to the over-riding plate. Models show that this closure of the window initially occurs through a lateral rollback process followed by a catastrophic re-initiation of subduction behind the colliding buoyant anomaly. This rollback leaves a characteristic, tightly rolled remnant in the mantle and significant rotation in the over-riding plate and the newly-docked block. The over-riding plate is thrown into extension perpendicular to the original orientation of the trench. This same situation applies at the late-stages of a closing ocean due to the passive margin geometry and the presence of debris collected from the closing ocean floor and it seems likely that these models can also be applied to the complicated geometry of subduction in such environments. Mason, W. G.; Moresi, L.; Betts, P. G. & Miller, M. S. Three-dimensional numerical models of the influence of a buoyant oceanic plateau on subduction zones Tectonophysics, 2010, 483, 71-79 P. Betts, W. Mason, L. Moresi, The influence of mantle plumes on subduction zone dynamics, Geology, 40, 739-742 (2012)

Moresi, Louis

2013-04-01

48

Acceleration spectra for subduction zone earthquakes  

USGS Publications Warehouse

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

Boatwright, J.; Choy, G.L.

1989-01-01

49

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

Microsoft Academic Search

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

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

2009-01-01

50

Fluid flow in ocean crust cools the Cascadia subduction zone  

Microsoft Academic Search

Temperatures along subduction zone plate boundary faults have been used to estimate the area and extent of the seismogenic zone. Recent studies of the well-constrained Nankai margin of Japan show that hydrothermal circulation in the subducting crust cools the subduction zone and widens the area of the plate boundary fault that is between the key temperatures of 150 and 350

B. D. Cozzens; G. A. Spinelli

2010-01-01

51

HYDRODYNAMIC SIMULATIONS OF HISTORIC TSUNAMIS FROM THE CASCADIA SUBDUCTION ZONE  

E-print Network

We use new geological and hydrodynamic models to simulate the sea floor deformation and tsunami runup and inundation in Cannon Beach, Oregon (Fig. 1) produced by Cascadia subduction zone (CSZ) earthquakes. We then compare the simulated inundation with inland extent of buried sand sheets inferred to be of tsunami origin. The CSZ exhibits similarities to other subduction zones

Y. Joseph Zhang Antonio Baptista

2007-01-01

52

Understanding Seismotectonic Aspects of Central and South American Subduction Zones  

Microsoft Academic Search

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

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

2004-01-01

53

Transportation of H 2O and melting in subduction zones  

Microsoft Academic Search

Material recycling in subduction zones, including the generation and migration of aqueous fluids and melts, is key to understanding the origin of volcanism in subduction zones and is also important for understanding the global circulation of materials. Recent knowledge concerning the phase relationships of hydrous peridotitic and basaltic systems allows us to model the fluid generation and migration in subduction

Hikaru Iwamori

1998-01-01

54

Interseismic coupling and asperity distribution along the Kamchatka subduction zone  

E-print Network

Interseismic coupling and asperity distribution along the Kamchatka subduction zone Roland Bu for the distribution of aseismic slip rate along the $15°NW dipping underthrust suggest a nonslipping plate interface anomalies over offshore forearc basins, while less coupled portions of the Kamchatka subduction zone

Hilley, George

55

The earthquake cycle in subduction zones  

NASA Technical Reports Server (NTRS)

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.

Melosh, H. J.; Fleitout, L.

1982-01-01

56

Strain accumulation along the Cascadia subduction zone  

USGS Publications Warehouse

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

Murray, M. H.; Lisowski, M.

2000-01-01

57

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

Microsoft Academic Search

The dynamic hydrogeology of subduction zones makes them important regions for geochemical cycling between the major reservoirs: seawater, oceanic crust, continental crust, and the mantle. The distillation and loss of some volatiles and fluid-soluble elements from the shallow slab not only affect reactions and processes within the seismogenic zone, but they also support the deep biosphere and play a central

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

2010-01-01

58

Permeability anisotropy of serpentinite and fluid migration in subduction zones  

NASA Astrophysics Data System (ADS)

Subduction zones are the place where water is transported into the Earth's interior and causes arc volcanism and seismic activities. Subducting slabs release most of the water to the mantle wedge by the dehydration reactions, and the expelled water reacts with mantle rocks, forming serpentinite at the plate interface. The existence of hydrous layer has been detected by low- velocity anomaly and high-Poison's ratio in several subduction zones (Kamiya and Kobayashi 2000 ; Brocher et al. 2003). The migration of water is generally considered to move upward by buoyancy in the mantle. However, if the hydrous layer is extensively deformed, the migration of water can be controlled by the deformation plane within such layer. In order to test this hypothesis, we analyzed the permeability anisotropy of serpentinite with a strongly-developed schistosity and discuss fluid migration in the subduction systems. Serpentinite samples were collected from Nishisonogi metamorphic terrane in Nagasaki, which schistosity is well-defined developed. Two types of experimental samples were prepared: one is parallel to schistosity and the other is perpendicular. We used intra-vessel deformation and fluid- flow apparatus (IVA) in Hiroshima University to measure the permeability. In this study, we measured gas permeability using nitrogen gas and water permeability under isotropic pressure. Gas permeability was measured using the constant flow method, and water permeability was similar to gas and the transient pulse method was also used. The experiments were conducted at confining pressures up to 50 MPa, pore pressures up to 8 MPa at room temperature. We converted gas permeability to intrinsic permeability with Klinkenberg effect. The permeability decreased with increasing confining pressure, and intrinsic permeability of samples parallel to schistosity were about 10^-20 m2 at confining pressure of 50 MPa. We observed two types of pressure effect: one is significant decline due to crack filling at low-pressure and the other is a gradual decline due to crystal grain boundary consolidation at high-pressure. Intrinsic permeability for sample perpendicular to schistosity was about 100 times lower than that parallel to schistosity. Porosity at atmospheric pressure was estimated about 0.5%. Assumeing a constant pressure derivative for porosity and permeability, flow velocity parallel to the foliation was 44cm/year. This result represent that fluid migration is much faster than the plate subducting rate. These experimental data show that fluid migration was influenced by not only water buoyancy but also by the schistosity of the rock (deformation geometry). In this case, released water from subducting oceanic slab can be migrated along plate interfaces.

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

2010-12-01

59

The dynamics of reactive fluid escape in subduction zones  

NASA Astrophysics Data System (ADS)

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.

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

2014-05-01

60

Exhumation in the Chugach-Kenai Mountain Belt above the Aleutian Subduction Zone, southern Alaska  

NASA Astrophysics Data System (ADS)

Convergent deformation systems in continental backstops are a common component of well-coupled subduction zones worldwide. The Aleutian megathrust offshore southern Alaska has many attributes in common with convergent subduction zones observed elsewhere, implying that significant permanent shortening could occur within the continental hanging wall. A continuous belt of rugged mountains occurs along the forearc of this subduction zone, from the Kenai Peninsula to the eastern Chugach Mountains near the Copper River, providing further evidence of active shortening. To test for long-term shortening and associated rock uplift along this forearc, we have analyzed bedrock samples from the Chugach and Kenai Mountains using low-temperature thermochronometry. Fourteen new apatite (U-Th)/He ages from this area are older than expected based on the rugged topography, and imply minimal exhumation in the late Cenozoic in response to shortening within the forearc. Ages on the leeward side of this mountain belt are ˜20-40 Ma and imply an average exhumation rate of ˜0.1 mm/a or less. Ages are younger along the coast (˜12-18 Ma), implying slightly more rapid exhumation or deeper incision relative to mean elevation, perhaps due to greater precipitation along the windward flank of the range. Younger ages (˜5 Ma) occur in the southeast near Cordova, and may result from local deformation associated with colliding and underthrusting of the Yakutat microplate. However, all of these cooling ages are older than ages from the Yakutat collision zone and along the transpressional Fairweather fault to the east, indicating that far less exhumation occurs in the backstop above the subduction zone. Based on these older ages and a mass balance interpretation of exhumation, we estimate that the long-term shortening in the forearc mountains above the Aleutian megathrust is less than 1 mm/a.

Buscher, Jamie T.; Berger, Aaron L.; Spotila, James A.

61

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

NASA Astrophysics Data System (ADS)

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.

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

1995-06-01

62

Mantle convection with strong subduction zones  

NASA Astrophysics Data System (ADS)

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.

Conrad, Clinton P.; Hager, Bradford H.

2001-02-01

63

Seismic Anisotropy Beneath the Sumatra Subduction Zone  

NASA Astrophysics Data System (ADS)

The Sumatra subduction zone is located on the eastern side of the Sunda Arc between the Sunda Strait and the Andaman Islands, where the Indo-Australian plate is subducting beneath the Eurasian plate. An important tool in understanding the style and geometry of deformation within a subduction zone is the measurement of seismic anisotropy, through observations of shear wave splitting, which provides information about the mantle flow. In Sumatra two temporary seismic networks were deployed within the Mentawai and Northern region, between December 2007 and October 2008 and April 2008 and February 2009, respectively. Here we use new splitting measurements from SKS and local S phases from these networks, to characterize the type and amount of anisotropy within the region. High quality SKS splitting results obtained from 16 stations show a coherent fast direction of NNW (350°) to SSE (170°) with delay times of 1.0 to 3.0s. The observations do not conform to the standard classification of trench parallel and trench perpendicular, instead they are trench oblique, approximately parallel to the direction of motion of the subducting Indo-Australian plate. The magnitude and direction of splitting suggests that the anisotropy is due too entrained flow in the asthenosphere beneath the subducting lithosphere. Local S wave splitting measurements were obtained from 85 rays, generated by earthquakes at focal depths of 15 km-200 km and recorded across 39 stations. The polarization of the fast shear wave is trench parallel along the islands and can be contributed to shape preferred orientation of cracks in the top of the subducting slab and overriding crust. In the forearc a rotation in fast direction to trench perpendicular is observed, with a clear positive correlation between the distance the ray has traveled in the mantle and delay time, suggesting anisotropy originates from entrained flow within the mantle wedge. In the Sumatra fault region and the back arc, events originating at the plate interface show both trench perpendicular and trench parallel fast directions with delay times of up to 0.4 s, while shallow events on the Sumatra fault indicate a clear fault parallel direction with delay times of 0.1 s to 0.18 s. This pattern of shear wave splitting suggests layers of different anisotropy, one deeper layer due to entrained flow within the Sumatran mantle wedge causing trench perpendicular direction, and the second shallower one within the overriding crust due to shape preferred orientation possibly caused by the Sumatran Fault.

Collings, R.; Rietbrock, A.; Nippress, S.; Lange, D.; Tilmann, F. J.; Natawidjaja, D.; Suwargadi, B.

2011-12-01

64

Shear stresses on megathrusts: Implications for mountain building behind subduction zones  

NASA Astrophysics Data System (ADS)

Shear stresses ? on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone. In this study, the temperatures and shear stresses on megathrusts in 11 subduction zones around the Pacific rim (Hikurangi, Tonga, Izu-Ogasawara, western Nankai, northeastern Japan, Aleutians, western Alaska, Cascadia, northern Chile, southern Chile) and SE Asia (northern Sumatra) have been determined. The main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses, in the plane of a trench- or arc-normal section. For a typical brittle and ductile megathrust rheology, frictional shear stress ? = ??gz, for depth z, and ductile shear stress ? = A exp (B/RT) at temperature T, where ?, A, B are rheological parameters treated as constants. Rheological constants common to all the megathrusts (?crust, ?mantle, B) are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure, using a simplex minimization algorithm, taking account of the induced mantle corner flow at depth (65 ± 15 km (2?)) and constant radiogenic heating (0.65 ± 0.3 ?W m-3 (2?)) throughout the crust. The A constants are solved individually for each subduction zone, assuming that the maximum depth of interplate slip earthquakes marks the brittle-ductile transition. The best fit solution shows two groupings of megathrusts, with most subduction zones having a low mean shear stress in the range 7-15 MPa (?crust = 0.032 ± 0.006, ?mantle = 0.019 ± 0.004) and unable to support elevations >2.5 km. For a typical frictional sliding coefficient ˜0.5, the low effective coefficients of friction suggest high pore fluid pressures at ˜95% lithostatic pressure. Tonga and northern Chile require higher shear stresses with ?crust = 0.095 ± 0.024, ?mantle = 0.026 ± 0.007, suggesting slightly lower pore fluid pressures, at ˜81% lithostatic. Ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency, with B = 36 ± 18 kJ mol-1. Amantle values are sensitive to the precise value of B but are in the range 1-20 kPa. The power law exponent n for mantle flow is poorly constrained but is likely to be large (n > 4). The brittle-ductile transition in the crust occurs at temperatures in the range 370°C-512°C, usually close to the base of the crust and in the mantle at much lower temperatures (180°C-300°C), possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties. In subduction zones where the subducted slab is older than 50 Ma, a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are ?300°C. In much younger subduction zones, the stress transmission is confined mainly to the crust. The shear stresses, particularly in the crust, may be kept low by some sort of lubricant such as abundant water-rich trench fill, which lowers the frictional sliding coefficient or effective viscosity and/or raises pore fluid pressure. The unusual high stress subduction zone in northern Chile lacks significant trench fill and may be poorly lubricated, with a mean shear stress ˜37 MPa required to support elevations >4 km in the high Andes. However, where the crust is thin in sediment-starved and poorly lubricated subduction zones, such as Tonga, the mean shear stress will still be low. Sediment may lubricate megathrusts accommodating underthrusting of continental crust, such as in the Himalayas or eastern central Andes, which have a low mean shear stress ˜15 MPa.

Lamb, Simon

2006-07-01

65

Seismotectonics of Central Mexico Subduction Zone From Crustal Deformation Studies  

NASA Astrophysics Data System (ADS)

The Pacific coast of Central Mexico is one of the most tectonically active plate margins. Large subduction thrust earthquakes up to Mw 8.0 repeat every 40-60 years. Crustal deformation measurements reveal seismotectonic processes resulting from this plate convergence. Crustal deformation studies in the Mexican subduction zone began in 1992 with GPS campaigns in Jalisco. High precision leveling and tilt meter observations commenced in 1995, and the first continuous GPS site was installed in 1997. Based on these data we estimate a variation of interseismic steady state coupling (?) along most of the subduction zone. On the shallow (less than 25 km depth) seismogenic zone beneath the coast ? = 0.9. This highly coupled zone begins 20 - 25 km from the trench and extends downdip for about 50 km. It coincides with the rupture zones of large thrust earthquakes. Deeper inland, the plate interface is not freely slipping; instead the continental North American and oceanic Cocos plates are partially coupled, ? = 0.4 - 0.7. In Guerrero state, a partially coupled zone extends for more than 100 km. This anomalous width can be attributed to the specific subhorizontal geometry of the plate interface. Continuous GPS observations and available tide gauge data have elucidated a sequence of slow transient slips or silent quakes (SQ) on the partially coupled portion of plate interface in Guerrero and Oaxaca in 1972, 1979, 1996, 1998, 2001, and 2006. The duration of these slow events ranges between 6-12 months. The maximum equivalent magnitude is Mw 7.5 for 2001-2002 SQ and probably significantly surpasses this value in the 1972 SQ. A modeled slip on the interplate fault for 2001-2002 SQ is 10-20 cm. We estimate the SQ propagation of more than 600 km SE from central Guerrero (101° W) during 1972 and 2001-2002 SQ. The propagation rate is about 1-2 km/day. A relation between large earthquakes and SQ is not yet clear. It seems that the 1996 SQ in Guerrero was triggered by the 14 September 1995, Mw 7.3 thrust Copala earthquake. The 2001-2002 SQ is linked to the unusual 8 October 2002, Mw 5.9, shallow normal-faulting Coyuca earthquake. Recently we have observed a number of low frequency non volcanic tremors (NVT) occurring at the edge of the partially coupled zone and the subsequent free slipping plate interface. The NVT high activity and it relation with the SQ is still uncertain.

Kostoglodov, V.; Larson, K. M.; Franco-Sánchez, S. I.

2006-12-01

66

Controls on earthquake rupture and triggering mechanisms in subduction zones  

E-print Network

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 ...

Llenos, Andrea Lesley

2010-01-01

67

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

NASA Astrophysics Data System (ADS)

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.

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

2007-12-01

68

The Sulfur Cycle at Subduction Zones  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

69

Thermo-Petrologic Structure of Subduction Zones and Its Implications for Fluid Availability at Depth (Invited)  

NASA Astrophysics Data System (ADS)

Aqueous fluids are critical to a number of subduction zone processes, such as episodic tremor and slip (ETS), intraslab earthquakes, mantle wedge serpentinization, and melt generation for arc volcanism. The availability of fluids at depth depends largely on the breakdown of hydrous phases in the subducting slab, which is controlled by the thermal structure of the slab. The slab’s thermal structure depends primarily on the age of the slab but are also influenced by slab-driven mantle wedge flow. Geophysical observations, such as surface heat flow, indicate that the shallow part of the forearc mantle wedge does not participate in this flow and thus is decoupled from the subducting slab. Using 2-D steady-state thermo-petrologic models for 17 subduction zones, we show that a common maximum depth of decoupling (MDD) of 70-80 km can explain a wide range of geological and geophysical observations at subduction zones. We find that the nonlinear mantle wedge rheology gives rise to a sharp downdip change from decoupling to coupling and consequently a sharp transition in the flow and thermal fields in the wedge. In all the subduction zones investigated, a cold, stagnant mantle is predicted updip of the MDD, providing stable thermal conditions for serpentinization. By contrast, immediately downdip of the MDD, the hot flowing mantle maintains high enough temperatures for melt generation and promotes dehydration in the underlying slab. In young and warm slabs (< ~15 Ma), such as in Cascadia and Nankai, peak slab dehydration is predicted to occur beneath the cold forearc mantle wedge before the slab reaches the MDD, providing fluids for serpentinization and ETS. In older and colder slabs, such as in NE Japan and Hikurangi, slab dehydration is predicted to peak beneath the hot flowing mantle at 80-140 km depths, providing ample fluids for melt generation. These model predictions are consistent with the observed metamorphic, volcanic, and seismological characteristics of the subduction zones and allow us to make qualitative inferences on the availability of fluids from the dehydrating slab. To quantify the fluid availability and distribution, we plan to model the process of dehydration in the slab on the basis of the thermal modeling results. In the new models, the total amount of fluids liberated from each vertical column of the slab will be calculated based on assumed compositions of the lithologies of the slab. These models will also account for fluid consumption by the hydration of anhydrous parts of the respective slab columns.

Wada, I.; Wang, K.; Behn, M. D.; Shaw, A. M.

2010-12-01

70

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

NASA Astrophysics Data System (ADS)

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

Harmon, N.; Rychert, C.

2013-12-01

71

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

NASA Astrophysics Data System (ADS)

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.

Schellart, W. P.; Rawlinson, N.

2013-12-01

72

Seismicity Within the West Sumatra Subduction Zone  

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

73

On subduction zone earthquakes and the Pacific Northwest seismicity  

SciTech Connect

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

Chung, Dae H.

1991-12-01

74

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)

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.

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

2013-02-01

75

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

NASA Astrophysics Data System (ADS)

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

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

2012-11-01

76

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

NASA Astrophysics Data System (ADS)

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

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

2012-07-01

77

Earthquake Dynamics in the Greater Antilles Subduction Zone  

NASA Astrophysics Data System (ADS)

The Greater Antilles subduction zone lies north of the densely populated islands of Hispaniola, Puerto Rico, and the Virgin Islands. A large earthquake on this subduction zone could cause significant structural damage and human suffering, and could furthermore trigger a large tsunami with equally devastating effects. The near-shore hanging wall of the subduction zone is cut by at least two major left-lateral strike-slip faults (the Septentrional Fault and Bunce Fault), which may have earthquakes both independently and in conjunction with the main plate boundary thrust. Further complicating matters, the subduction zone has a roughly 23 degree change in strike at the eastern edge of Hispaniola. To better understand the earthquake and tsunami hazard in this region, we use the 3D finite element method to perform dynamic spontaneous rupture models on this complex fault system. We find that the regional stress field is resolved quite differently on the various thrust and strike-slip fault segments, leading to large differences in the ability of rupture to propagate on the different segments. Static stress differences and dynamic stress interactions lead to asymmetrical behavior between the fault segments. In particular, rupture propagation from the strike-slip faults to the plate boundary thrust seems unlikely, but rupture propagation from the plate boundary thrust to the strike-slip faults appears more possible. We will discuss these results in the context of potential tsunamis in the region.

Oglesby, D. D.; Geist, E. L.; ten Brink, U. S.

2009-12-01

78

Contrasting styles of deformation along the Sumatra subduction zone  

Microsoft Academic Search

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

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

2006-01-01

79

Examining Structural Control on Earthquake Rupture Directivity in Subduction Zones  

Microsoft Academic Search

Structural features associated with fore-arc basins may strongly influence the rupture processes in large subduction zone earthquakes. Previous studies (Wells et al., 2003; Song and Simons, 2003) demonstrated that a significant percentage of the seismic moment release is concentrated beneath the gravity lows resulting from fore-arc basins. To better determine the nature of this correlation and to examine its effect

A. L. Llenos; J. J. McGuire

2005-01-01

80

Radiocarbon test of earthquake magnitude at the Cascadia subduction zone  

Microsoft Academic Search

THE Cascadia subduction zone, which extends along the northern Pacific coast of North America, might produce earthquakes of magnitude 8 or 9 ('great' earthquakes) even though it has not done so during the past 200 years of European observation1-7. Much of the evidence for past Cascadia earthquakes comes from former meadows and forests that became tidal mudflats owing to abrupt

Brian F. Atwater; Minze Stuiver; David K. Yamaguchi

1991-01-01

81

The Cascadia subduction zone: Two contrasting models of lithospheric structure  

Microsoft Academic Search

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 (Figure 1). The goal

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

1998-01-01

82

Evidence for large earthquakes at the Cascadia subduction zone  

Microsoft Academic Search

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

John J. Clague

1997-01-01

83

The Cascadia Subduction Zone: two contrasting models of lithospheric structure  

USGS Publications Warehouse

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.

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

1998-01-01

84

Geodetic Constraints on Fault Coupling on the Cascadia Subduction Zone  

Microsoft Academic Search

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

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

2002-01-01

85

Great earthquakes of variable magnitude at the Cascadia subduction zone  

Microsoft Academic Search

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

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

2006-01-01

86

Interseismic coupling and asperity distribution along the Kamchatka subduction zone  

Microsoft Academic Search

GPS measurements of interseismic horizontal surface velocities reveal the degree of kinematic coupling of the plate boundary thrust along the Kamchatka subduction zone from about 51° to 57°N latitude. Inversions for the distribution of aseismic slip rate along the ?15°NW dipping underthrust suggest a nonslipping plate interface in southern Kamchatka above ?50 km depth, along the segment that ruptured in

Roland Bürgmann; Mikhail G. Kogan; Grigory M. Steblov; George Hilley; Vasily E. Levin; Edwin Apel

2005-01-01

87

Interseismic coupling and asperity distribution along the Kamchatka subduction zone  

Microsoft Academic Search

GPS measurements of interseismic horizontal surface velocities reveal the degree of kinematic coupling of the plate boundary thrust along the Kamchatka subduction zone from about 51° to 57°N latitude. Inversions for the distribution of aseismic slip rate along the ~15°NW dipping underthrust suggest a nonslipping plate interface in southern Kamchatka above ~50 km depth, along the segment that ruptured in

Roland Bürgmann; Mikhail G. Kogan; Grigory M. Steblov; George Hilley; Vasily E. Levin; Edwin Apel

2005-01-01

88

Subduction zone anisotropy beneath Corvallis, Oregon: A serpentinite skid mark of trench-parallel  

E-print Network

Subduction zone anisotropy beneath Corvallis, Oregon: A serpentinite skid mark of trench: Rheology--crust and lithosphere; KEYWORDS: anisotropy, Cascadia, subduction Citation: Park, J., H. Yuan, and V. Levin (2004), Subduction zone anisotropy beneath Corvallis, Oregon: A serpentinite skid mark

89

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

E-print Network

in the northern Cascadia subduction zone, J. Geophys. Res. ,and slip on the Cascadia subduction zone: the chatter ofsubduction zones and intraplate faults has increased significantly, including southwest Japan (Obara, 2002), Cascadia (

Walter, Jacob Ineman

2012-01-01

90

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

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

91

Limits on great earthquake size at subduction zones  

NASA Astrophysics Data System (ADS)

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

McCaffrey, R.

2012-12-01

92

The Cascadia Subduction Zone and Related Subduction Systems | 133 Theoretical mineralogy, density, seismic wavespeeds  

E-print Network

The Cascadia Subduction Zone and Related Subduction Systems | 133 Theoretical mineralogy, density, seismic wavespeeds and H2 O content of the Cascadia subduction zone, with implications for intermediate hazardous seismicity the length of Cascadia. Introduction Much about subduction zones remains unknown be

Hacker, Bradley R.

93

Paleoseismicity of the Southern End of the Cascadia Subduction Zone, Northwestern California  

E-print Network

Paleoseismicity of the Southern End of the Cascadia Subduction Zone, Northwestern California Abstract The southern end of the Cascadia subduction zone (CSZ) in northwestern California poses a high) and Atwater (1987), the earthquake and tsunami threat along the Cascadia subduction zone (CSZ) was classified

Keller, Ed

94

USGS OPEN FILE REPORT #: Intraslab Earthquakes | 1 Heat sources in subduction zones  

E-print Network

of the southern Cascadia subduction zone system. Figure 1 illustrates the resultant tempera- ture­depth curveUSGS OPEN FILE REPORT #: Intraslab Earthquakes | 1 Heat sources in subduction zones: implications@passion.isem.smu.edu Introduction The shallow (upper 30­50 kilometers) thermal structure of subduction zones is well

Southern Methodist University

95

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

Microsoft Academic Search

Coastline deformation resulting from great shallow thrust earthquakes can provide information concerning the paleoseismicity of a subduction zone and thus information on the nature of potential seismicity. The Cascadia subduction zone is different from most other subduction zones in that it has been quiescent with respect to great earthquakes for at least the past 200 yr. The Washington-Oregon coastline also

Donald O. West; Dennis R. McCrumb

1988-01-01

96

GPS constraints on 34 slow slip events within the Cascadia subduction zone, 19972005  

E-print Network

GPS constraints on 34 slow slip events within the Cascadia subduction zone, 1997­2005 W. Szeliga,1 slip events located throughout the Cascadia subduction zone from 1997 through 2005. Timing of transient the Cascadia subduction zone, 1997­2005, J. Geophys. Res., 113, B04404, doi:10.1029/2007JB004948. 1

Miller, M. Meghan

97

Statistical Analyses of Great Earthquake Recurrence along the Cascadia Subduction Zone  

E-print Network

Statistical Analyses of Great Earthquake Recurrence along the Cascadia Subduction Zone by Ram. (2012) interpreted a 10,000 year old sequence of deep sea turbidites at the Cascadia subduction zone by strong shaking during great Cascadia subduction zone (CSZ) megathrust earthquakes (moment magnitude M 9

Goldfinger, Chris

98

Turbidite Event History--Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone  

E-print Network

of the Cascadia Subduction Zone By Chris Goldfinger, C. Hans Nelson, Ann E. Morey, Joel E. Johnson, Jason R--Methods and implications for Holocene paleoseismicity of the Cascadia subduction zone: U.S. Geological Survey Professional ......................................................................................3 Cascadia Subduction Zone and Great Earthquake Potential

Goldfinger, Chris

99

PhD offer : title : Impact of incoming oceanic plate on subduction zone interplate coupling  

E-print Network

. At the Cascadia subduction zone, the state of the down going JdF plate is of particular interest as relatively megathrust [e.g., Hyndman, 1988]. Some of the water entering the Cascadia subduction zone is transported of the down going plate. To understand the contribution of water to subduction zone processes at Cascadia

Nicolas, Chamot-Rooke

100

Mantle Flow in the Rivera-Cocos Subduction Zone  

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

101

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

102

Assessing the Seismic Potential Hazard of the Makran Subduction Zone  

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

103

Relating seismic observables to fluid migration in subduction zones  

NASA Astrophysics Data System (ADS)

Seismic images provide quantitative information about the physical state of the mantle. In subduction zones, a number of recent high-density broadband seismic field experiments provide images and analyses of the down-dip changes to the slab surface, the subarc melting region of the mantle, and the cold forearc nose. However, the implications of the measured quantities (P and S velocities Vp and Vs, attenuation Qp and Qs, and measures of seismic anisotropy) for the quantities of geodynamic interest (such as temperature, melt, water content, and composition) are not unique. We are embarking on an effort to systematically improve our understanding of the interrelationships between seismic and geodynamic parameters, leveraging seismic data collected from dense arrays in several subduction zones. These data compare well with arc chemistry, revealing complementary patterns between chemistry and seismology along strike in Central America and down dip in the Marianas. The slab surface can be imaged through a variety of reflections and mode conversions, including receiver functions. These show a variety of characteristics at different depths. Many subduction zones exhibit a transition from high amplitude conversions at the shallow thrust zone, indicating a weak and probably over-pressured subduction channel, to a deeper region that can be characterized by subducting uneclogitized crust. Elsewhere and deeper, the effects of steep thermal gradients become more significant in seismic images. Lower than expected velocities within the slab suggest the presence of partly (10-20%) serpentinized subducting mantle, at least offshore Nicaragua, but that remains to be shown in most subduction zones. In the wedge, low seismic velocities and high shear attenuation (1/Qs) indicate elevated temperatures, consistent with those recorded in the compositions of arc basalts. In some subduction zones seismic velocities (e.g., Vp/Vs) may indicate the presence of melt or high water content, although quantifying such interpretations remains difficult due to uncertainties in the mechanism by which melt affects seismic properties. Comparisons of 1/Qs, Vp, and Vs have potential to discriminate between the effects of melt, temperature and composition. Measured H2O contents in arc melt inclusions constrain concentrations in the mantle source region that strengthen such inferences, and can be shown to lead to self-consistent temperatures and fluid contents in some subduction zones, for example confirming imaged variations between Nicaragua and Costa Rica. Thus, seismic images are beginning to show evidence for the places at which fluids leave the slab, and the pathways they take to volcanic arcs.

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

2011-12-01

104

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

NASA Astrophysics Data System (ADS)

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.

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

2012-04-01

105

S wave velocity structure of the northern Cascadia subduction zone  

Microsoft Academic Search

The shear velocity structure across the northern Cascadia subduction zone is examined using three-component broadband digital seismographs. Locally generated P-to-S conversions are analyzed to estimate the S velocity structure to upper mantle depths and to constrain the subduction geometry of the Juan de Fuca plate. The oceanic crust is at 47-53 km beneath central Vancouver Island, 60-65 km beneath Georgia

John F. Cassidy; Robert M. Ellis

1993-01-01

106

S wave velocity structure of the Northern Cascadia subduction zone  

Microsoft Academic Search

Teleseismic receiver functions from an array of portable broadband seismograph stations located in southwestern British Columbia are interpreted to estimate the S wave velocity structure to upper mantle depths across the northern Cascadia subduction zone. At our westernmost station on central Vancouver Island, a prominent low-velocity zone (DeltaVs=-1 km s-1) with a high Poisson's ratio is estimated at 36-41 km

John F. Cassidy; Robert M. Ellis

1993-01-01

107

Absolute Gravity Measurements in the Cascadia Subduction Zone  

Microsoft Academic Search

Monitoring of temporal gravity change at GPS sites provides an independent means of verifying inter-seismic crustal deformation associated with a locked subduction zone. High-precision gravity measurements are theoretically sensitive to both the vertical displacement of the observation site and mass redistribution in the underlying, slowly-deforming crust. FG5 absolute gravity observations have been carried out a few times per year over

A. Lambert; N. Courtier; H. Dragert; T. S. James; M. Schmidt; K. Wang; J. He

2001-01-01

108

Postglacial rebound at the northern Cascadia subduction zone  

Microsoft Academic Search

Postglacial rebound is the response of the Earth to the decay of ice-sheets. A postglacial rebound model explains crustal tilting and rapid uplift at the northern Cascadia subduction zone that occurred during retreat of the Cordilleran ice-sheet. Observations explained by the model include the shoreline tilts of two proglacial lakes that formed at 13.5–14ka (14Cyr ago) and rapid sea level

Thomas S. James; John J. Clague; Kelin Wang; Ian Hutchinson

2000-01-01

109

Modeling of Mantle Convection in 3D Subduction Zones  

Microsoft Academic Search

The influence of 3D subduction zone geometries on mantle convection patterns is not well characterized. Subducting plates with complex 3D geometries often exhibit seismological signatures that cannot be explained by 2D flow patterns [Hoernle et al. Nature 2008\\/2009; Long and Silver Science 2008; Kneller et al. Nature 2007]. We use finite element methods to study the lateral transport and thermal

A. K. Bengtson; P. E. van Keken; S. Lin; E. A. Kneller

2010-01-01

110

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

PubMed

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

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

2014-01-01

111

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

NASA Astrophysics Data System (ADS)

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

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

2003-12-01

112

The Role of Viscous Dissipation on the Thermal Structure of Subduction Zones  

NASA Astrophysics Data System (ADS)

The thermal structure of subduction zones is related to mechanisms of magma generation and surface heat flow. For active arc volcanism and high heat flow in the back arc, the presence of high temperature magma (> 1200 °C) and mantle wedge are essential. To explain the thermal structure, frictional heating, exothermic metamorphism, and radiogenic heat production have been suggested but, they are insufficient to increase the temperature of the mantle wedge. Experiments using compressible fluid approximations show significant heat generation in the regions of downwelling (subduction) from viscous dissipation; however, most subduction experiments use incompressible fluid approximations (e.g. Boussinesq Approximation, BA hereafter) so that the effects of viscous dissipation on the thermal structure can not be assessed. In this study, we quantitatively evaluate the effects of viscous dissipation on the thermal structure of subduction zones using a compressible fluid approximation (Truncated Anelastic Liquid Approximation, TALA hereafter). We use a 50 km-depth overriding plate over the mantle wedge and a kinematically driven subducting slab with the dip of 45 degrees, the age of 130 Ma and the subduction velocity of 5 cm/year. We use the rheology of dry and wet olivine for the viscous mantle wedge by assuming that the mantle wedge is wet down to 200 km. At the corner of the mantle wedge, low viscosity of serpentinite and partial coupling between the subducting slab and the mantle wedge are assumed down to 70 km and 120 km, respectively. We vary the mantle rheology by considering diffusion creep, combined creep of diffusion and dislocation, and constant viscosity. Uniform radiogenic heat production of 3×10- 10 J/kg·s is assumed in the upper 20 km of the lithosphere. For comparison, we conduct the corresponding experiments using BA with the same parameters. Slight increases in temperature of the mantle wedge and the subducting slab as well as increases in surface heat flow are observed in the experiments using the rheology of diffusion creep and constant viscosity. However, there are significant differences between the thermal structures in the experiments using the rheology of the combined creep of diffusion and dislocation. The TALA experiments develop high temperature cornerflow but the BA experiments do not. The BA experiments produce low heat flow values in the arc. This suggests that viscous dissipation plays an important role in the thermal structure of subduction zones because dislocation creep is thought to be a major flow mechanism in the upper mantle. Like previous studies, we do not observe high heat flow in the back arc and thus require further study.

Lee, C.; King, S. D.

2007-12-01

113

Geochemical consequences of thermomechanical plumes in subduction zones. Implications for crustal making processes  

NASA Astrophysics Data System (ADS)

Crustal growth rates and geochemical consequences of composite plumes formed in subduction zones have been analysed using a thermo-mechanical numerical model of an oceanic-continental subduction zone. This model includes dehydration of subducted crust, aqueous fluid transport, partial melting and melt emplacement. Subduction of crustal material to sublithospheric depth results in the formation of tectonic rock melanges composed of basalts and sediments, which may trigger Rayleigh-Taylor instabilities atop the slab. Composite plumes are formed that rise through the mantle transporting subducted crustal materials (of varying composition) towards hotter zones of the mantle wedge. We have investigated the composition and the geochemical evolution of liquids derived from composite plumes by analysing the differing proportions of the endmembers in the source, i.e. basalts and sediments. Our results show that the proportions of the components are limited to short range variations over an interval of Xb(basalt/basalt+sediment) = 0.4 - 0.8 that allows for granodioritic melt production [1]. We have further calculated Sr and Nd isotopic initial ratios of the melange at any time during the simulations, based on the fraction of the components in the melange. Liquids derived from composite plumes inherit the geochemical characteristics of the parental magma and show distinct temporal variations of radiogenic isotopes. The decoupling between radiogenic isotopes and major elements is an interesting result, and may explain short range variations observed in some batholiths along the Cordillera. Batholiths formed along active continental margins display homogeneous major element composition but substanstial variation in radiogenic isotopic compositions, suggesting widely varying proportions of mantle and crustal components in their source that may be explained by melts derived from composite plumes. [1] Castro A., Gerya, T., García-Casco, A., Fernández, C., Díaz Alvarado, J., Moreno-Ventas, I., Löw, I. (2010). Melting relations of MORB-sediment mélanges in underplated mantle wedge plumes. Implications for the origin of cordilleran-type batholiths. Journal of Petrology, 51, 1267-1295.

Vogt, K.; Castro, A.; Gerya, T.

2011-12-01

114

Gravity anomalies, forearc morphology and seismicity in subduction zones  

NASA Astrophysics Data System (ADS)

We apply spectral averaging techniques to isolate and remove the long-wavelength large-amplitude trench-normal topographic and free-air gravity anomaly "high" and "low" associated with subduction zones. The residual grids generated illuminate the short-wavelength structure of the forearc. Systematic analysis of all subduction boundaries on Earth has enabled a classification of these grids with particular emphasis placed on topography and gravity anomalies observed in the region above the shallow seismogenic portion of the plate interface. The isostatic compensation of these anomalies is investigated using 3D calculations of the gravitational admittance and coherence. In the shallow region of the megathrust, typically within 100 km from the trench, isolated residual anomalies with amplitudes of up to 2.5 km and 125 mGal are generally interpreted as accreted/subducting relief in the form of seamounts and other bathymetric features. While most of these anomalies, which have radii < 50km, are correlated with areas of reduced seismicity, several in regions such as Japan and Java appear to have influenced the nucleation and/or propagation of large magnitude earthquakes. Long-wavelength (500 - >1000 km) trench-parallel forearc ridges with residual anomalies of up to 1.5 km and 150 mGal are identified in approximately one-third of the subduction zones analyzed. Despite great length along strike, these ridges are less than 100 km wide and several appear uncompensated. A high proportion of arc-normal structure and the truncation/morphological transition of trench-parallel forearc ridges is explained through the identification and tracking of pre-existing structure on the over-riding and subducting plates into the seismogenic portion of the plate boundary. Spatial correlations between regions with well-defined trench-parallel forearc ridges and the occurrence of large magnitude interplate earthquakes, in addition to the uncompensated state of these ridges, suggest links between the morphology of the forearc and the peak earthquake stress drop on the subduction megathrust. We present our classification of residual bathymetric and gravitational anomalies using examples from Sumatra, Kuril-Kamchatka, Mariana, Peru-Chile and the Tonga-Kermadec margin. We reassess proposed links between trench-parallel residual topography and gravity anomalies and subduction zone seismicity using global earthquake catalogs and a new compilation of published aftershock locations and distributed slip models from over 200 of the largest subduction zone earthquakes. Our results highlight the role of pre-existing structure in both the over-riding and subducting plates in modulating the along- and across-strike segmentation of subduction zones. Understanding the genesis of long-wavelength trench-parallel forearc ridges may provide further insights into links between forearc morphology, the rheology of the overriding and subducting plates and seismicity in subduction zones.

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

2012-12-01

115

Evolution and diversity of subduction zones controlled by slab width.  

PubMed

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

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

2007-03-15

116

Strong motions in Alaska-type subduction zone environments  

SciTech Connect

Peak accelerations of Alaska-Aleutian strong motion records are compared with those collected mostly in the western US. The most prominent difference is the larger scatter of Alaskan peak accelerations. The high scatter is attributed primarily to high variability of stress drops typical for some subduction zones. For critical engineering projects that must satisfy high probabilities of non-exceedence it implies that in Alaskan-type environments higher design peak accelerations may have to be adopted than under comparable cricumstances in the western US.

Jacob, K.H.; Mori, J.

1984-01-01

117

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

118

Metamorphism in Plate Boundary Zones  

NASA Astrophysics Data System (ADS)

Accretionary orogenic systems (AOS) form at sites of subduction of oceanic lithosphere; these systems dominate during supercontinent break-up and dispersal. Collisional orogenic systems (COS) form where ocean basins close and subduction ultimately ceases; these systems dominate during crustal aggregation and assembly of supercontinents. It follows that COS may be superimposed on AOS, although AOS may exist for 100s Ma without terminal collision. AOS are of two types, extensional-contractional AOS in dominantly extensional arc systems, and terrane-dominated AOS in which accretion of allochthonous elements occurs during oblique convergence. On modern Earth, regional metamorphism occurs in plate boundary zones. Blueschists are created in the subduction zone and ultra-high pressure metamorphic (UHPM) rocks are created in collision zones due to deep subduction of continental lithosphere; granulites are created deep under continental and oceanic plateaus and in arcs and collision zones [high-pressure (HP) granulites, ultra-high temperature (UHT) granulites]. In extensional-contractional AOS, basement generally is not exposed, primitive volcanic rocks occur through the history, rift basins step oceanward with time, and a well-defined arc generally is absent. LP-HT metamorphism is dominant, with looping, CW or CCW P-T-t paths and peak metamorphic mineral growth syn-to-late in relation to tectonic fabrics. UHT and HP granulites are absent, and although rare, blueschists may occur early, but UHPM is not recorded. Short-lived contractional phases of orogenesis probably relate to interruptions in the continuity of subduction caused by features on the ocean plate, particularly plateaus. Extensive granite (s.l.) magmatism accompanies metamorphism. Examples include the Lachlan Orogen, Australia, the Acadian Orogen, NE USA and Maritime Canada, and the Proterozoic orogens of the SW USA. At plate boundaries, oblique convergence is partitioned into two components, one directed more orthogonal to the strike of the trench than the convergence vector, and the other directed parallel to the strike of the trench. The orthogonal component is accommodated by subduction, but the margin-parallel component gives rise to block rotations and extension, strike-slip motion, and shortening within the upper plate. In some AOS, it has been argued that `paired' metamorphic belts characterize the metamorphic pattern. Commonly, this is a false construct that results from failure to recognize orogen-parallel terrane migration and the limitations of particular chronological datasets. Whereas a HP-LT (blueschist-eclogite) metamorphic belt may occur outboard, it is generally separated from a LP-HT (And-Sil type) metamorphic belt by a terrane boundary. These are terrane-dominated AOS. In some AOS an additional feature of the orogenic process is ridge subduction, which is reflected in the pattern of LP-HT metamorphism and the magmatism. Granulites may occur at the highest grade of metamorphism in the LP-HT belt, where granite (s.l.) magmatism is common, but UHPM occurs only rarely in the outboard HP-LT belt. Examples include the Mesozoic metamorphic belts of Japan and the North American Cordillera. COS commonly are characterized by syntectonic index minerals that record CW P-T-t paths and Barrovian-type metamorphic field gradients generated by thickening followed by exhumation. However, during the Neoproterozoic, ultra-high temperature granulite facies metamorphism is common in orogens that suture Gondwana, whereas during the Phanerozoic, metamorphism to high-pressure granulite/medium temperature eclogite facies and extreme UHPM conditions commonly occurs and may be more typical of younger COS; examples include the Alpides, the Qinling - Dabie Shan - Sulu orogens, the Variscides and the Caledonides.

Brown, M.

2005-12-01

119

UNIVERSITY OF CALIFORNIA, SAN DIEGO Cocos Plate Structure Along The Middle America Subduction Zone Off  

E-print Network

UNIVERSITY OF CALIFORNIA, SAN DIEGO Cocos Plate Structure Along The Middle America Subduction Zone: __________________________________________________ __________________________________________________ __________________________________________________ __________________________________________________ __________________________________________________ Chair University of California, San Diego 2003 #12;iv TABLE OF CONTENTS Signature Page

Dorman, LeRoy M.

120

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

NASA Astrophysics Data System (ADS)

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.

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

2008-12-01

121

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

NASA Astrophysics Data System (ADS)

The dynamic hydrogeology of subduction zones makes them important regions for geochemical cycling between the major reservoirs: seawater, oceanic crust, continental crust, and the mantle. The distillation and loss of some volatiles and fluid-soluble elements from the shallow slab not only affect reactions and processes within the seismogenic zone, but they also support the deep biosphere and play a central role in the longer-term global cycle of volatiles, such as the return of water and carbon dioxide to the ocean and atmosphere, to the depths of magmatism beneath volcanic arcs, and ultimately the mantle. Examples of key questions that have been addressed at these tectonic regimes through ocean drilling, are: what is the role of fluids in earthquake cycles; what are the global chemical and isotopic fluxes at subduction zones; how does fluid flow in the upper oceanic basement influence these global cycles? Motivated by these questions, sealed borehole hydrologic observatories (CORKs) were developed in 1989, with long-term instrumentation, to record background in-situ values of physical, chemical, and biological properties and transients. Two were deployed at the Costa Rica subduction zone; one in the upper oceanic basement ~0.3 km from the trench, the second along the décollement fault zone ~0.4 km arcward of the prism toe. Both were instrumented to continuously and simultaneously measure formation temperature, pressure, fluid chemistry and flow rate. The results of the first 7 years of deployment (2002-2009) in the oceanic basement, and 2 years in the décollement, constitute the first co-recorded hydrological, chemical, and physical databases and provided the first in-situ fluids from basement. These data have placed constraints on key questions, such as (1) How does fluid flow and chemistry vary spatially and temporally, and how do they change in response to tectonic events; (2) Can in-situ pressure, temperature, fluid flow, and chemistry be used to understand processes occurring within the seismogenic zone and be used to determine the temporal behavior of plate boundary ruptures? The continuous physical and geochemical records at the oceanic basement site show that the uppermost basement is highly permeable. The formation fluid composition, especially, the 87Sr/86Sr ratios, and the nitrate and sulfate concentrations, support mixing between seawater and a subduction zone fluid originating within the forearc, suggesting that the uppermost basement serves as an efficient pathway for fluid expelled from the subduction zone. Slow slip events were recorded at both sites during the monitoring period. These events were recorded both in pressure and flow rate, and the data indicate that they propagate up-dip and terminate at or near the trench.

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

2010-12-01

122

Modeling of Mantle Convection in 3D Subduction Zones  

NASA Astrophysics Data System (ADS)

The influence of 3D subduction zone geometries on mantle convection patterns is not well characterized. Subducting plates with complex 3D geometries often exhibit seismological signatures that cannot be explained by 2D flow patterns [Hoernle et al. Nature 2008/2009; Long and Silver Science 2008; Kneller et al. Nature 2007]. We use finite element methods to study the lateral transport and thermal structure in 3D subduction zones and their evolution over time. Realistic 3D geometries were created from seismological observations for both the Marianas and Central America, building upon the work of Kneller et al. [Nature 2007; Geochem. Geophys. Geosyst. 2008]. Highly refined meshes were created using spherical geometry on a Cartesian mesh, and slab motion was prescribed in a kinematic manner. The finite element code Sepran was used to solve the Stokes and heat equations using diffusion creep laws for viscosity. We present the velocities, temperatures, and pressures in the mantle wedge for the Marianas and Central America regions. Comparisons are made between 2D and 3D flow patterns to test the dependence of flow on 3D geometries, in addition to dependence of flow on plate direction. Resulting thermal and lateral structures are important for understanding mantle and slab mineralogy and seismic signatures. We find the lateral transport is particularly important in regions of strong obliquity and trench curvature.

Bengtson, A. K.; van Keken, P. E.; Lin, S.; Kneller, E. A.

2010-12-01

123

Gravity above subduction zones and forces controlling plate motions  

NASA Astrophysics Data System (ADS)

Short- and intermediate-wavelength gravity and geoid anomalies are used to provide constraints on the mechanical structure of subduction zones and on the forces involved. This study is based on 2D Cartesian dynamically self-consistent models with Newtonian or power law rheologies. We show that both strong decoupling of the two convergent plates (shear stresses of the order of 107 Pa) and weakened bending lithosphere are necessary to reproduce the observed geoid and gravity data. Good fits are found for relatively low failure stresses (?30-50 MPa). For all models providing reasonable predictions of gravity, only a small fraction of the downgoing slab weight is transmitted to the surface plates. About 10% of the energy is dissipated in the contact zone between the two plates, 10% to 20% in the bending region, and more than 70% in the sublithospheric mantle. The basal tractions (on the order of ?1-4 × 1012 N/m) induce a net motion of the plates, with the subducting lithosphere moving faster than predicted by the no-net-motion principle. A marked positive geoid anomaly is predicted above subduction zones at intermediate wavelength (? = 2000-4000 km) in the case of pure whole mantle convection. Such large geoid highs are not observed. Introducing "partial layering" (i.e., mass anomalies hampering mantle flow through the transition zone) is necessary to reconcile model predictions and observations for these wavelengths.

Krien, Y.; Fleitout, L.

2008-09-01

124

Water and the oxidation state of subduction zone magmas.  

PubMed

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 (Fe3+/SigmaFe), determined with Fe K-edge micro-x-ray absorption near-edge structure spectroscopy, and pre-eruptive magmatic H2O contents of a global sampling of primitive undegassed basaltic glasses and melt inclusions covering a range of plate tectonic settings. Magmatic Fe3+/SigmaFe 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 H2O 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. PMID:19644118

Kelley, Katherine A; Cottrell, Elizabeth

2009-07-31

125

Water and the Oxidation State of Subduction Zone Magmas  

SciTech Connect

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.

Kelley, K.; Cottrell, E

2009-01-01

126

Effects of active crustal movements on thermal structure in subduction zones  

NASA Astrophysics Data System (ADS)

In young suduction zones we observe steady uplift of island arcs. The steady uplift of island arcs is always accompanied by surface erosion. The long duration of uplift and erosion effectively transports heat at depth to shallower parts by advection. If the rates of uplift and erosion are sufficiently large, such a process of heat transportation will strongly affect thermal structure in subduction zones. First, we quantitatively examine the effects of uplift and erosion on thermal structure by using a simple 1-D heat conduction model, based on the assumption that the initial thermal state is in equilibrium. The results show that temperature increase, ?T, due to uplift and erosion can be approximately evaluated by ?T=?et? at depth, where ?e is the rate of uplift (erosion), t is the duration of uplift (erosion), and ? is the gradient of the geotherm in the initial state. Next, considering the effects of vertical crustal movements such as uplift and erosion in island arcs and subsidence and sedimentation in ocean trenches, in addition to the effects of radioactive heat generation in the crust, frictional heating at plate boundaries and accretion of oceanic sediments to overriding continental plates, we numerically simulate the evolution process of the thermal structure in subduction zones. The result shows that the temperature beneath the island arc gradually increases as a result of uplift and erosion as plate subduction progresses. Near the ocean trench, on the other hand, the low-temperature region gradually expands as a result of sedimentation and accretion in addition to direct cooling by the cold descending slab. The surface heat flow expected from this model is low in fore-arc basins, high in island arcs and moderately high in back-arc regions.

Fukahata, Yukitoshi; Matsu'Ura, Mitsuhiro

2000-05-01

127

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

USGS Publications Warehouse

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

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

1995-01-01

128

Banda Forearc Metamorphic Rocks Accreted to the Australian Continental Margin in Timor: Detailed Analysis of the Lolotoi Complex of East Timor  

Microsoft Academic Search

Petrologic, structural and age investigations of the Lolotoi Complex of East Timor indicate that it is part of a group of thin metamorphic klippe found throughout the region that were detached from the Banda forearc and accreted to the NW Australian continental margin during Late Miocene to Present arc-continent collision. Metamorphic rock types are dominated by (in order of abundance),

C. R. Standley

2006-01-01

129

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

NASA Astrophysics Data System (ADS)

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.

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

2012-12-01

130

Influence of Forearc Structure on the Extent of Great Subduction Zone Earthquakes  

Microsoft Academic Search

Structural features associated with forearc basins appear to strongly influence the rupture processes of large subduction zone earthquakes. Recent studies demonstrated that a significant percentage of the global seismic moment release on subduction zone thrust faults is concentrated beneath the gravity lows resulting from forearc basins. To better determine the nature of this correlation and examine its effect on rupture

Andrea L. Llenos; Jeffrey J. McGuire

2007-01-01

131

Seismoturbidite Record as Preserved at Core Sites at the Cascadia and Sumatra-Andaman Subduction Zones  

E-print Network

Seismoturbidite Record as Preserved at Core Sites at the Cascadia and Sumatra-Andaman Subduction and Sumatra- Andaman subduction zones. Source proximity, basin effects, turbidity current flow path Sumatra-Andaman and 11 March 2011 Tohoku- Oki subduction zone earthquakes and tsunamis, earthquake

Goldfinger, Chris

132

Seismoturbidite Record as Preserved at Core Sites at the Cascadia and Sumatra-Andaman Subduction Zones  

E-print Network

Seismoturbidite Record as Preserved at Core Sites at the Cascadia and Sumatra-Andaman Subduction and Sumatra- Andaman subduction zones. Source proximity, basin affects, turbidity current flow path Sumatra-Andaman and 11 March 2011 Tohoku- Oki subduction zone earthquakes and tsunamis, earthquake

Goldfinger, Chris

133

Teleseismic double difference relocation of earthquakes along the Sumatra Andaman subduction zone  

E-print Network

Teleseismic double difference relocation of earthquakes along the Sumatra Andaman subduction zone to relocate seismicity from the Sumatra Andaman region before and after the great earthquakes of 2004 and 2005 relocation of earthquakes along the Sumatra Andaman subduction zone using a 3 D model, J. Geophys. Res., 115

134

The Sumatra subduction zone: A case for a locked fault zone extending into the mantle  

E-print Network

The Sumatra subduction zone: A case for a locked fault zone extending into the mantle Martine of the Sumatra subduction zone where the downdip end of the LFZ can be well constrained from the pattern and rate: Creep and deformation; KEYWORDS: locked fault zone, interseismic deformation, Sumatra Citation: Simoes

Avouac, Jean-Philippe

135

Joint magnetotelluric and seismic investigation of the Cascadia subduction zone structure: Preliminary results and outlook  

Microsoft Academic Search

Using magnetotelluric data from EARTHSCOPE and EMSLAB, and passive seismic data from CAFÉ and EARTHSCOPE, we investigate the substructure of the Cascadia subduction zone. Through a series of two dimensional magnetotelluric inversions and migrated seismic sections, we explore some of the constraints to slab geometry, volatile release, and melting within the subduction zone structure. We consider what each geophysical method

R. S. McGary; R. L. Evans; S. Rondenay; G. A. Abers; K. C. Creager; P. E. Wannamaker

2009-01-01

136

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

Microsoft Academic Search

Chemosynthetic benthic communities, which live symbiotically with microbes capable of metabolizing nutrients dissolved in water seeping out of the seafloor, are widespread along the Cascadia subduction zone. These seeps and vents are therefore indicative of one mode of fluid migration out of the subduction zone sediments. The authors have used deep-towed seismic methods, including hydrophones mounted on Alvin, to examine

Brian T. R. Lewis; Guy C. Cochrane

1990-01-01

137

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

Microsoft Academic Search

Chemosynthetic benthic communities, which live symbiotically with microbes capable of metabolizing nutrients dissolved in water seeping out of the seafloor, are widespread along the Cascadia subduction zone. These seeps and vents are therefore indicative of one mode of fluid migration out of the subduction zone sediments. We have used deep-towed seismic methods, including hydrophones mounted on Alvin, to examine the

Briant T. R. Lewis; Guy C. Cochrane

1990-01-01

138

Influence of fore-arc structure on the extent of great subduction zone earthquakes  

Microsoft Academic Search

Structural features associated with fore-arc basins appear to strongly influence the rupture processes of large subduction zone earthquakes. Recent studies demonstrated that a significant percentage of the global seismic moment release on subduction zone thrust faults is concentrated beneath the gravity lows resulting from fore-arc basins. To better determine the nature of this correlation and to examine its effect on

Andrea L. Llenos; Jeffrey J. McGuire

2007-01-01

139

Resolution experiments for NW Pacific subduction zone tomography  

NASA Astrophysics Data System (ADS)

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.

Spakman, Wim; van der Hilst, Rob; Wortel, Rinus; Stein, Seth

1989-10-01

140

Slab melting versus slab dehydration in subduction-zone magmatism  

PubMed Central

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

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

2011-01-01

141

Late holocene tectonics and paleoseismicity, southern cascadia subduction zone.  

PubMed

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

Clarke, S H; Carver, G A

1992-01-10

142

Late Holocene tectonics and paleoseismicity, southern Cascadia subduction zone  

USGS Publications Warehouse

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

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

1992-01-01

143

Interseismic strain accumulation along the Sumatran Subduction Zone seismic gap.  

NASA Astrophysics Data System (ADS)

The south equatorial segment of the sumatra subduction zone (the Mentawai segment) is known to have produced giant earthquakes in 1797 and 1833 (Mw higher than 8.5). The northern adjacent segment (Nias) that broke in 1861 and again in March 2005 (Mw=8.7) highlights the potential of a future giant earthquake in the Mentawai segment. Paleogeodetic and GPS data from the Sumatran subduction zone provide an unusual opportunity to understand the physical parameters that control the behavior of a subduction interface. Interseismic strain measurements recorded over the last several decades by coral growth rings and GPS instruments are fit well by a simple model that assumes lateral variations in the depth of the updip and downdip limits of the locked fault zone (LFZ). The minimum width of the LFZ is about 100 km near the Equator and increases to about 200 km farther south. Near the Equator, where the width of the LFZ is about 100km, smaller earthquakes occurred (Mw=7.7 in Mw=1935 7.2 in 1984) than the area farther south where giant earthquakes happened in 1797 and 1833. The background seismicity also fit very well with the downdip end of the LFZ. This difference in both the seismic behavior and width of the LFZ might be related to the to the thermal structure of the plate interface and so to lateral variations in both the age of the subducting plate and the normal plate convergence rate. We find that the downdip end of the LFZ is everywhere between the 300C and 400C isotherms, corresponding to the stable sliding activation of quartzo-feldspathic rocks. If we extrapolate the last 50 years of strain accumulation in the Mentawai segment back to 1833, the seismic moment potential is found to be higher than 1.5*1022 N.m, the equivalent of a Mw higher than 8.7 earthquake.

Chlieh, M.; Avouac, J.; Sieh, K.; Natawidjaja, D. H.; Suwargadi, B. W.; Galetzka, J.

2005-12-01

144

Great earthquakes of variable magnitude at the Cascadia subduction zone  

USGS Publications Warehouse

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.

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

2006-01-01

145

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

NASA Astrophysics Data System (ADS)

The Calabrian subduction zone, situated southeast of the Italian 'boot' in the Ionian Sea, is the latest manifestation of African-Eurasian plate interaction. This plate interaction has been remarkably dynamic since the Mesozoic, hosting episodes of mountain belt and volcanic arc formation including, for example, the Alpine, Carpathian and Apennine orogenic belts and Hellanic and, most recently, Aeolian volcanic arcs. Subduction of cold oceanic lithosphere beneath Europe initiated around 80 Ma, and the last 30 Ma have been characterized by alternating episodes of rapid back-arc rifting and back-arc spreading (up to 6-8 cm/yr) mediated by dip-parallel and/or trench-parallel tears in the descending slab resulting from differential trench rollback (Wortel and Spakman 2000). Backarc extension effectively moved the plate boundary from the European continental margin in the north to the African continental margin in the south, creating the modern Western Mediterranean basins. The Tyrrhenian oceanic basin was opened during the latest episode of trench rollback, from 5-2 Ma, followed by initiation of the subduction-related Aeolian volcanism by 1.3 Ma (Beccaluva et al. 1982) and complete cessation of extension of the overriding plate around 0.8-0.5 Ma (Goes et al. 2004). The seven subaerial volcanoes of the Aeolian volcanic arc sit atop thin (16-30 km) continental crust, and collectively tap a heterogeneous mantle source. Slab geometry in the depth range of 150 to 500 km has been refined using the hypocenter relocation procedure of Engdahl et al 1998 for teleseismic events beneath the Tyrrhenian Sea, in conjunction with recent tomographic results. The thermal state of the Calabrian subduction zone at depths relevant to dehydration and magma genesis has been investigated using a 2-dimensional time-dependent thermal model of the descending slab and convecting mantle wedge based on seismic, geologic and geodetic observational data. Modeling methodology follows van Keken et al. 2002 and includes power-law olivine rheology and shear-heating scaled to fit forearc heat flow. The computed thermal model is used with a thermodynamically-based mineral solubility scaling relationship between fluid solute content and the relative permittivity of the fluid to infer the solute content of a slab-derived aqueous fluid existing in equilibrium with slab and mantle wedge mineralogy. The relative abundances of different solutes in slab-derived fluids are critical in determining the trace element characteristics of subduction zone magmas. Among some of the Aeolian arc volcanic centers, the recent (<30 ky) evolution in magma composition from calc-alkaline to high-potassium has been variably attributed to melting of distinct source regions, or the increasing abundance of aqueous fluids in a source region associated with a young/incipient arc. Our approach of thermal-field and solute content modeling can contribute to the resolution of this and other petrogenesis questions for Aeolian arc lavas.

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

2007-12-01

146

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

USGS Publications Warehouse

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

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

1996-01-01

147

High-resolution models of subduction zones: Implications for mineral dehydration reactions and the transport of water  

E-print Network

are used to reevaluate the thermal structure of the Honshu and Cascadia subduction zones using the more. The thermal structure of the Cascadia subduction zone is markedly different because of the age of the incomingHigh-resolution models of subduction zones: Implications for mineral dehydration reactions

van Keken, Peter

148

Late Holocene Rupture of the Northern San Andreas Fault and Possible Stress Linkage to the Cascadia Subduction Zone  

E-print Network

to the Cascadia Subduction Zone by Chris Goldfinger, Kelly Grijalva, Roland Bürgmann, Ann E. Morey, Joel E land and marine paleoseismic record from the southern Cascadia subduction zone includes a similar recurrence in both the Cascadia Subduction Zone, and on the northern San Andreas fault (NSAF) (Goldfinger et

Goldfinger, Chris

149

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

E-print Network

Shallow Thermal Conditions in the Central and Southern Cascadia Subduction Zone J.R. McKenna1 and D quiescent central and southern Cascadia subduction zone (CSZ) to gain insight into hypothesized great (Mw. The anomalous character of the Cascadia Subduction Zone (CSZ) is immediately obvious because, based

Southern Methodist University

150

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

NASA Astrophysics Data System (ADS)

The release and transport of fluids in subduction zones affect many fundamental Earth processes. The Catalina Schist subduction zone mélange is thought to have been a zone of fluid flow and mechanical mixing between the downgoing subducting slab and overlying mantle wedge. Study of rocks from this mélange zone can help elucidate processes of fluid release and transport occurring within subduction zones. Lithium is a fluid-mobile element which shows large variations in concentration and isotopic composition in subduction zone metamorphic rocks, thus providing a potentially useful tool in deciphering fluid flow pathways and mechanisms, fluid sources, and material transport. The well-studied Catalina Schist provides an opportunity to examine the effects of high-P/T devolatilization and other fluid-rock interactions on the behavior of Li in subduction zones. Whole-rock Li concentrations (10 to 45 ppm) and ?7Li (-2.3 to +4.9‰, all values reported relative to L-SVEC standard) of Catalina Schist metasedimentary rocks fall within the range of seafloor sedimentary rocks, and correlate with the chemical index of alteration, indicating that variations in Li at least partly reflect weathering processes. There is no discernible change in Li concentration or ?7Li in metasedimentary rocks with increasing metamorphic grade, an observation contrasting with that for B, Cs, and N, which show dramatic loss from metasedimentary rocks at higher grades. SIMS study of the same metasedimentary samples indicates the residency of Li in chlorite and phengite at lower grades and gradual transfer, at higher grades, to being dominantly in phengite (with chlorite breakdown). Metabasaltic rocks in the Catalina Schist are enriched in Li (10 to 32 ppm) relative to their MORB protoliths, showing a wide range of concentrations and, in most cases, ?7Li similar to that of nearby metasedimentary rocks (-6.7 to +1.8‰). These observations suggest metasomatic additions of Li mobilized during metasedimentary fluid-rock interactions. Lithium concentrations and ?7Li of metasomatized metamafic conglomerate cobbles (18 to 35 ppm, +1.2 to +3.5‰), veins (16 to 64 ppm, -2.6 to +2.2‰), and pegmatites (3 to 9 ppm, +1.1 to +3‰) also appear to reflect high-P/T mobility of Li in H2O-rich fluids or, at the highest grade, silicate melts. The majority of the metabasalts are relatively uniform in ?7Li, perhaps reflecting the fluid mixing and homogenization inferred from O, C, and N isotope systematics in the same complex. Several metabasaltic blocks in the amphibolite-grade mélange, and their metasomatic rinds, have lower ?7Li values (down to -6.7‰) that could reflect kinetic fractionation during the infiltrative/diffusive production of metasomatic gradients at the margins of the blocks.

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

2010-12-01

151

The Late Cenozoic geodynamic evolution of the central segment of the Andean subduction zone  

NASA Astrophysics Data System (ADS)

The presented model of the Late Cenozoic geodynamic evolution of the central Andes and the complex tectonic, geological, and geophysical model of the Earth’s crust and upper mantle along the Central Andean Transect, which crosses the Andean subduction zone along 21°S, are based on the integration of voluminous and diverse data. The onset of the recent evolution of the central Andes is dated at the late Oligocene (27 Ma ago), when the local fluid-induced rheological attenuation of the continental lithosphere occurred far back of the subduction zone. Tectonic deformation started to develop in thick-skinned style above the attenuated domain in the upper mantle and then in the Earth’s crust, creating the bivergent system of the present-day Eastern Cordillera. The destruction of the continental lithosphere is correlated with ore mineralization in the Bolivian tin belt, which presumably started at 16° S and spread to the north and to the south. Approximately 19 Ma ago, the gently dipping Subandean Thrust Fault was formed beneath the Eastern Cordillera, along which the South American Platform began to thrust under the Andes with rapid thickening of the crust in the eastern Andean Orogen owing to its doubling. The style of deformation in the upper crust above the Subandean Thrust Fault changed from thick- to thin-skinned, and the deformation front migrated to the east inland, forming the Subandean system of folds and thrust faults verging largely eastward. The thickening of the crust was accompanied by flows at the lower and/or middle crustal levels, delamination, and collapse of fragments of the lower crust and lithospheric mantle beneath the Eastern Cordillera and Altiplano-Puna Plateau. As the thickness of the middle and lower crustal layers reached a critical thickness about 10 Ma ago, the viscoplastic flow in the meridional direction became more intense. Extension of the upper brittle crust was realized mainly in gliding and rotation of blocks along a rhombic fault system. Some blocks sank, creating sedimentary basins. The rate of southward migration estimated from the age of these basins is 26 km/Ma. Tectonic deformation was accompanied by diverse magmatic activity (ignimbrite complexes, basaltic flows, shoshonitic volcanism, etc.) within the tract from the Western Cordillera to the western edge of the Eastern Cordillera 27-5 Ma ago with a peak at 7 Ma; after this, it began to recede westward; by 5 Ma ago, the magmatic activity reached only the western part of the Altiplano-Puna Plateau, and it has been concentrated in the volcanic arc of the Western Cordillera during the last 2 Ma.

Romanyuk, T. V.

2009-07-01

152

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

PubMed

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

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

2013-01-01

153

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

USGS Publications Warehouse

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; (

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; edited by Kayen, Robert

2012-01-01

154

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

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

155

Shallow subduction zone earthquakes and their tsunamigenic potential  

NASA Astrophysics Data System (ADS)

We have examined the source spectra of all shallow subduction zone earthquakes from 1992 to 1996 with moment magnitude 7.0 or greater, as well as some other interesting events, in the period range 1-20s, by computing moment rate functions of teleseismic P waves. After comparing the source spectral characteristics of `tsunami earthquakes' (earthquakes that are followed by tsunamis greater than would be expected from their moment magnitude) with regular events, we identified a subclass of this group: `slow tsunami earthquakes'. This subclass consists of the 1992 Nicaragua, the 1994 Java and the February 1996 Peru earthquakes. We found that these events have an anomalously low energy release in the 1-20s frequency band with respect to their moment magnitude, although their spectral drop-off is comparable to those of the other earthquakes. From an investigation of the centroid and body wave locations, it appears that most earthquakes in this study conformed to a simple model in which the earthquake nucleates in a zone of compacted and dehydrated sediments and ruptures up-dip until the stable sliding friction regime of unconsolidated sediments stops the propagation. Sediment-starved trenches, e.g. near Jalisco, can produce very shallow slip, because the fault material supports unstable sliding. The slow tsunami earthquakes also ruptured up-dip; however, their centroid is located unusually close to the trench axis. The subduction zones in which these events occurred all have a small accretionary prism and a thin layer of subducting sediment. Ocean surveys show that in these regions the ocean floor close to the trench is highly faulted. We suggest that the horst-and-graben structure of a rough subducting oceanic plate will cause contact zones with the overriding plate, making shallow earthquake nucleation and up-dip propagation to the ocean floor possible. The rupture partly propagates in sediments, making the earthquake source process slow. Two factors have to be considered in the high tsunami-generating potential of these events. First, the slip propagates to shallow depths in low-rigidity material, causing great deformation and displacement of a large volume of water. Second, the measured seismic moment may not represent the true earthquake displacement, because the elastic constants of the source region are not taken into account in the standard CMT determination.

Polet, J.; Kanamori, H.

2000-09-01

156

Melt Inclusions as Windows on Subduction Zone Processes - A Retrospective  

NASA Astrophysics Data System (ADS)

A.T. (Fred) Anderson, in a series of papers in the interval 1972-1984, presented evidence from melt inclusions for high dissolved water and Cl concentrations in many subduction zone basalts through andesites. His observations, subsequently shown to be correct, were not widely accepted because (1) phase equilibrium experiments on Paricutin and Mount Hood andesites indicated moderate water concentrations, and some workers reasoned that potentially parental basalts would have been drier still, (2) common basalts lack hydrous phenocrysts, and (3) water content estimates were indirect (water-by-difference) or involved difficult, unfamiliar measurements (single inclusion manometry) and thus were discounted. Subsequent development of techniques for the direct and precise measurement of water and CO2 in melt inclusions (SIMS, FTIR), new hydrous phase-equilibrium studies on arc basalts through rhyolites, and wider appreciation of the diversity of arc magmatic suites changed this situation. Melt inclusion evidence shows that subduction zone basalts can have pre-eruptive dissolved water concentrations as high as ~6 wt% (Sisson and Layne 1993 EPSL; Roggensack et al. 1997 Science), confirming predictions from phase-equilibrium experiments (Sisson and Grove 1993a,b CMP), and supporting the now standard model of water-fluxed melting to drive arc magmatism. An important discovery, presaged in the original Anderson data, is that there is a wide range of pre-eruptive water contents in arc basalts, with some as dry as MORB (Sisson and Bronto 1998 Nature). Nearly dry arc basalts can erupt at the volcanic front (Galunggung, Java) and sporadically along the arc axis over distances of hundreds of km (Cascades, USA), in some cases in proximity to demonstrably water-rich magmatic centers (Mt. Shasta, Crater Lake). To produce dry primitive basalts requires upwelling and pressure-release melting of peridotite in the mantle wedge at temperatures (~1300° C) well above those predicted by common geodynamic models. Pressure-release and water-flux melting probably act together to produce parental arc basalts, but presently there is no evidence establishing the relative dominance of these melt-producing processes in any arc or more local region. Regional studies of melt inclusions in arc basalts have potential for solving this question.

Sisson, T. W.

2002-12-01

157

Earthquake swarms in circum-Pacific subduction zones  

NASA Astrophysics Data System (ADS)

We systematically and manually search through clusters of earthquakes along circum-Pacific subduction zones to identify potential earthquake swarms. In total, we find 266 potential earthquake swarms: 180 we classify as megathrust and 68 we classify as volcanic due to their proximity to the megathrust or to volcanoes. We focus on the megathrust swarms and demonstrate that: (1) the number of events in a swarm is not a function of the largest earthquake in the swarm, (2) swarms exhibit an approximately constant rate of seismicity that lasts until after the mean timing of events in the swarm, (3) the timing of the largest earthquake in the sequence is no different than the timing of any other earthquake in the sequence, (4) our catalogs of earthquakes comprising swarms (~ 9000 events) have high b-values (1.5 to 2), and (5) when earthquake swarms are considered as single events using total duration and cumulative moment, they appear to be consistent with the slow earthquake magnitude-duration scaling law presented by Ide et al. (2007). The first three observations, along with the observation that swarms can span very large areas compared to their cumulative seismic moment, argue against static stress triggering as a driving mechanism for earthquake swarms. Along strike propagation velocities are observed for several swarms, showing epicentral propagation of ~ 10 km/day, similar to other documented slow slip events. Together, this evidence implies that aseismic slip along the megathrust is likely an important mechanism for the generation of megathrust earthquake swarms in circum-Pacific subduction zones. We then conduct a comparison of swarms and large megathrust earthquakes, finding evidence that the two are broadly anti-correlated: megathrust segments with large earthquake swarm gaps are more likely to experience large (Mw > 8) megathrust events. We characterize the ubiquity of megathrust swarms at different margins, and suggest that fault properties along Marianas-type margins may allow for earthquake swarms to occur regularly, but other margins may rely on other variables, such as the subduction of a ridge or seamount, to facilitate the generation of megathrust earthquake swarms.

Holtkamp, S. G.; Brudzinski, M. R.

2011-05-01

158

Geoid anomalies in the vicinity of subduction zones  

NASA Technical Reports Server (NTRS)

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.

Mcadoo, D. C.

1980-01-01

159

Radiocarbon test of earthquake magnitude at the Cascadia subduction zone  

USGS Publications Warehouse

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

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

1991-01-01

160

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

PubMed

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

Calvert, Andrew J

2004-03-11

161

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

USGS Publications Warehouse

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.

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

2003-01-01

162

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

USGS Publications Warehouse

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.

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

163

Supra-subduction zone ophiolites as favorable hosts for chromitite, platinum and massive sulfide deposits  

NASA Astrophysics Data System (ADS)

Supra-subduction zone ophiolites, as exemplified by the Zambales Ophiolite Complex, host extensive chromitite, volcanic-hosted massive sulfide deposits and, to a lesser degree, platinum-group minerals. In contrast, mid-ocean ridge basalt ophiolites are almost barren of economic mineral deposits. The high degree of partial melting, high water pressure, changes in oxygen fugacity, temperature, pressure and magma mixing could explain the chromitite deposits in zupra-subduction zone ophiolites. Platinum-group minerals are modeled to be derived from multi-stage melting events that also characterize supra-subduction zone ophiolites. The presence of capping rocks and the tectonics of emplacement are believed to be critical in the preservation of volcanic-hosted massive sulfides in ophiolites. Marginal basins (= SSZ ophiolites) are more easily emplaced than the large, open sea oceanic basin ophiolites which are usually subducted. Geochemical and tectonic controls point to supra-subduction zone ophiolites as more promising exploration targets.

Yumul, G. P.; Balce, G. R.

164

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

SciTech Connect

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.

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

1995-11-10

165

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

E-print Network

subduction zones may be a very important constituent in the cycle of large subduction thrust earthquakes without any geodetically (GPS) detectable SSE, and the reverse large, long-term SSE not always hal

166

Simulation of Tsunamis from Great Earthquakes on the Cascadia Subduction Zone  

Microsoft Academic Search

Large earthquakes occur episodically in the Cascadia subduction zone. A numerical model has been used to simulate and assess the hazards of a tsunami generated by a hypothetical earthquake of magnitude 8.5 associated with rupture of the northern sections of the subduction zone. Wave amplitudes on the outer coast are closely related to the magnitude of sea-bottom displacement (5.0 meters).

Max K.-F. Ng; Paul H. Leblond; Tad S. Murty

1990-01-01

167

Expected tsunami amplitudes and currents along the North American coast for Cascadia subduction zone earthquakes  

Microsoft Academic Search

Tsunamis are numerically modeled using the nonlinear shallow-water equations for three hypothetical Cascadia subduction zone earthquakes. Maximum zero-to-peak tsunami amplitudes and currents are tabulated for 131 sites along the North American coast. Earthquake source parameters are chosen to satisfy known subduction zone configuration and thermal constraints. These source parameters are used as input to compute vertical sea-floor displacement. The three

Paul M. Whitmore

1993-01-01

168

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

Microsoft Academic Search

The behavior of apparent stress for normal-fault earthquakes at subduction zones is derived by examining the apparent stress (taua=muES\\/M0, where ES is radiated energy and M0 is seismic moment) of all globally distributed shallow (depth, h < 70 km) earthquakes with normal-fault mechanisms that occurred in or near subduction zones between 1987 and 2001 for which ES and M0 are

George L. Choy; Stephen H. Kirby

2004-01-01

169

Thrust-type subduction-zone earthquakes and seamount asperities: A physical model for seismic rupture  

Microsoft Academic Search

A thrust-type subduction-zone earthquake of Mw 7.6 ruptures an area of ˜6000 km2, has a seismic slip of ˜1 m, and is nucleated by the rupture of an asperity ˜25 km 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

Mark Cloos

1992-01-01

170

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

NASA Astrophysics Data System (ADS)

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.

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

2013-08-01

171

179EARTHQUAKE CONTROL OF HOLOCENE TURBIDITE FREQUENCY, NORTHERN CALIFORNIA CONTINENTAL MARGINS EARTHQUAKE CONTROL OF HOLOCENE TURBIDITE FREQUENCY CONFIRMED  

E-print Network

intervals have been studied along the continental margins of the Cascadia Subduction Zone and northern San California margin. This difference in frequency of turbidites in a subduction zone compared to a transform ON THE CASCADIA AND NORTHERN CALIFORNIA ACTIVE CONTINENTAL MARGINS JULIA GUTIERREZ-PASTOR AND C. HANS NELSON

Goldfinger, Chris

172

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

USGS Publications Warehouse

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.

Spinelli, G. A.; Wang, K.

2009-01-01

173

Controls on the Migration of Fluids in Subduction Zones  

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

174

Rare earth element budgets in subduction-zone fluids  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

175

Heat-flow density across the Central Andean subduction zone  

NASA Astrophysics Data System (ADS)

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.

Springer, Michael; Förster, Andrea

1998-06-01

176

Rheology of magnesite and implications for subduction zone dynamics  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

177

GEOPHYSICS Periodic Slow Earthquakes from the Cascadia Subduction Zone  

E-print Network

Cascadia margin. Baseline lengths are from weekly station positions with respect to Penticton (DRAO). (Inset) Time series shows daily positions for Friday Harbor (SC02) for 2002 in the global reference, illustrating the onset of the latest of the slow earthquakes. Continuous geodetic measurements from convergent margins have shown that deep transient creep events can release large amounts of strain energy without detectable seismic shaking, and they are thus known as slow or silent earthquakes. Because subduction zones generate the largest earthquakes, seismic hazard assessment relies on understanding the degree to which slow earthquakes reduce the energy released by infrequent large earthquakes. We present data that indicate the periodicity of slow earthquakes in Cascadia and the current onset of such an event. Along the Cascadia margin, great earthquakes (moment magnitude, M w ? 8) have occurred over its 1500-km length from central British Columbia to northern California as the Juan de Fuca plate subducts beneath the North American plate (1–4). Since 1992, continuous global positioning system (GPS) monitoring of changes in station position have been used to monitor surface deformation associated with tectonic events (5, 6). In the fore arc that lies between the offshore plate boundary and the Cascade volcanoes, current interseismic deformation is dominated by northeast-directed contraction, the result of shallow convergence of the Juan de Fuca plate beneath the North American plate. During 1999, a sudden shift from fore arc contraction to short-lived extension led to identification of a deep slow earthquake (7). Our analysis of GPS results from the Pacific

M. Meghan Miller; Tim Melbourne; Daniel J. Johnson; William Q. Sumner; B Revia

178

Sources of Tsunami and Tsunamigenic Earthquakes in Subduction Zones  

NASA Astrophysics Data System (ADS)

We classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface (typical interplate events), earthquakes at the outer rise, within the subducting slab or overlying crust (intraplate events), and "tsunami earthquakes" that generate considerably larger tsunamis than expected from seismic waves. The depth range of a typical interplate earthquake source is 10-40km, controlled by temperature and other geological parameters. The slip distribution varies both with depth and along-strike. Recent examples show very different temporal change of slip distribution in the Aleutians and the Japan trench. The tsunamigenic coseismic slip of the 1957 Aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long. This asperity ruptured again in the 1986 Andreanof Islands and 1996 Delarof Islands earthquakes. By contrast, the source of the 1994 Sanriku-oki earthquake corresponds to the low slip region of the previous interplate event, the 1968 Tokachi-oki earthquake. Tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes; tsunami hazard assessments must include such events. Similarity in macroseismic data from two southern Kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights. Most moment release of tsunami earthquakes occurs in a narrow region near the trench, and the concentrated slip is responsible for the large tsunami. Numerical modeling of the 1996 Peru earthquake confirms this model, which has been proposed for other tsunami earthquakes, including 1896 Sanriku, 1946 Aleutian and 1992 Nicaragua.

Satake, K.; Tanioka, Y.

179

Permeability-porosity relationships of subduction zone sediments  

USGS Publications Warehouse

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

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

2011-01-01

180

Modelling stress accumulation and dissipation and the causes of intermediate depth seismicity in subduction zones  

NASA Astrophysics Data System (ADS)

The accumulation and dissipation of stress in the subducting lithosphere is directly responsible for Wadati-Benioff zone seismicity. Determining the development of these stresses remains as yet unresolved. Stress accumulation and dissipation within the subducting slab occurs through the superposition of contributions from slab-bending, slab-pull, thermal stresses and metamorphic phase changes, and ductile and brittle deformation. The resulting stress distribution has a complex relationship with depth, forming planes of alternating down-dip compression and tension as observed in double and triple Wadati-Benioff seismic zones. The formation of these double and triple seismic zones provides a powerful constraint on understanding stress accumulation and dissipation within the subducting slab and the cause of intermediate depth earthquakes. In this work we focus on stresses in the subducting slab down to 300 km whilst acknowledging that it is necessary to include deeper structures such as the 410km and 660km discontinuities in our model. We use coupled mechanical - thermal diffusion finite element code (powered by MILAMIN, Dabrowski et al 2008) to model a subduction zone. Temperature-dependent viscosity and density are used to calculate subduction flow velocities, deformation and stresses, which are used in turn to calculate the advection of temperature and slab material. The increases in mantle viscosity at the 410 km and 660 km phase transitions exert a strong control on subduction slab stress. We explore the evolution of subducting slabs and their stresses for a variety of imposed parameters including plate convergence velocity, subduction dip angle and roll-back velocity. From the analysis of steady-state and transient solutions, we hope to better understand the relative contributions of the different mechanisms to the accumulation and dissipation of stress in the subducting slab.

Fry, A.; Kusznir, N.; Dabrowski, M.; Rietbrock, A.; Podladtchikov, I.

2009-04-01

181

Structural evolution, metamorphism and restoration of the Arabian continental margin, Saih Hatat region, Oman Mountains  

Microsoft Academic Search

170 m.y. of relatively stable passive margin sedimentation along the northern continental margin of Arabia was abruptly terminated during the Cenomanian–Turonian (?95 Ma) when the Oman continental margin collapsed and subsided rapidly (Aruma basin) to accommodate obduction of the Semail ophiolite complex and underlying thrust sheets (Haybi and Hawasina complexes) in the Oman mountains. The ophiolite was emplaced at least

M. P Searle; C. J Warren; D. J Waters; R. R Parrish

2004-01-01

182

Modeling the controls on excess pore pressure by dehydration reactions in the slow-slip region of subduction zones  

NASA Astrophysics Data System (ADS)

Slow slip and tremor in subduction zones take place where there is abundant evidence for elevated, near lithostatic, pore pressures along the plate interface. In Japan and Cascadia, these depths (~30-45 km) are such that the main source of fluids must be attributed to chemical dehydration reactions. Here we model the consolidation of low porosity (~5%) oceanic crust subducting through the slow slip and tremor zone, and participating in pressure and temperature-dependent dehydration reactions. Pore pressures in excess of lithostatic values, and hence negative effective stresses, are a robust feature of preliminary work that employs parameters consistent with the geometry of the Cascadia subduction margin, and bulk permeabilities in the range 10-25 ? k ? 10-21 m2. In these calculations, the dehydration fluid source is parameterized using the predictions of previous studies that have calculated the amount of bound H2O lost from subducting oceanic crust as a result of broad metamorphic facies transitions. We also report the predictions of a revised model that includes a general kinetic law, which governs the rate of dehydration as a function of temperature and pressure, and provides constraints on the parameters necessary for such kinetic effects to be relevant for the fluid supply by dehydration reactions in the subducting crust. Finally, we investigate the role of lateral fluid migration by including a fluid sink term that is proportional to modeled horizontal pressure gradients. Our results indicate that processes must be active in the slow slip region of subduction zones that are responsible, perhaps periodically, for alleviating excess pore pressures generated by dehydration reactions. Candidate mechanisms include hydraulic fracturing within or below the plate interface, lateral and/or focused fluid migration, or increases in permeability during slow slip events.

Skarbek, R. M.; Rempel, A. W.

2013-12-01

183

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

NASA Astrophysics Data System (ADS)

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

Oncken, O.

2008-12-01

184

Mantle melting and crustal recycling in subduction zones  

NASA Astrophysics Data System (ADS)

Major element data for basalts from approximately 100 arc volcanoes are examined in order to test a model whereby the mantle melts to varying extents beneath different arcs. Because the downgoing plate is at a fairly constant depth (approximately 120 km) beneath arc volcanoes worldwide, the total length of the mantle column that is available for adiabatic melting is largely dependent on the thickness of the arc crust. Chemical parameters that reflect the degree of mantle melting might then correlate with crustal thickness. Major element data for arc basalts are corrected for the effects of differentiation by calculating values at 6 percent MgO. Na(6.0) and Ca(6.0) correlate strongly with the thickness of the arc crust: basalts erupted onto thick crust are rich in Na and poor in Ca. These characteristics are typical of low degree mantle melts. Thus, where the crust is thick, the mantle melting column is short and the mantle melts to a small extent, producing high Na and low Ca melts. Mantle melting variations can also explain the variations in some trace elements (e.g., Sc, Ce, Zr) but others (e.g., Ba, K, Sr) may reflect subducted sediment instead. In order to test the sediment subduction model, sediment fluxes into trenches are estimated. Over 250 new chemical analyses are reported for marine sediments from DSDP/ODP Sites 765, 595, 596, and 183. Based on evaluation of these reference sites, relationships are found between the geochemical and lithologic variations in sediments. This lithologic approach is used to calculate the bulk composition for eight trench sections (Java, Tonga, Aleutians, Antilles, Guatemala, Mexico, Vanuatu, and Marianas). The sediment input fluxes correlate well with the associated arc enrichments in K, Rb, Cs, Sr, Ba, U, and Th for the eight arc/trenches pairs examined. The subducted input and volcanic output fluxes can be balanced if the entire oceanic crust (sediment + basalt) loses elements to the arc. The results of this flux balance are that the sediment loses roughly 10-30 percent of all the elements to the arc, while the basalt loses more variable amounts (0 percent, 2.5 percent, 7-15 percent, and 30 percent of its Th, U, alkalis, and Ba, respectively). The relatively constant proportions from the sediment suggest that the transport phase to the mantle wedge is a sediment melt, while the variable proportions from the basalt suggest an aqueous fluid. In this model, elements are recycled in subduction zones via sediment melting and basalt dehydration.

Plank, Terry Ann

185

Geodynamic insights into patterns of shear wave anisotropy in subduction zones  

NASA Astrophysics Data System (ADS)

Shear wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that have trench-parallel fast directions and abrupt rotations. Several hypotheses have been proposed to explain these unexpected patterns. This work shows tests of the olivine fabric transition and three-dimensional flow hypotheses. The tool used to carry out this investigation is geodynamic modeling with seismological constraints on subduction zone geometry and rheological parameters based on rock deformation experiments. The geodynamic models presented in this work show that a transition to B-type olivine fabric can produce trench-parallel shear wave splitting with delay times greater than 1 s above the cold fore-arc mantle of subduction zones. The olivine fabric transition model adequately reproduces local-S splitting parameters associated with phases that sample the deep fore-arc mantle of the Ryukyu subduction system. The three-dimensional subduction zone models presented in this work show that both a transition to flat-slab subduction and strong trench curvature generate trench-parallel stretching in the warm are and back arc mantle. These models may explain trench-parallel shear wave splitting and abrupt rotations in fast direction in the are and back arc-mantle of the Andean and Marianas subduction systems. This thesis demonstrates that a variety of mechanisms may be necessary to account for trench-parallel anisotropy in subduction zones.

Kneller, Erik Arthur

186

Structure, metamorphism and timing of an exhumed Cretaceous subduction zone beneath the Oman Ophiolite  

Microsoft Academic Search

The Semail ophiolite in Oman was emplaced from NE to SW at least 200 km over the Arabian passive margin, probably over 450 km in total, during the late Cretaceous (95-70 Ma). The first phase of obduction involved NE-directed subduction of Triassic-Jurassic basalt at least 45-50 km beneath the ophiolite, whilst the crustal sequence was forming (U-Pb zircons from plagiogranites,

M. P. Searle; C. J. Warren; D. J. Waters; R. R. Parrish

2003-01-01

187

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

USGS Publications Warehouse

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.

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

1993-01-01

188

Serpentine rheology and its significance on subduction zone processes  

NASA Astrophysics Data System (ADS)

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.

Katayama, I.; Hirauchi, K.

2011-12-01

189

Mantle convection and crustal tectonics in the Tethyan subduction zone  

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

190

Effects of Fault Gouge Dilatancy on Subduction Zone Earthquakes (Invited)  

NASA Astrophysics Data System (ADS)

For fluid-saturated granular materials, pore expansion will result in a reduction in pore pressure p thus an increase in effective normal stress ?, a mechanism for inhibiting rapid deformations. In this study, we focus on the effects of dilatancy and pore compaction on slip in a subduction earthquake cycle simulated in the framework of rate and state friction (``ageing" evolution) with the radiation damping approximation, using laboratory friction data of granite and gabbro under hydrothermal conditions. We use the ``membrane diffusion" assumption that the slipping zone is bordered by a less permeable layer within which p on the sliding interface varies linearly to the ambient level in the surrounding rock mass. Under the fully undrained condition, fracture energy and mechanical energy release balance predicts that the ratio of instability nucleation size to that without dilatancy is a function of the friction parameter a/b and parameter E = f0(?/?)/(b?) that measures the relative contributions to the stress drop from pore suction and friction evolution. Here, ? is a dimensionless dilatancy coefficient, ? is a composite compressibility. E< 1-a/b is necessary for instability nucleation under the ``membrane diffusion" assumption. For a constant a/b<1, the ratio increases with E; the increasing rate is more rapid for larger a/b. The earthquake nucleation size from numerical simulations under nearly undrained condition agrees well with the analytical function. The second feature we study in relation to dilatancy is the percentages of slip released in the nucleation, coseismic and postseismic phases, respectively, in an earthquake cycle. When the time scales for pore pressure re-equilibration and state evolution are comparable, slip released in the seismogenic zone in the nucleation and postseismic phases both increase with E. Earthquakes under the influence of dilatancy experience exceptionally longer nucleation durations thus could release a significant amount of slip (e.g., ~ 40%) before seismic slip rate is reached. The large pre-seismic slip can be reduced by using much smaller, presumably more realistic, characteristic slip distance dc (e.g.10s ?m instead of 10s mm) in the seismogenic zone. Large postseismic slip (e.g. ~ 20%) in the seismogenic zone seems unrealistic based on available observations, and may be reduced by introducing thermal-weakening at the seismogenic depths. This model also includes a near-lithostatic fluid pressure zone, suggested to be related to the occurrence of episodic slow slip events and non-volcanic tremors in subduction zones, at the down-dip end of the seismogenic zone. Dilatancy stabilization effect becomes more significant at such extremely low ? condition. The resulted large values of E there can effectively arrest seismic rupture up-dip of the velocity-weakening to strengthening stability transition, a depth conventionally defined as the down-dip limit of the seismogenic zone in such calculations. Preliminary results show that at the depth of stability transition, at E~0.5, slip accumulated in the interseismic phase can be fully released in the nucleation and postseismic phases.

Liu, Y.; Rubin, A. M.

2009-12-01

191

Thermal structure and megathrust seismogenic potential of the Makran subduction zone  

NASA Astrophysics Data System (ADS)

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.

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

2013-04-01

192

Dependence of earthquake size distributions on convergence rates at subduction zones  

SciTech Connect

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).

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

1994-10-01

193

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

NASA Astrophysics Data System (ADS)

The pseudotachylytes (PT) were recently found in the Cretaceous Shimanto accretionary complex of eastern and western Shikoku, southwest Japan, but their microstructures under a backscattered electron image, chemical composition, and effects of frictional melting on co-seismic slip in the accretionary prism remains poorly understood. The PT bearing fault is the 1-2 m thick roof thrust of a duplex structure, which bounds the off-scraped coherent turbidites above from the imbricated melange below without a thermal inversion across the fault. The fault zone consists of foliated cataclasite of sandstone-shale melange in origin and dark veins. The PT commonly occurs as brecciated fragments in dark veins. The PT matrix is transparent under plane-polarized light and is optically homogeneous under cross-polarized light, similar to glass matrix. Under a backscattered electron image, the PT clearly shows the evidences for frictional melting and subsequent rapid cooling: rounded and irregularly shaped grains and vesicles in matrix and fracturing associated with grain margins. These textural features of the PT are very similar to those of experimentally generated PT. The EPMA analysis indicates that chemical composition of the PT matrix corresponds to illite with 5.7-9.9 wt% H2O and that partially melted grains are dominated by orthoclase and quartz. This indicates that the temperatures of the PT melt could reach the breakdown temperatures of orthoclase (1150 C) and quartz (1730 C), greater than the maximum temperature recorded in host rocks (170-200 C). We calculated the viscosity of friction melt, based on the chemical composition of the PT matrix and the volume fraction and aspect ratio of grains in the PT. We considered both Arrhenian and non-Arrhenian models for viscosity calculation. Our result demonstrates that the melt viscosity is much lower than PT in continental plutonic and metamorphic rocks: 10^3 Pa s (Arrhenian model) and 10^2 Pa s (non-Arrhenian model) even at 700 C and 10 Pa s (both models) at 1200 C. The extremely low melt viscosity is caused primarily by the formation of liquids (release of OH-) from hydrous illite, and secondarily by small volume fraction (< 20%) of grains in the PT. Because illite is commonly present in accretionary prisms, generation of a low viscosity melt from illite would lead to fault lubrication and hence control the efficiency of stored strain energy release and earthquake magnitude in subduction zones.

Ujiie, K.; Yamaguchi, H.

2004-12-01

194

Eocene supra-subduction zone mafic magmatism in the Sibumasu Block of SW Yunnan: Implications for Neotethyan subduction and India-Asia collision  

NASA Astrophysics Data System (ADS)

Geochemical and isotopic data for metabasic rocks in NW Yunnan constrain the nature of the mantle source beneath the East Himalayan Syntaxis during the Neotethyan subduction and the timing of initial collision between India and Asia. Our results indicate that these mafic rocks crystallized at 50-55 Ma and were metamorphosed at ~ 39 Ma, contemporaneous with magmatic flare-ups and high-grade metamorphism in South Tibet. The mafic rocks in NW Yunnan are divisible into three groups. Group 1, from Nabang, shows geochemical affinity to a MORB-like source modified by the recycled component with ?Nd(t) of + 5.1 to + 9.5 and ?Hf(t) of + 5.1 to + 11.6, and resembles back-arc basin basalt. Group 2 is mainly from Jinzhouzhai and Tongbiguan to the east of Group 1, and is marked by high Al2O3, enrichment in LILEs and depletion in HFSEs, along with negative ?Nd(t) (- 1.1 to - 5.6) and ?Hf(t) (- 4.6 to - 0.9) values. Group 2 is interpreted to have originated from the lithospheric mantle modified by input from a slab-derived component in a continental arc setting. Group 3, from Nanjingli, is located a considerable distance east from Group 1, and exhibits negative ?Nd(t) (- 6.7 to - 7.6) and ?Hf(t) (- 7.1 to - 2.2), and is inferred to be derived from the Tengchong lithospheric mantle but with limited involvement of a slab-derived component. Overall Nd-Hf isotopic compositions show a sharp decrease from Group 1 to Group 2 and then a more subdued decrease to Group 3, with the three groups corresponding to an overall spatial distribution from west to east. Our results suggest that early Eocene magmatism in NW Yunnan represents the eastward continuation of the Gangdese magmatic belt and that the Neotethyan subduction continued until ~ 50 Ma followed by the India-Asia collision. It is proposed that at least two eastward-dipping subduction zones are involved in the Neotethyan suprasubduction system prior to 55 Ma. The sudden decrease in convergence rate in the early Eocene (55-50 Ma), together with the inferred positive buoyancy of the Tengchong lithosphere, stimulated rollback of down-going slab and induced melting of heterogeneous mantle sources to result in a flare-up in magmatic activity.

Wang, Yuejun; Zhang, Limin; Cawood, Peter A.; Ma, Liyan; Fan, Weiming; Zhang, Aimei; Zhang, Yuzhi; Bi, Xianwu

2014-10-01

195

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

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

L. McNeill; T. Henstock; D. Tappin

2005-01-01

196

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

PubMed

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

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

1990-11-30

197

Plate rotation during continental collision and its relationship with the exhumation of UHP metamorphic terranes: application to the Norwegian Caledonides  

NASA Astrophysics Data System (ADS)

Lateral variation and asynchronous onset of collision during the convergence of continents can significantly affect the burial and exhumation of subducting material. We use 3D numerical models for continental collision to discuss how deep burial and exhumation of ultra-high pressure metamorphic rocks are enhanced by oblique convergence and resulting rotation of the colliding plates. Rotation during collision locally favours eduction, the inversion of the subduction process following ocean slab break-off, and may relate to the discontinuous distribution of ultra-high pressure (UHP) terranes along collision zones. For example the terminal (Scandian) collision of Baltica and Laurentia, which formed the Scandinavian Caledonides resulted in the exhumation of only one large high pressure/ultra-high pressure (HP/UHP) terrane, the Western Gneiss Complex (WGC), near the southern end of the collision zone. Rotation of the subducting Baltica plate during collision may provide a likely explanation for this distribution. We explore this hypothesis by comparing orthogonal and oblique collision models and conclude that an oblique collision can transport continental material up to 60km deeper, and heat material up to 300°C hotter, than an orthogonal collision. Our oblique collision model predicts that subducted continental margin material returns to the surface only in the region where collision initiated. The oblique collision model is consistent with petrological and geochonological observations from the Western Gneiss Complex and makes predictions for the general evolution of the Scandinavian Caledonides. We propose the collision between Laurentia and Baltica started at the southern end of the collisional zone, and propagated northward. This asymmetric geometry resulted in the counter clockwise rotation of Baltica and the northwards movement of Baltica's rotational pole with respect to Laurentia, consistent with paleomagnetic data from other studies. Our model has applications to others orogens with regional UHP terranes, such as the Dabie Shan and Papua New Guinea cases, where block rotation during exhumation has also been recorded.

Bottrill, Andrew; van Hunen, Jeroan; Cuthbert, Simon; Allen, Mark; Brueckner, Hannes

2014-05-01

198

Metamorphic rocks of the Yap arc-trench system  

Microsoft Academic Search

The Yap trench-arc is a link between the Mariana and Philippine arcs; the latter are both loci of acive volcanism and seismicity but the Yap arc is formed of metamorphic rocks and has had few historic earthquakes. It does not appear to be an active subduction zone. The 8-9 km deep Yap trench has a steep west well, it has

J. Hawkins; R. Batiza

1977-01-01

199

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

NASA Astrophysics Data System (ADS)

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.

Li, Tao; Hampel, Andrea

2013-04-01

200

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

201

Mantle Flow in the Rivera-Cocos Subduction Zone  

Microsoft Academic Search

Western Mexico, where the young and small Rivera plate and the adjacent large Cocos plate are subducting beneath the North American plate, is a unique region on Earth where tearing of subducting oceanic plates, as well as fragmentation of the overriding continental plate, is occurring today. Characterizing the mantle flow field that accompanies the subduction of the Rivera and adjacent

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

2009-01-01

202

Doubly Vergent Accretionary Wedge Active Tectonics in The Sumatra Subduction Zone  

NASA Astrophysics Data System (ADS)

The Sumatra subduction zone is a classical example of oblique subduction where the slip is partitioned between an orthogonal component along the megathrust and an arc parallel component along the Sumatran fault. It was previously suggested that the part of the arc parallel motion is accommodated along the Mentawai strike-slip fault. Using high-quality industry seismic reflection profiles spaced at 20 km spacing, we find that the Mentawai Fault Zone is not a strike-slip fault, but it is actually a backthrust system developed along the boundary between the retro accretionary wedge and the continental backstop. The active deformation along the western margin of the present day Mentawai forearc basin takes place along NW-SE trending parallel convex eastward anticlinal ridges. Beneath the anticlinal ridges, the deformation zone exhibits (1) main backthrust, as the upward continuation of the Mentawai Backthrust in forearc basin, (2) seaward verging imbricated thrusts developed in the accretionary wedge and (3) the landward verging thrust developed in the forearc basin. The Mentawai Backthrust developed in NW-SE direction along the eastern edge of the present outer arc islands, beneath the forearc basin sediments. This backthrust system formed as a long-lived boundary between the accretionary wedge and the continental backstop. This system was initiated by the translation and back-rotation of the closely spaced seaward-verging imbricated thrust, and developed through the continuous growth of the accretionary wedge. The compressional-dominated phase, active since the early Pliocene, initiated the Mentawai backthrust reactivation and deformed the basin fill sediments. To the west of the Main Backthrust, the deformation is interpreted to result from the back-rotation of the seaward verging imbricated thrusts in the accretionary wedge. Later, to the east of the Main backthrust, fault-related folding developed within sedimentary sequences and progressively moved to the east. The landward verging Mentawai Backthrust zone characterizes the major structural style in the Sumatra subduction system, and together with the Frontal Thrusts, formed the doubly vergent active tectonics in the Sumatra subduction system, which seems to be valid along much of the Andaman-Sumatra-Java margin.

Mukti, M. M.; Singh, S. C.; Hananto, N. D.; Permana, H.; Deighton, I.

2011-12-01

203

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

Microsoft Academic Search

Processes and inputs involved in generating arc magmas are reasonably well known but quantitative modeling is often overlooked when teaching about subduction zone magmagenesis. In order to appreciate these complexities, students need to be able to explore subduction zone magmagenetic processes using trace element compositions of igneous rocks. The MARGINS Subduction Factory experiment selected two endmember convergent margins, the Izu-Bonin-Mariana

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

2010-01-01

204

Seismic hazard from the Hispaniola subduction zone: Correction to "Historical perspective on seismic hazard to Hispaniola and  

E-print Network

Seismic hazard from the Hispaniola subduction zone: Correction to "Historical perspective), Seismic hazard from the Hispaniola subduction zone: Correction to "Historical perspective on seismic.1002/jgrb.50388. [1] In the paper "Historical perspective on seismic hazard to Hispaniola and the northeast

ten Brink, Uri S.

205

Mobile Belts, High Lithosphere Temperatures, and Subduction Zone Backarcs  

NASA Astrophysics Data System (ADS)

At many continental plate boundaries, there are mobile orogenic belts or zones of distributed deformation hundreds of kilometres wide. An important characteristic of "mobile belts" is a long geological history of ongoing deformation or of repeated deformation events that suggests long-term lithosphere weakness compared to cratons. Thus, they are "mobile" not just because they are subject to deforming forces, but also because they are weak. In contrast, Precambrian cratons have exhibited very little internal deformation for long geological periods, and are inferred to have long-term strength. We propose that most mobile belts are weak because they are hot, and they are hot because they are in backarcs or geologically recent backarcs. Most continental backarcs are observed to be hot; the temperature at the Moho is ~900C compared to ~400C for cratons, and the lithosphere thicknesses are 50-60 km compared to more than 250 km for cratons. The high temperatures result in thermal expansion and the common high elevations in backarc mobile belts, even where the crust is not thickened. Backarc mobile belt lithospheres are at least a factor of 10 weaker than those of cratons. Backarcs may be hot and have thin, weak lithospheres because of the water driven upward into the backarc mantle wedge from dehydration of the underlying subducting plate. The water reduces the effective asthenosphere viscosity and allows vigorous free convection. High backarc temperatures appear to decay after subduction stops, with a time constant of 50-100 m.y. Many continental mobile belts are hot enough for there to be a weak lower crust detachment so surface tectonics are decoupled from the upper mantle. For example, foreland thrusting at the craton edge of a mobile belt may be driven by continental margin plate boundary forces 100s of km away, with translation of the crust over a lower crust detachment.

Hyndman, R. D.

2003-12-01

206

Effect of Sediments on Rupture Dynamics of Shallow Subduction Zone Earthquakes and Tsunami Generation  

NASA Astrophysics Data System (ADS)

Low-velocity fault zones have long been recognized for crustal earthquakes by using fault-zone trapped waves and geodetic observations on land. However, the most pronounced low-velocity fault zones are probably in the subduction zones where sediments on the seafloor are being continuously subducted. In this study I focus on shallow subduction zone earthquakes; these earthquakes pose a serious threat to human society in their ability in generating large tsunamis. Numerous observations indicate that these earthquakes have unusually long rupture durations, low rupture velocities, and/or small stress drops near the trench. However, the underlying physics is unclear. I will use dynamic rupture simulations with a finite-element method to investigate the dynamic stress evolution on faults induced by both sediments and free surface, and its relations with rupture velocity and slip. I will also explore the effect of off-fault yielding of sediments on the rupture characteristics and seafloor deformation. As shown in Ma and Beroza (2008), the more compliant hanging wall combined with free surface greatly increases the strength drop and slip near the trench. Sediments in the subduction zone likely have a significant role in the rupture dynamics of shallow subduction zone earthquakes and tsunami generation.

Ma, S.

2011-12-01

207

FDM Simulation of an Anomalous Later Phase from the Japan Trench Subduction Zone Earthquakes  

Microsoft Academic Search

We investigated the development of a distinct later phase observed at stations near the Japan Trench associated with shallow, outer-rise earthquakes off the coast of Sanriku, northern Japan based on the analysis of three-component broadband seismograms and FDM simulations of seismic wave propagation using a heterogeneous structural model of the Japan Trench subduction zone. Snapshots of seismic wave propagation obtained

Shinako Noguchi; Takuto Maeda; Takashi Furumura

2011-01-01

208

Rayleigh Wave Tomography in the Nicaragua-Costa Rica Subduction Zone  

Microsoft Academic Search

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

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

2006-01-01

209

Geodetic observations of interseismic strain segmentation at the Sumatra subduction zone  

Microsoft Academic Search

Deformation above the Sumatra subduction zone, revealed by Global Positioning System (GPS) geodetic surveys, shows nearly complete coupling of the forearc to the subducting plate south of 0.5°S and half as much to the north. The abrupt change in plate coupling coincides with the boundary between the rupture zones of the 1833 and 1861 (Mw>8) thrust earthquakes. The rupture boundary

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

1997-01-01

210

Plate interaction in the NE Caribbean subduction zone from continuous GPS observations  

E-print Network

2012; published 19 May 2012. [1] Kinematic similarities between the Sumatra and Puerto Rico Trenches (10 ) to the NA-CA plate boundary, similar to that along Sumatra subduction zone. Following the 2004 Sumatra earthquake and tsunami, concern grew that the Puerto Rico Trench might be capable of producing

ten Brink, Uri S.

211

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

E-print Network

Distribution of slip from 11 Mw > 6 earthquakes in the northern Chile subduction zone M. E to constrain the relative location of coseismic slip from 11 earthquakes on the subduction interface both jointly and separately for the four largest earthquakes during this time period (1993 Mw 6.8; 1995

Simons, Mark

212

Stress drop as a criterion to differentiate subduction zones where Mw 9 earthquakes can occur  

E-print Network

Stress drop as a criterion to differentiate subduction zones where Mw 9 earthquakes can occur Tetsuzo Seno Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan a b earthquake Asperity Scaling relation Pore fluid pressure ratio Seismic coupling I propose a hypothesis

Seno, Tetsuzo

213

Shear wave anisotropy beneath the Cascadia subduction zone and western North American craton  

Microsoft Academic Search

We have examined shear wave splitting of SKS phases at 26 permanent broadband stations in western North America to constrain regional trends in anisotropy at the Cascadia subduction zone (CSZ) and adjacent regions. At forearc stations above the Juan de Fuca Plate, the fast directions are approximately parallel to the direction of absolute plate motion of the main Juan de

Claire A. Currie; John F. Cassidy; Roy D. Hyndman; Michael G. Bostock

2004-01-01

214

Tsunami inundation at Crescent City, California generated by earthquakes along the Cascadia Subduction Zone  

Microsoft Academic Search

We model tsunami inundation and runup heights in Crescent City, California triggered by possible earthquakes on the Cascadia Subduction Zone (CSZ). The CSZ is believed capable of producing great earthquakes with magnitudes of M w ~ 9.0 or greater. We simulate plausible CSZ rupture scenarios and calculate inundation using MOST. We benchmark our CSZ inundation projections against mapped flooded areas

Burak Uslu; José C. Borrero; Lori A. Dengler; Costas E. Synolakis

2007-01-01

215

Regional P wave velocity structure of the Northern Cascadia Subduction Zone  

Microsoft Academic Search

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

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

2006-01-01

216

Archaeological Evidence for Village Abandonment Associated with Late Holocene Earthquakes at the Northern Cascadia Subduction Zone  

Microsoft Academic Search

Geologic evidence suggests that great (magnitude 8 or larger) earthquakes, or series of such earthquakes, occurred six times in the past 3000 yr at the northern Cascadia subduction zone. The archaeological record, and native oral traditions, demonstrate that native villages along the adjacent coasts of southern British Columbia and Washington State were occasionally abandoned in the late Holocene as a

Ian Hutchinson; Alan D. McMillan

1997-01-01

217

Case study of GIS data integration and visualization in marine tectonics: The Cascadia subduction zone  

Microsoft Academic Search

A raster\\/vector geographic information system (GIS) has been developed at the College of Oceanic and Atmospheric Sciences at Oregon State University to support investigations of the active tectonics and earthquake potential of the Cascadia subduction zone. The Cascadia marine GIS uses Erdas Imagine as its core display and database software. Supporting software includes specialized processing packages for multibeam bathymetric data,

Chris Goldfinger; Lisa C. McNeill; Cheryl Hummon

1997-01-01

218

Tsunami inundation at Crescent City, California generated by earthquakes along the Cascadia Subduction Zone  

Microsoft Academic Search

We model tsunami inundation and runup heights in Crescent City, California triggered by possible earthquakes on the Cascadia Subduction Zone (CSZ). The CSZ is believed capable of producing great earthquakes with magnitudes of Mw ? 9.0 or greater. We simulate plausible CSZ rupture scenarios and calculate inundation using MOST. We benchmark our CSZ inundation projections against mapped flooded areas and

Burak Uslu; José C. Borrero; Lori A. Dengler; Costas E. Synolakis

2007-01-01

219

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

Microsoft Academic Search

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

Stephane Mazzotti; John Adams

2004-01-01

220

Correlation of tremor activity with tidal stress in the northern Cascadia subduction zone  

Microsoft Academic Search

We analyze hourly data from five tremor episodes in the northern Cascadia subduction zone over the period 2003–2005 provided by the Tremor Activity Monitoring System (TAMS). All five tremor episodes correspond to slow slip events observed by GPS. Fourier decomposition is used to separate the hourly tremor counts for each episode into “long-period” (0 < f < 0.8 cpd), “tidal”

Anthony Lambert; Honn Kao; Garry Rogers; Nicholas Courtier

2009-01-01

221

In-Slab Earthquakes at the North End of the Cascadia Subduction Zone  

Microsoft Academic Search

At the north end of the Cascadia subduction zone, in-slab earthquakes occur within the Juan de Fuca plate up to depths of 100 km and in two distinct concentrations. One concentration, ranging in depth from 25 to 35 km, lies beneath the west coast of Vancouver Island. First motion focal mechanism solutions from this group of earthquakes illustrate a complex

M. K. Bolton; G. C. Rogers; T. L. Mulder

2001-01-01

222

GPS constraints on 34 slow slip events within the Cascadia subduction zone, 1997-2005  

Microsoft Academic Search

Refinements to GPS analyses in which we factor geodetic time series to better estimate both reference frames and transient deformation resolve 34 slow slip events located throughout the Cascadia subduction zone from 1997 through 2005. Timing of transient onset is determined with wavelet transformation of geodetic time series. Thirty continuous stations are included in this study, ranging from northern California

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

2008-01-01

223

IODP Expedition 328: Cascadia Subduction Zone ACORK Observatory Site U1364 Summary  

E-print Network

IODP Expedition 328: Cascadia Subduction Zone ACORK Observatory Site U1364 Summary The operational documentation of the average state of pressure in the frontal part of the Cascadia accretionary prism and aseismic slip in this subduction setting. At a later date, the observatory will be connected to the NEPTUNE

224

Correlation of tremor activity with tidal stress in the northern Cascadia subduction zone  

Microsoft Academic Search

We analyze hourly data from five tremor episodes in the northern Cascadia subduction zone over the period 2003-2005 provided by the Tremor Activity Monitoring System (TAMS). All five tremor episodes correspond to slow slip events observed by GPS. Fourier decomposition is used to separate the hourly tremor counts for each episode into ``long-period'' (0 < f < 0.8 cpd), ``tidal''

Anthony Lambert; Honn Kao; Garry Rogers; Nicholas Courtier

2009-01-01

225

IODP Expedition 328: Cascadia Subduction Zone ACORK Observatory Week 1 Report (511 September)  

E-print Network

IODP Expedition 328: Cascadia Subduction Zone ACORK Observatory Week 1 Report (5­11 September of the average state of pressure in the frontal part of the Cascadia accretionary prism, the pressure gradients slip in this subduction setting. At a later date, the observatory will be connected to the NEPTUNE

226

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

Microsoft Academic Search

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

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

227

Recurrence of Great Earthquakes: Evidence of Double Periodicity Along the Cascadia Subduction Zone  

Microsoft Academic Search

Since the recognition that the Cascadia subduction zone in the US Pacific Northwest has produced large magnitude earthquakes in the past seven thousand years there has been considerable discussion centered on the dates and intervals between the earthquakes. Accurate information about the intervals between events improves the estimated date and magnitude of the next great earthquake and increases our ability

C. Jurney

2002-01-01

228

GPS constraints on 34 slow slip events within the Cascadia subduction zone, 1997–2005  

Microsoft Academic Search

Refinements to GPS analyses in which we factor geodetic time series to better estimate both reference frames and transient deformation resolve 34 slow slip events located throughout the Cascadia subduction zone from 1997 through 2005. Timing of transient onset is determined with wavelet transformation of geodetic time series. Thirty continuous stations are included in this study, ranging from northern California

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

2008-01-01

229

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

Microsoft Academic Search

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

Garry Rogers; Herb Dragert

2003-01-01

230

Deep electrical conductivity structure of the Cascadia subduction zone in Southern British Columbia  

Microsoft Academic Search

Long period magnetotelluric (MT) measurements were made in southwestern British Columbia in 2003 to investigate the electrical structure of the Cascadia subduction zone as part of the POLARIS project. Data were recorded in the period range 1 to 25000 seconds at 36 field sites using long-period MT systems with fluxgate magnetometers. The instruments used were the Geological Survey of Canada

W. Soyer; M. J. Unsworth; W. Xiao; G. J. Kovacs; C. Samson; P. Fernberg

2004-01-01

231

Geodetic and seismic signatures of episodic tremor and slip in the northern Cascadia subduction zone  

Microsoft Academic Search

Slip events with an average duration of about 10 days and effective total slip displacements of severalc entimetres have been detected on the deeper (25 to 45 km) part of the northern Cascadia subduction zone interface by observing transient surface deformation on a network of continuously recording Global Positioning System (GPS) sites. The slip events occur down-dip from the currently

H. Dragert; K. Wang; G. Rogers

2004-01-01

232

Shear wave anisotropy beneath the Cascadia subduction zone and western North American craton  

Microsoft Academic Search

SUMMARY We have examined shear wave splitting of SKS phases at 26 permanent broadband stations in western North America to constrain regional trends in anisotropy at the Cascadia subduction zone (CSZ) and adjacent regions. At forearc stations above the Juan de Fuca Plate, the fast directions are approximately parallel to the direction of absolute plate motion of the main Juan

Claire A. Currie; John F. Cassidy; Roy D. Hyndman; Michael G. Bostock

2004-01-01

233

The Apparent Stress Profile for Normal-Fault Earthquakes Across Subduction Zones  

Microsoft Academic Search

The behavior of apparent stress for normal-fault earthquakes across the various tectonic regimes of a subduction zone complex is derived by examining the apparent stress (tau a = mu ES\\/M_0 , where ES is radiated energy and M0 is seismic moment) of 139 large global earthquakes with depth < 70 km that occurred from 1987 to 2001. Using accurately determined

G. L. Choy; S. Kirby

2003-01-01

234

Composite Volcanoes, Stratovolcanoes, and Subduction-Zone Volcanoes (title provided or enhanced by cataloger)  

NSDL National Science Digital Library

This resource defines and describes composite volcanoes, stratovolcanoes, subduction-zone volcanoes and composite cones. The information is from different sources and therefore the site gives a broad picture of these forms. The shape of the volcano is described as a function of the type and frequency of eruption and its proximity to plate boundaries.

235

Subduction zone processes and the lithosphere-asthenosphere boundary: Electromagnetic Abstract by Samer Naif  

E-print Network

Subduction zone processes and the lithosphere-asthenosphere boundary: Electromagnetic insights with seismically observed lithosphere-asthenosphere boundary depths. A low degree of partial melt on the order of 1, we posit that the lithosphere is being decoupled from the deeper convecting mantle over a 25-km depth

Heaton, Thomas H.

236

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

237

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

238

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

NASA Astrophysics Data System (ADS)

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).

Nishikawa, T.; Ide, S.

2013-12-01

239

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

NASA Astrophysics Data System (ADS)

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.

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

2010-12-01

240

Deep low-frequency earthquakes in tectonic tremor along the Alaska-Aleutian subduction zone  

NASA Astrophysics Data System (ADS)

We 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 the Alaska Earthquake Information Center from 2005 to present. Visual inspection of waveform spectra and time series reveal dozens of 10 to 20 min bursts of tremor along the length of the Alaska-Aleutian subduction zone. We use autocorrelation to demonstrate that these tremor signals are composed of hundreds of repeating low-frequency earthquakes (LFEs). The tremor activity we characterize is localized in four segments, from east to west: Kodiak Island, Shumagin Gap, Unalaska, and Andreanof Islands. Although the geometry, age, thermal structure, frictional, and other relevant properties of the Alaska-Aleutian subduction zone are poorly known, these characteristics are likely to differ systematically from east to west. Locations near Kodiak Island are the most reliable because station coverage is more complete. LFE hypocenters in this region are located on the plate interface near the down-dip limit of the 1964 Mw 9.2 Alaska earthquake rupture area. LFE hypocenters in the remaining areas along the arc are also located down-dip of the most recent Mw 8+ megathrust earthquakes. Although these locations are less well constrained, our results support the hypothesis that tremor activity marks the down-dip rupture limit for great megathrust earthquakes in this subduction zone. Lastly, there is no correlation between the presence of tremor and particular aspects of over-riding or subducting plate geology or coupling. It appears that LFEs are a fundamental characteristic of the Alaska-Aleutian subduction zone.

Brown, Justin R.; Prejean, Stephanie G.; Beroza, Gregory C.; Gomberg, Joan S.; Haeussler, Peter J.

2013-03-01

241

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

USGS Publications Warehouse

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

Masterlark, T.

2003-01-01

242

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

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

Guibin Zhang; Shuguang Song; Lifei Zhang; Yaoling Niu

2008-01-01

243

Partitioning of Trace Elements Between Hydrous Minerals and Aqueous Fluids : a Contribution to the Chemical Budget of Subduction Zones  

NASA Astrophysics Data System (ADS)

Subduction zones are powerful chemical engines where the downgoing lithosphere reacts with asthenospheric mantle and produces magmas. Understanding this deep recycling system is a scientific challenge requiring multiple approaches. Among those, it appears that we lack basic information on the composition of the fluid that begins the process of material transfer in subduction zones. Indeed, no pristine fluid sample has yet been collected from this particular environment. Albeit challenging, the alternative would be experimental study of fluids under the appropriate conditions. Consequently, we developed an experimental protocol to measure the concentration of aqueous fluids equilibrated with minerals up to pressures (P) of 5 GPa, at least and temperatures (T) of 550 C. This includes syntheses at high-P and -T conditions, and determination of the fluid composition. Syntheses were performed in a large volume belt-type press at the conditions, 2-5 GPa and ca. 550 C. Oxides or minerals were loaded with water in a gold capsule sealed afterwards. Presence of free fluid during experiments could be confirmed by direct observation of fluid release from the sealed capsule upon puncturing. The composition in trace elements of the fluids that were equilibrated at high-P and -T with minerals was reconstructed from that of the precipitates deposited at the surface of minerals after evaporation of the capsule. The precipitates were dissolved and analyzed by a leaching technique detailed in Koga et al. (2005). Two hydrous minerals of prime interest for subductions were sofar investigated: the high-pressure variety of serpentine, antigorite, and talc. The partitioning coefficients of a series of trace-elements will be presented, as well as their evolution as a function of pressure. Consequences for the composition of the fluids released during the dehydration of hydrous metamorphic minerals will be drawn. Those measurements are unlikely to be feasible at pressures in excess of 5 GPa, since limited by the sample size. Hence, in order to gain similar data at higher pressures, we begun in situ measurements in an externally heated diamond anvil cell, that allows to reach 10 GPa. Composition of aqueous fluids could be measured at the ppm level by synchrotron X-ray fluorescence. The first results and the perspectives will be presented. Koga K.T., I. Daniel, B. Reynard (2005), Geochem. Geophys. Geosyst., 6, Q09014, doi:10.1029/2005GC000944

Daniel, I.; Koga, K. T.; Reynard, B.; Petitgirard, S.; Chollet, M.; Simionovici, A.

2006-12-01

244

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

NASA Astrophysics Data System (ADS)

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

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

2009-11-01

245

Areas of slip of recent earthquakes in the Mexican subduction zone  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

246

Marine Gravity Measurements at the Subduction Zone offshore Central Chile  

NASA Astrophysics Data System (ADS)

Gravity measurements were carried out during RV SONNE cruise SO-161 (SPOC) in late 2001 between 28° S and 44° S offshore Central Chile along a total length of about 17500 km. The mean accuracy of the data measured with the seagravimeter system KSS31M of BGR is better than 1 mGal. Further foreign marine gravity data were not included due to their considerable lower accuracy. Additional marine gravity data derived from satellite altimetry are needed to augment our data from the survey area. The SPOC data set was compared with 3 different satellite gravity data compilations. The data set with the best statistical results for the gravity differences was used for further gravity map compilations. The map of the freeair gravity is dominated by the anomalies of the main topographic features in the survey area. In the W the oceanic crust of the subducting Nazca Plate is characterized by weak positve gravity anomalies. Landward the anomalies decrease rapidly to less than minus 150 mGal in the Chilean trench. Further towards the coast extends a broad zone of alternating positve and negative freeair gravity anomalies. These could be interpreted either in terms of morphology of the continental slope or heterogeneous density distribution in the upper crust. Additionally Bouguer gravity anomalies were calculated. The anomalies on the Nazca Plate are strongly positive with a clear south - north trending increase of values, which reflect the increasing age of the oceanic crust. The effect of isostatic compensation was calculated assuming Vening-Meinesz models with different parameters. The gravity effect of the isostatic compensation root was eliminated from the Bouguer gravity anomalies and serves as a residual field. The interpretation of isostatic residual fields in this complicated tectonic environment leads to the detection of a series of offshore basins. In the N and the centre of the survey area the distribution of the profiles is rather uniform. For these areas 3D density models were developed. The models consider the results of the multi channel reflection seismic and especially the refraction seismic lines. The models show the density structure of the accretionary wedge which expands from N to S, the continental slope and various basins near to the coast. The possibility of a subduction channel in the southern area is discussed. The effect and trace of fracture zones before and after subduction were investigated.

Heyde, I.; Kopp, H.; Reichert, C.

2003-12-01

247

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

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.

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

1994-03-01

248

Subduction zone earthquake as potential trigger of submarine hydrocarbon seepage  

NASA Astrophysics Data System (ADS)

Methane, a potent greenhouse gas, is abundant in marine sediments. Submarine seepage of methane-dominated hydrocarbons is heterogeneous in space and time, and mechanisms that can trigger episodic seep events are poorly understood. For example, critical gas pressures have been predicted to develop beneath impermeable sediments that bear gas hydrates, making them susceptible to mechanical failure and gas release. Gas hydrates often occur in seismically active regions, but the role of earthquakes as triggers of hydrocarbon seepage through gas-hydrate-bearing sediments has been only superficially addressed. Here we present geochemical analyses of sediment cores retrieved from the convergent margin off Pakistan. We find that a substantial increase in the upward flux of gas occurred within a few decades of a Mw 8.1 earthquake in 1945--the strongest earthquake reported for the Arabian Sea. Our seismic reflection data suggest that co-seismic shaking fractured gas-hydrate-bearing sediments, creating pathways for the free gas to migrate from a shallow reservoir within the gas hydrate stability zone into the water column. We conservatively estimate that 3.26×108mol of methane have been discharged from the seep site since the earthquake. We therefore suggest that hydrocarbon seepage triggered by earthquakes needs to be considered in local and global carbon budgets at active continental margins.

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

2013-08-01

249

Fluid-Mediated Redox Processes at Subduction Zones (Invited)  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

250

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

NASA Astrophysics Data System (ADS)

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.

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

2014-01-01

251

Synthetic SKS splitting at subduction zones: reconciling observations with mantle flow  

NASA Astrophysics Data System (ADS)

The type of mantle viscous flow associated with the sinking of rigid oceanic plates is still poorly understood. One way to determine the geometry of the deformational field at subduction zones is by measuring shear wave splitting. The azimuth of the fast shear wave component is thought to be parallel to direction of mantle flow. However, recent studies have shown that this is true only in the case of simple shear deformation and that, hence, a more accurate analysis is needed. We compute the seismic anisotropy due to strain-induced lattice preferred orientation (LPO) in mechanical models of a rigid plate subducting in a non-newtonian mantle. Subsequently we compute seismogram synthetics and compare the SKS splitting parameters with those observed at subduction zones. Synthetic data reproduce quite well the observations, suggesting that the interpretation of seismic anisotropy, based on coupled flow models and LPO calculations, may be used to constrain the kinematic and dynamic behavior of the Earth's interior.

Faccenda, M.; Capitanio, F. A.:

2012-04-01

252

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

NASA Astrophysics Data System (ADS)

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.

Pommier, Anne

2014-12-01

253

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

Microsoft Academic Search

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

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

2004-01-01

254

Upper mantle structure of marginal seas and subduction zones in northeastern Eurasia from Rayleigh wave tomography  

Microsoft Academic Search

The upper mantle structure of marginal seas (the Seas of Japan and Okhotsk) and subduction zones in northeastern Eurasia is investigated, using the three-stage multimode surface wave tomography incorporating finite-frequency effects. Broadband waveform data from 305 events with magnitude greater than 5.5 from 1990 to 2005 recorded at 25 stations of the IRIS network in northeastern Eurasia and Japan and

E. Bourova; K. Yoshizawa; K. Yomogida

2010-01-01

255

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

Microsoft Academic Search

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

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

2004-01-01

256

Deformation and stress change associated with plate interaction at subduction zones: a kinematic modelling  

Microsoft Academic Search

The interseismic deformation associated with plate coupling at a subduction zone is commonly simulated by the steady-slip model in which a reverse dip-slip is imposed on the down-dip extension of the locked plate interface, or by the backslip model in which a normal slip is imposed on the locked plate interface. It is found that these two models, although totally

Shaorong Zhao; Shuzo Takemoto

2000-01-01

257

The water and trace element contents of melt inclusions across an active subduction zone  

Microsoft Academic Search

Water concentrations of olivine-hosted melt inclusions show no consistent variation across the northern part of the Central American subduction zone in southeastern Guatemala. Magmatic water contents remain moderately high (~2 wt%) throughout the back-arc region. Melt inclusions from some of these back-arc basalts also have notably high CO 2contents (>900 ppm CO 2). The B and B\\/Ce ratios of melt inclusions

James A. Walker; Kurt Roggensack; Lina C. Patino; Barry I. Cameron; Otoniel Matías

2003-01-01

258

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

Microsoft Academic Search

Strong ground motions are estimated for the Pacific Northwest assuming that large shallow earthquakes, similar to those experienced in southern Chile, southwestern Japan, and Colombia, may also occur on the Cascadia subduction zone. Fifty-six strong motion recordings for twenty-five subduction earthquakes ofMs=7.0 are used to estimate the response spectra that may result from earthquakesMwMw 9.5) is the largest event that

Thomas H. Heaton; Stephen H. Hartzell

1989-01-01

259

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

Microsoft Academic Search

Strong ground motions are estimated for the Pacific Northwest assuming that large shallow earthquakes, similar to those experienced in southern Chile, southwestern Japan, and Colombia, may also occur on the Cascadia subduction zone. Fifty-six strong motion recordings for twenty-five subduction earthquakes of M s>=7.0 are used to estimate the response spectra that may result from earthquakes M w<81\\/4. Large variations

Thomas H. Heaton; Stephen H. Hartzell

1989-01-01

260

Array measurements of deep tremor signals in the Cascadia subduction zone  

Microsoft Academic Search

Preliminary analysis of deep tremor recorded during July, 2004, in the Cascadia Subduction zone shows that small aperture arrays can resolve the slowness and back azimuth of seismic waves with a useful resolution. Data were collected by three dense arrays of short-period seismometers specifically deployed in the Puget Sound area under an US-Italy-Canada cooperative effort. Slowness analyses at the three

Mario La Rocca; Wendy McCausland; Danilo Galluzzo; Steve Malone; Gilberto Saccorotti; Edoardo Del Pezzo

2005-01-01

261

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

Microsoft Academic Search

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

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

2003-01-01

262

Tsunami exposure estimation with land-cover data: Oregon and the Cascadia subduction zone  

Microsoft Academic Search

A Cascadia subduction-zone earthquake has the potential to generate tsunami waves which would impact more than 1000km of coastline on the west coast of the United States and Canada. Although the predictable extent of tsunami inundation is similar for low-lying land throughout the region, human use of tsunami-prone land varies, creating variations in community exposure and potential impacts. To better

Nathan Wood

2009-01-01

263

Mantle wedge flow at the northern Cascadia subduction zone: Observational constraints and numerical models  

Microsoft Academic Search

We investigate mantle flow in the backarc of the northern Cascadia subduction zone (48-51\\\\deg N). Surface heat flow, seismic velocity, effective elastic thickness, thermal isostasy, and xenolith studies indicate that the backarc is extremely hot, with estimated temperatures of 800-1000\\\\deg C at the Moho (35 km depth) and a lithosphere thickness of only 50-60 km. The uniformly high temperatures are

C. A. Currie; R. D. Hyndman; K. Wang; J. F. Cassidy; J. He

2004-01-01

264

Crustal anisotropy in the forearc of the Northern Cascadia Subduction Zone, British Columbia  

Microsoft Academic Search

This paper aims to identify sources and variations of crustal anisotropy from shear-wave splitting measurements in the forearc of the Northern Cascadia Subduction Zone of southwest British Columbia. Over 20 permanent stations and 15 temporary stations were available for shear-wave splitting analysis on ˜4500 event-station pairs for local crustal earthquakes. Results from 1100 useable shear-wave splitting measurements show spatial variations

N. J. Balfour; J. F. Cassidy; S. E. Dosso

2011-01-01

265

Assessment of flexural analysis applied to the Sumatra-Java subduction zone  

Microsoft Academic Search

Indian Ocean subduction zone is one of the most active plate margins of the globe as evident from its vast record of great\\u000a magnitude earthquake and tsunami events. We use Bouguer admittance (Morlet isostatic response function) in Sumatra-Java subduction\\u000a zones comprising both the subduction and over-riding plates to determine the lithospheric mechanical strength variations.\\u000a We determine effective elastic thickness (T

R. T. Ratheesh Kumar; Tanmay K. Maji; Rajesh R. Nair

2010-01-01

266

Numerical simulation of seismic cycles at a subduction zone with a laboratory-derived friction law  

Microsoft Academic Search

We review recent studies of numerical modeling of seismic cycles at subduction zones in Japan. Laboratory-derived rate- and state-dependent friction laws are most useful for modeling the entire seismic cycles including aseismic sliding and healing. Among several versions of the rate- and state-dependent law, the composite law better fits experimental observations for a wide range of conditions and is most

Naoyuki Kato; Kazuro Hirahara; Mikio Iizuka

267

Coupling of Oceanic and Continental Crust During Eocene Eclogite-Facies Metamorphism: Evidence From the Monte Rosa Nappe, Western Alps, Italy  

NASA Astrophysics Data System (ADS)

Subduction of continental crust to HP-UHP metamorphic conditions requires overcoming density contrasts that are unfavorable to deep burial, whereas exhumation of these rocks can be reasonably explained through buoyancy-assisted transport in the subduction channel to more shallow depths. In the western Alps, both continental and oceanic lithosphere has been subducted to eclogite-facies metamorphic conditions. The burial and exhumation histories of these sections of lithosphere bear directly on the dynamics of subduction and the stacking of units within the subduction channel. We address the burial history of the continental crust with high precision U-Pb rutile and Lu-Hf garnet geochronology of the eclogite-facies Monte Rosa nappe (MR), western Alps, Italy. U-Pb rutile ages from quartz-carbonate-white mica-rutile veins that are hosted within eclogite and schist of the MR, Gressoney Valley, Italy, indicate that it was at eclogite-facies metamorphic conditions at 42.6 +/- 0.6 Ma. The sample area (Indren glacier, Furgg zone; Dal Piaz, 2001) consists of eclogite boudins that are surrounded by micaceous schist. Associated with the eclogite and schist are quartz-carbonate-white mica-rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins occurred at eclogite-facies metamorphic conditions (480-570°C, >1.3-1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. Lu-Hf geochronology of garnet from a chloritoid-talc-garnet-phengite-quartz-calcite-pyrite - chalcopyrite bearing boudin within talc-chloritoid whiteschists of the MR, Val d'Ayas, Italy (Chopin and Monie, 1984; Pawlig, 2001) yields an age of 40.54 +/- 0.36 Ma. The talc-chloritoid whiteschists from the area record pressures and temperatures of 1.6-2.4 GPa and 500-530°C (Chopin and Monie, 1984; Le Bayon et al., 2006) indicating near UHP metamorphic conditions. Based on the age, P-T, and textural data, the rutile age likely represents the prograde-leg of the eclogite-facies P-T path whereas the Lu-Hf garnet age likely represents higher grade metamorphic conditions. The timing of eclogite-facies metamorphism in the MR is within the same time interval as the duration of prograde metamorphism (~55-40) recorded in the structurally overlying Zermatt-Saas ophiolite (ZSO; e.g., Amato et al., 1999; Lapen et al., 2003; Mahlen et al., this meeting). In particular, the Lu-Hf garnet age from the MR is identical within error to a relatively young 40.8 +/- 1.8 Ma Lu-Hf garnet-whole rock-cpx age from a structurally low slice of the ZSO at Saas-Fee, Switzerland (Mahlen et al., this meeting). Not only do the ages of eclogite-facies metamorphism overlap between the MR and ZSO, but so do the P-T conditions (e.g., between 1.6-2.8 GPa; 500-600°C). These data, combined with the relative structural positions of the MR and ZSO in the western Alps, suggest that the MR and ZSO were likely juxtaposed within the subduction channel through underplating of the MR beneath the ZSO. The strong negative buoyancy of the MR has likely aided in the exhumation of sections of the ZSO. Therefore, coupling of continental and oceanic terranes in a subduction channel, perhaps a general feature in the western Alps, may be critical in preventing permanent loss of oceanic crust to the mantle.

Lapen, T. J.; Johnson, C. M.; Baumgartner, L. P.; Skora, S.; Mahlen, N. J.; Beard, B. L.

2006-12-01

268

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

NASA Astrophysics Data System (ADS)

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.

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

2014-12-01

269

The southwestern edge of the Ryukyu subduction zone: A high Q mantle wedge  

NASA Astrophysics Data System (ADS)

The lateral edge of a subduction zone is usually depicted as an opening to the asthenosphere where invigorated dynamics and amplified magmatism take place. In this study we present evidence from seismic data suggesting the presence of a cold and dynamically sluggish edge environment at the southwest end of the Ryukyu subduction system. We measured attenuation, or 1/Q, for P waves from subduction zone events at ˜100 km depths received by OBSs in the Okinawa trough and land stations in NE Taiwan. In the Okinawa trough 100-200 km from the edge, Q values are lower than 100. In the vicinity of the edge, Q values increase from 100 to over 1000 towards Taiwan. To reconcile arguments from geophysical and geochemical observations, we propose that the mantle wedge near the edge has high Q values due to low temperatures and probably low water content. These may result from coupling of the slab laterally with the thick Eurasian lithosphere, which inhibits back-arc rifting, retards subduction, and reduces the water supply to the mantle wedge. The SW Ryukyu subduction system represents a subduction-zone edge type distinct from more commonly documented free or warm edges.

Ko, Yen-Ting; Kuo, Ban-Yuan; Wang, Kuo-Lung; Lin, Shu-Chuan; Hung, Shu-Huei

2012-06-01

270

Hazards and climatic impact of subduction-zone volcanism: A Global And Historical Perspective  

NASA Astrophysics Data System (ADS)

Subduction-zone volcanoes account for more than 80 percent of the documented eruptions in recorded history, even though volcanism—deep and, hence, unobserved—along the global oceanic ridge systems overwhelmingly dominates in eruptive output. Because subduction-zone eruptions can be highly explosive, they pose some of the greatest natural hazards to society if the eruptions occur in densely populated regions. Of the six worst volcanic disasters since A.D. 1600, five have occurred at subduction-zone volcanoes: Unzen, Japan (1792); Tambora, Indonesia (1815); Krakatau, Indonesia (1883); Mont Pelée, Martinique (1902); and Nevado del Ruiz, Colombia (1985). Sulfuric acid droplets in stratospheric volcanic clouds produced by voluminous explosive eruptions can influence global climate. The 1815 Tambora eruption caused in 1816 a decrease of several Celsius degrees in average summer temperature in Europe and the eastern United States and Canada, resulting in the well-known "Year Without Summer." Similarly, the eruptions of El Chichon (Mexico) in 1982 and of Mount Pinatubo (Philippines) in 1991 lowered average temperatures for the northern hemisphere by as much as 0.2 to 0.5 °C, respectively. However, eruption-induced climatic effects of historical eruptions appear to be short-lived, lasting at most for only a few years.

Tilling, Robert I.

271

Seismicity and state of stress in Guerrero segment of the Mexican subduction zone  

NASA Astrophysics Data System (ADS)

We take advantage of a relatively dense network of seismic stations in the Guerrero segment of the Mexican subduction zone to study seismicity and state of stress in the region. We combine our results with recent observations on the geometry of the subducted Cocos plate imaged from receiver function (RF) analysis, an ultraslow velocity layer mapped in the upper crust of the subducted slab, and episodic slow slip events (SSEs) and nonvolcanic tremors (NVTs) reported in the region to obtain a comprehensive view of the subduction process. Seismicity and focal mechanisms confirm subduction of the Cocos plate below Mexico at a shallow angle, reaching a depth of 25 km at a distance of 65 km from the trench. The plate begins to unbend at this distance and becomes horizontal at a distance of ˜120 km at a depth of 40 km. Some of the highlights of the inslab seismicity are as follows: (1) A cluster of earthquakes in the depth range of 25-45 km, immediately downdip from the strongly coupled part of the plate interface, revealing both downdip compressional and extensional events. This seismicity extends from ˜80 to 105 km from the trench and may be attributed to the unbending of the slab. (2) The slab devoid of seismicity in the distance range of ˜105-160 km. (3) Sparse inslab seismicity revealing downdip extension in the distance range of 160-240 km. The NVTs are also confined to this distance range. The episodic SSEs occur on the horizontal segment, in the distance range of ˜105-240 km, where an ultraslow velocity layer in the upper crust of the slab has been mapped from waveform modeling of converted SP phases. Thus, in the distance range of ˜105-160 km, SSEs occur but NVTs and inslab earthquakes are absent. This suggests that metamorphic dehydration reactions in the subducting oceanic crust and upper mantle begin at a distance of ˜160 km, giving rise to both the inslab earthquakes and NVTs. No inslab earthquake occurs beyond 240 km. The receiver function images and P wave tomography suggest that the slab begins a steep plunge at a distance of ˜310 km, reaching a depth of 500 km around 340 km from the trench. The negative buoyancy of such a slab should give rise to large extensional stress in the slab. Yet inslab seismicity is remarkably low, which may be explained by a slab that is not continuous up to a depth of 500 km, but is broken at a shallower depth. The resulting slab window may permit subslab material to flow through the gap. This may provide an explanation for the recent rift-related basalts found near Mexico City. The fore-arc, upper plate seismicity, which during the period of study (1995-2007) consisted of a moderate earthquake (Mw5.8) near the coast (H = 12 km), its numerous aftershocks, and two shallow events farther inland, demonstrates a trench-normal extension in the upper plate near this convergent margin, a state of stress that may be explained by tectonic erosion and/or seaward retreat of the trench. Seismicity, location of the mantle wedge, and rupture areas of Mexican earthquakes suggest that the downdip limit of rupture during large/great earthquakes in Guerrero may be 105 ± 15 km. Shallow-dipping, interplate thrust earthquakes are not the only type of events that affect the seismic hazard in the region. The magnitudes of inslab downdip compressional and extensional earthquakes that occur within ˜20 km inland from the coast, in the depth range of 25-45 km, may reach 6.5 and 7.5, respectively. In addition, we now identify normal-faulting earthquakes in the upper plate. These sources need to be taken into account in the hazard estimation.

Pacheco, Javier F.; Singh, Shri K.

2010-01-01

272

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)

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.

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

2013-08-01

273

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

NASA Astrophysics Data System (ADS)

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

Cooper, Lauren Beth

274

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

275

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

276

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

PubMed Central

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

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

2007-01-01

277

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

SciTech Connect

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

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

2007-01-01

278

Structure and seismogenic properties of the Mentawai segment of the Sumatra subduction zone revealed by local earthquake traveltime tomography  

NASA Astrophysics Data System (ADS)

On 12 September 2007, an Mw8.4 earthquake occurred within the southern section of the Mentawai segment of the Sumatra subduction zone, where the subduction thrust had previously ruptured in 1833 and 1797. Traveltime data obtained from a temporary local seismic network, deployed between December 2007 and October 2008 to record the aftershocks of the 2007 event, was used to determine two-dimensional (2-D) and three-dimensional (3-D) velocity models of the Mentawai segment. The seismicity distribution reveals significant activity along the subduction interface and within two clusters in the overriding plate either side of the forearc basin. The downgoing slab is clearly distinguished by a dipping region of highVp (8.0 km/s), which can be a traced to ˜50 km depth, with an increased Vp/Vs ratio (1.75 to 1.90) beneath the islands and the western side of the forearc basin, suggesting hydrated oceanic crust. Above the slab, a shallow continental Moho of less than 30 km depth can be inferred, suggesting that the intersection of the continental mantle with the subducting slab is much shallower than the downdip limit of the seismogenic zone despite localized serpentinization being present at the toe of the mantle wedge. The outer arc islands are characterized by low Vp (4.5-5.8 km/s) and high Vp/Vs (greater than 2.0), suggesting that they consist of fluid saturated sediments. The very low rigidity of the outer forearc contributed to the slow rupture of the Mw 7.7 Mentawai tsunami earthquake on 25 October 2010.

Collings, R.; Lange, D.; Rietbrock, A.; Tilmann, F.; Natawidjaja, D.; Suwargadi, B.; Miller, M.; Saul, J.

2012-01-01

279

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

NASA Astrophysics Data System (ADS)

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

Casey, J.; Dewey, J. F.

2013-12-01

280

Trench-parallel flow in the mantle wedge: insights from integrating seismology and 3D subduction zone modeling  

NASA Astrophysics Data System (ADS)

The mantle wedge in many subduction zones is characterized by a cold, low attenuation fore-arc and a hot, high attenuation arc and back-arc. The transition from fore-arc to arc mantle correlates in the Honshu, Nankai and possibly the Ryukyu subduction zones with a change in orientation of shear wave splitting from trench-parallel to trench-normal. For the Marianas, Andes and Central America subduction zones the trench-parallel splitting occurs further into the arc as well. The differences between these fundamental seismological observations suggest that multiple mechanisms can be responsible for the formation of trench-parallel anisotropy. Proposed mechanisms include fabric transitions in olivine due to changes in hydration and stress, melt alignment, and 3D flow. We use high resolution 2D and 3D dynamical models of these subduction zones to investigate the role of 3D flow and olivine fabric transitions in generating the observed patterns of anisotropy. The slab geometry is determined from available shallow geophysics and Benioff zone seismicity. For Ryukyu we obtain satisfactory splitting magnitudes for models that are based strictly on 2D corner flow with the B-type fabric in the cold fore-arc mantle. The widespread trench-parallel anisotropy in the Marianas and the 30-34S segment of the Andean subduction zone is better explained by 3D flow driven by along-trench pressure differences induced by changes in slab dip (Andes) or curvature (Marianas).Initial models for the Nicaragua-Costa Rica and Cascadia subduction zones incorporating present-day slab shape are dominated by 2D cornerflow and cannot explain the observed anisotropy. Future modeling will explore whether the time-dependent evolution of this subduction zone can create three-dimensional flow patterns with significant arc-parallel flow.

van Keken, P. E.; Kneller, E. A.; Syracuse, E.; Long, M.; Abt, D.; Abers, G.; Fischer, K.

2004-12-01

281

Trench-parallel flow in the mantle wedge: insights from integrating seismology and 3D subduction zone modeling  

NASA Astrophysics Data System (ADS)

The mantle wedge in many subduction zones is characterized by a cold, low attenuation fore-arc and a hot, high attenuation arc and back-arc. The transition from fore-arc to arc mantle correlates in the Honshu, Nankai and possibly the Ryukyu subduction zones with a change in orientation of shear wave splitting from trench-parallel to trench-normal. For the Marianas, Andes and Central America subduction zones the trench-parallel splitting occurs further into the arc as well. The differences between these fundamental seismological observations suggest that multiple mechanisms can be responsible for the formation of trench-parallel anisotropy. Proposed mechanisms include fabric transitions in olivine due to changes in hydration and stress, melt alignment, and 3D flow. We use high resolution 2D and 3D dynamical models of these subduction zones to investigate the role of 3D flow and olivine fabric transitions in generating the observed patterns of anisotropy. The slab geometry is determined from available shallow geophysics and Benioff zone seismicity. For Ryukyu we obtain satisfactory splitting magnitudes for models that are based strictly on 2D corner flow with the B-type fabric in the cold fore-arc mantle. The widespread trench-parallel anisotropy in the Marianas and the 30-34S segment of the Andean subduction zone is better explained by 3D flow driven by along-trench pressure differences induced by changes in slab dip (Andes) or curvature (Marianas).Initial models for the Nicaragua-Costa Rica and Cascadia subduction zones incorporating present-day slab shape are dominated by 2D cornerflow and cannot explain the observed anisotropy. Future modeling will explore whether the time-dependent evolution of this subduction zone can create three-dimensional flow patterns with significant arc-parallel flow.

van Keken, P. E.; Kneller, E. A.; Syracuse, E.; Long, M.; Abt, D.; Abers, G.; Fischer, K.

2007-12-01

282

In-situ formation of Indian Mantle in global subduction zones  

NASA Astrophysics Data System (ADS)

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

Nebel, Oliver; Arculus, Richard; Davies, Rhodri

2014-05-01

283

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)

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.

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

2014-10-01

284

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

NASA Astrophysics Data System (ADS)

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.

Rajendran, C.

2013-05-01

285

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

286

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

NASA Astrophysics Data System (ADS)

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.

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

2014-11-01

287

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

USGS Publications Warehouse

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

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

1989-01-01

288

Deformation and mantle flow beneath the Sangihe subduction zone from seismic anisotropy  

NASA Astrophysics Data System (ADS)

Subduction of oceanic lithosphere is the most direct feedback between the Earth's surface and deep interior. However, the detail of its interaction with the broader convecting mantle is still unclear. Mantle flow around subduction zones can be constrained using seismic anisotropy, but despite many such studies, a simple global picture is lacking. The Sangihe subduction zone (where the Molucca Sea microplate is subducting westward beneath the Eurasian plate) is part of the tectonically complex Sulawesi-Philippine region, and an ideal natural laboratory to study complex subduction processes. We investigate the anisotropic structure of the Sangihe subduction zone with shear wave splitting measurements of local S and SKS phases at two stations (MNI in Sulawesi, DAV in the Philippines), as well as downgoing S phases at five stations at teleseismic distances. Combining different phases allows a better vertical resolution of anisotropic fabrics than is possible with a single phase. The broad depth distribution of local events (˜60-630 km) allows us to observe a change in splitting behaviour at ˜380 km depth: above, fast directions (?) are trench-parallel and delay times (?t) are ˜0.34-0.53 s with no increase with depth. We suggest this anisotropy is caused by aligned cracks, possibly melt-filled beneath the volcanic arc, and fossil anisotropy in the overriding plate. Below ˜380 km, ? is predominantly trench-normal and ?t are slightly higher (˜0.53-0.65 s). As no correlation is observed with inferred distance travelled inside the slab, we attribute this anisotropy to shear layers atop the slab, which are coherent from ˜200 to 400 km depth and perhaps extend into the transition zone. SKS and source-side measurements show larger ?t (˜1.53 and 1.33 s, respectively) and trench-parallel ?. Since these phases predominantly sample sub-slab mantle, we consider along-strike lateral flow associated with the double-sided subduction of the Molucca Sea microplate to be the most likely explanation. We thus infer three dominant regions of anisotropy at the Sangihe subduction zone: one within the overriding lithosphere, one along the slab-wedge interface, and one below the subducting Molucca Sea slab. The mantle wedge above 200 km depth and the slab itself do not seem to contribute notably to the measured anisotropy. This study demonstrates the insight seismic anisotropy can provide into mantle dynamics even in tectonically complex subduction systems.

Di Leo, J. F.; Wookey, J.; Hammond, J. O. S.; Kendall, J.-M.; Kaneshima, S.; Inoue, H.; Yamashina, T.; Harjadi, P.

2012-03-01

289

Thermal Regime of the Makran Subduction Zone and Relationship to Past and Future Megathrust Earthquakes  

NASA Astrophysics Data System (ADS)

The Makran subduction zone in the northern Arabian Sea is an end-member among global subduction zones due to the thickness of its input sediment section, a very wide (>200km) accretionary prism, the shallow dip of the subducting plate and relatively old subducting lithosphere (70-100Ma). In general, seismicity in the Makran is relatively low when compared to most other subduction zones such as Nankai or Chile. However, the margin experienced a tsunamigenic Mw 8.1 earthquake in 1945, possibly with a shallow thrust focal mechanism supported by previous dislocation modelling of uplift data. More recent smaller-magnitude offshore earthquakes also suggest seismicity on the plate interface. Seismicity on the shallow part of the megathrust beneath the outer accretionary prism at this margin is surprising considering conventional models for the position of the plate boundary seismogenic zone and the very thick sediment section on the subducting oceanic plate. The extreme thickness of incoming sediment (>7km) at the Makran has previously led to the suggestion that this margin may have a strong aseismic component, reasons being perceived presence of overpressured fluids and low strength of unconsolidated sediment. However, overpressure may in fact not be significant and the state of sediment consolidation depends on the in situ temperature and pressure conditions. To further understand the seismogenic potential of this margin we have developed a 2-D thermal model along a representative profile of the subduction zone, constrained by limited surface heat flow observations. The geometry of the model is constructed from seismic reflection data, seismicity where present, and the position of the volcanic arc. The estimated thermal structure and heat flow of the model are compared with heat flow data from the offshore prism, including direct probe measurements and values derived from depths to the base of the gas-hydrate stability zone (BSR). The primary results from the thermal modelling are twofold. 1) The high sediment cover on the incoming oceanic plate leads to high (~150°) plate boundary temperatures at the deformation front making the megathrust potentially seismogenic to a very shallow depth, and 2) If the downdip seismogenic limit is defined by a temperature of 350-450°C, the shallow dip of the subducting plate (and so shallow dip of the thermal contours) leads to a wide potential seismogenic zone. These results suggest that, though there may be an aseismic zone in the outermost accretionary prism, the thermal characteristics of the Makran do not preclude a significant megathrust rupture, with a possible shallow component. Potential earthquake rupture areas and subsequent magnitudes are generated based on these constraints of the seismogenic zone.

Smith, G. L.; McNeill, L. C.; Wang, K.; He, J.; Henstock, T.

2012-12-01

290

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

USGS Publications Warehouse

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

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

2004-01-01

291

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

NASA Astrophysics Data System (ADS)

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.

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

1996-05-01

292

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

PubMed

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

Rogers, Garry; Dragert, Herb

2003-06-20

293

Mineral rheological constraints on the stress limits in the subduction zone and its applications to deep earthquakes  

NASA Astrophysics Data System (ADS)

Most deep earthquakes occur in subduction zone and there are almost no earthquakes at the depth over 700 km. Some previous studies showed that the cufoff of the deepest earthquakes are related to the absence of the transformational faulting caused by metastable phase or loss strength of slab due to superplasticity. Here we report that the low stress limit in deep subduction zone caused by perovskite rheological properties can account for this issue. In this study, the possible limits of stress accumulation at different depth in subduction zone were obtained by considering both the dislocation creep of minerals and the elastic deformation of the subducted slab. Firstly, the basic condition of stress accumulation was supposed to be that the elastic strain rate generated by plate motion is no less than the creep rate of the minerals in subduction zone. Secondly, the temperatures in the subduction zone were obtained by simulating the slab using kinematic method and the slab subduction velocity was supposed to be 5~10 cm per year. The limits of elastic strain at different depths were determined by supposing that the plate displacement was completely converted to elastic deformation. Furthermore, the elastic strain was assigned at different depths according to the phase distribution and compressibility. Finally, the dislocation creep rate for the possible minerals (olivine, wadsleyite, ringwoodite and perovskite) in subduction zone against temperatures were plotted at a range of 0.1~1000 MPa. The stress accumulation limits at different depths in the subduction zone during one year were obtained from the above plots by constraining the temperatures and the elastic strain. Our results showed that in the subduction zone the stress limits decrease by one or two orders of magnitude with increasing depth and phase transformation. In the slab where olivine dominates, stress can be accumulated to over 1000 MPa if there are no earthquakes occurring. In the area where wadsleyite and ringwoodite exist, stresses are around 100 MPa. However, at the depth where perovskite dominates, the maximum is only around 10 MPa. If we apply the stress (10~30 MPa) by the geophysical observation as the usual values in the subduction zone, in perovskite phase area, there are no earthquakes because dislocation creep can dissipate the extra stress. However in other mineral phase areas, earthquakes are needed to keep the relatively constant stress. We think that the rheological characteristics of perovskite can account for the focal depth limit of earthquakes in subdcution zone and there is no need the transformational faulting caused by the olivine-spinel transformation or superplasticity of ringwoodite dissociation products.

Xu, J.

2013-12-01

294

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)

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.

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

2014-12-01

295

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

USGS Publications Warehouse

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.

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

2009-01-01

296

The Impact of Accretionary Prism Heterogeneity on Seafloor Displacement during Large Subduction Zone Earthquakes  

NASA Astrophysics Data System (ADS)

Large earthquakes often occur in sediment dominated subduction zones where large accretionary prisms have formed. Deformation within the prism creates heterogeneity that can be very difficult to incorporate into models of rupture and coseismic seafloor deformation. Finite element modeling was used to assess the impact of prism heterogeneity on coseismic seafloor displacement. The modeling was based on information from the Nankai Trough subduction zone, where a large splay fault system (the megasplay) carries older sediments over younger, less consolidated sediments. This region is being investigated as part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). The major fault zones were incorporated into the model as frictional contacts. Different frictional properties were assigned to the deep portion of the plate boundary fault, the shallow portion of the plate boundary fault, and the megasplay fault. To simulate the earthquake cycle, a lower coefficient of friction was applied to the deep portion of the megathrust during the earthquake than during the interseismic period. This approach allowed stress to build up in the model region prior to the earthquake. Seafloor displacement was compared for simulations with a homogeneous prism and for simulations in which the splay fault's footwall was weaker than the hanging wall. The weaker footwall resulted in greater horizontal seafloor displacement but less vertical displacement than the homogeneous case. This difference in seafloor deformation may have implications for tsunami hazards.

Screaton, E.; Ge, S.; Regueiro, R.

2010-12-01

297

Three-dimensional numerical models of the influence of a buoyant oceanic plateau on subduction zones  

NASA Astrophysics Data System (ADS)

We have investigated potential effects on a subduction zone during oceanic plateau-trench collision, using three-dimensional numerical models. An oceanic plateau of varied density was embedded in the slab, which was pinned at one end. The plateaus strongly influenced the shape of the trench. For a plateau with a higher density, the trench retreat rate was reduced in the region surrounding the plateau, and the plateau subducted along with the slab. For lower density plateaus, the trench in the region of the plateau advanced, and the plateau compressed and resisted subduction, spreading laterally along the trench. With a weaker slab rheology, the arcuate shape of the trench towards the free end of the trench was enhanced. Beneath the most buoyant plateaus, a tear formed in the subducted portion of the slab, soon after the slab tip reached the top of the lower mantle. We compare the model results with a region in the northwest Pacific, where the Ogasawara Plateau meets the trench of the Izu-Bonin-Mariana subduction zone.

Mason, W. G.; Moresi, L.; Betts, P. G.; Miller, M. S.

2010-03-01

298

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

NASA Astrophysics Data System (ADS)

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.

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

2014-06-01

299

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

NASA Astrophysics Data System (ADS)

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.

Morozov, I. B.; Zheng, H.

2005-12-01

300

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

USGS Publications Warehouse

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.

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

2001-01-01

301

2-D magnetotelluric investigation of fluid controls involved in ETS in the Cascadia subduction zone  

NASA Astrophysics Data System (ADS)

Episodic tremor and slip (ETS) occurring in the Cascadia subduction zone is characterized by periodic retrograde displacements occurring at the interface between the cold, locked upper plate interface and the slow-slip deep, hot plate interface. There are currently several hypotheses regarding the mechanisms controlling ETS, and while there is still debate as to what model best describes this phenomenon, most current hypotheses involve fluid at the plate interface. There are many unanswered questions regarding the role and source of fluids in ETS. While answering these questions can be difficult due to the depths at which ETS occurs, 25-45 km, electromagnetic geophysical methods offer the best method for fluid identification at the plate interface, in particular long-period magnetotellurics (MT) since electrical conductivity of crustal rocks is highly sensitive to the presence and interconnectivity of fluids that may be involved in ETS. As the first stage of a large scale 3-D MT model of the Cascadia subduction zone, a 2-D West to East transect across southwestern Washington was analyzed to provide initial results and interpretations. The West-East transect was obtained by 8 long-period MT stations with a spacing of ~15 km. The results of the 2-D model are expected to illuminate possible fluid controls involved with ETS, furthering our understanding of this relatively newly discovered tectonic process. With further understanding of the mechanisms controlling ETS, it is possible that better understanding of mega-thrust earthquakes will follow.

Blom, L.; Schultz, A.; Bowles-martinez, E.

2013-12-01

302

Subducting Seamounts and the Rupturing Process of Great Subduction Zone Earthquakes  

NASA Astrophysics Data System (ADS)

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.

Das, S.

2009-05-01

303

Automated Detection and Modeling of Slow Slip: Case Study of the Cascadia Subduction Zone  

NASA Astrophysics Data System (ADS)

The discovery of transient slow slip events over the past decade has changed our understanding of tectonic hazards and the earthquake cycle. Proper geodetic characterization of transient deformation is necessary for studies of regional interseismic, coseismic and postseismic tectonics, and miscalculations can affect our understanding of the regional stress field. We utilize two different methods to create a complete record of slow slip from continuous GPS stations in the Cascadia subduction zone between 1996 and 2012: spatiotemporal principal component analysis (PCA) and the relative strength index (RSI). The PCA is performed on 100 day windows of nearby stations to locate signals that exist across many stations in the network by looking at the ratio of the first two eigenvalues. The RSI is a financial momentum oscillator that looks for changes in individual time series with respect to previous epochs to locate rapid changes, indicative of transient deformation. Using both methods, we create a complete history of slow slip across the Cascadia subduction zone, fully characterizing the timing, progression, and magnitude of events. We inject the results from the automated transient detection into a time-dependent slip inversion and apply a Kalman filter based network inversion method to image the spatiotemporal variation of slip transients along the Cascadia margin.

Crowell, B. W.; Bock, Y.; Liu, Z.

2012-12-01

304

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

SciTech Connect

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.

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

1992-07-01

305

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

E-print Network

) 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

Paris-Sud XI, Université de

306

Geophysical signature of hydration-dehydration processes in active subduction zones  

NASA Astrophysics Data System (ADS)

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.

Reynard, Bruno

2013-04-01

307

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

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

308

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

NASA Astrophysics Data System (ADS)

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.

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

2012-12-01

309

Impact of fluid-rock and metamorphic reactions on style of rifting during formation of hyper-extended continental margins.  

NASA Astrophysics Data System (ADS)

It is now well established that the geometries of different rifted margins , specifically hyper-extended, cannot be explained by a simple scenario of extension of a horizontally uniform lithosphere. Indeed, over the last decade, the scientific community pushed forward models with heterogeneous initial conditions to simulate the impact of structural inheritance on the geometry and on the amount of pre-break up extension accommodated by passive margins. In this study, we develop a principally new model of structural inheritance where the latter is introduced in a physically-consistent way, as a consequence of metamorphic reactions and of the associated fluid pathways impacting crustal rheology during extension. The metamorphic reactions imply important mass transfers and occurrence of non-hydrostatic fluid pressure gradients during deformation. These two phenomena are known to have a considerable influence on the effective mechanical properties of the rocks within the shear zones. The observed P-T-t paths indicate that during extension, the rocks experience initial decompression, followed by a phase of reheating and ended , almost systematically, by a retrograde phase, in which temperature and pressure diminish with increasing deformation. The occurrence of retrograde reactions implies that at some stages the water re-enters the dehydrated parts of the system, resulting in rheological (re)-softening. Otherwise, without water, metamorphic reactions would not occur and the rocks would have preserve their dry (i.e. strong) rheologies. These last factors, i.e. meta-stable states and retrograde reactions in presence of water, are not accounted in current thermo-dynamically coupled thermo-mechanical models. However, they are crucial for localization of deformation and exhumation of rocks. Fluid circulation is also affected by rock porosity and permeability that in-turn are the functions of strain and of the degree of metamorphism. Using a new numerical approach that couples fluid flow and out-of-equilibrium thermodynamics, we undertake a parametric study of porosity and permeability using well calibrated metamorphic data from the Alps allowing for account for impact of metamorphic inheritance on rifting. We demonstrate that these factors influence the rheology of the lithosphere and deformation styles at large scale , in particular enabling migration of the level of necking during rifting.

Mezri, Leila; Le Lepourhiet, Leatitia; Burov, Evgene; Gorini, Christian

2013-04-01

310

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

NASA Astrophysics Data System (ADS)

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.

Reynard, B.

2011-12-01

311

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

312

Searching for slow-slip events at the Sumatran subduction zone  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

313

Slow slip events and seismic tremor at circum-Pacific subduction zones  

NASA Astrophysics Data System (ADS)

It has been known for a long time that slip accompanying earthquakes accounts for only a fraction of plate tectonic displacements. However, only recently has a fuller spectrum of strain release processes, including normal, slow, and silent earthquakes (or slow slip events) and continuous and episodic slip, been observed and generated by numerical simulations of the earthquake cycle. Despite a profusion of observations and modeling studies the physical mechanism of slow slip events remains elusive. The concurrence of seismic tremor with slow slip episodes in Cascadia and southwestern Japan provides insight into the process of slow slip. A perceived similarity between subduction zone and volcanic tremor has led to suggestions that slow slip involves fluid migration on or near the plate interface. Alternatively, evidence is accumulating to support the notion that tremor results from shear failure during slow slip. Global observations of the location, spatial extent, magnitude, duration, slip rate, and periodicity of these aseismic slip transients indicate significant variation that may be exploited to better understand their generation. Most slow slip events occur just downdip of the seismogenic zone, consistent with rate- and state-dependent frictional modeling that requires unstable to stable transitional properties for slow slip generation. At a few convergent margins the occurrence of slow slip events within the seismogenic zone makes it highly likely that transitions in frictional properties exist there and are the loci of slow slip nucleation. Slow slip events perturb the surrounding stress field and may either increase or relieve stress on a fault, bringing it closer to or farther from earthquake failure, respectively. This paper presents a review of slow slip events and related seismic tremor observed at plate boundaries worldwide, with a focus on circum-Pacific subduction zones. Trends in global observations of slow slip events suggest that (1) slow slip is a common phenomena observed at almost all subduction zones with instrumentation capable of recording it, (2) different frictional properties likely control fast versus slow slip, (3) the depth range of slow slip may be related to the thermal properties of the plate interface, and (4) the equivalent seismic moment of slow slip events is proportional to their duration (Mo??), different from the Mo??3 scaling observed for earthquakes.

Schwartz, Susan Y.; Rokosky, Juliana M.

2007-09-01

314

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

USGS Publications Warehouse

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

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

1997-01-01

315

An Investigation of Anatolian-African Subduction Zone in Southwestern Aegean:Lithospheric Structure Beneath Isparta Angle(IA)and the Surroundings from Surface Wave Tomography  

Microsoft Academic Search

Our primary objective is to obtain the lithospheric structure of Anatolian-African Subduction zone and the surroundings including the Isparta Angle(IA) from Phase velocity inversion of Rayleigh waves. IA is seismically active and formed by the intersection of two very different subduction zones: The Hellenic arc to the west and the Cyprian arc to the east. The Hellenic arc is characterized

U. M. Teoman; E. A. Sandvol; N. Turkelli

2009-01-01

316

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

E-print Network

; 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

Biggs, Juliet

317

The thermal effects of steady-state slab-driven mantle flow above a subducting plate: the Cascadia subduction zone and backarc  

Microsoft Academic Search

At subduction zones, geophysical and geochemical observations indicate that the arc and backarc regions are hot, in spite of the cooling effects of a subducting plate. At the well-studied Cascadia subduction zone, high mantle temperatures persist for over 500 km into the backarc, with little lateral variation. These high temperatures are even more surprising due to the juxtaposition of the

C. A. Currie; K. Wang; Roy D. Hyndman; Jiangheng He

2004-01-01

318

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

USGS Publications Warehouse

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

Mibe, K.; Chou, I. -M.; Bassett, W. A.

2008-01-01

319

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.

320

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

NASA Astrophysics Data System (ADS)

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.

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

2012-12-01

321

Geodetic Observations of Interseismic Strain Segmentation at the Sumatra Subduction Zone  

NASA Technical Reports Server (NTRS)

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

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

1997-01-01

322

Unusually large shear wave anisotropy for chlorite in subduction zone settings  

NASA Astrophysics Data System (ADS)

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.

Mookherjee, Mainak; Mainprice, David

2014-03-01

323

The termination of the southern New Hebrides subduction zone (southwestern Pacific)  

NASA Astrophysics Data System (ADS)

Recent data (bathymetric map and petrological investigations) as well as reappraisal of local seismological events for more than 20 years allow us to present a new geodynamic interpretation of the southern New Hebrides subduction zone. A multidisciplinary approach to this complex area, where transform movements relay subduction process, clearly shows the interrelationships between intermediate seismicity, lithospheric plates tectonics and volcanic activity. According to our model, two hinge zones, tearing the downgoing slab along directions which parallel the convergence vector (N70°E), reasonably account for most seismological and petrological data from the region. The approach of the Loyalty islands ridge towards the trench and its increasing influence on the subduction regime is beginning to complicate this scheme.

Monzier, M.; Maillet, P.; Herrera, J. Foyo; Louat, R.; Missegue, F.; Pontoise, B.

1984-01-01

324

The subduction zone flow field from seismic anisotropy: a global view.  

PubMed

Although the morphologies of subducting slabs have been relatively well characterized, the character of the mantle flow field that accompanies subduction remains poorly understood. To analyze this pattern of flow, we compiled observations of seismic anisotropy, as manifested by shear wave splitting. Data from 13 subduction zones reveal systematic variations in both mantle-wedge and subslab anisotropy with the magnitude of trench migration velocity |V(t)|. These variations can be explained by flow along the strike of the trench induced by trench motion. This flow dominates beneath the slab, where its magnitude scales with |V(t)|. In the mantle wedge, this flow interacts with classical corner flow produced by the convergence velocity V(c); their relative influence is governed by the relative magnitude of |V(t)| and V(c). PMID:18202286

Long, Maureen D; Silver, Paul G

2008-01-18

325

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

USGS Publications Warehouse

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

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

2009-01-01

326

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

NASA Astrophysics Data System (ADS)

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

Myhill, R.; Warren, L. M.

2011-12-01

327

2010 Maule earthquake slip correlates with pre-seismic locking of Andean subduction zone.  

PubMed

The magnitude-8.8 Maule (Chile) earthquake of 27 February 2010 ruptured a segment of the Andean subduction zone megathrust that has been suspected to be of high seismic potential. It is the largest earthquake to rupture a mature seismic gap in a subduction zone that has been monitored with a dense space-geodetic network before the event. This provides an image of the pre-seismically locked state of the plate interface of unprecedentedly high resolution, allowing for an assessment of the spatial correlation of interseismic locking with coseismic slip. Pre-seismic locking might be used to anticipate future ruptures in many seismic gaps, given the fundamental assumption that locking and slip are similar. This hypothesis, however, could not be tested without the occurrence of the first gap-filling earthquake. Here we show evidence that the 2010 Maule earthquake slip distribution correlates closely with the patchwork of interseismic locking distribution as derived by inversion of global positioning system (GPS) observations during the previous decade. The earthquake nucleated in a region of high locking gradient and released most of the stresses accumulated in the area since the last major event in 1835. Two regions of high seismic slip (asperities) appeared to be nearly fully locked before the earthquake. Between these asperities, the rupture bridged a zone that was creeping interseismically with consistently low coseismic slip. The rupture stopped in areas that were highly locked before the earthquake but where pre-stress had been significantly reduced by overlapping twentieth-century earthquakes. Our work suggests that coseismic slip heterogeneity at the scale of single asperities should indicate the seismic potential of future great earthquakes, which thus might be anticipated by geodetic observations. PMID:20829792

Moreno, Marcos; Rosenau, Matthias; Oncken, Onno

2010-09-01

328

Dislocation model of strain accumulation and release at a subduction zone  

SciTech Connect

Strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface. This process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone, consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip. Because steady state subduction does not contribute to the deformation at the free surface, deformation observed there is completely equivalent to that produced by the slip cycle alone. The response to that slip is simply the response of a particular earth model to embedded dislocations. For a purely elastic earth model, the deformation cycle consists of a coseismic offset followed by a linear-in-time recovery to the initial value during the interval between earthquakes. For an elastic-viscoelastic earth model (elastic lithosphere over a viscoelastic asthenosphere), the postearthquake recovery is not linear in time. Records of local uplift as a function of time indicate that the long-term postseimic recovery is approximately linear, suggesting that elastic earth models are adequate to describe the deformation cycle. However, the deformation predicted for a simple elastic half-space earth does not reproduce the deformation observed along the subduction zones in Japna at all well if stick slip is restricted to the main thrust zone. As recognized earlier by Shimazaki, Seno, and Kato, the uplift profiles could be explained if stick slips were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km, but independent evidence suggests that stick slip at such depths if unlikely.

Savage, J.C.

1983-06-10

329

Dehydration kinetics of talc and 10 Å phase: Consequences for subduction zone seismicity  

NASA Astrophysics Data System (ADS)

The process of dehydration embrittlement is usually proposed as an explanation for the presence of intermediate-depth earthquakes in subduction zones. It assumes that the release of water by hydrous mineral breakdown is fast enough to provoke brittle failure. We performed high-pressure, high-temperature, dehydration experiments of talc and 10 Å phase coupled with in situ measurement of reaction kinetics using synchrotron X-ray diffraction. Newly developed, X-ray transparent, pressure-sealed, titanium capsule ensured a closed thermochemical environment. From isothermal kinetics data fitted to the Avrami's equation and from the texture of reaction products, we conclude that dehydration rates of these minerals are limited by diffusion. Predicted minimum rates of fluid release range from 10 - 4 to 9 × 10 - 6 m 3fluid m - 3 rock s - 1 , and are fast enough to provoke hydraulic rupture since Maxwell relaxation rate of rocks relevant of subduction zones are slower than the rate of fluid release. These rates are comparable between talc, 10 Å phase and antigorite also [Perrillat, J.-P., Daniel, I., Koga, K.T., Reynard, B., Cardon, H., Crichton, W.A., 2005. Kinetics of antigorite dehydration: a real-time X-ray diffraction study. Earth Planet. Sci. Lett. 236, 899-913]. Consequently, we suggest that the dehydration of hydrous minerals may eventually be fast enough to trigger the intermediate-depth earthquakes, and that the deepest among intermediate-depth earthquakes may actually locate the limits for dehydration of hydrous minerals in the downgoing lithosphere.

Chollet, Mélanie; Daniel, Isabelle; Koga, Kenneth T.; Petitgirard, Sylvain; Morard, Guillaume

2009-06-01

330

Controls on the Morphological Variability of the Decollement in the Nankai Trough Subduction Zone  

NASA Astrophysics Data System (ADS)

Decollements form plate boundaries at subduction zones, hence the details of their initiation are of fundamental importance in the development of large fault systems. A key question at Nankai is does the decollement initiate and remain in a singular layer or jump around and if so why? To investigate this issue, we analyzed 3-D seismic reflection data collected off the Muroto Peninsula by a US-Japanese consortium in 1999. We examined seismic profiles and correlated our interpretations to core and log data from ODP sites 1173, 1174, and 808 to determine the physical properties of the decollement from the undeformed incoming sediment to the deformation front. These seismic profiles indicate that substantial along-strike variability characterizes the decollement in the Nankai Trough subduction zone. The decollement forms a series of overlapping splays above an elongate basement high that is approximately 1 km wide, rises 100-150 m above the adjacent low areas, and trends northwest across the survey area. Seismic profiles show the amount of overlap along the decollement above the basement high ranges from 80 m seaward of the deformation front to 40 m at the frontal thrust. These initial observations suggest the basement relief significantly influences the undulating character of the decollement, becoming less of a factor with increased subduction. Other possible influences on the decollement morphology include the variation in stress orientation caused by the basement high and differences in overpressure resulting from along-strike differences in sediment loading. Determination of these factors will provide a better understanding of the controls of decollement formation.

Adamson, S. P.; Moore, C.; Bangs, N. L.; Gulick, S. P.; Moore, G. F.

2003-12-01

331

Geodetic observations of an earthquake cycle at the Sumatra subduction zone: Role of interseismic strain segmentation  

NASA Astrophysics Data System (ADS)

We use survey mode and continuous GPS data from 1991 to 2007 to examine fault segmentation in the earthquake cycle at the Sumatra megathrust, site of the 26 December 2004 Mw 9.1 Sumatra-Andaman, the 28 March 2005 Mw 8.7 Nias-Simeulue, and the 12 September 2007 Mw 8.4 Mentawai earthquakes. These data, including new observations from 2006 and 2007, allow us to observe the final few years of one earthquake cycle and the beginning of the next. Our analysis reveals that the megathrust is segmented, a characteristic that may persist through multiple earthquake cycles. The Nias-Simeulue earthquake ruptured approximately the same region that broke in 1861, a 300 km long segment abutting the Sumatra-Andaman rupture zone. Farther southeast, the Mentawai segment of the megathrust (0.5°S-5°S), which produced M > 8 earthquakes in 1797 and 1833, is fully locked in the interseismic period but is flanked by two freely slipping regions, the Batu Islands in the NW and Enggano in the SE. The 12 September 2007 Mentawai earthquake sequence ruptured only the southern one third of the 1833 rupture zone. We model postseismic deformation from the Sumatra-Andaman and Nias-Simeulue earthquakes and find that afterslip was concentrated updip and downdip, respectively, from the main shocks. Comparing the velocity fields before and after 2001, we find the subduction zone underneath the Batu Islands and Enggano, which, prior to the earthquakes, was partially to fully coupled, appears now to be slipping freely. Thus, while the segmentation of the subduction zone is preserved, interseismic coupling on the subduction fault may vary with time.

Prawirodirdjo, Linette; McCaffrey, Robert; Chadwell, C. David; Bock, Yehuda; Subarya, Cecep

2010-03-01

332

Dehydration of subducting serpentinite: Implications for halogen mobility in subduction zones and the deep halogen cycle  

NASA Astrophysics Data System (ADS)

We investigated the halogen (Cl, F, Br, and I) chemistry of serpentinites that record progressive dehydration during subduction from shallow oceanic environments via increased pressure and temperature conditions to complete breakdown of antigorite. The aim is to evaluate the relevance of serpentinites for halogen recycling in subduction zones and for deep mantle recharge of these elements. The halogen compositions of the analyzed samples indicate input from seawater and sedimentary sources during initial serpentinization of either subducting lithospheric mantle during slab bending or forearc mantle by uprising slab fluids. During the first dehydration stage (antigorite + brucite ? olivine + H 2O), fluids with high Br/Cl and I/Cl ratios are released resulting in residual serpentinites with lower Br/Cl and I/Cl ratios. Veins associated with this event and with the final antigorite breakdown (antigorite ? olivine + orthopyroxene + H 2O) show higher halogen ratios compared to their adjacent wall rocks, and they are similar to those found in arc volcanoes (F/Cl and I/Cl between ca. 0.083-1.5, and ca. 0.00038-0.0013, respectively). All measured deserpentinization samples show a narrow range in ? 37Cl values (between - 0.42‰ and + 0.92‰) overlapping the ? 37Cl values of seafloor serpentinites and confirming that no significant Cl isotope fractionation occurs during subduction dehydration of serpentinites. Our findings document the conservative behavior of halogens during subduction. Mass balance constraints reveal that serpentinites strongly control the halogen chemistry of deep subduction zone fluids and that descent of rock residues after deserpentinization strongly affects the halogen budget of the mantle.

John, Timm; Scambelluri, Marco; Frische, Matthias; Barnes, Jaime D.; Bach, Wolfgang

2011-08-01

333

Interseismic deformation along the Philippine Fault system and Manila subduction zone  

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

334

Variations of b-values at the western edge of the Ryukyu Subduction Zone, north-east Taiwan  

E-print Network

University, Chung- Li 32001, Taiwan Introduction In the westernmost part of the Ry- ukyu subduction zone (Warren and Latham, 1970) and in the area of geothermal systems (Wiemer and Wyss, 1997; Wiemer et al et al. (2004). 3370 earthquakes located north of 23.5�N were relocated with the SIMUL2000 program

Lin, Andrew Tien-Shun

335

GEOPHYSICAL RESEARCH LETTERS, VOL. 0, NO. 0, PAGES 0-0, M 0, 2001 Homogeneous vs heterogeneous subduction zone  

E-print Network

subduction zone models: Coseismic and postseismic deformation T. Masterlark1 , C. DeMets, and H.F. Wang on coseismic and postseismic deformation predictions for the 1995 Colima-Jalisco Mw=8.0 earthquake. The FEMs the inverse problem of estimat- ing dislocation distributions from coseismic GPS displace- ments. Predictions

DeMets, Chuck

336

Modeling the influence of Moho topography on receiver functions: A case study from the central Hellenic subduction zone  

Microsoft Academic Search

Pseudospectral 2D modeling of wave propagation provides a fresh look at receiver function data from the Hellenic subduction zone. It is shown that distinct Moho topography, which is suggested for this complex subduction environment from various geophysical data sets, can provide a conclusive explanation for previously observed “inverted” Moho phases in the forearc as well as for a complex sequence

B. Endrun; L. Ceranna; T. Meier; M. Bohnhoff; H.-P. Harjes

2005-01-01

337

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

E-print Network

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

Demouchy, Sylvie

338

3D Modeling of Strong Ground Motion in the Pacific Northwest From Large Earthquakes in the Cascadia Subduction Zone  

Microsoft Academic Search

The Cascadia subduction zone in the Pacific Northwest, USA, generates Great (megathrust) earthquakes with a recurrence period of about 500 years, most recently the M~9 event on January 26, 1700. Since no earthquake of such magnitude has occurred in the Pacific Northwest since the deployment of strong ground motion instruments, a large uncertainty is associated with the ground motions expected

K. B. Olsen; A. Geisselmeyer; W. J. Stephenson; P. M. Mai

2007-01-01

339

Empirical ground-motion Relations for Subduction-Zone Earthquakes and Their Applications to Cascadia and other regions  

E-print Network

Abstract Ground-motion relations for earthquakes that occur in subduction zones are an important input to seismic-hazard analyses in many parts of the world. In the Cascadia region (Washington, Oregon, northern California, and British Columbia), for example, there is a significant hazard from megathrust earthquakes along the subduction interface and from large events within the subducting slab. These hazards are in addition to the hazard from shallow earthquakes in the overlying crust. We have compiled a response spectra database from thousands of strong-motion recordings from events of moment magnitude (M) 5–8.3 occurring in subduction zones around the world, including both interface and in-slab events. The 2001 M 6.8 Nisqually and 1999 M 5.9 Satsop earthquakes are included in the database, as are many records from subduction zones in Japan (Kyoshin-Net data), Mexico (Guerrero data), and Central America. The size of the database is four times larger than that available for previous empirical regressions to determine ground-motion relations for subduction-zone earthquakes. The large dataset enables improved determination of attenuation parameters and magnitude scaling, for both interface and in-slab events. Soil

Gail M. Atkinson; David M. Boore

2003-01-01

340

Simulations of Seismic Hazard for the Pacific Northwest of the United States from Earthquakes Associated with the Cascadia Subduction Zone  

Microsoft Academic Search

We investigate the impact of different rupture and attenuation models for the Cascadia subduction zone by simulating seismic hazard models for the Pacific Northwest of the U.S. at 2% probability of exceedance in 50 years. We calculate the sensitivity of hazard (probabilistic ground motions) to the source parameters and the attenuation relations for both intraslab and interface earthquakes and present

M. D. Petersen; C. H. Cramer; A. D. Frankel

2002-01-01

341

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

Microsoft Academic Search

Tidal-wetland stratigraphy reveals that great plate boundary earthquakes have caused hundreds of kilometers of coast to subside at the Cascadia subduction zone. However, determining earthquake recurrence intervals and mapping the coastal extent of past great earthquake ruptures in this region are complicated by the effects of many sedimento!ogic, hydrographic, and oceanographic processes that occur on the coasts of tectonically passive

Alan R. Nelson; Ian Shennan; Antony J. Long

1996-01-01

342

Simulations of Seismic Hazard for the Pacific Northwest of the United States from Earthquakes Associated with the Cascadia Subduction Zone  

Microsoft Academic Search

-- We investigate the impact of different rupture and attenuation models for the Cascadia subduction zone by simulating seismic hazard models for the Pacific Northwest of the U.S. at 2% probability of exceedance in 50 years. We calculate the sensitivity of hazard (probabilistic ground motions) to the source parameters and the attenuation relations for both intraslab and interface earthquakes and

MARK D. PETERSEN; CHRIS H. CRAMER; ARTHUR D. FRANKEL

2002-01-01

343

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

Microsoft Academic Search

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

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

2009-01-01

344

Interseismic locking of the plate interface in the northern Cascadia subduction zone, inferred from inversion of GPS data  

Microsoft Academic Search

We inverted GPS velocities from 20 continuous and 53 campaign sites in the northern Cascadia subduction zone using a Bayesian inverse method to estimate the locking state of the plate interface. The results are consistent with previous estimates based on thermal arguments and forward modeling. They suggest that the completely locked segment of the plate interface is offshore and that

Shoichi Yoshioka; Kelin Wang; Stephane Mazzotti

2005-01-01

345

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

Microsoft Academic Search

Tidal-wetland stratigraphy reveals that great plate boundary earthquakes have caused hundreds of kilometers of coast to subside at the Cascadia subduction zone. However, determining earthquake recurrence intervals and mapping the coastal extent of past great earthquake ruptures in this region are complicated by the effects of many sedimentologic, hydrographic, and oceanographic processes that occur on the coasts of tectonically passive

Alan R. Nelson; Ian Shennan; Antony J. Long

1996-01-01

346

Viscosity of the asthenosphere from glacial isostatic adjustment and subduction dynamics at the northern Cascadia subduction zone, British Columbia, Canada  

Microsoft Academic Search

Late glacial sea level curves located in the Cascadia subduction zone (CSZ) fore arc in southwestern British Columbia show that glacial isostatic adjustment (GIA) was rapid when the Cordilleran Ice Sheet collapsed in the late Pleistocene. GIA modeling with a linear Maxwell rheology indicates that the observations can be equally well fit across a wide range of asthenospheric thicknesses, provided

Thomas S. James; Evan J. Gowan; Ikuko Wada; Kelin Wang

2009-01-01

347

Historical tsunami in the Makran Subduction Zone off the southern coasts of Iran and Pakistan and results of numerical modeling  

Microsoft Academic Search

Tsunami hazard in the Makran Subduction Zone (MSZ), off the southern coasts of Iran and Pakistan, was studied by numerical modeling of historical tsunami in this region. Although the MSZ triggered the second deadliest tsunami in the Indian Ocean, among those known, the tsunami hazard in this region has yet to be analyzed in detail. This paper reports the results

Mohammad Heidarzadeh; Moharram D. Pirooz; Nasser H. Zaker; Ahmet C. Yalciner; Mohammad Mokhtari; Asad Esmaeily

2008-01-01

348

Influences of recurrence times and fault zone temperatures on the age-rate dependence of subduction zone seismicity  

Microsoft Academic Search

Correlations among subduction zone seismicity, convergence rate and subducting plate age are reassessed considering the possible roles of both recurrence times and fault zone temperatures. Distributions of earthquakes with respect to subducting lithosphere age and convergence rate are grossly explained by a recurrence relation when ages and rates at the world's trenches are taken into account. Correlations between maximum earthquake

Robert McCaffrey

1997-01-01

349

Erratum to Empirical Ground-Motion Relations for Subduction Zone Earthquakes and Their Application to Cascadia and Other Regions  

Microsoft Academic Search

Ground-motion relations for earthquakes that occur in subduction zones are an important input to seismic-hazard analyses in many parts of the world. In the Cascadia region (Washington, Oregon, northern California, and British Columbia), for example, there is a significant hazard from megathrust earthquakes along the subduction interface and from large events within the subducting slab. These hazards are in addition

Gail M. Atkinson; David M. Boore

2008-01-01

350

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

NASA Technical Reports Server (NTRS)

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.

Vassiliou, M. S.

1983-01-01

351

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

SciTech Connect

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.

Ponko, S.C.; Peacock, S.M. [Arizona State Univ., Tempe, AZ (United States)] [Arizona State Univ., Tempe, AZ (United States)

1995-11-10

352

Jurassic arc volcanism on Crimea (Ukraine): Implications for the paleo-subduction zone configuration of the Black Sea region  

E-print Network

Jurassic arc volcanism on Crimea (Ukraine): Implications for the paleo-subduction zone January 2010 Accepted 19 July 2010 Available online xxxx Keywords: Black Sea region Jurassic­Cretaceous 40 on the paleosubduction zone configuration of the southeastern European margin in the Jurassic, we report 40 Ar/39 Ar

Utrecht, Universiteit

353

High-precision Dating of Metamorphism and Melt Segregation in a Convergent Margin Setting: the North Cascades Continental Magmatic Arc  

NASA Astrophysics Data System (ADS)

Convergent plate margins represent areas where the crust has undergone intense physical and chemical changes that may be tracked through the use of accessory mineral chronometers. The Skagit Gneiss is located at the southernmost extent of the > 1500 km long Coast Plutonic-North Cascades arc system. The Skagit has experienced a protracted thermal and deformational history with the emplacement of plutons from ca. 96 to 45 Ma that overlaps a transition from transpression (ca. 73 to 58 Ma) to transtension (55-45 Ma). Migmatitic metapelites in the core of the Skagit record metamorphism during significant crustal thickening, heating, and possibly during decompression, with peak pressure-temperature conditions of 8-10 kbar and 650-725 °C. Electron backscatter diffraction (EBSD) was utilized to investigate the fabric of the leucosomes located throughout the Skagit core. The results show that the leucosomes were affected by an intense low-temperature deformation post-melt crystallization, with quartz results yielding basal-a and prism-a slip. In order to better understand the timescales of metamorphism, deformation and partial melting in the Skagit, both monazite and zircon were dated from leucosomes representing a variety of textures (stromatic/discordant; fine-grained/pegmatitic) and from the host metapelite. Zircons from the metapelite commonly yield Cretaceous dates, with a youngest date of ca. 60 Ma. Leucosomes yield zircon with concordant dates that range from 68 Ma to 47 Ma. In comparison, monazite from individual leucosomes yield a variety of dates, with one group clustering near 48 Ma and a second set of older dates from 69 to 65 Ma. The latter monazite dates are consistently older than the zircons from the same leucosome, consistent with the possibility that the older monazites record the timing of prograde to possibly peak metamorphism in the Skagit. Similar monazite dates have been revealed from the metapelite. The Cretaceous zircon results may either represent the timing of melt crystallization or may reflect inheritance from the melt source rock. The Eocene zircon and monazite dates are at the young end of the age spectrum for the North Cascades arc system and overlap with zircon dates from tuffs in adjacent transtensional basins, suggesting that the arc remained at high-temperature and was undergoing partial melting during at least the initial stages of transtension. Moreover, the abundance and the duration of magmatism, and the young metamorphic ages reveal that the Skagit underwent a high-grade thermal history throughout the Late Cretaceous to Eocene with major deformation outlasting melt crystallization.

Gordon, S. M.; Bowring, S.; Whitney, D.; Miller, R.; McLean, N.

2008-12-01

354

Fluid/melt activities during the exhumation of subducted continental crust in the Sulu UHP metamorphic terrane, China  

NASA Astrophysics Data System (ADS)

The widespread occurrence of coesite in the UHPM rocks in the Sulu terrane suggests a rapid exhumation process. The extremely low oxygen isotopic compositions of eclogites imply that the protoliths of eclogites resided at mantle depths for a short time and did not undergo significant water-rock interaction on a regional scale. However, we report here field observations and zircon SHRIMP and LA-ICP/MS dating results demonstrating important fluid/melt activities, at least on a local scale, during the exhumation of UHPM rocks in the Sulu terrane. In the northern Sulu terrane, widespread granitic layers or veins in centimeter to several meter scale can be observed in felsic gneisses, especially in biotite-rich felsic gneisses. The layers/veins are strongly deformed and occasionally show foliation in concordance with the surrounding gneisses. Zircons from a granitic gneiss in Weihai have a core-mantle-rim structure. Oscillatory zoning is well preserved in cores and rims, and irregular patch structure is present in mantle domains. The cores and rims yielded ages of 772 +/- 7 Ma and 195 +/- 4 Ma, respectively, and the mantles show a large age span. Zircons from a pegmatite within the gneiss gave two ages of 221 +/- 7 Ma and 199 +/- 3 Ma. We interpret the age of 221 Ma as the time of pegmatite crystallization and 199 Ma as a post-magmatic thermal event. In the southern Sulu terrane, muscovite-bearing quartz veins and zoisite-kyanite-paragonite quartz veins or nodules occur in eclogites and country gneisses. They have mica-WR Rb-Sr isochron ages of 226 to 215 Ma. Zircons from a granitic gneiss from Qinglongshan show a very complicated internal structure. This is probably due to complete metamorphic recrystallization in a condition of strong fluid activity. U-Pb isotope analyses of these zircons only revealed a unique metamorphic age of 218 +/- 5 Ma. This is in strong contrast with the published zircon U-Pb ages of the Dabie-Sulu UHP metamorphic rocks in which protolith ages of 600 to 800 Ma are commonly recorded. The age of 218 Ma probably corresponds to the time of a complete Pb loss, produced by the metamorphic fluid event.

Li, H.; Jahn, B.

2005-12-01

355

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

NASA Astrophysics Data System (ADS)

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.

Mori, Yasushi; Shigeno, Miki; Nishiyama, Tadao

2014-12-01

356

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

PubMed

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 approximately 20 million years, rapid ( approximately 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

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

1997-09-01

357

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

PubMed Central

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

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

1997-01-01

358

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

NASA Astrophysics Data System (ADS)

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.

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

1997-09-01

359

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

NASA Astrophysics Data System (ADS)

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.

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

2014-11-01

360

Continental subduction and a mechanism for exhumation of high-pressure metamorphic rocks: new modelling and field data from Oman  

Microsoft Academic Search

The physical model presented reveals two principal regimes of continental subduction: a highly compressional (HC) regime and a low compressional (LC) regime characterised by high and low pressure between the overriding and subducting plates, respectively. The pressure is inversely proportional to the pull force, which depends on the difference between average density of the subducting lithosphere and density of the

Alexander I. Chemenda; Maurice Mattauer; Alexander N. Bokun

1996-01-01

361

Frictional behaviour of exhumed subduction zone sediments from the Shimanto Belt, Japan, at in-situ P-T conditions and implications for megathrust seismogenesis  

NASA Astrophysics Data System (ADS)

Seismogenesis on subduction zone megathrusts is generally thought to be limited to a region between the ~100-150°C isotherms, at ~5-15 km depth, and the ~350°C isotherm, typically at ~40 km depth. This zone is bounded at its up-dip and down-dip limits by aseismic zones. However, in recent years it has been discovered that very low frequency earthquakes (VLFE) and non-destructive Slow Slip Events (SSEs) or slow earthquakes nucleate in these presumed aseismic regions. Slip on megathrusts is likely to localize in the weak subducted sediments along the plate interface, which implies that the fault material is derived at least in part from these sediments. Therefore, understanding the depth distribution of seismicity and SSEs on megathrusts requires knowledge of the frictional behaviour of metapelites. We investigated such behaviour by performing shear experiments on natural megathrust fault gouges, derived from exhumed subduction zone sediments and faults exposed in the Shimanto Belt on Shikoku Island, Japan. These gouges correspond to peak paleo-temperatures of 105°C to 280°C, representing different stages in the diagenetic and metamorphic evolution of the subducted sediments, covering the shallow aseismic zone as well as the seismogenic zone. The composition of all gouges was dominated by illite/muscovite, with smaller amounts of quartz, feldspar and chlorite. We sheared these gouges at low displacement rates (0.1-100 micron/s) to address the nucleation of megathrust earthquakes and SSEs, using either a double-direct (biaxial) shear machine or a rotary shear machine. The double-direct shear experiments were performed at room temperature, 5% relative humidity and 50 MPa normal stress. The rotary shear experiments, in turn, were conducted at the sample-specific, approximate peak in-situ P-T conditions, i.e. the P-T conditions corresponding to the maximum burial depth of these samples. At room temperature, samples from different peak paleo-temperatures showed similar frictional behaviour, with near-neutral velocity dependence, i.e. stable or aseismic behaviour. When deformed at their approximate in-situ peak P-T conditions, on the other hand, the samples showed a progressive transition from strong velocity-strengthening (stable) behaviour at 105°C (notably at 10-100 micron/s), to velocity-weakening (unstable) behaviour at 280°C. The results at elevated P-T conditions match previous results on simulated illite-quartz analogue fault gouges and imply a broad transition in the slip stability of subduction megathrusts from stable (velocity-strengthening), to unstable (velocity-weakening) with increasing depth, in agreement with seismological observations.

den Hartog, Sabine; Niemeijer, Andre; Saffer, Demian; Marone, Chris

2014-05-01

362

Seismic Anisotropy and SKS Splitting in the Sangihe Subduction Zone Predicted from 3-D Mantle Flow Models  

NASA Astrophysics Data System (ADS)

Observations of shear wave splitting are often interpreted as being due to strain-induced crystal alignment of olivine in the convecting upper mantle, and the polarization of the fast shear wave is frequently taken to directly indicate the direction of mantle flow. Caution must be exercised when making such inferences, as the relationship between olivine lattice-preferred orientation (LPO) and fast direction is dependent on many factors, including the entire deformation history. This is especially the case in regions where complex time-dependent mantle flow is expected, e.g., subduction zones. Observations of shear wave splitting at subduction zones are varied, ranging from trench-perpendicular to -parallel fast directions, or a combination of both. Rigorously interpreting this variety of observations requires modeling which properly accounts for LPO development in the near-slab mantle environment. To this end, we simulate olivine LPO evolution caused by defomation of polycrystalline aggregates as they deform and move along pathlines extracted from a 3-D mantle flow model at a subduction zone (Li & Ribe, 2012). The model is based on 3-D boundary-element numerical simulations of a dense fluid sheet (representing the slab) with a geometry approximating that of the Sangihe subduction zone in Indonesia, where trench-parallel fast directions have recently been measured and ascribed to trench-parallel sub-slab mantle flow (Di Leo et al., 2012). This subduction zone is unique in that it is part of the only double-sided subduction system on Earth. At the Sangihe trench, the Molucca Sea plate is subducting westwards beneath the Eurasian plate. However, this microplate is also subducting eastwards at the nearby Halmahera trench. To test whether the measured trench-parallel fast directions are due to sub-slab mantle flow, and whether this is only possible due to the double-sided geometry, we use two different flow models: one with single- and one with double-sided subduction. The effect of deformation and LPO development is simulated assuming the defomation of each crystal is governed by the motion of dislocations. Interactions between crystals are descibed using the visco-plastic self-consistent (VPSC) approach. Unlike previous studies, we consider the entire subduction history from subduction initiation onwards. This approach is necessary for the Sangihe subduction zone, as it is fairly young and the slab has only just reached the bottom of the mantle transition zone. In older subduction zones, early textures may eventually be destroyed. After calculating elastic properties associated with LPO at multiple depths, we estimate the resulting splitting parameters (fast direction ?, delay time ?t) for synthetic SKS phases. We compare these measurements with splitting observations in the Sangihe subduction zone. Our models show that complex behavior in ? appears in even apparently simple models of subduction zone mantle flow and that making robust dynamic inferences requires proper consideration of the geometry of subduction.

Di Leo, J. F.; Li, Z.; Walker, A. M.; Wookey, J.; Kendall, J.; Ribe, N. M.; Tommasi, A.

2012-12-01

363

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)

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.

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

2013-12-01

364

Seismicity, topography, and free-air gravity of the Aleutian-Alaska subduction zone  

NASA Astrophysics Data System (ADS)

The Aleutian-Alaska subduction zone, extending 3400 km from the Queen Charlotte Fault to Kamchatka, has been the source of six great megathrust earthquakes in the 20th Century. Four earthquakes have ruptured the 2000-km-long Aleutian segment, where the Cenozoic Aleutian arc overlies the subducting Pacific plate. These include the 1946 M 8.6 earthquake off Unimak Is., the 1957 M 8.6 and 1986 M 8.0 earthquakes off the Andreanoff Is., and the 1965 M 8.7 Rat Is. earthquake. The source regions of these earthquakes inferred from waveform inversions underlie the well-defined Aleutian deep-sea terrace. The deep-sea terrace is about 4 km deep and is underlain by Eocene arc framework rocks, which extend nearly to the trench. It is bounded on its seaward and landward margins by strong topographic and fee-air gravity gradients. The main asperities (areas of largest slip) for the great earthquakes and nearly all of the Aleutian thrust CMT solutions lie beneath the Aleutian terrace, between the maximum gradients. Similar deep-sea terraces are characteristic of non-accretionary convergent margins globally (75% of subduction zones), and, where sampled by drilling (e.g., Japan, Peru, Tonga, Central America), are undergoing sustained subsidence. Sustained subsidence requires removal of arc crust beneath the terrace by basal subduction erosion (BSE). BSE is in part linked to the seismic cycle, as it occurs in the same location as the megathrust earthquakes. Along the eastern 1400 km of the Alaskan subduction zone, the Pacific plate subducts beneath the North American continent. The boundary between the Aleutian segment and the continent is well defined in free-air gravity, and the distinctive deep-sea terrace observed along the Aleutian segment is absent. Instead, the Alaskan margin consists of exhumed, underplated accretionary complexes forming outer arc gravity highs. Superimposed on them are broad topographic highs and lows forming forearc basins (Shumagin, Stevenson) and islands (Kodiak, Shumagin). Two great earthquakes ruptured much of this segment: the 1938 M 8.3 earthquake SW of Kodiak and the 1964 M 9.2 earthquake, which ruptured 800 km of the margin between Prince William Sound and Kodiak Island. Large slip during the 1938 event occurred under the Shumagin and Tugidak basins, but slip in 1964 is thought to have occurred on asperities under Prince William Sound and the outer arc highs off Kodiak. Seismic profiling and industry drilling indicates sustained subsidence has also occurred along the Alaska margin. BSE is probably occurring there, but the terrace structure is buried by the high sedimentation rate. At present, the inherited accretionary structures, the ongoing collision of the Yakutat terrane, and uncertainties in finite fault modeling obscure correlation of slip with topographic and gravity signatures in the 1964 source region.

Wells, R. E.; Blakely, R. J.; Scholl, D. W.; Ryan, H. F.

2011-12-01

365

Repeating and not so Repeating Large Earthquakes in the Mexican Subduction Zone  

NASA Astrophysics Data System (ADS)

The rupture area and recurrence interval of large earthquakes in the mexican subduction zone are relatively small and almost the entire length of the zone has experienced a large (Mw?7.0) earthquake in the last 100 years (Singh et al., 1981). Several segments have experienced multiple large earthquakes in this time period. However, as the rupture areas of events prior to 1973 are only approximately known, the recurrence periods are uncertain. Large earthquakes occurred in the Ometepec, Guerrero, segment in 1937, 1950, 1982 and 2012 (Singh et al., 1981). In 1982, two earthquakes (Ms 6.9 and Ms 7.0) occurred about 4 hours apart, one apparently downdip from the other (Astiz & Kanamori, 1984; Beroza et al. 1984). The 2012 earthquake on the other hand had a magnitude of Mw 7.5 (globalcmt.org), breaking approximately the same area as the 1982 doublet, but with a total scalar moment about three times larger than the 1982 doublet combined. It therefore seems that 'repeat earthquakes' in the Ometepec segment are not necessarily very similar one to another. The Central Oaxaca segment broke in large earthquakes in 1928 (Mw7.7) and 1978 (Mw7.7) . Seismograms for the two events, recorded at the Wiechert seismograph in Uppsala, show remarkable similarity, suggesting that in this area, large earthquakes can repeat. The extent to which the near-trench part of the fault plane participates in the ruptures is not well understood. In the Ometepec segment, the updip portion of the plate interface broke during the 25 Feb 1996 earthquake (Mw7.1), which was a slow earthquake and produced anomalously low PGAs (Iglesias et al., 2003). Historical records indicate that a great tsunamigenic earthquake, M~8.6, occurred in the Oaxaca region in 1787, breaking the Central Oaxaca segment together with several adjacent segments (Suarez & Albini 2009). Whether the updip portion of the fault broke in this event remains speculative, although plausible based on the large tsunami. Evidence from the mexican subduction zone therefore suggests that even if the same segments breaks repeatedly, individual earthquakes may or may not be similar. Furthermore, at least some of the segments can participate in larger earthquakes involving adjacent segments. The near trench part has only broken in two known large events, 800 km apart, the 1995 Jalisco (Mw8.0) and the 1996 event in the Ometepec segment. If the near-trench fault area between these two events can rupture seismically, and participate in larger events together with downdip segments, there would be important implications for seismic and tsunami hazard. In this presentation we review the earthquake history of the region and demonstrate the similarity and non-similarity of earthquakes in repeatedly breaking subduction zone segments, with particular emphasis on our recent work on events in the Ometepec segment.

Hjorleifsdottir, V.; Singh, S.; Iglesias, A.; Perez-Campos, X.

2013-12-01

366

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

NASA Astrophysics Data System (ADS)

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